Fluidically Enhanced Chevrons for Supersonic Jet Noise Reduction

Abstract

An experimental and computational investigation of a combination of fluidic injection and chevrons (fluidically enhanced chevrons) for supersonic jet noise reduction was performed. Previous studies have shown that chevrons underperform in overexpanded flow due to reductions in effective penetration. Consequently, this study focuses on improvements gained by fluidic enhancement in the overexpanded regime. Acoustic results indicated the fluidically enhanced chevrons outperform the chevrons, with additional overall sound pressure level reductions of 2 dB in the upstream direction and nearly 1.5 dB in the downstream direction. Spectral results indicated that primary benefits were in shock and low-frequency noise. A small high-frequency penalty was the only observed detriment. Time-averaged velocity and turbulence quantities as well as shock cell spacing, were computed by large-eddy simulation and then compared to particle imaging velocimetry and shadowgraph results. These flowfield measurements indicated that the fluidically enhanced chevrons modify the flowfield by reducing the shock cell spacing and shock strength and increasing the upstream jet half-width, peak and integrated turbulence values, and shear-layer thickness. These modifications were caused by the introduction of streamwise vortices with over four times the magnitude of the unenhanced chevrons. Noise reductions were attributed to flowfield modifications through the use of several theoretical models, such as Powell’s screech formula.