Spatial-temporal stability analysis of confined compressible shear layers of a vortex-sheet type

In many open-jet aeroacoustic wind tunnels, large-amplitude and low-frequency vibration and pressure fluctuations can occur at certain jet velocities. To investigate the role played by the surrounding chamber, a spatial-temporal stability analysis is performed on confined compressible shear flows based on vortex-sheet assumptions. Results show that at low Mach number (less than 0.3), the effects of confinement on compressible shear flows are close to those of incompressible flows, i.e. confinement destabilizes the flow and causes the transition from convective to absolute instability to occurs at lower shear values, but the increase of Mach number stabilizes the jet while destabilizes the wakes. At high Mach numbers (such as M = 1), the effect of confinement on jets are significantly different from those at low Mach numbers. The stability of jets at high Mach numbers hardly changes with confinement when the flow is strongly confined; however, for weakly confined jets, its stability first increases and then decreases when confinement decreases. Confinement on wakes at high Mach numbers tends to destabilizes the flow.