Hydrodynamic Shockwave Simulation

[nbk2000]

I was just thinking that a lot of the more advanced techniques of explosive weapons designs (such as EFP, flying cutter, multipoint initiaton, etc) are too difficult to "guesstimate" or visualize in ones head.

The pros have huge budgets, test ranges, high speed cameras, supercomputers with hydromodeling programs, and all the other neat toys that we'd give our left nut to have, but don't. :(

So, how does the po' folk like us simulate an explosive shockwave in a manner that we can visualize?

Well, borrowing an idea from an Oct '55 Scientific American article, we'd make a simulator that uses a fluid and dye to simulate and visualize.

The article describes using a thin layer (~2mm) of flowing water with dye grains upstream to simulate the turbulance around wings and to visualize said turbulance.

I'm thinking a person can make a rectangular "tank" out of wood or metal sheeting, mount a plate of plexiglass a few millmeters above this, and seal the edges with screwed in or welded flats. The plexi is attaced with a screwed down frame and sealed to the tank with a gasket made from inner tube rubber.

<img src="http://server3001.freeyellow.com/nbk2000/Wave_Tank.gif" alt=" - " />

To simulate a shockwave flowing around a waveformer for use in an EFP (for instance), you'd first make your waveformer model from styrofoam sheet and place it in the tank. I'm thinking that it'd be made slightly thicker than the tank and, when the cover glass in screwed down, clamped into place.

The tank is then fixed vertically, filled with water, and allowed to settle. The top end is left open, but covered, to allow for water movement.

To "initate" your explosion, you whack a piston filled with dyed water (or something) with a sledgehammer. The dye piston is attached to the tank by a fitting sealed with a very thin piece of metal that acts as a pressure seal to retain the dyed fluid till whacked.

Once the dye piston is whacked, the seal ruptures and the dyed fluid flows through the tank and around the waveformer as any wave, explosive or water, will.

Being viewable, the wave can be videoed for later slo-mo analysis.

[megalomania]

That sounds good for simulating an ordinary blast, but how would you set up such a system to model, say, a flying cutter? I agree that unless you happen to have a Cray supercomputer laying around, you will not be modeling many blasts. The data from such people that do have the real thing is usually published. The Journal of Propellants, Explosives, and Pyrotechnics often has such articles.

[pyromaniac_guy]

is a supercomputer really needed???
dont forget that the computer power used to design a majority of the us nuclear arsenal can now be duplicated in a cell phone, or worst case, a high end scientific calculator....

Some time ago i could ahve sworn i saw a refrence to hydrocode in the public domain written by one of the national labs.. I wish i could remeber where i saw it... my book marks are such a mess...

[nbk2000]

Well, it's no coincidence that most of the worlds top 100 supercomputers are owned by the DoD (and NSA., but that's beside the point).

The largest SC in the world is owned by either Sandia or Lawerence Livermore (I'm always getting the two mixed up) to model H-bomb designs.

There are programs available that will model explosions (as illustrated in the "Multipoint initiation and Asymetric effects" topic by me), but I've never found a warez copy, and buying it is out of the question, what with a multi-thousand $ price range.

One thing at a time. First, you have to be able to generate visible waves. Once you have that, then comes the problem of simulating materials. I'm thinking of flexible gelatin or very thin metal shims that would bend ubder the pressure wave.

Unfortunately, the melting that metals undergo under the intense pressure of an explosion would be very difficult, I think, to simulate. Though, there may be possibility in the use of fluids insoluble in each other, and of different densities, to simulate different material densities.

For instance, water is (I believe) 400x more dense than air. So this is totally going to skew things in the sim tank. What's needed is a fluid that is of a different density as water, but that would have the same difference ratio as air has to (the effect being modeled). Does that make sense?

Anyways, while this isn't the end all and be all of the science of simulatiing, I think that it'd be of great help to furthuring our understanding of wave effects and their application to our field.

[pyromaniac_guy]

why use water, use air.... schleiren photography is a method of shadowography that shows even miniscule changes in the density of air... normally this is used on a much mure subtle scale, if you will, then whats involved in explosive research, but a quick google search shows that aparently it's used to investigate wavefront propagation and the like... might be something you would want to check into..

[vulture]

How about using steam or air saturated with water? In the highest pressure zones the water will condense. Only problem is that certain materials will propagate condensation (steel, glass,...)

[Polverone]

I'm quite confident that a high-end PC can produce reasonable simulations in a tolerable amount of time, no supercomputer required.

Hmmm, this page has some interesting software:

" Free and Low-Cost CFD Software: Explosives, Weapons Effects, and Impact Engineering"

<a href="http://capella.colorado.edu/~laney/softboom.htm" target="_blank">http://capella.colorado.edu/~laney/softboom.htm</a>

Packages written and used at U.S. national laboratories, yours for $1,000 and under. Sure, it's more expensive than a CD of clip art, and most of the packages won't run on Windows, but what do you expect? I'm guessing that even if you obtain these packages, it will take a lot of knowledge and patience to be able to use them properly. I would still take them over a water tank, though. But unless you already have expertise in this area, how will you know if your simulation results were reasonable or you totally buggered something up? This is an issue that comes up with a lot of scientific software. There are no dancing paperclips, no handy presets for common tasks, no gentle video tutorials. There's the source code, a manual that's 2 revisions out of date, and the expectation that the end-user will be an expert in his/her field and immediately know what all the parameters and acronyms mean.

[Mr Cool]

Just a thought for simulating the refraction of a wave as it goes through regions of different density:
A region of high density would be a cut-out shape (the shape of the region of high density, obviously), that is, for example, half as thick as the thickness of fluid in the tank (I can tell this isn't going to make sense...). This would be stuck into the tank. As the wave of dye reaches it, it will still flow over it, but more slowly than it does through the rest of the tank since the space it can flow through is more narrow = more resistance to flow. This would simulate a shock wave slowing down as it passes through... something.
See what I mean? If not I'll knock up a little picture. I'm not sure if it'd work, but I think it would.

P.S. - what's a "flying cutter"? Sounds like fun.

[nbk2000]

Water is better than air simply because it's much easier to handle. You can easily visualize dyed water with the naked eye.
Whereas, with schleiren shadography, you need a pinpoint source of extremely bright light, high speed cameras, mirrors, and filters. This is even more complicated than water tank sim.

Mr. Cool, I understand what you're saying. By leaving a small gap between the model and the cover plate, this would allow the dyed water to flow "through" the model, but at a slower rate then the unimpeded dyed water. Only problem I see is the need for very fine tolerances and some math to appropriately scale the gap to model similiar effects as the real thing. You'd have to take into account thin layer boundry turbulance and such esoteric things.

A "flying cutter" is similar in concept to an EFP except, instead of being a point penetrator (bullet), it's an area penetrator (wedge). Think of a flying ax...make a long cut, instead of a hole, through steel, concrete, and rebar.

[vulture]

I saw a decorative humidifier in Germany a few days ago. It produces fog out of water with the help of ultrasonic sound. However, the process does not work by evaporation of the water, because the fog feels very cold. The water than dissolves into the air when it reaches warmer air. Maybe this would be of some use?

[Microtek]

I think that it wouldn't be impossibly difficult to come up with the physics part of the simulation software. Of course it would be somewhat simplified, but even something as simple as assuming that every differential volume of explosive initiates a spherical wave, will enable you to simulate the propagation of detonation in single point initiations, including the flow around inert inserts.
Certainly, things like colliding waves and edge-effects would be missed, but I think it would be just as good as the other alternatives.