Analysis of Tower Shadow Effects on the UAE Rotor Blades
A leading obstacle hindering the development of wind turbines to extreme scale is the structural integrity of the blades. Downwind rotors have been shown to give structural advantages for larger systems. However, there is an added aerodynamic complication from the tower shadow. This paper presents and analyzes a previously unpublished subset of data collected by NREL during an extensive wind tunnel campaign for Unsteady Aerodynamic Experiment Phase VI (UAE Phase VI). The experimental data includes relative flow fields, aerodynamic blade forces, and root blade flapwise bending moments, from upwind turbines, downwind turbines and downwind turbines with the use of an aerodynamic tower fairing. It is shown that the tower shadow can have a severe and negative effect on these variables, leading to higher bending stresses. The use of a tower fairing can greatly reduce these detrimental effects. To better interpret this data, predictions using an aeroelastic wind turbine code, FAST, was used to model the experimental conditions. The differences between the experimental data and the computational predictions are attributed to unsteady effects of the wake. This suggest that wake modeling for downwind turbines may require modifications to capture physically realistic tower shadow effects.