• Energy Research

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.

This dataset is intended to support the publication "On the wake re-energization of the X-Rotor vertical-axis wind turbine via the vortex-generator strategy" by Bensason, Sciacchitano, and Ferreira, now in preprint with Wind Energy Science at 10.5194/wes-2025-3. The dataset includes wake data obtained using Particle Image Velocimetry (PIV) in the open-jet wind tunnel facility of the TU Delft (OJF) for a scaled X-Rotor vertical-axis wind turbine (VAWT) with a diameter of 0.6m. The wake at cross-stream planes is measured up to a distance of 6 diameters at increments of 1 diameter. Three fixed-pitch offsets are applied to the upper blades of the X-Rotor to demonstrate a wake recovery mechanism known as the "vortex-generator" method. This research and dataset aim to provide an experimental database for the future and ongoing development of numerical models for this novel X-Rotor VAWT.

Abstract This study explores the performance and near-wake dynamics of a VAWT-based Multi-Rotor System in both its original configuration and in the presence of external lift-generating devices, specifically employed for wake control operations. The wake of a scaled VAWT-based MRS was measured in a wind tunnel using Particle Tracking Velocimetry. Lift-generating devices, including a 3-element cascading wing on top and a single-element wing in the middle of the MRS, were used to enhance wake control and deflection. Measurements in the near-wake revealed notable differences between configurations with and without these devices. Without them, the wake remained concentrated in the actuator surface’s projected downstream area, with minimal crossflow diffusion. Conversely, the configuration with lift-generating devices exhibited significant wake deformation, including axial expansion and lateral contraction, promoting streamwise momentum recovery. As a result, increased power recovery was found downstream of such a system compared to the clean MRS. These findings underscore the potential of such systems, particularly when equipped with lift-generating devices, to manipulate wake dynamics and enhance wind farm efficiency, thereby advancing innovative wind energy solutions.

This report describes the experimental results conducted for the VAWT model (Generation 1) around the impact of secondary actuator mesh disks on the wingtip vortices as well as the aerodynamic loading behavior of the rotor.

We present a method to adapt Greenberg's potential flow model for coupled pitching and surging flow such that it can be applied to predict the loads on a vertical-axis wind turbine blade. The model is extended to compute loads on a blade undergoing multi-harmonic oscillations in effective angle of attack and incoming flow velocity by formulating the blade kinematics as a sum of simple harmonic motions. Each of these functions is a multiple of the main turbine rotational frequency, associated with an individual amplitude, as suggested by Greenberg. The results of the adapted model are compared with experimental data from a scaled-down model of a single-bladed H-type Darrieus wind turbine. The comparison between the predictions by the Greenberg model and experimentally obtained phase-averaged radial force evolutions show that the inviscid Greenberg model predicts well the loads at the start of the upwind portion and the maximum loads during upwind, but fails during the downwind portion when flow separation occurs. The proposed application of Greenberg's model to vertical-axis wind turbine kinematics shows a great potential to diagnose regions of separated flow and for quantifying the relative influences of dynamic stall and intrinsic turbine kinematics on the blade loading. Future research can readily extend this method to any airfoil undergoing an arbitrary combination of pitching, surging, and heaving, following a kinematic profile that can be approximated by a Fourier series.

This dataset is intended to support the findings of the publication "A study of the near wake deformation of the X-Rotor vertical-axis wind turbine with pitched blades" by Bensason, Sciacchitano, and Giri Ajay, and Ferreira (DOI: 10.22541/au.170664317.75532826/v1). The dataset includes wake data obtained using Particle Image Velocimetry (PIV) in the open-jet wind tunnel facility of the TU Delft (OJF) for a scaled X-Rotor vertical-axis wind turbine (VAWT). The near wake of the rotor is measured with fixed-pitch offsets applied to the blade to demonstrate a wake recovery mechanism for the X-Rotor concept. Furthermore, numerical simulation data is provided to highlight the impact of this wake recovery strategy on the blade level loading. This research and dataset aim to provide an experimental database for the future and ongoing development of numerical models for this novel X-Rotor VAWT. Furthermore, it demonstrates the wakerecovery strategy for VAWTs, which uses fixed blade pitch offsets.

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.

Abstract The Horizon 2020 European Commission-funded project - X-ROTOR - proposes a radical rethink of the traditional vertical-axis wind turbine geometry. The X-Rotor vertical axis wind turbine relies on blade-tip mounted rotors, referred to as secondary rotors, for power generation and takeoff. This study examines the aerodynamic effects of secondary rotors on a scaled X-Rotor model’s loading in an open-jet wind tunnel. Particle image velocimetry measurements are taken at two cross-stream planes within the volume of rotation of a scaled turbine model at two phase-locked positions. The measurements are compared with cases without secondary rotors present to understand the local impact of the blade-tip mounted devices on the wake and vortex strengths. The results indicate an accelerated turbulent diffusion of the trailing tip-vortex of the X-Rotor, and the subsequent local in-plane velocity gradients induced by the trailing tip-vortex are diminished. These insights and experimental database contribute to the development and validation of numerical models of the X-Rotor with blade-tip mounted rotors.