Deep cavity noise reduction by exploiting the strategically embedded aeroacoustic-structural interaction of elastic panels

Abstract

The phenomenon of flow over a rectangular cavity has garnered significant research interest due to its prevalence in diverse engineering applications across the low and high-speed environments. Under specific operating conditions, unsteady flow over a cavity can initiate self-sustained oscillations that couple with acoustic modes within the cavity, leading to pronounced extreme noise response owing to these flow-induced cavity oscillations which can precipitate premature mechanical failures upon longtime exposure. Existing noise reduction strategies, including cavity shape modifications and the use of plasma actuators or leading-edge blow systems, are flow-invasive and often introduce significant disturbances, altering the cavity fundamental flow characteristics. This can result in unintended aerodynamic consequences such as increased turbulence, flow-induced drag, and higher actuation energy requirements, alongside potentially inducing extraneous noise in frequency ranges absent in the original flow. Such implications have not been comprehensively addressed in the existing literature. This study explores a new passive control method for reducing cavity tonal noise using flow-induced surface vibrations, employing an elastic panel mounted across the cavity walls. The primary objective is to decrease tonal noise while keeping the shear layer over the cavity opening largely unchanged, minimizing any negative impact on cavity aerodynamic performance. The application of an elastic panel is envisaged to invoke aeroacoustic-structural interaction, which could alter the phase and intensity of acoustic waves inside the cavity, inherently modify the aeroacoustic coupling receptivity pattern at the cavity leading edge and ultimately reduce the cavity noise emission. In the first part of the study, the concept of utilizing localized surface compliance is tested to suppress deep cavity aeroacoustics at low Mach number with a single elastic panel embedded across the cavity walls. The concept is studied using high-fidelity, two-dimensional Direct Aeroacoustic Simulation at a freestream Mach number of 0.09 and a Reynolds number, based on the cavity length, of 4 x 104. The investigation of the baseline rigid cavity (without panel) case deciphers that the aeroacoustic feedback process in deep cavities consists of five distinct processes, each supported by the corresponding cavity walls. Having confirmed the key aeroacoustic processes in the numerical solution through careful validation and investigation, localized surface compliance in the form of an elastic panel is strategically introduced to target each aeroacoustic constituent process at five different cavity walls. The natural frequency of the panel is set equal to the feedback loop characteristic frequency to facilitate its flow-induced structural resonance for energy absorption. Suppression of cavity noise pressure and power levels by 3.8 dB and 4.8 dB, respectively, is successfully achieved with an aft wall-mounted panel case, together with an unforeseen reduction in cavity drag by almost 19%. To corroborate the numerical findings and the potential of aeroacoustic suppression induced by the elastic panel, an experiment was conducted in a closed-circuit type open-jet anechoic wind tunnel. The experimental study observed a significant decrease in pressure fluctuations across the cavity base, shear layer, and far-field region with the application of the elastic panel. Additionally, the peak frequency shifted, suggesting a considerable alteration in the shear layer-cavity mode coupling phenomenon in the elastic panel case. Compared to the baseline rigid cavity, the cavity-panel configuration demonstrated a promising reduction in tonal noise by 16.1 dB. The phase pattern across the shear layer and cavity base was also modified in the case of the elastic panel, resulting in reduced noise radiation due to the changed interaction. Finally, we leverage further noise reduction potential of the cavity-panel configuration by employing a localized surface compliance mechanism realized through an arrangement of strategically designed multiple elastic panels. Each panel in this arrangement is tailored to target a certain constituent process of the deep cavity aeroacoustic mechanism. With the synergistic action of its flow-induced panel vibration, every panel is expected to maximize noise reduction potential. The underlying principle of the proposed approach is to harness flow-induced panel resonant vibrations, which are set to absorb the incident flow energy to alter or decouple the aeroacoustic feedback mechanisms driving the fluid-resonant oscillations with the combined action of strategically placed panels across the cavity walls. The most effective configuration gives a remarkable noise power reduction of 15 dB from the rigid cavity, inadvertently reducing cavity drag by almost 15%. Simultaneous reduction of both cavity noise and drag is doubly advantageous. In the most effective tested multi-panel configuration, the vertical panel acts to curtail the efficacy of coupling between the growing shear layer and cavity acoustic modes, whose sustenance is further impeded by an acoustically induced resonant panel at the cavity bottom. The proposed methodology is confirmed to be feasible yet effective, holding great potential for fluid-moving applications in which a quiet and energy-efficient cavity configuration is desired.