High Frequency Direct Excitation of Small-Scale Motions in Planar Shear Flows

Abstract

The effect of direct, small-scale excitation on the evolution of a plane shear layer which forms at the edge of a backward facing step is investigated experimentally using high resolution particle image velocimetry and hot-wire anemometry. Actuation is effected at frequencies that are over an order of magnitude higher than the characteristic (or natural) formation frequency of the layer by a spanwise array of piezoelectrically-driven synthetic jet actuators that are placed near the edge of the step. The actuation has significant effects on the evolution of both large- and small-scale motions within the shear layer inducing an increase in small-scale dissipation and simultaneous suppression of turbulence production. While the fundamental instabilities that lead to the formation of large scale motions are typically suppressed, low-frequency amplitude-modulation of the actuation signal allows the formation of large scale motions and entrainment which, in concert with the small-scale actuation, lead to enhancement of the turbulent shear stresses throughout the shear layer. Amplitude modulation is also used to assess the effect of flow transients that are induced by step or low duty cycle actuation. The present findings suggest strategies for controlled suppression or enhancement of mixing in the near field of the shear layer.