How fast does a blastwave travel?

[Squizzie]

The shockwave will lose speed as it travels, eventually turning into a normal sound wave that moves at the speed of sound. I don't know how quickly the shock wave in the video loses speed though.

I think the time in which a shockwave's speed decays to the speed of sound is demonstrated in the Kinney & Graham extract above. It's pretty short!

 

[Squizzie]

It stops being called a "shockwave" when the wavefront goes subsonic (relative to local atmosphere).

I think you'll find from the Kinney & Graham reference above, that the wavefront never goes subsonic. After the initial few millisecs, the blast wave, shock front or shockwave proceeds at the speed of sound.

 

[Drakkith]

I prefer my view that at any survivable distance from an explosion, the blast wave, shock front or shockwave will travel from the site of the explosion to your position at Mach 1.

There appears to be two types of shockwaves that I've seen discussed. The first, generated by something traveling supersonic (like a bullet or supersonic aircraft) travels at the speed of sound. The second, generated by rapid expansion, such as in explosions, travels faster than sound (at least initially). Unfortunately both of these appear to share the same name, leading to confusion.

 

[hmmm27]

I think you'll find from the Kinney & Graham reference above, that the wavefront never goes subsonic. After the initial few millisecs, the blast wave, shock front or shockwave proceeds at the speed of sound.

Thanks for the graph and excerpt. "Going subsonic" (my post(s)) was a temporary brainfart, now cleaned up somewhat : apologies for any confusion caused.

Kinney and Graham appear to use "blast wave" and "shock front" interchangeably.

They aren't, neither do they appear to ; reread the snippet you quoted.

 

[Baluncore]

I prefer my view that at any survivable distance from an explosion, the blast wave, shock front or shockwave will travel from the site of the explosion to your position at Mach 1.

Everyone prefers their own view.
Your private definition of "shock front or shockwave" is causing a problem in this physics thread.

A shock front always travels at a speed greater than the speed of sound. That is, by definition, because compressive heating of the air, accelerates the back of the shock, to catch up with the front of the shock.

It is the momentum of the sub-sonic blast "wind" that follows, that does most of the physical damage. Survivability comes down to your landing, after being thrown 10 metres in the air, and pelted with a hailstorm of broken glass and bricks.

 

[Squizzie]

There appears to be two types of shockwaves that I've seen discussed. The first, generated by something traveling supersonic (like a bullet or supersonic aircraft) travels at the speed of sound. The second, generated by rapid expansion, such as in explosions, travels faster than sound (at least initially). Unfortunately both of these appear to share the same name, leading to confusion.

Yes, there is indeed confusion.
Following Kinney and Graham, we should probably refer to the explosion phenomenon as a "blast wave" having a "shock front" and leave the shock wave (or shockwave) to describe the phenomenon experienced from a supersonic plane or bullet.
Kinney et. al. do muddy the water slightly with the use of "shock wave" on p.55 when discussing the thickness of the shock front and on p. 57 when discussing explosive overpressure, otherwise they refer to "shocks", "shock fronts" and "blast waves".
I remain curious as to the nature of the two "shock waves" referenced by the Slo Mo Guys in my OP,
as well as the nature of the white dome and the silver disk observed in the Beirut explosion videos.

 

[Baluncore]

I remain curious as to the nature of ..... the white dome and the silver disk observed in the Beirut explosion videos.

The white dome, that follows the blast wave, has a pressure lower than normal atmospheric pressure. The momentary white cloud forms in the partial vacuum, pulled by the outward momentum of the explosion.

The expanding silver disk, marks the progress of the shock front, as it blows the tops off, small waves on the water. In video of atom bomb tests, it can be seen approaching the camera, arriving just before the momentum of the blast wave.

 

[Squizzie]

The white dome, that follows the blast wave, has a pressure lower than normal atmospheric pressure. The momentary white cloud forms in the partial vacuum, pulled by the outward momentum of the explosion.

