How fast does a blastwave travel?

[Squizzie]

And that gas is very hot and expanded like a fireball. At first, it pushes air away from the site of the explosion, then the combustion products of the explosive cool and condense, and the air comes flowing back in.

Are you offering this as a complete explanation for the source of the negative overpressure?

 

[Baluncore]

Are you offering this as a complete explanation for the source of the negative overpressure?

No. Just the part played by the fireball seen in the video of big explosions.

 

[snorkack]

I think that we could return to the example of squib load.
Yes, as you pointed out, if the bullet is stuck at the breech and the explosive load is normal then the pressure at the chamber will be above the normal live load pressure, which may result in barrel burst.
However, consider a squib which is a squib because it has abnormally small amount of explosive, but still some explosive. For example a cartridge that has just primer but no powder.

When the bullet is stuck at the breech and the explosion is not strong enough to either dislodge the bullet or burst the barrel, there will be no negative phase. Because the overpressure in the chamber will be released by slowly leaking out of the windage. Not enough inertia to produce overshoot and underpressure.
When the bullet slides freely but the explosion is too small to fill the length of the barrel, the bullet will travel partway along the barrel, and then continue travelling by inertia while underpressure is left behind. So the bullet is sucked back.

If the explosive load is sufficient then the smoke in the barrel is still under overpressure when the bullet exits the muzzle, and smoke will continue to exit the muzzle as a muzzle blast. But the inertia of muzzle blast will cause the smoke to overshoot, leading to underpressure in the barrel, after which air is sucked back into the barrel. This is demonstrated in that when a target is near the muzzle, pieces of target are sucked into the barrel, such as blood and pieces of brain.

 

[Squizzie]

I think that we could return to the example of squib load.

At the risk of repetition, can we keep to the topic: blastwaves?

 

[Squizzie]

No. Just the part played by the fireball seen in the video of big explosions.

I'm pretty sure that the condensation cloud does not appear until the blastwave has cleared the fireball.

 

[snorkack]

At the risk of repetition, can we keep to the topic: blastwaves?

I believe it very much includes a blastwave and illustrates how a blastwave forms and propagates! When smoke exits the muzzle as a muzzle blast, it is a blastwave. When a blank shot propagates along the barrel, it is a blastwave. When a squib load is stopped by a bullet seized in the breech, the blastwave is prevented from forming, or perhaps stopped after it has propagated across empty cartridge. The barrel, muzzle and free air outside the muzzle allow examining blast wave propagation in 1D and in 3D, and the transition from 1D to 3D.

 

[Squizzie]

Hot off the press, a response from Dr. Rigby:
"Particle velocity is a common term in fluid dynamics. It doesn't refer to velocity on the molecular level, more so it describes what happens to a "packet" of air (or "parcel", but I prefer the term "packet"!). Think differential calculus rather than statistical mechanics. You don't need to go down to that sort of scale when you have wavelengths in the order of metres. It's important to remember that the wave is moving, but the particles have considerable momentum and therefore move themselves, as can be seen in that YouTube video I linked in the previous email.
Sam"
I'm not sure how to reconcile "think differential calculus" with "you don't need to go down to that scale when you have wavelengths in the order of metres" .
Comments?

 

[snorkack]

Hot off the press, a response from Dr. Rigby:
"Particle velocity is a common term in fluid dynamics. It doesn't refer to velocity on the molecular level, more so it describes what happens to a "packet" of air (or "parcel", but I prefer the term "packet"!). Think differential calculus rather than statistical mechanics. You don't need to go down to that sort of scale when you have wavelengths in the order of metres. It's important to remember that the wave is moving, but the particles have considerable momentum and therefore move themselves, as can be seen in that YouTube video I linked in the previous email.
Sam"
I'm not sure how to reconcile "think differential calculus" with "you don't need to go down to that scale when you have wavelengths in the order of metres" .
Comments?

Again consider for example firing a blank.
The length of the barrel is tens of cm, and that will be "wavelength" of the blastwave.
But the front of the blastwave will be very much thinner. In the rear, there will be the smoke accelerating along the barrel; but in the front there will be the air that was in the barrel and is pushed ahead by the smoke.

The size of air molecule is in the order of 0,1 nm. The free path is in the order of 100 nm. A few of the molecules in the blastwave will be accelerated to speeds well above the speed of the blastwave; but they will collide with the molecules ahead of blastwave and be slowed down until the mass of ambient molecules get accelerated. So a region of, say, 1000 nm across will behave as a "packet". And it can be accelerated, compressed etc...

