2020

Pondělí 2. listopadu ve 13:00, online
Design and analysis of membrane structures
Dr. Rostislav Lang, Ústav stavební mechaniky VUT Brno a FEM consulting, s.r.o.
Odkaz pro připojení k přednášce je k dispozici zde.

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.

Středa 16. září v 10:00, posluchárna B
Corrosion study in subcritical and supercritical water: An electrochemical approach
Prof. Jan Macák, Ústav energetiky, Fakulta technologie ochrany prostředí VŠCHT Praha

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.

Středa 13. května 2020, 13:00, přednáška v režimu online
High-order methods in simulations of fluid dynamics problems
Dr. Jan Pech, Ústav termomechaniky AV ČR
Odkaz pro připojení k přednášce je k dispozici zde.

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.

Středa 6. května 2020, 13:00, přednáška v režimu online
Advanced Titanium Alloys for Medical Applications
Dr. Josef Stráský, Matematicko-fyzikální fakulta UK
Odkaz pro připojení k přednášce je k dispozici zde.

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.

Středa 29. dubna 2020, 13:00, přednáška v režimu online
Achievements, agreements and quarrels of forefathers of mechanics
Prof. Miloslav Okrouhlík, Ústav termomechaniky AV ČR
Odkaz pro připojení k přednášce je k dispozici zde.

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.

Úterý 21. dubna 2020, 13:00, přednáška v režimu online
Application of boundary element type methods in computational aerodynamics
Dr. Chandra Shekhar Prasad, Ústav termomechaniky AV ČR

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.

 

Středa 15. dubna 2020, 13:00, online
Models of „semi-compressible“ fluids as a compromise between incompressible fluids and compressible gasses
Prof. Tomáš Roubíček, Ústav termomechaniky AV ČR

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.

Středa 11. března 2020, 10:00, posluchárna B
Thermomechanics in optical fibre drawing, splicing, and everyday use
Prof. Pavel Honzátko, Ústav fotoniky a elektroniky AV ČR

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.

Čtvrtek 27. února 2020, 14:00, posluchárna B
Structural Design and Analysis at OHB System AG
Dr. Markus Geiß, Structural and Thermal Development Engineer, OHB System AG, Weßling, Německo

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.

Pondělí 17. února 2020, 10:00, posluchárna B
Experimental and Numerical Procedures for Calibration of Advanced Phenomenological Models of Metal Plasticity
Ing. Slavomír Parma, Ph.D, Ústav termomechaniky AV ČR, v. v. i.

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.

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)

Středa 8. ledna 2020, 10:00, posluchárna B
First-principles calculations of elastic constants for complex systems
Ing. Martin Zelený, Ph.D., fakulta strojní VUT v Brně

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

Pondělí 9. prosince 2019, 13:00, posluchárna 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, Španělsko

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.

Pátek 6. prosince 2019, 11:00, posluchárna 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.

Středa 4. prosince 2019, 10:00, posluchárna B
Properties of open thermodynamic systems as the consequence of their stability
Prof. František Maršík, DrSc., Ústav termomechaniky AV ČR, Západočeská univerzita, a Fakulta tělesné výchovy a sportu Univerzity Karlovy

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.

Pátek 15. listopadu 2019, 10:00, posluchárna 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.

Středa 13. listopadu 2019, 13:00, posluchárna B
Non-coaxiality between two tensors: Application to stress rate decomposition and non-coaxial invariants
Prof. Yannis F. Dafalias, Ústav termomechaniky AV ČR

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.

Úterý 5. listopadu 2019, 10:00, posluchárna A
Notes on Experimental Research on Transonic Compressor Blade Cascades
Dr. David Šimurda, Ústav termomechaniky AV ČR

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.

Úterý 22. října 2019 v 12:00, posluchárna 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.

Pondělí 21. října 2019 v 10:00, posluchárna B
Lessons to be learned from German Attempts to Reduce Atmospheric CO2-Emissions
Prof. Dr-Ing. Roland Span, Ruhr University Bochum, Německo

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.

Středa 2. října 2019 v 10:30, posluchárna B
High Strain-rate Experiments Using Hopkinson Bar: Application on Cellular Metals and Additively Manufactured Auxetic Structures
Ing. Tomáš Fíla, Fakulta dopravní ČVUT

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.

Pondělí 23. září 2019, 14:00, posluchárna A
Twin mortar method: A new unbiased mesh tying formulation
Ing. Ján Kopačka, Ph.D., Ústav termomechaniky AV ČR

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)

Pondělí 23. září 2019, 13:00, posluchárna 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.

Středa 18. září 2019, 10:00, posluchárna B
Film-based shear stress sensor
Ing. Zuzana Broučková, Ph.D., Ústav termomechaniky AV ČR

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)

Středa 9. srpna 2019, 10:00 posluchárna B
The Martensitic Transformation in In-Tl Alloys Revisited
Prof. Trevor R. Finlayson, University of Melbourne, Austrálie

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.

Středa 31. července 2019, 10:00, posluchárna 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.

