The Wakes of Novel Vertical-Axis Wind Turbines

Abstract

Offshore wind energy has gained prominence due to its favorable wind resources, turbine scaling potential, and vast installation areas. However, wake losses caused by closely spaced wind turbines remain a significant challenge, reducing overall wind farm efficiency. This thesis explores the application of vertical-axis wind turbines (VAWTs) geometries for maximizing energy density through passive wake recovery techniques. In addition to traditional Darrieus-type VAWTs, the efficacy of passive wake control techniques for novel multi-rotor systems (MRS), namely the X-Rotor and block design concepts, is assessed through experimental studies and proof-of-concept demonstrations.

The thesis begins with a large-scale experimental investigation into the wake dynamics of a high-energy-density VAWT wind farm, providing the first comprehensive dataset of three-dimensional, time-averaged flowfield measurements. A dense grid of nine Htype VAWTs with fixed spacing was analyzed, exploring a passive wake control strategy known as the "vortex generator" mode, where blade pitch is modified to accelerate wake re-energization. Two pitch configurations were tested: positive (pitched-in) and negative (pitched-out). The positive pitch case exhibited significant momentum influx from above and below the rotor, along with lateral wake deflection. In contrast, the negative pitch case induced upwash while injecting high-momentum flow from the sides. Wake recovery was quantified by assessing available power, showing a maximum of 72.4% recovery three diameters downstream in the positive pitch case, 6.4 times higher than the baseline. The negative pitch case reached a 53% recovery four diameters downstream, a 2.1-fold improvement over the baseline. These findings highlight the potential of passive wake control strategies to enhance wind farm energy density.
The X-Rotor introduces an innovative design featuring an X-shaped VAWT, referred to as the "primary rotor," and blade-tip-mounted HAWTs, known as the "secondary rotors." This design employs an "aerodynamic gearbox" mechanism, where the primary rotor extracts mechanical power while the secondary rotors drive electrical generators at the blade tips. This thesis presents the first experimental wake measurements of the X-Rotor, revealing that its wake remains concentrated within its projected frontal area, shaped by the coned blades. The shed vorticity follows an elliptical pattern, inducing crossflow components along the height. This dataset provides a baseline for evaluating the secondary rotors’ impact on wake evolution near the bottom blade tips and demonstrating the aerodynamic gearbox mechanism.