Coherent structure interactions in a two-stream plane turbulent mixing layer with impulsive acoustic excitation

Abstract

Periodic plane acoustic waves consisting of four discrete pulses are used to trigger the Kelvin–Helmholtz (KH) instability at the origin of an initially laminar plane mixing layer. The resulting coherent large-scale structures (CLSS) which grow and interact with their neighbors are followed downstream with hot-wire probes traversed across the mixing layer to a Reynolds number Re=1.5×106. The Reynolds number here is based on downstream distance x and the velocity difference (U1−U2). The continuous hot-wire time record is conditionally sampled with respect to the tone bursts and the samples are phase averaged to reveal the CLSS footprints. Optimum phase averages are obtained using an iterative correlation technique that corrects for phase jitters due to turbulence. An enhanced study of the CLSS is therefore made possible. The ensembles reveal a vivid picture of vortex pairing between the initial eddies as well as the significant role played by the CLSS in momentum transport; hence turbulence mixing.