Effect of Surface Roughness Geometry on Boundary-Layer Transition and Far-Field Noise

Surface roughness elements are often used to force laminar to turbulent transition in aerodynamic and aeroacoustic wind-tunnel experiments. The statistical features and spectral content of the pressure fluctuations in the resulting turbulent boundary layer at the trailing edge can affect far-field noise. To elucidate this dependence, boundary-layer transition induced by randomly distributed roughness elements and a zigzag strip of the same height over a NACA 0012 airfoil is investigated experimentally. The effects of roughness geometry on the near-field flow topology, transition location, and far-field noise are addressed in the common experimental setting for the first time. For a fixed roughness height, distributed roughness elements are less effective in forcing transition than the zigzag strip at low freestream velocity (u∞20 m/s, the earlier onset of transition with the distributed surface roughness leads to a lower noise level than that with the zigzag strip. The data confirm that an adequate characterization of the boundary-layer transition is necessary when measuring the far-field noise during wind-tunnel experiments.