Investigation of reduced frequency and freestream turbulence effects on dynamic stall of a pitching airfoil

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.Graphical abstract