Experimental Investigation of Flow-Acoustic Coupling in a Deep Axisymmetric Cavity

In this paper, the phenomenon of self-sustained pressure oscillations due to the flow past a deep, circular, axisymmetric cavity is investigated. In many engineering applications, such as flows through open gate valves, there exists potential for coupling between the vortex shedding from the upstream edge of the cavity and a diametral mode of the acoustic pressure fluctuations. In the present study, the unsteady pressure was measured at several azimuthal locations at the bottom of the cavity walls, and the associated acoustic mode shapes were calculated numerically for the four representative cases of the internal cavity geometry, which involved a reference case with sharp, 90°edges as well as several modifications that involved chamfers of various length of the upstream and the downstream edges of the cavity. In addition, the flow velocity in the vicinity of the cavity opening in selected cases was measured using digital particle image velocimetry (PIV). The optical access to the highly confined internal flow was provided by implementing an endoscope attached to the camera. This global, quantitative imaging approach yielded patterns of velocity, streamlines and out-of-plane vorticity component. Instantaneous and time-averaged flow patterns provided insight into the mechanism of the flow tone generation. Among the considered cavity geometries, the configuration that corresponded to the most efficient noise suppression was identified.