It's a pleasure to me to share with the DIY Community this Instructable, where I'll teach you guys how to make a blast wave (the same blast waves present in actual explosions) in your home, in a (sort of) controlled way! These waves are used in the industry to secure buildings against explosions and also in supersonic aircraft development. So how cool is that?
You can watch the video above where I blow up some flowers taken (with my grandma's permission!) from our garden and go through some of the build details. Or you can just read through, it's a fairly easy build and you should have no difficulties building your own version!
This is a potentially unsafe project if proper safety precautions are not taken. Shockwaves can be deadly and will produce sounds above 191dB (because that's a definition of a shockwave). Wear double ear protection, earmuffs and earplugs. Don't point the barrel at any living thing (the flowers were dead already, OK??), because the waves can hurt quite a lot or even kill smaller livestock!
Seriously, this is not a toy.
Step 1: So What's a Shock Wave and How to Make One?
A shock wave is a sound wave that went wild. When the intensity of the sound wave is very large (very loud sound), a positive feedback phenomenon occurs that makes the wave sharper and sharper until it becomes a "discontinuity" in space, a few molecules thick. It travels faster than sound, and the more intense it is, the faster it travels.
It's basically the stuff that makes explosions powerful as they are! So how do we make one? There are quite a lot of ways of making shock waves, but the most famous in the scientific community is definitely to build a shock tube. It's a device, as shown in the picture, where we have two sections: A driver section (named there as "4") and a driven section (named as "1").
The driver section will hold gas at a larger pressure, whereas the driven section will have lower pressure. They are separated by a membrane, that somehow "suddenly" disappears. In a real shock tube, the membrane is a plastic film (or, for larger pressures, an aluminum/steel sheet metal) that is designed to burst at a definite pressure. Another way to do it is to puncture the membrane with a sharp edge, forcing it to burst whenever the operator wants to.
So, in summary, its a tube that's rigid and sealed enough to hold some gas pressure and that will burst in a specific place. This bursting will create the shock wave, which can be guided in the driven section to a "test subject", that will receive the blast.
Step 2: The Materials
The bill of materials I used is as follows. Bear in mind I'm in Brazil, where PVC piping is metric. You can use whatever you want, as long as it's strong not to burst before the membrane (that would be catastrophic!!!). ***SERIOUSLY, YOUR TUBE CANNOT BURST, THIS WILL MOST LIKELY INJURE YOU***
2 units - 500mm of glued PVC tube (Ø40mm OD)
2 units - Glued PVC adaptor Ø40mm OD female to Ø1.1/4" male thread
2 units - PVC flange Ø1.1/4"
4 sets - 1/4" bolt and nut set
1 unit - Ø40mm PVC cap
1 unit - Bike tire fillup nozzle
1 unit - 2mm smooth rubber mat (for the gasket)
1 roll - PVC film for air conditioner hose finishing
That costed me (in Brazil) roughly $15. In America I believe it will be way more expensive (maybe $40?) as my experience was that this kind of material is quite expensive - at least in the East Coast!
Step 3: Some Details
Drill a hole (my case Ø8mm) and assemble the bike pump adaptor. Glue the PVC cap and threaded adaptors as shown in the images. The flange in the driven section was glued to improve safety and leak protection.
Cut the rubber mat with a pair of scissors in the shape of the flange. And that's basically it. This can be built in less than two hours without a doubt!
Step 4: Preparing the PVC Membrane
I used as a membrane a 70um thick PVC film. I actually used 3 layers of it in the blasts shown in the video, as it bursts at 4.5 bar. It's quite linear, though, bursting at 1.5 bar with one layer and 3 bar with two layers. These layer/pressure combinations are quite geometry specific and might lower if you use larger pipes.
Again, be careful not to make the membrane too strong. IT MUST BREAK AT A PRESSURE WAY LOWER THAN THE PRESSURE THE PIPE IS RATED FOR. I cannot stress it enough, this is a dangerous experiment.
Other materials I used, without as much success:
>> Aluminum foil: Bursts well, but the pressures were too low (<0.5 bar even with 3 layers)
>> Stretch film for packaging: Bursts at way too low pressures (<0.5 bar for 7 layers)
>> Balloon rubber: Bursts at ~1 bar for 2 layers, but stretches too much before rupturing.
>> Packaging tape: Bursts at a very low pressure (the pressure gage didn't even move)
>> Office paper: Leaks too much as it wrinkles under stress
>> Photographic paper: Way too strong, didn't burst at 5 bar (I deemed unsafe to go further)
My PVC pipes are rated for 7 bar operating pressure, so I was fine using them. Be careful to review the ratings and make sure the adaptors/connectors/flanges are also rated to an acceptable pressure!
Step 5: Blowing Up Stuff!
So the footage shown in the video (Step 1) is quite interesting, but here I wanna share some analysis of what a regular (non high-speed) camera can capture in the frame-by-frame view. Sorry, I cannot afford a high speed camera!!
The water experiment shows a big mess made by splashing water everywhere and the frame-by-frame cannot show us anything other than water spilling over at very high speeds everywhere.
The flower experiment is more interesting. The shock wave literally smashed the flower into the wall! The second frame shows a (surprisingly) transparent flower, signing that the flower was only there during a fraction of the exposure time. Then it was rammed into the wall in <33 milliseconds! For this overpressure I used in this experiment, the Mach relationships suggest a Mach number of ~1.5 (the wave would travel 1.5x faster than sound, or 510 m/s approximately).
This is fascinating and I'll definitely ram shockwaves onto other objects with this device (just for the fun of it!). If anything turns out interesting, I'll update this instructable!
Thanks for reading through, and if you make your version, share the experiments you made, I'll be quite happy to see what you guys want to blow up! Play safe!