The terminal velocity assumption in simulations of long-range ember transport.

Ember transport and the subsequent development of spot fires is a significant mode of wildfire spread, particularly in extreme conditions. An important simplifying assumption made in early research into ember transport is the terminal-velocity assumption, in which embers are assumed to always fly at their terminal velocity relative to the wind field. With increases in computational power, it is now possible to directly simulate the atmospheric conditions resulting from wildfires and such simulations can resolve the larger of the turbulent processes involved. Because of the time-scales at which these processes occur, the terminal-velocity assumption may not be justified when modelling ember transport using these simulations. In this study we use a large eddy simulation of a turbulent plume to examine the validity of the terminal-velocity assumption when modelling the long-range transport of non-combusting embers. The results indicate that the use of the terminal-velocity assumption significantly overestimates the density of ember landings at long range, particularly for embers with higher terminal fall speeds.

Figure 3. Modelled embers in flight (black points), and after landing (grey points), 15 min into the ember transport phase of the modelling. In this case trajectories were computed using Eq. (NTVA) with u ∞ = 6 m s−1 . Only part of the computational domain is shown.