CFD Studies on Wind Turbine Interactions with the Turbulent Local Flow Field Influenced by Complex Topography and Thermal Stratification

Abstract

Abstract. This paper shows the results of CFD studies of turbulent flow fields and their effects on a wind turbine in complex terrain. As part of the WINSENT project a research test site comprising four meteorological masts and two research wind turbines is currently being constructed in the Swabian Alps in Southern Germany. This work is an essential part of the research of the Southern German wind energy research cluster WindForS. The terrain site is characterised by a densely forested escarpment and a flat plateau downstream of the slope. The met masts and wind turbines are built on this plateau. In the first part, high-resolution CFD simulations are performed to separately investigate the effects of the forested escarpment and of thermal stratification on the flow field and on the wind turbine accordingly. In the second part, all the examined effects are combined for a real-life case dated March 2021. There, unstable conditions prevailed and the forest shows low leaf area densities due to the wintertime. It is shown that atmospheric turbulence, forests, orographies, and thermal stratification must be considered when assessing the impact of wind turbines in complex terrain. All of these effects influence the flow field both at the turbine position as well as in its wake. Turbulent structures of the forest wake cross the rotor plane temporarily and thereby affect the turbine inflow. Moreover, convective conditions and upward flows caused by the orography have an impact on the performance output as inclined flows result in asymmetric torque distributions. The wind turbine wake and the forest wake mix further downstream, resulting in a fast decay of the turbine wake. The paper also describes how the turbulent flow in the wake changes in the presence of thermal stratification.

High-fidelity aeroelastic analyses of wind turbines in complex terrain: fluid–structure interaction and aerodynamic modeling

“…Additionally in Table 2 the dimensions of the different backgrounds are shown in terms of distances from the tower bottom. The complex-terrain mesh is clearly much wider than it is long, and this is done to avoid any influence of the interaction between the periodic boundary condition and the topography, as explained in Letzgus et al (2021). The mesh discretization occurs via hanging grid nodes, ensuring a resolution of 0.25 m in the closest area to the turbine and increasing up to 2 m for a large area of the domain to ensure a sufficient physical resolution.…”

“…In the observed time period the atmosphere was neutrally stratified. More information on how the turbulence is injected inside the computational domain is given in Letzgus et al (2021). The CFD model of the complex terrain is depicted in Fig.…”

“…The tower passage effect can also be seen in the FMC case, although the load distribution between the upper and lower side of the rotor is not symmetric anymore. This is explained by the retreating blade effect (Letzgus et al, 2021) by which the blade faces an inclined flow due to escarpment of the orography. This is in phase with the blade movement on the left-hand side (LHS) of the rotor and opposite to the phase in the right-hand side (RHS) of the rotor.…”