Abstract

A variety of wildfire models are currently used for prescribed fire management, fire behaviour studies and decision support during wildfire emergencies, among other applications. All these applications are based on predictive analysis, and therefore require careful estimation of aleatoric and epistemic uncertainties such as weather conditions, vegetation properties and model parameters. However, the large computational cost of high-fidelity computaional fluid dynamics models prohibits the straightforward utilization of traditional Monte Carlo methods. Conversely, low-fidelity fire models are several orders of magnitude faster but they typically do not provide enough accuracy and they do not resolve all relevant phenomena. Multifidelity frameworks offer a viable solution to this limitation through the efficient combination of highand low-fidelity simulations. While high-fidelity models provide the required level of accuracy, low-fidelity simulations are used to economically improve the confidence on estimated uncertainty. In this work, we assessed the suitability of multifidelity methodologies to quantify uncertainty in wildfire simulations. A collection of different multifidelity strategies, including Multilevel and Control Variates Monte Carlo, were tested and their computational efficiency compared. Fire spread was predicted in a canonical scenario using popular simulators such as the Wildland-Urban Interface Fire Dynamics Simulator (WFDS) and FARSITE. Results show that multifidelity estimators allow speedups in the order of 100× to 1000× with respect to traditional Monte Carlo.

Abstract

This work presents a coupling between two numerical models, one for mesh generation and the other for heat transfer analysis, used to characterize the tree trunk thermal behaviour located in a forest fire environment. The geometry of the tree trunk is developed, using adaptive mesh generation, to evaluate the view factors, used to calculate the heat exchanges by radiation, between the virtual trunk tree and a fire front. The fire front represents the existing forest fire. The trunk tree thermal behavior numerical model is based on energy equations. The energy balance integral and differential equations consider the heat exchange by conduction inside the trunk tree, the heat exchange by convection between the trunk tree external surfaces and the environment and the heat exchanges by radiation between the trunk tree external surfaces and the environment and fire front. The numerical simulation is characterized by the propagation of a fire front at a constant fire spread rate of 0.01 m/s from a distance of 5 m upstream of the tree trunk. With an irregular shape, the tree trunk has a height of 2 m and an external diameter of 0.2 m. The fire front has a tilt angle of 45º, 10 m wide and 2 m high. The temperature distribution in the tree trunk was obtained for a wind speed of 0.1 m/s, an average flame temperature of 500ºC, an environmental air temperature of 20ºC and an air relative humidity around the trunk tree of 50%. The temperature distribution obtained allows to identify the areas of the tree trunk tissues that were most affected by the passage of fire.