Study of Supersonic Jet Noise Reduction using LES

Abstract

Increases in air traffic and denser population around airports have led to stricterregulations on aircraft noise. High noise levels from high-speed aircraft can causehearing damage in pilots and the airfield personnel. The engine is the main sourceof 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 therebyincrease the engine lifetime. In this thesis, the response of the radiated noise froma supersonic jet emitted from a converging diverging nozzle to steady-state, pulsedand flapping fluidic injection is studied using Large Eddy Simulation (LES), andcomparisons are made with experimental data. An investigation is also presentedin which actions were taken to reduce the internal shock strength by modifying thenozzle throat, and thereby reduce the radiated noise. The optimized nozzle nearlyeliminates the internal shock, which reduces the double diamond structure in thejet plume but increases the strength of the shock at the nozzle exit. It has lowerturbulence levels at the nozzle exit due to a weaker shock interaction with theshear layer. The optimized nozzle provides equal thrust to the sharp nozzle with4 % less pressure without any acoustic penalty. The pulsed injection showed thatthe 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 thenoise reduction of steady-state injection with a lower net mass flow of the pulsedinjection. However, increased noise was noted at the downstream observers. Theflapping injection cases that were investigated did not show improvements overthe corresponding steady injection cases. These are positive findings, since steadyinjection should be simpler and more robust to apply to real jet engines. Theinjection 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 flowwhereas the pressure thrust increased due to a shock shift at the nozzle exit. Thework 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 fluidicinjection