Středa 12. června 2019, 10:00, posluchárna B
Stress waves and people in the Institute of Thermomechanics
Prof. Miloslav Okrouhlík, Ústav termomechaniky AV ČR

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

Pondělí, 10. června 2019, 13:00, posluchárna 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, Francie

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.

Středa 3. dubna 2019, 10:00, posluchárna B
A parallel multilevel domain decomposition solver and its application to adaptive finite element method
Dr. Jakub Šístek, Matematický ústav AV ČR

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.

Čtvrtek 21. března 2019, 10:00, posluchárna B
Decomposition of waves, stresses and forces in rotating disks
Prof. Izhak Bucher, Mechanical Engineering, Technion, Haifa, Izrael

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.

Čtvrtek 21. března 2019, 11:00, posluchárna 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.

Středa 6. března 2018, 10:00, posluchárna B
Rekonstrukce minulých klimatických změn z měření teploty v hlubokých vrtech
RNDr. Jan Šafanda, CSc., Geofyzikální ústav AV ČR

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.

Středa 13. února 2019, 10:00, posluchárna B
Theoretical and implementation problems of the multi-dimensional Fokker-Planck equation analysis using the Finite Element Method
Dr. Jiří Náprstek, Ústav teoretické a aplikované mechaniky AV ČR

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.

Středa 9. ledna 2019, 10:00, posluchárna B
Dynamical damage and phase-field fracture models
Tomáš Roubíček, Ústav termomechaniky AV ČR

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

Úterý 18. prosince 2018, 11:00, posluchárna 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, Norsko

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.

Středa 5. prosince 2018, 10:00, posluchárna B
Využití FEM pro virtuální zajištění funkčnosti při vývoji karoserie vozu ve 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.

Cyklus přednášek:
Úterý 6. listopadu 2018, 14:00, posluchárna B
Středa 7. listopadu 2018, 14:00, posluchárna B
Pátek 9. listopadu 2018, 14:00, posluchárna B
Lecture series on Computational Plasticity
Prof. Nikolaos Aravas, University of Thessaly, Řecko

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.

Středa 7. listopadu 2018, 10:00, posluchárna B
Evropa bez USA – nejen v energetice
Václav Bartuška, zvláštní vyslanec ČR pro otázky energetické bezpečnosti, MZV ČR

Abstrakt:

Největším zlomem v „klasické“ energetice posledních desetiletí je nástup nových metod těžby (fracking) v USA. V minulé dekádě se USA z dovozce zemního plynu staly vývozcem; v této dekádě se totéž odehrává v ropě. Trend dále pokračuje: podle odhadů International Energy Agency (IEA) má v příštích třech letech plných 80 % nové těžby ropy na světě pocházet z USA. Jak se projevuje tato změna ve vzájemném vztahu Evropy a USA?

Středa 7. listopadu 2018, 10:30, posluchárna B
New role of silicon thin films in advanced photovoltaics
Antonín Fejfar, Fyzikální ústav AV ČR

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.

Středa 3. října 2018, 10:00, posluchárna 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.

Středa 3. října 2018, 10:40, posluchárna B
Nanoparticle synthesis by spark discharge
Tomáš Němec, Ústav termomechaniky AV ČR, Praha

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.

Čtvrtek 7. listopadu 2018, 10:00, posluchárna B
New role of silicon thin films in advanced photovoltaics
Antonín Fejfar, Fyzikální ústav AV ČR, v. v. i.

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.

Čtvrtek 6. září 2018, 10:00, posluchárna B
Predicting low and high friction in rotating mechanisms
Juan Carlos Jauregui, Universidad Autonoma de Queretaro, Mexiko

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).

Pátek 22. června 2018 od 12:00, posluchárna B
A nonlinear continuum theory of finite deformations of elastoplastic media
Doc. Ing. Ladislav Écsi, PhD., Faculty of Mechanical Engineering, Slovenská technická univerzita v Bratislavě

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.

Středa 6. června 2018, 11:00, posluchárna 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, Technická univerzita v Košicích

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.

Pátek 1. června 2018 od 10:00, posluchárna 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.

Úterý 10. 4. 2018, 13:00, posluchárna B
Behaviour of brittle materials under dynamic loading.
Jaroslav Buchar a Jan Trnka, Ústav termomechaniky AV ČR, v. v. i.

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.

Středa 28. 3. 2018, 11:00, posluchárna 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, Španělsko

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.

Středa 28. 3. 2018, 14:00, posluchárna B
Brief introduction to optimization and topology optimization
Dr. Paulo Salvador Britto Nigro, Software Developer and Researcher of Virtual.PYXIS optimization, São Paulo, Brazílie

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.

Středa 28. 3. 2018, 10:00, posluchárna 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.

Středa 7. 3. 2018, 10:00, posluchárna B
Catch the yield surface, experimentally, theoretically, and computationally
Dr. Li-Wei Liu, Department of Civil Engineering, National Taiwan University, Taipei 10617, Tchaj-wan / Ústav termomechaniky AV ČR, v. v. i.

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.

15. 2. 2018, 14:00, posluchárna 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, Rumunsko

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.

