Wind energy-related atmospheric boundary layer large-eddy simulation using OpenFOAM

The focus of this work is on large-eddy simulation of the atmospheric boundary layer, which will be used in future work as inflow for large-eddy simulations of flow through wind turbine arrays. Worldwide deployment of wind turbines is growing and is expected to continue at a substantial rate in the coming decades. These turbines are often multi-megawatt in size with blades that currently extend 150 m into the atmospheric boundary layer, and the size of machines is expected to grow. Commonly, turbines are installed in large arrays on the order of a hundred turbines. Because of the size of the turbines and the extent of the turbine arrays, their interaction with the atmospheric boundary layer has become an important practical problem that needs to be better understood.

Current models used by wind farm developers to predict wind turbine array performance are empirical and, therefore, computationally efficient but were developed in the 1980s and 1990s when turbines were smaller. It has been recently observed that these models over-predict large turbine array performance by roughly 10%, which has significant monetary implications for wind farm operators and investors. An important part of this over-prediction is attributed to the inadequacy of these models in properly representing turbine wakes and array-atmospheric boundary layer interactions. There is a need to better understand these flow phenomena in order to develop better engineering models. Large-eddy simulation is an important tool in gaining a better understanding of the interaction of the atmospheric boundary layer with wind turbines and their wakes in an array.

The first step in performing such a simulation is to create an accurate representation of the atmospheric boundary layer and its turbulence, which is used as the inflow for the wind turbine array simulations. Others who have performed large-eddy simulations of wind turbine arrays have either used a stochastic model of turbulence in the atmospheric boundary layer with limited success or performed full precursor large-eddy simulations limited to the neutral stability case. Because the precursor approach appears to create a better representation of the inflow turbulence and because wind turbine arrays often operate in non-neutral boundary layers, the focus of this work is to perform large-eddy simulations of both the buoyant and the shear-driven atmospheric boundary layers. These will serve as precursors to full turbine array simulations in future work. These atmospheric boundary layer simulations are performed using a solver created with the open-source OpenFOAM toolbox. It differs from more traditional atmospheric solvers because it is a spatially and temporally second-order accurate unstructured finite-volume solver and uses a co-located grid with Rhie-Chow-like interpolation. The OpenFOAM toolbox is unique in its extreme flexibility in creating or modifying new solvers, sub-filter scale models, and boundary conditions due to its object-oriented code structure.

This work is important in establishing the performance of an OpenFOAM-based solver in predicting atmospheric boundary layer flows and also in beginning to create best practices for this solver that is gaining popularity in the wind energy community. Its performance will be compared against that of more traditional spectral and finite-difference solvers. Ultimately, it is hoped that this work will serve as the basis for wind turbine array large-eddy simulations that will provide important flow details concerning the interaction of the turbine wakes and the entire array with the atmosphere so that better engineering models can be created and used by wind farm developers.