Altering Kinetic Energy Entrainment in Large Eddy Simulations of Large Wind Farms Using Unconventional Wind Turbine Actuator Forcing

Abstract

In this study, horizontally periodic large eddy simulations (LES) are utilized to study turbulent atmospheric boundary-layer flow over wind turbines in the far-downstream portion of a large wind farm where the wakes have merged and the flow is fully developed. In an attempt to increase power generation by enhancing the mean kinetic energy (MKE) entrainment to the wind turbines, hypothetical synthetic forcing is applied to the flow at the turbine rotor locations. The synthetic forcing is not meant to represent any existing devices or control schemes, but rather acts as a proof of concept to inform future designs. The turbines are modeled using traditional actuator disks, and the unconventional synthetic forcing is applied in the vertical direction with the magnitude and direction dependent on the instantaneous velocity fluctuation at the rotor disk; in one set of LES meant to enhance the vertical entrainment of MKE, a downward force is prescribed in conjunction with a positive axial velocity fluctuation, whereas a negative axial velocity fluctuation results in an upward force. The magnitude of the forcing is proportional to the instantaneous thrust force with prefactors ranging from 0.1 to 1. The synthetic vertical forcing is found to have a significant effect on the power generated by the wind farm. Consistent with previous findings, the MKE Energies 2015, 8 371 flux to the level of the turbines is found to vary along with the total power produced by the wind turbine array. The reverse strategy of downward forcing of slow axial velocity flow is found to have almost no effect on the power output or entrainment. Several of the scenarios tested, e.g., where the vertical force is of similar magnitude to the horizontal thrust, would be very difficult to implement in practice, but the simulations serve the purpose of identifying trends and bounds on possible power increases from flow modifications through action at the turbine rotor.

Dynamic Strategies for Yaw and Induction Control of Wind Farms Based on Large-Eddy Simulation and Optimization

“…In practice, this can be achieved either through individual pitch control, tilt control or through yaw control [15]. Notwithstanding the fact that individual pitch control and, to a lesser extent, tilt control both show potential for increasing power and/or reducing loads [16][17][18], we will focus on wake redirection for power optimization through yaw control in the remainder of this discussion. Although nominally wind turbines are operated such that the rotor is perpendicular to the incoming flow, an intentional yaw misalignment can be used to induce transversal forces on the incoming flow.…”

Model‐based receding horizon control of wind farms for secondary frequency regulation

“…Given the complexity of the aerodynamic interactions between turbines, more work is needed to develop effective controls for secondary frequency regulation. This area of research is just beginning to be explored by studying the potential for control using time‐dependent models and aerodynamic control variables, such as thrust modulation through pitch or generator torque control, yawing or tilting. Receding horizon control techniques have been used successfully for similar problems, including drag reduction in turbulent boundary layers and power maximization of wind farms.…”

Unraveling the Mysteries of Turbulence Transport in a Wind Farm

“…The Reynolds stress component, R12, or the statistical correlation between u' and v', is an indicator of the meandering behavior of the wake; the correlation between u' and w', i.e., R13, can be used to understand the entrainment from more energetic regions of the ABL above the turbine and wake region. This has been discussed extensively in the work by VerHulst and Meneveau [19]. The combined effect of the self-correlation of fluctuations in different velocity components (Rii) is a proportional measure of the turbulent kinetic energy (TKE).…”

“…Load statistics, spectra, and wavelet analysis representations for different space and time resolutions were studied. VerHulst and Meneveau [19] studied the kinetic energy entrainment in a large wind farm using LES and the lower-order actuator disk method. They found that synthetic vertical forcing at the turbines can have a significant effect on the power extraction and the mean kinetic energy entrainment in a large wind turbine array.…”