Large Eddy Simulation study of fully developed thermal wind-turbine array boundary layers

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 atmospheric boundary layer (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 recently been studied1 using Large Eddy Simulations (LES) under neutral stability conditions. The simulations showed the existence of two log-laws, one above and one below 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 also modeled using the classical “drag disk” concept are performed but for non-neutral conditions. 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. By considering various turbine loading factors, surface roughness values and different atmospheric stratifications, it is possible to analyze the influence of these into the induced surface roughness, and the sensible heat roughness length. These last two can be used to model wind turbine arrays in simulations of atmospheric dynamics at larger (regional and global) scales2, where the coarse meshes used do not allow to account for the specifics of each wind turbine. Results from different sets of simulations under stable and unstable conditions will be presented, for which also the corresponding effective roughness length-scales will be determined. Simulations use imposed heat flux at the bottom or an imposed temperature. The simulation results will be analyzed to determine how stratification affects momentum and scalar transport processes in the wind turbine wakes.