I guess you are referring to the low pressure described in Kinney:

When a gas expands adiabatically, its temperature drops, and in the case of the Beirut explosion, the temperature dropped below the dew point.

 

[Baluncore]

I guess you are referring to the low pressure described in Kinney:

I was not referring to Kinney.
I was referring to the partial vacuum that follows, immediately after an explosion.

 

[Squizzie]

I was not referring to Kinney.
I was referring to the partial vacuum that follows, immediately after an explosion.

Apologies, I thought we were on the same page. Which partial vacuum follows?
I don't see any reference to that in Kinney. Do you have a reference please?

 

[Baluncore]

Apologies, I thought we were on the same page. Which partial vacuum follows?
I don't see any reference to that in Kinney. Do you have a reference please?

Look it up in Kinney.

 

[Squizzie]

Look it up in Kinney.

Sorry you've lost me. I can find no reference in Kinney to partial vacuum.
There is a reference to a reversed blast wind:
"These blast wave effects then decrease quasi-exponentially with time until the pressure reaches atmospheric (zero overpressure) after which there is a slight negative phase, as at point C, along with a reversed blast wind."
along with these two diagrams :

to which I referred in my earlier post.

 

[Baluncore]

https://en.wiktionary.org/wiki/partial_vacuum

 

[vanhees71]

It the plural really "partial vacuums" and not "partial vacua"? And isn't this the "end of civilization"?

 

[Baluncore]

What distinction are you trying to make with negative overpressure?

Where did I refer to "negative overpressure"?

 

[vanhees71]

https://en.wikipedia.org/wiki/Sonic_boom

 

[Squizzie]

Kinney et. al. refer to "negative overpressure" only once, in Figure 1.1 C , reproduced in post #53, in the context of the pressure distribution in blast wave where, as explained on p. 90 :
"The overall pressure configuration then consists of an abrupt pressure discontinuity followed by positive and negative pressure phases as indicated by Curve 4 of Figure 6-2. ".
I suspect that the white cloud observed in the Beirut explosion images is the condensation that occurs due to the temperature drop associated with the adiabatic expansion of humid air to below its dew point in the "negative pressure phase" described above. However I can't find any reference to that phenomenon in Kinney or the textbooks I have read.
I would be interested to hear of any source that describes it in a scientific manner.

 

[renormalize]

I suspect that the white cloud observed in the Beirut explosion images is the condensation that occurs due to the temperature drop associated with the adiabatic expansion of humid air to below its dew point in the "negative pressure phase" described above. However I can't find any reference to that phenomenon in Kinney or the textbooks I have read.
I would be interested to hear of any source that describes it in a scientific manner.

Wikipedia states (https://en.wikipedia.org/wiki/Condensation_cloud):
"A transient condensation cloud, also called a Wilson cloud, is observable surrounding large explosions in humid air."
And there is a link to this article: https://cen.acs.org/safety/industrial-safety/chemistry-behind-Beirut-explosion/98/web/2020/08.
With this info, perhaps you can find references to the scientific literature you seek.

 

[Baluncore]

However I can't find any reference to that phenomenon in Kinney or the textbooks I have read.

The condensation cloud is only visible after large explosions, say about 500 kg TNT. It does not get attention because it is transient and a minor part of the explosion.

The condensation cloud has a smooth surface, marked by the transition to lower pressure. It appears to expand in place, from the air, without mass flow of material. Shortly after, it dissolves back into the air, again without mass movement. What could it be but atmospheric moisture?

https://en.wikipedia.org/wiki/Cloud_chamber#Invention

 

[Squizzie]

