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

Abstract

Abstract. 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 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 direction of the actuator disc, whereas the direction of the disc rotation only marginally modifies the wake if no veer is present. The differences result from the amplification or weakening/reversion of the spanwise and the vertical wind components due to the effect of the superposed disc rotation. They are also present in the streamwise wind component of the wake and in the total turbulence intensity. In the case of an counterclockwise rotating actuator disc, the spanwise and vertical wind components increase directly behind the rotor, resulting in the same rotational direction in the whole wake while its strength decreases downwind. In the case of a clockwise rotating actuator disc, however, the spanwise and vertical wind components of the near wake are weakened or even reversed in comparison to the inflow. This weakening/reversion results in a downwind increase in the strength of the flow rotation in the wake or even a different rotational direction in the near wake in comparison to the far wake. The physical mechanism responsible for this difference can be explained by a simple linear superposition of a veering inflow with a Rankine vortex.

How does the rotational direction of an upwind turbine affect its downwind neighbour?

“…2(c, d, k, l)) in case of a counterclockwise rotating rotor. This agrees with previous studies [5,4], which discuss the same nighttime veering inflow condition interacting with a single scaled wind turbine. The wake deflection angle difference results in larger (smaller) streamwise velocity values at the downwind rotor position for y < 0 (y > 0) in case of a CCR upwind rotor, especially pronounced at hub height and in the upper rotor half (Fig.…”

“…The veering of the wind in the stable boundary layer (SBL) is described by the Ekman spiral and can be attributed to the interaction between the Coriolis force and friction. Previous studies show an impact of the rotational direction of a wind turbine on its wake under veering inflow [5,3,4], whereas it is of minor importance for an unidirectional wind over the whole rotor [6,5]. The wake under veering inflow has a larger streamwise elongation, a narrower spanwise width, and a smaller deflection angle in case of a common clockwise rotating rotor in the Northern Hemisphere.…”

“…The turbulent inflow is integrated into the wind-turbine simulations with open streamwise boundary conditions by a synchronized coupling. For a more detailed description of this method we refer to [5]. The horizontal averaged atmospheric inflow profiles of the zonal and meridional wind components u and v, the potential temperature Θ, and the turbulent intensity T I including the three individual components (T I x , T I y , T I z ) are shown in Fig.…”

“…This impact of the rotational direction under veering inflow results from an amplification of the spanwise velocity component of the near wake in case of a counterclockwise rotating rotor. In case of a clockwise rotating rotor, the spanwise velocity component is weakened and can even reverse sign [5].…”

“…These previous studies [5,4] are limited to the wake of a single wind-turbine rotor. This approach is extended in this study, which focus on the wakes of a pair of rotors, aiming to answer the following question: How does the rotational direction of an upwind turbine affect its downwind neighbour?…”