Unsteady flow mechanisms of thin airfoils at high angles

翼型上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。所有这些力的数学模型都假定升力与分离点的瞬时位置之间存在一种关系。这个关系式是从薄平板上流动的经典基尔霍夫解推导出来的。尽管最近光学风速测量技术的进步使得这种研究成为可能，但它还没有得到测试。该项目将研究在大迎角范围内，一些薄翼型上的非定常升力和阻力，特别强调低速(严格的低雷诺数)。其主要应用是提高小型风力涡轮机的性能。当叶片静止时，流动角度大，雷诺数低，众所周知，翼型必须很薄(约为弦长的10%或更少)才能在这些条件下良好地工作。还将对薄平板进行测试。即使在叶片启动后，由于风的大小和方向不断变化，它们仍然经历着不稳定的流动。改进的非定常行为知识和建模将被用来制定详细的涡轮机行为计算机模型，从中将开发出更好的控制算法，以从风能中提取最大可能的功率。主要任务将是对一些翼型进行一系列稳定和非稳定的升力和阻力测量，这些翼型在一定范围的角度和降低的频率上呈正弦俯仰。正弦俯仰是最常见的产生非定常流动的方法，特别适合于风力涡轮机的应用。测量将在一个新的风洞中进行，使用专门设计的力平衡，将于2011年底完成并投入使用。具有代表性的案件将通过已经存在的粒子图像测速(PIV)设备进行调查。激光光片将照射在俯仰的翼型或平板上的播种气流。一个镜子系统将把运动粒子的图像提供给安装在俯仰轴上但远离隧道的高速摄像机。得到的图像将被处理，以解决相同角度和大量重复实验的图像集合的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.