An aerodynamic/acoustic splitting technique for hybrid CAA applications

Hybrid CAA-approaches, where the computational domain is split into an aerodynamic source domain and an acoustic propagation region, are commonly used for aeroacoustic engineering applications and have proven to be of acceptable efficiency and accuracy. The different coupling techniques tend to give erroneous results for a number of applications, which are mainly encountered in confined environments. Acoustic analogies are inaccurate, if the acoustic variables are of the same order of magnitude as the flow variables and an acoustic continuation of the source domain simulation using the latter solution as acoustic boundary conditions is only possible if no vortical outflow is occurring. These inaccuracies can be avoided by using appropriate filtering techniques where the source domain solution is split into an acoustic and an aerodynamic fluctuating part. In this paper, such an aerodynamic/acoustic splitting technique is developed and validated for some simple test cases. The filtering method is valid for low-Mach number applications, assuming that all compressibility effects are caused by the irrotational acoustic field while the incompressible aerodynamic field is responsible for the vortical movement of the flow field. Under these assumptions, it is shown that the aerodynamic and acoustic fields at every time step are obtained by solving a system of Poisson equations driven by the fluctuating expansion ratio and vorticity, obtained form the source domain simulation. For hybrid CAA-approaches this filtering technique, general applicable for both free-field and confined flow applications, is able to provide more accurate coupling information and improves the knowledge of aerodynamic noise generating mechanisms.