25. 1. 2018, 14:00 posluchárna 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
Doc. Ing. Zdeněk Trávníček, CSc., Ústav termomechaniky AV ČR, v. v. i.

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
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.

14. 11. 2017
Deformation-Induced Anisotropy in Porous Metals: Constitutive modeling and computational issues
Nikolaos Aravas, Department of Mechanical and Industrial Engineering, University of Thessaly, Řecko

Abstract:

A constitutive model for a porous metal subjected to general three-dimensional finite deformations is presented. The model takes into account the evolution of porosity and the development of anisotropy due to changes in the shape and the orientation of the voids during deformation. The pores are initially ellipsoidal and distributed randomly in an elastic-plastic matrix (metal). Under finite plastic deformation, the voids are assumed to remain ellipsoids, and to change their volume, shape, and orientation. At every point in the homogenized continuum, a “representative” ellipsoid is considered with principal axes defined by the unit vectors n(1)n(2)n(3) = n(1) n(2) and corresponding principal lengths a, b and c. The homogenized continuum is locally orthotropic, with the local axes of orthotropy coinciding with the principal axes of the representative ellipsoid. The basic “internal variables” characterizing the state of the microstructure at every point in the homogenized continuum are given by the local equivalent plastic strain , the local void volume fraction or porosity , the two aspect ratios of the local representative ellipsoid (w1=c/a and w2=c/b) and the orientation of the principal axes of the ellipsoid (n(1)n(2)n(3)). A methodology for the numerical integration of the elastoplastic constitutive model is developed. The problem of ductile fracture near the tip of a blunt crack is studied by using the finite element method and comparisons with traditional constitutive models that assume isotropic behavior are made. A “plastic strain-gradient” version of the model will be also presented and issues associated with its numerical implementation will be 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, Slovinsko

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, 10:00
On gravitational waves and 2017 Nobel Prize for Physics
Jiří Chýla, Fyzikální ústav AV ČR

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 Complicated Wave Structures
Prof. Andrus Salupere, Department of Cybernetics, School of Science, Tallinn University of Technology a 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, Německo (spolupracující: 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, Austrálie

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, Brazílie

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, Izrael
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
Nestandardní tlumené oscilátory
Doc. RNDr. Dalibor Pražák, Ph.D, Katedra matematické analýzy, Matematicko-fyzikální fakulta, Univerzita Karlovy v Praze

Abstrakt:

Tlumené oscilátory typu x'' + a(x)x' + b(x) = f(t) patří k elementárním problémům v mechanice. Pro přiměřeně hladké funkce a(.), b(.), např. C1 nebo Lipschitzovské, existuje rozsáhlá a klasická matematická teorie. Tzv. nestandardní analýza (NSA) je velmi silný a abstraktní logický rámec, který umožňuje vložit libovolnou matematickou teorii do rozšířeného univerza, které typicky obsahuje nestandardní („ideální”) prvky. Nejjednodušším a nejznámějším příkladem jsou nekonečně malá a velká čísla - čísla, s nimiž se moderní analýza před cca 150 lety možná k oboustranné škodě rozešla.

Pokusíme se ukázat, že určité nestandardní volby funkcí a(.), b(.) přirozeně vedou k popisu „nestandardních” mechanických jevů: Coulombovo tření, neroztažitelná struna, či, obecněji, náraz tělesa na pevnou stěnu. Díky jazyku NSA zároveň zůstaneme v rámci klasické teorie diferenciálních rovnic.

5. 4. 2017, 10:00
Implozivní magnetokumulativní generátor pro účinnou přeměnu energie
Ing. Jiří Šonský, Ph.D., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Historie magnetohydrodynamických generátorů sahá až do roku 1832, kdy Michael Faraday začal s prvními experimenty. Magnetokumulativní generátory byly vyvinuty Andrejem Sacharovem již na začátku padesátých let minulého století, ale stále nejsou využívány v civilní energetice a zůstávají na experimentální, navíc často vojenské úrovni vývoje. Proto jsme vyvinuli nový zdroj termického plazmatu pro magnetohydrodynamické nebo magnetokumulativní generátory vhodné pro obecné použití v energetice. Plazma je vytvořeno z hořlavé směsi implozí – tedy sférickou kompresí konvergentní detonační vlnou. Konvergentní detonační vlna je spuštěna přechodem deflagrace do detonace po zapálení elektrickou jiskrou v detonační trubici. Konvergentní polyedrální tvar detonační vlny je vytvarován velkým počtem větvících se zátravek ústících do hemisférické spalovací komory. Vzniklé plazma vytryskne vysokou rychlostí tryskou ve středu zařízení a je sledováno vysokorychlostní kamerou. Postup detonační vlny je také sledován ionizačními sondami. Konstrukce implozivních zdrojů plazmatu a možnosti extrakce elektrické energie z kinetické energie plazmatu působením na počáteční magnetické pole bude v této přednášce také probrána

1. 3. 2017, 10:00
Mezní vrstva atmosféry: vlastnosti a metody výzkumu v kontextu mechaniky kontinua
prof. RNDr. Zbyněk Jaňour, DrSc., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Převážná část tekutin na zemském povrchu se nachází v atmosféře a oceánech. Těmito tekutinami se zabývá tzv. geofyzikální mechanika tekutin. Při jejím pohybu, v jisté analogii s klasickou teorií mechaniky tekutin, je její oblast přiléhající zemskému povrchu označována jako Mezní vrstva atmosféry. Její vlastnosti, metody výzkumu, s přihlédnutím jejich nedostatků, budou naznačeny v následujících bodech.