 

[Baluncore]

I believe it very much includes a blastwave and illustrates how a blastwave forms and propagates!

The best way to understand and analyse one dimensional shock fronts and blast waves is with the well understood technology of shock tubes, not by analogy to the interior ballistics of guns.
See:
https://vesselmechanics.mech.utah.e...Computational-shock-wave.-Thesis-2010-Lib.pdf

 

[Drakkith]

You may be right, you may be wrong, but don't you see that a discussion on this opens a further detour from the present subject which is the source of the negative phase of a blastwave, which is generally accepted to have been generated by a chemical or nuclear reaction?

No, I don't see my post as a detour.

 

[Baluncore]

I'm pretty sure that the condensation cloud does not appear until the blastwave has cleared the fireball.

The blast wave air is outside the fireball. The transparent shock front is initiated at the surface of the growing opaque fireball. The shock front then moves away from the fireball surface, leaving the massive outward blast wave of air, between the shock front and the fireball surface.

It is the later inward condensation collapse of the fireball, coupled with the outward momentum of the blast wave air mass, that pulls the partial vacuum and rarefies the air, that precipitates formation of the Wilson cloud. Note that the Wilson cloud grows at a rate dependent on the RH and the distribution of the partial vacuum. The Wilson cloud surface has a phase velocity, not a flow velocity. The cloud surface grows from the point of minimum pressure, inwards and outwards at the same time, but you only see the opaque outer surface coming towards you. Since the cloud forms where the slowly moving air is cooling, a shock front step cannot form, though the phase velocity of the cloud surface formation can be infinite in places.

Following the formation of the cloud, another lower step shock front may form, but this time travelling back from the blast wave, towards the original site of the explosion. As the rarefied air is heated, and the return shock front builds and accelerates towards the site of the original explosion, the Wilson cloud condensate is rapidly dissolved again into the air.

I believe that reasoning sufficiently explains the rapid formation of, and the rapid disappearance of, the opaque Wilson cloud, and why it is seen momentarily in the place that it is.

 

[Squizzie]

.
OK it's in a tunnel, so there are lots of reflections.
Use your "." and "," keys to step frame by frame (I measured the frame rate at 30/sec) through the second between 11:16 and 11:17.
You can see the fuse burn in two or three frames.
And then there are at least three clouds behind the first boom and cloud (reflections?)
[PS] take no notice of the commentary.

 

[Baluncore]

And then there are at least three clouds behind the first boom and cloud (reflections?)

I do not think that video adds anything to the discussion, unless you want to confuse the issue.
That is an uncontrolled experiment, deliberately misleading, for entertainment purposes only.

 

[Squizzie]

I think I have found the original video from a portuguese source that appears to be genuine. The sound is much better, and there are some interesting comments.

 

[Baluncore]

sound is much better, and there are some interesting comments.

How can you tell the difference between cloud formation and the refraction of light due to the change in density of the air as a positive pressure wave passes?

What can be added to this discussion by watching or discussing that unscientific video?

 

[Drakkith]

How can you tell the difference between cloud formation and the refraction of light due to the change in density of the air as a positive pressure wave passes?

It looks pretty clear to me to be cloud formation in the tunnel. A change in the refraction of light is basically not visible without specialized techniques (like Schlieren photography).

 

[Squizzie]

It looks pretty clear to me to be cloud formation in the tunnel. A change in the refraction of light is basically not visible without specialized techniques (like Schlieren photography).

Actually I think I can detect refraction caused by the secondary waves (but not the initial one). Look at the first light down the tunnel and step through the frames (".") towards the end of 0:03 (from about frame 20) and see the distortion of the cable hanging down.

Just for giggles, I timed the duration from the initial det-cord ignition frame to the arrival of the cloud at 30 frames = 1000 ms +/- 30. With det-cord detonation speed of 6,400 m/s and sound at 340 m/sec, I reckon the charge was around 300 metres down the tunnel.

 

[Squizzie]

I have been wondering if the negative phase observed in the blast wave from a detonation is the inevitable evolution of a sharp high pressure pulse into a longitudinal pressure wave according to wave mechanics, and a colleague with whom I have been sharing my thoughts showed me this demonstration and explanation of the Kamifusen Japanese paper balloon.
It seems it's possible to generate a negative phase from a mechanically induced pressure pulse.

