Self-excitation of acoustic resonance in axisymmetric cavities can lead to a complex flow–acoustic coupling, which may result in severe noise generation. In this work, a large eddy simulation is performed to model the different flow–sound coupling mechanisms during the self-excitation of various excitable acoustic modes in an axisymmetric shallow cavity configuration with an aspect ratio of L/d = 1 over the lock-in region. The compressible Navier–Stokes equations are solved at a resolution sufficient to capture the flow and the acoustic dynamics. The excitation of three acoustic modes of different aerodynamic characteristics over the range of the tested flow velocities was observed. These modes are a stationary diametral mode, a spinning diametral mode, and a longitudinal mode. The initiation and separation of vortices over the cavity mouth accompanying the self-excitation of each mode involve different dynamics. If two antisymmetric series of vortical crescents separate successively at the leading edge, a stationary acoustic mode is excited. The formation of a continuously rotating helical vortex, connecting the leading edge and the trailing edge, leads to the excitation of the diametral spinning mode. The excitation of the longitudinal mode is associated with symmetric rings of vortices. Complex patterns of flow velocities and Reynolds stresses in the circumferential direction are observed for the diametral modes but not for the longitudinal mode. In all cases, the excitation of acoustic resonance requires a synchronization of the vortex separation and impingement processes, which is necessary for efficient feedback to sustain the flow–sound coupling mechanism.
The excitation of acoustic resonance by flow over a rectangular cavity can generate acute noise, cause damage to equipment, and interrupt operation. In this work, a passive control technique to suppress the excitation of acoustic resonance by the flow over rectangular cavities is experimentally investigated. A span-wise rod that generates high-frequency vortices is mounted upstream of the cavity leading edge to prevent the flapping of the shear layer. The effect of the rod parameters on the mechanism of acoustic resonance suppression is identified by means of acoustic pressure and particle image velocimetry (PIV) measurements. It is found that the effectiveness of this control technique is significantly dependent on the streamwise location of the rod with respect to the cavity leading edge, the gap between the rod and the wind tunnel wall, and the cavity aspect ratio. In addition, PIV measurements revealed that, in effective rod configurations, the vortices generated in the gap between the control rod and the wall alter the development of the shear layer. Moreover, analysis of the Reynolds stresses showed that fluctuations in the wake of the rod prevent the shear layer from impinging on the cavity downstream edge. Consequently, this interaction interrupts the initiation of the feedback mechanism responsible for the onset of acoustic resonance excitation. Finally, a universal criterion is developed to predict an optimum region to implement the control rod upstream of the cavity leading edge to effectively suppress the acoustic resonance excitation.
Flow over rectangular cavities can become unstable and excite the acoustic modes of the surrounding duct, resulting in severe noise and vibration. In this work, acoustic resonance excitation by two opposite and aerodynamically isolated rectangular cavities is experimentally and numerically investigated to identify the effect of the flow-acoustic coupling on the synchronization of shear layer instabilities. Compressible unsteady Reynolds-averaged Navier–Stokes simulation is used to model the self-excitation of resonance and characterize the fully coupled flow and acoustic fields. Moreover, the location and the strength of the acoustic sources and sinks are evaluated using Howe's integral formulation of the aerodynamic sound. It is revealed that double symmetric cavities generate a higher rate of acoustic energy transfer due to the synchronization of the shear layer instabilities over the two cavities in an antisymmetric pattern, leading to a stronger acoustic resonance than all other cases. On the other hand, the two shear layers over two opposite cavities with different aspect ratios were mismatched in phase and vortex convection velocity. As a result, the net energy transfer in an asymmetric cavity configuration occurred at a similar rate to a single rectangular cavity, driving a weaker acoustic resonance excitation.
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