Wake dynamics and coherence modes of vertical-axis wind turbines : the role of atmospheric boundary layer

Abstract

Vertical-axis wind turbines (VAWTs) are increasingly recognized as the preferred choice for large-scale wind energy harvesting, particularly in offshore environments, due to their unique advantages, including omni-directional capability and lower installation and maintenance costs. Variations in terrain and different phases of the diurnal cycle create distinct atmospheric boundary layer (ABL) conditions, which inevitably have a significant impact on the aerodynamic and wake characteristics of VAWTs. To systematically observe these effects, this study employs large-eddy simulation to explore the influence of four representative ABL scenarios on VAWTs. The results indicate that ABL influences on VAWT aerodynamic performance are sensitive to installation height. ABLs with higher wind shear coefficients (WSCs) result in greater velocity deficits (VDs) in near-wake regions, while turbulence intensity (TI) fluctuations increase with rising WSC. Higher installation heights facilitate a faster recovery of both VD and TI. Furthermore, vortex stability is affected by ABL conditions and installation height, as higher WSCs or lower heights bring unsteady vortex break positions closer to the rotor, thus enhancing turbulence. Modal decomposition reveals that the dominant mode frequency across various ABL conditions corresponds to twice the VAWT rotation frequency, highlighting the dynamic evolution of wake vortices. These findings provide valuable insights for the optimization of VAWT wind farm design, particularly in integrating ABL variability into the determination of hub height and turbine spacing strategies, thereby maximizing energy harvesting.