Airfoil boundary-layer development and transition with large leading-edge roughness

An experimental study of the effects of large distributed roughness located near the leading edge of an airfoil has been performed to determine the effect on boundary-layer development and transition. Boundary-layer measurements were carried out on a two-dimensional NACA 0012 airfoil with a 53.34-cm chord through the use of hot wire anemometry at Reynolds numbers of 0.75 X 10(6), 1.25 X 10(6), and 2.25 X 10(6), These measurements included mean and fluctuating velocity, turbulence intensity, flowfield intermittency, and associated integral parameters, The roughness used was of the type and density observed to occur during the initial glaze ice accretion process, Results have shown that the transitional boundary layer induced by large distributed roughness is markedly different from the smooth model Tollmein-Schlicting induced transition process, No fully developed turbulent boundary layers were observed to occur near the roughness location. Instead, the large distributed roughness was observed to trigger a transitional boundary layer at or very near the roughness location, This transitional boundary layer required a substantial chordwise extent to obtain a fully developed turbulent state, Streamwise turbulence intensity levels in the roughness induced transitional region were observed to be relatively low as compared with the smooth model transitional region.