2020

Monday, November 2, 2020 at 13:00, online
Design and analysis of membrane structures
Dr. Rostislav Lang, Faculty of Civil Engineering, Brno University of Technology and FEM consulting, s.r.o.
Online meeting links

Abstract:

The subject of the presentation are lightweight structures, composed mostly of membrane or cable parts. First, the special aspects during the design and analysis process will be introduced, which leads to the form finding and cutting patterns generation process, as well as to using the special wrinkling assumption in the analysis. Further, the methods for those processes will be presented and finally demonstrated by the calculation tool, developed during the doctoral research and implemented into the commercial software.

Wednesday, September 16, 2020, 10:00 CET, lecture room B
Corrosion study in subcritical and supercritical water: An electrochemical approach
Prof. Jan Macák, Department of Power Engineering, Faculty of enviromental technology, University of Chemistry and Technology Prague

Abstract:

Interest in supercritical water (SCW) is motivated by its use for different purposes: supercritical water is used as a working fluid and coolant in fossil-fueled power plants, supercritical water oxidation systems are designed for destruction of dangerous waste and a supercritical water-cooled reactor (SCWR) was selected as one of the six Generation IV International Forum concepts selected for further investigation. The experimental corrosion data obtained for SCW supported the corrosion model assuming a superposition of two parallel corrosion processes: a "chemical oxidation" (CO) mechanism and an "electrochemical oxidation" (EO) mechanism. Validity of this model was confirmed by in-situ electrochemical impedance spectroscopy measurements.

Tuesday, May 13, 2020, 1pm CET, online lecture
High-order methods in simulations of fluid dynamics problems
Dr. Jan Pech, Institute of Thermomechanics, Czech Academy of Sciences
Online meeting links

Abstract:

Solution accuracy is often a limiting factor for correct reproduction of physical phenomenons in numerical simulations. In the field of fluid dynamics, numerical inaccuracy may result in wrong flow instabilities or degeneration of flow structures. In previous decades, lower-order accurate methods were dominating for its robustness and ease of implementation, while resolution was compensated through sophisticated models.

Today, libraries implementing spectral/high-order (finite) elements are developed sufficiently (e.g. Nektar++) and bring an ambition to solve with high accuracy such systems as is the Navier-Stokes's. An additional benefit of the high-order approach is in insight to coefficient spectra over individual elements, since the decay of expansion coefficient spectra brings instant and computationally cheap information about quality of solution approximation in spatial coordinates.

The talk will introduce significant features of the spectral/hp element method and its application to problems currently investigated in Laboratory of CFD (D1), as is flow of heated/cooled fluid with variable material properties, fluid-structure interaction or flow separation in transition to turbulence.

Tuesday, May 6, 2020, 1pm CET, online lecture
Advanced Titanium Alloys for Medical Applications
Dr. Josef Stráský, Faculty of Mathematics and Physics, Charles University
Online meeting links

Abstract:

Titanium alloys, despite originally developed for aerospace application, are considered as a 'material of choice' for various types of medical implants. In the last two decades, dedicated Ti alloys for the biomedical use have been developed, such as Ti-35Nb-6Ta-7Zr alloy. These alloys must meet several requirements including good biocompatibility, satisfactory strength and low modulus of elasticity to avoid so-called stress shielding effect. In designing Ti alloys we typically face trade-off between high strength and low modulus. Our important finding is that alloying by oxygen leads to a significant interstitial strengthening. However, simultaneously, oxygen addition leads to an adverse increase of elastic modulus. We now understand that the body centered cubic parent matrix (beta phase) exhibits the lowest elastic modulus when the stability of the beta phase is low due to the 'proximity' to martensitic beta to alpha'' transformation (anomalous softening). Oxygen causes relative beta phase stabilization resulting in increased stiffness. We therefore designed several alloys with reduced stability of beta phase by reducing the content of niobium and tantalum which are beta stabilizing elements. We successfully broke the strength-modulus trade-off and achieved biocompatible Ti-based material with unparalleled combination of yield stress exceeding 900 MPa and modulus of elasticity below 70 GPa.

Tuesday, April 29, 2020, 1pm CET, online lecture
Achievements, agreements and quarrels of forefathers of mechanics
Prof. Miloslav Okrouhlík, Institute of Thermomechanics of the CAS
Online meeting links

Abstract:

The purpose of the lecture is to show that the original procedures, leading to discovery, or rather to invention, of mechanical laws and principles, that are taken for granted today, were lengthy, complicated and far from being straightforward.

Following lifetime destinies of a few forefathers of mechanics we try to unveil the difficulties and complications that they witnessed in derivations of their laws and formulas and to show how their personal grievances and bitter quarrels, that sometimes lasted for decades, complicated not only their mutual relations but sometimes made obstacles in scientific communications between nations.

We will concentrate on lives and works of Galileo Galilei, Johannes Kepler, Robert Hooke, Christiaan Huygens, Isaac Newton and Gottfried Wilhelm Leibniz trying to bring their narratives into proper relations.

Tuesday, April 21, 2020, 1pm CET, online lecture
Application of boundary element type methods in computational aerodynamics
Dr. Chandra Shekhar Prasad, Institute of Thermomechanics of the CAS

Abstract:

Boundary elements type methods (BEM) are well known method in many engineering application of numerical simulation. BEM for the aerodynamic or hydrodynamic flow modeling was first introduced in 1940s in NASA by Falkner as vortex lattice method (VLM) and after 60s by Hess & Smith and it is termed as Panel method (PM), since then it became very popular in aircraft industry and NASA has setup special research department for PM research. In BEM or PM unlike field based CFD methods, only solid boundary in the flow field is needed to be discretized which greatly reduce the computational cost. These methods are good compromise of speed and accuracy and being used by many aircraft manufactures for aerodynamics and aero-elasticity calculation. During the talk mathematical back ground, numerical implementation and some example will be discussed.

Wednesday, April 15, 2020 at 13:00, online
Models of “semi-compressible” fluids as a compromise between incompressible fluids and compressible gasses
Prof. Tomáš Roubíček, Institute of Thermomechanics of the CAS

Abstract:

Starting from a so-called quasi-incompressible fluid models developed originally as an approximation of incompressible models, various models will be developed which are slightly compressible but, anyhow, not fully compressible as full gas-dynamical models. Being called "semi-compressible", particular models will enjoy the attributes:
1) propagation of pressure waves is allowed and the velocity dispersion is controlled in a specific way,
2) the energy balance is (at least formally but sometimes rigorously) preserved,
3) the pressure is well defined in a reasonable sense (in some models even on the boundary),
4) the equations are consistently written in Eulerian coordinates, the model being thus fully convective,
5) in some models, uniqueness of weak solutions holds even in the physically relevant threedimensional cases.
Various gradient theories will be applied and particular presented models will exhibit normal or anomalous dispersion. Also coupling with other phenomena will be mentioned, specifically the diffusion governed by the chemical-potential gradient possibly also anisothermal, or electrically-conductive enhancement leading to magneto-hydrodynamics. In specific situation, Bernoulli's principle will be seen, too.

 

Wednesday, March 11, 2020 at 10:00, Lecture Room B
Thermomechanics in optical fibre drawing, splicing, and everyday use
Prof. Pavel Honzátko, Institute of Photonics and Electronics of the Czech Academy of Sciences

Abstract:

Overview of the optical fibre drawing technology in the Institute of Photonics and Electronics of the Czech Academy of Sciences (ÚFE) will be presented. Some future trends in the fibre technology will be discussed together with related thermomechanical problems which include preform shaping by laser ablation, capillary drawing, hollow core and air-clad fibre drawing, polymer and metal coating, thermomechanical polymer stripping, fibre fusion splicing and tapering, splice testing, fibre cooling, and mitigation of acousto-optical effects.

Thursday, February 27, 2020 at 14:00, Lecture Room B
Structural Design and Analysis at OHB System AG
Dr. Markus Geiß, Structural and Thermal Development Engineer, OHB System AG, Weßling, Germany

Abstract:

This lecture gives an introduction to the field of structural design and analysis of optical space instruments at OHB System AG. OHB System AG is one of the three leading space companies in Europe which specializes in design and manufacturing of high-tech solutions for space, science and industrial applications. To start the lecture, special considerations of spacecraft structures are discussed and the typical tasks of a structural development engineer are presented. Next, design highlights of optical instrument structures and their analysis methodologies are reviewed. After a brief discussion of topology optimization and additive manufacturing used for design and manufacturing of optical space instruments, thermos-elastic aspects of spacecraft structures are addressed. Finally, the process of assembly, integration and testing (AIT) is discussed, which typically concludes the hardware development-cycle of space structures.

Monday, February 17, 2020 at 10:00, Lecture Room B
Experimental and Numerical Procedures for Calibration of Advanced Phenomenological Models of Metal Plasticity
Dr. Slavomír Parma, Institute of Thermomechanics, Czech Academy of Sciences

Abstract:

The talk summarizes the research done within author’s internship at the Northern Arizona University, AZ, USA. Elastic domain of metals is bounded by the yield surface. When the material is loaded over the elastic limit, changes in size, position, and shape of the yield surface are observed. The new experimental procedure that employs hollow tubular specimens was developed to detect these phenomena. In the experiment, specimens are axially preloaded over the initial elastic limit to develop a distorted shape of the yield surface and consequently loaded by a sequence of combined axial load and torque to carefully probe the boundary of the elastic domain. This experimental methodology requires (i) a real-time evaluation of the effective plastic strain, and (ii) a real-time conditional control of the experiment.

The numerical part of this research is focused on the calibration of current models of phenomenological plasticity on the experimental data of multiaxial ratcheting. These kinds of models usually feature about 10–20 material parameters and need to be calibrated numerically. The proposed calibration procedure employs numerical integration of models combined with the optimization method based on the gradient descent. The algorithm is coded in FORTRAN language.

Wednesday, January 8, 2020, 10:00, Lecture Room B
First-principles calculations of elastic constants for complex systems
Ing. Martin Zelený, Ph.D., Brno University of Technology

Abstract:

First principles or ab initio means to perform the calculations of the properties of a system from fundamental quantum mechanics with no parametrization or knowledge of experimental data. The Density Functional Theory (DFT) forms the basis for most of current first-principles methods and it is able to provide the exact information about the electronic structure of the system under study and subsequently about its internal energy. Nowadays, employing DFT for calculations of elastic properties is routine task for system with ideal crystal lattices. On the other hand, estimation of elastic properties is still challenging for systems with chemical disorder or with defect in crystal lattice despite large computational resources available to the scientific community. The recently introduced stress-strain method together with the DFT package VASP is able to provide the full matrix of elastic constants with reasonable computational cost also for large supercells describing lattices with defects, e.g. grain boundaries or twin boundaries. Large supercells are also required for description of alloys with chemical disorder by the supercell-based quasirandom structures (SQS). Further directional optimization of SQS (do-SQS) supercell is necessary to obtain proper matrix of elastic constants that will reflect the symmetry of the simple ordered lattice. Using of the stress-strain method and do-SQS approach will be demonstrated for Ni3Si and Ni3Al intermetallics, CoCrNi high entropy alloy and Ni2MnGa magnetic shape memory alloy.

2019

Monday, December 9, 2019, 13:00, Lecture Room B
Application of the method of localized Lagrange multipliers to the partitioned solution of large-scale structural dynamic systems: The AFETI algorithm
Prof. José González, Universidad de Sevilla, Spain

Abstract:

In this talk, we will discuss about classical and new numerical techniques used for parallel/partitioned computations in structural mechanics and also multi-physics or coupled-field problems. In this field, methods based on classical Lagrange multipliers, like the two-level FETI-DP method, have been de facto the preferred parallel algorithms in solid and structural mechanics for decades. However, as we will see, classical Lagrange multipliers also present some limitations. The method of localized Lagrange multipliers (LLM) is a more general coupling technique, that introduces an explicit definition of the problem interface and brings some important advantages. Under this LLM framework, new partitioned algorithms like AFETI-C method are derived from variational principles. AFETI-C uses a combination of rigid-body modes and dominant substructural deformation modes in enforcing the interface force equilibrium equation as constraint conditions. In addition, a regularization of heterogeneities of partitioned systems is appended to AFETI-C that makes it competitive with FETI-DP. These methods, their derivation and their performance will be described, providing a full understanding of the potential of LLM in the solution of coupled problems.

Friday, December 6, 2019, 11:00, Lecture Room Klub
Cavitation and separation during water entry and exit
Alexander Korobkin, Professor in Applied Mathematics

Abstract:

First part of the presentation will explain the pressure distributions over the wetted surfaces of a parabolic contour in 2D and an elliptic paraboloid in 3D for oblique water impact. The pressures are calculated by the Wagner model of water entry with focus on zones of the wetted surface, where the hydrodynamic pressure is below the ambient pressure, and the zones, where the pressure approaches the vapour pressure. When the zones of low hydrodynamic pressure approach the contact line of the body surface, the surrounding air flows into this area separating the liquid surface from the body and leading to ventilation. Several models of ventilation and cavitation for 2D problem of oblique impact of rigid and elastic plates will be introduced. The nonlinear 2D problem of oblique impact of an elliptic cylinder onto a thin liquid layer with multiple bouncing of the cylinder from water will be presented. The second part of the presentation is about the water exit problems including the problem of an elastic disc lifted from water surface. The corresponding exit models will be applied to the 3D problem of a rigid ellipsoid gliding on water surface. Comparison of the obtained results with 3D CFD results will be shown. Our exit model of elastic bodies will be compared with the experimental results for large accelerations of the body lifting.

