An experimental investigation of flow-induced cavity oscillations
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Eric F. Spina
Acoustic near field, Shear layer, Flow-induced, Cavity, Oscillations
Aerospace Engineering | Engineering | Mechanical Engineering | Structures and Materials
The flow over a cavity is characterized by a complex feedback process that leads to large-amplitude, self-sustaining oscillations of the pressure, velocity, and density in and around the cavity. These oscillations are undesirable in a number of engineering applications ( e.g ., aircraft landing gear and weapons bays) as they can induce structural fatigue and vibration, noise radiation, and drag on bodies possessing the cavity. In light of the strong current interest in active control of cavity oscillations, research focused on improving the understanding and modeling of the cavity flow physics is critical and timely.
Research on cavity flowfields during the past 40 years has concentrated primarily on the acoustic environment in and around the cavity. Few investigations have been focused specifically at the cavity shear-layer physics and the mechanisms for noise generation at the cavity trailing edge. Flow-field data are particularly scarce at moderate subsonic Mach numbers. Therefore, detailed measurements of the cavity shear layer, the internal region of the cavity, and the acoustic near field were performed in this dissertation. Cavity length-to-depth ratios of 2 and 4 were considered in these surveys at freestream Mach numbers of 0.25 and 0.4, and 0.6, respectively. Two experimental techniques were used in this investigation: a quantitative schlieren technique known as "optical deflectometry" and hot-wire anemometry (normal and cross wires). The amplitudes and phases of the modal disturbances in the cavity flow field were studied using these high frequency response, high spatial resolution data. The modal disturbances (amplitude and phase components) were extracted from the unsteady flow-field signals using the pressure signal acquired from a transducer embedded in the cavity rear wall and cross-spectral analysis.
A secondary objective in this dissertation was to apply joint time-frequency methods and schlieren visualization of the instantaneous cavity shear-layer structure to determine whether "mode switching" occurs between the multiple cavity modes.
The detailed flow-field measurements have increased the current understanding of the cavity flow-field physics that cause and maintain the self-sustaining oscillations. Disturbance measurements in the shear layer and internal region of the shallow cavities considered in this study indicate that the wave field is comprised of an upstream-traveling acoustic wave and a downstream-traveling instability wave, both at the same frequency. Disturbance-field measurements in the trailing-edge region of the cavity have provided insight to the nature of the shear-layer/corner interaction. This interaction gives rise to a sound source that provides the feedback necessary to maintain the cavity oscillations. These measurements are expected to aid in the development of theories concerning the sound source and future design of control systems for the suppression of self-sustaining oscillations.
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Kegerise, Michael Aaron, "An experimental investigation of flow-induced cavity oscillations" (1999). Mechanical and Aerospace Engineering - Dissertations. Paper 31.