Understanding the Influence of Façade Rough Elements on Ventilation and Heat Removal in Urban Canyons

Description

The coupling between climate change and the urban heat island effect has intensified extreme heat events in cities globally. Effective ventilation, facilitated by urban canyons as wind pathways, is crucial for transporting urban heat away. Solar heating on urban surfaces and anthropogenic heat emissions, e.g., from air conditioning led to higher air temperatures in urban canyons, which induces a buoyancy effect that drives the hot air upwards and away. Past studies have shown that the buoyancy-driven and wind-driven effects collectively shape the ventilation and heat removal process. Our current understanding of the heat removal mechanisms is based on simplifying assumptions, treating buildings as shoe boxes with smooth surfaces. It fails to account for common façade rough elements such as louvers, balconies, window sills, mullions, and overhangs. In subtropical regions, these façade rough elements commonly exist to provide solar shading and access to the natural environment. Previous studies have investigated specific rough elements under solely wind-driven conditions and highlighted the potential error of neglecting them in predicting ventilation. Nonetheless, when buoyancy and wind jointly drive urban ventilation, thermal effects contribute to the mixing and transport processes, and our knowledge regarding the impact of façade rough elements is lacking.In this project, a complementary approach, consisting of physical (wind-tunnel measurement) and numerical modeling will be sought to gain a fundamental understanding of the influence of façade rough elements on the ventilation and heat removal in urban canyons. Critical factors will be evaluated, including the transformation from smooth façades to highly rough façades, and the transition from solely wind-driven ventilation to combined wind- and buoyancy-driven ventilation. The air velocity and temperature will be measured under typical scenarios in the wind tunnel. Based on the wind tunnel measurements, we will develop, verify, and validate feasible numerical approaches to simulate heat removal in urban canyons with façade rough elements. The numerical approach will be employed to identify influential geometric features of façade rough elements and unravel how they affect ventilation and heat removal in various canyon morphologies.This project addresses an important gap in our understanding of heat removal mechanisms in cities. It is essential for a detailed understanding of the urban heat island effect, enhancing the parameterization of momentum and heat exchange between urban canopies and the above atmosphere. It is deemed to inspire design strategies that promote the removal of urban anthropogenic heat, upgrade building energy efficiency, and improve pedestrian thermal comfort.