Does the rotational direction of a wind turbine impact the wake in a stably stratified atmospheric boundary layer?

Kurzfassung

Stably stratified atmospheric boundary layers are often characterized by a veering wind profile, inwhich the wind direction changes clockwise with height in the Northern Hemisphere. Wind-turbine wakes re-spond to this veer in the incoming wind by stretching from a circular shape into an ellipsoid. We investigate therelationship between this stretching and the direction of the turbine rotation by means of large-eddy simulations.Clockwise rotating, counterclockwise rotating, and non-rotating actuator disc turbines are embedded in windfields of a precursor simulation with no wind veer and in wind fields with a Northern Hemispheric Ekman spiral,resulting in six combinations of rotor rotation and inflow wind condition. The wake strength, extension, width,and deflection depend on the interaction of the meridional component of Ekman spiral with the rotational direc-tion of the actuator disc, whereas the direction of the disc rotation only marginally modifies the wake if no veeris present. The differences result from the amplification or weakening/reversion of the spanwise and the verticalwind components due to the effect of the superposed disc rotation. They are also present in the streamwise windcomponent of the wake and in the total turbulence intensity. In the case of an counterclockwise rotating actuatordisc, the spanwise and vertical wind components increase directly behind the rotor, resulting in the same rota-tional direction in the whole wake while its strength decreases downwind. In the case of a clockwise rotatingactuator disc, however, the spanwise and vertical wind components of the near wake are weakened or even re-versed in comparison to the inflow. This weakening/reversion results in a downwind increase in the strength ofthe flow rotation in the wake or even a different rotational direction in the near wake in comparison to the farwake. The physical mechanism responsible for this difference can be explained by a simple linear superpositionof a veering inflow with a Rankine vortex.