The impact of the rotational direction of a wind turbine on its wake

Wind turbines operating in a stably stratified atmospheric boundary layer often interact with a veering wind, which is characterized by a clockwise wind direction change with height in the Northern Hemisphere. The rotational direction of the wind turbine rotor has a significant impact on the flow field in the wake in case of a veering wind, whereas it is of minor importance if the wind direction is the same over the whole rotor.

The impact of the rotational direction in a stably stratified atmospheric boundary layer results in contrasting rotational directions of the near and far wake in case of a common clockwise rotating rotor, whereas in case of a counterclockwise rotating rotor the rotational direction of the wake persists in the whole wake. The change of the rotational direction of the wake at a downstream location, which is related to the transition from the near wake to the far wake region, results in a larger streamwise wake elongation and a narrower spanwise wake width. In the lower and upper part, the wake deflection angle is also influenced by the rotational direction of the blades, resulting in a smaller wake deflection angle in case of a common clockwise rotating rotor in the Northern Hemisphere. In the Southern Hemisphere, the situation is reversed, an effect related to the Coriolis force impact on the Ekman spiral.

As the rotational direction impacts the inflow velocity, it effects the produced power of a downwind turbine and likewise the loads acting on a downwind turbine. For a hypothetical downwind turbine with a staggered spacing of 7 D, the power output difference would be up to 23% in idealized simulations, whereas the power output difference for a counterclockwise rotating rotor instead of a clockwise one also depends on atmospheric conditions like the strength of stratification, the strength of the veering wind, the rotor fraction impacted by a veering wind, and wind speed.