Effect of finite cavity width on the self-sustained oscillation in a low-mach-number cavity flow

Cavity flow is known to be associated with high-level-noise generation, strong vibration and substantial increase in the drag force on the object containing the cavity. Most of the studies of the cavity flow are based on a two-dimensionality assumption. The three-dimensional features of cavity unsteadiness/oscillation are rarely investigated. The current study is focused on examining the effect of the cavity width and side walls on the self-sustained oscillation in a low-Mach-number cavity flow with a turbulent boundary layer at separation.;An axisymmetric cavity geometry is employed in the present research. The axisymmetric configuration provides a distinct advantage in studying the side-wall effects in that the configuration provides a reference condition that is free from any side-wall influence; yet, the cavity could be partially filled to form a finite-width geometry. Unsteady surface pressure, on the cavity floor along the streamwise direction and on the downstream wall along the azimuthal direction, is acquired using microphone arrays. The pressure data are recorded over a range of flow and geometrical parameters. In particular, the Reynolds number based on cavity depth and free-stream velocity is changed from Re = 4067 to 12200, and cavity length-to-depth ratio L/D from 2.6 to 4.1 for both the axisymmetric geometry and finite-width cavities with width-to-depth ratio W/D in the range 2.5 to 7.4. Based on the outcome of the analysis of the unsteady surface-pressure field, velocity measurements using a two-component Laser Doppler Anemometer (LDA) system are performed simultaneously with array measurements in different azimuthal planes for a cavity with L/D = 3.3 and W/D = 7.4 at Re = 12200 to explore the effect of the side wall on the mechanism driving the cavity oscillation. Evolution of coherent structures generating the pressure oscillation on the downstream wall of the cavity is evaluated using linear stochastic estimation (LSE) of the velocity field based on the wall-pressure signature.;The results show that while no prominent oscillation is found in axisymmetric cavities without side walls, strong harmonic pressure oscillation different from any of the known modes in the literature is observed in finite-width cavities at an azimuthal location of about one cavity depth away from the side wall. Analysis of the mean three-dimensional flow inside the cavity and the stochastic estimation results lead to the hypothesis that the flow structures in the symmetry plane of the finite-width cavities, although have a weak pressure signature, they interact with the flow near the side wall, providing the driving mechanism for the establishment of the oscillation near the side wall. Follow up experiments to validate this hypothesis are described.