An Analytical Model Including the Tip Loss Effects to Predict the Sectional Pressure Drag for a Wind Turbine Blade Oscillating in Pitch

The tip loss is one of the most challenging issues in the aerodynamic performance of wind turbine blades. In this paper a simple analytical model has been proposed, based on response surface methodology, RSM to accurately predict the sectional pressure drag while the blade was oscillating in pitch in terms of the mean angle of attack, oscillation amplitude and frequency and of course, the spanwise location. In wind turbine blades, the pitching motion is usually encountered in practice and is due to the blade elastic behavior and the torsional moments. Extensive wind tunnel tests were performed on a rectangular wind turbine blade planform to construct the regression model and support the results. In this model, the combinational effects of the tip loss and induced drag as well as those associated with oscillation frequency and amplitude have been considered. The periodic upwash and downwash velocities arising from the downstroke and upstroke motions have also been included in the model in the form of a change in the effective angle of attack seen by the blade. Remarkable agreements have been achieved between the experimental results and those predicted by the present regression model in a broad range of the angles of attack and oscillation frequencies. Similar model with subtle modifications and different regression coefficients can be used for various wind turbine blade sections and planforms. No analytic model has already been proposed in the literature to predict the sectional drag for a pitching 3-D blade. Most of the existing models for tip loss work as a correction factor for CFD- or BEM-based codes.