Wind tunnel and computational fluid dynamics study of wind conditions in an urban square

Recognizing the role of urban squares in city life, the paper focuses on wind conditions at squares, with the objective to understand the building-scale flow phenomena influencing the wind comfort of pedestrians and the dispersion of pollutants. Wind tunnel testing of a selected square has been carried out at two wind directions, as well as accompanying computational fluid dynamics (CFD) simulations were performed using the MISKAM microscale model. The high spatial resolution experimental and CFD data allowed the identification of flow structures, like separation bubbles, vortices, high-speed zones in and around the square. Based on the analysis of numerical and experimental data obtained, the MISKAM model is able to resolve the flow field in a complex urban setting with some limitations. In a second step, the model was then used to run further numerical simulations, to compare fully built-up areas to an area with a square, and to assess the influence of tree plantings on the square. Results regarding the latter indicate that below tree crown height, flow velocity and turbulent kinetic energy are both decreased by about one fourth due to the vegetation. It is also shown that the presence of the square increases wind speed in connecting streets and induces longitudinal flows even in streets perpendicular to the approaching wind direction. The time-resolved wind tunnel measurements also allowed presenting the local velocity statistics as wind roses. Using these, we identified locations with non-isotropic turbulence and alternating wind directions.