Computational Study of Roughness-Induced Boundary-Layer Noise

As a first step toward predicting rough-wall boundary-layer noise, the sound radiated from a single hemispherical roughness element and a pair of roughness elements in a turbulent boundary layer at Re-theta = 7500 is investigated. The roughness height is 3.6% of the boundary-layer thickness, or 95 wall units. The flowfield is obtained from large-eddy simulation, and the results are validated against experimental measurements. Acoustic calculations are performed based on the Curie-Powell integral solution to the Lighthill equation for an acoustically compact hemisphere. The sound radiation is dominated by unsteady drag dipoles and their images in the wall. It is found that the spanwise dipole, which has been overlooked in previous studies of roughness noise, is of larger or similar strength compared with the streamwise dipole, and the viscous contribution to the drag dipoles is negligible in comparison with the pressure contribution. Important flow features contributing to sound radiation are identified by examining the unsteady surface-pressure held and the surrounding flow structures. Pressure fluctuations are strongest on the upstream part of the hemispheric surface near the base due to impingement of incoming turbulent eddies and their interaction with horseshoe vortices. On the back surface of the hemisphere, pressure fluctuations are relatively weak, indicating that shear-layer separation and vortex shedding do not produce significant self-noise from the hemisphere. In the case with two hemispheres, the wake of the upstream hemisphere is found to significantly enhance sound radiation from the downstream hemisphere, particularly in the streamwise direction and at high frequencies.