Large eddy simulation study of scalar transport in fully developed wind-turbine array boundary layers

become larger, they begin to attain scales at which two-way interactions with the atmospheric boundary layer (ABL) must be taken into account. Several studies have shown that there is a quantifiable effect of wind farms on the local meteorology, mainly through changes in the land-atmosphere fluxes of heat and moisture. In particular, the observed trends suggest that wind farms increase fluxes at the surface and this could be due to increased turbulence in the wakes. Conversely, simulations and laboratory experiments show that underneath wind farms, the friction velocity is decreased due to extraction of momentum by the wind turbines, a factor that could decrease scalar fluxes at the surface. In order to study this issue in more detail, a suite of large eddy simulations of an infinite (fully developed) wind turbine array boundary layer, including scalar transport from the ground surface without stratification, is performed. Results show an overall increase in the scalar fluxes of about 10%–15% when wind turbines are present in the ABL, and that the increase does not strongly depend upon wind farm loading as described by the turbines’ thrust coefficient and the wind turbines spacings. A single-column analysis including scalar transport shows that the presence of wind farms can be expected to increase slightly the scalar transport from the bottom surface and that this slight increase is due to a delicate balance between two strong opposing trends.

Study of a Fully Developed Wind Turbine Array Boundary Layer

Marc Calaf Bracons

Wind harvesting is fast becoming an important alternative source of energy. As wind farms become larger, they begin to attain scales at which two-way interactions with the atmospheric boundary layer (ABL) must be taken into account. Several studies (Baidya-Roy et al. (2004); Baidya-Roy and Traiteur (2010)) have shown that there is a quantifiable effect of wind farms on the local meteorology, mainly through changes in the land-atmosphere fluxes of heat and moisture. Also, it is well-known that when wind turbines are deployed in large arrays, their efficiency decreases due to complex interactions among themselves and with the ABL. For wind farms whose length exceeds the height of the ABL by over an order of magnitude, a "fully developed" flow regime can be established. In this asymptotic regime, changes in the stream-wise direction can be neglected and the relevant exchanges occur in the vertical direction. Such a fully developed wind-turbine array boundary layer (WTABL) has not been studied systematically before. Now, a suite of Large Eddy Simulations (LES), in which wind turbines are modeled using the classical "drag disk" concept, are performed for various wind turbine arrangements, turbine loading factors, and surface roughness values. Further, simulations including scalar transport from the ground surface without stratification, are also performed. The results are used to quantify the vertical transport of momentum and kinetic energy across the boundary layer. It is shown that the vertical fluxes of kinetic energy are of the same order of magnitude as the power extracted by the forces modeling the wind turbines. In the fully developed WTABL, the kinetic energy extracted by the wind turbines is transported into the wind turbine region by vertical fluxes associated with turbulence. The results are also used to develop improved models for effective roughness length scales experienced by the ABL. The effective roughness scale is often used to model wind turbine arrays in simulations of atmospheric dynamics at larger (regional and global) scales. Results from the LES are compared with several existing models for effective roughness lengths. Based on the observed trends, a modified model is proposed showing improvement in predicted effective roughness length. Results also show an overall increase in the scalar fluxes of about 10-15% when wind turbines are present in the ABL, and that the increase does not strongly depend upon wind farm loading as described by the turbines' thrust coefficient and the wind turbines spacings. Similiarly as for the effective roughness length scales, a single-column analysis including now scalar transport, confirms that the presence of wind farms can be expected to increase slightly the scalar transport from the bottom surface. This slight increase is due to a delicate balance between two strong opposing trends: an increase of the friction velocity above the wind turbines, together with a decrease underneath.

EPFL2011

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 study of a fully developed thermal wind-turbine array boundary layer

Marc Calaf Bracons, Charles Vivant Ignacio Meneveau, Marc Parlange

When wind turbines are arranged in large wind farms, their efficiency decreases significantly due to wake effects and to complex turbulence interactions with the atmospheric boundary layer (ABL) [1]. For large wind farms whose length exceeds the ABL height by over an order of magnitude, a “fully developed” flow regime may be established [1, 2, 3]. In this asymptotic regime, the changes in the stream-wise direction are small compared to the more relevant vertical exchange mechanisms. Such a fully developed wind-turbine array boundary layer (WTABL) has recently been studied [2] using Large Eddy Simulations (LES) under neutral stability conditions. The simulations showed the existence of two log-laws, one above (characterized by: uhi ∗ , zhi o ) and one below (ulo ∗ , zlo o ) the wind turbine region. This enabled the development of more accurate parameterizations of the effective roughness scale for a wind farm. Now, a suite of Large Eddy Simulations, in which wind turbines are modeled as in [2] using the classical drag disk concept are performed, again in neutral conditions but also considering temperature. Figure 1 shows a schematic of the geometry of the simulation. The aim is to study the effects of different thermal ABL stratifications, and thus to study the efficiency and characteristics of large wind farms and the associated land-atmosphere interactions for realistic atmospheric flow regimes. Such studies help to unravel the physics involved in extensive aggregations of wind turbines, allowing us to design better wind farm arrangements. As a first step, temperature is treated in a passive mode, allowing us to focus the study on the influence of a large WFABL into the scalar fluxes. By considering various turbine loading factors, surface roughness values and different atmospheric stratifications, it is possible to analyze the influence of these parameters on the induced surface roughness, and the sensible heat roughness length. These last two parameters can be used to model wind turbine arrays in simulations of atmospheric dynamics at larger (regional and global) scales [4], where the coarse meshes used do not allow to account for the specifics of each wind turbine. Results from different sets of simulations are presented, for which also the corresponding effective roughness length-scales can be determined. The results also help our understanding of how wind turbines affect scalar transport processes in the turbine wakes. By using a simple drag disk approach for modeling the wind turbines, it is found that the surface heat flux inside the thermal wind-turbine array boundary layer is increased. This is the result of two competing effects: (1) a major increase on u∗,hi; (2) a smaller decrease due to lower u∗,lo near the ground.

Springer2011