Detachment of the dynamic-stall vortex above a moving surface

Dynamic stall occurs on helicopter blades and pitching airfoils when the dynamic-stall vortex, which forms as a result of an unsteady boundary-layer eruption near the leading edge, detaches from the surface and convects into the wake of the airfoil. The dynamic-stall vortex is modeled as a thick - core vortex above an infinite plane surface. Numerical solutions of the unsteady Navier-Stokes equations are obtained to determine the nature of the unsteady separation and vortex detachment processes and the influence of a moving wall. Whereas the unsteady separation process evolves very differently within two Reynolds number regimes, the detachment process is observed to be very similar over the range of Reynolds numbers considered. A moving wall has a significant influence on both processes. It is found that increasing the wall speed toward a critical value, which is equal to the magnitude of the maximum inviscid velocity at the surface, results in solutions that are indicative of flows at lower Reynolds numbers. Eventually, for wall speeds close to this critical value, formation of the recirculation region and unsteady separation are suppressed altogether, and detachment of the vortex is prevented.