Date of Award
Doctor of Philosophy (PhD)
Dr. Nigel Berkeley Kaye, Committee Chair
Dr. Abdul A. Khan
Dr. William C. Bridges, Jr.
Dr. Weichiang Pang
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.
Tohidi, Ali, "Experimental and Numerical Modeling of Wildfire Spread via Fire Spotting" (2016). All Dissertations. 1681.