Effects of Reduced Frequency and Freestream Turbulence on Dynamic Stall of the Pitching Airfoil

• Main Authors: Jui-MingYu, 游瑞名
• Other Authors: Tzong-Shyng Leu
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/g3md6w

博士 === 國立成功大學 === 航空太空工程學系 === 105 === The strong growth of a trend toward utilizing wind energy in the past decade has stimulated extensive research on wind turbine technology. Recently, a great deal of attention has been paid to investigating the aerodynamic performance of vertical-axis wind turbines (VAWTs) due to their potential applications in urban environments. However, straight-blade VAWTs face a critical challenge, especially when operating at low tip speed ratios. Straight-blade VAWTs often operate continuously with a considerable number of flow separations and blade stalls because the cyclic motion of the blades as well as changes in wind velocity and direction induce large variations in the angle of attack on the blades, which in turn leads to unsteady aerodynamics and stall effects. These can be important contributors to blade airload and reductions in wind turbine performance. The phenomenon of the dynamic stall of straight-blade VAWT blades is significant and complex at low tip speed ratios. In this study, an airfoil pitching waveform was created under conditions calculated from the angle of an attack histogram of a straight-blade vertical axis wind turbine. In the wind tunnel experiments, self-made MEMS thermal flow sensors were designed and fabricated on a flexible skin. The steady laminar separation was investigated on a two-dimensional LS(1) 0417 airfoil by using thermal flow sensors at various angles of attack, with validation obtained using hot wires and flow visualization. The unsteady flow on the pitching airfoil was experimentally investigated to simulate the dynamic stall condition of a straight-blade VAWT. Based on variations in the mean value and standard deviations of the thermal flow sensor signals, nine stages of unsteady flow-developing events were identified with further evidence from flow visualization. It was found that as the reduced frequency (k) was increased, this delayed an incipient transition to higher angles of attack during the pitch-up motion and postponed re-laminarization to lower angles of attack during the pitch-down motion. The hysteresis was more pronounced at higher frequencies of k, where the oscillating time scale played a more significant role in determining the unsteady flow pattern than the convective time scale. The phase difference between transition and re-laminarization was enlarged from 4.9 o for k=0.009 to 13.5 o for k=0.027 at Re=6.3x104. The dynamic stall evolutions of the NACA 0015 airfoil were investigated using PIV (particle image velocimetry) in a water channel with the Reynolds number Re=4.5x103 based on the chord length. By using PIV, the instantaneous vorticity contours and streamlines could be revealed. Based on the formation of the leading edge vortex, the stall angle could be explored at reduced frequencies of k=0.09, 0.18, and 0.27. It was found that the stall angle was delayed from the angle of attack α=16o to α=30o as the frequency was increased from k=0.09 to 0.27. Moreover, the freestream turbulence effect on the pitching airfoil was investigated with turbulence intensities TI=0.5% and 6.9%. In the case of high turbulence intensity, the stall angles were delayed to higher angles of attack. The phase differences between TI=0.5% and 6.9% were ∆α=8o, 4o, and 4o for k=0.09, 0.18, and 0.27, respectively. For TI=6.9%, enhanced turbulence mixing reduced the velocity deficit (u/U〈1) and flow reversal (u/U〈0). In addition, the maximum velocity was reduced from umax/U=1.8 for TI = 0.5% to 1.2 for TI = 6.9%, and the S-shaped velocity profile was either diminished or weakened at k=0.27. Thus, the dynamic stall was further delayed to the downstroke. The circulation values increased rapidly to a maximum and then dropped quickly after the dynamic stall for k = 0.18 and 0.27.