Passive methods for suppressing acoustic resonance excitation in shallow rectangular cavities.

Abstract

The flow-excited acoustic resonance in shallow rectangular cavities can be a source of severe noise and/or excessive vibration. This phenomenon is excited when one of the acoustic modes in the accommodating enclosure is coupled with the flow instabilities resulting from the shear layer formation at the cavity mouth. In this thesis, two passive methods for suppressing the flow-excited acoustic resonance phenomenon are addressed. The first passive method considers the edge geometry effect on the phenomenon. Several edge geometries including chamfered, round, and different configurations of spoilers are considered. The effect of the spoilers dimensions is investigated to provide criteria that help designing and optimizing spoilers. Some of the spoilers are found to be effective in suppressing the acoustic resonance excitation, while some other edges including chamfered and round edges result in shifting the resonance excitations to higher velocities with amplification in the acoustic pressure. To enrich the understanding of the suppression mechanism introduced by these passive methods, hotwire measurements are performed revealing the existence of orthogonal vortices interacting with the shear layer at the cavity mouth. The second passive method investigated is the effect of placing a high frequency vortex generator (control cylinder) in vicinity of the upstream edge of the cavity on the acoustic resonance excitation. The method is investigated experimentally and numerically. The effectiveness of the control cylinder method is studied by investigating different cylinder diameters and locations on both horizontal and vertical directions. It is found that locating the cylinder at relatively small height from the bottom wall and with a distance of 25.4 mm upstream the leading edge can significantly suppress the resonance excitation. To further understand the interaction between the cylinder vortex shedding and the shear layer at the cavity mouth and the influence on the shear layer thickness, a 2D numerical simulation using K-epsilon and Detached Eddy Simulation (DES) models has been carried out and compared to the experimental results. For both passive methods, the study included two cavities with different aspect ratios (L/D=1.0 and L/D=1.67, L: cavity length, D: cavity depth) to address the effectiveness of the methods with respect to the cavity depth. The methods are investigated in flow with Mach number up to 0.45. All different configurations investigated are compared to the base case which is the bare cavity with sharp edges installed upstream and downstream.