Tornado Generator Box

TORNADO GENERATOR BOX                 (c)1996 William J. Beaty

  SCIENCE FAIR NOTES: If you don't need the device to last forever, you
  could build this out of cardboard or "gatorfoam" instead of plexiglas.
  Use duct tape instead of glue, and use incense sticks to make "smoke"
  instead of a humidifier.  The bottom panel should not be cardboard,
  since it's a fire hazard with incense, or humidifier mist will make it
  soggy after awhile.  If you use opaque materials, then make a big hole
  in one of the side panels and tape clear plastic over it to form a
  window.  The fan creates the tornado, and the mist or incense smoke
  makes it visible.  Instead of using an ultrasonic humidifier, you can
  use burning incense sticks or cones, but supply a large dish or pan on
  the bottom so the burning incense cannot become a fire hazard!  If
  desired, put a small (15W) light bulb at the rear of the chamber to
  light up the mist tornado, and paint the inside of the chamber black to
  give good contrast.  Try using the "12-volt micro fan" from Radio
  Shack, part number 273-240, and run it from a 9V battery.  However,
  if the slots in the side of your tornado box are too wide, you'll need
  a more powerful fan.

Top view of the tornado box looks like this (if the square top panel is
removed) :
                                                                 ||                   four 14" x 24" panels,
   ===========================   ||                  at least one is clear
                                                                 ||                   (glass, acrylic, or
         ||                                                       ||                   cut a hole and cover it
         ||                                                       || TOP VIEW with stretched plastic
         ||                                                       ||                     film.)
         ||                                                       ||
         ||                                                       ||
         ||   =============================     panels are positioned to
         ||                                                                        form four slots at the
         ||   (-------- about 14" --------)                          corners.

The four vertical panels are fastened to square top and bottom plates, but
are not fastened to each other.  Gaps between the vertical panels form the
slots.  With 18in square top and bottom panels, and 14in walls, the slots
are 1in across, and the square central chamber is 12in.  Air rushes in
through the narrow slots at the four corners, swirls inwards to form the
tornado, then exits through the the top. A small fan was installed in the
center of the top plate (with a hole cut for the fan.)  The device can be
any size, keeping the scale of the slots about the same of course:  with a
12"  square inner chamber, the slots end up being about 1" across.  The
total height is up to you.  My device was about 2ft. tall.  If you make
yours lots taller, you either need to use a fairly strong fan, or you need
to make the width of the slots smaller.

[Oblique View .gif]

The top plate of the version I built was a flat panel with a 2"  hole in
the center, with a high speed 120volt 3" box fan screwed over the hole.
Fan air direction was aimed out of the box of course.  This type of fan is
available at surplus electronics outlets, and I think Radio Shack carries
one version.

In order that it stand up to many weeks of wetness, I made the bottom of
mine from a heat-formed plexi sheet, formed into a shallow funnel shape in
an oven, w/crude frame to hold the edge, and a piece of ABS pipe used
temporarily to punch the "funnel" shape into the hot, soft sheet.  A hole
was cut at the bottom of the funnel shape, and the edge of the hole was
glued to a short piece of ABS sewer pipe.  A perforated plexi disk covers
the funnel mouth and provides the flat surface that the tornado needs in
order to form properly.  The ABS pipe led down into an ultrasonic
humidifier which provided the marker-mist and made the vortex visible.

                   _____ boxer fan (cooling fan from mail order surplus)
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   |                                        | |             SIDE VIEW
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   ---___  ======  __----
            --_    _--     Shallow funnel-shape for water return, with
                 |  |        a disk supported in the center so that the
                 |  |        tornado isn't disrupted by the open pipe.
                 |  |
         ____|  |____
          |                |
          |                |  "Cool Mist" Ultrasonic Humidifier
          |                |  (can be found in some drug stores,
          |                |  or garage sales)
          |                |

Ultrasonic humidifiers normally emit a blast of mist rather than a slow
flow.  To eliminate the blast, I plugged the little air-jet aperture down
inside the humidifier's mist compartment with a plexiglas block, with a
few small holes drilled to point downwards.  (Pools of water develop
everywhere inside the humidifier, so properly angled air holes in the
small plexiglas plug are needed to prevent water buildup from filling
the air holes.)  When the holes are the right size, dense white mist fills
the humidifer and pours out the top like a white fluid.  The quick
version: crimp some aluminum foil over the air-jet aperature inside the
humidifier, wrap rubber bands around it to hold it in place, then stab it
with a pencil point to make small holes in the foil.  See the
"Touch the
exhibit article for more info on humidifiers.

