**5. Prospects for the future**

22 Computational and Numerical Simulations

Figure is modified from Lu and Porté-Agel [59].

(a) (b)

is due to the enhancement in mean shear at the top-tip level.

Porté-Agel [59].

**Figure 18.** Vertical distribution of turbulent kinetic energy (measured over the last 15 min) obtained from the baseline case (solid), the 8D case (dashed) and the 5D case (dash dotted) through the axis of the turbine at different downwind locations.

**Figure 19.** Vertical distributions of (a) x-direction momentum flux and (b) buoyancy flux. Figure is modified from Lu and

turbines are placed in the boundary layer, turbulence is reduced below the turbine bottom and significantly enhanced in the wake region; this observation agrees with single turbine experimental results [19]. As revealed in other researches on wind-turbine wakes in shear flows including experiments [19, 92] and simulations [90], the large values of turbulence intensity are found at both the top-tip and bottom-tip levels, and the turbulence intensity above the hub-height is generally larger than that below the hub-height. It is argued that this

Besides extracting kinetic energy and generating turbulence, wind-turbine blade motions also mix fluid parcels. The investigation of fluxes is of interest because local meteorology is considerably affected by the overall exchanges of momentum, heat, moisture, etc. Figure 19 shows the vertical distributions of mean total (resolved part plus SGS part) vertical flux of axial momentum and heat. The results obtained from the baseline case are consistent with This chapter gives an overview of our recent research efforts aimed at improving parameterizations and making LES a more reliable technique to planetary boundary layer research. Large-eddy simulation has shown its capabilities in simulations of high-Reynolds-number flows that, at present, could not be solved by DNS. It has been proved to be very useful in understanding the turbulent exchange in atmosphere and ultimately in parameterization improvement in traditional meteorological models; and also it assists theoreticians and weather/climate modelers with reliable information about the averaged vertical structure of the ABL, as well as with better estimations of key ABL parameters. The outlook for using LES in planetary boundary layer modeling is very good. The number of high-quality LES studies is rapidly increasing.

The need for accurate simulation has provided much of the impetus for the development of numerical methods in turbulence research. The proposed new nonlinear formulation has been examined in LESs of several types of turbulent flows. The new SGS closure presents a significant improvement with respect to simple eddy-viscosity/diffusivity-type models, also delivers more accurate representation of the energy cascade in the inertial sub-range.

Possible future model modifications of the new SGS closure include the development of dynamic and scale-dependent dynamic procedures to optimize the values of the model coefficients using information of the resolved scales. Moreover, one could develop and assess more advanced modifications (e.g., one-equation models), which could offer alternatives to relax some of the model assumptions.

Further, the next stage of wind-energy application will encompass more realistic physics and a variety of atmospheric conditions. These include the consideration of other inflow and surface boundary conditions, wind-farm configurations, and the effects of topography, air moisture, and the like. Future studies will use the LES framework to further study the effects of wind-farm size, atmospheric stability (neutral, convective and stable), topography, and wind-farm configuration. Also, there is a need for reliable coupling of LES with weather models to account for the effects of large-scale atmospheric forcing.
