Time Resolved Measurements of the Pressure Field Generated by Vortex-Corner Interactions in a Cavity Shear Layer

Abstract

A 2D open cavity shear layer flow, especially its interaction with the trailing corner of the cavity, is investigated experimentally in a water tunnel, at a Reynolds number of 4.0×104, based on cavity length. Time-resolved PIV, at an image sampling rate of 4500 fps is used to simultaneously measure the instantaneous velocity, material acceleration and pressure distribution. The pressure is obtained by spatially integrating the material acceleration (Liu and Katz [19]). A large database of instantaneous realizations enables detailed visualization of the dynamic changes to the shear layer vortices, including convection, deformation and breakup as they impinge on the cavity trailing corner, as well as their interactions with the freshly generated boundary layer around the corner walls. The vorticity on top of the corner is originated from advected shear layer vortices and locally generated vorticity associated with the local pressure gradients. The resulting periodic recirculating flow above the corner generates the lowest mean pressure in the entire flow field. Two mechanisms with distinct characteristic frequencies affect the periodic variations in vorticity and pressure around the corner. The first is caused by streamwise transport of the large shear layer vortices. For example, when these vortices are located just upstream of the corner, they induce a downwash, which periodically eliminates the recirculating flow there. This recirculation and the associated low pressure reappear after the vortex climbs over the corner. The second mechanism involves low frequency undulations of the entire shear layer, which affect its interaction with the corner, and causes substantial variations in the pressure maximum along the vertical wall of the corner, and pressure minimum above it. These undulations are self sustained since the corner pressure fluctuations alter the recirculation speed in the cavity, which in turn change the shear layer elevation during initial rollup, and even modify the boundary layer upstream of the cavity. The time resolved pressure distribution can also be used for estimating the dipole noise radiated from the corner. The characteristic frequencies of the hydraulic and acoustic fields can be traced back to specific flow phenomena. Analysis shows that the unsteady surface pressure along the vertical wall of the trailing corner is a major dominant source of low frequency noise.