Suppression of deep cavity aeroacoustics at low Mach number by localized surface compliance

Abstract

A unique concept of utilizing localized surface compliance is proposed to suppress deep cavity aeroacoustics at a low Mach number. The core idea is to provide local absorption of the energy of aeroacoustic processes supporting cavity flow self-sustained feedback loop responsible for tonal noise generation. The concept is studied with a flow past cavity of length-to-depth ratio of 0.4 at freestream Mach number 0.09 and Reynolds number based on cavity length 4 × 104 using high-fidelity, two-dimensional direct aeroacoustic simulation. Having confirmed the replication of key aeroacoustic processes in the numerical solution through careful validation, localized surface compliance in the form of an elastic panel is strategically introduced to modify every process for cavity noise suppression. The panel natural frequency 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 and 4.8 dB, respectively, is successfully achieved, together with an unforeseen cavity drag reduction by almost 19%. Comprehensive wavenumber-frequency analyses of the coupled aeroacoustics and flow-induced panel vibration are conducted to uncover the physical mechanism of noise suppression. The results show that the same type of aeroacoustic feedback loop occurs, but its efficacy is significantly reduced due to the exhaustion of aeroacoustic process energy to the flow-induced vibrating panel. The proposed concept is confirmed to be feasible in terms of giving remarkable cavity noise and drag suppression, yet it retains the basic problem geometry intact, which are considered important in many practical applications.