Shear layer dynamics of a cylindrical cavity for different acoustic resonance modes

This paper investigates the interaction between the shear layer over a circular cavity and the flowexcited acoustic response of the volume to shear layer instability modes. Within the fluid-resonant category of cavity oscillation, most research has been carried out on rectangular geometries and where cylinders are considered, side branch and Helmholtz oscillators are most common. In these studies, focus is generally restricted to either longitudinal standing waves or to Helmholtz resonance. In practical situations however, where the cavity is subject to a range of flow speeds, many different resonant mode types may be excited. The current work presents a cylindrical cavity design where Helmholtz oscillation, longitudinal resonance and also azimuthal acoustic modes may all be excited upon varying the flow speed. Experiments performed show how lock-on between each of the three fluid-resonances and shear-layer instability modes can been generated. A circumferential array of microphones flush mounted with the internal surface of the cavity wall was used to decompose the acoustic pressure field into acoustic modes and has verified the excitation of higher order azimuthal modes by the shear layer. One of these interior pressure signals was also used to provide a phase signal for averaging flow field measurements of the shear layer acquired using PIV. Observation of the PIV images provides insight into these acoustically coupled oscillations.