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Rotor blades of wind turbines in the atmospheric boundary layer regularlyexperience the aerodynamic phenomenon of dynamic stall consisting of atemporary overshoot of lift and detrimental fatigue loads. Particularly theformation of dynamic stall under three-dimensional inflow conditions raisesopen questions. Aerodynamic behavior of a DU 91-W2-250 wind profile undergoinglight dynamic stall is thus analyzed in a wind tunnel. Effects of a gust withstreamwise and spanwise periodic variation are investigated by comparing totaland local lift generation with flow formation above the airfoil. The observedstall cycle is divided into five stages of which one reveals lift overshoot ofup to $16~\%$. The aerodynamic response of the airfoil shows a delay of about$1/8$ period between evolution of local angle of attack and lift giving acounterclockwise dynamic polar. A proper orthogonal decomposition (POD)analysis of the flow field contributes to understand aerodynamic consequencesof the three-dimensional gust. Local inflow, total lift as well as certain liftevents are captured by one POD eigenmode, respectively. Obtained results leadto the conclusion that the flow and particularly the stalled wake of an airfoilfacing a three-dimensional gust are strongly coupled in the spanwise direction.This yields to flow stabilization, inhibition of stall, and in turn,counterclockwise dynamic polar along with augmented total lift.

Dynamic stall of an airfoil under tailored three-dimensional inflow conditions