Advances in wind-plant control have often focused on more effectively balancing power between neighboring turbines. Wake steering is one such method that provides control-based improvements in a quasi-static way, but this does little to fundamentally change the wake recovery process, and thus, it has limited potential. This study investigates use of another control paradigm known as dynamic wake control (DWC) to excite the mutual inductance instability between adjacent tip-vortex structures, thereby accelerating the breakdown of the structures. The current work carries this approach beyond the hypothetical by applying the excitation via turbine control vectors that already exist on all modern wind turbines: blade pitch and rotor speed control. The investigation leverages a free-vortex wake method (FVWM) that allows a thorough exploration of relevant frequencies and amplitudes of harmonic forcing for each control vector (as well as the phase difference between the vectors for a tandem configuration) while still capturing the essential tip-vortex dynamics. The FVWM output feeds into a Fourier stability analysis working to pinpoint candidate DWC strategies suggesting fastest wake recovery. Near-wake length reductions of >80% are demonstrated, although without considering inflow turbulence. Analysis is provided to interpret these predictions considering the presence of turbulence in a real atmospheric inflow.
The complexity and associated uncertainties involved with atmospheric-turbine-wake interactions produce challenges for accurate wind farm predictions of generator power and other important quantities of interest (QoIs), even with state-of-the-art high-fidelity atmospheric and turbine models. A comprehensive computational study was undertaken with consideration of simulation methodology, parameter selection, and mesh refinement on atmospheric, turbine, and wake QoIs to identify capability gaps in the validation process. For neutral atmospheric boundary layer conditions, the massively parallel large eddy simulation (LES) code Nalu-Wind was used to produce high-fidelity computations for experimental validation using high-quality meteorological, turbine, and wake measurement data collected at the Department of Energy/Sandia National Laboratories Scaled Wind Farm Technology (SWiFT) facility located at Texas Tech University's National Wind Institute. The wake analysis showed the simulated lidar model implemented in Nalu-Wind was successful at capturing wake profile trends observed in the experimental lidar data.
Advances in wind plant control have often focused on more effectively balancing power between neighboring turbines. Wake steering is one such method that provides control-based improvements in a quasi-static way, but this fundamentally does not change the downstream wake deficit and thus, can only provide limited improvement. Another control paradigm is to leverage the turbine as a flow actuator to dynamically excite unstable modes in the wake, thereby producing accelerated wake breakdown and recovery. Taking a more applied approach than some studies in the wake instability area, this article investigates the use of dynamic wake control (DWC) from two existing turbine control vectors, blade pitch and rotor speed, to incite rapid breakdown of the tip vortex structures. Both control vectors can be dynamically manipulated to make a significant difference on the wake structure and breakdown. The mid-fidelity free-vortex wake method (FVWM) used below allows a thorough search of the parametric space while still capturing the essential physics of the mutual inductance instability. The parameters for investigation include the frequency, amplitude, and phase of the harmonic forcing for both control vectors. The output from the FVWM is the basis for a Fourier stability analysis, which is used to pinpoint and quantify candidate forcing strategies with the highest instability growth rates and shortest near-wake lengths. The strategies, including dynamic rotor speed, blade pitch, and a novel tandem configuration, work to augment the initial tip vortex instability magnitude, leading to near-wake length reductions of greater than 80%, though without considering inflow turbulence. Analysis is provided to interpret these predictions considering the presence of inflow turbulence in a real atmosphere.