I am indebted to Wikipedia for its reference to Glasstone, Samuel and Philip J. Dolan. The Effects of Nuclear Weapons, U.S. Dept. Of Defense/ Dept. Of Energy; 3rd Edition (1977) for its analysis of this cloud (also referred to as a "Wilson cloud").
That paper associates the cloud's appearance to the blast wave propagating through humid air, and the cloud being formed in the "negative phase" of the blast wave where the air pressure drops below the ambient pressure, as illustrated on p.84 :

and, on p.43:
"During the compression (or blast) phase, the tem-perature of the air rises and during the decompression (or suction) phase it falls. For moderately low blast pressures, the temperature can drop below its original, preshock value, so that fi the air contains a fair amount of water vapour, condensation accompanied by cloud formation will occur"

It is left for the reader to connect a) the existence of the temperature drop with that associated with the adiabatic expansion of gas, and b) the formation of the cloud due to the temperature falling below the dew-point of the humid air, but I think the formation of the cloud is well explained.
The mystery for me now is the source of this "negative phase". The presence of the overpressure is easily imagined as the result of the rapid expansion of the hot detonation gases . But what is the source of the negative phase, where the pressure drops for about the same duration as the compression phase, and only starts to appear at some time and distance from the detonation?

It is referenced in both Kinney and Glasstone, but is not explained.

 

[renormalize]

The mystery for me now is the source of this "negative phase". The presence of the overpressure is easily imagined as the result of the rapid expansion of the hot detonation gases . But what is the source of the negative phase, where the pressure drops for about the same duration as the compression phase, and only starts to appear at some time and distance from the detonation?

Search for info on blast waves:
https://en.wikipedia.org/wiki/Blast_wave
"In fluid dynamics, a blast wave is the increased pressure and flow resulting from the deposition of a large amount of energy in a small, very localised volume. The flow field can be approximated as a lead shock wave, followed by a self-similar subsonic flow field. In simpler terms, a blast wave is an area of pressure expanding supersonically outward from an explosive core. It has a leading shock front of compressed gases. The blast wave is followed by a blast wind of negative gauge pressure, which sucks items back in towards the center. The blast wave is harmful especially when one is very close to the center or at a location of constructive interference. High explosives that detonate generate blast waves."

 

[Squizzie]

Search for info on blast waves:
https://en.wikipedia.org/wiki/Blast_wave
"In fluid dynamics, a blast wave is the increased pressure and flow resulting from the deposition of a large amount of energy in a small, very localised volume. The flow field can be approximated as a lead shock wave, followed by a self-similar subsonic flow field. In simpler terms, a blast wave is an area of pressure expanding supersonically outward from an explosive core. It has a leading shock front of compressed gases. The blast wave is followed by a blast wind of negative gauge pressure, which sucks items back in towards the center. The blast wave is harmful especially when one is very close to the center or at a location of constructive interference. High explosives that detonate generate blast waves."

I guess my question is along the lines of conclusion 4 of the paper[12] linked in the Wikipedia article :

It would appear that the Friedlander equation provides a close correlation with the experimental data, but it doesn't explain what appears to me anyway, as wholly counterintuitive, why the high pressure zone of the blast wave is followed by this low pressure zone.

 

[Baluncore]

The mystery for me now is the source of this "negative phase". The presence of the overpressure is easily imagined as the result of the rapid expansion of the hot detonation gases . But what is the source of the negative phase, where the pressure drops for about the same duration as the compression phase, and only starts to appear at some time and distance from the detonation?

Atmospheric pressure is greater than zero, so there can be positive and negative pressure excursions. After a disturbance, while an equilibrium is being restored, the pressure can be expected to oscillate about atmospheric pressure. The positive and negative phases have the same periodic duration, simply because they are parts of the same oscillation.

 

[renormalize]

...but it doesn't explain what appears to me anyway, as wholly counterintuitive, why the high pressure zone of the blast wave is followed by this low pressure zone.

Do you think it's counterintuitive that a region of rarefaction (low pressure) follows every region of compression (high pressure) in an ordinary sound wave in a fluid? If not, shouldn't an impulsive (shock) wave in that fluid similarly have low pressure follow high?

