Large eddy simulation and linear stability analysis of active sway control for wind turbine array wake

Abstract

The convective instability of wind turbine wakes allows specific upstream forcing to amplify downstream, leading to increased wake meandering and replenishment, thereby providing a theoretical basis for active wake control. In this study, the active sway control—a strategy previously proven to enhance wake recovery at the single wind turbine level—is analyzed at the turbine array level. The similarity and differences between individual turbine wakes and the wake array are analyzed using large eddy simulations and linear stability analysis, considering both uniform and turbulent inflow conditions. For cases with uniform inflow, large eddy simulations reveal significant meandering motion in the wake array induced by active sway control at a motion amplitude of 1% rotor diameter, consistent with previous studies of standalone wind turbine wakes. Nevertheless, the sensitive frequency for the wake array extends down to St = 0.125 below the limit of St > 0.2 for a single wake, and the optimal control frequency for the standalone turbine wake becomes suboptimal for the array. Linear stability analysis reveals the underlying mechanism of this frequency shift as changes in the shear-layer instability due to the overlap of upstream and downstream wakes and is capable to provide fast estimation of optimal control frequencies. When inflow turbulence intensity increases, the gain of active sway control is reduced, underscoring the importance of low-turbulence environment for successfully implementing the active sway control. The reduction in wake response is captured by the linear stability analysis if the base flow accounts for the faster wake expansion caused by inflow turbulence.