Upwind vs downwind: Loads and acoustics of a 1.5 MW wind turbine

Abstract. This paper discusses the motivation, preparation, risk mitigation, execution, and results of a full-scale experiment where the rotor of a 1.5 MW wind turbine was operated in a downwind configuration. The experiment took place at the National Renewable Energy Laboratory Flatirons Campus in Colorado, USA, and involved the collection of loads and power together with acoustic measurements from an array of four microphones. 410 min of downwind operation and 960 min of conventional upwind operations are used to validate the numerical predictions of the aeroelastic solver OpenFAST in terms of loads and performance. In the wind speed range from 4.5 to 12.5 m s^-1 the downwind rotor generates higher damage equivalent loads for the blade root flapwise moment, blade root edgewise moment, and tower-base fore-aft moment. Numerical predictions match well the experimental observations. OpenFAST is also seen underpredicting a power gain in the downwind rotor. In terms of acoustics, the overall sound pressure levels recorded in the field are similar between the upwind and downwind cases, but downwind operation worsens the metrics describing amplitude modulation. The paper closes with the recommendation to further investigate the potential of downwind rotor technology for floating wind applications, where the tilt angle of downwind rotors can compensate for the pitching of the floating platform.