Tornado

The Volvo Commercial

This was our first really big artificial tornado. Tony Kaye directed the commercial, which won a pile of awards (Cannes, Clios, etc.). Tony believes in doing things for real: They hired a real tornado chaser, Gene Rhoden (email:grhoden@why.net, to play the chaser in the commercial.

We actually did all the special effects (including falling boats, trees, and immense amounts of blowing sand, debris, etc.) in the spot, which aired in the U.K. in the summer of 1995. Hundreds of horsepower driving a modified helicopter rotor produced the updraft, and more hundreds of horsepower created a rotating airflow to feed the vortex. We went from "green light" on the project to having the rig up and running in our parking lot in about 3 weeks for testing, then dismantled it and moved it to the desert for the shoot, which lasted a week.

Our real problem was that because it operated outdoors, we were subject to ambient winds. March in California is probably the windiest month, and our machine wouldn't operate in winds much over 5-7 mph. When we were setting up, we had thunderstorms (making working on a 50 foot tower all the more fun), 50 mph winds and driving rain. However, we did get some relatively calm windows of several hours each to run the machine and generate some truly amazing footage.

We measured wind speeds in excess of 70 mph at the core, which was about a foot in diameter. It easily picked up run of the mill debris, like straw, cloth, paper, and dust. It also could pick up and fling heavier things, like 2x2 foot pieces of 3/4" plywood (ask Steve Triplett, who got hit by one while we were testing). Tony wanted a dark and menacing effect for the tornado, so we used coal dust to produce a black debris cloud. We added fuller's earth to the coal to reduce the explosion hazard. The black dust cloud from the top of the rig extended several thousand feet into the air, and was visible 10 miles away.

If you are a storm chaser and you run across Gene, you might ask how he uses the telescope at the Griffith Observatory in his work.

A fact sheet we produced on the tornado for inquiring minds.

A photo of the rig in operation. That is a full size Volvo 850 next to the debris cloud.

An article that appeared in a newspaper in England that summer with an interview with two guys from the ad agency that was responsible for the spot.

Coming Soon: a video clip of the spot.

Smaller Tornadoes

Photographic Applications

Several TV commercials and programs have photographed our 10 foot tornado as a visual element which was later composited into another image. An episode of Picket Fences used this kind of image to create a fairly realistic looking tornado off in the distance which had realistic movement dynamics. Since the vortex dynamics and appearance are controlled by the same aerodynamics as a real tornado, the appearance is quite realistic. In the Picket Fences episode, they talk about the tornado as an F4, when in actuality, from movement and scale, it looks more like an F2.

Coming soon: A single frame from the video

The actual 10 foot tornado appeared as Urkel's Science Project in an episode of Family Matters that aired Twister opening week in May 1996. We set up the machinery on stage and fired it up on cue. This is a pretty neat effect, because you see the vortex core organize and form a nice ropy looking vortex. The live audience thought it was pretty cool, as well.

Coming soon: A single frame from the video.

The 10 footer, visualized with Liquid Nitrogen fog, was used as a novel projection screen for a Miller beer commercial. The fog forms a very even, distinct wall around the core with the appropriate flow parameters.

Photographing the Clear center

If the vortex generator is properly set up, smoke injected outside the core remains there, with a clear core remaining. A camera can be placed within the core producing an interesting image of a hollow tube of smoke. Obviously, the camera needs to be of an appropriate size and shape to not perturb the flow excessively. The same phenomenon occurs with burning fuel, producing a hollow luminous tube of fire, which is visually quite impressive and surreal. For this case, the camera needs to be heat resistant, or sacrificable. (We view this as an appropriate sacrifice to the tornado gods).

There is a nice sequence down the center of the fire vortex in the Entertainment Tonight segment that aired xxxx.

Liquid Fuels - Fire Vortices

Many people have discovered (some accidentally) that running a vortex over a pool of liquid fuel can produce an extremely intense vortex phenomenon. The high air speed over the surface of the liquid, along with the high shear, causes the liquid to be atomized quite effectively. The atomized particles then burn quite vigorously and impressively. In a sufficiently large size, the heated air created from the combustion is sufficient to drive the vortex.

The trick is in finding fuels that have a high enough flame speed to not blow out in the high wind speeds encountered. For instance, kerosene is not nearly flammable enough. We have had the best luck with things like petroleum ether (a mix of pentane and hexane), dimethyl ether (which has some handling problems), and blends of methanol and nitromethane. You can, of course, use a fuel with high hydrogen/carbon ratio (like ether) and mix inorganic ions like strontium or barium in to produce colored fire whirls. The usual hydrocarbons burn with a very luminous sooty flame which obscures the color from the ions.

A photo of petroleum ether in a 10 foot tall, 1 foot diameter vortex. Note the hollow appearance, due to a strong downflow of clear air in the center of the vortex . This particular vortex burned about 100 cc of the pentane/hexane mix in about 30 seconds.

Some experiments with larger fire vortices

We decided to experiment with thermally driven vortices with the classic fire whirl (just to get our feet wet, so to speak). We set up a rotating air flow around a 4x8 foot aluminum tray about 2 inches deep. We then put several gallons of gasoline into the tray and lit it on fire remotely (Did you think some brave soul was going to run in there and do it by flicking a Bic?) This produced a very spectacular whirling column of intense fire about 40 feet tall and 3-4 feet in diameter. Our experiments were terminated when the aluminum pan melted and burned.

Sorry, we don't have any pictures of these experiments, but next time the weather is suitable, we'll try it again using a better fluid pan. We'll also remember to turn on the video camera the next time, too.

Research Opportunities

We haven't had much chance to do good quantitative research with these vortices. However, if someone is interested, we do have some Research Opportunities in which you would be interested.

Other Tornado Web Sites and Resources of Interest

You have to go see the Tornado Project at http://www.tornadoproject.com. They put out a set of the best collection of tornado videos around, named (oddly enough): Tornado Video Classics Vol I, II, and III.

The classic Scientific American column "The Amateur Scientist" featured some vortex apparatus and fire whirls in 1963. You might find more at http://www.sciam.com

Laboratory Simulations of Tornadoes, Robert P. Davies-Jones, in Proceedings of the Symposium on Tornadoes: Assessment of Knowledge and Impolications of Man, pp. 151-173, American Meteorological Society, Boston, Mass. 1976.

Laboratory Models of Tornadoes, Christopher R. Church and John T. Snow, in The Tornado: Its Structure, Dynamics, Prediction, and Hazards. Geophysical Monograph 79, American Geophysical Union, 1993.

The American Meteorological Society is, of course, a resource for issues about weather in general, and severe local storms in particular.