Wednesday, December 4, 2019, 10:00, Lecture Room B
Properties of open thermodynamic systems as the consequence of their stability
Prof. František Maršík, DrSc., Institute of Thermomechanics, The Czech Academy of Sciences, University of West Bohemia, Faculty of Physical Education and Sport, Charles University

Abstract:

The presented concept deals with production of entropy generated by the nonequilibrium processes in consequence of the mass and energy transfer. Often used concept of endoreversible thermodynamics is based on non-realistic conjecture that the entire entropy production is realized at the system boundary. In this contribution, the open system in the thermodynamically non-equilibrium state is assumed. Production of entropy is generated due to the non-equilibrium processes accompanied by the energy conversion. The non-equilibrium steady state is maintained by a negative entropy flux. The stability conditions of the state with the minimum of entropy production are used to replace the endoreversibility concept.

This theory is applied to three different open non-equilibrium systems.

i) Efficiency of thermal machines and chemical reactors.
Hydrogen fuel cell with polymer electrolyte membrane are studied in details. The transport coefficients for reactants inlet, i.e. hydrogen and air, and for the products outlet, i.e. water, are connected with the actual electric efficiency. The calculated efficiency qualitatively and quantitatively corresponds to the experimentally obtained values. The further research shall focus on the relation of the parameters characterizing the membrane and transport of reactants and products to the power output.

ii) Energetic limitations of population growth.
Entropy production is characterized by general form of chemical reaction based on the mass action law. This law is usable for description of dynamics of population biology, e.g. cells, species. Moreover, this law can be even used to study dynamics of ecological systems. The reproduction process is spontaneous process with increase of entropy. The entropy increase is compensated by the negative entropy flux from the Sun. From the thermodynamic point of view, the sex reproduction is more advantageous as the cellular division because of it is reached by the lower Gibbs free enthalpy. This is probably the reason why sex reproduction is evolutionarily more advantageous.

iii) Dynamics of ecological system with migration.
Influence of reproduction and migration dynamics is evident on example of two competitive ecological systems (in general two auto catalytic reactions) of type predator and prey. The migration decreases the frequency of dynamical state of system. Due the migration this dynamical state can change to the stationary state, when time period is high enough. In principle, it is a diffusion reaction system in which a stationary spatial change of concentrations can occur. An example may be the presence of colored stripes on the body of some animals, such as cats, some fish, hornets, and the like.

Friday, November 15, 2019, 10:00, Lecture Room B
Model reduction for the FEM of solids applied to the Rayleigh-Ritz computation of the free vibration spectrum
Prof. Petr Krysl, University of California, San Diego

Abstract:

Modal expansion is a workhorse used in many engineering analysis algorithms. One example is the coupled boundary element-finite element computation of the backscattering target strength of underwater elastic objects. To obtain the modal basis, a free-vibration (generalized eigenvalue) problem needs to be solved. This tends to be expensive when there are many basis vectors to compute. In the above mentioned backscattering example it could be many hundreds or thousands. Excellent algorithms exist to solve the free-vibration problem, and most use some form of the Rayleigh-Ritz (RR) procedure. The key to an efficient RR application is a low-cost transformation into a reduced basis. In this work we show how a cheap a priori transformation can be constructed for solid-mechanics finite element models based on the notion of coherent nodal clusters. The inexpensive RR procedure leads to not insignificant speedups of the computation of an approximate solution to the free vibration problem.

Wednesday, November 13, 2019, 13:00, Lecture Room B
Non-coaxiality between two tensors: Application to stress rate decomposition and non-coaxial invariants
Prof. Yannis F. Dafalias, Institute of Thermomechanics of the Czech Academy of Sciences, Prague

Abstract:

Given a stress tensor and its rate, what are the analytical expressions of the parts of the stress-rate tensor that are (a) coaxial with the stress; (b) non-coaxial with the stress; (c) proportional with the stress; (d) non-proportional but coaxial with the stress; (e) orthogonal and coaxial with the stress and (f) orthogonal and non-coaxial with the stress? To answer the foregoing questions the coaxial and totally non-coaxial parts of a tensor in regard to another reference tensor are derived in closed analytical form based on representation theorems of tensor-valued isotropic functions. In the process a new interpretation is obtained for a singular case of representation theorems. The particular application of rotational shear is presented where analytical expressions are obtained for the parts of a stress rate tensor that induce (1) change of stress principal axes at fixed principal stress values, and (2) change of stress principal values at fixed stress principal axes such that the deviatoric stress orbit is circular on the π-plane. Additional application in mechanics are discussed such as the definition and role of invariants related to the non-coaxial and orthogonal parts.

Tuesday, November 5, 2019, 10:00, Lecture Room A
Notes on Experimental Research on Transonic Compressor Blade Cascades
Dr. David Šimurda, Institute of Thermomechanics of the Czech Academy of Sciences, Prague

Abstract:

Despite ever increasing capabilities of CFD, experimental research on compressor blade cascades still plays important role in design and operation of gas turbines and other turbomachinery. This is true particularly in case of first stages of today’s large output gas turbine compressors and aircraft engine fans which operate at transonic range of relative inlet velocities. However, due to specific features of the flow past compressor cascades at transonic regimes, namely unstarted supersonic flow, experimental modelling is relatively complicated. Aim of the lecture will be to point out main difficulties connected with the cascade tests arising from the nature of compressor cascade flow and to provide ways of dealing with these problems.

Tuesday, October 22, 2019, 12:00, Lecture Room A
The Energy-Sampling Stabilization of Nodally Integrated Continuum Elements for Dynamic Analyses
Prof. Petr Krysl, University of California, San Diego

Abstract:

Nodally integrated elements exhibit spurious modes in dynamic analyses (such as in modal analysis). Previously published methods involved a heuristic stabilization factor, which may not work for a large range of problems, and a uniform amount of stabilization was used over all the finite elements in the mesh. The method proposed here makes use of energy-sampling stabilization. The stabilization factor depends on the shape of the element and appears in the definition of the properties of a stabilization material. The stabilization factor is non-uniform over the mesh, and can be computed to alleviate shear locking, which directly depends on the aspect ratios of the finite elements. The nodal stabilization factor is then computed by volumetric averaging of the element-based stabilization factors. Energy-sampling stabilized nodally integrated elements (ESNICE) tetrahedral and hexahedral are proposed. We demonstrate on examples that the proposed procedure effectively removes spurious (unphysical) modes both at lower and at higher ends of the frequency spectrum. The examples shown demonstrate the reliability of energy-sampling in stabilizing the nodally integrated finite elements in vibration problems, just sufficient to eliminate the spurious modes while imparting minimal excessive stiffness to the structure. We also show by the numerical inf-sup test that the formulation is coercive and locking-free.

Monday, October 21, 2019, 10:00, Lecture Room B
Lessons to be learned from German Attempts to Reduce Atmospheric CO2-Emissions
Prof. Dr-Ing. Roland Span, Ruhr University Bochum, Germany

Abstract:

With the goal to limit the increase of the average global temperature to 1.5 to 2 K, the governments of almost all developed countries agreed to drastically reduce atmospheric CO2 emissions. Though the common political will is clearly declared, concepts how the emission of greenhouse-gas emissions can be drastically reduced are very different and seem only partly realistic and appropriate. In this context Germany has launched the “Energiewende” program. This program relies essentially on a drastic increase in renewable power-production with wind and sun being the main energy sources and on reduced energy consumption in industry, private households, and traffic. Even though the availability of both wind and sun is rather limited in Germany, the program can so far be considered a success with regard to renewable power-production. The increase in renewable power-productions exceeded the politically formulated goals. However, currently the further increase is slowed down by different political measures, because a number of (mostly non-technical) limitations became obvious, that have not been addressed properly. And the required energy savings in the different sectors could not be realizedto date. Aspects of sectorial coupling have not been considered properly and problems in the areas of mobility and heat supply were underestimated. The presentation will briefly address the observed limitations and will formulate a number of theses derived from these findings. Not all of the factors influencing the further development of renewable power-production in Germany are relevant in other countries as well, but still the derived theses may serve as a starting point for general discussions on options for energy systems with largely reduced atmospheric CO2-emissions.

Wednesday, October 2, 2019, 10:00, Lecture Room B
Digital image correlation: from static tests to X-ray tomography and high strain-rate loading
Prof. Ondřej Jiroušek, Faculty of Transportation Sciences, Czech Technical University in Prague

Abstract:

In the short lecture an overview of digital image correlation (DIC) applied to strain measurement of samples loaded in mechanical tests in general will be given. Examples of using the method in several engineering applications will be given, extension of the method that enables to use it in 3D (using time-lapse tomography) will be introduced and examples of experiments performed on a wide range of materials, ranging from trabecular bone samples, whole bone samples (vertebral bodies) to metallic foams will be presented. A special attention will be given to high-strain rate loading using Split Hopkinson Pressure Bar technique.

Wednesday, October 2, 2019, 10:30, Lecture Room B
High Strain-rate Experiments Using Hopkinson Bar: Application on Cellular Metals and Additively Manufactured Auxetic Structures
Ing. Tomáš Fíla, Faculty of Transportation Sciences, Czech Technical University in Prague

Abstract:

Cellular solids, such as metal foams, hybrid foams, 3D printed lattices or additively manufactured auxetic structures are complex lightweight cellular materials with high energy absorption capabilities and possible functionally graded material properties. Thus, mechanical behavior of the materials under the representative loading conditions (i. e., dynamic impact, blast) has to be well understood. In this study, results of several experimental campaigns covering high-strain rate testing of cellular solids using conventional Split Hopkinson Pressure Bar (SHPB) and direct impact Open Hopkinson Pressure Bar (OHPB) are presented. High-speed imaging together with custom digital image correlation (DIC) technique are introduced as vital techniques for a complex experimental analysis of the materials at high strain-rates. Examples covering the evaluation of the displacement and strain fields, different methods for evaluation of Poisson’s ratio, and the analysis of the digital image correlation reliability are shown. Comparison of the digital image correlation results with the other methods (e. g. strain-gauges), its limitations and the actual challenges in this field are discussed. Overview of the experiments conducted at low and elevated temperatures observed using high-speed thermal imaging will be provided as well.

Monday, September 23, 2019, 14:00, Lecture Room A
Twin mortar method: A new unbiased mesh tying formulation
Ing. Ján Kopačka, Ph.D., Institute of Thermomechanics, Czech Academy of Sciences

Abstract:

This work was motivated by the author's six-month stay in the Aerospace Mechanics Research Center of the University of Colorado Boulder. The author joined the Multi-Physics Design Optimization group focusing on the level-set eXtended Finite Element Method (XFEM) topology optimization. The main aim was to revise existing interface formulations and come up with a new one that would be robust and stable enough to be used with the level-set XFEM. The mesh tying is an important issue encountered in the finite element analysis of complex structures. It enables to join the adjacent dissimilarly meshed parts or their regions. This problem is even more pronounced in the case of isogeometric analysis that is a modern spatial discretization technique which instead of Lagrange shape functions utilizes NURBS basis functions. Conventional mesh tying methods are based on the master-slave concept that leads to a biased algorithm. Consequently, results are influenced by the selection of the master and the slave interface. Inspired by the two-pass dual formulations, we come up with a new formulation which inherits all appealing properties of the mortar method. Namely, it preserves optimal convergence rates and is variationally consistent. At the same time, the newly proposed mesh tying formulation is unbiased, i.e. the formulation is independent on the selection of the master and slave side. As a result, it substantially simplifies the definition of mesh tying interface and has a great potential for the solution of the self-contact problems.

Tato přednáška se koná v návaznosti na projekt OP VVV EF16_027/0008500 - Podpora zahraničních stáží pracovníků Ústavu termomechaniky AV ČR (2018-2020, MSM/EF)

Monday, September 23, 2019, 13:00, Lecture Room A
Recent Advances in Wave Propagation and Large-Step Transient Analysis Procedures
Prof. K. C. Park, Ann & H.J. Smead Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA

Abstract:

In recent years, there have been several notable advances both in wave propagation and explicit transient structural dynamic analysis procedures. These include: (1) accurate wavefront tracking algorithms that can handle material heterogeneities; (2) accurate explicit algorithm employing improved non-diagonal inverse mass matrices; (3) large-step explicit integration of low and medium-frequency response analysis by filtering out mesh frequencies, among others. These advances offer structural dynamicists several options in wave propagation and transient analysis for capturing the predominant physics of the problems at hand, with drastically increased computational efficiency and robustness. In this talk, we will go over some salient features of these advances, and offer potential topics for further research.

