Cross-scale aerodynamics and wake coherent structures of vertical-axis wind turbines via data-driven modal decomposition

Abstract

This study investigates the aerodynamic characteristics and wake evolution of vertical-axis wind turbines (VAWTs) at laboratory and engineering scales. The improved delayed detached eddy simulation method is employed, with aerodynamic validation first performed against published experimental data for a small-scale two-blade VAWT. Subsequently, the aerodynamic performance of a small-scale three-blade configuration is evaluated. A megawatt-scale three-bladed VAWT is then conceptually designed based on the Weibull wind model and offshore wind-resource conditions. Finally, proper orthogonal decomposition (POD) and spectral POD (SPOD) are employed to characterize the wake coherent structures of the two configurations. The small-scale configuration reaches a maximum power coefficient of 0.285, whereas the megawatt-scale configuration reaches 0.36 and exhibits stronger leading-edge suction and altered separation behavior under its design condition. The higher Reynolds number contributes to the changed boundary-layer behavior, while the concurrent differences in solidity, aspect ratio, geometric proportions, and optimal tip-speed ratio also affect the aerodynamic response. At the macroscopic level, the small-scale wake exhibits gradual outward dissipation, whereas the large-scale wake demonstrates significant centripetal momentum transport and intense core mixing. POD analysis reveals that large-scale energy-containing vortices break down faster, significantly dispersing modal energy, with the first-order mode energy proportion dropping from 39% to 24%. Furthermore, SPOD analysis demonstrates that small-scale wake evolution is dominated by mechanical rotational forcing at 1P (2.419 Hz) and 3P (7.258 Hz) frequencies. In contrast, the large-scale wake exhibits a separation of characteristic spatial and temporal scales. The blade-passing-related coherent structures remain concentrated near the rotor, whereas slowly evolving large-scale structures become more prominent in the mid-to-far wake. These findings characterize the aerodynamic and wake phenomena of representative VAWTs operating in two distinct engineering-scale regimes.