Suppression of Acoustic Resonance in Rectangular Cavities Using Spanwise Control Cylinder

Flow over cavities can be a significant source of noise in many engineering applications when a coupling occurs between the flow instabilities at the cavity mouth and one of the acoustic cross-modes in the accommodating enclosure. In this paper, a passive noise control technique using a spanwise cylinder located at the cavity upstream edge is investigated experimentally for two different cavities with aspect ratios of L/D = 1.0 and 1.67, where L is the cavity length and D is the cavity depth. The effect of both the location of the cylinder and its diameter on the flow-excited acoustic resonance is investigated in air flow with Mach number up to 0.45. This passive control technique is found to be effective in suppressing the acoustic resonance excitation when compared to the base case where no cylinder is attached. It is observed that using the optimum cylinder location and diameter reduces the acoustic pressure to less than 140 Pa, compared to the base case with values exceeding 2000 Pa. Moreover, a shift in the onset of acoustic resonance to higher velocities is observed. Localized hot-wire measurements of the free shear layer at the cavity mouth during the off-resonance conditions reveal that attaching a spanwise cylinder at the cavity upstream edge reduces the spanwise correlation of the free shear layer which, in turns, reduces its susceptibility to acoustic excitation. To further understand the interaction between the cylinder’s vortex shedding and the free shear layer at the cavity mouth, a numerical simulation of the flow field using a detached eddy simulation (DES) model has been carried out. The simulation shows that the suppression occurs due to a disturbance of the cavity shear layer by the vortex shedding from the cylinder which results in altering the impingement point at the downstream edge of the cavity, and thereby weakening the feedback cycle that controls the acoustic resonance excitation.