Wednesday, September 18, 2019, 10:00, Lecture Room B
Film-based shear stress sensor
Ing. Zuzana Broučková, Ph.D., Institute of Thermomechanics, Czech Academy of Sciences

Abstract:

The talk will summarize the six-month stay of the author at the School of Mechanical, Aerospace and Civil Engineering of the University of Manchester, UK. The stay was focused on the research and development of the novel experimental technique for evaluation of the shear stresses on the wall washed by the fluid flow. This technique is based on attachment of a sensor made out of elastic material on a surface/wall (film-based shear-stress sensor). Subsequently, the shear stress is evaluated from the deformation of the sensor under loading (both instantaneous and time-average shear stresses). The advantage of this method is its possibility to measure very low shear stresses, i.e. to use it in flows with very low velocities. Moreover, it is also possible to use it e.g. in water-channels and on curved surfaces, where many other commonly used methods are unable to work.

Tato přednáška se koná v návaznosti na projekt OP VVV EF16_027/0008500 - Podpora zahraničních stáží pracovníků Ústavu termomechaniky AV ČR (2018-2020, MSM/EF)

Wednesday, August 9, 2019, 10:00, Lecture Room B
The Martensitic Transformation in In-Tl Alloys Revisited
Prof. Trevor R. Finlayson, University of Melbourne, Australia

Abstract:

The traditional view for the martensitic transformation in In-xat%Tl alloys, for 15.5 ≤ x ≤ 30.5 was via a double shear such as: (101)[10-1]; (011)[01-1], on the basis of optical microscopy observations and measurements of the (c11 - c12)/2 elastic constant. These early results, together with a calculation of the phonon dispersion relations based on a model pseudopotential and the measured elastic constants as input parameters, suggested that the transformation was driven by the softening of low-ζ [ζζ0][ζ-ζ0] phonons, which provided the motivation for a measurement of the phonon dispersion relations using neutron, inelastic scattering. This now historical background for the transformation in In-Tl alloys will be reviewed.
However, the suggested low-ζ [ζζ0][ζ-ζ0] phonon softening has never been observed experimentally, despite phonon measurements to as low as ζ = 0.02 rlu on the [ζζ0][ζ-ζ0] branch. An alternative model for the formation of coherent nuclei and growth along conjugate {111} planes was once proposed by Geisler. This model is consistent with some electron diffuse scattering data as well as yielding identical x-ray pole figure results as those for the double-shear mechanism. Appropriate nuclei could be generated by <111><11-2> atomic displacements.
To test such an idea, we have measured the [ζζζ]T phonon branch for a good quality In-Tl crystal in a recent experiment using the cold triple-axis instrument, SIKA, at the Australian OPAL Research Reactor. The initial results have shown that the zone-boundary, [ζζζ]T phonon softens with decreasing temperature, which may provide the dynamical behaviour consistent with the Geisler model for the transformation. Further experiments are planned to investigate this softening and the consequential microstructural behaviour.

Associate Professor Trevor Finlayson has been an Honorary Principal Fellow at the University of Melbourne since February, 2007, following an academic career at Monash University where he had been engaged to introduce and teach Materials Science as an undergraduate discipline during the early 1970s. His research has covered a range of projects in the field of condensed matter physics/materials science, including aspects of superconductivity, magnetism, ferro- and piezo-electricity, phase transformations and the direct measurement of stresses in materials using diffraction techniques. His current projects involve studies on martensitic alloys and magnesia-partially-stabilized zirconia, using neutron scattering.

Wednesday, July 31, 2019, 10:00, Lecture Room B
Control of Grid-side Converters under Grid Imbalance
Prof. Yongsug Suh, Ph.D., Chonbuk National University, Jeonju, Korea

Abstract:

The growing penetration of renewable energy sources into the present power grid requires that renewable energy sources provide the similar electrical characteristics as those of classical thermal power plants. In order to meet this requirement, active front-end converters; grid-side converters of renewable energy sources have been evolving to offer various control features to properly regulate the active and reactive output power. Recently, grid codes about LVRT and operation under unbalanced grid become very strict. In general, unbalanced current is caused by unbalanced grid conditions, and it leads to unbalanced voltage at PCC (Point of Common Coupling). These unbalanced voltage conditions generate a significant ripple and distortion of dc-link and ac input current of grid-side converters which eventually undermine various control features of grid side converter. This seminar covers the latest requirements on the gridside converter of renewable energy sources particularly under grid imbalance. The impact of grid imbalance on the operation of grid-side converters is analyzed based on the positive and negative sequential component theory of unbalanced electrical network. The various control techniques to properly compensate for the generation of harmonics are introduced. These control techniques are aimed to enhance the grid-friendly electrical characteristics of renewable energy sources. As a result, these control techniques are expected to play a positive role in growing penetration of renewable energy sources into the present power grid.

Wednesday, June 12, 2019, 10:00, Lecture Room B
Stress waves and people in the Institute of Thermomechanics
Prof. Miloslav Okrouhlík, Institute of Thermomechanics of the Czech Academy of Sciences, Prague

Abstract:

The lecture will cover the following topics:
- Historical overview
- Analytical approach to stress wave propagation
- Dispersion
- Musing about threshold
- Computational
- Experimental
- Continuum limits
- Case studies
- Shell – experiment vs. FE analysis
- Rock drilling – how much of impact energy is lost in the rock
- How to make torsional waves out of axial ones
- Impacted rod with spiral slots – FE vs. experiment
- Cheep wisdom (or triviality) at the end

Monday, June 10, 2019, 13:00, Lecture Room B
Shock wave propagation in complex media: an experimental contribution to dynamic behavior of materials at very high strain rates
Prof. Michel Arrigoni, ENSTA Bretagne, Brest, France

Abstract:

High added value technologies, as well as critical infrastructures in service, are more and more subjected to severe loadings. In order to increase their survivability in harsh environment, structures and materials have to be characterized under dynamic conditions such as crash test, ballistic impact and blast loading. During these extreme events, it is not always easy to implement sensors able to catch the evolution of physical parameters. The presented work exposes an experimental contribution to the characterization of shock wave effects and propagation in materials and on structures. Cases of study are split into two categories: soft impacts and hard impacts. This includes the use of instruments developed for this intention, such as shock pressure gauges and laser Doppler velocimeters and non-destructive techniques for damage assessment.

Wednesday, April 3 2019, 10:00, Lecture Room B
A parallel multilevel domain decomposition solver and its application to adaptive finite element method
Dr. Jakub Šístek, Institute of Mathematics of the CAS

Abstract:

In the first part of the talk, a brief overview of the development of nonoverlapping domain decomposition methods will be given. The focus will be on the iterative substructuring methods using primal unknowns. The Balancing Domain Decomposition based on Constraints (BDDC) by C. Dohrmann will be used for describing these concepts. Next, two extensions of the original BDDC method will be discussed. The first is an adaptive generation of the coarse space to enhance its robustness, e.g. for finite element problems with variable coefficients. The second is an extension of the method to multiple levels, an approach to improving scalability of the method for parallel computations. Our open-source implementation of this Adaptive Multilevel BDDC method, the BDDCML library, will be presented.
In the second part of the talk, we will discuss combination of this solver with the finite element method using an adaptive mesh refinement (AMR). AMR is challenging in the context of distributed memory parallel FEM in general. The treatment of hanging nodes will be also described. Of particular interest is the effect of disconnected subdomains, a typical output of the employed mesh partitioning based on space-filling curves.
The talk will be concluded with numerical results for benchmark Poisson and linear elasticity problems.

Thursday, March 21, 2019, 10:00, Lecture Room B
Decomposition of waves, stresses and forces in rotating disks
Prof. Izhak Bucher, Mechanical Engineering, Technion, Haifa, Israel

Abstrakt:

The present talk will focus on rotating disk dynamics by introducing a novel signal-processing method geared towards capturing the dynamics of such systems. The method exploits multiple sensors and is thus capable of handling spatially complex transient dynamics. Rotating disks identification methods rely on special features of rotating elements, e.g. cyclic-symmetry, gyroscopic effects, directional whirling and circumferentially traveling deformations, all have a physical meaning and are exploited in the proposed approach.
The ‘eyes’ of ‘Smart Rotating Machines’ are the sensors and the accompanied, real-time signal processing methods play the role of a ‘brain’ in the assessment of measured data. Indeed ‘smart’ also means combining advanced sensing capabilities with an electronic brain which is aware of the underlying physics laws to which the model obeys. At the moment, it seems that the pendulum leans heavily towards numerical modeling. Finite Element models are the basis for analysis and design, while testing and measurements provide only limited verification means for some of the model parameters due to poor deployment and simplistic signal processing procedures. The new method narrows the gap between models and experiment and it illustrates what can be gained when they are added.
The presentation will highlight the advantages of model-based signal processing over past and presently used methods and will try to point to a path leading from older methods and techniques towards present, state-of-the-art methods and further into the future where smart machines will have ‘eyes’ and ‘brains’.
Specifically, the presentation will describe spatial, temporal and directional decomposition of rotating machine vibrations during rapid rotational accelerations. Real time signal processing methods that exploit Hilbert transform based decompositions; directional order-tracking and time-frequency maps will be demonstrated via simulations and experiments. The spatial and temporal decomposition method enables a Smart-Machine to assess true stress and strain on parts rotating relative to an array of sensors and thus help to enhance safety.
One additional topic will be briefly shown if time allows: active detection of imbalance for high-speed modes, using slow rotation data.

Thursday, March 21, 2019, 11:00, Lecture Room B
Thermodynamical modeling via GENERIC: from quantum mechanics to semiconductor devices
Prof. Alexander Mielke, Weierstrass Institute for Applied Analysis and Stochastics, and Humboldt University Berlin

Abstract:

We discuss the thermodynamically consistent modeling of semiconductor devices from the mathematical point of view. The task lies in coupling of several physical effects that occur on different temporal or spatial scales, namely optics via the Maxwell equations, charge transport
via drift-diffusion models and quantum mechanical processes in embedded quantum dots, wires or layers.
Using the framework of GENERIC, which is an acronym for General Equations for Non-Equilibrium Reversible Irreversible Coupling, we construct suitable hybrid models that are thermodynamically consistent in the sense that for the isolated system we have energy conservation and positive entropy production. The conservative dynamics is driven by a Hamiltonian structure involving the energy, whereas the dissipative dynamics is driven by an entropic gradient system.

Wednesday, March 6, 2019, 10:00, Lecture Room B
Rekonstrukce minulých klimatických změn z měření teploty v hlubokých vrtech
Jan Šafanda, Institute of Geophysics of the CAS

Abstrakt:

Jedním z často diskutovaných témat současnosti je charakter a příčiny oteplování podnebí pozorované v posledních 100–150 letech a předpověď jeho budoucího vývoje. Odpovědi na tyto otázky se hledají zejména pomocí klimatických a meteorologických modelů vycházejících ze současného (nedokonalého) stavu poznání procesů v atmosféře, hydrosféře i litosféře. Ke kalibraci modelů se vedle observatorních dat používají i proxy data o historii klimatu v delších časových obdobích. Jednou z paleoklimatických metod poskytujících proxy data je rekonstrukce historie povrchové teploty z křivek vyjadřujících chod teploty s hloubkou. Přednáška se zaměří na principy a výsledky této metody.

Wednesday, February 13, 2019, 10:00, Lecture Room B
Theoretical and implementation problems of the multi-dimensional Fokker-Planck equation analysis using the Finite Element Method
Dr. Jiří Náprstek, Institute of Theoretical and Applied Mechanics of the CAS

Abstract:

Fokker–Planck equation is one of the most important tools for investigation of dynamic systems under random excitation. Finite Element Method represents very effective solution possibility particularly when transition processes are investigated or more detailed solution is needed. However, a number of specific problems must be overcome. They follow predominantly from the large multi-dimensionality of the Fokker–Planck equation, shape of the definition domain and usual requirements on the nature of the solution which are out of a conventional practice of the Finite Element employment. Unlike earlier studies it is coming to light that multi-dimensional simplex elements are the most suitable to be deployed. Moreover, new original algorithms for the multi-dimensional mesh generating were developed as well as original procedure of the governing differential and algebraic systems assembling and subsequent analysis. Finally, an illustrative example is presented together with aspects typical for the problem with large multi-dimensionality.

Wednesday, January 9, 2019, 10:00, lecture room B
Dynamical damage and phase-field fracture models
Tomáš Roubíček, Institute of Thermomechanics of the CAS

Abstract:

Damage is a phenomenon/concept in continuum mechanics of solid materials undergoing various degradation processes with numerous applications in engineering and in computational mechanics and (geo)physics. Combination with inertial effects may be important modelling issue to prevent various undesired effects otherwise occuring in quasistatic models. Various damage models and their variants as a phase-field fracture will be overviewed. Also, several numerical approaches will be presented, amenable to compute vibrations or waves emitted during fast damage/fracture, together with various extensions of the basic scenario, combining mass or heat transfer, or plasticity.

