Reducing Vortex Shedding Sound from a Trailing Edge Above a Wall Jet

Abstract

Large eddy simulation (LES) was used to simulate vortex shedding behind a blunt trailing edge located directly above a curved wall jet. The spectrum from the LES of fluctuating wall pressures at the trailing edge showed a fundamental tone that was consistent with an audible tone observed in an experiment using the same trailing edge and wall jet configuration. The LES showed that spanwise vortices shed from the blunt trailing edge were responsible for the tone. The lower surface of the trailing edge which bounded the wall jet was then contoured in an attempt to diffuse the shear layer above the wall jet downstream of the trailing edge, thereby reducing the intensity of the vortex shedding. Wall pressure spectra from an LES of the contoured trailing edge showed a significant reduction in the amplitude of the fundamental tone from the shed vortices, and also the broad band spectrum. The spanwise shape of the trailing edge was also changed to a sawtooth configuration in an attempt to break up the strong spanwise vortices. Wall pressure spectra from an LES of the sawtooth configuration showed some areas of the trailing edge with reduced tonal amplitude, but other areas where the amplitude was increased, when compared to the original blunt trailing edge. Calculated far field sound spectra confirmed the reduction in tonal and broad band amplitudes for both the contoured and sawtooth trailing edge shapes. The far field sound spectra were calculated using the Ffowcs-Williams Hawkings approach with stationary, compact sources. In the LES technique, the filtered, incompressible Navier-Stokes equations were discretized using a second order, bounded central difference scheme. Time advancement was implicit and second order accurate using a fractional step method. The sub-grid scale turbulence was accounted for with a dynamic Smagorinsky model.