Predicting outdoor thermal comfort in urban environments: A 3D numerical model for standard effective temperature

•An improved methodology of predicting outdoor thermal comfort is introduced.•Standard effective temperature is taken as the metric of outdoor thermal comfort.•SET at the pedestrian height is highly variable in the street canyon due to variation in velocity and Tmrt.•In addition to shading pattern, sky view factor and visibility of urban surfaces significantly affect SET.•The wind speed is more important in SET for higher Tmrt values particularly in the wake regions.

With the rapid rate of urbanization, outdoor thermal comfort is a growing health concern in densely-built areas. Accordingly, in order to achieve a comprehensive solution to urban environmental problems, more detailed and accurate prediction of outdoor thermal comfort is needed alongside building energy and urban wind flow analyses. To address this need, this study introduces an improved methodology of predicting outdoor thermal comfort and its spatial variability in urban streets using the comprehensive index standard effective temperature (SET). The improvement of the thermal comfort calculation is twofold. First, CFD simulations of the flow field dynamically coupled with the realistic urban surface heating are used to provide the input variables for the SET calculation. The CFD results provide detailed information on the heterogeneous urban flow field as a critical determinant of human comfort. Second, the SET calculations are improved by introducing a detailed model of mean radiant temperature that incorporates a) the visibility of urban surfaces to the pedestrians at any point, b) the spatial distribution of sky view factor, and c) inter-building shadowing and shortwave radiation effects on thermal comfort. These improvements allow for evaluating the spatial distribution of SET. Additionally, several sensitivity studies are carried out for an idealized configuration representing a compact low rise urban zone. The SET evaluated at the pedestrian level shows that urban density, wind patterns, and solar position concurrently influence thermal comfort. A sensitivity study on the effect of urban density reveals that higher urban concentration can favourably impact thermal comfort in warm climates due to the increased shading, despite the associated increase in air temperature and building energy consumption. The current study also demonstrates the critical importance of a comprehensive thermal comfort model that considers the flow field patterns as well as the realistic heating distribution of urban surfaces. We find that even in the shaded areas in the street canyon, SET changes up to 10 °C due to the wind sheltering in the urban roughness. Following this methodology, more complex scenarios can be evaluated in order to evaluate the effect of urban design on thermal comfort, and ultimately achieve a climate-conscious design.