Schlieren Studies of Compressibility Effects on Dynamic Stall of Airfoils in Transient Pitching Motion

Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0 - 60 degrees over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experiments] documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers, the flow was dominated by a single vortex. The delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value. Increasing the pitch rate monotonically delayed stall to angles of attack as large as 27 degrees.AFOSR-MIPR-87-0029 and 88-0010NAVAIRAR

On the Prediction of Dynamic Stall Onset on Airfoils in Low Speed Flow

Proceedings of the 8th International Symposium on Unsteady Aerodynamics and Aeroelasticity of Turbomachines, Ed. T. H. Fransson, Kluwer Academic Publications, Dordrecht, The Netherlands, 1998, pp. 797-812.A computational approach is described for the rapid and systematic prediction and evaluation of the onset of dynamic stall due to rapid incidence changes or unsteady pitch or plunge motions. The method combines an unsteady, two-dimensional panel code with a two-dimensional boundary-layer code. The panel code provides incompressible, inviscid flowfields about arbitrary airfoils undergoing prescribed motions. The boundary-layer code computes laminar, transitional and turbulent regimes. Presented results demonstrate that the delay in dynamic stall onset is directly related to the dynamic pressure lag, in agreement with previous Navier-Stokes simulations. Also, results are presented showing the effect of airfoil shape and Reynolds number on the onset of dynamic stall