CFD modeling of wind turbine wakes

Accurate predictions of the atmospheric properties are of paramount importance for many engineering applications, such as wind turbine wakes. The simulations of the atmospheric boundary layer (ABL) is a big challenge due to the fact that, with an exception of the ground boundary, it does not have any physical borders (e.g. such as flows in pipes) and consequently reasonable distances from the geometry of interest must be taken. The simulation of the flow near the ground also is challenging because, usually, ground consists of anomalies such as small vegetation, various sized stones etc. which renders impossible the simulation of it. Another big challenge in ABL simulations for both RANS (Reynolds Averaged Navier – Stokes) simulations and LES (Large Eddy Simulations) is the maintenance of inflow conditions throughout the computational domain. The streamwise distance of the computational domain can be of the order of magnitude of many kilometers, especially in wind farm simulations, and consequently huge distortions of the inlet profiles may occur. A fully developed inlet profile must be imposed at the inlet and be maintained throughout the domain, at least for a uniform roughness terrain. While the problem appears to be similar for both RANS and LES, the treatment is far different due to the different nature of RANS and LES approaches. RANS deals with average quantities and the problem with the maintenance of the inlet quantities is a boundary condition and turbulence modeling problem. LES on the other hand, does not include any extra equations (for the resolved part of the flow), consequently the maintenance of the inlet properties appears to be only a boundary condition problem. However, it is not only a boundary condition problem but the way that the turbulence is generated at the inlet plays a vital role, as well as the grid resolution, which makes the situation even more complicated. This thesis is specialized in the simulations of the surface layer of the neutral ABL in both RANS and LES and interactions of the properties of the neutral atmosphere with wind turbine wakes are investigated. Modifications to various RANS models are considered to make the models be mathematically consistent with the properties of neutral atmosphere in order to successfully simulate the neutral ABL by eliminating any streamwise gradients within the empty domain. Then, the adequacy of the modified models is investigated for the simulation of 2 different small wind turbine wakes. Finally, the problem of the inflow generation with synthetic methods for the simulation of the surface layer of the ABL is presented for LES. Periodic boundary conditions are employed to overcome this difficulty along with an extension of a shear stress boundary condition at the upper boundary of the domain in order to maintain the turbulence within the domain. The results of the proposed models show good agreement when compared with available measurements and also improvements when compared with other models found in the literature, especially in the far wake region of the wind turbines as they have the correct recovery of the wake for 2 different wind turbines for various different inlet conditions.