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Nonballistic transport of particles in a canopy-plume system

Erika S. MacDonald1,*, Hayoon Chung1, Laura K. C. Sunberg2, Nicholas T. Ouellette1, and Jeffrey R. Koseff1

  • 1The Bob and Norma Street Environmental Fluid Mechanics Laboratory, Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, USA
  • 2Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, Colorado 80303, USA

  • *Contact author: emacd@stanford.edu
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Phys. Rev. Fluids 10, 053801 – Published 6 May, 2025

DOI: https://doi.org/10.1103/PhysRevFluids.10.053801

Abstract

Wildfires pose an increasing threat, with spotting being a key mechanism of their spread. In this process, small pieces of burning material, known as firebrands, can travel long distances and start new fires ahead of the fire front. However, modeling and predicting the behavior of firebrands in the complex flows that typify large wildfires remains a significant challenge. To clarify the impact of the flow physics on firebrand transport, we experimentally investigated the transport of model firebrands in a controlled canopy-plume system. We examined four experimental cases, varying the canopy length and plume buoyancy. The distributions of the downstream position at which the particles land have long tails in all the experimental cases, motivating a segmentation of particle population into a population that travels a short distance and one that travels a long distance. We investigate differences in the particle trajectories in these two populations, including their maximum heights and velocities, and we consider the physical parameters that are influential in determining their landing position. We find that particles that land further downstream tend to remain within the plume for longer time periods. We also evaluate the ballistic-trajectory and terminal-velocity assumptions that are often invoked in firebrand transport models under our experimental conditions, and we find that they do not fully capture the observed dynamics in our experimental system. Finally, we run reduced-order stochastic simulations to determine if knowledge of the particle characteristics, mean flow properties, and Reynolds stresses is sufficient to capture the complex dynamics in the experimental data. We find that the simulation results lack the extreme cases of the experimental data (particles that land unusually far or unusually close to the particle release). We attribute this discrepancy to the lack of spatiotemporal coherence in the simulated flow field, and we suggest that spatiotemporal coherence is necessary to accurately model these extreme trajectories.

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Phys. Rev. Fluids 10, 053801– Published 6 May, 2025
  • Received 1 October 2024
  • Accepted 14 April 2025
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