Unsteady flow mechanisms of thin airfoils at high angles

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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.