Abstract
In this study, the dynamic stall evolutions were investigated using particle image velocimetry (PIV) in a water channel with Reynolds number Re = 4.5 × 103 based on the chord length. The airfoil pitching waveform was performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). Using PIV, the instantaneous vorticity contours and streamlines can be revealed. Based on the formation of the leading edge vortex, the stall angle can be explored at reduced frequency k = 0.09, 0.18, and 0.27. It was found that the stall angle was delayed from the angle of attack α = 16° to α = 30° as reduced frequency increased from k = 0.09 to 0.27. The hysteresis effect of stall angle delay was more pronounced for high reduced frequency. Moreover, the freestream turbulence effect on the pitching airfoil was investigated with turbulence intensity TI = 0.5 and 6.9 %. As found, the stall angles were postponed to higher angles of attack for the high turbulence intensity. The phase difference between TI = 0.5 and 6.9 % were ∆α = 8°, 4°, and 4° for k = 0.09, 0.18, and 0.27, respectively. For TI = 6.9 %, enhanced turbulence mixing reduces the velocity deficit (u/U < 1) and flow reversal (u/U < 0). In addition, the maximum velocity is reduced from u/U = 1.8 to 1.2 and the S-shaped velocity profile is diminished or weakened for TI = 6.9 %. Thus, the dynamic stall is further delayed to the downstroke. The circulation values increase rapidly to maximum and then drop quickly after dynamic stall for k = 0.18 and 0.27.
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Acknowledgments
The authors would like to acknowledge the funding support by Ministry of Science and Technology, Taiwan, under the contract number MOST 104-3113-E-006-012-CC2 for this research work. This work is partially supported by the Robert M. and Mary Haythornthwaite Foundation, when Mr. Jui-Ming Yu was an international exchange student, under the supervision of Dr. Jim S. Chen, in the Department of Mechanical Engineering, Temple University, USA.
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Yu, J.M., Leu, T.S. & Miau, J.J. Investigation of reduced frequency and freestream turbulence effects on dynamic stall of a pitching airfoil. J Vis 20, 31–44 (2017). https://doi.org/10.1007/s12650-016-0366-6
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DOI: https://doi.org/10.1007/s12650-016-0366-6