2018

Tudesday, December 18, 2018, 11:00, Lecture Room B
Active grids as a tool for turbulence and wind energy studies
Jason Hearst, Department of Energy and Process Engineering, Faculty of Engineering, Norwegian University of Science and Technology, Trondheim, Norway

Abstract:

Traditional wind tunnel testing has been limited by our ability to accurately control the incoming flow conditions. While a wind tunnel offers simple control of the Reynolds number (velocity), conducting measurements in different turbulent flows is significantly more challenging due to severe limitations on our ability to produce bespoke turbulent shear flows in a wind tunnel. Over the last quarter-century, active grids have become popular tools to overcome this limitation. An active grid is a motorized device placed at the inlet of a wind tunnel that produces a transient blockage. By controlling the time variation of this blockage, a user can exert some degree of control authority over the incoming turbulence conditions. It will be demonstrated that using this methodology, the turbulence intensity and the mean shear can be adjusted independently, offering unprecedented control authority over the experimental turbulence conditions. The application of this approach to wind energy will then be illustrated via particle image velocimetry measurements of the near-field of a model horizontal axis wind turbine. This will be compared to simpler measurements of a vertical axis wind turbine in more basic turbulent flows produced by conventional meshes. Ultimately, increasing turbulence intensity is shown to mitigate Reynolds number effects, and impact wake recovery.

Wednesday, December 5, 2018, 10:00, Lecture Room B
Use of FEM to provide virtual functionality for car body development in ŠKODA AUTO a. s.
Ing. Jan Korouš, Ph.D., ŠKODA AUTO a.s.

Abstract:

Metoda konečných prvků (FEM) je již řadu let nedílnou součástí všech fází procesu vývoje vozu, karoserie a jejích komponent ve ŠKODA AUTO a.s. V počátečních fázích vývoje nahrazuje a simuluje reálné zkoušky a umožňuje prověřit velké množství konstrukčních variant, které musí splňovat mnoho často protichůdných požadavků. Neustále rostoucí kapacita výpočetních clusterů umožňuje díky paralelizaci nejen zvládnout permanentně zvyšující se počet výpočtů, ale taktéž popsat chování virtuálního modelu stále ve větších detailech. Přednáška přestaví filozofii použití (FEM) při dimenzování karosérie vozu a přehled portfolia výpočtů, které pokrývají nejen statické zátěžné stavy, ale především velké množství crash testů, kterým jsou dnes moderní vozy podrobovány. Zmíněny budou základní metodiky modelování, ale i současné trendy pro popsání procesů odehrávajících ve struktuře vozu při dynamických nárazových testech, které souvisí např. s porušováním materiálů.

The lecture will be presented in the Czech language.

Lecture series:
Tuesday, November 6, 2018, 14:00, Lecture Room B
Wednesday, November 7, 2018, 14:00, Lecture Room B
Friday, November 9, 2018, 14:00, Lecture Room B
Lecture series on Computational Plasticity
Prof. Nikolaos Aravas, University of Thessaly, Greece

Abstract:

Prof. Nikolaos Aravas is a world-recognized specialist in the field of Computational Mechanics of Materials. His almost 33-years academic career has been associated with the University of Thessaly in Greece and the University of Pennsylvania. Prof. N. Aravas has made significant contributions in the fields of computational plasticity, non-linear fracture mechanics, strain-gradient elasticity theories, and modelling of mechanical behaviour of human tissue. His current research interests include non-linear homogenization theories for multi-phase media and the analysis of electromechanical problems including piezoelectricity and flexo-electricity.

Lecture 1

General form of elastoplastic constitutive equations. Rate-dependent versus rate-independent models. The elastoplastic boundary value problem. The weak formulation of the problem.

Lecture 2

Finite element formulations. Methods of solution of non-linear finite element problems. Consistent linearization. Algorithms for the numerical integration of general elastoplastic models. Backward versus forward Euler methods.

Lecture 3

Applications: von Mises plasticity, pressure-dependent plasticity, the Gurson model, general isotropic plasticity, J3-dependence, kinematic hardening, rate-dependent models, implementation in general purpose commercial finite element codes, e.g., ABAQUS.

Wednesday, November 7, 2018, 10:00, Lecture Room B
Europe without USA – not just in the energy sector
Václav Bartuška, Special envoy of the Czech Republic for Energy Security, Ministry of Foreign Affairs of the Czech Republic

Abstract:

The biggest breakthrough in the "traditional" energy sector of the last decades is the emergence of new methods of mining (fracking) in the USA. Over the past decade, the US has transformed from a natural gas importer into an exporter; in this decade, the same is happening for oil. The trend continues: International Energy Agency (IEA) estimates that in the next three years 80% of new oil production in the world will come from the United States. How is this change manifested in the relationship between Europe and the US?

Wednesday, November 7, 2018, 10:30, Lecture Room B
New role of silicon thin films in advanced photovoltaics
Antonín Fejfar, Institute of Physics of the Czech Academy of Sciences, Prague

Abstract:

Currently photovoltaics is becoming an established industrial field with the global installed capacity over 400 GWp, with perspective of reaching the terawatt installed capacity within the following decade. The field is dominated by silicon wafer based cells which reached the unforseen low system prices. The advantages of silicon thin film based photovoltaics of lower consumption of semiconductors and shorter energy payback time was not sufficient to overcome the disadvantage of lower efficiencies (record is 14 % for Si thin films is about half of the best Si wafer based cell). The most recent record efficiencies are due to the combination of the two technologies: the interdigitated back contacted silicon heterojunction based cells reached 26.7 % efficiency by combining high quality wafer with very thin silicon films for preparing passivating selective contacts. In another parallel development silicon thin films make part of silicon nanowire based solar cells which unite the concept of geometrically thin – optically thick films with simple manufacturing. In our group we have contributed to the field by developing optical profilometry for nanometer thin films based on Raman spectroscopy, microscopic methods for characterizing the local properties of the silicon nanostructures or for exploring photovoltaic materials and we explore new ways of junction engineering by inserting 2D materials or self-assembled dipolar molecule monolayers.

Wednesday, October 3, 2018, 10:00, Lecture Room B
The development of Fuel Cell & Energy Storage technologies in ITRI – Status and Prospective
Wen-Sheng Chang, Director, Division of Energy Storage Technology, Green Energy and Environment Research Laboratories, ITRI

Abstract:

Taiwan Government has set the goal of promoting energy transformation to achieve the vision of non-nuclear country by 2025. In addition to energy security and carbon reduction, Taiwan Government is looking forward to develop advanced energy technologies through the promotion actions and policy.

With the advantages of high efficiency, distributed, and environmental protection, fuel cell industries have been booming in recent years and the market for electric vehicles and power stations are continuously growing. With the support and demonstration by the government, Taiwan stationary power generation has successful popularized. Not only certain key technologies and related industrial chains have been established, industries also try to expand the overseas markets. In addition to promote the distributed power sources, fuel cell can be used with renewable energy as a fuel storage option.

Energy storage is one of the major focuses as the infrastructure of green energy for government in the green energy industry. It is also considered as one of the solutions to the problem caused by high penetration rate of renewable energy. In response to the 20% development goal of renewable energy in 2025, utilization of energy storage technology to strengthen the renewable energy is expected. Energy storage can stabilize intermittent power output of renewable energy, eliminates transient fluctuation of grid power, and improves reliability of power grid.

ITRI has devoted to developing core technologies of PEMFC, Aluminum ion battery, Vanadium Redox flow battery for distributed energy supply and storage. Hoping that this meeting achieves strengthen cooperation between ITRI and CAS and jointly creates innovative research and application on the hydrogen energy and energy storage area.

Wednesday, October 3, 2018, 10:40, Lecture Room B
Nanoparticle synthesis by spark discharge
Tomáš Němec, Institute of Thermomechanics of the Czech Academy of Sciences, Prague

Abstract:

The idea of nanoparticle synthesis in the gas phase is to first evaporate a solid material, e.g. by spark discharge, and second, to condense the vapor in a stream of carrier gas. As a result, solid nuclei are formed by homogeneous nucleation and the nuclei grow to sizes of several nanometers in diameter.

Nanoparticle synthesis in the gas phase is advantageous for certain applications. It is typically a continuous process that offers high purity of product nanomaterials, reduced waste formation, and straightforward scale-up possibilities. Cooling of the gaseous systems can be well controlled and therefore, the morphology and size distribution of the nanoparticles can be tailored to specific applications. Also, nanoalloys can be generated by this technique.

We constructed a spark discharge generator, which achieves nanoparticle production rate of tens of miligrams per hour. This production rate allows us to generate enough material for sample analysis, but also represents usable amounts of nanopowders for various applications. Our target application is the use of platinum-based nanomaterials as catalysts in hydrogen fuel cells. We synthesized nanoparticles from platinum, iridium, tungsten, and characterized the materials by TEM and XRD techniques. .

Thursday, November 7, 2018, 10:00, Lecture Room B
New role of silicon thin films in advanced photovoltaics
Antonín Fejfar, Institute of Physics of the Czech Academy of Sciences, Prague

Abstract:

Currently photovoltaics is becoming an established industrial field with the global installed capacity over 400 GWp, with perspective of reaching the terawatt installed capacity within the following decade. The field is dominated by silicon wafer based cells which reached the unforseen low system prices. The advantages of silicon thin film based photovoltaics of lower consumption of semiconductors and shorter energy payback time was not sufficient to overcome the disadvantage of lower efficiencies (record is 14 % for Si thin films is about half of the best Si wafer based cell). The most recent record efficiencies are due to the combination of the two technologies: the interdigitated back contacted silicon heterojunction based cells reached 26.7 % efficiency by combining high quality wafer with very thin silicon films for preparing passivating selective contacts. In another parallel development silicon thin films make part of silicon nanowire based solar cells which unite the concept of geometrically thin – optically thick films with simple manufacturing. In our group we have contributed to the field by developing optical profilometry for nanometer thin films based on Raman spectroscopy, microscopic methods for characterizing the local properties of the silicon nanostructures or for exploring photovoltaic materials and we explore new ways of junction engineering by inserting 2D materials or self-assembled dipolar molecule monolayers.

Thursday, September 6, 2018, 10:00, Lecture Room B
Predicting low and high friction in rotating mechanisms
Juan Carlos Jauregui, Universidad Autonoma de Queretaro, Mexico

Abstract:

  • Professor at Universidad Autonoma de Queretaro - conducts research in design and dynamics of machinery.
  • Responsible for the design of a large number of automatic tailor made machines which are been installed in different industries. Involved in the development of monitoring systems based on vibration analysis.
  • Author of two books: “Mechanical Vibrations of Discontinuous Systems” (Nova Publishers) and “Parameter Identification and Monitoring of Mechanical Systems under Nonlinear Vibrations (Elsevier).
  • More than 70 papers in international journals and congresses.
  • Member of many professional organizations such as ASME (American Society of Mechanical Engineers), Mexican Society of Mechanical Engineering, Academy of Engineering (Mexico), National Research System (Mexico).
  • Chair of the Technical Committee for Vibrations at IFToMM (The International Federation for the Promotion of Machines and Mechanisms).

Friday, June 22, 2018, 12:00, Lecture Room B
A nonlinear continuum theory of finite deformations of elastoplastic media
Doc. Ing. Ladislav Écsi, PhD., Faculty of Mechanical Engineering, Slovak University of Technology in Bratislava

Abstract:

Contemporary flow plasticity theories in finite-strain elasto-plasticity are either based on an additive decomposition of a strain rate tensor into an elastic part and a plastic part, or on a multiplicative decomposition of the deformation gradient tensor into an elastic part and a plastic part. While the former theories are considered to be ad hoc extensions of small-strain flow plasticity theories into the area of finite deformations to cover large displacements, but small strains in the material of the deforming body, the latter are now generally accepted as true finite-strain flow plasticity theories. Unfortunately, none of the theories entirely satisfies the requirements of thermodynamic consistency, and as a result, the material models and their analysis results, when used in numerical analyses, are dependent on the description and the particularities of the material model formulation. Recently a nonlinear continuum mechanical theory of finite deformations of elastoplastic media has been developed, which allows for the development of objective and thermodynamically consistent material models. This means that the plastic flow, including ‘normality rules’ can be described in a thermodynamically consistent manner in terms of different stress measures and strain rates or their objective derivatives, which are conjugate with respect to the mechanical power, using various instances of the yield surface defined in the above stress spaces. A few results of the modified hypoelastoplastic and hyper-elastoplastic material models based on the aforementioned nonlinear continuum
mechanical theory will be presented and discussed.

Wednesday, June 6, 2018, 11:00, Lecture Room B
IMPLEMENTATION AND APPLICATIONS OF A GENERAL MODEL FOR VARIOUS CONTACTS WITH ADHESION AND/OR FRICTION
Doc. Ing. ROMAN VODIČKA, PhD., Faculty of Civil Engineering, Technical University of Kosice

Abstract:

A general model covering a large variety of adhesive or cohesive contact interfaces with friction between visco-elastic bodies is presented. A semi-implicit time discretisation advantageously decouples the solved system and, after a spatial discretisation, it enables an efficient numerical implementation by the boundary element method. The model is illustrated by various examples documenting its wide applicability.

