A Study of a Separated Turbulent Boundary Layer in Stalled-Airfoil-Type Flow Conditions

Abstract

The experimental study of the turbulent boundary layer and its separation under external flow conditions similar to those found on the suction side of airfoils in trailing-edge post-stall conditions has been performed. The flow is characterized by a very narrow suction peak with a minimum pressure coefficient of -16 at a Reynolds numbers of 1.5×10 6 , based on an effective chord length of 2.5 m. Special care was taken to achieve a nearly twodimensional mean flow. Detailed boundary layer measurements were carried out with a PIV system and a two-sensor wall probe. They cover the region downstream of the suction peak where the boundary layer is subjected to a very strong adverse pressure gradient and has suffered from an abrupt transition from strong favorable to strong adverse pressure gradients. The experiments show that in spite of these severe conditions, the boundary layer is able to recover a state of equilibrium and maintain it up to the separation point. In the equilibrium zone, the mean velocity and all the measured Reynolds stress components share common self-similarity scales, namely δ and Ueδ*/δ. These similarity scales are not valid for the separated zone. These findings support the conclusion of Castillo and Wang (J. Fluids Eng., Vol. 126, 2004) that most nonequilibrium flows are actually able to reach a local equilibrium state.