Experimental study of noise emitted by circular cylinders with large roughness

A set of experiments of rough circular cylinder flow as well as rough wall turbulent boundary layer (TBL) flow were performed with the goal of studying the effect of large roughness on the aerodynamic noise emitted by single bluff bodies. Roughness consisted of uniformly distributed hemispherical and cylindrical elements with normalised heights of 0.031 and 0.035, respectively, as well as cable helicoidally wrapped with normalised heights of 0.031 and 0.047. The Reynolds numbers in the circular cylinder tests ranged between 1.6×10⁵ and 3.2×10⁵. A broadband spectral peak has been identified mainly in the hemispherical roughness configurations, centred at frequencies well predicted by Howe's roughness noise prediction model that was extended to the case of a circular cylinder. The peak level was overpredicted and for higher frequencies the difference increased. The helicoidal cable configurations didn't generate significant roughness noise and, furthermore, its far field noise spectra have been found to collapse with the smooth cylinder spectra for Strouhal numbers greater than 0.7. The observed effect of roughness at lower frequencies was to increase the vortex shedding peak level and to reduce the peak Strouhal number, which is in accordance with the observations of previous studies.