Prediction of aerodynamic forces on horizontal axis wind turbines in free yaw and turbulence

A free vortex lattice model is used to stimulate the flow field of a rotor at either a steady yaw angle to the wind or undergoing a changing yaw angle, for which cases the flow field itself and the aerodynamic forces on the rotor are unsteady. The model is run from an impulsive start and continues until an asymptotic (typically periodic) state is reached. This technique is used to analyse three situations: steady yaw, sinusoidally oscillating yaw and a random time history of yaw generated by an incident turbulent wind. Because of the computational expense of the oscillating yaw and turbulence computations, an investigation is carried out to study the accuracy of using the force time histories from steady yaw computation in indicial fashion to simulate the unsteady yaw cases. It is shown that very good predictions are obtained by this method up to yaw rates that are rapid from a practical point of view. Results from the steady yaw cases are compared with experimental and wind tunnel measurements and the comparisons discussed. The applications of the unsteady yaw results to free yawing rotor behaviour and to the forces induced by incident turbulence are also discussed.