UC Irvine

Complex turbulence patterns emerge from the interaction of a wildland fire-front with its surrounding atmosphere in a variety of terrain and ambient conditions. This interaction, referred to as fire-atmosphere interaction, frequently begets unexpected fire behavior that is difficult to control and threatens communities residing at the boundaries of wilderness. Fire–atmosphere interactions also inform ember-driven ignitions and smoke dispersion patterns that affect respiratory health and visibility downwind. Despite the progress made in recent decades and the operational utility of several fire models, a lot still needs to be understood about fundamental fire behavior to improve model predictive capabilities and expand their range of application.This dissertation begins with the examination of wind velocity and temperature data collected during burn experiments ranging from small scales to operational scales for the turbulence processes characteristic of the presence of a fire in vegetative environments. At small scales, the analysis is akin to the microscopic observation of a fire in its infant stages: tracking velocity vectors in the vicinity of a propagating flame at high temporal and spatial resolutions allows a meticulous examination of the characteristic organized (coherent) flow structures. Besides being able to quantify the presence of counter-rotating vortex pairs along the fire-front, which play a key role in fire spread, potential mechanisms for the interaction between neighboring spot fires are illuminated. Next, data from operational-scale burn experiments (prescribed burns) is leveraged to differentiate between turbulence patterns during fires in grassland and forested environments, under differing ambient wind-forcing conditions. Such comparisons can help simplify the complex equations that govern fire physics, thereby expediting model predictions. A follow-up study utilizes a wavelet-based approach to track individual fire-modulated ramp-cliff-like structures in the measured temperature signal, in a forest canopy, as they evolve in time, along with their constitutive frequencies. These are known to find application in computing sensible heat fluxes within forested environments, which can be used to initialize and evaluate fire-behavior models. Given the spatial restraints on field measurements, we further attempt to investigate flow structures characteristic of buoyant-plume–canopy interaction under cross-wind forcing through large-eddy simulations (LES) using a low-complexity, ‘no-flame’ setup. The study replicates characteristic structures observed during field-scale experimental fires while also revealing differences in the spatial structure of the turbulent fluxes within the plume in both canopy and no canopy environments. By varying ambient wind speeds and heat-source strength, we then attempt to construct scaling laws informing plume deflection in canopy environments.Comprehensive knowledge of these processes can help develop improved parameterizations for the process-level phenomena in coarser-resolution, fast-running, predictive fire-behavior and plume-dispersion models. Improved models can be utilized for the careful planning of prescribed burns intended to reduce hazardous fuels and forewarning fire managers regarding conditions conducive to unexpected fire behavior, leading to efficient wildland fire management practices.