The Effect of a Boundary Layer on Engine Noise propagating to the Fuselage at Flight Conditions

Abstract

Engine noise transmitted through the fuselage causes a significant part of the cabin noise. In a previous work a Computational Aeroacoustics (CAA) technique was introduced that takes into account the effect of non-stationary fluctuating boundary layer on sound propagation. It showed a considerable change of the sound field reaching the fuselage due to refraction in the boundary layer. The sound propagation is calculated solving the linearised Euler equations over a viscous Reynolds-averaged Navier-Stokes (RANS) flow. Turbulent scattering is considered by augmenting the steady flow computation with turbulent fluctuations reconstructed stochastically using the Random-Particle-Method (RPM) based on RANS turbulence statistics. In this study realistic engine fan modes at 2850 Hz are used as a sound source together with a free flow Mach number of 0.85. The noise emission from the engine intake exhibits a distinct directivity with highly increased sound radiation in direction normal to the axis of rotation of the fan. However the refraction shows a strong influence on the sound pressure level. The effect of the scattering is more subtle including the build-up of spectral haystacks (broadband humps next to the tone frequency) as well as a frequency shift of the spectral maximum towards higher frequencies. Especially the frequency shift can be very important for the assessment of the structural response of the fuselage skin to the acoustic surface pressure.