Unsteady flow mechanisms of thin airfoils at high angles

翼型上的非定常升力和阻力在包括直升机、风力涡轮机和微型飞行器在内的许多应用中是重要的。 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关系。 这种关系是从经典的Kirchhoff解中推导出来的。 它还没有被测试，尽管最近的进展，光学风速计，现在使这样的调查成为可能。 这个项目将研究在大迎角范围内，特别是在低速（严格低雷诺数）时，一些薄翼型上的非定常升力和阻力。 主要应用是提高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，翼型必须很薄（约为翼弦的10%或更小）才能在这些条件下良好工作。 还将测试薄平板。即使在叶片已经启动后，它们仍然会随着风力大小和方向的不断变化而经历不稳定的流动。 改进的知识和不稳定行为的建模将用于制定详细的涡轮机行为的计算机模型，从中将开发出更好的控制算法，以从风中提取最大可能的功率。主要任务是对在一定角度范围内和降低频率下正弦俯仰的翼型进行一系列定常和非定常升力和阻力测量。正弦变桨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，使用一个专门设计的力平衡，将在2011年底完成并投入使用。 代表性的情况下，将调查的粒子图像测速（PIV）设备，已经存在。激光片将照亮俯仰翼型或板上的引射气流。一个反射镜系统将把运动颗粒的图像传送到安装在俯仰轴上但远离隧道的高速摄像机。将处理所得到的图像，以解决与时间相关的流场和分离点的位置，在相同的角度和大量的重复实验的图像集合。

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.