Abstract
Roughness noise generation mechanisms are investigated using data from large-eddy simulations of turbulent boundary layers at Re� = 3065 over arrays of 10 × 4 roughness elements. Three types of roughness elements, including hemispheres, short cylinders and cuboids are considered, which share the same height of 0.124�, the same element-to-element spacing of 0.727� and the same flow blockage area. The streamwise dipole sound spectra share a common peak at fh/U∞ = 0.17. The surface pressure spectra in the roughness fetch are elevated over all frequencies and have smaller correlation length and time scales than those in the unperturbed smooth-wall boundary layer. The correlation coefficients and coherence between acoustic roughness dipoles and the pressure fluctuations on the rough wall are calculated. Analysis of the results confirms the importance of the impingement of upstream turbulence to dipole sound radiations, especially in the streamwise direction. For roughness shapes with sharp frontal edges, the edge-induced unsteady flow separations and vortex shedding also play important roles to sound generation. Large scale structures in the boundary layer have relatively low influence on roughness dipoles, except for the first rows of elements.
Published Version
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