On the Use of Filtering Techniques for Hybdrid Methods in Computational Aero-Acoustics W. De Roeck, G. Rubio, W. Desmet K.U.Leuven, Department of Mechanical Engineering, Celestijnenlaan 300 B, B-3001, Leuven, Belgium e-mail: wim.deroeck@mech.kuleuven.be Abstract Hybrid CAA-approaches are commonly used for aeroacoustic engineering applications. In this kind of com- putational 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 hy- brid 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 acous- tic 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. 1 Introduction Aeroacoustics is a research area of research of growing interest and importance over the last decade. In the transportation sector, the interest for this field has emerged during the last few years, due to various reasons. In aeronautics, for example, strict noise regulations around airports are forcing aircraft manufacturers to reduce the noise emissions during landing and take-off operations. In automotive industry, customer surveys identify wind noise as a regular complaint. With the increase in computational power, the direct computation of aerodynamic noise has become feasible for academic cases [1, 2, 3]. Such a direct approach solves the compressible Navier–Stokes equations, which describe both the flow field and the aerodynamically generated acoustic field. Due to the large disparity in energy and length scales between the acoustic variables and the flow variables, which generate the acoustic field, and since acoustic waves propagate over large distances, the direct solution of the Navier–Stokes equations (DNS) for computational aeroacoustics (CAA) problems is only possible for a limited number of engineering applications [4]. In order to meet the required design times without excessive the costs, hybrid methods are proposed. In these methods, the computational domain is split into different regions, such that the governing flow field (source region) or acoustic field (acoustic region) can be solved with different equations, numerical techniques, and computational grids. As such, prediction of the acoustic field at large distances from the sound source is enabled. There exists a large number of hybrid methodologies differing from each other in the type of 595