Flow transitions on a cambered airfoil at moderate Reynolds number

Abstract

A combined experimental and numerical study is performed to investigate the flow field and associated aerodynamic forces on a cambered airfoil. The Reynolds number is low enough to ensure importance of viscous dynamics, and high enough so that instability and transition to turbulence can occur. The flow fields are complex and their correct description is essential in understanding the nonlinear curves describing the variation of lift and drag coefficients with angle of attack, α. As α is increased from 0, the flow states go through a number of qualitatively distinct phases. At low to moderate α, the laminar boundary layer separates before the trailing edge, and as the separation point moves forward, instabilities of the detached shear layer form coherent vortices over the upper (suction) surface. At a critical angle, αcrit, instabilities in the shear layer grow fast enough to transition to turbulence, which then leads to reattachment before the trailing edge. In this flow state, lift is increased and drag decreases. Hence, in order to understand the aerodynamics at this scale, we need to understand the viscous dynamics of the boundary layer, as elegantly described and analyzed by Frank White.