Gust-like flow behavior is simulated using pitching airfoils torepresent the unpredictable nature of atmospheric turbulence onthe blades of wind turbines. Angle of attack, reduced frequencyand both NACA 0021 and NACA 0012 airfoils are investigatedusing particle image velocimetry to assess their influence on thedeveloped flow structure resulting from the ramp-up constantangular velocity motion. The pitch motion was shown to delay theonset of the stall vortex to high angles of attack, which is linked toincreased lift. Moreover, increasing the reduced frequency reducedthe rate of vortex growth as the angle of attack was increased.Development of a rear separation bubble with low velocity is notedduring initial development of the dynamic stall process. Once thecritical angle of attack is reached, initiation of the formation of thedynamic stall vortex is observed. Increased angular displacementresulted in the annihilation of the trailing edge vortices, by theinitial stall vortex, which also coincided with vortex-inducedseparation leading to bluff body separation. Results from thecurrent work show the presence of delayed separation and vortexformation on the upper surface of the airfoil characteristic to thedynamic stall process. The current work highlights the flowfeatures responsible for enhanced lift, whilst shedding light on thedevelopment process for constant-pitch-rate motion about thickand thin airfoil sections.
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