 

[Squizzie]

Do you think it's counterintuitive that a region of rarefaction (low pressure) follows every region of compression (high pressure) in an ordinary sound wave in a fluid? If not, shouldn't an impulsive (shock) wave in that fluid similarly have low pressure follow high?

Yes, counterintuitive.
Especially as we now have the Friedlander equation that doesn't have any wave-like e^{ix}, e^{-ix} components.

 

[Baluncore]

Especially as we now have the Friedlander equation that doesn't have any wave-like e{ix}, e{-ix} components.

Do not forget that; e^iz = cos(z) + i⋅sin(z)
https://en.wikipedia.org/wiki/Euler's_formula

 

[Squizzie]

Do not forget that; e^iz = cos(z) + i⋅sin(z)
https://en.wikipedia.org/wiki/Euler's_formula

Precisely, but Friedlender's equation has no i term!

 

[Baluncore]

The Friedlander equation is an approximation to the pressure of a blast wave. Blast waves do not follow the Friedlander equation as a law.

It seems that Friedlander simplifies the analysis by assuming the wave is damped to one cycle. It predicts the rarefaction, and we should expect the integral of the pressures above and below atmospheric pressure will be proportional.

 

[sophiecentaur]

The shockwave will lose speed as it travels

But will the wave maintain its profile until it slows to sonic speed. The 'wave' is surely more of a pulse which will disperse as soon as it's formed and you then have to ask which bit of the wavefront counts in the speed calculation?
If you observe a pulse, launched from a supersonic event, at various distances from its formation then is it not true that the pulse will spread out in time / distance? Can there be an answer to the OP question? I know that many people claim that the sonic boom they hear is actually a shock wave (don't ask for references) but PF has discussed this several times and my memory tells me that the wave acquires sonic speed very near the plane's path.

Any images of suitable graphs available?

 

[Baluncore]

If you observe a pulse, launched from a supersonic event, at various distances from its formation then is it not true that the pulse will spread out in time / distance?

The thing that defines a shock wave is that it is self-sharpening, as the higher pressure and temperature at the back of the pressure step, continuously catches up with the lower temperature at the front.

Once the compression and heating is sufficiently attenuated by inverse square law, the sharp step, falls to sonic velocity, and begins to take the form of a ramp, with attenuation of the high audio frequencies in the spectrum occurring more rapidly than the low.

By definition, the speed of an explosively generated shock-front is supersonic, due to the local heating in the front.

 

[hmmm27]

Once the compression and heating is sufficiently attenuated by inverse square law, the sharp step, falls to sonic velocity, and begins to take the form of a ramp, with attenuation of the high audio frequencies in the spectrum occurring more rapidly than the low.

I think the confusion is why form an (outwards) ramp. If it were an "ideal gas" all'round, without any of the normal variations in molecular velocity, would it stay a sharp step ? (gradually attenuating).

 

[Baluncore]

Sound waves are assumed to travel at sonic speeds, and do not assume heating of the air, during passage of the pressure wave.

If a pressure step increases the temperature, the pressure wave will be self-sharpening, with a supersonic speed.

There must be a transition defined between those two distinct models.

 

[Baluncore]

https://en.wikipedia.org/wiki/Rankine–Hugoniot_conditions

 

[sophiecentaur]

Once the compression and heating is sufficiently attenuated by inverse square law, the sharp step, falls to sonic velocity,

The inverse law must depend on the shape of the generating object - a sphere if it's an explosion but not for all shock waves. (Close to an aircraft or in a tube) This is hard stuff - that link of yours is not bed-time reading.

 

[Baluncore]

This is hard stuff - that link of yours is not bed-time reading.

The theory gets hard when you try to apply it to every possible case of supersonic flight, shock tube, and explosion.

If we can limit discussion to supersonic explosions, near the Earth's surface, then it can be discussed in this thread, without too much confusion.