Numerical prediction of size, mass, temperature and trajectory of cylindrical wind-driven firebrands

Abstract

Mathematical models and numerical solution procedures for predicting the trajectory, oscillation, possible rotation, and mass and size time-evolution of cylindrical wind-driven firebrands are described and discussed. Two test problems and the results, used for validating the mathematical models, are presented. In one, experimental measurements of non-burning cylindrical particles falling in still air are compared to numerical predictions and in the other, predictions of time-evolution of mass and size of stationary burning particles in air flows are compared with experimental results reported in the literature. Results yielded by the proposed models for a demonstration problem involving cylindrical wind-driven firebrands, with the same initial volume, mass and position, but different initial aspect ratios and distinct initial orientations relative to the wind velocity, are then presented. These results show the following: the horizontal distance travelled by the firebrand from release to landing locations is an increasing function of its initial aspect ratio; and the initial orientation of the firebrand, and its subsequent oscillations including possible rotation, have a significant influence on its trajectory, thus it is important to account for them in mathematical models formulated for predicting the spread of fires by spotting.

Deposition Characteristics of Firebrands on and Around Rectangular Cubic Structures

“…The firebrand equations are expressed and solved in the Lagrangian framework. Firebrands are assumed to be cylinders with a large ratio of length to diameter, undergoing both translational and rotational motions (Yin et al, 2003;Oliveira et al, 2014;Anand et al, 2018) and thermal degradation as a result of pyrolysis and charring (Morvan and Dupuy, 2004;Anand, 2018).…”

“…They observed that for higher wind speeds, the change in the initial vertical velocity of the convective column did not affect the mean or standard deviation of the heights where the firebrands lofted or the distances they traveled to land. Yin et al (2003), Oliveira et al (2014) developed numerical models for the firebrand transport accounting for the drag, lift and gravitational forces and their effect on the rotation of firebrands to model both translational and rotational motions of cylindrical firebrands. To validate their model, Oliveira et al (2014) performed computations and experiments for a cylindrical firebrand (balsa wood) falling from an elevated point under a no ambient flow condition.…”

“…Yin et al (2003), Oliveira et al (2014) developed numerical models for the firebrand transport accounting for the drag, lift and gravitational forces and their effect on the rotation of firebrands to model both translational and rotational motions of cylindrical firebrands. To validate their model, Oliveira et al (2014) performed computations and experiments for a cylindrical firebrand (balsa wood) falling from an elevated point under a no ambient flow condition. The influence of different formulations for the distance between center of pressure and center of mass of a cylindrical object in motion was explored in the modeling by Rayleigh (1876), Marchildon et al (1964), Rosendahl (2000), Yin et al (2003).…”

Evaluating the terminal-velocity assumption in simulations of long-range inert ember transport

“…The investigation of its validity for long range ember transport is beyond the scope of this paper, however we note that it is supported to some extent by the studies of Ellis (2010) and Tarifa et al (1967); in drop tests, embers either assumed an orientation of maximum drag, or tumbled but fell at a speed as if they had assumed such an orientation. Some numerical studies, including those of Koo et al (2012) and Oliveira et al (2014), include the rotational terms in the equations of motion.…”

Calculation of Spotting Particles Maximum Distance in Idealised Forest Fire Scenarios

“…Physics based on coupled fire-atmosphere models consider approximations of the governing equations from the fluid dynamics, the combustion, and the thermal degradation of solid fuel (see, e.g., [64][65][66]) aiming to preclude the use of existing simplified empirical wildfire models because they do not predict general fire behaviour; however the high-resolution and the high-fidelity combustion are not currently appropriate because of their computational cost. Several physics based on coupled fire-atmosphere studies have been conducted (see [67][68][69][70][71][72]) and some of these studies have been applied to the fire spotting problem. Among them [71] has considered particle combustion of cylindrical and disk-shaped firebrands for several geometrical parameters.…”