How do blast waves and shock waves relate in an explosion?

[blastbasics]

I would like some help in understanding the basics of a blast caused by an explosion. The company I work for supply instrumentation systems which are then connected to accelerometers to measure shock and air blast sensors to measure the pressure wave caused by any ballistic blast. I would like to have a better understanding of the shock wave and how it is formed and the relationship between the blast wave and the shock wave,,,is one possible without the other? Anyone one who can help or suggest the best place to read up on this?

 

[Simon Bridge]

A start:
http://en.wikipedia.org/wiki/Blast_wave
... will help you out with the concepts and how the blast wave is related to the shock wave.

 

[boneh3ad]

Except the Wikipedia article isn't 100% accurate. For one, the flow behind the shock is not necessarily subsonic relative to a stationary observer. It merely has to be subsonic relative to the advancing wavefront.

 

[Simon Bridge]

Yeah - wikipedia is not the end of the search just a beginning.
Perhaps you can suggest another reference?

 

[boneh3ad]

Well I mean, a blast wave is really just an (initially) spherical shock wave created by an explosion, so it is a subtopic of compressible gas dynamics and any text on the subject would be able to give you a fair amount of insight. Something like https://www.amazon.com/dp/0486419630/?tag=pfamazon01-20 by Liepmann and Roshko is a good source and is really cheap on Amazon to boot. It is a textbook, however, so it is pretty "mathy" and it does assume the reader is familiar in some way with fluid mechanics beforehand.

That would give the basic understanding of shockwaves necessary to delve deeper into those produced by a high explosive. After that you would likely need to find sources that are more directly focused on explosives.

 

[Bobbywhy]

blastbasics, Welcome to Physics Forums!

You’ve already received some good suggestions from members here for learning more about explosive blast waves. Since your company supplies measurement systems for these phenomena it seems there probably are persons right there inside your company who’ve designed and developed these products. Have you checked with them for learning resources? They might suggest textbooks, journals, or loan you technical papers on the subject.

Also, here on Physics Forums last year there was a thread you may find illuminating (pun intended). The subject was atomic bomb shockwaves. Be sure to check all the references therein.
https://www.physicsforums.com/showthread.php?t=591191&highlight=shock+wave+atomic+blast

Bobbywhy

 

[blastbasics]

Many thanks

Many thanks for all your useful comments. I understand that the basic shape of the blast wave would be spherical, if we ignore for now any ground reflections etc. Let's say that at 5 meters distance from a 1Kg charge we would see a 1 atmosphere (15 psi) over pressure for a 360 degree radius from the blast centre. But I would like to reproduce that for just a small part of the over all spherical blast wave, let's say just 1mSq of the resulting 15 psi blast at 5m. This would allow me to test new instruments whilst not having to set up big, expensive and dangerous tests. Does anyone know how this would be possible to do, what size of charge you would still need? So we could imagine a blast wave of conical shape, which at 5m from its source would be approx 1msq. I would not be so interested in the shock wave part, the over pressure would be the most interesting part.

I would be very interested to hear anyones comments if this is possible

 

[Bobbywhy]

blastbasics,

I cannot imagine how to create a blast wave of conical shape. What would constrain the wave from spreading spherically in the atmosphere?

Overpressure may be calculated using Wiebull's formula. See: http://en.wikipedia.org/wiki/Overpressure

The Sadovsky formulas also allow characterization of the blast wave, and from that, the overpressure could be derived (I guess).

See this pdf document that defines terminology for detonation wave, blast wave, shock wave, and overpressure:

“Explosion and Blast-Related Injuries, Part 3: Modeling and Mechanisms of Primary Blast Injury, “Blast Wave Dynamics and Forces

An explosion is caused by the rapid exothermic oxidation of a solid or liquid material into gaseous reaction products resulting in a large energy release in the form of increased pressure and temperature within the explosive compound. That reaction and pressurization propagation process within the explosive is known as the detonation wave. In solids and liquids, detonation waves propagate from the center of ignition outward at supersonic speeds of 6 to 8 km/s (6.8 km/s for TNT) (Henrych 1979). These reaction gases expand violently, compressing and forcing out the surrounding air. A pressure wave, blast wave, is formed, spreading in air radially outward. The blast wave consists of a microns-thin pressure wave, known as the shock wave, followed closely by the blast wind. There is a dramatic increase in pressure across the shock wave. Based on the Sedov–Taylor blast wave self-similar solution (Taylor 1950), the pressure-time history of a spherical blast wave can be expressed in the form of the Friedlander equation (Baker 1973; Kinney & Graham 1985; Sedov 1993).

