CFD methods in the atmospheric roughness layer at different scales

The wind-related processes in the atmospheric roughness layer strongly affect our comfort and health and can have a significant influence on our lives, especially in urban areas. The flow patterns are the main drivers of dispersion and transport of pollutants and particles. However, their evolution in the roughness layer is complex because of disturbances introduced by topology or wind flows in the ABL. Experimental approaches, such as wind-tunnels can significantly improve the understanding of characteristics and be used for the improvement and validation of model approaches. Nevertheless, they are conducted under ideal conditions and do not represent the complex influences occurring in the atmospheric roughness layer. Field measurements are therefore a very practicable approach to study the chaotic behaviour of environmental flow patterns. The effort required to obtain field measurements and their small spatial significance however make it difficult to obtain large spatial monitoring of the phenomena. In the past, numerical studies often used simplified and idealized geometries or methods to model roughness elements and only few studies were conducted in real environments using field measurements to improve the knowledge of roughness layer flows, especially in urban areas. While the different approaches have frequently been used separately, their combination could lead to significant knowledge and new insights into the complex processes. This thesis introduces the theory and major steps taken to model the relevant roughness elements: sub-grid elements, such as surface texture, vegetation elements, such as trees, and impermeable obstacles, such as buildings. The numerical domains were based on real environments where either field or experimental studies had been conducted providing data with which the numerical methods could be validated, highlighting the advantages of a combined, multidisciplinary approach. The bridge between the engineering derived CFD methods and their application in environmental flow case-studies is introduced, as well as methods to model urban areas from a micro to a neighborhood scale. Ways in which to deal with problems, such as meshing complex geometrical data, as well as an alternative approach for obtaining approximate transient results are shown using examples from case studies in different environments. Four case studies, all validated with either experimental wind-tunnel or field measurements were conducted for the relevant roughness elements: sub-grid, porous (e.g. trees) and impermeable (e.g. buildings). The case studies also create a bridge between the engineering dominated CFD methods and today's applications in environmental flows. In each of the case studies, the application of dispersion or transport is studied for gaseous fluids or solid-particle detachment, dispersion and transport. Numerical problems occurring, especially in urban areas, where the topology is complex and heterogeneous, were addressed and the successful solution demonstrated. The implementation used commercial software and OpenFOAM, an open-source code that was found to be an ideal software package for research purposes because of its parallel capabilities and flexibility for future studies.