基于高频PIV的偏航对风力机叶片尾迹膨胀和叶尖涡耗散影响

为了揭示叶片尾迹结构随偏航角变化的响应特征,该文以直径为1.4m的水平轴风力机为模型,利用高频PIV开展了尾迹流场特征的试验测试,探究了尾迹膨胀、叶尖涡耗散与来流风速、接入负载(即叶片转速)、偏航角度间的关联性和关联规律。研究结果揭示:不偏航时,随着发电机接入负载的增加,尾迹流动向风轮外侧膨胀的趋势变大,外流场与尾迹流场间的掺混效应加剧,从而导致叶尖涡耗散速率加快;偏航时,偏航行为会使尾迹流场向风轮内侧收缩,且收缩速率会随着偏航角的增加而变大,此时外侧流场与尾迹流场间的掺混效应减弱,从而导致叶尖涡扩散速率减小。测试结果同时揭示:在叶尖涡脱落的初始阶段,涡量值存在先增大后减小的规律性变化。同时,偏航状态下,叶片转速的增加会促使最大涡量值点提前出现,且提前出现的趋势会随偏航角的增大而加剧。该文以试验测试的方法揭示了叶片的尾迹膨胀和叶尖涡耗散特征,相关成果对于叶片尾迹结构组成和输运规律的深入探究具有较重要的参考价值。

Effects of yaw on wake expansion and tip-vortex dissipation of wind turbine blades based on high-frequency PIV

The complex spatial vortices are generated when the airflow passes through the high-speed rotating blades. The generation, growth and expansion of vortices is not only an important basis for analyzing the structural characteristics of the blade wake flow field, but also a key problem for accurately grasping the wake flow resistance, aerodynamic noise and vibration inducement of downstream wind turbines. Therefore, it is of great value to carry out relevant research work. As the junction between wake flow field and external flow field, tip vortices'' propagation characteristics are an important basis for analyzing the structure of wake flow field and an important topic that researchers have always paid close attention to. However, due to the lag of research and development of high-speed flow field monitoring equipment and insufficient attention paid by previous researchers to yaw conditions, the relevant research is still in its infancy, which leads to the uncertainty of the sensitivity of blade wake expansion to tip vorticity dissipation, and the influence of yaw behavior on the structure characteristics of wake flow field is still not clear. In particular, the regularity and mechanism of its influence on the generation and dissipation of tip vortices remain to be revealed. In order to reveal the response characteristics of blade wake structure with yaw angle, an experimental test on the near wake flow field characteristics of a small horizontal axis wind turbine with a diameter of 1.4 m was carried out using the high frequency PIV flow field measurement device, which investigated the correlations and correlations between wake expansion, tip vortex dissipation and incoming wind velocity, access load (i.e. blade velocity) and yaw angle. The results showed that, in the non-yaw state, with the increase of generator load, the trend of wake flow expanding towards the outside of wind turbine became larger, and the mixing effect between the outside flow field and wake flow field intensified, which led to the acceleration of the dissipation rate of tip vortices. The yaw behavior made the wake flow field shrink to the inside of the wind turbine, and the shrinkage rate increased with the increase of yaw angle. At this time, the mixing effect between the outer flow field and the wake flow field was weakened, which led to the decrease of tip vortex diffusion rate. The test results also showed that in the initial stage of tip vortex shedding, there was a regular change of vorticity value which increases first and then decreases. The discovery provided an exact answer to the controversy that the variation of tip vorticity value obtained by numerical simulation was not the same. At the same time, in yaw condition, the increase of blade velocity would cause the maximum vorticity point to appear ahead of time, and the trend would be aggravated with the increase of yaw angle. In this paper, the characteristics of wake expansion and tip eddy dissipation of blades were revealed by means of experimental measurements. Relevant results have important reference value for the further study of the wake structure and transport law of blades.