Work

Flows across 3D urban street canyons: Revisiting Reynolds number independence by large eddy simulation

Year: 2025

Type: article

Abstract: The Reynolds number ( Re ) independence assumption allows flows across urban street canyons to be studied at much lower Re in reduced scale experiments relative to Re at full scale. Re of 10,000 is often taken to be the critical Re ( Re c ) to achieve Re -independent flows, but recent studies suggest that this Re c is not universal. Moreover, most previous research has focused on Re -independence in 2D street canyons of infinite length, whereas real urban street canyons are inherently 3D, formed by buildings with varied lengths. This study investigates the influence of height-to-width ( H/W ) and length-to-width aspect ratio ( L/W ) of canyons on flow structures and Re c in urban street canyons, using validated large eddy simulations (LES). Our flow pattern analysis reveals that vortex structures in 3D urban street canyons differ significantly from those in 2D canyons. Unlike 2D canyons, where vortex cores are centralized, the vortex cores in 3D urban street canyons shift due to the interaction between spanwise flow and vertically aligned vortices. The results show that Re c increases with H/W and L/W . For example, a canyon with H/W of 1 and L/W of 1 has Re c between 15,000 and 30,000, while a canyon with H/W of 3 and L/W of 10 has Re c between 180,000 and 360,000. These findings indicate that Re c required to achieve Re -independent flows in 3D urban street canyons depends on the geometrical parameters such as H/W and L/W . Care should be taken when applying reduced-scale results at relatively low Re to full-scale scenarios.

Source: Building and Environment

DOI: 10.1016/j.buildenv.2025.113635

Authors: Sirui Zhu, Lup Wai Chew

Institution: National University of Singapore

Language: English

Topic: Wind and Air Flow Studies

Subfield: Environmental Engineering

Field: Environmental Science

Domain: Physical Sciences

SDG: Sustainable cities and communities

Open Access status: hybrid

Estimated APC paid: $4,280