Indices to evaluate ventilation efficiency in newly-built urban area at pedestrian level

Abstract

This paper aims to numerically evaluate the effects of building arrangements on average ventilation efficiency in newly-built residential areas at pedestrian level. It proposes a practical design parameter as a practical guide for urban ventilation design. A reference urban model was designed according to a typical residential area in Shanghai. Other urban models with constant floor area ratio and various urban building arrangements with various building coverage ratios, passage widths, building arrays and building height variations were considered. The ventilation efficiency was evaluated by spatially-averaged wind speed ratio, spatially-averaged normalized concentration, and visitation frequency. Flow flux analysis was conducted to explain the ventilation mechanism and the relationships among ventilation efficiency indices. The results showed that small building coverage ratios and large passage widths resulted in higher ventilation efficiency in most cases. The staggered array showed better ventilation efficiency than the aligned array when the wind direction was 0° and 45° and the advantage of building height variation was distinct in the densest case for 90°. A general relationship was found among the three indices: a large spatially-averaged wind speed ratio and a low visitation frequency resulted in a low spatially-averaged normalized concentration, although there were a few exceptions. A practical design parameter (passage ratio) for urban ventilation was proposed based on the calculation results.