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SUMMARYSelf-sustained oscillations are present in a wide variety of flows. For flows passing cavities, acoustics can play a role in the physical mechanisms triggering and maintaining the oscillation phenomena. This is the case for the flow over rectangular cavities, which is studied in this paper. The compressible LES Navier-Stokes equations are solved for cavities with an upstream laminar boundary layer. The flow passing rectangular cavities is computed, varying two different parameters related to the physics of the problem, Mach number and length of the cavity divided by the momentum thickness of the boundary layer at the leading edge. This paper intends to prove that the switch between shear-layer oscillation mode, characterized by an acoustic feedback process, and wake oscillation mode, characterized by a large-scale vortex shedding with Strouhal number independent of Mach number, can be identified using large eddy simulation, and consequently, with much less computational effort than in other studies in literature, where direct numerical simulation has been applied.

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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.