Book Description

Abstract: Grazing flow over a shallow cavity is characterized by a shear layer instability feedback loop that induces strong pressure fluctuations at discrete frequencies. Examples include flow over aircraft weapons bays, aircraft landing gear openings and transmission or reception optical ports on aircraft. In the first application, this behavior can result in decreased accuracy of weapons delivery, fatigue failures and lowered efficiencies. Because of the variety of unwanted effects inherent to cavity flows, their control is of interest to both military and commercial aircraft applications. The complex nature of cavity flow has been explored since the mid-20th century with accelerated study over the past 25 years in hopes of understanding and controlling major pressure fluctuations within the cavity. A key to developing a successful control scheme for this fluidic system is a detailed understanding of the physics that govern the flow-acoustic coupling mechanisms. This work provides both qualitative and quantitative measurements for use in characterizing the fluid and acoustic physics that govern the resonance phenomenon. Focus is placed on the subsonic regime with an emphasis on Mach 0.30 flow and its control. Results include the analysis of surface pressure spectra and spatial correlations, flow visualization and velocity measurement using particle image velocimetry (PIV) in various controlled and baseline (unforced) cases. These results show that a well chosen actuation scheme at or near the receptivity region of the cavity shear layer can interrupt or weaken the acoustic feedback loop and significantly reduce the sound pressure level in the cavity. Physically, this amounts to modifying the development and evolution of the coherent structures spanning the cavity and thereby changing the frequency at which these vortices impact the trailing edge of the geometry. In addition, simultaneous dynamic surface pressure and planar velocity measurements are used in low dimensional modeling for closed-loop controller design purposes. Modeling using this method allows design of accurate models from experimental results rather than relying on numerical data, which is incapable of resolving the complex nonlinear production of acoustics in the low subsonic cavity flows at this time.