A study on axisymmetric cavities in supersonic flow

Abstract

The research presented in this dissertation focuses on understanding and passively controlling the unsteadiness of the flow that occurs in cavity flows. Flow passing over a cavity generates turbulent structures that result in pressure fluctuations in the cavity that exhibits itself as noise at selected tonal frequencies. The unsteadiness and the presence of the turbulent structures can be strong enough to damage the equipment that is stored in these cavities. Understanding the flow unsteadiness and determining the means to control it is an important research area that has been undertaken over the past several decades. High speed cavity flow presents several problems when used in practical applications. At lower speeds, vibrations can be controlled with ramps and/or flaps; however, there is a current need to provide a better understanding of supersonic flow around a cavity to allow future aircraft to release payload at high speed. Most of the previous research on cavity flows has been done on rectangular cavities and has provided guidelines for determining dominant frequencies and other flow properties over a cavity. This project focuses on understanding the flow unsteadiness in more of a basic configuration, namely over an axisymmetric cavity, to eliminate the effects of the side walls on the flow development. Such research is scarce in the literature, although it is a building block in understanding the nature of the cavity flows. The results of the current research agree well with the previous research conducted on rectangular cavities. The measured tonal frequencies match the predicted Rossiter modes that are well defined for rectangular cavities. The pressure data obtained at the cavity walls has shown a decrease in sound pressure levels at the various tonal frequencies when the cavity is lengthened. It is also shown that replacing the back wall of the cavity with a half-height wall results in both positive and negative effects to the tonal frequencies. At the front wall pressure fluctuations are reduced at all cavity lengths; however, the rear wall shows significant increase in pressure fluctuations for the medium and large cavity lengths.