Introduction
Working principle
Necessary materials
- Materials
- Tools
Description of materials
Metric system conversion factors
Construction
- Cutting the wooden disks and support blocks
- Making the lower plexiglass disk
- Making the upper plexiglass disk
- Cutting the PVC sheeting
- Making the upper and lower compartments
- Preparing the PVC tube
- Assembly
- Painting
- Attaching the aluminium strips
Installing the fan and airflow channels
Installing the mist generator
Installing the light
Finishing touches
Dimensional modification
Running the generator
Operation tips
Troubleshooting
Summer 1998 I went to Paris for a holiday. There I visited the La Cité museum,
where a 3 feet tall tornado generator was setup. I was fascinated by the beauty
of the tornado vortex, and the open design of the generator: I could stick
my hand right through the front of the generator box, and touch the misty vortex
of the tornado.
A few weeks later, at home, I designed my own generator box and built the thing,
this one too with open front. When I got the generator working, the tornado it
produced was of the same beauty as that one in Paris. Now I want to describe to you
my design, so you can build one yourself and be amazed by the tornado. The design
may also serve for research purposes.
The tornado will be about 0.9m tall (about 3 feet) when fully developed. The
generator design uses a variable-speed fan, for the generator to be able to simulate the F1 - F3 range of tornado structures.
At very low fan speed, the tornado will be like a rope, almost stationary positioned
in the generator box. When run at high speed, the tornado will be a swirling vortex, about 130mm (5") thick,
going beserk in the generator.
Costs to build this generator are about $150, mostly due to the high cost of
the mist generator it uses ($100), the fan (about $25) and the power supply for
the fan ($20). Most of the rest of the materials will be lying about in your workshop.
You can build the generator first, without buying the mist generator, to save money.
You'll not have a visible tornado vortex, but you can make this visible with smoke
or soldering fumes. If the device really works and a tornado is produced, you can buy the
expensive mist generator.
I will assume you have some basic modelling experience, to successfully build the
device. It is important to be accurate in cutting and sawing materials and the like,
for it is a rather expensive project, and the generator is a tall object when setup
in a living room, for example, so you'll want to let it look nice.
If we want to simulate a real tornado in small scale, we need to rotate air and suck the
air out at the top of this circulation, to get a low-pressurized core. Since angular momentum
of the air is preserved, the air
rotating around will not enter the core (in the ideal case... but even in everyday life
the momentum is preserved pretty well...). So air wants to flow to the low-pressure core
of the vortex but if it has rotation it will keep rotating around it, faster and faster. At
a real tornado, air will start rotating around the core by itself due to the Coriolis force
due to earth rotation. At small scaled tornados this effect is not enough to start air rotation
by itself.
I used this principle of rotating air and suction at the top in my generator. Other generator designs
I have seen consist of a box with a fan at the top, sucking air out of the box. Air is reentering the
generator via some slots at its sides. These slots are each orientated so that the
air entering will flow around the mid section of the box. This design works great but
you have to look through a window at the tornado and you can't put your hands inside in a
comfortable manner.
I constructed an open-design tornado generator, a cylindrical box about 0.4m in diameter, with
an open front about 0,3m wide, 0.8m tall. Since this open "window" will let air in immediately,
letting it flow right to the low pressure, no tornado will ever form using the slots principle
described above. So I had to think of another way to let the air rotate: to use the fan at the
top of the generator not only for suction, but also for blowing the air back in the generator
again, by means of a punched PVC tube at the side of the generator. This tube I positioned
so to blow the air entering at the open front sideways. This way the open front
of the generator acts as one huge slot. If you're sceptical, I need to say I constructed this
design and it works pretty well, even when putting my hands in the slot.
To build the generator I used the following materials:
The dimensions of my design are in mm (millimeter). If you're more at home in inches and feet, here's the conversion
from the metric system (for convenience I put both-way conversions in here):
m (meter) and mm (millimeter = 1/1000 meter)
1 mm = 0.0394 in (thus nearly 1/25 in)
1 m = 39.4 in
1 m = 3.28 ft
1 mm = 0.00328 ft
So, for example, the tornado generator will be 1200mm tall, this is 1200 x 0.0394in = 47.24in or 1200 x 0.00328ft = 3.94 ft., etc.
First, draw two 400mm diameter circles on the wood using a pair of compasses and cut out the two wooden disks.
