Unsteady simulations of fluid inserts for supersonic jet noise reduction

Hearing damage due to noise produced in the jet exhaust of high performance military aircraft is a concern to the US Navy. This has led to the investigation of several noise reduction methods ranging from the installation of chevrons to the use of corrugated seals. The majority of these noise reduction techniques are termed as passive, as they cannot be turned off or modified during flight and can cause performance penalties. The fluid insert technology is an active noise reduction technology that is being developed at Penn State for supersonic jet noise reduction. The goal of the fluid insert method is to achieve noise reduction for low bypass ratio turbofans with minimal influence on engine performance. The fluid inserts blow air into the divergent section of the nozzle to provide an on-demand noise reduction that can be turned off or modified depending on the flight regime. This method has been shown to successfully reduce both mixing and broadband shock associated noise. Although extensive research in the form of noise measurements and RANS (Reynolds Averaged Navier-Stokes) calculations have been performed to attempt to improve on the fluid insert technology, the reason why these inserts work is still not understood completely. The analysis of existing RANS data alone has been found to be insufficient to correlate the changes in the flow-field with the corresponding changes in the noise. This suggests the use of unsteady scale-resolving simulations to provide additional insights into the flow-field and help understand the noise reduction mechanisms. This paper is a step in that direction as it presents unsteady Large Eddy Simulations that have been carried out for baseline and fluid insert nozzle variants using the commercially available code STAR-CCM+. These simulations are accompanied by correlation studies and the use of the proper orthogonal decomposition technique in order to quantify the effect of the fluid inserts on the flow-field and the corresponding noise reduction.