Influence of flow parameters on cavity-tone control using a rear-wall protrusion

This study investigated the effects of flow parameters on cavity-tone suppression using a rear-wall protrusion, which is a passive control device previously optimized for low-Mach-number conditions. Three key nondimensional parameters were systematically varied to clarify the robustness and limitations of the protrusion’s control performance: the cavity length to boundary layer momentum thickness ratio (L/θ), Reynolds number (Re), and cavity depth to length ratio (D/L). High-fidelity three-dimensional direct aeroacoustic simulations based on the lattice Boltzmann method were conducted over a wide range of flow regimes. The results showed that the protrusion effectively suppresses shear-layer oscillations and reduces tonal radiation when L/θ≤100, Re≤2.0×104, or D/L>0.5. Under these conditions, the device inhibits the development of internal recirculation, stabilizes the shear layer, and significantly weakens the coherent vortex impingement on the trailing edge, yielding up to 18.6 dB reduction in the overall sound pressure level. In contrast, thin shear layers (large L/θ) and strongly three-dimensional flows at high Re or low D/L destabilize the shear layer and promote interactions with the protrusion, generating residual tonal components that limit the achievable noise reduction. These findings provide practical design guidance for the application of rear-wall protrusions in engineering systems that require reliable and efficient cavity-tone suppression.