A POPULATION STUDY OF SMALL-SCALE SPANWISE VORTICES IN TURBULENT CHANNEL FLOW

摘要

Instantaneous, high-resolution velocity field measurements are made in the streamwise-wall-normal plane of turbulent channel flow using particle-image velocimetry (PIV) with the intent of studying the population trends and characteristics of small-scale spanwise vortices. Spanwise vortices with clockwise rotation (negative spanwise vorticity) are believed to be the heads of hairpin-like vortices, the building-blocks of large-scale vortex organization (vortex packets) in the outer-layer of wall turbulence. Swirling strength, an unambiguous vortex-identification criterion and hence a local marker of rotation, is used to identify small-scale spanwise vortex cores with both clockwise and counter-clockwise rotation in the instantaneous PIV fields. The present results indicate that the very-near-wall region (y < 0.1h) is densely populated by spanwise vortices with clockwise (negative) rotation, which is consistent with the character of the mean shear and indicates that hairpin-like vortices originate at the wall. In contrast, counter-clockwise (positive) spanwise vortices are scarce in the very-near-wall region, but their presence steadily increases within the logarithmic layer. Beyond y/h = 0.25, the number of both positive and negative vortices decreases, although the percentage of each as a function of wall-normal position remains constant. The diameter of these vortices is found to roughly scale with inner units, although the negative vortices have a larger size distribution than the positive vortices. Finally, instantaneous velocity and swirling strength fields indicate that the positive vortices may be created by the local shear generated between two consecutive negative vortices within a packet.