Aerodynamic Hysteresis of a Low-Reynolds-Number Airfoil

The aerodynamic hysteresis near the static stall angle of low-Reynolds-number airfoil was investigated. A high-resolution particle image velocimetry (PIV) system was used to make quantitative flowfield measurements to quantify the occurrence and behavior of boundary layer transition and/or separation on the airfoil when aerodynamic hysteresis occurs. The lift and drag coefficients of the airfoil were estimated by numerically integrating the pressure distribution around the airfoil. The flow obtained along the increasing angle branch of the hysteresis loop results in an almost attached flow with small unsteadiness, higher lift and lower drag. The hysteresis was found to be closely related to the behavior of the laminar boundary-layer transition and/or separation on the airfoil.