Counter‐rotating vortex shedding generated by acoustic excitations in confined mixing layers

Abstract

Ultrafast mixing enhancement has been realized in confined mixing layers at an optimal forcing frequency, under which the flow exhibits the strongest mixing. The fast mixing is accompanied by the so-called counter-rotating vortices (CRVs). In this investigation, by using particle imaging velocimetry (PIV) and laser-induced fluorescence, the conditions of generating CRVs in confined mixing layers are theoretically and experimentally investigated. To generate CRVs, (a) the acoustic particle displacement must be larger than the summation of the curvature radius of the trailing edge and the thickness of the Stokes layer, and (b) the temporal derivative term of the vorticity equation should be larger than the convective term of the mean vorticity generated by the mean flow. This study enhances our understanding on vortex shedding dynamics and enables novel designs of fast mixers for continuous operations in process industry.