 

[Squizzie]

By consolidating shock propagation data from two distinct sources [1][2] into a single chart using a log scale for both axes, the contrasting speeds associated with two distinct phases of an explosion, which contribute to the formation of a shock front, become clearly evident.

The detonation phase involves the expansion of hot gaseous products at an initial hypersonic speed. This speed eventually decays to the speed of sound, marking the transition to the blast wave. The blast wave detaches upon the completion of the detonation phase and then propagates as a pressure wave at the speed of sound in the surrounding air, or Mach 1.0.

The timing and location of this transition depend on the type and size of the explosion, with the blast wave originating within milliseconds of the detonation of a chemical explosion up to a few seconds of a nuclear explosion. In all cases, the blast wave propagates at Mach 1.0.

[1] Kenney G. F and Graham K. J, (1985) Explosive Shocks in Air, Springer Science+Business Media, LLC, Table XI
[2] Bethe H. A, Fuchs K., Hirschfelder J, Magee J., Pearls R. and von Neumann J. (1958) Blast Wave U. S. Atomic Energy Commission, p. 185 ("The Los Alamos report")

 

[Baluncore]

What is your definition of "Mack 1.0" and "the speed of sound"?

During the detonation phase, the shock wave accelerates, then it begins to decelerate as it expands and moves away, with the fireball well behind it. Why do you include the decelerating supersonic shock wave, as part of the detonation wave, when it is usually treated as being outside the detonation, as an early part of the blast wave?

 

[Squizzie]

What is your definition of "Mack 1.0" and "the speed of sound"?

(my emphasis)
What makes you think I am using a definition of Mach 1.0 and the speed of sound in any way other than established in physics textbooks?

During the detonation phase, the shock wave accelerates,

Unless you are referring to a behaviour in the first 0.007 milliseconds (7 microseconds) that is not documented in Kinney's data, can you identify any stage in either dataset that indicates an acceleration in the shock front?

Why do you include the decelerating supersonic shock wave, as part of the detonation wave,
when it is usually treated as being outside the detonation,

I have not found that to be the case in the literature I have accessed.

 

[Baluncore]

What makes you think I am using a definition of Mach 1.0 and the speed of sound in any way other than established in physics textbooks?

In all cases, the blast wave propagates at Mach 1.0.

Your post is really confused, because your classification of the distinct phases of an explosion, or your definition of Mach 1.0 must be wrong.

What is it with your fixation on, or worship of, Mach 1.0 ?
Mach is only needed, or useful, when it has a value greater than one.

 

[Squizzie]

This is all a bit vague and depends on what you mean by 'blast wave'. If we take a blast wave to be an explosively generated shock wave/front and the entirety of all of the effects seen as it passes by a region, then sure, it can last several seconds. I'd guess the sounds of a blast wave are generated mainly by the following (data taken from a 100g charge, so times may vary as the charge increases):

• The initial shock front, which, for small explosion sources at least, rises from ambient to peak pressure virtually instantly (much less than 1 ms).
• The decay of the shock front, which takes 3-4 ms.
• Sound waves generated by the interaction of the shock front with other objects as it passes.
• Vibrations induced in the audio equipment or your ear from the interaction of the shock front.
• Reflections of the shock front and other sound waves by terrain and local objects.

If you're far enough away not to have your eardrums or microphones blown out by the blast, then the end result should be a sharp crack as the shock front passes you followed by several seconds or more of induced and/or reflected sounds depending on the surrounding terrain. As the distance increases the sound should decay in amplitude and start to lose its high frequencies, ending up as a low-frequency 'rumble' that's often heard if you're very far away.

Reference for time values: https://www.mdpi.com/2076-3417/12/5/2691
Again, note that I don't know how the times change as explosive power increases. It could be that the durations increase, which is what I naively expect, but I don't know.

As you can see, after just 20 meters the blast wave has fallen from nearly 7,000 m/s to roughly the speed of sound at 340 m/s. Graph referenced from this article on Injury and death to armored passenger-vehicle occupants and ground personnel from explosive shock waves.

I have added the data from your Viano reference to the plot (in yellow) which shows a high correlation with the Kinney and Los Alamos data.

 

[Squizzie]

Your post is really confused, because your classification of the distinct phases of an explosion, or your definition of Mach 1.0 must be wrong.

