A numerical investigation of the interplay between fireline length, geometry, and rate of spread

The current study focuses on coupled dynamics and resultant geometry of fireline segments of various ignition lengths. As an example, for ignition lines of length scales typical for field experiments, fireline curvature is the result of a competition between the head fire and the flanks of the fire. A number of physical features (i.e. buoyancy and wind field divergence for example) arise in and around an incipient fire that defines the shape and spreading pattern of the flame zone. These features are explored using a numerical atmospheric dynamics model HIGRAD, and wildfire combustion physics model FIRETEC. HIGRAD/FIRETEC was designed to investigate wildfires and their interactions with the environment. In this study, the model was used to simulate grass fires that were initiated with a finite length, straight ignition line in homogeneous fuels. The dynamic evolutions of these firelines were analyzed to understand the individual events that evolve a wildfire. By understanding each individual process and how it interacts with other processes, information can be extracted to develop a theory about the mechanisms that combine to produce the observed wildfire behavior. In the current study, the flow field in the region of the simulated fires developed structures consistent with multiple buoyancy-induced vortex pairs. The series of stream-wise vortex pairs produce a regular alternating pattern of up-wash and down-wash zones, which allow air to penetrate the flame zone through troughs created in downwash regions. Consequently, this periodicity in the flow field within the fire resulted in a pattern of residual combustion where prolonged burning occurred in the up-wash zones separated by near-complete fuel depletion in the downwash zones. Some explanation is provided for why increased ignition line length leads to increased rate of spread (ROS) with some asymptotic limit. Published by Elsevier B.V.