David Spain wrote:
David Spain writes:

Peter Fairbrother writes:

(etc, see below)

Do you know physical principle is behind ZND theory?
Brisance is a description of the phenomena, but I don't find it
a very satisfying explanation of physically what is happening.
Since the shock-wave is propagating at supersonic speed, I have
to believe the physical force at work is electrical. Do you
know if this is the case?

No, it's just atoms or molecules bumping into each other. If they get
hot enough (= bump fast enough) they can give off light when they bump,
but that's mostly incidental.

Compared with a sound wave, the mechanism of transmission of a shockwave
through a material is similar except in that in the case of a sound wave
the transfer is almost perfectly elastic (and energy conserving),
whereas in the case of a shockwave the transfer is more inelastic - this
is because the elastic limits in the material have been overcome by the
high pressure in the shockwave.

The speed-of-sound limitation no longer applies (the "sound barrier" has
been "broken" because of the high energy levels involved), and the
actual speed of transmission depends not only on the material condition,
but also the maximum pressure, the energy, and the detailed shape of the
shockwave.

Because the transfer is inelastic some of the energy in the shockwave is
inevitably lost, converted to heat (or sound, or shattering of solids).

An unusual example of this can be seen sometimes when a shockwave from a
powerful explosion meets the surrounding air (and no actual material
from the explosion has reached that far yet) - the air glows briefly in
a wave, because the energy lost from the shockwave heats it up to
several thousand degrees. You need high-speed photography to see the
wave progress though, usually it's just a glow.



Well, you addressed this question someone in your footnote #2
where you talk about 'opacifiers' being added to explosives to
change chemical propagation by 'light'. I'll leave it at that.

Opacifiers are used mostly to ensure the energetic coupling between
nearby parts of an exploding material is good - if opacifiers were not
used and if a lot of the energy was given off as light then it might
spread out and not reach the next bit of explosive efficiently enough to
cause it to detonate/deflagrate.

[...]

So is it fair to say that brisance determines the material's
ability to change to gaseous state *before* the chemical reaction
which is necessary for the supersonic propagation of the shockwave
relative to the solid material?

Wow, that's a hard question, like "have you stopped beating your wife?"
- it assumes many things which ain't necessarily so.

First off, the chemical reaction is not necessary for the supersonic
propagation of a shockwave. Shockwaves can propagate through any kind of
material, whether BEC, solid, liquid, gas, sparse or dense plasma. Hope
you have got that part now.

In a high explosion the chemical reaction does however drive the
detonation shockwave so it doesn't lose energy and fade out, in fact it
makes it stronger (constant strength shockwaves in explosions are
unstable, and don't happen - this is how the firework guys make whistle
noises BTW).

[ for most high explosives an initial shockwave is necessary to cause
detonation, otherwise if ignited many (eg TNT) will merely deflagrate,
while some others will undergo a deflagration-to-detonation transition -
which is a whole entire different subject, and it's verra complicaaated
indeed, Capt'n. ]


Second, in CJ theory, the fine details of the reaction - how long it
takes,etc, - are not relevant, we are only interested in the outcome.
They may be relevant in ZND theory, but we are nowhere near that
detailed in our understanding as yet.


Third, in a way you might be considered to be partly right, in that
perhaps the high energy shockwave [256] causes the material to turn to
gas before the reaction properly ends - but as above, for simple CJ
theory purposes it doesn't matter how long the reaction takes, within
reason (ie as long as it has happened before the expansion reaches the
speed of sound).

[256] though not the brisance, which is just the shattering effect a
shockwave can have - in this case it's the heating, not the shattering,
effect of the shockwave which causes the transformation to gas.

And if enormous pressures are generated in the shockwave, what
about the temperature within the shockwave?

It can get very high indeed, see the example about air glowing above. In
an explosion it then decreases rapidly to the CJ temperature (which is
still high, usually about 3,000-4,000K).

Since temperature
can effect the speed of sound in a gas and according to your
footnote #3 the pressure is variable why not the temperature?

The temperature is variable during the reaction, and during the
expansion to CJ conditions when the speed of sound matches the speed of
the shockwave - but the CJ temperature is fixed (actually it's not
exactly fixed, but the variation caused by the amount of energy given to
the shockwave is small).


And if so, wouldn't that make the shockwave speed also variable?

Yes, but only to a small extent, a percent or two, as above - and there
are usually other factors, like shape and confinement, which can cause
it to vary by 20% or more.

Also the small change above is usually offset by another change I'm too
lazy to go into just now, and the two changes almost exactly cancel out.


-- Peter Fairbrother



Dave