Assessment of a Noise Reduction Concept for a Hot Supersonic Jet Using Large Eddy Simulation

Noise emissions of a hot, supersonic jet at laboratory scale are predicted with and without a noise reduction concept. The hot jet is representative of an over-expanded, low bypass ratio military gas turbine engine exhaust. The reduction device consists of twelve non-penetrating chevrons that are equilateral triangles, extending tangentially from the inner nozzle wall and to the flow at the nozzle exit. The chevrons are idealized and not a result of any design or testing procedures. Hybrid Reynolds Averaged Navier Stokes and Large Eddy Simulation (HRLES) is performed at both 5.5 million and 128 million cell grid resolutions. A novel structured topology is used to perform the HRLES with and without the chevrons using the same grid, and the chevrons are turned on and off through a boundary condition switch. The noise emissions are then predicted on a circular array one hundred jet exit diameters from the nozzle exit with the permeable Ffowcs Williams and Hawkings equation method. The chevrons modify the entrained flow at the over-expanded power setting, impacting the shear layer and associated far-field noise. The chevrons stabilize the jet shock cell structure, and also reduce turbulent fluctuations throughout the length of the shear layer. An overall sound pressure level reduction of between one and four decibels is predicted at all angles to the jet, including the elimination of jet screech relative to the baseline jet with no chevrons. A significant reduction in broadband noise is also predicted downstream. The jet with chevrons is also predicted with the higher resolution grid and noise levels are compared with the low resolution grid. Upstream noise level predictions are increased due to enhanced resolution of fine scale turbulence.