Large eddy simulation of wake mixing control strategies in offshore wind farms

Abstract

Wake-turbine interaction reduces power output in wind farms, especially in aligned layouts where downstream turbines operate in the wake of upstream units. This study evaluates established wake mixing control strategies, including dynamic induction control (DIC), baseline dynamic individual pitch control (DIPC), and a new higher-order DIPC method for enhancing wake recovery in floating offshore wind turbines. Wall-modeled large eddy simulation, coupled with an actuator line model, is employed to simulate a two-turbine wind farm under a neutrally stratified turbulent atmospheric boundary layer. The flow fields are analyzed using proper orthogonal decomposition, mean kinetic energy flux, and Fourier transformation techniques, which revealed that the DIPC-based methods significantly enhance wake mixing through a higher energy entrainment and tip-vortex breakdown, especially in below-rated wind region. The higher-order DIPC method, which combines two sinusoidal blade pitch signals, exhibits greater energy flux as well as a notable wake deflection that can improve downstream turbine performance. Compared to the conventional Greedy method, the wake mixing control methods generally increase the growth rates of mutual inductance instability modes that accelerate tip-vortex breakdown. In terms of wind turbine performance, the higher-order DIPC method achieves a 7.9% increase in total power relative to the Greedy case and 2.3% more than the baseline DIPC, while having slightly higher structural load and comparable platform motions. These findings demonstrate the potential of wake forcings to increase wake recovery and power yield in offshore wind farms, showing the dominant modes in the blade pitching frequency and their effect on exciting floating platform motions.