Tornado experiments

Robert P. Davies-Jones states that all laboratory scaled experimental apparatus work by concentrating artificially produced vorticity that already has a vertical orientation. This is not true and I will describe various vortex generators that have a variety of vorticity sources.

Apparatus making use of pre-existing vertical vorticity.

Ward gave a very complete description of this apparatus and experiments he ran using it. His device consisted of three cylindrical tanks placed atop one another. The lowest of the three allowed air to enter the apparatus through a rotating screen and directional vanes. This allowed him to vary the angular momentum of the entering flow. This layer simulates the inflow beneath the cloud deck of a thunderstorm.

The second layer is about 4ft in diameter. Flow through it is generally vertical. It simulates the convection cell of a thunderstorm.

The final layer pulls air from below through a honeycomb screen which dissipates all of the rotation. The air is drawn off through a fan. This layer represents the anvil of a thunderstorm. And the screen prevents rotation of the fan from creeping upstream into the central chamber.

This is a graphic based on Ward's original paper. The conclusions he drew from experiments with this device are quite interesting and form the basis of some of the evolution of a vortex as described in the section on stability and evolution.

An apparatus making use of horizontal vorticity.

This design comes from a Scientific American article in the early 1980s, and works very much as Maxworthy describes the workings of a dust devil. A box fan placed 12 or more feet from the apparatus blows air across a lab bench. The air flow at the bench top is halted by friction; whereas a few inches above the benchtop it is a free stream. Therefore the air has horizontally oriented vortex threads just above the bench top.

The apparatus consists of only an open inlet from a shop vac suspended above a thin slab of dry ice. The suction from the shop vac lifts vortex threads from the cross flow and draws them away. As the drawn air accelerates, the inflow stretches vortex threads and intensifies them. The dry ice provides condensation to demark the intense vortexes which last only a few tens of seconds at a time.

Thus, the box fan creates wind shear similar to that above a desert playa surface, or perhaps a little like the environment of a thunderstorm, while the shop vac suction simulates the thunderstorm updraft. One modification that I suggest making to this apparatus is adding a screen at the inlet to the dry vac tank. This prevents rotation within the tank from making its way back down the hose as a thin vortex filament. This could easily cause confusion in the source of the tiny tornado rotation.

Click on the thumbnail of the tornado, below, to fetch a 10 second video of one of these tiny tornadoes in action. The clip is from a meteorology class that I taught at LCCC.

An apparatus that uses bubbles rising through liquid.

The apparatus below consists of a transparent tank of liquid placed on a rotating platform. The liquid rotates with the tank eventually, and this is the source of vorticity to drive the tornadoes. A stream of bubbles exiting a small tube rise in the fluid and pull some of the fluid with them. This simulates the buoyancy of unstable air. This, in turn, causes fluid to converge on the pipe.

Each of these apparatus are useful in one way or another for studying the sorts of conditions in the atmosphere that lead to the formation of a tornado.

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