Active Control of Laminar Separation: Simulations, Wind Tunnel, and Free-Flight Experiments

Abstract

When a laminar boundary layer is subjected to an adverse pressure gradient, laminar separation bubbles can occur. At low Reynolds numbers, the bubble size can be substantial, and the aerodynamic performance can be reduced considerably. At higher Reynolds numbers, the bubble bursting can determine the stall characteristics. For either setting, an active control that suppresses or delays laminar separation is desirable. A combined numerical and experimental approach was taken for investigating active flow control and its interplay with separation and transition for laminar separation bubbles for chord-based Reynolds numbers of Re approximate to 64,200-320,000. Experiments were carried out both in the wind tunnel and in free flight using an instrumented 1:5 scale model of the Aeromot 200S, which has a modified NACA 64(3)-618 airfoil. The same airfoil was also used in the simulations and wind tunnel experiments. For a wide angle of attack range below stall, the flow separates laminar from the suction surface. Separation control via a dielectric barrier discharge plasma actuator and unsteady blowing through holes were investigated. For a properly chosen actuation amplitude and frequency, the Kelvin-Helmholtz instability results in strong disturbance amplification and a "roll-up" of the separated shear layer. As a result, an efficient and effective laminar separation control is realized.