<p>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.</p><p>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.</p><p>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.</p>
Answers to Anonymous Referee 2 on the paper 'Does the rotational direction of a wind turbine impact the wake in a stably stratified atmospheric boundary layer?'
