Aerodynamic Control of Laminar Separation on a Wall-Bounded Airfoil at Transitional Reynolds Numbers

Experiments were conducted in a low-turbulence wind tunnel to investigate the efficacy of localised acoustic forcing upon the dynamics and stability of the flow on a cambered, wall-bounded airfoil over a range of Reynolds numbers (Re) where the flow state can switch between two limits -- a low-lift state (SI) where separation continues beyond the trailing edge and a high-lift state (SII) where the separated flow is closed off to form a laminar separation bubble. The switching between SI and SII can occur close to a critical angle of attack ($\alpha_{\textrm{crit}}$) which varies with $\textrm{Re}$. The most effective forcing frequencies are found at a constant value of a rescaled Strouhal number, $\textrm{St}^* = \textrm{St}/\textrm{Re}^{1/2}= 0.027$, which indicates that though the primary unstable modes of the separated shear layer are of the inviscid, Kelvin-Helmholtz type, these modes are seeded by length scales that originate in the laminar (viscous) boundary layer. The most effective chordwise forcing location varies with $\textrm{St}/\textrm{Re}^{1/2}$ and incidence angle, $\alpha$, and is always upstream of the separation point. Although the boundary layer flows are far from two-dimensional, forcing at a fixed chord location across all spanwise locations is effective in controlling the SI -- SII transition. Strategies for active and passive feedback control are suggested.