Postdoctoral Scholar Ali Tohidi Receives IAFSS Best Thesis Award

Ali Tohidi, a Postdoctoral Scholar in the Department of Fire Protection Engineering, has recently been honored with the International Association for Fire Safety Science (IAFSS) 2017 Best Thesis Award for the Americas. Tohidi, who joined FPE after completing his PhD at Clemsen University in 2016, is one of three applicants and the only one in the Americas, to receive this award. Recipients will receive a plaque, a grant of $2,000 to cover travel costs related to the recipient’s attendance at the 12^th Annual IAFSS Symposium in Lund, Sweden, and free registration for the Symposium. According to IAFSS, his dissertation entitled, Experimental and Numerical Modeling of Wildfire Spread via Fire Spotting, makes “significant contributions to wildland fire science by improving the current understanding of firebrand transport during wildland fires.”

Tohidi is currently working with FPE Associate Professor Michael Gollner and his group on the generation, transport and ignition by firebrands in the Wildland-Urban Interface, in addition to related problems in fluid mechanics.
Abstract:

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.

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