Explosions and Unobtainium, as pertains to video game physics

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Aikanaro
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Explosions and Unobtainium, as pertains to video game physics

Postby Aikanaro » Mon Jan 30, 2017 3:17 am UTC

So I have another odd hypothetical question....

I'm assuming pretty much everyone here has played enough video games to know that if there's a powerful explosive detonating next to you, and you step around a corner, then barring destructible environments, you typically will take zero damage from the blast. This has held true from Bomberman to present day; all damage radiates out from the center of the explosion, and if there's anything blocking that line-of-sight, then you're completely unharmed.

However, game physics don't take into effect things like air currents, thermal convection, etc. So I was wondering, let's say that I have a a building-sized block of adamantium or some similar substance, with The Blob from Marvel comics sitting on top of it, to guarantee it can't be moved. Let's say I'm standing next to the north face of the block, near the east corner. If a non-nuclear, but powerful bomb (we'll say something around the power level of the OK City bombings) is detonated on the east face of the block, near the north corner, will shock waves, thermal currents, etc., liquefy me? Or would it all simply be directed north, past me?

EDIT: Let's also assume the experiment is out in a flat desert somewhere, so there will be neither rubble, nor nearby extrusions from the landscape for shockwaves to reflect off of.

Hypothetical Explosion.png
Hypothetical Explosion.png (7.72 KiB) Viewed 2084 times
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Re: Explosions and Unobtainium, as pertains to video game physics

Postby pogrmman » Mon Jan 30, 2017 4:57 am UTC

I believe the shockwave would diffract around the corner. I'm not sure the extent of the diffraction, so I can't tell you exactly what it would do to you.

I imagine that depending on how close you are, it would be somewhat attenuated.

EDIT: This may be of some interest. (It's about exactly what you are asking -- the diffraction of a shockwave around a sharp 90° corner.) [In case you don't have access, to sum it up, there is some attenuation, but not much. If you could survive, you'd see neat things like vortices produced by the passage of the shockwave.]

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Re: Explosions and Unobtainium, as pertains to video game physics

Postby jaap » Mon Jan 30, 2017 1:23 pm UTC

Here's another data point. The Mythbusters tested a scene from Lethal Weapon 2:
https://en.wikipedia.org/wiki/MythBusters_(2011_season)#Episode_178_.E2.80.93_.22Toilet_Bomb.22
It turns out that that a rather heavy metal bathtub can protect you from a blast that would otherwise have been lethal (180 psi reduced to 8 psi).

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Re: Explosions and Unobtainium, as pertains to video game physics

Postby Zamfir » Mon Jan 30, 2017 7:10 pm UTC

There is a bit of truth in the 'line of sight' model of damage. Bomb fragments are typically the major source of injuries, with a large radius of effect. Standing behind a corner will shield you from most of that.

Another major source of injuries is building damage - collapse, or secondary fragments like glass splinters. Those are excluded from your hypothetic case.

Next factor : direct injuries from overpressure from the blast, especially collapsed lungs. This is a factor for high explosives, but not so much for low explosives (roughly put, bombs vs accidental explosions ) . The blast wave is the 'edge' of the expanding volume of air around the bomb, and it will go around corners in (roughly) the same way that sound does.

Th human body can take a remarkable amount of blast pressure - lethal effects occur at a few bar of overpressure. For a bomb as Oklahoma, that's about 25 to 50 meters away, if I did the calculations right. Standing around the corner has little effect on that.

Next effect: heat. This is also relevant for low explosives. I am less sure about the relevant formulas here, but a simple assumption might be that the explosion uniformly heats all air within a quarter-sphere of air (because of the high building). For 25 meter and OKC, that gives me 400K temperature rise, for 50 meters 50K. Looks similar to the overpressure in injury terms. Again, little benefit from the corner- the hot gas will expand around it, and the blast itself heats the air.

Finally, being thrown around. For your scenario, this might actually be the largest danger. Those overpressures come with wind speeds of many hundreds of kilometers/hr (though only lasting half a second). Even if the corner is further away than 50 meters, you might still end up slammed into the unobtainium building by the vortices around the corner. Edit: after the blast has passed, the displaced air will be sucked back in. As you can imagine, that is not a good moment to be standing near that edge of a building.

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Re: Explosions and Unobtainium, as pertains to video game physics

Postby poxic » Mon Jan 30, 2017 7:15 pm UTC

You know an awful lot about this.
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Re: Explosions and Unobtainium, as pertains to video game physics

Postby Aikanaro » Mon Jan 30, 2017 11:37 pm UTC

Thanks, all! So basically, for a people-killing bomb, ducking around a (incredibly durable) corner might protect you....but for a building-killing bomb, you're still screwed.

Also, I deliberately imagined a setting that's OUTSIDE, since I know that if you're ducking around a corner of a hall, the blast will still end up being channeled towards you.
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Re: Explosions and Unobtainium, as pertains to video game physics

Postby LaserGuy » Tue Jan 31, 2017 7:31 pm UTC

I imagine that if rather than being just around the corner, the block was actually between you and the explosion, you'd probably be much better off, even if the block were proportionately quite a bit smaller. It's much less likely that the dangerous air currents would make it around two 90 degree corners with too much intensity. I think it would mostly just be the heat would be the problem, which would depend a lot on the thermal conductivity of the material.

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Re: Explosions and Unobtainium, as pertains to video game physics

Postby morriswalters » Wed Feb 01, 2017 2:43 am UTC

Aikanaro wrote:EDIT: Let's also assume the experiment is out in a flat desert somewhere, so there will be neither rubble, nor nearby extrusions from the landscape for shockwaves to reflect off of.
If the drawing is correct there will be reflection from one source, the rigid wall, it's set up like a perfect mirror. And debris from the ground. The crater in Oklahoma was 30 foot in diameter and 8 foot deep. It looks like if it's close enough to the corner the blast might travel under the wall at the corner.

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Re: Explosions and Unobtainium, as pertains to video game physics

Postby Zamfir » Wed Feb 01, 2017 12:55 pm UTC

LaserGuy wrote:I imagine that if rather than being just around the corner, the block was actually between you and the explosion, you'd probably be much better off, even if the block were proportionately quite a bit smaller. It's much less likely that the dangerous air currents would make it around two 90 degree corners with too much intensity. I think it would mostly just be the heat would be the problem, which would depend a lot on the thermal conductivity of the material.

Here's a picture of a blast wave going around a (3d) cube. Picture randomly from the net.The wave is flat, generated somewhat 'upstream', not quite the same as the situation in the OP with a blast centre close to the building. Colors indicate pressure.
Image
There's a red (extra high) edge to the high-pressure zone, that's typical of detonation blasts. In front of the block, pressure increases even further as the impulse of the blast reflects against it.

The parts of the wave of that diffract around the back of the block are a tad weaker than the main wave (it lost that red edge), but still strong. Eventually they will meet in the middle of the block and reflect off each other, creating a pressure peak again.

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Re: Explosions and Unobtainium, as pertains to video game physics

Postby morriswalters » Wed Feb 01, 2017 2:52 pm UTC

Such lovely toys. I'm assuming the center for the blast wave is off the image. In the OP's setup all propagation is away from the wall. Wouldn't the reflection wave be indicative of the OP's explosion? The pressure wave has to diffract twice. Wasn't this setup something like a space craft propelled by small nukes. Project Orion?


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