An Experimental Investigation on Aerodynamic Hysteresis of a Low-Reynolds Number Airfoil

An experimental study was conducted to investigate the aerodynamic characteristics of a NASA low speed GA(W)-1 airfoil at the chord Reynolds number of ReC=160,000. Aerodynamic hysteresis was observed for the angles of attack close to the static stall angle of the airfoil. In addition to mapping surface pressure distribution around the airfoil, a high-resolution PIV system was used to make detailed flow field measurements to quantify the occurrence and behavior of laminar boundary layer separation and transition on the airfoil when aerodynamic hysteresis occurs. The flow field measurements were correlated with the airfoil surface pressure measurements to elucidate underlying fundamental physics. For the same angle of attack in hysteresis loop, the flow obtained along the increasing angle branch was found to result in an almost attached flow with small unsteadiness, higher lift and lower drag, whereas the one with decreasing angle of attack branch was associated with large unsteadiness, lower lift, and higher drag. The hysteresis was found to be closely related to the behavior of the laminar boundary layer separation and transition on the airfoil. The ability of the flow to remember its past history is believed to be responsible for the hysteretic behavior.