Friday, June 1, 2018, 10:00, Lecture Room B
Developments in Fluid-Structure Interaction Modeling and Analysis
Prof. K. C. Park, Center for Aerospace Structures and Department of Aerospace Engineering Sciences University of Colorado, Boulder

Abstract:

The talk offers some recent developments in modeling, analysis and some applications of external and internal fluid-structure interaction (FSI) problems, largely based on the speaker’s experience. We begin by reviewing classical internal flow characterizing sloshing and its interaction with the liquid containers. We then introduce the origin of a staggered solution procedures to tackle external FSI solution tracing back to the 1970s. We introduce a modern continuum mechanics-based formulation of incompressible and/or nearly incompressible flows interacting structures. Finally, we discuss some improvements in approximate modeling of external acoustic-structure interaction problems by the boundary element method and its computational performance.

Tuesday, April 10, 2018 at 13:00, Lecture Room B
Behaviour of brittle materials under dynamic loading
Jaroslav Buchar and Jan Trnka, Institute of Thermomechanics, Czech Academy of Sciences

Abstract:

The lecture deals with some achievement on description of brittle materials behavior at highstrain-rate loadings such as: air blast loading or percussive drilling of rocks, ballistic impact against ceramic armour or transparent windshields, plastic explosives used to damage or destroy concrete structures, soft or hard impacts against concrete structures and many others in civil and military applications.

The most popular dynamic testing techniques used for this which are based on the use of split Hopkinson pressure bar methodologies and/or plate impact testing methods are briefly described. The influence of the strain rate on the material strength is discussed. Some constitutive equations are presented. Some of them are used in the numerical simulation of some ballistic loading of ceramics.

Wednesday, March 28, 2018 at 11:00, Lecture Room B
Finite Fracture Mechanics and its Applications to Composite Materials
Vladislav Mantič, Department of Continuum Mechanics and Structural Analysis, School of Engineering, University of Seville, Spain

Abstract:

The basic assumption of Finite Fracture Mechanics (FFM) is to allow crack growth by (possibly) finite steps, in opposite to the hypothesis of crack growth by infinitesimal steps adopted in classical Linear Elastic Fracture Mechanics (LEFM). The coupled (stress and energy) criterion of FFM introduced by D. Leguillon (2002) requires that both stress and energy conditions are simultaneously fulfilled for such a finite crack advance. A quite general formulation of the coupled criterion of FFM leading to an optimization problem is introduced. Several examples of applications of this coupled criterion to the prediction of damage initiation in form of cracks at micro- and meso-scale in composites are presented.

Wednesday, March 28, 2018 at 14:00, Lecture Room B
Brief introduction to optimization and topology optimization
Dr. Paulo Salvador Britto Nigro, Software Developer and Researcher of Virtual.PYXIS optimization, São Paulo, Brazil

Abstract:

The lecture will address the following topics in Topology Optimization (OT):

  • Type of Optimizations – a shortly review of several case of optimizations methods, as parametric and
  • shape optimization;
  • Topology Optimization – introduction to basic concepts of OT, as material law and Filter;
  • Topology Optimization Software – The concept how the software works;
  • Design Response – Principal tools of the software: Compliance, Frequency, Volume, Mass, Displacement
  • and Internal Force;
  • Manufacturing tools - Symmetry, Casting and Extrude;
  • Objective function Definition – options to deal with multi-objective functions, as minmax and KS
  • functions;
  • Nonlinear Optimization Methods- An explanation of nonlinear optimization with equality and inequality
  • constraint;
  • Method of Explicit Convex Approximation – Introduction to OC and MMA.

Dr Paulo Salvador Britto Nigro is a Software Developer skilled in Numerical Simulation applied to computer simulation industry. He has strong background in Model Order Reduction, Nonlinear Optimization Methods and C++. Doctor of Philosophy (Ph.D.) focused in Structural Engineering from Universidade de São Paulo.

Wednesday, March 28, 2018 at 10:00, Lecture Room B
Micromechanics of Martensitic Laminates
Doc. Ing. Hanuš SEINER, Ph.D., Institute of Thermomechanics, Czech Academy of Sciences / Visiting Fulbright Scholar at the University of Minnesota, Minneapolis, USA

Abstract:

The talk will summarize the main theoretical aspects of mechanics of geometrically ordered microstructures appearing in single crystals of shape memory alloys, called martensitic laminates. It will be shown that the formation of the laminates can be explained based on the concept of non-linear elasticity and energy-minimizing sequences.

The applicability of this theoretical framework will be illustrated on two technologically important examples: i) branched laminates at the phase interfaces; ii) highly mobile laminate-laminate interfaces in the ferromagnetic shape memory alloys. For both cases, explicit constructions of energy upper-bounds will be shown, and the implications of the theoretical findings for designing of new alloys with advanced functionalities will be discussed. The development of these upper-bounds and the exploration of their properties are the main subjects of the current speaker’s research at the University of Minnesota, done in collaboration with prof. R.D. James and his research group.

Wednesday, March 7, 2018 at 10:00, Lecture Room B
Catch the yield surface, experimentally, theoretically, and computationally
Dr. Li-Wei Liu, Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan / Institute of Thermomechanics of CAS, v. v. i., Prague

Abstract:

The yield surface of a material is the boundary of the elastic region where every stress point inside the region result from the elastic response of the material. The experimental evidence shows that the yield surface changes position, size, shape, and orientation during the material undergoing the plastic loading which results in the permanent deformation. Based on the experimental observation, the modelling of the yield surface evolution is a key point to completely simulate the plastic behavior of the material. Most experiments of yield surface detection were conducted in the two-dimensional space (axial-torsional or bi-axial). Due to the complete stress space is six dimensional, detecting the yield surface in the space whose dimension is more than two can collect more detail of the yield surface evolution. For the experiment of yield surface detection, the determination of yield point underpins the accuracy of the geometry of the yield surface. Nowadays, test machines used for the experiments of yield surface detection are usually servo-controlled hydraulic system, hence the scatter of data should be taking into account in the determination of yield point. To this end, an automated yield stress determination based on the Weibull distribution is introduced. After conducting the experiment in the axial-torsional-hoop stress space, yield points are obtained according to the yield-stress determination and designed probing paths. To further capture the global information from these yield points and observe the evolution of yield surface during different pre-loading paths, a convex-closed-cubic polynomial, which is capable of description of the yield surface evolution, including translation, expansion/ contraction, rotation, affine deformation, and distortion in the three dimensional space, is proposed and the corresponding three-stage estimation for parameters of the polynomial is developed. This polynomial enable us to observe the yield surface evolution from the three dimensional point of view and it can also be a candidate of potential yield functions. Furthermore, the computation of elastoplastic models needs more attention to the special mathematical structure of the model containing ordinary differential equations, algebraic equations, and inequalities. Exploring the underlying structure of elastoplastic models shows part of them possesses internal symmetry that is the pseudo-sphere of real pseudo-Euclidean space Rp,q on which the proper orthochronous pseudo-orthogonal group SOo(p,q), a sub group of the Lie group, leaves acts. Based on the internal symmetry, a return-free integration is developed and it keeps the computed stress point on the yield surface automatically and exactly without any extra enforcement during the plastic deformation.

Thursday, February 15, 2018 at 14:00, Lecture Room B
Complementary near field technique for assessment of materials with added value
Dr. Adriana Savin, Head of Nondestructive Testing Department, National Institute of Research and Development for Technical Physics, Iasi, Romania

Abstract:

The National Institute of Research and Development for Technical Physics (NIRDTP) is a part of the national institutes R&D network coordinated by the Ministry of Research and Innovation - National Authority for Scientific Research and Innovation. Institute performs basic and applied research in the field of advanced materials with novel structures and properties, devices (i.e., sensors, transducers, actuators, measuring systems) based on advanced materials, new preparation methods and characterisation techniques, including non-destructive evaluation and magnetometry, electrical and magnetic separation, and devices for applications in engineering, healthcare, and biotechnology.

Nondestructive Testing Department (NDT) performs theoretical and applicative research in the field of electromagnetic testing of cylindrical and plate products including composite materials; calculation of the fields scattered by material discontinuities located at different areas of the multilayered medium by solving the forward problems; theoretical optimization of the operation of different types of sensors. Department also performs ultrasonic testing, development of specific methods for ultrasonic signal processing with FFT, digital filtering, neuro-fuzzy networks, development of the algorithms for defects localization and automaticclassification of flaws.

In this lecture, a new possibility of using sensor with metamaterial lens for the nondestructive evaluation of metallic strip gratings and carbon fiber reinforced plastics will be presented. The sensor has enhanced spatial resolution due to the apparition of evanescent waves in the space between strips and between carbon fibers respectively, during the excitation by transversal electromagnetic field polarized along z-axis. The evanescent waves can be manipulated by a lens made from two conical Swiss rolls that act as a field concentrator. The detection has spatial resolution better than λ/2000.

January 25, 2018, 14:00 Lecture Room B
Evolution and Verification of a Kinematic Hypothesis for Splitting of the Strain Energy
Prof. Herbert A. Mang, Institute for Mechanics of Materials and Structures, Vienna University of Technology

Abstract:

Splitting of the strain energy into its “non-membrane” and membrane percentage provides insight into the load-carrying mechanism of structures, subjected to proportional loading. It may be useful, for example, for sensitivity analysis of the initial post-buckling behavior of beams, arches, plates, and shells, and assemblies of such structures. The task of this work is to determine this percentage without computing insignificant numbers such as the values of the strain energy and its membrane part. It is hypothesized that this percentage is proportional to the acceleration of a fictitious particle, moving along a curve on the unit sphere. The curve is described by the vertex of the normalized “fundamental eigenvector” of the so-called “consistently linearized eigenvalue problem”. The proportionality factor is obtained from the initial condition for the “non-membrane” percentage of the strain energy, hypothesized as twice the initial velocity of the particle. The lower bound of this factor signals the constancy of this percentage with increasing load, whereas the upper bound indicates a monotonic increase or decrease up to its ab initio predictable value at a stability limit or to an unphysical asymptotic limiting value. The proof of the universal validity of the two hypotheses begins with their verification for the special cases of a membrane stress state and pure bending. The assertion that this is a sufficient condition for the universal validity of these hypotheses is subsequently verified for an example with a monotonically increasing “non-membrane” percentage of the strain energy. It is finally confirmed by an indirect proof of their validity for a non-monotonic course of this percentage. A by-product of this work are conditions for extreme values of the stiffness of structures, subjected to proportional loading.

2017

6. 12. 2017, 10:00
Research on synthetic jets
Assoc. Prof. Zdeněk Trávníček, Institute of Thermomechanics, v.v.i., CAS, Prague

Abstract:

Synthetic jets are fluid flows which are generated from periodically oscillating fluid. In spite of zero time-mean flux at the actuator, a non-zero time-mean jet flow can be generated (synthesized) from a train of individual fluid “puffs”. These flows have many perspective applications such as active control of flowfields and thermal fields (external and internal aerodynamics, cooling, mixing, etc.). The basic advantage is the simplicity – neither fluid source (compressor, blower, pump) nor supply piping is required. Therefore, the synthetic jet has been subject of intensive investigations recently.

The topic has been investigated at the Institute of Thermomechanics since 2001. For example, the following particular tasks have been solved: (1) Impinging synthetic jet and heat transfer enhancement, (2) newly proposed principle: "hybrid synthetic jet", (3) formation criterion of synthetic jets and identification of flow regimes, and (4) geometry optimization.

15. 11. 2017, 10:00
Modelling of yield surface distortion in the finite strain range
Prof. A.V. Shutov, Lavrentyev Institute of Hydrodynamics, Novosibirsk State University

Abstract:

The talk is devoted to the phenomenological modelling of the stress response of metallic materials subjected to non-proportional loading conditions. As a preliminary step, a class of two-dimensional rheological models is introduced, capable of capturing the initial and strain-induced anisotropies of the analyzed material. The rheological models mimic the effect of a combined isotropic-kinematic-distortional hardening; the essential part of the approach is a direction-dependent friction element, which allows us to describe an arbitrary sharpening of the yield surface in the loading direction, accompanied by arbitrary flattening on the opposite side. Two different specific definitions of the direction-dependent friction are provided. The first approach is based on a certain interpolation between the initial yield surface of the von Mises type and a fully saturated yield surface exhibiting maximum distortion. The second approach allows interpolating between a sequence of pre-defined symmetric yield surfaces. Both approaches are practical and flexible. They guarantee that the yield surface remains convex and smooth at any stage of the deformation process, which is important for stable and robust computations. Next, basing on these results, a system of constitutive equations is constructed for a general multiaxial loading. The description of the finite strain kinematics is based on the nested multiplicative split of the deformation gradient. The resulting model is objective, thermodynamically consistent, w-invariant; it is free from shear stress oscillations. Finally, an efficient and robust numerical implementation of the model is discussed.

