Computational fluid dynamics simulations of local wind in large urban areas

The present report details the methodology and parameters used for high-fidelity computational fluid dynamics (CFD) simulations of wind in large urban areas. In particular, a 150 km² area of Oslo is used as an example, and local flows resulting from 18 different meteorological wind directions have been simulated. To the knowledge of the authors, this is the first documented case of large-eddy simulations of complex urban geometries larger than approximately 25 km² with geometry-conforming tetrahedral meshes.

The work is presented in the context of the FFI-project “UNOS”, which concerns the development of a high-quality operational hazmat dispersion tool for use in Norwegian cities. Such a tool is valuable for risk assessment, situational awareness and improved preparedness for emergency events involving urban dispersion of chemical or biological agents. That said, the guidelines and workflow discussed are relevant to many cases in which CFD simulations of a geographical area are of interest. Examples include wind-load computations, wind-comfort simulations, and air-pollution simulations. Choices of sufficient mesh resolutions, wind inflow formulations and other parameters relevant to the present context are discussed.

Reported results mainly exemplify flow data from selected simulated global wind directions, and the examples show how such data can be presented. However, in determining the necessary problem parameters, several relevant results are also found: Although a mesh resolution of 1 m close to solid surfaces is recommended, a 2 to 4 m resolution suffices for the specific context of this work. Further, it is shown that the details of a turbulent inflow is not essential. On the other hand, the domain height and wall boundary-conditions have noticeable impacts on the solution; a domain height of 4 km was sufficient for the present case, and weakly imposed wall-boundary conditions perform somewhat better than strongly imposed conditions for the finite-volume solver used here.

Applying the methodology described here, 18 different simulations of urban wind in Oslo were successfully computed and used in the generation of the operational hazmat dispersion tool, CT-Analyst® Oslo.