Low-frequency components and modulation processes in compressible cavity flows

The modulated character of cavity-flow oscillations is investigated through a so-called modulation analysis of spectral distributions. The approach is based on a recently proposed viewpoint on the Rossiter formula and concerns mid- to high-subsonic flows in shallow cavities. For this type of configuration, the spectra are mainly characterized by the presence of several dominant peaks (Rossiter modes) that are not in a harmonic relation but uniformly spaced at a distance equal to the fundamental frequency of the oscillation mechanism (aero-acoustic feedback loop). This feature is interpreted as the result of an amplitude modulation process and related to variations in the vortex–corner interaction in the downstream part of the cavity (γ modulation). A lower frequency modulation is identified through the secondary peak distribution. A detailed analysis of the spectral structure confirms the presence of a component at the corresponding frequency value (Δf mode). The assumption of a specific coupling between the two modulation processes is investigated. It leads to a new approximate form for the γ-modulation ratio that allows an explicit expression of the Rossiter constant γ related to aspect ratio of the cavity (L/D).