10. 11. 2017, 10:00
Cellular structures and materials – fabrication, properties characterisation and applications
Zoran Ren, Srečko Glodež, Matej Vesenjak and Nejc Novak, University of Maribor, Faculty of Mechanical Engineering, Maribor, Slovenia

Abstract:

The presentation will give a short overview of cellular materials in general. Initially, their properties, fabrication procedures and application possibilities will be discussed. Then their geometrical characterization, experimental testing and computational modelling within the finite element method of various cellular metal types will be described. The geometrical characterisation is based on the analysis of micro computed tomography scans and proper recognition of their internal cellular structure, taking into account the statistical distribution of morphological and topological properties. The results of conducted geometrical analysis provided means to develop methodology for proper 2D and 3D geometrical modelling of irregular cellular materials and consequent formation of computational models. The numerical models were validated by quasi-static and dynamic mechanical experimental tests supported by infrared thermography.

In the next part of the presentation, auxetic cellular structures, which exhibit negative Poisson’s ratio, will be discussed. Negative Poisson’s ratio is a consequence of internal structure deformation. This effect is useful for many different applications to enhance properties in density, stiffness, fracture toughness, energy absorption and damping. Several 2D and 3D auxetic structures will be introduced. Experimental results of some selected auxetic structures, tested under quasi-static and dynamic loading conditions, will be presented. Furthermore, representative discrete computational models built with the beam finite elements and homogenised computational models that were validated by experimental data will be shown as well. They were developed to explore the auxetic response at different loading conditions and material distribution (including porosity variation).

1. 11. 2017, 11:00
On gravitational waves and 2017 Nobel Prize for Physics
Prof. Jiří Chýla, Institute of Physics of the Czech Academy of Sciences

Abstract:

Four weeks ago Nobel Prize for Physics had been awarded to three leading scientists from LIGO-VIRGO Collaboration “for decisive contributions to the LIGO detector and the observation of gravitational waves”. In this seminar I will first recall basic facts about the origin and detection of gravitational waves in general and then discuss in nontechnical terms the construction and amazing sensitivity of LIGO detector as well as the way how the gravitational waves are recorded and presented.

The crucial role of the three Nobel Prize Laureates will be emphasized and all five signals of gravitational waves so far recorded by LIGO will be briefly described.. Particular attention will be paid to the very recent one, announced on October 16, which originated from the collision of two neutron stars and which has also its optical counterpart as Gamma Ray Burst, observed by two space-based telescopes. Finally, future observatories, that would significantly extend the capabilities of LIGO-VIRGO, will be briefly discussed.

2. 10. 2017, 11:00
Internal Variables associated with Microstructure
Dr. Arkadi Berezovski, Department of Cybernetics, School of Science, Tallinn University of Technology

Abstract:

Prediction of the response of microstructured materials on an external loading can be achieved by means of various methods. In (quasi)statics, homogenization methods are suitable in most situations, but this is not the case for functionally graded materials, e.g. Strain gradient models are quite sufficient if only the influence of a microscale length is taken into account. The most general approach is provided by generalized continuum theories, which include microdeformation into consideration. One more possibility is the introduction of internal variables for the description of microstructure.

In the paper, we compare different descriptions of microstructured solids on the simple example of wave propagation in the one-dimensional setting. In the classical continuum mechanics the existence of a microstructure is neglected. Thus, the classical wave equation needs to be modified to include the observed dispersive effects due to the microstructure. We consider modifications of the wave equation which follow from homogenization, continualization of lattice models, and from generalized continuum theories. The linear version of the Boussinesq equation for elastic crystals, the Love-Rayleigh equation for rods accounting for lateral inertia, the Maxwell-Rayleigh model of anomalous dispersion, etc., are compared with dispersive wave equations obtained by means of single and dual internal variables.

2. 10. 2017, 10:00
2D Discrete Spectral Analysis – A Tool for Examining of omplicated Wave Structures
Prof. Andrus Salupere, Department of Cybernetics, School of Science, Tallinn University of Technology (in collaboration with Mart Ratas)

Abstract:

In case of 1D wave propagation the discrete spectral analysis is very helpful method in order to analyze the space-time behavior of different wave structures. Here we generalize the method to 2D case. The Kadomtsev–Petviashvili equation is applied as a model equation. For numerical integration the pseudo-spectral method is applied. We demonstrate how 2D spectral characteristics can be applied for analysis of complicated wave structures that can be formed from different initial pulses in case of the Kadomtsev–Petviashvili equation. Recurrence phenomenon, temporal periodicity and temporal symmetry of the solution will be discussed.

17. 8. 2017, 11:00
Recent advances in reciprocal mass matrices
Dr. Anton Tkachuk, Institute for Structural Mechanics, University of Stuttgart, Stuttgart, Germany (in collaboration with Anne Schäuble, Prof. Manfred Bischoff)

Abstract:

Standard explicit dynamic simulation relies on diagonal or lumped mass matrices. Lumped mass enables a trivial computation of the nodal accelerations from the total force vector. Moreover, critical time step estimators and contact-impact algorithms for such mass types are well understood and developed. A disadvantage of the exlicit time integration with the lumped mass is huge number of the time steps even for short time dynamics. Recently, several approaches for reciprocal mass matrix that allows higher time steps and reduction of the total computational cost were proposed. A reciprocal mass is a sparse inverse of mass matrix that usually has a mask/structure of consistent mass or stiffness matrix. It can be constructed directly and cheaply either with variational or with algebraic methods. Achievable speed-up with respect to lumped mass is from 20% to 50%.

In this talk, an overview of existing approaches of construction reciprocal mass matrices is given and recent advances in reciprocal mass matrices for impact algorithms, time step estimation and assessment of the error in heterogeneous materials are presented.

17. 8. 2017, 10:00
Multi-Scale Structural Gradients Optimize the Bio-Mechanical Functionality of the Spider Fang
Dr. Benny Bar-On, Laboratory for the Mechanics of Complex Materials, Department of Mechanical Engineering, Ben-Gurion University of the Negev

Abstract:

The spider fang is a natural injection needle, built as a multi-scale composite material with outstanding mechanical properties. In this study we introduce a hierarchical modeling for the spider fang, based on computer tomography and SAXS measurement, and analyze the correlation between the fang architectural motifs and its macroscopic elastic behavior. Analytical methods and Finite-Element simulations are used for the mechanical analysis and the effects of small- and large-scale structural gradients on the macroscopic mechanical properties are investigated.

It is found that the multi-scale structural gradients of the spider fang optimize its performances in term of load-bearing stiffness and strength, and that the naturally evolved fang architecture provides optimal mechanical properties compared to other alternative structural configurations.

26. 6. 2017, 10:00
Modelling extreme deformation and dynamic behaviour of materials using mesh-less methods
Dr. Raj Das, Sir Lawrence Wackett Aerospace Research Centre, School of Engineering, RMIT University, Australia

Abstract:

The seminar will present overview of computational mechanics research at the Centre for Multifunctional and Composite Materials of RMIT University, Australia. Our research covers both fundamental and applied aspects of material behaviour and failure processes. This presentation will encompass computational modelling of material deformation, damage and fracture using multi-scale techniques in conjunction with mesh-less methods, novel composite materials development and damage tolerance structural optimisation.

Multi-scale modelling of damage and fracture progression linking nano to macro scales and associated development of coupled computational modelling tools will be highlighted. The strengths of mesh-less methods will be illustrated with reference to both low to high-speed impact induced fractures and small to large scale problems. These include several dynamic fracture and fragmentation processes, such as hypervelocity impact fracture, nano-scale machining, large scale geo-mechanical failures (magma intrusion, caving, slope stability, etc).

One of our core areas to be presented is novel impact and blast resistant, light weight composite material developments for aerospace components subjected to high-speed loading and extreme deformations, as occurs in the cases of debris impact on spacecrafts, bird strike on aircraft engines, blast induced failures, etc. Lastly novel shape and topology optimisation methodologies for damage tolerance optimisation, i.e. maximising the residual strength and fatigue life, of aero-structures will be highlighted. Case studies from projects with Royal Australian Air Force and Defence Science and Technology Organisation will be presented to demonstrate the practical implementation and utilities of the developed design and analysis methodologies.

19. 6. 2017, 10:00
Additive Manufacturing of metals: Past, today and tomorrow
Dr. Edson Costa Santos, SENAI Innovation institute in Laser Processing, Joinville, Santa Catarina, Brazil

Abstract:

The lecture will be addressed the following topics in Additive Manufacturing (AM) of metals:

  • Draw some observations from various attitudes to AM world-wide.
  • Review shortly various additive manufacturing technologies, their virtues and drawbacks.
  • Address cases, in which additive can/cannot replace conventional manufacturing - problems with distortions, variability in micro-structure and consequences, etc.
  • Comparison of additive and conventional micro-structures and their impact on macro-mechanical properties: strength, fragility, fatigue, impact resistance, etc.
  • Use of additive manufacturing for meta-materials (auxetic and other) for the purposes of "energy absorption or distribution" and "mechanical strength with low weight".
  • Additive manufacturing process certification and/or serial production of components - competitiveness in terms of both function and price.
  • Design of components for Additive Manufacturing - material only there "where needed" and the related development of software (e.g. topology optimization).
  • Future of AM - visions and expectations.
  • Describe and introduce FIESC - SENAI focus in AM.

Dr. Edson Costa Santos spent more than a decade in various laboratories related to Additive Manufacturing in Europe, South America and Japan. In the presentation, it will be drawn from his experience, and present a view of current AM layout – technologies, directions and main leaders.

15. 6. 2017, 10:00
Quasibrittle Failure Probability and Scaling
Prof. Zdeněk P. Bažant, Northwestern University, Evanston, Illinois, USA

Abstract:

The size effect on structural strength and its probability distribution function (pdf) is a complex problem for quasibrittle materials because their failure behavior transits from quasi-plastic at small sizes to brittle at large sizes. These are heterogeneous materials with brittle constituents in which the size of inhomogeneity, or representative volume element (RVE), is not negligible compared to the structure size. Aside from concrete, the archetypical example, they include fiber composites, coarse-grained ceramics, rocks, sea ice, snow slabs, wood, bone, foam, stiff soil, dry snow,ccarton, etc., and on the micro- or nano-scale, all brittle materials become quasibritle. Since the break probability is known exactly only for interatomic bonds (being equal to frequency), Kramer’s rule of transition rate theory is applied to nano-crack jumps. Based on proving the rules of multiscale transition of tail probabilities of break to material scale, the probability distribution function (pdf) of strength of one macro-scale representative volume element (RVE) is shown to have a Weibullian tail, calibrated to reach to probability circa 0.001, the rest being Gaussian. On the structure scale, only Type 1 failure is considered, i.e., the structure fails as soon as the first RVE fails. Hence the weakest-link model applies on the structure scale. But, crucially, the number of links is finite, because of non-negligible RVE. For increasing structure size, the Weibullian portion gradually spreads into the Gaussian core. Only in the infinite size limit the distribution becomes purely Weibull, but, importantly, with a zero threshold. Based on an atomistic derivation of the power law for subcritical macro-crack growth, a similar Gauss-Weibull transition is shown to apply to structure lifetime. The theory is then extended to the size dependence of Paris law and Basquin law for fatigue fracture, to statistics of fatigue lifetime, and to residual strength after a period of preload. The theory is shown to match the existing experimental results on the monotonic strength, residual strength after preload, static and fatigue crack growth rates, and static and fatigue lifetimes, including their distributions and size effects on the distributions. There are three essential consequences: 1) The safety factors must depend on structure size and shape; 2) To predict the pdf of strength, the size effect tests of mean strength suffice; 3) To predict the static and fatigue lifetimes, it suffices to add tests of initial subcritical crack growth rate. An interesting mathematical analogy predicting the lifetime of nano-scale high-k dielectrics is also pointed out. Finally, a new “fishnet” statistics for strength of biomimetic nacre-like lamellar structures, modelled as a square fishnet pulled diagonally, is presented. This simple model differs from the weakest-link model as well as the fiber bundle model. The pdf is found again to transit from Gaussian to Weibullian, but in a different way.

30. 5. 2017, 15:00
ISG-Israel Smart Grid consortium and Large-Scale Power System Dynamics
Prof. Yuval Beck, Head of Power Engineering, Faculty of Engineering, Holon Institute of Technology, Israel
Prof. Yoash Levron, Professor of Electrical Engineering, Faculty of Electrical Engineering, Technion – Israel Institute of Technology

Abstract:

A Smart Grid demonstrator was implemented within the framework of the "Israel Smart Grid – ISG" Magnet project. The goal of the project was to implement and develop technologies for optimizing and controlling Smart Grids. The main achievement of the project is its operation system which is hierarchical in nature. Namely, the control and commands are not centralized but rather distributed from top levels downwards. Every such control level can also be selfcontained. The project consists of a demonstration of rout of electric power that is delivered to a modern "Procumer" (Producer and Consumer), precisely upon its request, with minimum power failures, energy optimization and minimal electricity costs. The demonstrator is constructed in various sites and controlled by a virtual network. The virtual network consists of controllable loads and some generators. Some of the actual controllable loads are motors, air conditioning chiller, air treatment units and others. The loads are controlled by an Intelligent Home Gateway Unit (IHGU) which operates in accordance to the contract between the grid and the prosumer or consumer. The system also controls, via web services, two remote sites of the Israel Electric Company – IEC, consisting of a virtual neighborhood.