There are several sets of equations for predicting peak static overpressure, pS~Z, developed using both numerical and experimental techniques. The best known semiempirical models are the Brode and Henrych equations (Brode 1955; Henrych 1979; Smith & Hetherington 1994). Brode (1955) proposed two equations for near and far field. The Henrych model, which is more accurate in the near zone, divides the analysis into a near, middle, and far field zones and expresses the pS (in kPa) as: (formula)”

http://c2.api.ning.com/files/g64JBXRPh0yB6BK99PdSBQxUuRMpWdptsmMioQR2f-*zLxjnErJRhj*sOxFB6X6omQH1XWe2B8bUgdi9P1DjTqC31DXIcXmu/ExplosionandBlastRelatedInjuries.pdf

It would seem anyone in your field should have this seminal textbook:
Henrich J. The dynamics of explosions. Elsevier, Amsterdam, 1979. (spelled “Henrych” in the above reference document)

Finally, you have not commented on my proposal made in post number six above concerning consulting with engineers in your own company regarding product testing. Do you think someone at your company might offer you some useful information for your project?

Bobbywhy

 

[blastbasics]

Many thanks for your thoughts Bobbywhy, I was hoping to use some sort of barrel with a smaller amount of HE to give the directed blast wave, like an open ended shock tube if you like. But I have no real ideal how quickly any blast wave would disperse in open air once it had left the confines of the barrel. From what I have read up on, the use of a conical type tube would give the most realistic blast wave. But again I have no idea of what size of barrel to what size of charge would give me the best results. And also if a smaller diameter barrel then expanding into a larger diameter would allow smaller amounts of HE to be used.

Again our goal here is to produce a 15psi pressure wave in open air about 5m from the ignition point, we would want the 15spi to cover a 2 dimensional area of about 1mSq at 5m distance, but the speed and duration of the blast wave itself are not so important.

I understand your comment about engineers we work with but please understand the people who are experts in measuring this type of phenomenon are not the people who are experts of creating it in the first place it really is a very different skill set.

 

[boneh3ad]

If you are setting off explosives in a confined space, then there is no point in saying you want it to have your measurements at 5m from the ignition point if the goal is to compare it to the effect of the same explosive free in the air. You would need to quantify the focusing effect of the confinement vessel quite accurately to get an accurate comparison.

If the goal is simply to test new instruments, what prevents you from simply using a shock tube, which is safer and more repeatable?

 

[blastbasics]

boneh3ad, many thanks, but the goal is not to compare the same blast, we would only require a small segment of the full 360 blast wave that would be created by a standard open air test. So we hope by only looking at one segment of the over all 360 degree effect we can cause the same readings with a much smaller amount of explosive.

We could not use a shock tube as we would ideally like to test the instruments in situ. During any explosive test we would have a number of pressure sensors at different heights and different distances from the ignition point. If we can "project" a 1Msq wall of pressure towards then we could test a number of sensors and the complete data acquisition set up at the same time.

 

[boneh3ad]

That's what I am saying. You could almost certainly do that sort of thing with a shock tube. A blast wave is just a shock wave anyway so you could create a shock wave in a tube that propagates down the tube without the need for explosives that gives you the same overpressure and Mach number fairly easily. From what you have said so far I see no reason that wouldn't work. Maybe I'm misunderstanding your goals?

 

[blastbasics]

Hi Boneh3ad, maybe my understanding is incorrect but I understood that any pressure wave produced inside a shock tube is dissipated very quickly when in open air rather than the confines of the shock tube. What I want to be able to do is "fire" that pressure blast a distance of 5M. So what need to be able to calculate is what size of shock tube, what size of charge would allow me to do this, and then if it is practically possible to make that shock tube easily portable. So ultimately would it be possible by playing with the size and configuration of the tube and the type of excitation used (explosive blast, or compressed air source etc) to make such a thing small enough to carry by hand?
Again this is not my area of expert ease, the measurement side is, but if we had a volume of gas at high pressure inside a shock tube as it leaves the tube i assume the air blast area would spread and of course at the same time the pressure would reduce, so would you end up with a conical area of pressure coming from the end of the shock tube? I know this would be a large volume of gas (v=.3(pi)(r)sq* height...so...v=.3(3.14)(.25)*5= 1.3 m cubed,,is that correct?). I would be very interested to hear your comments. Many thanks

 

[cjl]

blastbasics: I think what boneh3ead is suggesting is that you put your instruments in a shock tube, not that you use a shock tube to somehow "fire" a shock wave at your instruments. I have to agree with boneh3ad that this would be the best way to repeatably get a shock wave of whatever magnitude you desired with relatively low cost, setup, and without significant quantities of explosive. Is there any reason this method couldn't be used?

 

[blastbasics]

I am sorry I must not be explaining myself well, we want to test the instruments in situ of the actual test place. When tests like these are carried out it can days to set up the test, all the sensors (100's) sometimes, have to be placed, measured out precisely, documented, and then tested. The cabling back to the data acquisition systems has to be laid out, protected and connected, it is a huge under taking. If we can test the sensors once they are in place after the set up it would help ensure tests are set up correctly and hence save much time and effort if mistakes are found before the real testing.