How urban geometry influences air flow and heat stress

For individuals who live in urban areas, rising temperatures translate to increased heat stress when outdoors. To develop strategies for reducing these unwanted effects, civil and environmental engineers require a greater understanding of the fluid dynamics of air flow within urban streets and the overall urban space.

Mouzourides et al. experimentally investigated how air flow within urban streets is shaped under different influences. They considered the natural synoptic wind above streets, which acts as an inertial forcing, as well as the different heating of buildings and walls under direct solar radiation, which promotes a buoyancy-driven flow.

“Urban streets become the lungs of a city, undertaking the breathability function like a living organism,” said co-author Marina K.-A. Neophytou. “The breathability determines the capacity of a street or city to remove heat, pollution, and moisture produced from the human activities within, and therefore determines the comfort level and risk of mortality for citizens.”

The study used a water channel apparatus that contained scaled-down physical models of an idealized symmetric urban canyon. A particle image velocimetry system measured the time-resolved flow velocity field. Three model street-canyon cavities were examined with varying height-to-width (aspect) ratios.

Mouzourides et al. found that the two forcing influences combine to manifest a resulting air flow pattern which may completely differ depending on how narrow or wide the street is compared to the height of the surrounding buildings.

“In some cases, the fluid dynamics condemns the air to a double recirculating pattern, thus suppressing severely the breathability capacity,” said Neophytou. “For those situations, local actions within the street, such as planting trees, are the only refuge.”

Source: “Urban street canyon air flows under combined wind forcing and thermal buoyancy,” by Petros Mouzourides, Costas Marakkos, and Marina K.-A. Neophytou, Physics of Fluids (2022). The article can be accessed at http://doi.org/10.1063/5.0090642 .