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Old 2020-08-05, 20:05   #3928
xilman
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Quote:
Originally Posted by kruoli View Post
A long time ago, we had a similar explosion in Germany: http://en.wikipedia.org/wiki/Oppau_explosion
I well remember Flixborough, though I lived too far away to experience it first-hand.

A question on a finals paper 2 or 3 years before my examinations used it to see whether wannabee chemists understood thermodynamics well enough to apply their knowledge to real-world scenarios. It was given to me and my cohorts as practice for our up-coming exams.
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Old 2020-08-05, 20:50   #3929
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Serge, my master thesis was on pine dust/air cloud explosions to characterize the minimum ignition temperature (thermal auto-ignition temperature). Picture below part of the study for dust dispersion analysis before jumping to real business...lol
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Last fiddled with by pinhodecarlos on 2020-08-05 at 20:52
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Old 2020-08-05, 21:57   #3930
chalsall
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Quote:
Originally Posted by pinhodecarlos View Post
Picture below part of the study for dust dispersion analysis before jumping to real business...lol
Cool. It sounds like you got a *real* education. Somewhat rare, nowadays. ("Inside voice; outside voice.")

I posted this several months ago, but (as usual) SpaceX is doing some very interesting work, including in GPU CFD. Please see this article, and this YouTube video.
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Old 2020-08-05, 22:06   #3931
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Quote:
Originally Posted by chalsall View Post
Clearly, the wavefront is super-sonic for a good second or so (the radius covering over a km) (all guestimates).
By definition, shockwaves are supersonic - the stronger the shock, the higher the Mach number. A nasty thing about shockwaves - you may see them coming, but you'll never hear them coming. Oh, I see you mentioned computational fluid dynamics - shocks are one of the most difficult things to do well with CFD, due to their extreme narrowness - real shocks are just a few molecular mean free paths thick, literally microscopic. So you want to model airflow around, say, an entire wing section of a fighter jet or high-subsonic airplane, and suddenly you've got this microscale fluid phenomenon to deal with. In my PhD program at U. Michigan we were fortunate to have 2 of the world's leading experts on so-called "approximate Riemann solvers" ('Riemann' refers to said mathematician's work on the 1-D 'shock tube' fluid problem), Bram Van Leer and Phil Roe, on the faculty. My work was in non-shock fluid problems, but a number of my grad-school colleagues were doing cutting-edge work on things like adaptive multigrid meshing for modeling moving shock waves. Challenging, fascinating stuff.

BTW, 2,750 tons of Ammonium Nitrate is roughly 1,000 times the amount used by the late Timothy McVeigh in his 1995 bombing of the Oklahoma City Federal Building, though in McVeigh's case the stuff was mixed with fuel oil and thus would have had a relatively higher explosive yield.

When I first heard about the nearby Lebanese grain-silage I thought a graindust explosion might have been responsible for the blast, though the scale of the blast and the distinctive rust-colored smoke plume associated with Am-Nitrate quickly put that to rest.

Last fiddled with by ewmayer on 2020-08-05 at 22:24
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Old 2020-08-05, 22:07   #3932
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Quote:
Originally Posted by chalsall View Post
Cool. It sounds like you got a *real* education. Somewhat rare, nowadays. ("Inside voice; outside voice.")

I posted this several months ago, but (as usual) SpaceX is doing some very interesting work, including in GPU CFD. Please see this article, and this YouTube video.
I was pitched by a salesman on the prospects of doing whole-car-engine simulations including flow and combustion. In 1980. I managed not to laugh out loud in his face. Barely.
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Old 2020-08-05, 22:23   #3933
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Quote:
Originally Posted by ewmayer View Post
By definition, shockwaves are supersonic - the stronger the shock, the higher the Mach number. A nasty thing about shockwaves - you may see them coming, but you'll never hear them coming.
The speed of sound is the velocity of an asymptotically low pressure ratio shock wave.
I've heard the shock waves of many an aircraft or smaller projectile, from well off the trajectory. Slightly supersonic projectiles have a quite different sound than Mach 2-3.
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Old 2020-08-05, 22:28   #3934
chalsall
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Quote:
Originally Posted by ewmayer View Post
By definition, shockwaves are supersonic - the stronger the shock, the higher the Mach number.
You are commingling the nomenclature.

Quote:
Originally Posted by ewmayer View Post
A nasty thing about shockwaves - you may see them coming, but you'll never hear them coming.
Well, duh...

Somewhat similar to soundwaves. Photons are pretty quick, in comparison to just about anything else (when available).
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Old 2020-08-05, 22:57   #3935
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Quote:
Originally Posted by chalsall View Post
Photons are pretty quick, in comparison to just about anything else (when available).
Just about. Subatomic particles at high speed when entering a higher refractive index material make electromagnetic shock waves, producing Cherenkov radiation. Electrons above ~400KeV entering glass, or cosmic rays entering the upper atmosphere will do it.
https://en.wikipedia.org/wiki/Cherenkov_radiation
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Old 2020-08-05, 23:06   #3936
chalsall
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Quote:
Originally Posted by kriesel View Post
Subatomic particles at high speed when entering a higher refractive index material make electromagnetic shock waves, producing Cherenkov radiation.
Sure. Empirically demonstrated.

Hawking radiation is similar, but different.

Physics is fun. I wish I could discuss it more with people in "meat space".
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Old 2020-08-05, 23:14   #3937
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Satellite photo indicates which Beirut port warehouse held the ammonium nitrate. It's the one where there's now an approximately 70 meter radius flooded crater. https://www.foxnews.com/world/new-sa...rut-explosions

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Old 2020-08-05, 23:49   #3938
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another set of photos with a slider

https://www.lemonde.fr/international...8234_3210.html

Last fiddled with by firejuggler on 2020-08-05 at 23:49
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