This article presents a comparison study of different aerodynamic models for an X-shaped vertical-axis wind turbine and offers insight into the 3D aerodynamics of this rotor at fixed pitch offsets. The study compares six different numerical models: a double-multiple streamtube (DMS) model, a 2D actuator cylinder (2DAC) model, an inviscid free vortex wake model (from CACTUS), a free vortex wake model with turbulent vorticity (from QBlade), a blade-resolved unsteady Reynolds-averaged Navier–Stokes (URANS) model, and a lattice Boltzmann method (from PowerFLOW). All models, except URANS and PowerFLOW use the same blade element characteristics other than the number of blade elements. This comparison covers the present rotor configuration for several tip-speed ratios and fixed blade pitch offsets without unsteady corrections, except for the URANS and PowerFLOW which cover a single case. The results show that DMS and 2DAC models are inaccurate – especially at highly loaded conditions, are unable to predict the downwind blade vortex interaction, and do not capture the vertical/axial induction this rotor exhibits. The vortex models are consistent with each other, and the differences when compared against the URANS and PowerFLOW mostly arise due to the unsteady and flow curvature effects. Furthermore, the influence of vertical induction is very prominent for this rotor, and this effect becomes more significant with fixed pitch offsets where the flow at the blade root is considerably altered.

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.

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.

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 article presents a comparison study of different aerodynamic models for an X-shaped vertical-axis wind turbine and offers insight into the 3D aerodynamics of this rotor at fixed pitch offsets. The study compares six different numerical models: a double-multiple streamtube (DMS) model, a 2D actuator cylinder (2DAC) model, an inviscid free-wake vortex model (from CACTUS), a free-wake vortex model with turbulent vorticity (from QBlade), a blade-resolved URANS model, and a Lattice-Boltzmann method (from PowerFLOW). All models, except URANS and PowerFLOW use the same blade element characteristics other than the number of blade elements. This comparison covers the present rotor configuration for several tip-speed ratios and fixed blade pitch offsets without unsteady corrections, except for the URANS and PowerFLOW which cover a single case. The results show that DMS and 2DAC models are inaccurate – especially at highly loaded conditions, are unable to predict the downwind blade vortex interaction, and do not capture the vertical/axial induction this rotor exhibits. The vortex models are consistent with each other and the differences when compared against the URANs and PowerFLOW mostly arise due to the unsteady and flow curvature effects. Furthermore, the influence of vertical induction is very prominent for this rotor and this effect becomes more significant with fixed pitch offsets where the flow at the blade root is considerably altered.

Vertical-axis wind turbines (VAWTs) are gaining research attention in offshore energy due to their ability to operate in omnidirectional wind, simpler design characteristics, and 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. 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) against 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 three-dimensional aerodynamics introduced into the wake by the turbine’s coned blades. Results indicate that CACTUS effectively replicates the flowfield within the rotor volume and the very near wake when no pitch offsets are applied. 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 three-dimensional 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.