Management of streamwise vortices in near-wall turbulence

Abstract

In this paper we describe our series of studies aimed at finding a way to control turbulence in wall-bounded flows. First we discuss, as the basis of control mechanism, sustenance of a streamwise vortex which plays a key role in all the events in turbulence in the near-wall layer. We demonstrate that of two sources of production of streamwise vorticity, i.e., stretching and re-orientation of vortex filaments, the latter is far more important than the former. Although stretching effect contributes to enhance vorticity in the core of a streamwise vortex, it does not augment circulation. The unique source of supplying new streamwise vorticity is re-orientation of wall-normal vorticity which is generated by folding of a vortex sheet of spanwise vorticity to a streamwise one. Second, two cases of attenuating streamwise vortices are presented in order to exemplify that destruction of the above-mentioned mechanism of growth of a streamwise vortex causes drag reduction. The first one is to assume anisotropic viscosity which is stronger in the spanwise direction than in the others. The second one is to introduce beads-spring elements convected with flow with slight relative velocity. This is proved to provide a reasonable explanation for drag reduction, known under the name of Toms effect, and related observations in experiment.