Shockwave Calcs

[Agnaa]

Please, someone help resolve my confusion about shockwave calcs.

I found this accepted calculation, where the energy of a shockwave was found by applying KE, using the mass of the air the shockwave traveled through, and the velocity that the shockwave traveled through that air. This sounds invalid to me since that entire volume of air isn't being moved at that speed, small portions of the air are being jostled while the frontier travels through that volume of air at that speed.

I found this accepted calculation, where the energy of a shockwave was found by assuming that the frontier of the shockwave had the same amount of energy as a sonic boom, and then applying that energy level to the surface area of the shockwave's frontier. This sounds invalid to me since that's a very high energy level to be assuming. It's justified in that calc (imo poorly) because they're created through pure physical force (that doesn't tell us that they're as energetic as sonic booms) and because the characters are mach-speed through other feats (that tells us nothing about the energy of this feat). (I should note that this method does seem fine for the Godzilla calc that was based off of; that character's explicitly described as attacking by generating sonic booms.)

Ideally I'd like to use a method like the latter one, but with more reasonable assumptions about energy at the shockwave's frontier.

Both of those calcs are made weird with what I know of physics; if I remember correctly, for the same material at the same temperature, shockwaves travel through that material at the same speed. A shockwave being more energetic just lets it travel for longer; it's not made any faster.

I then found this thread from a year ago, where a certain formula was made illegible for shockwaves, as it was created to derive the energy yield of atomic bombs from their shockwave. A certain formula was still allowed to be used for general shockwaves, as long as that shockwave produces lethal-level destruction at the frontier being measured. However, DT acknowledged that many fictional shockwaves just end, with everything before it ends presumably being lethal to ordinary humans. Yet that there may be some more realistic shockwaves that disperse and should be handled more carefully.

My takeaway from this is that the only truly accepted method for shockwaves is the general explosion formula (with fireball/nuclear portions of the yield removed), and that this can only be used in cases where there's a relatively accurate level of destruction shown within the shockwave being measured.

If that's accurate, then shockwave calcs like the first two linked (such as the second calc here) will need to be removed from verse/character pages, and will need to stop being accepted.

 

[DontTalkDT]

I mean, the first is obviously bogus. Wave energy has very few to do with expansion speed.

The second one is, idea wise, interesting. Not sure in which cases it would be a good idea to use it, though.

 

[LIFE_OF_KING]

I found this accepted calculation, where the energy of a shockwave was found by assuming that the frontier of the shockwave had the same amount of energy as a sonic boom, and then applying that energy level to the surface area of the shockwave's frontier. This sounds invalid to me since that's a very high energy level to be assuming. It's justified in that calc (imo poorly) because they're created through pure physical force (that doesn't tell us that they're as energetic as sonic booms) and because the characters are mach-speed through other feats (that tells us nothing about the energy of this feat). (I should note that this method does seem fine for the Godzilla calc that was based off of; that character's explicitly described as attacking by generating sonic booms.)

I used the same method here. It would be usable since it has more context, right?

 

[DontTalkDT]

I used the same method here. It would be usable since it has more context, right?

Is that a sonic boom? The scan in the calc doesn't say anything of the sort. Couldn't it just be a soundwave?
Also, one consideration one has to do is where exactly the dB value is measured. I assume when 213 dB is measured that is pretty much at the source. Don't think it applies to the edges of the expanded shockwave.

 

[KLOL506]

I mean, the first is obviously bogus. Wave energy has very few to do with expansion speed.

Don't we have a different formula for omnidirectional expansion? (1/12) * mass * speed^2? Perhaps we could swap that in instead.

 

[Antoniofer]

For more realistic shockwaves, wouldn't be more appropriated the use the the ground level explosion calc? Long time ago, I broke up the different levels of destruction in relation of the power of overpressure, although we would somehow remove the termal part form the equation.

 

[DontTalkDT]

Don't we have a different formula for omnidirectional expansion? (1/12) * mass * speed^2? Perhaps we could swap that in instead.

This is not about expansion, though. A shockwave is a wave. For waves, the kinetic energy isn't dependent on the propagation speed.
Basically, no mass is moving outside with the wave. Only an impulse is travelling and hence the kinetic energy works differently in practice.

For more realistic shockwaves, wouldn't be more appropriated the use the the ground level explosion calc? Long time ago, I broke up the different levels of destruction in relation of the power of overpressure, although we would somehow remove the termal part form the equation.

Maybe. There are also 4 other constants that can be used for the nuclear airblast formula, in order to compute weaker shockwaves. The requirement is that some destruction still happens, of course.

 

[KLOL506]

This is not about expansion, though. A shockwave is a wave. For waves, the kinetic energy isn't dependent on the propagation speed.
Basically, no mass is moving outside with the wave. Only an impulse is travelling and hence the kinetic energy works differently in practice.

Understandable.

 

[Agnaa]

The second one is, idea wise, interesting. Not sure in which cases it would be a good idea to use it, though.

I think it's a good candidate for general adoption if we could somehow find a decibel-equivalent for the edge of a visible shockwave. That should make it applicable for almost every shockwave feat, even without corresponding destruction, right?

