On the wake re-energization of the X-Rotor vertical-axis wind turbine via the vortex-generator strategy

Abstract. Wake losses are a significant source of inefficiencies in wind farm arrays, hindering the development of high-energy density wind farms offshore. Studies have demonstrated the potential of vertical-axis wind turbines (VAWTs) to achieve high-energy density configurations due to their increased rate of wake recovery compared to their horizontal-axis counterparts. Recent works have demonstrated a wake control technique for VAWTs that utilizes blade pitch to accelerate the wake recovery, hereinafter referred to as the "vortex-generator" method. The present work is an experimental investigation of the wake topology using this control technique for the novel X-Rotor VAWT. The time-averaged wake topology of the X-rotor has been measured by stereoscopic particle-image velocimetry at three fixed-pitch conditions of the top blades, namely a pitch-in, pitch-out, and a baseline case with no pitch applied. The results demonstrate the wake recovery mechanism linked to the streamwise vorticity system of the rotor and the mechanisms that lead to a streamwise momentum recovery, where the pitched-in case injects high momentum flow from above the rotor while ejecting the wake from the sides. In contrast, the pitched-out case operates in a mirrored fashion, with high momentum flow injected into the wake from the sides while low-momentum flow is ejected out axially above the rotor. These modes of operation demonstrate a significant increase in the available power for hypothetical downstream turbines, reaching as high as a factor of 2.2 two rotor diameters downstream compared to the baseline case. The pitched-in case exhibits a higher rate of momentum recovery in the wake compared to the pitch-out configuration.