Characterization of near-wall structures in the log-region of a turbulent boundary layer by means of conditional statistics of tomographic PIV data

Abstract

The random-in-appearance turbulent flow of boundary layers seem to display packages of organized coherent structures near the wall. The understanding of the generation and interaction of these structures and their contribution to the production and dissipation of turbulence is of great relevance to the modelling and control of wall-bounded flows. The present work is a contribution to the knowledge about organized coherent structures at the logarithmic region of turbulent boundary layers at relatively high Reynolds number. From a six-camera tomo-PIV experiment, conducted in a wind tunnel with a Reynolds number based on the momentum thickness of 8500, conditional statistics were obtained that allowed the characterization of low- and high-speed regions, ejections, sweeps and vortices. The analyses confirmed findings from previous works in the literature regarding ejection and sweep structures as being major contributors to the turbulent production, while vortices are responsible for dissipating this energy in the self-sustaining turbulent process. The vortical structures were found to be close to low-speed regions, being responsible to maintain these regions and to induce ejection and sweep events. The conditional statistics on the coherent structures at the log-region obtained in the present work were in good agreement with the literature, including statistics at the inner and outer regions of the canonical flow of a flat plate boundary layer, which suggest some possible general structure organization.