Vertical-axis wind turbines (VAWTs) are gaining research attention in offshore energy due to their ability to operate in omnidirectional wind, the simpler design characteristics, and the potential for faster wake recovery. As part of this interest, a novel X-shaped VAWT (X-Rotor) has been proposed to minimise the levelised cost of energy by minimising capital and operational expenditures. While existing studies on the X-Rotor rely on numerical tools to analyse rotor performance, experimental validation remains limited, making it essential to assess the accuracy of these models in predicting the flowfield around the rotor. This study compares a free-wake vortex model (CACTUS) to stereoscopic particle image velocimetry (PIV) results for a scaled X-Rotor. Both qualitative and quantitative comparisons are performed, examining flowfield features with and without blade pitch offsets. Additionally, the study provides insights into the 3D aerodynamics introduced into the wake by the turbine's coned blades. Results indicate that CACTUS is able to predict the flowfield to a reasonable extent within the rotor volume and in the very near wake when no pitch offsets are applied, with discrepancies attributed to the uncertainty of the polars at the low Reynolds numbers. However, with pitch offsets, significant deviations from experimental data are observed, suggesting the need for careful model tuning for full-scale X-Rotor analysis. Furthermore, the introduction of coned blades enhances the 3D effects, generating notable upwash and downwash in the wake. These findings highlight the importance of using 3D aerodynamic tools over 2D approaches in future X-Rotor analyses to accurately capture vertical flow components.

This study assesses the wake recovery mechanism between an H-type Darrieus and an X-type vertical-axis wind turbine, named H-Rotor and X-Rotor respectively for different blade pitch offset configurations. The analysis is conducted in OpenFOAM using the actuator line method to model the turbines to present qualitative (velocity and vorticity contours) and quantitative (available power) studies for three different fixed blade pitch offsets. The results demonstrate that the H-Rotor recovers the wake much faster than the X-Rotor at positive pitch offsets. Overall, applying fixed pitch offsets to VAWT blades helps recover the wake faster than having no blade pitch offset.

Recent studies have revealed the large potential of vertical-axis wind turbines (VAWTs) for high-energy-density wind farms due to their favorable wake recovery characteristics. The present study provides an experimental demonstration and proof-of-concept for the wake recovery mechanism of the novel X-Rotor VAWT. The phase-locked flowfield is measured at several streamwise locations along the X-Rotor's wake using stereoscopic particle image velocimetry (PIV) with fixed-pitch offsets applied to the blades. The streamwise vortex system of the upper half of the X-Rotor is first hypothesized and then experimentally verified. The induced wake deformations of the vortex systems are discussed in comparison with previous studies concerning traditional H-type VAWTs. The results suggest that positive blade pitch is more favorable for accelerated wake recovery due to the dominant tip-vortex generated on the upwind windward quadrant of the cycle. Utilizing theoretical blade load variations along the span explains distinct unsteady flow features in the near wake generated at select quadrants of the rotor rotation, shedding light on the potential of the two pitch schemes.

In contemporary wind farm design, the primary focus has traditionally been on reducing wake interference to optimize energy capture from horizontal wind flows. However, with the scaling up of wind farms, their interaction with the Atmospheric Boundary Layer (ABL) evolves, making vertical entrainment the main mechanism for the exchange of momentum and energy. This study introduces a methodical approach to augment the efficiency of large-scale offshore wind farms by actively controlling this vertical entrainment of momentum within the ABL. The strategy involves the precise engineering of advection fluxes to alter wind flow dynamics, utilizing turbines as effective vortex generators, toward a process of "regenerative wind farming."This setup aims to create a vorticity and vertical flux system akin to those observed in highly unstable ABLs. Expanding upon previous studies that focused on single Vertical Axis Wind Turbines (VAWTs), our research explores the implementation of multi-rotor systems equipped with lift-generating wings. These systems are designed to exert forces perpendicular to the prevailing wind direction, thus creating trailing vortices and directing the flow orthogonally for improved vertical advection. This research is part of a comprehensive investigative framework that combines experiments and multifidelity simulations. The current study extends those findings to wind farm simulations, aiming to assess the impact of ABL control on a full wind farm scale. The first part of the work validates an established analytical wind farm performance model against real wind farm data for thirty-one wind farms in the North Sea and Baltic Sea. The results confirm the predicted trend of decreased performance with increased wind farm size and density. The model is used to calculate the performance of a wind farm for varying regimes of vertical entrainment due to the creation of large-scale circulatory systems. The results are compared against 3D vortex simulations of the full wind farm in "regenerative wind farming"mode. Our results demonstrate a notable improvement in wind speeds at the turbine hub height and the potential to double the feasible density of wind farms without compromising efficiency compared to traditional setups. These findings suggest a promising pathway towards a more sustainable and profitable future in wind energy, achieved through the strategic manipulation of ABL momentum, regenerating the energy in the wind farm.

Vertical-axis wind turbines (VAWTs), particularly in offshore wind farms, are gaining attention for their capacity to potentially enhance wake recovery and increase the power density of wind farms. Previous research on VAWT wake control strategies have demonstrated that the pitch offset is favorable for VAWT wake recovery. In the present study, an investigation on the wake recovery and its mechanisms for an H-Rotor and a novel X-Rotor VAWTs with fixed blade pitch offsets is conducted through qualitative and quantitative methods. The actuator line method is utilized in this study. Results indicate that the two rotors produce distinct vortex systems that drive the wake recovery process—which is augmented with pitch offsets. Through quantitative studies, the contribution of wake recovery due to advection increases dramatically with pitch offsets in the near wake. With pitch offsets, the inline available power increases up to 2.3 times for the rotors when compared to when there is no pitch offset. The mean kinetic energy flux occurs mostly above and below the rotors as well as the windward side, suggesting the mechanism of power replenishment for these rotors with pitch offsets. These results encourage further research into the effectiveness of wake recovery in the wind-farm level with the ground and atmospheric boundary layer influences.