Validation of two RANS solvers with flow data of the flat roof Michelstadt case

To further increase the confidence in the results of numerical simulations of flow and dispersion phenomena in urban environments thorough verification and validation of the numerical models is required. Wind tunnel measurements of an idealized Central-European city, Michelstadt, which are publicly available in the CEDVAL-LES database were used here to validate the open source code OpenFOAM® 1.7 and the commercial code Ansys® Fluent 13. The results of the flow field computations were compared graphically and with the metric hit rate to the experimental data. Unstructured tetrahedral and polyhedral meshes were used with different resolutions to investigate the advantages and shortcomings of the polyhedral meshing. With both codes and mesh types the qualitative agreement between numerical simulation and experiment is good for the mean velocities, with a minimum hit rate of 0.64 for the coarse polyhedral mesh. The Reynolds stresses on the other hand are consistently under-predicted with both codes on all meshes. Dispersion simulations were also investigated with one of the meshes. Validation of the two codes with this realistic urban geometry shows the capabilities and shortcomings of RANS CFD modelling for regulatory purposes in urban air quality modelling.