Universal Fire Shape and Whirls

In this wildfire simulation, as the fire spreads, it evolves into a shape well-known to fire managers. The "head" is the leading edge of the fire where the heat is focused. Along the side are two "flanks", where the winds blow parallel to the edge of the fire. The "Back" is the slowest moving part of the fire and creeps against the wind.

In addition, an animation of the vertical vorticity (rotation of air) shows the presence of fire whirls. Wildfires can contain fire whirls of greatly varied intensity, duration and size - from a few meters in diameter, to where the entire smoke-filled updraft (perhaps 1 km wide) is rotating. Most are small, but they have been known to break off trees more than a meter in diameter.

QuickTime | Real | MPEG 9 MB

QuickTime | Real | MPEG 14 MB

The fire starts as a line with constant easterly winds of 3 m/s driving the fire from behind. The misty field is smoke, denser and more red where the fire is burning most intensely.

The heat from the fire rises in updrafts that the winds focus at the head of the fire. These updrafts draw warm air into their base from all directions, guiding the wind to flow along the flanks and focus the heat at the front. In this way, the interaction of the fire with environmental winds creates a self-perpetuating universal shape that is observed in fires in many conditions all around the world.

As the fire grows, perturbations (seen in the vorticity animation to be fire whirls) occur along the fire flank and are brought forward to the head, which gets stronger. These fire whirls change the airflow all around the fire, directing some parts of the fire into fresh fuel, creating local heat, and even creating more fire whirls. This increase in intensity is not due to the environment, which remains constant, or to the fuel, which is the same throughout the domain, but purely the fire-induced winds.

The red whirls represent positive vertical vorticity (counter-clockwise rotation when viewed from above the fire). Blue whirls are negative (clockwise).

This animation shows fire whirls at the head of the fire, and forming along the flanks at the intersection of the edge of the fire with unburned fuel. This is where the strongest gradients are, where the winds created by the fire bring them forward to the head. Note the suddenly strong and erratic wind vectors as the whirls move by.

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