Unsteady flow mechanisms of thin airfoils at high angles

机翼的非定常升力和阻力在许多应用中都很重要，包括直升机、风力涡轮机和微型飞行器。这些力的所有数学模型都假定升力与分离点的瞬时位置之间存在关系。这种关系来自于经典的基尔霍夫解，用于解决薄平板上的流动问题。它还没有被测试，尽管最近光学风速测量的进步，现在使这样的调查成为可能。该项目将研究在大迎角范围内的一些薄翼型的非定常升力和阻力，特别强调低速（严格低雷诺数）。主要应用是改善小型风力涡轮机的性能。当叶片静止时，气流角高，雷诺数低，众所周知，在这些条件下，翼型必须很薄（约弦的10%或更少）才能很好地工作。薄平板也将进行测试。即使在叶片启动后，由于风的大小和方向不断变化，它们也会继续经历不稳定的流动。改进的非定常行为知识和建模将用于制定涡轮机行为的详细计算机模型，从中将开发更好的控制算法，以从风中提取最大可能的功率。主要任务将是一系列的稳定和非稳定的升力和阻力测量的一些翼型是正弦倾斜在一定范围内的角度和降低频率。正弦俯仰是产生非定常流的最常用方法，特别适用于风力涡轮机应用。测量将在一个新的风洞中进行，使用一个特殊设计的力平衡，该风洞将于2011年底完成并投入使用。代表性的案例将调查粒子图像测速（PIV）设备已经存在。激光光片将照亮种子气流在俯仰翼型或板。反射镜系统将把运动粒子的图像传送到安装在俯仰轴上但远离隧道的高速摄像机上。对于相同角度和大量重复实验的图像集合，将得到的图像进行处理，以求解随时间变化的流场和分离点的位置。

The unsteady lift and drag on airfoils are important in many applications including helicopters, wind turbines, and micro-aerial vehicles. All mathematical models for these forces postulate a relationship between lift and the instantaneous location of the separation point. This relationship derives from the classical Kirchhoff solution to the flow over a thin flat plate. It has not been tested despite recent advances in optical anemometry that now make such an investigation possible. This project will study the unsteady lift and drag on a number of thin airfoils over a large range of angle of attack with particular emphasis on low speeds (strictly low Reynolds numbers). The primary application is to improve the performance of small wind turbines. When the blades are stationary, the flow angles are high and the Reynolds number low and it is well known that airfoils must be thin (around 10% of the chord or less) to work well in these conditions. Thin flat plates will also be tested. Even after the blades have started they continue to experience unsteady flow as the wind changes continually in magnitude and direction. Improved knowledge and modeling of unsteady behaviour will be used to formulate detailed computer models of turbine behaviur from which will be developed better control algorithms to extract the maximum possible power from the wind. The main task will be a series of steady and unsteady lift and drag measurements for a number of airfoils that are sinusoidally pitched over a range of angles and reduced frequenices. Sinusoidal pitching is the most common method of generating unsteady flow and is particularly appropriate to wind turbine applications. The measurements will be done in a new wind tunnel using a specially-designed force balance that will be completed and operational by the end of 2011. Representative cases will be investigated by particle image velocimetry (PIV) equipment that already exists.. A laser light sheet will illuminate the seeded air flow over the pitching airfoil or plate. A system of mirrors will feed the images of the moving particles to a high speed camera mounted on the axis of pitching but away from the tunnel. The resulting images will be processed to resolve the time-dependent flow field and the location of the separation point for ensembles of images at the same angle and a large number of repetitions of the experiments.