Wind tunnel experiments on the flow over rectangular cavities at subsonic and transonic speeds

Measurements have been made of the time average and unsteady pressures acting on the roof and behind a series of rectangular cavities set in the roof of the 2 ft x 1tr ft transonic tunnel. It was found that the unsteady pressures contain both random and periodic components. The random component predominates in the shallower cavities (length/depth ratio> 4) and is most intense near the rear wall. The periodioc component predominates in the deeper cavities (length/depth ratio < 4) and may form standing wave patterns. It is suggested that the periodic component is due to an acoustic resonance within the cavity excited by a phenomenon similar to th at causing edge-tones. It may be suppressed by fixing a small spoiler at the front of the cavity.