 

[DontTalkDT]

I think it's a good candidate for general adoption if we could somehow find a decibel-equivalent for the edge of a visible shockwave. That should make it applicable for almost every shockwave feat, even without corresponding destruction, right?

Sounds fine in principle. Well, provided we are ok with assuming that shockwaves in fiction are actually visible and not just visualized the way sound attacks are at times.

And we should be careful not to confuse the edge of a visible shockwave with the vapor cone, of course.

---

That aside, the 213 dB pdf doesn't open for me. Anyone else having that problem?

 

[Agnaa]

The current link doesn't work for me, but I found this archived version of it.

 

[Spinoirr]

So we need to recalc any outdated shockwave calcs if this goes through I assume

 

[DemonGodMitchAubin]

So basically get rid of the shockwave calcs that don’t use the main methods

 

[Agnaa]

Also, is the part of Antoniofer's blog where shockwaves need to demonstrate realistic effects on objects to get those values something that's implemented? It sounds like the standard explosion calc (assuming 20 psi) requires demolition of heavy reinforced concrete buildings, which doesn't feel like a very common effect in the fictional shockwaves I know of. If it's not damaging reinforced concrete at all then it wouldn't hit 6 psi. There's a lot of times shockwaves are shown without causing any damage, merely being a visual effect causing no damage to the environment.

So basically get rid of the shockwave calcs that don’t use the main methods

What are the main methods you speak of?

 

[DemonGodMitchAubin]

What are the main methods you speak of?

Airburst Explosions and Ground level Explosions

 

[Antoniofer]

Also, is the part of Antoniofer's blog where shockwaves need to demonstrate realistic effects on objects to get those values something that's implemented? It sounds like the standard explosion calc (assuming 20 psi) requires demolition of heavy reinforced concrete buildings, which doesn't feel like a very common effect in the fictional shockwaves I know of. If it's not damaging reinforced concrete at all then it wouldn't hit 6 psi. There's a lot of times shockwaves are shown without causing any damage, merely being a visual effect causing no damage to the environment.

Yeah, I've noticed that people does not pay much attention to overpressure values, a recent example was with a Curious George's calc when a minor combustion happened and the airburst calc (that uses 20 psi op value) was used despite the destruction caused was minimal. In general, people scale the fireball (that does not only involve a diffirent equation, but it may vary depending of the explosive's compound) to calculate explosion instead of the shockwave generated.

 

[DontTalkDT]

Also, is the part of Antoniofer's blog where shockwaves need to demonstrate realistic effects on objects to get those values something that's implemented? It sounds like the standard explosion calc (assuming 20 psi) requires demolition of heavy reinforced concrete buildings, which doesn't feel like a very common effect in the fictional shockwaves I know of. If it's not damaging reinforced concrete at all then it wouldn't hit 6 psi. There's a lot of times shockwaves are shown without causing any damage, merely being a visual effect causing no damage to the environment.

20psi is for "basically all humans in the area will die"-level or near-total fatalities as the nuke calculator likes to call it.

Usually, fictional explosions give us that and it hence it rarely mentioned, but there are some cases where we might need to be more careful.

 

[Antoniofer]

We hardly can prove than an explosion will kill everyone in certain radius unless people actually died in the scene (although at the same time, one may prove that the explosion is not that powerful if people survived within that radius), and that is death by barotrauma, so does not count for debris or such. Using environmental destruction as reference to measure the yield of the explosion seems more accurate.

 

[DontTalkDT]

We hardly can prove than an explosion will kill everyone in certain radius unless people actually died in the scene (although at the same time, one may prove that the explosion is not that powerful if people survived within that radius), and that is death by barotrauma, so does not count for debris or such. Using environmental destruction as reference to measure the yield of the explosion seems more accurate.

It often is... rather obvious. Like

I'm fairly sure any normal human in that explosion's range died and it's not even because of the debris flying around.

Like, in general, if you're standing within the fireball (or its energy explosion equivalent) you're most likely dead.

 

[Antoniofer]

To be fair, these kind of aoe attacks does not behave as conventional explosions; in principle, whe shouldn't use explosion equations to calculate how much these aoe attacks yield, is just that we have no other choise. Plus, not all "explosions" are like that, several calculations involve more reduced explosions.

 

[DontTalkDT]

If you scale the fireball/energy ball (like one would in the above case), I think using 20psi is fine.
If you have a more "reduced" explosion and scale the actual shockwave I can agree that one needs to be more careful.

 

[Agnaa]

I just remembered this thread. A lot of the calcs mentioned as flawed appear to still be on relevant pages, and no real changes to the standards were implemented.

 

[Agnaa]

It often is... rather obvious. Like

I'm fairly sure any normal human in that explosion's range died and it's not even because of the debris flying around.

Like, in general, if you're standing within the fireball (or its energy explosion equivalent) you're most likely dead.

They may have died, but if it is just a spherical expansion of heat so bright that it's visible, it doesn't really make sense to apply a formula based on pressure to it, imo. ofc in your example there is debris flying around, but you said you'd be fine using explosion formula even if debris isn't shown.