Numerical Simulation of Aeroacoustic Problems

The present paper investigates the numerical methods used to predict aerodynamically generated noise, more specifically airframe noise. Here, airframe noise is understood as noise, which is generated due to the interaction of turbulence-related vorticity disturbances with geometric inhomogeneities of an aerodynamic body. Such interaction may take place directly or indirectly through a previously occuring hydrodynamic instability of the flow field. First, the main mechanisms of airframe noise are discussed. Based on these physical considerations the potentially suitable computation approaches to airframe noise are identified and assessed w.r.t. limitations. Modern CAA (Computational Aeroacoustics) techniques are presented. Since (as opposed to using acoustic wave equations) the dynamics of all types of waves (pressure-, vorticity-, entropy-) can be simulated using Euler's equations, generally, approaches based on the Euler equations can be employed to compute not only the noise propagation but its very generation as well. The numerical solution of Euler's equations on a computational grid generally necessitates the use of some sort of artificial selective damping in order to render the solutions stable. The paper discusses the role, artificial selective damping may play for the sound generation. The effect of these terms are understood when using one of the aeroacoustic analogy equations or e.g. Möhring's acoustic wave equation. Examples are given, showing how artificial selective damping "contributes" to computational noise. Typical aeroacoustic problems relating to airframe noise are solved numerically employing the DRP-scheme of Tam