Abstract
The sound radiation by turbulent boundary-layer flow over an array of 10× 4 roughness elements is studied using large-eddy simulation and Lighthill’s theory. Two types of roughness elements, hemispheres and cuboids of the same height, are considered. The roughness height is 12.4% of the boundary layer thickness and 17% of the spacing between neighboring elements in both directions. The Reynolds number based on momentum thickness is 3065. The acoustically compact roughness elements and their images in the wall radiate primarily as acoustic dipoles in the plane of wall. The acoustic characteristics are found to be strongly dependent on the roughness shape. With the hemispherical array, similar to previous findings with a pair of hemispheres, the dipole sources are mainly generated by the interaction of hemispheres with incoming turbulent eddies and vortical structures formed around the bases of elements. Spanwise dipoles are stronger than streamwise dipoles, and wake turbulence enhances sound radiation from downstream elements. The leading row of hemispheres produces the weakest sound. In contrast, in the case of cuboid elements, unsteady separation and reattachment around the front edges are important acoustic sources, the spanwise dipoles are slightly weaker than the streamwise dipoles, and the leading row produces the strongest radiation. The spatial distributions of the roughness dipole sources are investigated. Correlations between dipole sources associated with neighboring roughness elements are found to be relatively weak.
Published Version
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