Airfoil Boundary-Layer
Development and Transition with Large Leading-Edge
Roughness
Kerho, M.F. and Bragg, M.B.
University of Illinois, Urbana, Illinois 61801
ABSTRACT
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 53.34cm chord through the use of hot-wire anemometry
at Reynolds numbers of 0.75x106, 1.25x106, and 2.25x106.
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 to the smooth model
transitional region.