A time-resolved particle image velocimetry investigation of a cavity flow with a thick incoming turbulent boundary layer

The present paper aims for a better understanding of the flow in a rectangular cavity at very low Mach number and with a thick incoming turbulent boundary layer. In the present experiment the boundary layer is defined as thick because the ratio of the cavity length to the momentum thickness Θ is low (L/Θ = 18) with respect to the claimed threshold (L/Θ>80) for cavity self-sustained oscillations to take place. Time-resolved particle image velocimetry (PIV) measurements were taken in planes normal and parallel to the wall over a cavity having aspect ratio L/H = 4, at ReΘ = 1041. The PIV data were analyzed to give the mean characteristics and the time behavior of the flow. The irregular vortex shedding observed in the shear layer was analyzed and interpreted with reference to the coherent structures in the incoming turbulent boundary layer. The unsteady load on the cavity walls and the consequent oscillating drag forces were studied by analyzing the instant momentum flux across the cavity mouth. A low frequency mode of about 2 Hz, corresponding to a Strouhal number based on the cavity length of StL = 0.2, was revealed. This oscillation mode is due to the build up of the flow inside the cavity and not to the vortex shedding in the shear layer.