On a Mechanism of Stabilizing Turbulent Free Shear Layers in Cavity Flows

Abstract

Turbulent free shear flows are subject to the well-known Kelvin-Helmholtz type [1] instability, and it is well-known that any free shear flow which approximates a thin vorticity layer will be unstable to a wide range of amplitudes and frequencies of disturbance. In fact, much of what constitutes flow control in turbulent free shear layers consists of feeding a prescribed destabilizing disturbance to these layers. The question in the control of free shear flows is not whether the shear layer will be stable, but whether you can influence how the layer becomes unstable. In most cases, since these flows are so receptive to forcing input, and naturally tend toward instability, large changes in flow conditions can be achieved with very small amplitude periodic inputs. Recently, it has been discovered that turbulent free shear flows can also be stabilized using periodic forcing. This is, at first glance, counter-intuitive, considering our long history of considering these flows to be very unstable to forcing input. It is a phenomenon not described in modern fluid dynamic text books. The forcing required to achieve this effect (which we will call turbulent shear layer stabilization) is of a much higher amplitude and frequency than the more traditional type of shear layer flow control effect seen in the literature (which we will call turbulent shear layer destabilization). A numerical study is undertaken to investigate the effect of frequency of pulsed mass injection on the nature of stabilization, destabilization and acoustic suppression in high speed cavity flows. An implicit, 2nd-order in space and time flow solver, coupled with a recently-developed hybrid RANS-LES (Reynolds Averaged Navier Stokes - Large Eddy Simulation) turbulence model by Nichols et. al. [2], is utilized in a Chimera-based parallel format. This tool is used to numerically simulate both an unsuppressed cavity in resonance, as well as the effect of mass-addition pulsed jet flow control on cavity flow physics and ultimately, cavity acoustic levels. Frequency (and in a limited number of cases, amplitude) of pulse is varied, from 0 Hz (steady) up to 5000 Hz. The change in the character of the flow control effect as pulsing frequency is changed is described, and linked to changes in acoustic levels. Limited comparison to 1/10th scale experiments is presented. The observed local stabilization of the cavity turbulent shear layer, when subjected to high frequency pulsed blowing, is shown in simulation to be the result of a violent instability and break-down of a pair of opposite sign vortical structures created with each high frequency “pulse”. This unique shear layer stabilization behavior is only observed in simulation above a certain critical frequency. Below this critical frequency, pulsing is shown in simulation to provide little benefit with respect to suppression of high cavity acoustic levels.