Global response of cavity-flow shear layer to low and high amplitude acoustic forcing*

Aeroacoustic instabilities in deep cavities subject to grazing flows are governed by the nonlinear response of the shear layer to transverse acoustic velocity forcing. We characterise experimentally the shear layer dynamics with and without acoustic forcing using time-resolved particle image velocimetry (PIV) and acoustic measurements. Considering the Navier-Stokes equations linarized around the mean flow from the PIV, we predict the harmonic response of the flow for a range of acoustic forcing amplitudes and frequencies.  A global stability analysis is also carried out, showing that the modes, which are marginally stable, define the hydrodynamic feedback responsible for the aeroacoustic instabilities. These analysis are performed for low Mach and highly turbulent conditions, for different cavity opening geometries, with sharp and round corners. These results allow us to explain the self-sustained aeroacoustic oscillations for a range of grazing flow velocities and predict the associated limit cycle amplitudes.