On the wind flow patterns under neutral and unstable conditions in an urban area

Wind flow, turbulence and pollutant dispersion characteristics within an urban street canyon depend largely on the thermal behaviour of the materials, e.g., the street floor under the sun and on the geometrical characteristics of the surrounding buildings under low wind conditions. They have a detrimental effect on the stability conditions within and above street canyons, which in turn affect the wind flow patterns and the local air quality. The purpose of the present work is to study experimentally and numerically the flow field developed under neutral and unstable stratification conditions within a typical square urban street canyon. A two-dimensional set up is employed including several canyons in order to simulate an urban environment. Experiments were carried out with the use of a triple hot sensor anemometer upwind above and downwind an isolated building. Measurements were taken under neutral stratification conditions for a wind speed of 3ms^-1. Two dimensional numerical simulations have also been conducted for typical urban street canyons. The numerical results have been obtained by the solution of the time averaged Navier-Stokes equations coupled with the standard k-ε turbulence model. The performance of the CFD algorithm is increased by the unstructured Cartesian grid. The numerical code was firstly validated with the experimental results derived under neutral stratification conditions. The effects of stability within a typical urban area were also studied numerically. Spatial distributions of wind velocity, turbulence and pollutants, within street canyons, were analyzed under neutral and unstable conditions for Richardson number R_b=0.17. Under unstable conditions a great deal of mixing exists within the street canyons. The buoyancy induced flow affects the air exchange between the street canyons and the free surface layer. Vertical air motions extend beyond the level of the street canyons causing instability to increase. It was found, that the stratification effects on the flow can be successfully predicted by the CFD numerical code.