Spanwise differences in static and dynamic stall on a pitching rotor blade tip model

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Merz, C.B.
Wolf, C.C.
Richter, K.
Kaufmann, K.
Mielke, A.
Raffel, M.
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An experimental investigation of static and dynamic stall on a rotor blade tip model with a parabolic tip geometry and aspect ratio 6.2 at a chord Reynolds number of 900,000 and a Mach number of 0.16 is presented. The resulting flow is analyzed based on unsteady surface pressure measurements and quantitative flow visualizations by high-speed particle image velocimetry. The flow separation is found to be delayed near the parabolic blade tip for static angles of attack as well as for sinusoidal angle of attack motions. The maximum effective angle of attack prior to stall is shifted to approximately two-thirds of the span outboard from the root because of a positive twist of the model with an increasing geometric angle of attack towards the tip. The stall onset is observed near the section with the maximum effective angle of attack, with a subsequent spanwise spreading of the flow separation. Different stages of flow separation for static angles of attack are identified one of them with the occurrence of two stall cells. During dynamic stall, the leading edge vortex formation starts near the maximum effective angle of attack and the pitching moment peak resulting from the passage of the dynamic stall vortex is higher at this section. Further inboard the maximum aerodynamic loads are of comparable magnitude whereas the outboard section shows reduced peaks due to the influence of the wing tip vortex.