On the Use of Filtering Techniques for Hybdrid Methods in Computational Aero-Acoustics

Hybrid CAA-approaches are commonly used for aeroacoustic engineering applications. In this kind of computational techniques, the numerical domain is split into a noise generating region, where an aerodynamic field generates the acoustic sources, and an acoustic propagation region. Nowadays a large variety of hybrid approaches exist differing from each other in the way the source region is modeled; in the way the equations are used to compute the propagation of acoustic waves in a non-quiescent medium; and in the way the coupling between source and acoustic propagation regions is made. The coupling between source and propagation region is usually made using equivalent sources (acoustic analogies) or acoustic boundary conditions (Kirchhoff’s method). For certain applications both coupling approaches tend to give erroneous results: acoustic analogies are inaccurate if the acoustic variables are of the same order of magnitude as the flow variables, which is the case for flow-acoustic feedback phenomena such as cavity noise or when acoustic resonance occur which happens for duct aeroacoustics applications; acoustic boundary conditions are sensitive to hydrodynamic pressure fluctuations when a vortical flow passes through the Kirchhoff’s surface. These inaccuracies can be avoided by using appropriate filtering techniques where the solution in the source domain is split into an acoustic and a hydrodynamic part. This paper illustrates the need for such filtering techniques for CAA-applications and starts with the theoretical development of a new filtering technique based on an aerodynamic-acoustic splitting.