An Experimental Investigation of Cavity Flow

Understanding and controlling the flowfield in and around a cavity has challenged researchers for a number of years. The presence of a cavity on a body has been observed to significantly increase the mean drag, cause flow-induced vibrations and give rise to strong acoustic production. Of particular interest here are the flow structure and dynamics associated with open shallow rectangular cavities at low Mach numbers for various length-to-depth ratios. At the Reynolds number investigated, it is the presence of convective instabilities through the process of feedback disturbance that gives rise to a globally unstable flowfield. Using an instrumented wing model with a cut-out an experimental investigation of a cavity flowfield exhibiting `fluid-dynamic' phenomenon has been completed. A postprocessing module for the PIV image data was constructed which optimised the data fidelity and accuracy while improving upon velocity spatial resolution. These improvements were necessary to capture the flow scales of interest and minimise the measurement error for the presentation of velocity, velocity-derivative and turbulent statistics. It is shown that the hydrodynamic instability that is intrinsic to the cavity flowfield at these inflow conditions organises the oscillation of smalland large-scale vortical structures. The impingent scenario at the downstream edge is seen to be crucially important to the cavity oscillation and during the mass addition phase a jet-edge is seen to form over the rear bulkhead and floor. In some instances this jet-like flow is observed to traverse the total internal perimeter of the cavity and interact with the shear layer at the leading edge of the cavity, this disturbs the normal growth of the shear layer and instigates an increase in fluctuation. The coexistence and interplay between a lower frequency mode dominant within the cavity zone and the shear layer mode is seen to shed large-scale eddies from the cavity. This modulation imposes a modification to the feedback signal strength such that two distinct states of the shear layer are noted. Concepts for the passive reduction of internal cavity fluctuation are successful although modifications to the shear layer unsteadiness are encountered; an increase in drag is implied.