Influence of geometric parameters of façade protruding ribs on turbulent flow statistics in street canyons: A large-eddy simulation study

• A wide range of protrusion ribs' geometric parameters is investigated by LES. • The geometric parameters that govern the flow structure transition are identified. • The size and height of the top canyon vortex are affected by top two windward ribs. • The pronounced secondary vortices in-between ribs enhance the primary vortex. • Deep ribs enlarge the contact area and restrict the vertical air exchange. Façades with appurtenances are typical building features. While previous studies have demonstrated that the ribs-like horizontal protrusions significantly influence flow features in urban canyons, systematic investigations into the rib geometric parameters remain limited. This study employs large eddy simulations (LES) to examine the geometric influence of horizontal protruding ribs on turbulent flow in streamwise periodic street canyons with a height-to-width aspect ratio of 1. First, computationally efficient domain sizes for LES simulation of street canyons are investigated. Subsequently, the impact of two key geometric parameters, the ratio of separation distance to the building height ( s*, s* = s / H ) and the ratio of protrusion depth to the canyon width ( d*, d* = d / W ), on mean flow structures and turbulence statistics is evaluated. Results reveal that the central vortex dynamics—including their number, strength, and size—within the canyon are highly sensitive to both s* and d* . Notably, a flow regime transition of the central vortex from single- to multi-vortex dominance occurs as s* increases, and the size of the top vortex is affected by the top two protrusions on the windward side. For dense ribs ( s* ≤ 0.083), pronounced secondary vortices between the ribs enhance the central vortices, which can lead to an increase in the canyon-averaged mean velocity magnitude by up to 36% compared to configurations with lower contact area. Compared to s*, d* is less influential on the velocity magnitude of the central vortex. These findings provide insights into façade design strategies that affect ventilation in urban canyons.