INTENSE VORTICAL STRUCTURES IN GRID-GENERATED TURBULENCE

This paper presents a set of experiments aimed at investigating the features and the statistical frequency of intense vortical structures (sometimes called ''filaments'', or ''worms'') as manifested by a migrating bubble technique in a mean shear free, homogeneous, isotropic, stationary turbulence generated by oscillating grids in a water tank for R(lambda) reaching up to 300. It is found that the nucleation of filaments at the surface of the walls of the tank, where boundary layers are liable to destabilize is much more frequent than in the homogeneous bulk of the tank where one filament is typically detected each hundred large scale turnover time. This distinction between the wall surface and the bulk activity, supplemented with the fact that the size of the filaments and their lifetime compare with the length and time-scales of the largest structures of the flow leads us to formulate an elementary model explaining the origin and the geometrical features of these intense vortical structures in turbulent flows for arbitrary Reynolds numbers. (C) 1995 American Institute of Physics.