Wind farm flow control aims at mitigating wake effects in order to maximize power production in wind farms. This work mostly focuses on the Helix strategy, which relies on individual pitch control to radially offset the application point of the thrust force from the rotor center and to dynamically change its azimuthal position. Previous studies have shown that power gains for a downstream turbine are higher for a counter-clockwise (CCW) rotation of the application point than for a clockwise (CW) one. In the CCW case, the wake develops as a right-handed helix, while in the CW case, a left-handed helix is observed. Using Large Eddy Simulations, this paper shows that the helix handedness in the wake matters due to its interaction with the wake swirl. Results of the CCW and CW helix first highlight the formation of streamwise vorticity in the near wake, which is transformed into strong coherent vortices in the far wake. Those vortex structures, to some extent similar to the counter-rotating vortex pair in the wake of yawed wind turbines, are responsible for (i) displacing the wake thanks to their induced velocities and (ii) deforming the shape of the wake.

Dynamic flow control strategies are raising interest for wake mitigation purposes. Among the different strategies, the so-called helix one relies on individual pitch control (IPC). The numerical simulation of the helix is thus readily performed by means of discrete-blade capturing methods. Yet, if this control strategy is considered at the scale of wind farms, the resolution required by such methods becomes prohibitive and actuator disk (AD) models should be envisioned. It is however not trivial to translate IPC strategies to an AD framework which by definition considers rotor-averaged effects. This work assesses the ability of an AD method to simulate the helix strategy by comparing it to a higher fidelity approach relying on a discrete-blade capturing model. Results show that the disk-type approach supplemented with a disk-adapted IPC scheme is able to capture both the forced motion of the wake at low turbulence and the faster wake recovery at moderate turbulence. From a quantitative perspective, the disk-type approach predicts bigger power gains, compared to those foreseen by the discrete-blade type approach, for a downstream turbine in the wake of a helix-operated one.