An Experimental Study of Stall Hysteresis of a Low-Reynolds-Number Airfoil

An experimental study was conducted to investigate static stall hysteresis of a NASA GA(W)-1 airfoil at the chord Reynolds number of Re = 162,000. In addition to mapping the surface pressure distribution around the airfoil, a digital PIV system was used to make detailed flowfield measurements to quantify the occurrence and behavior of laminar boundary layer separation and transition on the airfoil when static stall hysteresis occurs. The measurement results revealed clearly that, for a same angle of attack in the hysteresis loop, incoming flow streams were found to be able to attach to the airfoil upper surface in general with a thin separation bubble formed near the airfoil leading edge when the angle is at the increasing branch of the hysteresis loop. The attached flow pattern resulted in higher lift and lower drag acting on the airfoil as well as a lower Reynolds stress level and smaller unsteadiness in the wake of the airfoil. When the angle of attack is at the decreasing branch of the hysteresis loop, the laminar boundary layer was found to separate from the airfoil upper surface for good at a location very near to the airfoil leading edge. The turbulence transition of the separated laminar boundary layer was found to take place rapidly accompanied by periodical shedding of strong Kelvin-Helmholtz vortex structures in the wake of the airfoil. Large-scale flow separation was found to take place on almost the entire upper surface of the airfoil. As a result, the lower lift and higher drag acting on the airfoil were found with Reynolds stress and turbulence kinetic energy levels in the wake of the airfoil increased significantly. The present study elucidates quantitatively that static stall hysteresis of a low-Reynolds number airfoil is closely related to the behavior of laminar boundary layer separation and transition on the airfoil. The ability of the flow to remember its past history is responsible for the stall hysteretic behavior.