Study of Supersonic Jet Noise Reduction using LES

Doctoral thesis, 2014

Increases in air traffic and denser population around airports have led to stricter regulations on aircraft noise. High noise levels from high-speed aircraft can cause hearing damage in pilots and the airfield personnel. The engine is the main source of noise of all jet aircraft and is therefore a key component for improvement. Decreasing jet engine noise can in some cases reduce sonic fatigue and thereby increase the engine lifetime. In this thesis, the response of the radiated noise from a supersonic jet emitted from a converging diverging nozzle to steady-state, pulsed and flapping fluidic injection is studied using Large Eddy Simulation (LES), and comparisons are made with experimental data. An investigation is also presented in which actions were taken to reduce the internal shock strength by modifying the nozzle throat, and thereby reduce the radiated noise. The optimized nozzle nearly eliminates the internal shock, which reduces the double diamond structure in the jet plume but increases the strength of the shock at the nozzle exit. It has lower turbulence levels at the nozzle exit due to a weaker shock interaction with the shear layer. The optimized nozzle provides equal thrust to the sharp nozzle with 4 % less pressure without any acoustic penalty. The pulsed injection showed that the radiated noise is sensitive to the pulsation characteristics and the pulsation fre- quency. It was shown that the noise reduction with pulsed injection can equal the noise reduction of steady-state injection with a lower net mass flow of the pulsed injection. However, increased noise was noted at the downstream observers. The flapping injection cases that were investigated did not show improvements over the corresponding steady injection cases. These are positive findings, since steady injection should be simpler and more robust to apply to real jet engines. The injection was shown to impact the jet thrust, as expected. The net jet thrust in- creased with increased injection mass flow, whereas the specific thrust decreased. The momentum thrust was shown to decrease with increased injection mass flow whereas the pressure thrust increased due to a shock shift at the nozzle exit. The work presented in this thesis adds to the body of knowledge found in the liter- ature about supersonic jet noise generation and its noise reduction using fluidic injection.