We illustrate transport through firebrand travel distances and trajectories from a set of simulations configured with various wind speed and particle properties (diameter, temperature, and density). In Figures \ref{938404} and \ref{470341}, the parameters for the sensitivity tests are chosen to be representative of real-world scenarios. More specifically, firebrand density and diameter are chosen to result in typical firebrand mass of 0.01 g to 1 g based on the available data in the literature \cite{Filkov_2017,Tohidi_2015}. Due to unavailable data for firebrand temperature, we selected a range based on reasonable values for smoldering and flaming combustion. The wind speed upper range is limited by the small simulation domain, in that in higher wind speeds, the fire reaches the domain boundaries, forcing the termination of the simulation.
The travel distance sensitivities due to particle properties and wind speed is shown in Figure \ref{938404}. Each parameter is tested with three different values, using both coupled and uncoupled configurations. In the uncoupled scenarios, travel distances among firebrands generated at the same heights do not vary within each simulation because firebrands are generated with fixed-value properties, the atmosphere profile is homogeneous, and the wind vector is constant. In the coupled scenarios, travel distances vary due to turbulence, which develop as the fire heat perturbs the wind field. Firebrands will travel farther if generated where the local flow is upward and forward.
In the uncoupled simulations, the travel distances are illustrated by the dashed vertical lines. These simulations show that for the same wind speed (10 m s-1) and for the range of parameters simulated, cooler and smaller, firebrands burn out faster. On average, the median travel distance increases by 1.44 m for each 100 K warmer, and by 3.8 m for each mm larger. The effect of particle density (spherical particles in this case) is more subtle - the median distance decreases by 0.015 for each kg m-3 denser. Although subtle, the overall sensitivity to particle density indicates that a higher number of lower density particles travel longer distances. Lastly and as expected, wind speed is a primary factor affecting firebrands’ reach - the median travel distance increases by 3.3m for each m s-1 increase.
Because firebrand generation is a function of fire ROS, which in turn is a function of wind speed, the number of firebrands generated increases as we increase the background wind speed. In the simulations for the coupled scenario, 828, 844, and 1832 firebrands were generated for 5, 10, and 15 m s-1 wind speed, respectively, whereas in the uncoupled scenario simulations, the number of firebrands was 598, 624, and 652 firebrands.