Control of transonic cavity flow instability by streamwise air injection

A time-dependent numerical model of a turbulent Mach 1.5 flow over a rectangular cavity has been developed, to investigate suppression strategies for its natural self-sustained instability. This instability adversely affects the cavity form drag, it produces large-amplitude pressure oscillations in the enclosure and it is a source of far-field acoustic radiation. To suppress the natural flow instability, the leading edge of the two-dimensional cavity model is fitted with a simulated air jet that discharges in the downstream direction. The jet mass flow rate and nozzle depth are adjusted to attenuate the instability while minimising the control mass flow rate. The numerical predictions indicate that, at the selected inflow conditions, the configurations with the deepest nozzle (0.75 of the cavity depth) give the most attenuation of the modelled instability, which is dominated by the cavity second mode. The jet affects both the unsteady pressure field and the vorticity distribution inside the enclosure, which are, together, key determinants of the cavity leading instability mode amplitude. The unsteadiness of the pressure field is reduced by the lifting of the cavity shear layer at the rear end above the trailing edge. This disrupts the formation of upstream travelling feed-back pressure waves and the generation of far-field noise. The deep nozzle also promotes a downstream bulk flow in the enclosure, running from the upstream vertical wall to the downstream one. This flow attenuates the large-scale clockwise recirculation that is present in the unsuppressed cavity flow. The same flow alters the top shear layer vorticity thickness and probably affects the convective growth of the shear layer cavity second mode.