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Computational Study of Boundary-Layer Noise Due to Surface Roughness

As a first step toward predicting rough-wall boundary layer noise, the sound radiation from a single hemispherical roughness element and a pair of roughness elements in a turbulent boundary layer at Re� = 7500 is investigated. The roughness height is 3.6% of the boundary layer thickness, or 95 wall units. The flow field is simulated with large-eddy simulation, and the results are validated against experimental measurements. Acoustic calculations are performed based on the Curle-Powell integral solution to the Lighthill’s 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 stronger than the streamwise dipole, and the viscous contribution to the drag dipole is negligible in comparison to that of the pressure contribution. The source mechanisms are discussed in terms of diffraction and turbulence generation by a roughness element. The presence of an upstream wake is found to significantly enhance sound radiation, particularly in the streamwise direction and at higher frequencies.