Large-scale dynamics models of power systems are mostly based on time-varying phasors. However, with increasing integration of distributed and renewable sources into existing power grids, the assumption of time-varying phasors (or quasi-static models) becomes less accurate, and may even lead to misleading conclusions regarding the system dynamics and stability. During the lecture I will briefly review and compare several types of dynamic models, describe several paradoxes that result from misuse of these models, and describe our group's approach to this problem.

3. 5. 2017, 10:00
Non-standard damped oscillators
Prof. Dalibor Pražák, Department of Mathematical Analysis, Faculty of Mathematics and Physics, Charles University in Prague

Abstract:

Damped oscillators of the form x'' + a(x)x' + b(x) = f(t) are classical models in mechanics and for regular enough a(.), b(.), say C1 or Lipschitz, the mathematical theory is very well understood. Non-standard analysis (NSA), on the other hand, is a rather strong and abstract logical framework. Using NSA, various mathematical theories can be embedded into larger universes with non-standard ("ideal") elements. The simplest and most famous examples are infinitely large and small numbers (which are thought by some advocates of NSA to be fatally missing from Calculus for nearly 200 years by now.)

Curiously enough, some nonstandard choices of the functions a(.) and b(.), taking infinitely large values, or with infinitely steep growth, are natural models of some "non-standard" mechanical elements: damper with Coulomb's friction, inextensible string, or more generally, collision of a moving mass with a wall.
In our talk, we will see how these situations can be modelled within the framework of NSA. We show that interesting dynamics can occur and even more, new interesting questions can be asked.

5. 4. 2017, 10:00
Implosive magnetocumulative generator for effective energy conversion
Dr. Jiří Šonský, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

History of magnetohydrodynamic generators goes back to 1832 when Michael Faraday tried first experiments. Magnetocumulative generators were developed by Andrei Sakharov at the start of the 1950s, but up to these days such devices are not used for public energetics and remain in experimental, often military development. Therefore we have developed new thermal plasma source for magnetohydrodynamic or magnetocumulative generator suitable for general energetics. The thermal plasma is created from combustible mixture by implosion - spherical compression driven by convergent detonation wave. The detonation wave is initiated by a weak spark and by means of deflagration to detonation transition in detonation tube. Convergent polyhedral shape of the detonation wave is formed by large number of vents opened to hemispherical combustion chamber. Propagation of the detonation wave and its multiple branches is tracked by an array of ionization probes. Resulting high velocity plasma is ejected from a nozzle near the geometrical center of the device. The plasma is observed by capturing emitted light by hi-speed camera to determine plasma velocity. Construction of the implosion plasma source and possible variants of extraction of electrical energy from kinetic energy of the plasma by interaction of high velocity plasma with seed magnetic field will be also discussed in this presentation.

1. 3. 2017, 10:00
Atmospheric Boundary Layer: main characteristics and methods of the research in context of continuum mechanics
Prof. Zbyněk Jaňour, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The bulk of the fluid on the Earth's surface can be found in the atmosphere and the oceans. Geophysical Fluid mechanics investigate it. The flow inside the area near the Earth's surface is called Atmospheric Boundary Layer in a certain analogy with the classical theory of fluid mechanics. Its properties, methods of research, taking into account their shortcomings; will be discussed in the following paragraphs.
1) Introduction: the introduction of the concept of and the reasons for its monitoring;
2) The basic characteristics (Equations of motion in continuum mechanics approximation, Flow in a rotating coordinate system, The temperature stratification, Turbulence and determinism)
3) Modelling (experimental methods, numerical methods)
4) Tasks solved in the Laboratory of Environmental Aerodynamics;
5) New problems to solve:
Verification and validation of mathematical models,
Many scales problem;
6) Conclusion: applications

14. 2. 2017, 10:00
Modelling of complex processes in nanopowder fabrication using thermal plasma flows
prof. Masaya SHIGETA, Joining and Welding Research Institute, Osaka University, Japan

Abstract:

Thermal plasmas have been expected as a promising tool for mass-production of nanopowders [1] because thermal plasmas offer a distinctive thermal-fluid field involving high temperature, high chemical reactivity and variable properties. Furthermore, thermal plasmas have steep temperature gradients at their fringes where many small nanoparticles are produced rapidly from the material vapour as a result of the highly supersaturated state. However, it is still difficult to investigate the formation mechanism of nanoparticles generated in/around a thermal plasma because the process involves remarkably intricate mass transfer of phase conversions in micro-second scales. Moreover, the plasma fringe is fluid-dynamically unstable and consequently it forms a turbulent mixing field composed of multiscale eddies [2]. The growing nanoparticles are transported by the complicated convection as well as diffusion and thermophoresis. In this lecture, several modelling works to simulate those complex processes are explained.

1. 2. 2017, 10:00
Highlights of plasma spraying in the life of one researcher
Dr. Tomáš Chráska, Institute of Plasma Physics of the CAS, v. v. i.

Abstract:

Thermal spraying techniques are coating processes in which melted or heated materials are sprayed onto a surface. There is a great variety of feedstock materials that can be thermally sprayed including solid powders and suspension liquids. There is also a great variety of thermally sprayed coatings used for many different applications including for example the thermal barrier coatings in jet engines. Plasma spraying process is a member of the thermal spraying family of techniques. It uses plasma gun to generate a plasma jet that melts feedstock materials. This talk is not going to present a complete overview of plasma sprayed coatings and their applications. It will rather present a set of interesting and sometimes intriguing examples of what can achieved by plasma spraying. The examples will include nanopowders, epitaxial growth of crystals in plasma sprayed coatings, amorphous and nanocomposite coatings, spraying of suspension and more.

4.1. 2017, 10:00
Usage of time reversal signal processing in nondestructive diagnostics of materials and structures
Dr. Zdeněk Převorovský, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

Time reversal processing of acoustic and ultrasonic signals (TRA) is very effective tool for complicated problems solution in many fields like nondestructive testing and evaluation (NDT/NDE) of materials and structures. TRA enables waves focusing in time and space and therefore precise localization and reconstruction of wave sources in strongly inhomogeneous, anisotropic, and dispersive media. Properties of TRA may be used to signal processing in acoustic emission (AE), nonlinear elastic wave spectroscopy (NEWS), and also e.g. in seismology, medicine, telecommunications, etc.

TRA principles will be mentioned in the talk, and its potentials in AE source location and identification will be discussed. Outlined will be also some questions of a new approach to that inverse problems solution by using the ultrasonic signal transfer from a real body onto its laboratory and/or numerical model where they can be analyzed more easily.

2016

28 December 2016, 11:00
Modelling of lotus-type porous structures: bi- and multi- axial loading
Assoc. prof. dr. Matjaž Šraml, University of Maribor, Slovenia

Abstract:

A computational modelling of the low-cycle fatigue behaviour of lotus-type porous material subjected to multiaxial loading cycles is presented. The considered computational models have the same porosity but different pore topology patterns. Multiaxial loading conditions in the direction perpendicular to the longitudinal axis of pores are assumed to be proportional (in-phase) and non-proportional (out-of-phase) loading paths in numerical simulation. The fatigue life analysis is performed using a damage initiation and evolution law, based on the inelastic strain energy approach. The computational results show that a different fatigue life is obtained in the models with the same porosity but with different pore topology at the same loading level. Furthermore, the results of computational simulations show a qualitative understanding of the loading path on low-cycle fatigue failures of lotus-type porous material under multiaxial loading conditions.

7 December 2016, 10:00
Iron. Static and “dynamic” phase diagrams and transformation kinetics
Prof. Eugene B. Zaretsky, Department of Mechanical Engineering, Ben Gurion University, Beer Sheva, Israel

Abstract:

Our civilization is closely acquainted with iron for some 4500 years, iron polymorphism is known for some 100 years, and it is some 50 years since the iron static phase (P,T) diagram has been established with reasonable accuracy.

The talk describes some recent experimental results aimed to establishing the borders of existence of iron phases when the iron is compressed by shock. Such "dynamic" phase diagram is found to differ strongly from the static one, i.e. the shock-generated metastable phase can subsist for a time longer than the experiment duration (microseconds) while the time required for the phase formation (transformation kinetics) is extremely short, few tens of nanoseconds.

November 9, 2016, 10:00
Regularized Models for Softening Materials
Prof. Milan Jirásek, Czech Technical University in Prague, Faculty of Civil Engineering

Abstract:

For many materials, the deformation process at some stage leads to propagation and coalescence of existing defects and to initiation of new ones. If the defects grow sufficiently fast, the material can exhibit, on the macroscopic scale, a decrease of the averaged stress even at increasing strain. This phenomenon, referred to as softening, is one of the destabilizing factors that can, under certain conditions, lead to localization of inelastic deformation processes into narrow bands. An objective description of localized strain patterns in the framework of continuum mechanics requires special adjustments of material models, because for traditional models the width of the localized band can become arbitrarily small and, consequently, the numerical solution exhibits a pathological sensitivity to the discretization (e.g., to the size of finite elements).
This lecture provides an overview of various regularization techniques that can serve as localization limiters. In view of their diversity, localized solutions will be analyzed for a one-dimensional model problem only. We will identify which specific regularization techniques are suitable for elastoplastic models with softening and for damage models, and we will compare the corresponding localization conditions and localized profiles of plastic strain or damage, including their subsequent evolution. Such analysis will reveal why certain specific formulations based on nonlocal averaging or on gradient enhancements serve as reliable localization limiters while other formulations fail or suffer by various deficiencies.

5 October  2016, 10:00
Experimental study and simulation on localization of phase transformation in shape memory alloys
Ing. Petr Sedlák, Ph.D., Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The remarkable properties of shape memory alloys (SMA) that are utilized in a number of useful applications are due to a phase transformation between austenite and martensite. In many situations, the transformation does not proceed homogenously, but in the form of macroscopic transformation bands, i.e. in a highly localized manner. Since these inhomogeneities influence the mechanical response and reduce the fatigue performance of SMA products, the roots and mechanisms of localization have been investigated by material scientists and engineers for many years. In a unique experiment utilizing the advanced technique of three-dimensional X-ray diffraction (3D-XRD), complete strain and stress states of the polycrystalline grains close to the macroscopic transformation band front have been resolved on a grain-by-grain basis for the first time. Results show substantial heterogeneity of stress between grains – implied by anisotropy of both elastic and transformation properties – and a striking redistribution of macroscopic (homogenized) stress near the interface. Analysis of the experimental data allowed the team to adapt an established constitutive model tailored for NiTi SMA so that non-local, gradient effects could be included. Consequent numerical simulation of the propagating transformation band demonstrated how the internal stress redistributes close to the phase interface within the wire causing the macroscopic localization to occur.

15 September  2016, 10:00
Effects of the nozzle exit boundary layer on hot-jet mixing
Ing. Jan Lepičovský, DrSc., Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The main motivation of the research effort reported here was passive protection of airplanes against an enemy’s heat seeking missiles. The stated goal was to reduce the temperature of a jet engine exit flow by means of the hot-jet rapid mixing with the ambient air, and no penalty of the jet engine thrust was allowed. The focus of the first phase of this project was to explore the properties of high-speed free jets and demonstrate that it is possible to enhance jet flow mixing by flow excitation. Inconsistencies in hot-jet responses to flow excitations were investigated in the second phase of this project. It was shown that the jet receptivity to flow excitation is strongly dependent on a character of the nozzle exit boundary layer. Finally, it was proven that the decisive factor controlling the jet receptivity is the velocity gradient across the exit boundary layer. There were also side byproducts of this research effort. First, it was an improvement in the high-frequency stroboscopic visualization of large-scale turbulent structures in free jet flows. The second innovation was the development of a new methodology for conditional sampling of random laser velocimeter data.

17 August  2016, 10:00
Rotation-free parametrization and isogeometric analysis of shear deformable plates and shells
Prof. Dr.-Ing. habil. Manfred Bischoff, Institut für Baustatik und Baudynamik, Universität Stuttgart

Abstract:

Structural theories for static and dynamic analysis of shear deformable plates and shells (Reissner-Mindlin type) usually employ independent degrees of freedom for displacements and rotations. It is shown how equivalent models can be developed based on displacement degrees of freedom only. In the context of finite element formulations this has the advantage that transverse shear locking can be intrinsically avoided within a standard displacement-based concept, regardless of the underlying function spaces used for discretization.

As in this context higher continuity of the shape functions is required, a natural way is to incorporate such theories into the isogeometric concept, using NURBS (non-uniform rational B-splines) as shape functions. Corresponding shear-deformable shell finite element formulations for geometrically linear and non-linear applications are presented and their performance is demonstrated with the help of numerical examples.

