A study of noise reduction mechanisms of jets with fluid inserts

The fluid insert technology is an active noise reduction technique that has been shown to successfully reduce supersonic jet noise in model scale nozzles. The goal of this paper is to better understand the noise reduction mechanisms of fluid insert jets using Doak's Momentum Potential Theory and Spectral Proper Orthogonal Decomposition applied to an LES database. It is found that the use of fluid inserts results in a lower convection speed of the wavepacket-like coherent structures in the jet shear layer resulting in a shallower Mach wave radiation angle. It is also shown that the use of fluid inserts results in a reduction of length scales of the turbulent fluctuations in the shear layer, which correlates correctly with the observed noise reductions.