Power losses at waked turbines due to the energy extraction of upstream turbines from the flow pose a major risk to the economic feasibility of wind farms. Helix active wake control has proven its potential to mitigate these wake-induced power losses by accelerating the recovery of the individual turbine wakes. This method leverages individual pitch control to induce a non-uniformly distributed force perturbation that rotates either in a clockwise (CW) or counterclockwise (CCW) direction around the rotor center. This deforms the wake into a helical shape that recovers faster than the wake of a conventionally controlled turbine. The CCW-oriented helix achieves higher power gains than the CW helix. Previous studies have identified a system of counter-rotating vortices to drive the wake recovery enhancement and the difference between CW and CCW helix. Nevertheless, a causal explanation for the creation of these vortices is still pending. This work contributes to understanding their creation by isolating the effect of the helix force perturbation on a symmetric wake from the impact of blade-related features like tip-vortices, hub vortex, or wake swirl. For this purpose, we perform Particle Image Velocimetry (PIV) measurements of a porous disc (PD) model in a wind tunnel. The PD is modified to mimic the helix but does not inherit the blade-related features present in a wind turbine wake. We observe the formation of two counter-rotating vortices in the far wake that deform the wake cross-section into a kidney shape, analogous to the structures present in the wake when helix active wake control is applied to a wind turbine. A conceptual comparison of PD wake and wind turbine wake implies that the wake swirl present in the turbine wake causes asymmetric reactions in several characteristics of the vortex system to changes in the rotational direction of the helix perturbation. Consequently, the dynamic, non-uniform helix perturbation alone is sufficient to activate the governing mechanisms that enhance the wake recovery when using helix active wake control, while blade-related phenomena are not fundamental to the principal processes.

A promising method to reduce wake effects in offshore wind farms is the Helix approach, which increases the mixing of the wake with the surrounding flow by exciting the individual blade pitch. This increases the wind speed in the wake, resulting in a higher power output at a downstream turbine. Wind tunnel testing is crucial to gather further understanding of the governing mechanisms behind the Helix and its efficiency in larger wind farm arrays. However, model turbines are expensive and complex. Porous Discs (PD) have proven to supply a less expensive and less complex alternative for wake-focused wind tunnel studies. In this study we present a novel PD model to mimic the Helix. The fundamental idea is to mimic the non-uniform, unsteady energy extraction over the rotor plane as observed at a Helix-controlled turbine. For this purpose, we derive a non-uniform porosity distribution over the PD, based on Large Eddy Simulations of a three-bladed turbine controlled with the Helix approach, and the actuator disc theory. The resulting non-uniform PD rotates at the excitation frequency described by the Strouhal number to mimic the Helix. We verified the novel experimental setup with smoke visualisation techniques and thrust measurements at a second PD in the wake and observed the typical characteristics of the Helix wake of a model turbine: First, the wake was deformed into a helical shape, and second, the wake velocity increased depending on the excitation Strouhal number.