Make sure the disks are perfectly round, for every inconsistency will show on the finished generator.
Out of one disk, cut a smaller disk, 300mm in diameter (this will be some sort of hatch to access the fan compartment later on).
Sand the three pieces and lay them aside.
Next, cut the 16 wooden blocks to length and sand them. Verify that all are of the same length. Variations of at most 1mm are acceptable.
Now take the sheet of plexiglass where the disks will be cut from. Lay the sheet on top of the plate of wood where the wooden
disks were cut from, and position the part (of the plexiglass) which you want to cut a disk from on top of the gap in the wooden plate.
Clamp the two sheets together using glue clamps or something. Be careful with clamping, the plexiglass may crack
easily, especially this 2mm thin sheet. Support the two sheets at the underside with some scrap material or books to lift the
sheets about 20cm (8") above the working area.
With a permanent marker, draw a circle 400mm in diameter on the underside of the plexiglass sheet (that is, use the wood as a stencil).
Next, with the plexiglass sheet on top, use a ruler to locate the center of the disk and dash it. Rotate the ruler 90 degrees and repeat,
you now have a cross which is at the center.
Locate the 75mm round mould (I used an empty paint can 75mm in diameter), and fire the air heating gun. Set it on most powerful setting (mine
was able to set a piece of wood on fire at close distance). Now heat the plexiglass sheet over the matrix, going around with the heat gun
starting at the edge of the disk and spiralling towards the center and back, and repeat some times. Make sure you heat the center part
more than the outer edge of the disk, and make sure there occur no "hot spots". So heat the plexiglass evenly, but more and more towards
the center. Take care you don't heat it too much to let it form bubbles - some bubbles are okay but it needn't get any hotter.
Turn off the gun, and take the 75mm mould. Use the center cross mark to position the mould over the center of the disk, set it on the
plexiglass surface and press. Press it about 100mm - 120mm down, if you heated the plexiglass right you will see the shape in figure [2]
forming. Make sure you're holding the mould horizontally positioned, so the 75mm bottom is parallel to the sheet. Hold it in place for about
1 or 2 minutes to let the plexiglass harden.
If you're not able to press the mould in the plexiglass deep enough, reheat the plexiglass until it
does. Make sure no holes form in the plexiglass (this will become the lower plexiglass disk which will be filled with water during
generator operation).
Remove clamps and cut out the deformed disk using the drawn circle as a guideline. Take care you do the cutting slowly... the plexiglass will
crack easily, maybe spoiling all the work. Saw the plexiglass with speed of 3 to 4 mm/s at most. Also, take care not to saw too slow,
because the plexiglass will run hot by the saw, and may stick to the blade, resulting in a mess (and a jammed saw blade).
For the upper disk, proceed in the same way. But now after heating use the 100mm mould, and press it for only 50mm deep in the plexiglass.
After it has hardened, remove the clamps, turn the plexiglass sheet over, and reheat the center part (in the 100mm area). Heat this part
as hot as possible. Then take the 30mm mould and press it 70 - 90mm down, to obtain the shape as in figure [3]. You may have to repeat
the last step (heating and pressing), for this is a difficult shape. This time it's no big problem if any holes form in the plexiglass,
you can put some tape over it if necessary. The upper disk needn't be water tight. It's also not bad if the shape is not exactly
the same as in figure [3].
When done, cut the disk out and sand it. Take a 6mm drill and drill a hole through the center cross marking (drill slowly else the
plexiglass will crack - due to heating and deforming the glass may be as thin as 0.5 mm in this area). Next get the file and
carefully file the drilling hole to 30mm size as in figure [3]. Make sure the hole gets round and not much smaller or larger than 30mm.
And take your time when drilling and filing, if things go wrong you'll spend a lot more time making a new disk!
Finally mark and cut the hole for the PVC air blowing tube in the disk as shown in figure [4]. Dimensions of this hole are the diameter
of the air blower tube you use plus 1mm width, and diameter plus 3mm in length. This extra mm are for installing the tube when the
generator is ready (the tube can't be put through the hole straight, and may have to be removed and reinstalled later on), and for
connecting the tube to the fan later on. If your PVC tube is not 32mm in diameter make the hole to fit that tube.
Note: cutting out this hole can best be done with a file, it will take some time to carefully file the hole out but else the plexiglass
could also crack. If it happens to crack, use instant glue to correct the problem.