  1. Úvod: zavedení pojmu a důvody jejího sledování;
  2. Základní vlastnosti. (Pohybové rovnice v rámci aproximace mechaniky kontinua, Proudění v rotující soustavě souřadné, Teplotní zvrstvení, Turbulence a determinismus)
  3. Metody výzkumu (Experimentální, Numerické)
  4. Případy řešené v Laboratoři aerodynamiky prostředí;
  5. Nové problémy k řešení: Verifikace a validace matematických modelů, Problém mnoha měřítek;
  6. Závěr: možnosti aplikace získaných poznatků.

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
Vrcholné kousky plazmového stříkání v životě jednoho výzkumníka
Ing. Tomáš Chráska, Ph.D., Ústav fyziky plazmatu AV ČR, v. v. i.

Abstrakt:

Žárové stříkání je proces vzniku vrstev a povlaků, při kterém se roztavený nebo ohřátý materiály nastříká na povrch. K dispozici je široká škála primárních surovin, které mohou být žárově stříkané včetně prášků a suspenzních kapalin. K dispozici je také široká škála žárových nástřiků používaných pro mnoho různých aplikací, včetně například tepelných bariér v proudových motorech. Plazmové stříkání patří do skupiny technik žárového stříkání. Využívá plazmový hořák pro vytvoření proudu plazmatu, který taví materiál vstupní suroviny. Tato přednáška nebude předkládat kompletní přehled plazmově stříkaných povlaků a jejich aplikací. Místo toho představí řadu zajímavých a někdy i fascinujících příkladů toho, čeho lze dosáhnout pomocí plazmového stříkání. Příklady budou obsahovat nanoprášky, epitaxní růst krystalů v plazmových nástřicích, amorfní a nanokompozitní povlaky, stříkání suspenzí a další.

4. 1. 2017, 10:00
Využití procedury časové reverzace signálů v nedestruktivní diagnostice materiálů a konstrukcí
Ing. Zdeněk Převorovský, CSc., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Procedura časové reverzace akustických a ultrazvukových signálů ("Time Reversal Acoustics", TRA) je efektivním nástrojem řešení složitých problémů v mnoha oblastech jako jsou nedestruktivní zkoušení a hodnocení materiálů a konstrukcí (NDT/NDE), neboť TRA umožňuje fokusaci vln v čase i prostoru a přesnou lokalizaci a rekonstrukci zdrojů signálu i v silně nehomogenních, anizotropních a dispersních prostředích. Vlastnosti TRA lze využít při zpracování signálů v akustické emisi (AE) a nelineární spektroskopii elastických vln (NEWS), ale také např. v seismologii, medicíně, telekomunikacích apod.
V přednášce budou zmíněny principy metody TRA a diskutovány zejména její možnosti při lokalizaci a identifikaci zdrojů AE a nastíněny problémy nového přístupu k řešení těchto inverzních úloh pomocí přenosu ultrazvukových signálů z nepřístupného reálného tělesa na laboratorní resp. výpočetní model, kde mohou být snáze analyzovány.

2016

28. 12. 2016, 11:00
Modelling of lotus-type porous structures: bi- and multi- axial loading
Assoc. prof. dr. Matjaž Šraml, University of Maribor, Slovinsko

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. 12. 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, Izrael

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.

9. 11. 2016, 10:00
Regularizované modely pro materiály se změkčením
prof. Ing. Milan Jirásek, DrSc., Stavební fakulta ČVUT v Praze

Abstrakt:

Pro mnoho materiálů dochází v jisté fázi přetvárného procesu k šíření a spojování již existujících defektů a vzniku nových. Pokud je rozvoj defektů dostatečně dramatický, může z makroskopického hlediska nastat pokles zprůměrovaného napětí při rostoucí deformaci. Tento jev, označovaný jako změkčení (softening), je jedním z destabilizujících faktorů, které za určitých okolností způsobují lokalizaci nepružného přetváření do úzkých pásů. Objektivní popis lokalizované deformace v rámci mechaniky kontinua vyžaduje speciální úpravy materiálových modelů, protože při použití klasických modelů je tloušťka lokalizovaných pásů libovolně malá a numerické řešení pak vykazuje patologickou závislost na diskretizaci (například na velikosti použitých konečných prvků).
V přednášce podáme přehled nejrůznějších regularizačních technik, které mohou být použity jako tzv. omezovače lokalizace. Vzhledem k jejich rozmanitosti bude analýza lokalizovaných řešení provedena pouze pro modelovou úlohu v jedné prostorové dimenzi. Ukážeme, jaké regularizační postupy jsou vhodné pro pružnoplastické modely se změkčením a jaké pro modely poškození (damage models). Provedeme porovnání podmínek lokalizace a vzniklých lokalizovaných profilů plastické deformace či poškození, včetně jejich následného vývoje. Přitom objasníme, proč jisté konkrétní formulace založené na nelokálním průměrování nebo na přidání gradientních členů fungují jako spolehlivé omezovače lokalizace, zatímco jiné selhávají nebo trpí různými neduhy.

