Refraction and Scattering in High Mach Number Boundary Layers

Abstract

A significant part of the cabin noise of airplanes comes from the engines. The noise field of the engines is considerable changed due to the fuselage’s boundary layer before it hits the fuselage skin. In this work we study the influence of the mean boundary layer as well as of the instationary turbulent fluctuating boundary layer on the sound field on the fuselage using a point noise source. The flow field is calculated with the Reynolds-averaged Navier-Stokes (RANS) equation and the turbulent fluctuations are reconstructed stochastically by means of a Random-Particle-Mesh method (RPM). The sound propagation is calculated with a computational aeroacoustic (CAA) approach solving the linearized Euler equations. At high Mach numbers we find a strong refraction effect due to the mean boundary layer and a substantial scattering effect of the turbulent vortices. The surface pressure completely differs from the free acoustic field: The frequency is increased, the pressure field is spatially incoherent and a considerable part of the original signal is reflected by the boundary layer. According to our knowledge, the combined effect of refraction and scattering of high Mach numbers boundary layers was not studied before. We think that the results are very important for the assessment of the structural response of the fuselage skin to the acoustic surface pressures.