Aeroacoustic Source Distribution Around Four Cylinders Orientated in a Square

An experimental investigation of the flow-acoustic coupling of four cylinders arranged in a square configuration with a spacing ratio in the proximity interference range subject to forced acoustic resonance is presented. The aeroacoustic characteristics and the flow field structures are investigated at various sound pressure levels to study its influence on the “lock-in” behaviour of the separated flow and the corresponding distribution of the resonant acoustic sources. Two mainstream flow velocities were selected for testing that corresponded to flow field conditions before acoustic-Strouhal coincidence of the vortex shedding frequency with the natural acoustic frequency of the duct and to flow field conditions after acoustic-Strouhal coincidence. Increasing the sound pressure level was found to slightly increase the range of flow velocities with which the acoustics could entrain the vortex shedding regime. Increasing the sound pressure level was also found to shorten the length of the most intense vortical structures in the shear layers emanating from the upstream cylinders and hence also shifted the dominant acoustic sources upstream. Spatial distributions of the net acoustic energy suggests that the mechanism triggering acoustic resonance of the four cylinders is shear layer instability, which is similar to that observed for two tandem cylinders.