Numerical Simulations of Noise Reduction Devices for Aero Engines

Licentiate thesis, 2012

Increasing air traffic and denser population around airports have led to stricter regulations on aircraft noise. The engine is the main source of noise of jet aircraft. Decreasing jet engine noise can in some cases reduce sonic fatigue and thereby increase the engine lifetime. In this thesis the performance of a novel low-frequency acoustic liner concept is investigated using unsteady Reynolds- Averaged Navier-Stokes simulations (URANS). The results are compared with those of an analytical model and experiments. The liner is designed to reduce fan noise upon placement on the outlet guide vanes. Furthermore, the response of the radiated noise from a supersonic jet emitted from a converging diverging nozzle to steady-state and pulsed fluidic injection is tested using Large Eddy Simulation (LES). 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 resulting noise. The optimized nozzle was evaluated further using LES and experimental techniques. The acoustic liner study showed that the resonance frequency of the liner obtained by the URANS compared within 200Hz to the measured resonance frequency. It was shown that the analytical model can be tuned with a single parameter to match the URANS simulations over a wide range of frequencies. Simulations of the sharp throat CD-nozzle with and without fluidic injection compared within 2 dB to the measured values of the overall sound pressure level (OASPL) for all observers. The pulsed injection showed that the radiated noise is sensitive to the pulsation characteristics and the frequency. It was shown that noise reduction with pulsed injection can equal the noise reduction of steady-state injection with lower net mass flow of the pulsed injection. However, an increased noise was noted at the downstream observers. 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. Good comparison is obtained with RANS, LES and experiments.