2 June 2016, 10:00
Mathematics of fluids in motion
Prof. Eduard Feireisl, Institute of Mathematics of the CAS, v. v. i.

Abstract:

We discuss the recent development of the mathematical theory of fluid dynamics, classical and new open problems and possibilities of their solution. Special attention is paid to the recent results on well and/or ill-posedness of problems describing inviscid fluids. In particular, several modern concepts of solutions are examined: Weak, very weak, measure-valued etc.

4 May 2016, 10:00
Development of Human Artificial Vocal Folds
Dr. Jaromír Horáček, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

Voice production is a complex physical process, which involves airflow coming from the lungs, selfoscillating vocal folds and acoustics of the resonance cavities of the human vocal tract. The vocal folds, excited by the airflow, generate a primary sound which propagates in the airways of the vocal tract modifying its spectrum and producing the final acoustic signal radiated from the mouth. Understanding basic principles of voice production is important for detection of laryngeal pathologies and treatment of laryngeal disorders. The physical models of voice production are important tools not only for experimental verification of computational 3D finite element models of phonation, but also for development of the vocal folds prosthesis. The study compares results of in vitro measurements of phonation characteristics performed on originally developed 1:1 scaled artificial models of human vocal folds. The measured aerodynamic, vibration and acoustic characteristics of the last models are comparable with the values found in humans..

6 April 2016, 10:00
Stochastic Self-Organization in Inner Structure of Vehicular Systems
Prof. Milan Krbálek, Faculty of Nuclear Sciences and Physical Engineering, Department of Mathematics, Czech Technical University in Prague

Abstract:

History of traffic science is relatively short. Generally, it is meant that the first scientific article is the essay of Professor Bruce Greenshields dated to 1934. The factual beginning of systematic scientific discipline (called Transportation Science) is the year 1992, when the field accelerated by many famous publications. Nowadays Transportation Science is very well anchored in the portfolio of scientific disciplines (associated MIF is 1.377). For the purposes of this seminar talk we choose a theme of predictions for statistical properties of traffic flows and detection of surprising relations in traffic microstructure.
In this talk we will show that macroscopic self-organization of traffic streams (e.g. spontaneous traffic congestions) is projected into evolution of stochastic properties detected for vehicular micro-quantities. Furthermore, we will demonstrate that there exists a smart and uncomplicated way how to predict such microscopic effects.

2 March 2016, 13:30
Animal Flight
Dr. Rudolf Dvořák, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

Unlike airplanes, animals must have either flapping wings (birds, bats) or oscillating wings (insects). Only such wings can produce both lift and thrust, provided the animal has sufficient muscle power.
To fly, wings impart downward momentum to the surrounding air and obtain lift by reaction. How this is achieved under various flight situations (cruise flight, hovering, landing, etc.), and what is the role of the wing-generated vortices in producing lift and thrust is discussed (both for birds and insects).
Bird wings have several possibilities how to obtain the same functions as airplane wings. Birds have the capabilities of adjusting the shape of the wing according to what the immediate flight situation demands, as well as of responding almost immediately to conditions the flow environment dictates.

3 February 2016, 10:00
FLUIDIC OSCILLATORS FOR ALGAE CULTIVATION and their role in geopolitic stability
Prof. Ing. Václav Tesař, CSc., Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

Our civilisation is extremely dependent on cheap liquid fuel used for transportation. Until roughly the end of 19th century people used to work in their respective dwelling places. Now they commute in huge numbers every day. Food and other goods travels hundreds (if not thousands) of kilometres between production and use. This model is increasingly adopted by developing most populated countries (China, India). Fossil fuel sources, on which this all depends, is produced – at an increasingly high cost – in politically unstable regions. No wonder research grant providers are willing to support financially the research promising renewable petrol as its result. The starting point are algae – primitive, often unicellular plants capable to produce by photosynthesis - from H in water and CO2 taken from air - hydrocarbon compounds, processing of which into biofuels brings no difficulty in principle – after all, the fossil oil was produced the same way from algae millions of years ago. Additional benefit would be the whole process being carbon neutral so that removal of CO2 from the atmosphere would suppress the global warming. Algae may be also a starting point of a food chain, solving another global problem.
The difficulty is so far the price of the crude oil from algae being higher than the fossil one. The key factor for success is making more efficient every step in the process. One of perhaps small but nevertheless important contribution towards the goal is more efficient diffusion transport of CO2 into the algae in bioreactors. Suggested solution is generation of sub-millimetre sized microbubbles by placing a fluidic oscillator into the gas inlet. The research grant project investigated in the Institute of Thermomechnaics enabled recently testing a number of alternative oscillator designs.

6 January 2016, 10:00
Discontinuous Galerkin method for the solution of elasto-dynamic, compressible flow and fluid-structure interaction problems
Prof. Miloslav Feistauer, Department of Numerical Mathematics, Faculty of Mathematics and Physics, Charles University in Prague

Abstract:

This lecture will be concerned with the numerical solution of dynamic elasticity and compressible flow. We consider the linear case as well as the nonlinear St. Venant-Kirchhoff model. The space Discretizat on is carried out by the discontinuous Galerkin method (DGM). For the time discretization several techniques are proposed and tested. As the best method the DG discretization both in space and time appears. The discontinuous Galerkin method is also used for the numerical solution of compressible flow in time-dependent domains, formulated with the aid of the arbitrary Lagrangian-Eulerian (ALE) method. It will be shown that this method allows the solution of compressible flow with a large range of the Mach number. Then the developed methods are combined and used for the numerical simulation of vibrations of elastic bodies induced by compressible flow. The applicability of the developed techniques will be demonstrated by several numerical experiments.

The results were obtained in cooperation with M. Balázsová, J. Česenek, M. Hadrava, A. Kosík and J. Horáček.

2015

2 December 2015, 10:00
Fractography and Failure Analysis
Prof. Jan Siegl, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague

Abstract:

1) Basic methodology of the fractographic analysis
2) Methods of quantitative fractography – reconstructions of fatigue crack growth kinetics (materials characterization and failure analysis of aircraft structures)
3) Analysis of operational failures and brakedowns (case study for turbines)

4 November 2015, 10:00
Scalable algorithms for the solution of contact problems with 10e9 unknowns
Prof. Zdeněk Dostál, Faculty of Electrical Engineering and Computer Science, VŠB – Technical University of Ostrava

Abstract:

An overview of the theoretical results on optimal algorithms for the solution of special problems of quadratic programming and QCQP will be given together with the adaptation of TFETI domain decomposition method for the solution of contact problems. Then will be presented a short summary of theoretical results showing the asymptotically linear complexity of the presented algorithms for frictionless, Tresca, and transient problems. The theoretical results will be illustrated by the solution of large real world contact problems and academic problems discretized by over 10e9 variables.

12 October 2015, 13:30
Aerodynamic Control using Virtual Surface Modification
Prof. Ari Glezer, Woodruff School of Mechanical Engineering, Georgia Institute of Technology

Abstract:

A novel, scalable approach to flight control by distributed fluidic modification of the apparent aerodynamic shape of the lifting surfaces, or virtual aerosurface shaping will be discussed. Robust control of attached and separated flows is engendered by leveraging the generation, accumulation, and advection of vorticity concentrations near the surface to alter its aerodynamic shape and thereby the aerodynamic forces and moments without mechanical control surfaces. Actuation is effected by the interactions of arrays of surface-integrated jets with the local cross flow such that the actuation time scale is typically considerably lower than the relevant characteristic time scale of the flow. The presentation will also describe applications of virtual aerosurface shaping to manoeuvring, drag reduction, and structural stabilization.

7 October 2015, 10:00
Multi-Phase Electric Machines and Drives
Prof. Miroslav Chomát, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The recent advances in power electronics have enabled the employment of some unconventional types of electric machines in drives. In this regard, increasing attention has been paid to machines with the number of phases higher than three. The lecture will focus on the advantages, disadvantages and specific properties that need to be taken into account when designing and developing drives with these machines.

3 June 2015, 10:00
Ultrasonic characterization of advanced material
Dr. Michal Landa, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

Motivation - investigation of relationships between the mechanical properties and microstructures of advance materials; determination of anisotropic elasticity, and detection of phase transformations and thermally activated processes. Why do we need an ultrasound? Necessity of contactless measurements. Advantages of recent all-optical techniques in ultrasonics (laser-ultrasonics).

6 May 2015, 10:00
Application of contact algorithm in creep analysis of high-pressure casing DSPWR
Dr. Dušan Gabriel, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

First, the laboratories of the Department D4 - Impact and Waves in Solids will be briefly introduced. Next, the contribution will be aimed at the long-term development of a general three dimensional contact algorithm for the solution of complex engineering problems including the effects of material and geometric non-linearities. The key feature of this algorithm is that the contact search is performed at the Gaussian points rather than the nodes. The method was shown to be consistent with the variational formulation of a continuum problem, which enabled easy incorporation of higher-order elements with midside nodes to the analysis. The test problem involved creep analysis of high-pressure casing of the DSPWR steam turbine. Material properties were described by the probabilistic exponential model with damage. The FE prediction of permanent displacements in time 10 thousand and 200 thousand hours and a deformed shape of the dismantled casing was obtained. The analysis was performed in the FE code PMD (Package for Machine Design) developed at Department D4. The results were compared with FE code ANSYS.

1 April 2015, 10:00
Experimental and theoretical research of friction coupling effect on dynamics of bladed discs
Dr. Luděk Pešek, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The laboratories of the department Dynamics and Vibration will be briefly introduced at the beginning. Then the contribution will be aimed at the development of experimental and computational methods for explanation of the friction coupling effect in rotary bladed machines. By means of the friction couplings in the blading, the reduction of dynamical straining of blades of rotary wheels can be achieved. Therefore new experimental procedures on the test wheel model including theoretical analytical-numerical methods enabling parametric optimization of dynamic model of bladed wheels will be presented.

4 March 2015, 10:00
The Department of Thermodynamics at a glance and research of the homogeneous nucleation of droplets
Dr. Jan Hrubý, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The lecture will consist of two parts. In the first part, the research activities of the Department of Thermodynamics will be overviewed. Most of the research is performed in three laboratories. Density and surface tension for newly appearing fluids such as ionic liquids are precisely determined and correlated in the Laboratory of Thermophysical Properties of Fluids. Transport phenomena in various flow configurations including synthetic jets and microfluidic devices are investigated in the Laboratory of Heat and Mass Transfer. Metastable states of fluids, such as supercooled water or supersaturated steam, and nucleation of new phases are subject of research in the Laboratory of Phase Transition Kinetics.

In the second part of the lecture, the problem of homogeneous nucleation of droplets from supersaturated vapors will be introduced. The research is motivated by applications in steam turbines, natural gas processing, carbon dioxide separation and storage, and atmospheric phenomena. Experimental techniques will be reviewed with an emphasis on expansion-based devices for high nucleation rates. Although the so-called classical nucleation theory (CNT) is available for decades, no breakthrough happened allowing to quantitatively predict correct temperature and pressure dependencies of the nucleation rates. Own research will be introduced, considering the effects of finite thickness of the phase interface and its undulation by capillary waves, coupled to the universal critical scaling relations.

4 February 2015, 10:00
Aerodynamic Research on the Tip Sections of Long Rotor Turbine Blades
Dr. Martin Luxa, Institute of Thermomechanics of the CAS, v. v. i.

Abstract:

The lecture is focused on systematic aerodynamic research on the flow structures and flow parameters of tip section cascades at transonic and supersonic regimes of operation. The analysis of the flow is based on numerous detailed wind tunnel measurements of five tip blade cascades of different design. The blade cascades represent tip sections of the last stage rotor bladings with length of blades 860 mm, 1080 mm, 1220 mm, 1375 mm and 1525 mm.

Aerodynamic experimental research of the tip blade cascades is oriented to providing data on high-speed flow of compressible fluid past tip sections of blades. The analysis of experimental data is aimed to transonic effects namely expansion over sonic conditions, aerodynamic choking, development of supersonic flow, boundary layer development, the flow past trailing edge, exit shock waves, interaction of shock wave with boundary layer, wakes, etc.

Results of experiments are invaluable not only for improvements of new machine results but also for numerical modeling of transonic flow and numerical solution of flow in flow paths of turbomachines

2014

Wednesday, December 3, 2014, 10:00
Mikrokosmos a makrokosmos: Záhady a souvislosti
Prof. Jiří Chýla, Fyzikální ústav AV ČR, v. v. i., Praha

Abstract:

Jeden z nejpozoruhodnějších rysů současné fyziky je skutečnost, že zákonitosti mikrosvěta a makrosvěta jsou úzce propojeny. Kvantová povaha zákonů mikrosvěta i základní stavební kameny hmoty hrají klíčovou roli při pochopení zrodu, vývoje a dnešního stavu našeho vesmíru. Poznatky získávané zkoumáním astrofyzikálních objektů a částic přicházejících z vesmíru naopak zásadním způsobem ovlivňují naše znalosti vlastností elementárních částic a sil mezi nimi působících. V přednášce budou výše uvedená tvrzení dokumentována na konkrétních příkladech.