An evaluation of fire-plume properties simulated with the Fire Dynamics Simulator (FDS) and the Clark coupled wildfire model

Abstract

Before using a fluid dynamics physically based wildfire model to study wildfire, validation is necessary and model results need to be systematically and objectively analyzed and compared to real fires, which requires suitable data sets. Observational data from the Meteotron experiment are used to evaluate the fire-plume properties simulated by two fluid dynamics numerical wildfire models, the Fire Dynamics Simulator (FDS) and the Clark coupled atmosphere–fire model. Comparisons based on classical plume theory between numerical model and experimental Meteotron results show that plume theory, because of its simplifying assumptions, is a fair but restricted rendition of important plume-averaged properties. The study indicates that the FDS, an explicit and computationally demanding model, produces good agreement with the Meteotron results even at a relatively coarse horizontal grid size of 4 m for the FDS, while the coupled atmosphere–fire model, a less explicit and less computationally demanding model, can produce good agreement, but that the agreement is sensitive to surface vertical-grid sizes and the method by which the energy released from the fire is put into the atmosphere.

Simulation of a Large Wildfire in a Coupled Fire-Atmosphere Model

“…In this respect, simulated updrafts appear in agreement with [45], who observed updraft values on the order of 20-30 m s −1 , and [46], who reported peaks of 32-60 m s −1 for crown fires occurring at the highest detectable heights (about 50 m). The largest simulated downward motion occurs in the upper plume behind the fire front (maximum of −7 m s −1 ), consistent with previous observations [47] and simulations [48]. (Figure 10d), which fall within the range of sensible heat fluxes as reported by [49].…”

“…The horizontal grid size is 2400 m for the outer domain covering 600 km × 720 km. The inner computational grids have grid increments of respectively 600, 200 and 50 m, covering a total area of respectively 144 km × 240 km, 48 …”

The Spotting Distribution of Wildfires

“…As mentioned above, coupled fire-atmosphere models [2,17,18] may provide an improvement to static fluid dynamic computations currently employed in operational front prediction software (for North American examples see [19,20]). Indeed, Large Eddy Simulations (LES) coupled with firebrand dispersal have shown that the fire plume may be quite different from the plume used in standard models like the Baum and McCaffery plume [21], leading to different firebrand trajectories [22][23][24]. In addition, there is an extensive literature covering empirical measurement of plume characteristics; extensive measurement of plume heights and characteristics in [25] compared a variety of plume models (different from Baum and McCaffery) against measurements for approximately 2000 wildfire plumes.…”

“…Hence to determine the spotting distribution, we can apply the ignition operator to the total landed mass, where, as in the previous paragraph, the landing distribution's mass component is evaluated in terms of the inverse combustion operator at the landing time. The exact formula is given in Equation (23).…”

“…Any fire plume is more turbulent than laminar, and our knowledge about plumes is mostly experimental [61]. There is a complicated interaction between the atmosphere and the fire, hereafter referred to as fire-atmosphere interactions, which has been extensively studied from experiments, and physical modelling [1,2,17,18,[22][23][24]31,[62][63][64][65][66][67].…”