Wind Energy: The Pressure is On*

Wind turbine aerodynamics presents unique challenges as it combines extremely high Reynolds numbers with additional time scales imposed by the rotation. This implies that resolved numerical solutions are too computationally expensive and investigations in conventional wind tunnels are impossible due to the flow speeds and rotational rates needed in order to satisfy the dynamic similarity requirements. Most of the previous studies at high Reynolds numbers are either performed in the field, or in extremely large wind tunnels. At Princeton, we simulate the conditions a large wind turbine experiences by compressing the air around it. By using a wind tunnel with pressurized air at up to 238 bar, we study both steady and unsteady 2D airfoil performance as well as rotating 3D vertical and horizontal axis rotors. This allows us to investigate the rotating systems as well as evaluate modeling approaches, as they often rely on 2D data as input. A model wind turbine setup has been developed that allows—for the first time—well-controlled laboratory tests of their performance and wake dynamics, at dynamic similarity to the field scale machines. The effect of Reynolds number on the performance of wind turbines is characterized and compared to field measurements and it is shown that the machine reaches a Reynolds number invariant state above a critical Reynolds number. We also find that the conventional modeling approaches for vertical axis rotors can be significantly improved by incorporating a new model for the drag on porous plates.