Now try to be neat and clean the filings and saw dust out of your working area. Especially the plexiglass filings and sawing rubble are
sharp and bad for your health. (I got such a plexiglass filing in my eye when cutting the disks, this caused a lot of pain. Make sure you
wear glasses or something when sawing plexiglass!)
Introduction
Working principle
A tornado consists of a low pressure core with air rotating around this core. The
strength of a tornado will depend on the low pressure at the core: the lower the pressure,
the more swiftly the air will rotate around it. In a real tornado, air is usually
rotating with speeds of 100 km/h to 500 km/h. The strength of a tornado is measured
in the Fujita scale F1 to F5. An airflow speed of about 0-100 km/h applies to an F1 tornado,
and so on, to about 400-500 km/h for an F5 tornado. (This is only my observation, but the rule
stands very well).
Materials
and the following tools to assemble the thing:
Description of materials
RPM drivers are sold as kits to be built by yourself, or as already built-up units. Cost: about $10 - $15.
If your fan runs on line output voltage 230 VAC (115 VAC in the U.S.), use as RPM driver a Variac(TM) (variable output transformer).
I believe the mini-sized mist generator type I used may also be available at drugstores, warehouses or electronics shops.
Metric system conversion factors
1 in = 25.4 mm
1 in = 0.0254 m
1 ft = 0.3048 m
1 ft = 304.8 mm
Construction
Cutting the wooden disks and support blocks
Save the plate of wood the disks are cut out of, you will use the circular gaps as a matrix to deform the plexiglass disks.
Making the lower plexiglass disk
Figure [2]
When done, sand the shaped disk and put it in a safe place.
Making the upper plexiglass disk
Figure [3] 
Figure [4]
 Lower assembly - Figure [5] |
 Upper assembly - Figure [6] |
From scrap material, cut three small wooden blocks and mount these partly over the inner edge of the wooden ring, to act as locks for the inner disk (hatch) to be supported (visible in figure [6]). Don't forget this step (like I did), for it's almost impossible to mount the scrap blocks on the ring if the generator is finished (using screws, that is).
Using the screws, assemble the upper and lower compartments as shown in figures [5] and [6]. Take care you use the upper plexiglass disk in combination with the wooden ring (else you'd end up with a hatch in the bottom of the generator). Also take into account the proper orientation of the plexiglass disks, refer to figures [7] and [8] for cross-sectional views of the compartments.
 Figure [7] |
 Figure [8] |
Tip: it's best to pre-drill holes in the support blocks, too, else the blocks risk getting split when screwing in the screws. For this drilling use a drill about 0.5mm or 1mm smaller than the screws. When assembling the compartments, take care no blocks protrude at the edges of the wooden disks and plexiglass disks.
Note: Don't forget to use the washers with the screws, else the plexiglass disks may crack (I think, tonight, you'll dream about that warning "The plexiglass may crack" :)
From PVC tubing, cut a tube 1000mm long, and sand the ends. Next, mark a line all along the tube, and mark drill locations every 25mm
along this line, as in figure [9]. Leave 150mm on one end clear (this will be the upper part of the tube in the generator).
The tube will have the appearance of a pan flute or something.
Next, use a sharp tool to punch the mark locations a bit (this is to prevent drill swerving when drilling the holes). Next take a 6mm
drill and drill the holes. Take extra care you drill all holes straight, all holes must be essentially parallel to eachother and
straight to the tube. I state "essentially" here, because the appearance of the tornado vortex depends on the hole orientations. If one
hole is drilled slightly tilted to the other holes, the air exits this hole in the same "invalid" direction and the air rotation in
the generator will not be exactly
cylindrical, thus the tornado will not be at the center but curved slightly to one side. Note this effect at the photos on this page -
these are photos of my generator. I drilled the holes not exactly straight to eachother, and thus the tornado vortex is curved a little
bit. I like the effect, so I didn't care about making another PVC tube with straight air exit holes.
So drill the holes essentially straight (straight for the eye). Then your tornado vortex will not be perfectly straight, but curved and
a little eccentric (about 5cm, 2") to the generator's center. But if you prefer a perfectly concentric tornado vortex, by all means use
a drill stand.
Note: this curved vortices story only applies to the case that the generator's fan is running low-speed. In that case the tornado vortex
is most sensitive for disturbances (like a tilted air exit hole at the tube). If the fan is running full-speed, the tornado will be at
the center, for then all air is rotating very fast, thus cylindrically.