5. 10. 2016, 10:00
Experimentální studium a simulace lokalizované transformace v materiálech s tvarovou pamětí
Ing. Petr Sedlák, Ph.D., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Martenzitická transformace, jež stojí za nevšedními vlastnostmi slitin s tvarovou pamětí, neprobíhá vždy v těchto slitinách homogenně. V určitých režimech zatěžování se tato transformace vzorkem šíří nehomogenně, ve formě makroskopických pásů. Otázka jak a proč k této lokalizaci v polykrystalických materiálech dochází, trápí jak teoretiky, tak i inženýry, neboť lokalizovaná transformace nejen značně mění mechanickou odezvu těchto materiálů, ale především negativně ovlivňuje jejich únavové vlastnosti. V unikátním experimentu realizovaném na synchrotronu v ESRF se podařilo s využitím metody 3D rentgenové difrakce (3D-XRD) zmapovat stavy napjatosti v polykrystalických zrnech mikrometrických rozměrů v okolí čela deformačního pásu. Ukázala se jak výrazná heterogenita napětí na úrovni jednotlivých zrn daná anizotropií jejich elastických a transformačních vlastností, tak i dramatické přerozdělení makroskopických (homogenizovaných) napětí v okolí rozhraní. Analýza těchto výsledků umožnila adaptaci konstitutivního modelu popisujícího chování materiálů s tvarovou pamětí, především zahrnutí tzv. nelokálních, gradientních efektů. Numerická rekonstrukce šířícího se rozhraní pak objasnila mechanizmus postupného přerozdělování vnitřních napětí, a tím vysvětlila podstatu lokalizované deformace v NiTi.

15. 9. 2016, 10:00
Vliv mezní vrstvy na směšování horkých volných proudů
Ing. Jan Lepičovský, DrSc, Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Hlavní motivací výzkumu byla pasivní ochrana letounu proti tepelně řízeným střelám. Úkolem bylo snížit teplotu výstupního horkého proudu z motoru rychlým mísením s okolím bez jakékoliv ztráty tahu proudového motoru. První fáze se soustředila na objasnění rychlostních charakteristik volných proudů a ukázala, že vybuzováním a zvyšováním teploty proudu se zvýší intenzita směšování. V druhé fázi byly analyzovány nesrovnalosti v odezvě horkých volných proudů na buzení a vyšetřena úloha výstupní mezní vrstvy z trysky při směšování. Závěrem bylo prokázáno, že příčný rychlostní gradient výstupní mezní vrstvy je dominantním faktorem rozhodujícím o intenzitě směšování volných proudů s okolím. Během práce na tomto úkolu byla zdokonalena metoda vysokofrekvenčního stroboskopického zviditelňování periodických struktur v proudění. Dále byla vypracována a ověřena metoda podmíněného vzorkování náhodných signálů laserového anemometru.

17. 8. 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. 6. 2016, 10:00
Matematika tekutin v pohybu
Prof. RNDr. Eduard Feireisl, DrSc. Matematický ústav, AV ČR, v. v. i.

Abstrakt:

Budeme se zabývat současným vývojem matematické teorie pohybu tekutin, klasickými i novými otevřenými problémy a možnostmi jejich řešení. Zaměříme se zejména na úlohy spojené s řešitelností a jednoznačnou závislosti na datech pro úlohy nevazkého proudění. Budeme též diskutovat různé moderní přístupy k řešením: Slabá, velmi slabá i řešení v mírách.

4. 5. 2016, 10:00
Vývoj umělých hlasivek člověka
Ing. Jaromír Horáček, DrSc., Ústav termomechaniky, AV ČR, v. v. i.

Abstrakt:

Hlas člověka je složitý fyzikální proces, který zahrnuje proudění vzduchu přicházejícího z plic, samobuzené kmitání hlasivek a akustiku rezonančních prostor vokálního traktu člověka. Hlasivky buzené proudem vzduchu generují primární zvuk, který se šíří vzduchovými kavitami vokálního traktu, které modifikují spektrum tohoto signálu, a vytvářejí konečnou podobu akustického signálu vyzařovaného z úst člověka do okolního prostoru. Porozumění základním principům tvorby hlasu
člověka je důležité pro detekci patologických poruch a léčení onemocnění hrtanu. Fyzikální modely fonace jsou důležitým nástrojem nejen pro verifikaci současně vytvářených výpočtových 3D MKP modelů, ale i pro vývoj hlasových protéz. Prezentace zahrnuje porovnání výsledků měření fonačních charakteristik in vitro prováděných na originálně vyvíjených modelech lidských hlasivek v měřítku 1:1. Naměřené aerodynamické, vibrační a akustické charakteristiky nejnovějších modelů jsou srovnatelné s hodnotami naměřenými v lidských hlasivkách.

