Influence of Edge Geometry on the Noise Generated by a Deep Cavity With a Turbulent Grazing Flow

An exhaustive analysis of the modifications on the cavity flow dynamics due to a change of either cavity size and edge geometry is proposed in this article. In a previous study, a strong tonal noise has been found for a cavity flow configuration with sharp edges. A deep cavity (D/L = 3 depth-to-length ratio of 3) was submitted to a high subsonic flow at a Mach number M = 0.85 with a large Reynolds number based on the momentum thickness of the boundary layer, Re = 1.23 × 10^5. The present study will use this validated flow configuration and change the cavity size and/or the upstream edge geometry. The purpose of the project is based on the investigation of a real-life configuration: the tonal noise of a cavity located around the door of an aircraft. First, using experimental data, the influence of the Mach number and of the upstream edge geometry are studied, plotting tonal acoustic resonances maps to predict the mechanisms affecting the cavity tonal noise emissions. Then, large-eddy simulations (LES) are performed for a new cavity size of D/L = 6 and with two upstream corner geometries: a sharp one and a rounded one. For all cases, fluctuating pressure spectra are found to be in reasonable agreement with experimental measurements. It was observed that the sharp edge configurations (D/L = 3 and D/L = 6) both exhibit a strong tonal noise radiation, and that the round edge one (D/L = 6) presents a higher broadband noise level and almost no tonal resonance. This change of the broadband noise level seems partly due to the impact of the shear layer, that is deviated with the curve of the round edge, on the downstream edge, and partly due to the round corner that increases the hydrodynamic instabilities in the shear layer. Interestingly, for the cavity size D/L= 6, the use of a round upstream edge also changed the dominant mode from a five-quarter-wave mode to a three-quarter wave mode. Finally, using the former configuration D/L = 3, which has a strong tonal noise with a coupling between hydrodynamics instabilities and acoustic waves (both those traveling in the cavity and those traveling upstream), we proved that the configurations simulated here, with D/L = 6, did not presented any vortex-noise coupling. Indeed, both edge geometries showed a lack of vortex periodicity in the shear layer, thus the emerging quarter-wave modes are not triggered by a periodic vorticity but by the turbulent boundary layer that act as a broadband noise source and trigger acoustic modes of the cavity, without any feedback.