After drilling, clean the inside of the tube, there will be a lot of drilling scrap in there which you must get rid of. Then sand the surface
of the tube to remove the rough rims of the drilling holes, and to obtain a good painting surface.
Now close the lower end of the tube off (you can use hot glue for this, in combination with some small scrap wood). The lower end of the tube
is the end with the drill holes, the left end in figure [9].
When done, paint the tube black, using the silk matt paint for the generator's interior. Don't use too much paint, else it will flow
in the air exit holes too much. You'll need two coats, at least, to paint the tube.
Preparing the PVC tube
Figure [9]
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Before attaching the aluminium strips (and screwing down the corners of the PVC sheet), paint the generator body. You can paint the
whole thing in one time, except for the under surface where the generator stands on. Before painting, slightly sand the PVC surface to
make it rough and to remove any dirt. Also sand the inner surface (inside the generator), and the plexiglass disks. Paint the inside of the generator using the silk matt black paint. You don't need to paint the interior of the compartments, these will not be visible when the generator is finished. So paint the inner surface of the PVC sheet and the two plexiglass disks. For the upper plexiglass disk, leave a sector on the disk, at the rear of the generator, free of paint, this will be necessary for the light which will be installed in the upper compartment. I used a painting scheme for the upper plexiglass disk as in figure [15]. Tip: start painting the upper plexiglass disk, next the remainder of the interior and last the exterior. Paint the outside of the generator in the color you like. I used computer-grey paint (RAL ...). Note: The PVC sheet is hard to paint in one time, it's best to use little paint (to prevend sagging), using a soft brush.
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 Figure [15] - Top view of plexiglass disk, paint on other side!
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When the paint has dried, slightly sand the painted surfaces, and repaint it. After this second coat has dried, inspect to decide whether it needs painting a third time. I didn't need to paint the generator for a third time, however.
If you want to, you can paint the bottom of the generator, too. I was impatient to see the generator finished and operating, so I didn't paint the bottom wooden disk.
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Temporarily install the fan with air intake downwards on the upper surface of the upper plexiglass disk, the air intake will more or less
fit around the hole in the disk, as in figure [18]. Now I'll describe the easiest way to make an airflow channel: Take an old sock of yours (not *that* old - it must not have any holes in it), cut off the toes-end to end up with the airflow channel (that's it). Connect one end of the sock to the air outlet of the fan, put some tape, rope or fishline around it to secure it to the fan exit, and reinstall the fan. Now put tape around the air intake where it touches the plexiglass disk. If necessary hold the fan in place with some scrap wood or something. Finally connect the other end of the sock to the end of the PVC tube, and put tape around (this is hard to do, however, because the tube nearly touches the PVC sheet of the generator - I used some fishline to connect the sock). Make sure the sock isn't twist, this will prevend air flowing through the sock to the pvc tube and no tornado may form.
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 Figure [18]
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If you're ambitious, by all means use genuine tubing to fabricate the airflow channel, like that of a vacuum cleaner. For my generator, an old sock was best, because a sock isn't air-tight. This is advantageous, because the fan needs to release (compressed) air. If it can't, it won't suck any more air, either, and the fan will go run like hell, doing no work at all. It's okay to experminent with airflow channels, when you get the generator working. But start out with the old sock method, this works well in any case.
Finally lead the electrical wires out of the compartment, so you can connect the power supply (if applicable to your fan) easily. Then close off the upper compartment with the hatch.
There is much to say about this. The mist generator I used is very small, and will be placed at the bottom center of the tornado chamber,
thus on the lower plexiglass disk. Lead the power connector wire out of the generator via the back side or something, so it isn't
that much visible,
and tape down the wire to the plexiglass and/or PVC sheet, so it won't disrupt the airflow too much. Next, use some hot glue to
fasten the mist generator to the plexiglass, else it may fall over. Then fill the plexiglass disk with clean water (distilled water
is best for the mist generator), until the water is about 30-40mm over the ultrasonic mist generator.
