Aerodynamic Noise from Sparse Surface Roughness

The noise produced by fetches of 3mm hemispherical and cubic roughness in a turbulent boundary layer was studied. The wall pressure was recorded in and around 42 element fetches of the roughness. Far field measurements were made using a linear phased microphone array. The microphone array data was analyzed using a conventional beamforming method as well as a least-squares analysis technique which can separate individual source strengths in the multi-element fetches. The cubic elements produce much stronger far field pressure fluctuations than the hemispherical elements. This radiated noise is produced by the inhomogeneous wall pressure fluctuations on the surface of the roughness elements. Examination of different roughness noise directivity models confirms the presence of separate spanwise and streamwise dipoles at each roughness element location. The relative strengths of the sources for a single 3mm cubic element vary with frequency and therefore produce a directivity pattern that varies with frequency. Normalized estimated mean square unsteady drag spectra for cubic and hemispherical elements have a similar shape and magnitude to those computed using LES by Yang & Wang (2011). For wall jet nozzle velocities of 50 and 60m/s, a single cubic element’s estimated spanwise dipole is significantly weaker than the streamwise dipole at lower nondimensional frequencies. Above fh/U=2, estimated streamwise strengths are similar to the spanwise dipoles. Source strength estimations of the multi-element fetches show that the addition of trailing cuboidal elements enhance the streamwise noise produced by upstream elements. Also, the estimated spanwise dipole strength of cubic elements was enhanced by the presence of spanwise adjacent elements.