6. 4. 2016, 10:00
Stochastická samoorganizace vnitřní struktury dopravních systémů
doc. Mgr. Milan Krbálek, Ph.D., Fakulta jaderná a fyzikálně inženýrská, ČVUT v Praze, Katedra matematiky

Abstrakt:

Studium dopravních systémů je velice mladou vědeckou disciplínou. Za první vědecký článek se všeobecně pokládá studie profesora Bruce Greenshieldse z roku 1934. Za faktický počátek systematické vědní disciplíny nazývané v anglické literatuře Transportation Science se pak pokládá rok 1992, kdy tento obor zaznamenal prudký nárůst zájmu a kdy vyšly přelomové publikace oboru. V dnešní době je Transportation Science již velice dobře ukotvena v portfóliu vědeckých disciplín (její aktuální oborový medián je 1,377 impaktního bodu). Z celé šíře dopravní problematiky je pro účely semináře vybráno téma predikce statistických vlastností dopravního proudění a odhalení zajímavých zákonitostí v dopravní mikrostruktuře.

V příspěvku ukážeme, že se makroskopická samoorganizace dopravního proudu (např. spontánní vytvoření dopravní kongesce) promítá do adekvátních změn stochastických vlastností dopravních mikroveličin a že existuje velmi hezký a nepříliš komplikovaný způsob, jak tyto mikroskopické efekty samoorganizace predikovat.

2. 3. 2016, 13:30
Létání živých tvorů (ptáci, hmyz, člověk?)
Ing. Rudolf Dvořák, DrSc., Ústav termomechaniky, AV ČR, v. v. i.

Abstrakt:

Létat znamená vytvořit sílu, která vyrovná sílu tíže (vztlak), a sílu, která uvede toto těleso do pohybu atmosférou a udělí mu potřebnou rychlost (tah). Aby mohl vzniknout vztlak na tělese s pevnými křídly (např. letadle) musí být těleso uvedeno do pohybu vůči okolnímu prostředí nějakou vnější silou,pohonnou jednotkou, která vytvoří potřebný tah. Letci ze živočišné říše jsou odkázáni jen sami na sebe. Aby mohli létat, musí mít pohyblivá křídla, která kromě toho, že vytvářejí potřebný vztlak, vytvářejí i tah, a dostatečný svalový výkon.

Aby vznikl vztlak (síla, která při letu směřuje od Země) musí křídlo udělit tekutině hybnost směřující dolů (tj. k Zemi). Podle principu akce a reakce je pak vztlak reakcí tekutiny na tuto sílu a směřuje nahoru (tj. od Země). Podobně je tomu u jakéhokoli pohybu živočichů v tekutinách (např.ve vodě). Docílit toho lze různými způsoby, např. odkloněním proudu obtékaným profilem, nebo tím, že pohyblivým křídlem (stejně tak i pádlem) vytvoříme vírovou dvojici, která udělí tekutině hybnost směřující dolů pod letící objekt. V přednášce je tento mechanizmus popsán jak pro ptačí křídla (mávající křídla), tak pro křídla hmyzu (kmitající křídla).

Popsána bude i stavba křídel ptáků a hmyzu a různé jejich úpravy používané pro okamžité zvýšení vztlaku, nebo zvládnutí různých letových situací (např. let na místě, start nebo přistání, únik před predátorem, let mezi překážkami, aj.).

3. 2. 2016, 10:00
FLUIDICKÉ OSCILÁTORY PRO PĚSTOVÁNÍ VODNÍCH ŘAS.
Mohou snad nějak ovlivnit světovou politickou rovnováhu?
prof. Ing. Václav Tesař, CSc., Ústav termomechaniky, AV ČR, v. v. i.

Abstrakt:

Naše civilizace je v současnosti extrémně závislá na levném kapalném palivu pro dopravu. Ještě zhruba do konce 19. stol. lidé pracovali přímo tam. kde bydleli. Dnes se v ohromných počtech dopravují do zaměstnání. Také zboží se dopravuje stovky a tisíce kilometrů od výrobců k prodejcům a pak ke spotřebitelům. Tento model nyní ještě přebírají nejlidnatější rozvojové země, Čína a Indie. Fosilní zdroje, na nichž to vše závisí, se těží za stále rostoucí cenové náklady v politicky nejistých oblastech. Není divu, že grantové agentury v západních zemích jsou ochotny financovat myšlenky na obnovitelný benzin. Východiskem jsou právě řasy — primitivní, často jen jednobuněčné rostliny schopné z vody a CO2 ve vzduchu produkovat fotosyntézou uhlovodíkové sloučeniny. Z těch pak není principiální problém produkovat biopaliva — ostatně nafta právě takto z řas kdysi vznikala. Navíc by nenarůstalo v atmosféře CO2 jako skleníkový plyn a tím by se zastavilo globální oteplování. Řasy také nakonec mohou být i výchozím článkem potravinového řetězce.

