Learning to teach about vorticity with a tornado vortex model

Tornadic Vortex Model

We have successfully created an open room, floor to ceiling tornado vortex simulation many times under many different circumstances. Colleagues have replicated it dozens of times in different rooms at a variety of locations. This model is utilized to teach about tornados and vorticity to a variety of audiences including students in my introductory weather and climate classes as well as teacher participants in the American Meteorological Society's DataStreme Water in the Earth System course. This paper will discuss how the vortex is created, why it works and how it can be used for teaching. Video clips of the actual vortex will be shown.

The model is created by getting the air in the room to rotate. Once this rotation gets established it is entrained as a spiraling flow of air into a upward drawing fan in a ceiling open space. After letting the circulation go for a while an equilibrium motion is created that is gentle yet persistent with eddies and other turbulent motions superimposed or imbedded in the overall flow. Fans on the floor contribute to this overall circulation. They force inward the air in the open vortex demonstration space where it is dawn upward under the upwards-blowing fan in the ceiling and provide shear which gets translated into rotation.

Adding dry ice (solid carbon dioxide) makes the air near the floor chill rapidly creating a cold air pool that is forced to converge by the side fans. The side fans are positioned to create shear that causes an initial spin and lift in the denser cold air that seems to be necessary to create the funnel. This initial cold spin gets “captured” by the bottom of the somewhat broader circulation created with the fans above and is drawn up. As the cold air is drawn up it chills the air immediately surrounding it causing it to tighten up. The tightening due to chilling and the inward flow of air caused by the draw from the fan above causes the rotation to rapidly speed up by conserving angular momentum. This provides the rapid, tight rotation from the floor to ceiling. You can actually see it go from a rotation that is slower to one that is faster as it tightens.

The condensate funnel is produced if the room humidity is high enough and there is sufficient hygroscopic nuclei in the air. Water vapor is added using atomizers and humidifiers. Nuclei are provided by burning incense or candles. With sufficient quantities of both, the condensate funnel will abruptly establish, usually from the bottom up, but sometimes from the top down.

As the rotation continues, the condensate funnel appears and disappears with changed conditions. At times the entire funnel will collapse downward or a bulge will appear in it and work its way down to the floor. There is usually a clear space in the center of the funnel that is easily seen if the vortex is thin. This clear space is symmetrical with the rest of the vortex always staying approximately in the center and usually proportionately the same size, about half the diameter of the funnel, no matter how wide or narrow the vortex. The clear space tapers with the diameter of the funnel and bends with the funnel when it “ropes out.” If the vortex is thick the condensation tends to be less transparent and you can't see inside the vortex.

There never appears to be “suction” of debris by the vortex in this experiment. Upward motion of debris (a monopoly house or chip of dry ice for example) seems to occur when an upward vector of air hits the debris at a low angle and propels it upward. The displacement which results from upward “tossing,” and an arcing return to the surface of a 0.6 gram Monopoly “house” is sometimes as much as 50 centimeters and with a bit of bouncing and sliding can easily result in the house resting a meter away from where the vortex tossed it. How far debris are “tossed” seems to depend on the angle that the individual “suction vortex” hits the “house”.

While the funnel is readily observed and descriptions of what is happening can be made from real time observation, the true nature of motion is much more easily discerned if one video tapes the vortex and then plays back the images at a slower speed. Further investigation is planned using a digital video by analyzing motions of the vortex and objects moving in the field of view using VideoPoint video analysis software.

This open room model is very useful for teaching the basic elements of vorticity and the relationship between pressure and condensation. Other physical principles can also be taught. The hands on nature of the experiment with the students building the various elements and the unique mesmerizing visual impact provides keen student attention and enhanced learning. Learning vorticity from this model is far more impactual than from similar models done in various small chambers and is much more realistic.