Hysteresis in flow past a naca 0012 airfoil

Computations for two-dimensional flow past a stationary NACA 0012 airfoil are carried out with progressively increasing and decreasing angles of attack. The incompressible, Reynolds averaged Navier-Stokes equations in conjunction with the Baldwin-Lomax model, for turbulence closure, are solved using stabilized finite element formulations. Beyond a certain angle of attack the flow stalls with a sudden loss of lift and increase in drag. Hysteresis in the aerodynamic coefficients is observed for a small range of angles of attack close to the stall angle. This is caused by the difference in the location of the separation point of the flow on the upper surface of the airfoil during the increasing and decreasing angles of attack. With the increasing angle, the separation point moves gradually towards the leading edge. With the decreasing angle, the movement of the separation point away from the leading edge is abrupt.