Potíž je zatím v tom, že z řas syntetizované palivo vychází dražší než z fosilních zdrojů. Klíčovým faktorem je zefektivnění každého kroku z nichž proces sestává. Jedním z drobných ale podstatných příspěvků je efektivnější difúzní transport CO2 do vody v bioreaktorech. Ukazuje se, že cestou je tvorba submilimetrových mikrobublin, která je dosažena zařazením fluidického oscilátoru do přívodu plynu. V probíhajícím grantovém projektu byla podrobně zkoumána řada alternativních oscilátorů.

6. 1. 2016, 10:00
Galerkinova metoda pro řešení dynamické elasticity, stlačitelného proudění a interakce tekutin a struktur
Prof. RNDr. Miloslav Feistauer, DrSc., Dr.h.c, Katedra numerické matematiky, Matematicko-fyzikální fakulta Univerzity Karlovy, Praha

Abstrakt:

Přednáška je věnována numerickému řešení problémů dynamické elasticity a stlačitelného proudění. Uvažujeme lineární pružnost a nelineární St. Venantův-Kirchhoffův model. Prostorová diskretizace je realizována pomocí nespojité Galerkinovy metody. Pro časovou diskretizaci bylo navrženo a testováno několik technik. Jako nejpřesnější se ukazuje časoprostorová nespojitá Galerkinova metoda. Tato metoda byla rovněž aplikována na řešení problému stlačitelného proudění v časově závislých oblastech formulovaného pomocí tzv. ALE (arbitrary Lagrangian-Eulerian) metody. Bude ukázáno, že tato metoda umožňuje řešení stlačitelného proudění s velkým rozsahem Machova čísla. Vyvinuté metody byly aplikované na numerickou simulaci vibrací elastických struktur indukovaných stlačitelným prouděním. Použitelnost těchto metod bude demonstrována ukázkami numerických experimentů.

Výsledky byly získány ve spolupráci s následujícími spolupracovníky: M. Balázsová, J. Česenek, M. Hadrava, A. Kosík a J. Horáček.

2015

2. 12. 2015, 10:00
Fraktografie a analýza poruch
doc. Ing. Jan Siegl, CSc., Katedra materiálů, Fakulta jaderná a fyzikálně inženýrská, České vysoké učení technické v Praze

Abstrakt:

1) Základní metodické postupy při fraktografické analýze
2) Metody kvantitativní fraktografické analýzy – rekonstrukce kinetiky šíření únavových trhlin (výzkum vlastností materiálů, porušování letadlových konstrukcí)
3) Analýza provozních poruch a havárií (turbíny)

4. 11. 2015, 10:00
Škálovatelné algoritmy pro kontaktní problémy s miliardami neznámých
prof. RNDr. Zdeněk Dostál, DSc., Fakulta elektrotechniky a informatiky, Vysoká škola báňská — Technická univerzita Ostrava

Abstrakt:

V úvodní části přednášky budou shrnuty teoretické výsledky o optimálních algoritmech pro řešení speciálních úloh kvadratického programování a QCQP problémů a adaptace metody rozložení oblasti typu FETI na řešení kontaktních úloh. Dále budou shrnuty teoretické výsledky ukazující asymptoticky lineární složitost výsledných algoritmů pro řešení kontaktních úloh s mnoha tělesy, a to bez tření, s Trescovým (daným) třením, a dynamických úloh. Přednáška bude doplněna numerickými experimenty s řešením rozsáhlých reálných úloh a akademických úloh s miliardami neznámých demonstrujících paralelní škálovatelnost algoritmů.

12. 10. 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. 10. 2015, 10:00
Vícefázové elektrické stroje a pohony
doc. Ing. Miroslav Chomát, CSc., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Vývoj v oblasti výkonové elektroniky umožnil využívat v elektrických pohonech i některé netradiční typy elektrických strojů. V této souvislosti je v poslední době věnována značná pozornost strojům s počtem fází vyšším než obvyklé tři. Přednáška bude zaměřena na výhody a nevýhody těchto strojů a na některé zvláštnosti, které je třeba při jejich využití v elektrických pohonech brát v potaz.

3. 6. 2015, 10:00
Výšetřování mechanických vlastností pokročilých materiálů ultrazvukem
Ing. Michal Landa, CSc, Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Motivace – studium vztahu mechanických a mikrostrukturních vlastností pokročilých materiálů; vyšetřování anizotropní elasticity, detekce fázových transformací a tepelně aktivovaných procesů. Proč zjišťovat mechanické vlastnosti právě ultrazvukem? Nezbytnost plně bezkontaktních metod. Jaké možnosti poskytují současné charakterizační metody založené na optických technikách generování a detekce ultrazvuku.

