Shear-layer characteristics across a cavity with a horizontal top plate

The influence of a horizontal top plate on self-sustaining oscillation over a rectangular cavity is investigated experimentally. From both the mean and the spectral analysis of the cavity shear flow, the presence of the top plate does provide a powerful external perturbation to modify the oscillatory characteristics of the cavity shear layer. While the leading-edge of the top plate is located right above the most sensitive region (or the point of flow separation) of the cavity shear layer, the self-sustaining oscillation of the shear layer is promoted most effectively. However, the effectiveness decreases as the leading-edge of the top plate moves downstream away from the most sensitive region. Insertion of the horizontal top plate not only produces a favorable streamwise pressure gradient across the cavity mouth, it also creates a stronger feedback effect from the downstream edge of the cavity. Significant flow acceleration near the most sensitive region of the cavity shear-layer reduces the local momentum thickness and enhances the receptivity of the separated shear layer to the feedback effect. Therefore, stronger feedback from the downstream edge of the cavity and high sensitivity (or receptivity) to the feedback at the most sensitive region of the cavity shear layer are the key mechanisms to promote the oscillation of the shear-layer across the cavity mouth while the horizontal top plate is inserted. (C) 1998 The Japan Society of Fluid Mechanics Incorporated and Elsevier Science B.V. All rights reserved.