Airfoil Boundary-Layer Development and Transition with Large Leading-Edge Roughness

An experimentalstudy of the effects of largedistributed roughness located neartheleading edgeofan airfoilhas been performed to determine the effect on boundary-layer development and transition. Boundary-layer measure- ments werecarried out on a two-dimensional NACA0012 airfoil with a 53.34-cm chord through theuse of hot-wire anemometry at Reynolds numbers of 0 .75 £ 10 6 , 1.25 £ 10 6 , and 2.25 £ 10 6 . These measurements included mean anductuating velocity, turbulence intensity, ¯ ow® eld 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 asubstantialchordwiseextentto obtaina fully developedturbulentstate.Streamwiseturbulenceintensity levels in the roughness induced transitional region were observed to berelatively low as compared with the smooth model transitional region.