| The unsteady pressure and optical properties of subsonic flow over open and partially covered rectangular cavities were investigated through a series of wind-tunnel experiments. When the cavity was open, the well known "Rossiter" modes were excited, but when the cavity was partially covered, peak frequencies arose which were lower than the Rossiter modes for a cavity with the same opening length. A new "long-path" frequency prediction formula was developed in order to estimate these lower frequencies in a manner similar to the Rossiter formula. When one of these new "long-path" mode frequencies coincided with the natural frequency of the cavity, the shear-layer motions were dominated by a resonant frequency. Phase-averaged schlieren images were used to examine the suitability of these resonant shear-layer motions for aero-optic applications. Additionally, the acquisition of time-resolved schlieren images of the shear layer allowed the development of a novel form of schlieren velocimetry, which revealed that coherent optical structures in the shear layer traverse the length of the cavity at speeds lower than those presupposed by the Rossiter or long-path formulations. |
