Prediction of Dynamic Separation Characteristics Using a Time-Stepping Viscid-Inviscid Approach

Flow separation on the lifting surfaces of a vehicle at high angle of attack is always complicated by a certain degree of unsteadiness, but when the vehicle itself is undergoing unsteady motion or deformation, or if it enters a different flow field rapidly, then the complexity of the separated flow is even greater, and culminates in the phenomenon of dynamic stall. If the angle of attack oscillates around the static stall angle, the fluid-dynamic forces and moments usually exhibit large amounts of hysteresis, and a condition of negative aerodynamic damping often develops during part of the cycle. This can lead to the condition of flutter in single-degree-of-freedom oscillating rigid-body motion. (Normally, in attached flow, flutter only occurs when the body motion includes multiple degrees of freedom—e.g., combined bending and torsion of an aircraft wing.) During a rapid increase in angle of attack, the static stall angle can be greatly exceeded, resulting in excursions in the dynamic force and moment values that are far greater than their static counterparts. The consequences of dynamic stall are far-reaching and lead to such problems as wing drop, yaw (sometimes leading to spin entry), wing rocking, and buffeting as well as stall flutter.