Effect of surface mounting of upper plate on oscillating flow structure within cavity

Abstract

This study investigates the effect of a surface mounting located on the upper plate on the oscillating flow structure within the cavity. All the experiments were performed in a recirculating water channel. The Reynolds numbers, based on the uniform velocity and the momentum thickness at the upstream edge of the cavity, vary from 45 to 1200. It is found that the surface mounting on the upper plate creates a streamwisely elongated and transversely expanded recirculating region underneath the upper plate. This enlarged recirculation region further deflects the shear layer into the cavity and leads to stronger feedback on the shear layer itself. In presence of a surface mounting on the upper plate, significant flow acceleration between the upper plate and the cavity is found to be primarily responsible for reducing the momentum thickness at the upstream edge of the cavity. In the present study, the mechanisms responsible for the enlarged oscillating amplitude and mode shift are three folds. First, the separated shear layer at the upstream edge of the cavity is triggered by high velocity fluctuation of the enlarged recirculating region underneath the upper plate. Second, stronger feedback provides synchronous condition on the initial separated shear layer and contributes to the enhancement of oscillating amplitude across the cavity. Third, depending upon the relative location of surface mounting and the upstream edge of cavity, the oscillating amplitudes are enhanced to different extents and the shift of oscillating mode occurs at different inflow speed.