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

Surface roughness elements are often used to force laminar to turbulent transition during aerodynamic and aeroacoustic experiments in wind tunnels. The statistical features and frequency content of the forced turbulent boundary layer can influence the far-field trailing edge noise. To study 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 type on the near-field flow topology, transition location, and far-field noise are addressed. At a fixed roughness height, the distributed roughness elements are less effective in forcing transition than the zigzag trip at low freestream velocity (u∞ < 20 m/s). As u∞ increases, the transition front for the distributed roughness elements moves close to the roughness location, reaching the same or even further upstream location compared with the zigzag strip. The far-field noise intensity depends on the transition location. For u∞ < 20 m/s, a higher noise level is measured for the distributed roughness with respect to the zigzag strip. Whereas, for u∞ <20m/s, the earlier onset of transition leads to a lower noise level for the distributed roughness elements with respect to the zigzag strip. The data confirms that an adequate characterization of the state of the boundary layer is necessary when measuring far-field noise in wind tunnel experiments.