Mixing Effects Between Self-Sustained and Unstable Hydrodynamic Oscillations Near Injecting Walls

Abstract

Self-sustained oscillations in a shear layer were studied in a confined chamber with injecting walls. Experiments were carried out over a wide range of Reynolds numbers (0.86 x 10 5 ≤ Re ≤ 1.94 x 10 5 ) and of Mach numbers (0.05 ≤ M ≤ 0.227) in order to underline the effect of dynamic conditions on the pressure amplifications and on the excited acoustic modes. The presence of an obstacle generated a highly sheared flow in which vortex-shedding frequency was set by an acoustic-hydrodynamic coupling. The Mach number controlled the selectivity process of the vortex-shedding frequency, but turbulence dissipation weakened this coupling by a loss of structure coherence. As this did not affect the pressure fluctuation level, a second noise source was detected; vortices near the injecting wall, resulting from a mixing effect between shear-layer oscillations and injection flow through the porous wall, were identified