风力机翼型气动力非定常特性对湍流的敏感性

    Sensitivity of the aerodynamic force unsteady characteristics of wind turbine airfoil to turbulence

    • 摘要: 大气湍流是风力机非定常特性的主要诱因,该研究基于CDRFG(consistent discretizing random flow generation)方法生成湍流入口边界,采用大涡模拟(large eddy simulation, LES)研究风力机翼型气动力非定常特性对湍流的敏感性。结果表明:翼型前缘区域对湍流来流较为敏感,而中部及尾缘区域几乎不受湍流的影响。攻角分别为2°、8°和14°时,吸力面从前缘点到约0.5、0.3和0.1倍弦长位置处表面压力的标准差较均匀来流时幅值增大,表明小攻角时翼型吸力面上压力脉动受湍流影响的区域较大。来流湍流强度分别为9.3%、6.5%和4.8%时,2°攻角下翼型升力系数的标准差是其均匀来流时的6.36、5.42和4.90倍;8°攻角下是其均匀来流时的3.95、3.33和3.02倍;14°攻角下是其均匀来流时的1.78、1.63和1.40倍;表明小攻角时湍流引起的升力系数脉动特性较大攻角时更加显著。翼型前缘点脉动压力的功率谱曲线与湍流来流速度的功率谱曲线在整个频域区间趋势一致,表明前缘点压力的脉动特性主要取决于湍流来流的脉动特性,沿翼型弦向逐渐往后,吸力面上的压力脉动只在低频区域对湍流来流有响应,而高频处的压力脉动取决于自身的流动特性。研究结果可为降低机组功率波动和非定常载荷提供参考。

       

      Abstract: Atmospheric turbulence is the main inducer of the unsteady characteristics in wind turbines. This study aims to explore the sensitivity to turbulence in the aerodynamic unsteady characteristics of the wind turbine airfoil using the large eddy simulation (LES). The turbulent inlet boundary was also generated using the Consistent Discretizing Random Flow Generation (CDRFG). The calculation domain and grid were then established, according to surveying and mapping of the outfield true blade. A series of wind tunnel experiments were carried out to verify the numerical simulation. Specifically, the airfoil was numerically calculated at 2°, 8°, and 14° angles of attack, where the turbulence intensity of the incoming flow was set as 9.6%, 6.5%, and 4.8%, respectively. Among them, the turbulence scale was of the same order of magnitude as the chord length of the airfoil. The results show that the turbulence increased the unsteady characteristics of the lift coefficient of the wind turbine airfoil. Specifically, the higher the turbulence intensity was, the stronger the unsteady characteristics were. The leading edge of the airfoil was much more sensitive to the incoming flow of turbulence, whereas, there was no influence of the turbulence on the middle and trailing edge of the airfoil. Once the turbulence intensities of incoming flow were 9.3%, 6.5%, and 4.8%, the standard deviations of the lift coefficient of airfoil were 6.36, 5.42, and 4.90 times that of the uniform incoming flow at 2° angle of attack, respectively, while the standard deviations of the lift coefficient of airfoil were 3.95, 3.33, and 3.02 times of uniform flow at 8° angle of attack, respectively, and the standard deviations of lift coefficient of airfoil were at 1.78, 1.63, and 1.40 times that of the uniform incoming flow at 14° angle of attack, respectively. It infers that there were much more significant fluctuation characteristics of the lift coefficient caused by the turbulence at the small angles of attack, compared with the large angles. When the angles of attack were 2°, 8°, and 14°, about 0.5, 0.3, and 0.1 times increases were found in the chord length for the standard deviations of the pressure on the suction surface from the leading-edge point to the position, compared with the uniform incoming flow, indicating the turbulence influence of pressure fluctuation on the suction surface in a larger area at a small angle of attack. The reason was also that the more significant characteristics were observed in the lift coefficient fluctuation caused by turbulence at the small angle of attack. Furthermore, the pressure fluctuations on the suction surface were larger than those on the pressure surface. As such, the suction surface greatly contributed to the fluctuations of the lift coefficient, indicating the relatively higher sensitivity to the incoming turbulent flow. A consistent trend was found in the power spectrum curve between the fluctuating pressure at the leading edge of the airfoil and the turbulent incoming velocity in the entire frequency domain region. Therefore, the turbulence of the incoming flow was dominated by the fluctuating characteristics of the pressure at the leading edge. Going back along the chord of the airfoil, the fluctuation pressure of the airfoil suction surface only responded to the incoming flow turbulence in the low-frequency region, while the pressure fluctuation at the high frequency depended on the own flow characteristics. Anyway, the aerodynamic sensitivity of wind airfoil to turbulence can be expected to reduce the power fluctuation for the better control strategy of the wind field.

       

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