Does the Rotational Direction of a Wind Turbine Impact the Wake in a Stably Stratified Atmospheric Boundary Layer?

Stably stratified atmospheric boundary layers are often characterized by a veering wind profile, in which the wind direction changes clockwise with height in the Northern Hemisphere. Wind-turbine wakes respond to this veer in the incoming wind by stretching from a circular shape into an ellipsoid. We investigate the relationship 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 wind fields with no wind veer or in wind fields with an Ekman spiral representative of the Northern Hemisphere, resulting in six combinations of rotor rotation and inflow wind condition. The impact of the Coriolis force via the Ekman spiral depends on the rotational direction of the actuator disc, whereas the direction of the disc rotation exerts little impact if no veer is present. The differences result from the interaction of the actuator rotation with the Ekman spiral and are present in the zonal, the meridional, and the vertical wind components of the wake. The interaction of the Ekman spiral with both rotational directions lead to two different flow fields characterizing the wake. In the case of a counterclockwise rotating actuator disc, the rotational direction of the wake persists in the whole wake. In case of a clockwise rotating actuator, however, the rotational direction is different in the near wake in comparison to the far wake. The physical mechanism responsible for this difference is explained by a simple linear superposition of the inflow wind field, characterized by vertical wind shear and wind veer, with a wind-turbine model including a Rankine vortex, representing the rotational effects imposed on the flow by the rotating blades.