Interaction between large wind farms and the atmospheric boundary layer

Accurate prediction of atmospheric boundary layer (ABL) flow and its interactions with wind turbines is of great importance for optimizing the design and efficiency of wind farms. This study first focuses on recent efforts to develop and validate a large-eddy simulation (LES) framework for wind-energy applications. The subgrid-scale turbulent fluxes of momentum and heat are parameterized using tuning-free dynamic models. The turbine-induced forces are parameterized using two types of models: an actuator disk model that allows for non-uniform force distribution and includes rotational effects, and an actuator line model. The LES framework is validated against wind-tunnel measurements collected inside and above a large model wind farm. Further, this framework is used to study wind-farm effects. Comparison of simulations of flow through both aligned and staggered wind farms shows important effects of farm layout on the flow structure and wind-turbine performance. We also investigate the impacts of wind farms on a stable ABL and a convective ABL.

Hao Lu, Fernando Porté Agel, Yu-Ting Wu

2012

Large-eddy simulation of atmospheric boundary layer flow through wind turbines and wind farms

Hao Lu, Fernando Porté Agel, Yu-Ting Wu

Accurate prediction of atmospheric boundary layer (ABL) flow and its interactions with wind turbines and wind farms is critical for optimizing the design (turbine siting) of wind energy projects. Large-eddy simulation (LES) can potentially provide the kind of high-resolution spatial and temporal information needed to maximize wind energy production and minimize fatigue loads in wind farms. However, the accuracy of LESs of ABL flow with wind turbines hinges on our ability to parameterize subgrid-scale (SGS) turbulent fluxes as well as turbine-induced forces. This paper focuses on recent research efforts to develop and validate an LES framework for wind energy applications. SGS fluxes are parameterized using tuning-free Lagrangian scale-dependent dynamic models. These models optimize the local value of the model coefficients based on the dynamics of the resolved scales. The turbine-induced forces (e.g., thrust, lift and drag) are parameterized using two types of models: actuator-disk models that distribute the force loading over the rotor disk, and actuator-line models that distribute the forces along lines that follow the position of the blades. Simulation results are compared to wind-tunnel measurements collected with hot-wire anemometry in the wake of a miniature three-blade wind turbine placed in a boundary layer flow. In general, the characteristics of the turbine wakes simulated with the proposed LES framework are in good agreement with the measurements in the far-wake region. Near the turbine, up to about five rotor diameters downwind, the best performance is obtained with turbine models that induce wake-flow rotation and account for the non-uniformity of the turbine-induced forces. Finally, the LES framework is used to simulate atmospheric boundary-layer flow through an operational wind farm. (C) 2011 Elsevier Ltd. All rights reserved.

Elsevier2011

Large-Eddy Simulation of Turbulent Boundary Layer Flow through Wind Turbines and Wind Farms

Yu-Ting Wu

In this thesis, we develop a new large-eddy simulation (LES) framework to simulate atmospheric boundary layer (ABL) flows through wind turbines and wind farms. A Lagrangian scale-dependent dynamic model is used to compute the Smagorinsky model coefficient dynamically based on the local dynamics of the resolved velocity field. The turbine-generated power outputs and the turbine-induced forces (e.g., thrust, lift, drag) are parameterized using two actuator-disk models: (a) the traditional actuator-disk model without rotation (ADM-NR), which uses 1D momentum theory to relate the power output and the uniform thrust force distribution with an incoming velocity; and (b) the actuator-disk model with rotation (ADM-R), which adopts blade element theory to calculate the lift and drag forces (that produce thrust, rotor shaft torque, and power) based on the local blade and flow characteristics. The performance of the LES framework is first evaluated through comparisons with high-resolution velocity measurements collected in the wind-tunnel experiments with the single wake of a stand-alone miniature wind turbine [Chamorro and Porté-Agel, 2010a] as well as the multiple turbine wakes in a model aligned wind farm [Chamorro and Porté-Agel, 2011]. Emphasis is placed on the structure and characteristics of the simulated turbine wakes using the ADM-NR and the ADM-R. In general, the ADM-R yields improved predictions compared with the ADM-NR in the wake regions, where the ADM-NR tends to underestimate the velocity deficit and the enhancement of turbulence intensity in the wakes. Next, the LES framework is used to model multiple turbine wakes and associated power losses in a large wind farm. Here, we propose a new dynamic procedure coupled with the ADM-R to predict turbine power output based on a turbine-model-specific relationship between the shaft torque and the blade angular speed. The Horns Rev offshore wind farm is chosen as a case study since both power data from eighty Vestas V80 wind turbines and flow data from three nearby meteorological masts were available [Barthelmie et al., 2009]. The torque-speed relationship for the V80 turbine is obtained from a series of stand-alone turbine simulations. In the wind farm simulations, the ADM-R gives the best power prediction. Finally, the validated LES framework is used to study atmospheric turbulence effects on wind-turbine wakes in neutrally-stratified ABL over homogeneous flat surfaces with four different aerodynamic roughness lengths. In this study, the simulation result shows that the different turbulence intensity levels of the incoming flow lead to considerable influence on the spatial distribution of the mean velocity deficit, turbulence intensity, and turbulent shear stress in the wake region. In particular, when the turbulence intensity level of the incoming flow is higher, the turbine-induced wake (velocity deficit) recovers faster, and the locations of the maximum turbulence intensity and turbulent stress are closer to the turbine.

EPFL2013

Large-Eddy Simulation of Turbulent Boundary Layer Flow through Wind Turbines and Wind Farms

Yu-Ting Wu

EPFL2013

Large-eddy simulation of atmospheric boundary layer flow through wind turbines and wind farms

Hao Lu, Fernando Porté Agel, Yu-Ting Wu

Elsevier2011