Pedestrian level wind and thermal environment around buildings : wind tunnel investigations

Abstract

Constructing a new building will always affect the microclimate at the building and its surrounding area. This can lead to low airflow or poor outdoor ventilation around the building blocks, which can negatively influence indoor air quality, pollutant dispersion in the surroundings, and airborne transmission of infectious diseases. Conversely, high wind speeds can also be encountered in densely built up areas that can introduce discomfort or danger. The air flow patterns at pedestrian level around buildings, particularly high-rise buildings, are generally complicated. And there are insufficient studies focusing on the pedestrian-level wind environment for different building designs, especially those with optimizations. One of the design options for optimizations, which is called lift-up design has been proposed in this work. Three tall building configurations, a singular building (SB), a row of buildings (RB) and a row of building with podium (PB) were selected from a systematic study by Tsang et al (2012) that resulted in the lowest wind speed zones. Buildings with a lift-up design may have a number of impacts on the pedestrian-level wind and pollutant dispersion environment. A 3.5m lift-up in prototype scale was added to each of the three configurations producing three lift-up designs for comparison. Scale models of the designs were studied in the wind tunnel to ascertain the lift-up design influence on airflow and ventilation around the buildings. Undesirable areas of low wind speed leading to poor air ventilation and discomfort due to strong wind conditions at the other extreme are both identified in the results, and their practical implications are discussed. In this series of study, wind direction is normal to the building facade. For the building configurations without the lift-up design, the wind hits the windward facade of the building and a downwash flow is generated. This downwash results in a backflow in front of the building at pedestrian level. When the two opposing windward and backflows encounter each other, a low wind speed zone is created at the upstream of buildings. For the buildings with lift-up designs, the low or poor ventilation situation of the upstream near-field has been improved due to a 3.5m lift-up area underneath of these buildings. This is because part of the downwash and the approaching wind can flow through the lift-up area of the buildings where there is reduced blocking of flow. Consequently less backflow wind and approaching wind meets in front of the building at the pedestrian level. This study also focuses on the pedestrian-level pollutant dispersion from an upstream or downstream line source representing vehicular emission from a line of stopped buses. Pollutant dispersions from the respective line sources upstream and downstream were compared, and the practical implications to the wind and thermal comfort and pollutant dispersions are discussed. Lift-up design in general results in higher pollutant concentrations when upstream line source is implemented. Even higher pollutant concentrations are found when downstream line source is used. Building geometry and sizes can affect the pollutant dispersion dramatically. Larger building results in lower pollutant concentrations, and vice versa. Lift-up design for podium building is the most effective building type studied to reduce the normalized pollutant concentration when the pollutant source is located downstream side of the building. Because the 3.5m lift-up height is much lower than the building height, the 3.5m lift-up design has not much effect on the upper part of the building. Consequently, this study only emphasized on the lower part of the building. The surface pressure, which potentially affects cross the ventilation potential at the lower part of the building for different building configurations has been evaluated. It has been discovered that the lift-up design can all increase the cross ventilation potential for SB, RB and PB. But the RB with lift-up design has the highest ventilation potential among the three building configurations. Thermal sensation of SB, RB and PB with and without the lift-up design has also been investigated. Two predictive formulas from Cheng et al's (2010) study have been utilized to analyses the thermal sensation and overall comfort, which is based on PET. Summer and winter have been considered for this research work respectively. In summer, only positive thermal sensation is found for the pedestrians for singular building with and without the lift-up design for all the zones. For a row of buildings and podium building, there are negative thermal sensation values for some zones. Lift-up design is the most effectively improve the thermal environment for pedestrians for singular building and a slab of building among three building types. This design can improve the downstream thermal environment for a row of buildings. But it is not so useful for podium building at all. In winter, all the zones for singular building, a row of buildings and podium building are all under negative thermal environment. But due to the cold environment in winter, lift-up design does not benefit the pedestrians significantly. In practice, architects or engineers can use singular building or a slab of buildings with lift-up design to achieve better mean wind speed and thermal comfort if these aspects are the aspects they need to consider in the architectural design. If pollution is the aspect that architects or engineers need to considered, it would be better to have a large scale building such as a podium building and locating the pollutant source downstream when the lift-up design is used to reduce the pollutant effect. But for the real situation inside the street canyon, it is suggested to conduct a simulation first, such as CFD simulations, as there are so many combinations of street canyons and cities.