Open-Loop Flow-Control Investigation for Airfoils at Low Reynolds Numbers

Abstract

Within the scope of a flight research project involving dynamically scaled models, active flow control was investigated for a modified NACA airfoil. At low-Reynolds-number conditions, the aerodynamic performance of the modified NACA airfoil is considerably reduced by flow separation. Computational-fluid-dynamics simulations of the natural (uncontrolled) and controlled flow were carried out and provide the basis for a detailed analysis of the underlying physical mechanisms. The simulation results for the natural uncontrolled flow compare well with wind-tunnel measurements. Simulations and experiments show a large trailing-edge separation at low angles of attack and a leading-edge separation bubble at high angles of attack. To control the laminar separation and improve the overall performance of the airfoil at the low-Reynolds-number conditions, time-periodic blowing and suction through a spanwise slot is employed. A strong dependence of the effectiveness of the active flow control on the forcing parameters, such as forcing frequency and forcing slot location, is found. In addition, linear stability analysis is employed to identify the instability mechanisms that are responsible for the effectiveness of the flow-control approach.