Dynamic Stall Life Cycle on a Rotating Blade in Steady Forward Flight

Stereoscopic particle image velocimetry is used to study the three-dimensional flow field over a retreating rotor blade in dynamic stall. The dynamic stall event is characterized from the inception of flow dynamic separation through dynamic reattachment on the rotating blade. First, the separation event is studied by characterizing the dynamic separation mechanism in action and the effect of advance ratio on the onset of dynamic separation. Next, the vortex dynamics and the associated cycle-to-cycle variations during the dynamic stall event are characterized. The radial component of flow (from root to blade tip) during the dynamic stall event is shown to be significant in the rotating environment and strongly affected by advance ratio. The unsteady nature of the flow field is characterized by using proper orthogonal decomposition. Comparisons of the phase-averaged and instantaneous velocity fields shows the dynamic stall vortex to be spatially diffused. These results imply significant effects on the pitching moment variation from cycle to cycle since the effective center of the vortex appears to occur at different locations in every cycle. Finally, the last phase of dynamic stall, the dynamic reattachment phase on the rotating blade is investigated. Dynamic reattachment was observed to be greatly affected by advance ratio with an increase in advance ratio resulting in advancement of dynamic reattachment.