直立葉片式垂直軸風力機的葉片於運轉時具非定常空氣動力特性，係由複雜的流體運動行為所主導，而造成此動態氣動力特性的原因，主要來自隨著相對攻角與速度向量。當葉片處於低尖端速度比的情況下，攻角變化幅度相當劇烈，尤其是攻角增大時流體將從葉片表面分離而產生動態失速。此現象會造成葉片氣動力性能之下降，使得風力機轉速與發電效率無法提升。
本研究利用直立葉片式垂直軸風力機葉片之俯仰運動來探討風力機葉片因攻角改變所產生之動態失速。研究方法上將分別使用風洞實驗與水槽實驗來探討俯仰機翼之動態失速特性。風洞實驗上，利用自製之可撓式熱膜流體感測器來探討翼型為LS(1)0417在不同攻角時之穩態層流分離現象與非定常之動態失速。除此之外，藉由熱膜流體感測器之電壓輸出訊號，暫態動態失速現象可區分為九個主要階段。其結果顯示，機翼進行上仰運動時，層紊流轉換初始發展會隨著折合頻率延後至高攻角；於下俯運動時，再層流化現象則會延遲至低攻角時才發生。在高折合頻率時，機翼上下擺盪的時間尺度對於暫態滯後現象之影響相較於對流時間尺度之影響更加顯著。當折合頻率由k= 0.009加快至0.027時，其層紊流轉換與再層流化間的相位差也由 4.9 o加大由 13.5 o
在水槽實驗上，利用粒子影像測速儀來觀察翼型為NACA 0015之動態失速過程。藉由粒子影像測速儀的使用，可清楚得展現出流場之瞬時渦量與流線。翼尖渦流的形成可做為判斷失速攻角的依據。實驗結果發現，當折合頻率從k=0.09 加快至 0.27時，失速攻角會從α=16o 增加至 α=30o。除此之外，自由來流之紊流強度分別為TI=0.5% 與6.9%用以探討紊流效應對於動態失速之影響。其結果顯示，在高紊流強度TI=6.9%時，動態失速會延後到高攻角才形成。比較兩者之動態失速形成攻角，其相位差為∆α=8o, 4o與 4o，其所對應得折合頻率分別為k=0.09, 0.18, and 0.27。在折合頻率k=0.27 時，當紊流強度從TI=0.5% 提高到6.9%，增強的紊流混合會減低速度缺陷 (u/U<1)與逆流(u/U<0)。除此之外，最大的速度會從umax/U=1.8降低到umax/U=1.2，輪廓為S型的速度剖面也會消失或著趨於平坦。這意味著在高紊流強度下流場具有較佳的混合條件。另外，在高折合頻率時，流場的環流量會迅速達到最大並且在動態失速後快速降低。本研究結果可提供直立葉片式垂直軸風力發電機在葉片最佳化設計時之參考。

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.

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