Mean flow and turbulence measurements around a 2-D array of buildings in a wind tunnel

In order to predict the dispersion of harmful materials released in or near an urban environment, it is important to first understand the complex flow patterns which result from the interaction of the wind with buildings and, more commonly, clusters of buildings. Recent advances in the application of computational fluid dynamics (CFD) models to such problems have shown great promise, but there is a need for high-quality data with which to evaluate CFD models. This study was performed to fill that need for a limited range of conditions. High-resolution measurements of the three components of the mean and turbulent velocity statistics were obtained around a 2-D array of model buildings in the USEPA meteorological wind tunnel. Seven rectangular blocks (0.15 x 0.15 x 3.8m) were placed in the wind tunnel with their long face perpendicular to the flow and with a spacing of one building height (Hb) between the buildings. A pulsed-wire anemometer was used to measure mean velocity, turbulence intensity and some higher-order velocity statistics within and around the array. Multiple vertical profiles were taken from 3.3Hb upstream of the building array to 7.5Hb downstream of the array. Five vertical profiles were measured above each building rooftop and in the street canyon between the buildings in order to fully characterize the flow structure within the array. Surface pressure coefficient was measured on the upstream, rooftop and downstream faces of each building. The velocity profile at the furthest upstream position showed that the flow at this position is already being influenced by the building array. A small rotor just upstream of the first building was observed to result in reverse flow within approximately 0.5Hb of the array. Rooftop measurements showed that the flow separates over the top of the first building, but not over the remaining downstream buildings. The separation which occurs on the top of the first building resulted in a substantially different velocity defect near the top of the building and in the acceleration of the flow above about 1.3Hb. The flow at the surface, however, was nearly identical in all street canyons, where there was a strong recirculating region with reverse flow extending upward to about 0.6Hb. The downstream profiles showed a separation cavity extending to approximately 3.5Hb, followed by a gradual readjustment toward the approach-flow velocity profile.