The shallow funnel-shape allows the water which builds up in the bottom of
the tornado chamber to run back into the humidifier, rather than pooling
up in the bottom of the chamber and growing algae.  Over the heat-formed
funnel I placed a 6" dia. perforated plexiglas disk with 1/8" legs around
the edge.  The legs raise up the circular plate so water can run past the
edge, but the slot is narrow enough that debris from museum visitors won't
end up in the humidifier water. The perforations in this flat plate were
closely-spaced 1/4" holes made in a large circle, and the mist from the
humidifier exits through these holes.  It is important to provide a
central, flat, unperforated surface for the vortex to "work against", and
therefore this plate has an unperforated center, with a circle of holes
about 3" dia. to feed mist into the vortex from around its perimeter.

I found that optical contrast is important, so I made the two back walls
opaque black, the bottom funnel and plate black, and two front walls
transparent, then I lighted the whole thing heavily from the top and from
slightly behind.  If I had to do it over again, I might try installing a
flourescent tube running the length of the box.  This tube would need to
be in the rear from the viewpoint of users, but with some sort of light
shields added, so it illuminates the mist column from behind, but doesn't
shine directly in the user's eyes.

To allow people to stick their hands inside, I cut the vertical slot in
the front of the box larger at the bottom, as shown below.  This distorts
the vortex, but allows "hands on" access.  If you use a tiny, weak fan,
you probably should skip this part, since the large hole will divert air
from the vertical slots and prevent the vortex from forming.

                   _____ boxer fan
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   |                                        | |
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   |                                        | |
   |                                       |  |  ENLARGED SLOT FOR HAND ACCESS
   |                                      |   |
   |                                    |     |
   |                                    |     |
   |                                    |     |
    ---___  ======  __----
                --_    _--   
                     |  |

And last, I built a small electronic controller which allowed users to
vary the fan speed by pushing and holding a "faster" and "slower" button.
(Speed control knobs tend to get wrecked pretty fast in an exhibits
environment, while pinball machine flipper-buttons are long-lasting.)  As
the fan runs faster and faster, the tornado suddenly undergoes transition
to turbulence, changing from an onion-layered smoke column into a whirling
turbulent cloud.  To preserve the contents of the water tank, the
controller would disable the humidifier if none of the buttons were pushed
for about two minutes.  Once the mist had started, there was one last
button which allowed the user to turn the mist off and on, and even to
make "pulses"  of mist which would travel upwards in the vortex.

The speed of the fan and the size of the slots must be adjusted correctly
in order to create a robust vortex.  I used a handheld incense stick to
inject smoke into the air so I could see if the vortex was working.  If
your fan is too powerful, the vortex will be turbulent and won't create
beautiful complicated "onion layers" of laminar flow in the smoke pattern.
The fan's air stream can be slowed by partially blocking its exit with
cardboard and duct tape.  If the side slots of the main chamber are too
large and the fan too weak, the vortex will form very slowly and will
vanish at the slightest disturbance.  If this occurs, either move the
chamber walls to make the side slots smaller, or find a more powerful fan.

Forever on my "wish list" was to install a scanned laser beam "sheet-of-
light" generator.  This would allow visual cross-sections of the mist to
be created.  Simply aiming a handheld laser through the rear slot of the
chamber and waving the beam rapidly back and forth through the mist caused
momentary but spectacular "wind tunnel" turbulence patterns to appear, but
I never went any further with this.  Shielding the beam would be an issue,
since to be efficient, the illumination must be directed through the mist
column from behind, and towards (but not hitting) the users' eyes.

Other ideas for variations: a sculptor in New York converted an entire
room into a tornado chamber.  He/she provided a large exhaust fan in the
center of the ceiling, then arranged a large circle of vertical pipes into
the shape of a skeletal cylinder, with the center of the cylinder pattern
aligned with the ceiling fan. These pipes extended from floor to ceiling,
and each pipe presumably contained its own fan, and a series of holes or
slots running one along side of the pipe.  The pipe slots blew air
sideways so all the air in the room rotated slowly, and the central fan
pulled air towards the center of the circle.  The large "cylinder" of
pipes acts as the walls of the chamber and provides tangential air jets,
so any air which flows towards the center of the room is forced to spiral
inwards.  A powerful vortex forms in the center of the room, extending
from floor to ceiling.  A big pile of leaves, shredded plastic, etc.
completes the exhibit.

The Exploratorium museum in San Francisco contains one large, famous
example of a tornado generator chamber. (  Only two
walls are provided, the walls being made of huge curved sheets of
plexiglas, with a fan in the ceiling of the exhibit.  Ultrasonic
humidifiers provide mist in the floor of the exhibit.  Children can walk
into the device and interrupt the spinning mist vortex.