6. 5. 2015, 10:00
Aplikace kontaktního algoritmu v creepové analýze vysokotlaké skříně DSPWR
Ing. Dušan Gabriel, Ph.D., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Na začátku presentace bude stručně představena výzkumná činnost v jednotlivých laboratořích oddělení D4. Vlastní příspěvek bude věnován obecnému trojrozměrnému kontaktnímu algoritmu pro řešení komplexních inženýrských problémů zahrnující vliv materiálových a geometrických nelinearit. Klíčovým rysem algoritmu je, že vyhledávání kontaktu je realizováno v bodech Gaussovy integrace na stěnách prvků a nikoliv v uzlech MKP sítě. Navržená metoda je konzistentní s variační formulací mechaniky kontinua, což umožňuje využití vyšších tvarových funkcí prvků se středovými uzly v numerické analýze. Dále bude představena aplikace uvedeného kontaktního algoritmu v creepové analýze vysokotlaké skříně turbíny DSPWR. Pro popis materiálových vlastností byl použit pravděpodobnostní exponenciální model s poškozením. Hlavním cílem práce bylo vyhodnocení zbytkových deformací v čase 10 tis. a 200 tis. hodin ve vybraných místech dělicí roviny po odlehčení, vychladnutí a demontáži skříně vysokotlakého tělesa. Výpočty byly provedeny MKP programem PMD (Package for Machine Design) vyvíjeném v oddělení D4 a nezávisle porovnány s výsledky MKP programu ANSYS.

1. 4. 2015, 10:00
Experimentální a teoretický výzkum vlivu třecích vazeb lopatek na dynamiku lopatkových kol
Ing. Luděk Pešek, CSc., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Na začátek prezentace bude stručné seznámení s laboratořemi oddělení Dynamika a vibrace. Vlastní příspěvek je zaměřen na vývoj experimentálních a výpočtových metod pro objasnění vlivu třecích vazeb lopatek v rotačních lopatkových strojích. Pomocí třecích vazeb lopatek dochází ke snížení dynamického namáhání lopatek oběžných kol. Za tímto účelem budou prezentovány nové experimentální postupy na modelovém zkušebním kole včetně teoretických analyticko-numerických metod umožňujících optimalizaci parametrů dynamického modelu lopatkových kol.

4. 3. 2015, 10:00
První pohled na oddělení Termodynamika a výzkum homogenní nukleace kapek
Ing. Jan Hrubý, CSc., Ústav termomechaniky AV ČR, v. v. i.

Abstrakt:

Přednáška se bude skládat ze dvou částí. V první části bude podán přehled badatelských činností v oddělení Termodynamika. Výzkum je převážně soustředěn do tří laboratoří. Hustota a povrchové napětí nově se objevujících médií, jako např. iontových kapalin, jsou přesně určovány a korelovány v Laboratoři termofyzikálních vlastností tekutin. Transportní jevy v různých proudových uspořádáních, včetně syntetických proudů a mikrofluidických přístrojů, jsou zkoumány v Laboratoři sdílení tepla a hmoty. Metastabiliní stavy látek (např. podchlazená voda nebo přesycená pára) a nukleace (vznik nové fáze) jsou předmětem výzkumu Laboratoře kinetiky fázových přechodů.

V druhé části přednášky bude uvedena problematika homogenní nukleace kapek z přesycených par. Výzkum je motivován aplikacemi v parních turbínách, zpracování zemního plynu, zachycováním a ukládáním oxidu uhličitého a atmosférickými jevy. Bude podán přehled experimentálních metod s důrazem na přístroje založené na expanzi, umožňující studium vysokých nukleačních rychlostí. Ačkoli tzv. klasická nukleační teorie (CNT) je k dispozici po mnoho desetiletí, zatím nebyl učiněn průlom vedoucí na kvantitativně správnou předpověď závislosti nukleační rychlosti na teplotě a tlaku. Bude představen vlastní výzkum, založený na uvažování konečné tloušťky fázového rozhraní a jeho zvlnění kapilárními vlnami ve spojení s univerzálním kritickým škálováním.

4. 2. 2015, 10:00
Aerodynamický výzkum špičkových řezů dlouhých rotorových turbínových lopatek
Ing. Martin Luxa, Ph.D., Ústav termomechaniky AV ČR

Abstrakt:

Přednáška se zabývá systematickým aerodynamickým výzkumem parametrů proudění a struktur proudového pole ve špičkových řezech lopatkových mříží při transsonických a supersonických provozních stavech. Analýzy jsou založeny na základě rozsáhlých experimentů, které byly provedeny na pěti profilových mřížích různého tvaru. Tyto modely representují špičkové řezy oběžných kol posledních stupňů nízkotlakých částí parních turbín velikého výkonu. Jde o řezy lopatek dlouhých 860mm, 1080mm, 1220mm, 1375mm a 1525mm.

Aerodynamický výzkum špičkových řezů poskytuje důležité informace o vysokorychlostním proudění stlačitelné tekutiny v těchto řezech oběžných kol. Analýza experimentálních dat je zaměřena především na problematiku expanse okolo zvukové čáry, problematiku aerodynamického ucpání, problematiku vývoje supersonického proudového pole, dále na proudění v oblasti odtokové hrany, výstupní rázové vlny, interakce rázových vln s mezní vrstvou, úplav, atd.

Výsledky experimentů jsou nedocenitelné nejenom pro vývoj nových strojů, ale též pro vývoj a testování metod výpočetní mechaniky tekutin, které modelují i transsonické proudění a proudění průtočnými kanály turbostrojů.

2014

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

Abstrakt:

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.