Water-Table Visualization of Flow-Induced Pressure Oscillations in Shallow Cavities for Simulated Supersonic Flow Conditions

Abstract

Intense periodic pressure fluctuations can occur in the fluid in and above a cavity that is exposed to high-speed external airflow. Water table visualization provides an excellent tool to study the flow pattern in and near a cavity because all nonstationary phenomena are slowed by a factor of several hundred. Direct observation by eye or high-speed movie camera reveals for an oscillating cavity (1) the periodic deflection of the shear layer above the cavity, (2) the cavity internal upstream propagation of an “acoustic” wave originating at the trailing edge bulkhead, (3) the downstream propagation of a pressure wave in the cavity originating at the leading edge bulkhead, and (4) the propagation of a “shock wave” pattern away from the cavity into the surrounding fluid. The effect of geometric and fluid dynamic changes on these non-stationary phenomena is most easily visualized. In particular, oscillation-suppression techniques can be readily implemented and studied, before elaborate wind-tunnel and flight tests are conducted to substantiate findings and evaluate the feasibility and effectiveness of certain devices. A motion picture of water table visualized pressure oscillations in shallow cavities for simulated supersonic flow conditions shown during the presentation, illustrates the effects of the following geometric changes: length-to-depth ratio, curvature of leading-edge and trailing-edge bulkheads, upstream and downstream spoilers, attached and detached upstream and downstream cowls, and upstream and downstream bulkhead impedance and floor impedance. [This work has been supported in part by the Air Force Flight Dynamics Laboratory, Wright-Patterson AFB, Ohio 45433.]