Note: this is only applicable to the mist generator I used!! Other ultrasonic humidifiers will do perfectly, too, but these aren't
operated under water, like mine! You may have to modify the lower plexiglass disk, by drilling a hole in it, for instance, to put
through the mist exit nipple of the humidifier,
if the humidifier you use is a larger machine and you need to install it in the lower compartment. In any case, make sure the mist
produced by the generator will indeed enter the generator's mid section, where the tornado will form, and preferably enters it on the
bottom (thus at the plexiglass disk) at the center (then the inner structure of the tornado will be best visible, else
mist at the outer edge of the generator will obscure the tornado vortex).
The correct installation I have drawn in figure [19a], for if you use the same sort of mist generator I used, and in figure [19b]
for a 'genuine' ultrasonic humidifier, with different generator dimensions (see below:
dimensional modification).
Note, in this case you might want to have a differently shaped plexiglass disk, to fit the
mist generator in the lower compartment. If your mist generator is taller than 130mm you'll need to have a larger compartment! Or
if you already made the tornado generator at given dimensions, you'll have to use the mist generator outside the tornado generator,
and connect to it using a small tube. However, I'm not sure whether the mist will be dense enough, then, so keep connection as short
as possible.
Installing the mist generator
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For the light, use a low wattage lamp! For instance, a 5 Watt bulb or something. Install this in the upper (fan) compartment,
over the window in the plexiglass disk. You can make a switch in the PVC sheet to easily switch the light on and off.
When using a too high wattage light, the lamp will heat the plexiglass disk and may deform it. Also, the PVC sheet is not
even heat resistant and will immediately deform, which is hard/impossible to fix neatly (I had that problem, and ended up with
a bumpy patch of sheeting at the side of the generator).
When using not any lighting at all, the upper part of the tornado will be poorly visible, because the upper front of the
generator will cast its shadow on the misty vortex. The light is to lighten op the tornado from the top. However, I didn't bother
yet to install any lighting in my generator. A floor lamp or something put in front of the generator chamber will light up the
vortex nicely, too.
Rotate the PVC tube so that the air exit holes point to the generator's front opening, as in figure [20], and hot-glue the tube
in straight upward position to the lower plexiglass disk, so it won't rotate by itself or slip.
Next, kit the edges of both plexiglass disks, where they touch the PVC sheeting, with (preferably black) silicone chaulk, to make
these locations water tight. Also, kit around the PVC tube where it touches the upper plexiglass disk.
Let the chaulk harden out, this may take some time. After it has set, check the fan compartment to see whether the sock has twisted
(because of rotating the PVC tube), if it has, untwist the sock.
Cross sectional views of the generator are in figures [19a] and [19b].
This is about adjusting the size of the generator, with the purpose of letting your mist generator fit into the lower compartment.
If you want to use a large ultrasonic humidifier, it is best to install it in the lower compartment (I'll call this the mist compartment
from now on). If it won't fit in this design, or you want to be sure, then adapt to these modifications:
That's it, now you'll end up with a 1.4 meter tall generator with a mist compartment over 300mm in height, which will fit most common
ultrasonic humidifiers. One note about the wooden support blocks: omit the 2 support blocks (that's why you need only 6) on the back
side of the mist compartment
assembly, and after the painting stage, cut a hole in the PVC sheet over the missing supports, to gain access to the mist compartment
at the back of the generator, so you can install the humidifier (the access gap must not be visible at the front side, this really looks
ugly!).
Have fun!
Some fascinating things to do with the generator:
If the generator doesn't produce any tornado, check these items:
Harald Edens
Installing the light
Finishing touches
Figure [20]
Dimensional modification
Running the generator
Now you can test run the generator. Connect the fan to the RPM driver, and this driver to the power supply (or connect the fan to the
Variac(TM) and this to a wall outlet, in case your fan runs on standard line output). Next, pour water in the lower plexiglass disk,
if you use the same mist generator as mine, or fill up the humidifier or whatever. Turn the mist generator on, and if some mist has
accumulated at the bottom of the tornado chamber, run the fan. You should now see the mist start rotating around, and after 5 - 10 seconds
to 1 minute (depending on fan speed), you'll see the tornado develop upwards to the fan suction.
Operation tips
Troubleshooting
With other problems or unsolved problems, don't sledge the thing... you can email me at
edens@multiweb.nl.
You can also email me for hints, suggestions, remarks, typos etc.
If you decide to build the generator, have fun with it and if you like, tell me about it or send me some images of it at work!
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All pictures and photos on this page are copyright Harald Edens. This page may not be charged money for except for copying costs,
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