Effect of periodic perturbations on the turbulence statistics in a backward-facing step flow

Abstract

Turbulence budget for natural and periodically perturbed backward-facing step flows was studied using particle image velocimetry data, aiming to provide more evidence for the mechanisms of the turbulence production under perturbation. The flow has a Reynolds number of 9630 based on the step height. A synthetic jet actuator deployed at the step corner was used to perturb the flow with a wide range of parameters including different frequencies and amplitudes. The results show that the sizes of the turbulence production terms are enhanced with efficiency when flow is perturbed at a frequency near the shear layer mode frequency. The enhancement is associated with vortical structures produced by the perturbation. The length scale of the structures is similar to the step height. In contrast, the low-frequency perturbation (with a frequency which is 25% of the shear layer mode frequency) has less impact on the turbulence production despite the large amplitude vertical flapping motion of the whole shear layer and large increases in the streamwise Reynolds normal stress occur.