Effects of Urban Boundary Layer Turbulence on Firebrand Transport

TL;DR

This study uses large-eddy simulations to show how urban topography-induced turbulence influences firebrand transport, affecting landing patterns and increasing spotting risks during wildfires.

Contribution

It demonstrates the significant impact of urban boundary layer turbulence on firebrand dispersion, landing distribution, and spotting risk, providing new insights into wildfire behavior near urban areas.

Findings

Urban topography enhances horizontal firebrand dispersion.

Larger firebrands land closer together and near the release point.

Urban turbulence increases firebrand landing temperatures and spotting risk.

Abstract

This study investigates the role of topography-induced turbulence, generated by an idealized urban region, in the transport of firebrands and risk of spotting. Flight dispersion, deposition, and smoldering state of tens of thousands of individual mass and size-changing firebrands were investigated in the atmospheric boundary layer turbulence, which was obtained using Large-eddy simulations. Firebrands were assumed to be smoldering spherical particles of Stokes numbers ranging from 30 to 175. Results indicate that the presence of urban topography significantly affects the firebrand flight behavior, landing distribution, and risk of spotting. Compared to a case with flat topography, horizontal dispersions of the smallest size firebrands were significantly enhanced when urban topography was presented, while the largest firebrands landed closer to each other and closer to the release point. Consequently, a notably different and more compact spotting risk map was achieved. Within the urban boundary layer turbulence, firebrands had shorter flight and smoldering times in comparison with the flat case. As a result, firebrands landed with larger temperatures, which contributed to a higher risk of spotting in the presence of urban topography.