Topical Problems of Fluid Mechanics


Institute of Thermomechanics AS CR, v.v.i.	CTU in Prague Faculty of Mech. Engineering Dept. Tech. Mathematics	MIO Université du Sud Toulon Var - AMU - CNRS - IRD	Czech Pilot centre ERCOFTAC

Fractal Intermittent Wakes in a Wind Tunnel
J. M. Redondo, J. Tellez, J. Kopec, K. Kwiatkowski, P. Lopez, M. Tijera, S. Strihjak, N. Malik, O. B. Mahjoub, S. Usama
Abstract:
Flat 2D fractal grids are known to alter turbulence characteristics downstream of the grid as compared to the regular grids with the same blockage ratio and the same mass flow. Recently, a new 3D multi-scale grid design has been proposed [1, 2] such that each generation of length scale of turbulence grid elements is held in its own frame, in overall co-planar This produces a sparse grid system whereby each generation of grid elements produces a turbulent wake pattern that interacts with the other wake patterns downstream. A motivation of the new fractal wake experiments reported here is that the effective blockage ratio in the 3D Sparse Grid Turbulence (3DSGT) design seems lower than in the 2D counterpart, with the new configurations, the blockage ratio could be reduced from say 30% in 2D flat grid to 30%-10% in the 3DSGT. This is a reduction, which, if realized in practice values of the network. High resolution one point velocity measures obtained with hot wire probes are shown and compared with Atmospheric wind data. The advantadge of using long high resolution point velocity data is that structure functions of higher order and intermittency may be calculated. In addition, the analysis allows to set bounds on the parameters to be used. Examples of fluxes are presented to show the behaviour of the grid Numerical DNS compared to Eulerian and Lagrangian experiments will also be used to examine these systems in an accessible range of parameter space. Some experiments on the 3DSGT has already been carried out under an EU Hit Large Turbulent Facility project at the Max Planck Institute in Gottingen, the actual 3DSGT that was constructed at the MPI has three recurrence square grid levels. Other results and analysis from these high resolution experiments, as well as with several traditional configurations, such as square and Sierpinski Carpet designed grids with higher solidity will be reported soon.
Keywords:
fractal wakes, sparse grids, mixing, turbulent intermittency, dispersion, structure functions

Fulltext: PDF DOI: https://doi.org/10.14311/TPFM.2018.033

In Proceedings Topical Problems of Fluid Mechanics 2018, Prague, 2018 Edited by David Šimurda and Tomáš Bodnár, pp. 243-250 ISBN 978-80-87012-65-9 (Print) ISSN 2336-5781 (Print)