Experimental and Numerical Modeling of Wildfire Spread via Fire Spotting

Wildfire spread via fire spotting phenomenon has three major stages, namely formation and break-off of firebrands from vegetative structures, lofting and transport of them through the ambient velocity field, and finally deposition of firebrands upon landing and ignition of spot fires. This dissertation develops novel models in different areas related to fire spotting phenomenon and integrates them to improve understanding of the firebrand flight through a multiphysics model. In this regard, a mechanical break-off model for the formation of cylindrical firebrands from coniferous trees is proposed; And by geometric scaling analysis, it is shown that the firebrand surface area scales on the mass raised to the 2/3rds power. By applying a non-linear regression model to the available experimental data on firebrands, a predictive statistical model for estimating mass and shape distribution of firebrands is proposed, that can be used as realistic input into the current fire spotting models. Further, the aerodynamic behavior of the cylindrical firebrands is characterized by conducting free-fall experiments where it is shown that the governing equations of the transport are highly sensitive to the initial conditions of the release. On this matter, near field dynamics of highly buoyant bent-over plumes are thoroughly characterized and, it is shown, analytically, that the steep trajectories of wildfire plumes necessitate for the inclusion of the boundary layer shearing effects in the mathematical models of the velocity field. Moreover, for the first time, the most extensive large scale wind tunnel experiments of the lofting and downwind transport of non-combusting model firebrands is conducted. It is found that the normalized landing location of firebrands with their maximum rise height have similar probability density functions (PDF) regardless of the aspect ratio. This implies that unlike previous studies the lofting and transport cannot be decoupled. Given the wind tunnel experiment results, a highly scalable coupled stochastic parametric model for firebrand flight is developed by synthesizing OpenFOAM and MATLAB solutions. This model couples the fine resolution time-varying Large Eddy Simulation (LES) resolved velocity field of the jets/plumes in non-uniform cross-flow boundary layers with the fully deterministic 3D 6-D.O.F. firebrand transport model. Comparisons between the experiments and corresponding numerical simulations with this model show very good agreement in estimating the average statistics of the flight. Also, it is shown that the transport equations are highly sensitive to the spatial and temporal variations in the ambient velocity field