Making a wind tunnel

A wind tunnel, in essence, is a tunnel with wind going down it.

While the name might speak for itself, the science that goes into making a functional and practically useful one is incredibly in depth, which is why I decided to attempt making one with my limited knowledge of aerodynamics.

To begin with, before even worrying about how the wind tunnel itself works, I needed to find a large hollow tube that was see-through in some section, as this is vital to the final plan of having the iconic smoke trails of a windtunnel:

 

 The initial plan was to use some sort of oil vapouriser, not unlike the ones used in vapes, to make the thick smoke, then a clear acryclic tube as the body of the system. A hairdryer could be used as a vaccuum fan for testing, and then a cheap PC fan implemented as a permanenet solution. 

The issue with using a fan as a wind source is the inherent nature of how they move air, with a spinning blade. This means the wind coming out of the fan is actually spinning, quite fast, so instead of smooth linear air, it blows rapidly vortexing air that will cause turbulence and not allow the smooth trails shown above.

For a rough plan, I sketched out a bottle with straws in one end to act as a flow straightener, the end cut off, and some sort of device to generator smoke.

generate

 

As the fan in this test, I used a hairdryer, turned around to suck air. A honeycomb was made of cut straws to straighten the air flow. I did some digging and discovered glycerin makes smoke when heated. Luckily, I live near many drugstores and managed to get 200ml for under 2 pounds, and one drop of glycerin made thick smoke for 5 minutes straight when heated up. To heat it up, I took two tealight candles, and removed the wax block of one. I bent an old bicycle spoke into a shape where the candle without wax can be placed on a candle with wax. The candle with wax can then be lit, and its heat rises and heats up the metal shell of the other wax..

Apologies for the low quality.

 

 

With this extremely rudimentary setup, I still managed to get some incredible pictures:

I started 3d modelling the wind tunnel, so that I could 3d print the walls, but I soon found out the constraint of my friend's 3d printer, with a max bed size of 30cm and max height size of 15cm. The issue with the curved wall above was that a 3d printer would need a huge amount of 'support' material to print it horizontally, which is wasteful and annoying to remove. I realised that with such a relatively powerful fan, I did not need to have the wind tunnel converge-it could just be rectangular and would suffice. I tried to find a plastic box with around the right dimensions but always fell short, so instead I attempted to make an outer casing for the wind tunnel myself, out of wood. I had recently found some wood from a construction site directly next to my house, thrown out in a skip, so I lifted the plank into my attic and marked down the exact measurements of the new 3d model. 

 

 

I then cut them out and nailed, then glued the peices together

 

While this was being done, I realised there were some issues with the approach of using glycerin as a smoke source. For one, it had caught on fire after a few minutes of running, perhaps because the water I added to the solution had evaporated off - nonetheless, it was scary learning that glycerin was flammable when heated - and I needed to be able to bring in this wind tunnel into a school, where any form of fire, including candles werent allowed. To fix this, I ordered a dirt-cheap ultrasonic speaker for under £3. When immersed in water, the membrane vibrates well over 20,000 times per second. This causes a standing wave on the water's surface, which makes water droplets rise in some places and fall in others (see my post on making a Ruben's tube with standing waves). 

 

This creates an incredibly fine mist, which is akin to smoke, which leaves no residue, and only requires water and 12V to run. Using this to generate smoke, I tried a similar setup as before, just using the water vapour and a hairdryer: