Assessing the Pedestrian Wind Environment: The Role of Urban Morphology

The evaluation of the urban wind environment has been receiving increasing attention as it has been associated with issues related to the quality of the urban climate, such as heat island intensity and air pollution, affecting health and well-being in large metropolitan areas. Urban morphology influences almost exclusively the wind conditions at the street level. Therefore, since large metropolitan cities around the world are dealing with a rapid development over the last decades, with increasing building heights and built densities, the evaluation of the urban wind environment has become a major concern. Focusing on the context of London, this study aims to assess the impact of the building geometrical and morphological characteristics on the pedestrian level wind environment. The influence of the contextual parameters such as the wind climatic data, the landscape as well as the implementation of the wind criteria for comfort and safety was also assessed. For that purpose, a comprehensive morphological and landscape analysis was accomplished with GIS software for Greater London in both uniform and non-uniform grids of various sizes, while extensive validation, verification and sensitivity CFD studies were performed to optimise the model settings and maximise accuracy. The impact of buildings on wind characteristics is given by the wind speed ratio (Ur) which is the ratio of the simulated wind velocity at pedestrian level height over the wind speed at a known reference height upwind to the site, where wind velocity is not influence by any buildings. The impact of the entire urban environment, consisting of both buildings and terrain on wind characteristics is given by the total wind amplification factor (γ); necessary for pedestrian wind comfort assessment. The pedestrian wind comfort assessment is affected by a number of parameters, such as the number of wind directions tested, the wind data used, the wind comfort criterion and the roughness of the ground surfaces which intervene between the area under investigation and the meteorological station. The importance of such parameters has been highlighted and their effect on ground level wind conditions has been evaluated according to the wind comfort criteria from the Building Research Establishment (BRE) for the different types of human activities. The results from the sensitivity studies show that no significant discrepancies on ground level mean wind speed ratio (Ur) and wind comfort conditions are observed by decreasing the number of wind directions tested from 16 to 8 for the additional computational time required to be worthwhile but simulations for the additional wind directions might be necessary for studies focusing on specific wind directions or outdoor spaces. The wind data used, however, as well as the ground surface roughness and the selected wind comfort criterion influence the pedestrian level wind conditions significantly, indicating the importance of their careful consideration and selection in wind comfort evaluation studies. For the final simulations, almost 40 cases with dimensions of 500x500m and 1,000x1,000m were examined through CFD simulations. They demonstrate a variety of building configurations and typologies, while a mixture of building morphological characteristics were examined. For each case study, eight wind directions were tested and the wind comfort criteria from BRE were implemented. The results reveal strong connections between the ground level mean wind speed ratio (Ur) and building morphology as well as a strong association between the mean Ur and the development of acceptable or tolerable wind conditions for the different pedestrian activities, independently of the wind data and the ground roughness characteristics. The intensity of the impact of the buildings on the pedestrian level wind conditions and the importance of the terrain characteristics and the wind data are directly related to the category of the plan area ratio in which the site of interest corresponds. For that purpose, London was divided into three plan area ratio categories: low, medium and high plan area ratio. Depending on the plan area ratio category, the building morphological characteristics affect differently the pedestrian level wind environment. Several building morphological characteristics, mostly those related to the vertical and the three dimensions of buildings influence the pedestrian wind comfort conditions, especially in the areas where buildings occupy a smaller proportion of the outdoor space. As a result, those characteristics should be primarily consider by architects, designers and planners during the design of new buildings and developments