Modelling air flows around buildings in urban environment

In urban areas, main air flows from prevailing winds are strongly modified, depending on constructions’ morphology and urban microclimate’s effects. The meteorological data on the wind speed are often available from outside city centres whereas this wind speed is tough to assess in the urban canopy layer. The meteorological models which are developed from regional level to urban canopy layer level are insufficient to assess the air flows around buildings and from streets to the urban canopy. The development of a local model is detailed in this presentation through the study of wind flows in the specific case of an urban canyon street. From the experimental data collected during the European project Urbvent (Georgakis and Santamouris, 2005) by the University of Athens (UOA), in five canyon streets, an qualitative analysis is conducted. Then, an empirical model is developed in order to assess the wind air flows within the street from the wind air flow above the urban canopy. The obtained results are discussed, regarding the measurements for the studied streets. The model is developed with the outlook of the development of a more complete tool to study building interaction with the urban microclimate. 1. WIND IN URBAN CONTEXT In urban context, the knowledge of air flows around buildings appears to be necessary for the calculation of the buildings’ climatic loads in air conditioned studies (Bozonnet, 2005) as in the case of naturally ventilated buildings. In summertime, the specific phenomenon of urban heat island (UHI) is a source of thermal discomfort and can be linked to an energy demand increase due to air conditioning as to health’s and pollution’s risks. Then, the assessment of heat and mass transfer around buildings, at a street scale, is valuable especially for the design of natural ventilation systems. At a local scale, there are perturbations due to anthropogenic activities and solar radiation. A first approach of the effect of solar radiation in a canyon street was proposed through a zonal model (Bozonnet et al., 2005b). The effect of prevailing winds at the canyon street scale is studied here in order to complete this approach. As for pollution problems, low wind velocities in summertime are critical in the reinforcement of heat urban island effect. In these conditions thermal and anthropogenic perturbations can be non negligible, and the prevailing wind impact can be tough to isolate. Prevailing winds are observed at large scales, several hundred of kilometres, and their time scale is in the order of one day. Locally, some specific winds can occur, as sea or mountain’s breezes. These breezes are linked to temperature variations on small distances depending on local geography, and it can even be observed at an urban scale, from the surrounding to the city centre where warm air due to heat island effect rises. The profile of wind mean velocity ū(z) can be assessed by a logarithmic profile near the soil for homogeneous zones, typically in rural areas. This wind profile characterizes an atmospheric boundary layer which is disturbed in the city area, where an urban boundary layer (UBL) can be defined (Oke, 1987) as outlined Figure 1. The mean wind profile observed in rural areas is shifted up and distorted in the UBL. In a 2 International Workshop on Energy Performance and Environmental Quality of Buildings, July 2006, Milos island, Greece similar way as forest canopy, the constructions’ obstruction moderates the prevailing winds’ velocity and characterizes the urban canopy. The building surrounding corresponds to the urban canopy scale, and the UBL conditions above this canopy account for the boundary conditions. 2. ASSESSMENT OF WIND AT URBAN