Fire devil (vortex)

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Wildfire with fire devil
Experiment in the phæno

Fire devils are small bombs that arise over or near fires . Fire devils are sometimes incorrectly referred to as fire tornadoes, which are large bombs . In extreme cases, however, a fire devil can develop into a fire tornado. Fire devils are triggered by strong updrafts caused by the fire mixed with existing air turbulence. These are then concentrated and amplified into a column of rotating air. Their extent ranges from eddies with a diameter of less than one meter and wind speeds of 10 m / s to eddies with a diameter of 3 km and wind speeds of 50 m / s. Smaller fire devils appear quite often over wildfires, while larger ones are rare.

Emergence

For a fire devil to develop, strong updrafts must exist over a fire together with already rotating air. There are many different mechanisms that can make air rotate. The most important are vertical wind shear and baroclinicity . The resulting vortices do not have to rotate around a vertical axis like the fire devil themselves, but can be erected from the horizontal. Strong updrafts develop over the fire. To replace the air that has risen, new air flows in from the sides with the vortices. There the air is captured by the updrafts. The axis of rotation of the air is pulled upwards and straightened up until it is completely vertical.

The updrafts accelerate the air in the vortex and thereby stretch it. In doing so, it contracts and due to the conservation of angular momentum , the pirouette effect occurs and the speed of rotation increases.

structure

Fire devils evolve around a rising and rotating core of fuel-rich gases. Within this core, the air behaves like a rotating solid . The angular velocity remains constant, the tangential velocity increases with the distance from the axis of rotation. Outside this core, the air behaves more like in a potential vortex . The tangential velocity is therefore inversely proportional and decreases with the distances from the rotational axis. In summary, the entire system can be approximated as a Rankine vortex .

Inside the core, the rotation of the air also leads to a greatly reduced turbulence . This effect comes about because the pressure gradient force acts as a centripetal force in that it accelerates all air particles towards the center and thus holds most of the air particles firmly on their circular path. When the rotational speed increases, so does the centripetal acceleration, which is necessary to keep a particle on its orbit. At the same time, however, the air pressure inside is also reduced , which in turn increases the pressure gradient force so that the circular path of the particles can be maintained. Overall, this leads to an almost cyclostrophic flow . As a result, there is a greatly reduced exchange of air between the core and its surroundings and the vortex can retain a large part of its momentum and is slowed down less. Fuel and oxygen are also retained in the core. The low turbulence is one of the most important effects that stabilize the fire devil and thus enable him to have a long life and high rotation speeds.

There is little turbulence, however, only at a sufficiently large distance from the ground. In the vicinity of the floor, the balance of forces is disturbed by friction, which allows new air to flow into the center. Friction and obstacles to be flown around, coupled with the high wind speed, can cause strong turbulence near the ground. If the incoming air is already rotating, this rotation is also transported into the vortex and can strengthen it. With the air flow, new fuel is sucked into the center and help to keep the fire going. In the middle, the air is then strongly heated and accelerated vertically.

Only at high altitudes, when the rising air has slowed down enough due to friction or when it hits a stable atmospheric layer , the cyclostrophic flow can no longer be maintained. The pressure gradient in the core decreases, the radius of the vortex increases and the entire vortex slows down until it dissolves.

Effects

Within a vortex of fire there are significantly increased rates of burn, which amplifies the overall fire. In laboratory experiments, depending on the type of artificially created vortex, a 2- to 7-fold increase was found. This effect can probably be attributed to better mixing of air and fuel and increased heat transfer. Which mechanism of heat transfer increases the combustion rate has not yet been conclusively clarified, but most researchers suspect that convective heat transfer , caused by strong turbulence near the ground, is the main cause.

The strong winds that develop in the center can throw burning objects into the air and fall back to the ground further away from the fire. This makes it possible for firebreaks to be overcome and new sources of fire to arise.

In extreme cases, the wind speed can equal that of a normal tornado , which means that most of the objects caught by the fire devil are destroyed.

Although the basic mechanism of the formation is known, it is not yet possible to predict the formation of fire devils. It can only be determined which situations are prone to emerge. This brings difficulties in fighting the fire and endangers fire-fighting forces who are on duty.

Fire tornadoes

Pyrocumulus or Pyrocumulonimbus often form over fires . If a fire devil connects with the updrafts associated with these clouds and becomes large enough to reach the cloud layer, a large current / tornado emerges from the small current . Alternatively, a fire tornado can arise directly from the winds of Pyrocumulus or Pyrocumulonimbus. In these cases it is real fire tornadoes. Their structure remains very similar to that of a fire devil. Real fire tornadoes are very rare.

Examples

Carr Fire

The Carr Fire was a major forest fire in California . On July 26, 2018, it created a fire tornado northwest of the city of Redding in the Sacramento Valley . This reached a diameter of 300 m, a height of approx. 4.8 km and wind speeds of up to 265 km / h. Although it only existed for about half an hour from 7:30 p.m. to 8:00 p.m., during this period it destroyed a power pole, uprooted some trees, killed four people and further injured a few.

Weather models show fast, cold winds for this day that poured from the Pacific over the adjacent mountains into the Sacramento Valley. They are the most likely source of the rotating air masses needed. After crossing the mountains, the cold, fast air sank along the mountainside, accelerated by gravity (see catabatic wind ). Down in the valley, this current met the slow, warm air there and created strong turbulence with a breaking wave. Together with strong rotating winds in the pyrocumulonimbus, a fire tornado formed.

Web links

Individual evidence

  1. ^ A b c d Jason M. Forthofer, Scott L. Goodrick: Review of Vortices in Wildland Fire . In: Journal of Combustion . 2011, ISSN  2090-1968 , doi : 10.1155 / 2011/984363 .
  2. ^ A b Paul A. Werth, Brian E. Potter, Craig B. Clements, Mark A. Finney, Scott L. Goodrick: Synthesis of knowledge of extreme fire behavior: volume I for fire managers . US Department of Agriculture, Forest Service, Pacific Northwest Research Station, Portland 2011, p. DOI = 10.2737 / pnw-gtr-854 .
  3. a b California’s Carr Fire spawned a true fire tornado. In: Science News for Students. November 14, 2018, accessed August 1, 2020 (American English).
  4. ^ A b Richard HD McRae, Jason J. Sharples, Stephen R. Wilkes, Alan Walker: An Australian pyro-tornadogenesis event . In: Natural Hazards . tape 65 , no. 3 , October 12, 2012, ISSN  0921-030X , p. 1801-1811 , doi : 10.1007 / s11069-012-0443-7 .
  5. ^ Wagenbrenner, N .; Forthofer, J .: The Potential Role of a downslope windstorm and Associated Hydraulic Jump in the formation of a Fire Tornado during the 2018 Carr Fire in Redding, CA . In: American Geophysical Union, Fall Meeting 2019, abstract # A21R-2687 . December 2019.