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Large-eddy simulation (LES) of a wind turbine under uniform inflow is performed using an actuator line model (ALM). Predictions from four LES research codes from the wind energy community are compared. The implementation of the ALM in all codes is similar and quantities along the blades are shown to match closely for all codes. The value of the Smagorinsky coefficient in the subgrid-scale turbulence model is shown to have a negligible effect on the time-averaged loads along the blades. Conversely, the breakdown location of the wake is strongly dependent on the Smagorinsky coefficient in uniform laminar inflow. Simulations are also performed using uniform mean velocity inflow with added homogeneous isotropic turbulence from a public database. The time-averaged loads along the blade do not depend on the inflow turbulence. Moreover, and in contrast to the uniform inflow cases, the Smagorinsky coefficient has a negligible effect on the wake profiles. It is concluded that for LES of wind turbines and wind farms using ALM, careful implementation and extensive cross-verification among codes can result in highly reproducible predictions. Moreover, the characteristics of the inflow turbulence appear to be more important than the details of the subgrid-scale modeling employed in the wake, at least for LES of wind energy applications at the resolutions tested in this work.

We study dynamic induction control for mitigating the wake losses of a pair of inline wind turbines. In order to explore control strategies that account for unsteady interactions with the flow, we employ optimal control and adjoint-based optimization in combination with large-eddy simulations. The turbines are represented with an actuator line model. We consider a simple uniform inflow case with two NREL 5 MW turbines spaced 5 diameters apart and find that optimal control leads to 25% gains compared to standard Maximum-Power-Point Tracking (MPPT). It is further found that only the control dynamics of the first turbine are changed, improving wake mixing, while the second turbine controller remains close to the MPPT control. We further synthesize the optimal generator torque and blade pitch controls of the first turbine into a signal that can be periodically used as an open-loop controller, with a Strouhal number of 0.38, while realizing the same gains as the original optimal control signal. Further analysis of the improved wake mixing resulting from the open-loop signal reveals periodic shedding of a three-vortex ring system, which interacts and merges downstream of the first turbine, increasing entrainment of high-speed momentum into the wake. The sensitivity of the open-loop signal to inlet turbulence levels and turbine spacing is also investigated. At low to medium turbulence levels, the control remains effective, while at higher levels, the coherence of the vortex rings degrades too fast for them to remain effective.

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