What is it with your fixation on, or worship of, Mach 1.0 ?

I'm not sure it rates as "fixation" or "worship" , but the fact that it defines the speed at which a blast wave (or any pressure, density or temperature disturbance) propagates through air is pretty important.

Mach is only needed, or useful, when it has a value greater than one.

Then I wonder why the Mach number is displayed on an airliner's instrument panel.

 

[Baluncore]

Then I wonder why the Mach number is displayed on an airliner's instrument panel.

The Mach number there refers to the speed of an aerodynamic vehicle, not to the speed of sound wave propagation. The aerodynamics of the vehicle change as it approaches the speed of sound, which is slower in the cold at higher altitudes.

 

[Squizzie]

The Mach number there refers to the speed of an aerodynamic vehicle, not to the speed of sound wave propagation. The aerodynamics of the vehicle change as it approaches the speed of sound, which is slower in the cold at higher altitudes.

Yes, I know.
All I'm trying to do is establish that once the blast wave has detached from the detonation phase, i.e. after a few milliseconds, or, in the case of a nuclear detonation a second or so, under normal atmospheric conditions, such as the case of the Beirut explosion, the blast wave then propagates at approximately 340 m/sec, or Mach 1.0.

 

[Baluncore]

All I'm trying to do is establish that once the blast wave has detached from the detonation phase, i.e. after a few milliseconds, or, in the case of a nuclear detonation a second or so, under normal atmospheric conditions, such as the case of the Beirut explosion, the blast wave then propagates at approximately 340 m/sec, or Mach 1.0.

Why do you try to establish that falsehood?

Outside the explosive combustion products, the supersonic shock front, is part of the blast wave.

https://en.wikipedia.org/wiki/Brandolini's_law
Brandolini's Law: “The amount of energy needed to refute bullshit is an order of magnitude bigger than that needed to produce it”.

 

[Squizzie]

Outside the explosive combustion products, the supersonic shock front, is part of the blast wave.

Could you please specify where and when the blast wave starts propagating from the detonation centre using the attached figure and data?

 

[Baluncore]

You are plotting three sets of data there, each is a different magnitude, varying over a factor of maybe 1000. There is no one point on that plot, and because it is a Log-Log plot, the traces cannot be moved into alignment.

 

[Squizzie]

You are plotting three sets of data there, each is a different magnitude, varying over a factor of maybe 1000. There is no one point on that plot, and because it is a Log-Log plot, the traces cannot be moved into alignment.

Could you please specify where and when the blast wave starts propagating from the detonation centre on any of the three data sets?

 

[Squizzie]

You are plotting three sets of data there, each is a different magnitude, varying over a factor of maybe 1000. There is no one point on that plot, and because it is a Log-Log plot, the traces cannot be moved into alignment.

Here are my estimates of the (time, distance) coordinates of the shock fronts' transition to Mach 1.0.

Please can you indicate where you believe the blast waves originate?

 

[Baluncore]

Please can you indicate where you believe the blast waves originate?

The blast wave begins with the first recorded point on your graphs, at the point of maximum shock front velocity in air. The point you mark on the blast wave, is where the shock front ceases to exist. The blast does not begin at your marker, nor does it end there.

When it comes to the definition of a blast wave, the 1947, Los Alamos paper "Blast Wave", LA-2000, discusses shock fronts that are certainly the most interesting part of the blast wave.

The problem with specifying the start of the blast wave, is that no local instrumentation can survive in proximity with the detonation. Observation of that critical point must be remote and optical, perpendicular to the opaque detonation surface.

The precise origin point of the blast wave, that I would select, is where the shock front, in "transparent air", first separates from the "opaque combustion products". Before that point the contact surface is accelerating, after that point it is a decelerating shock front.

 

[Squizzie]

The blast wave begins with the first recorded point on your graphs, at the point of maximum shock front velocity in air. The point you mark on the blast wave, is where the shock front ceases to exist. The blast does not begin at your marker, nor does it end there.

When it comes to the definition of a blast wave, the 1947, Los Alamos paper "Blast Wave", LA-2000, discusses shock fronts that are certainly the most interesting part of the blast wave.

The problem with specifying the start of the blast wave, is that no local instrumentation can survive in proximity with the detonation. Observation of that critical point must be remote and optical, perpendicular to the opaque detonation surface.

The precise origin point of the blast wave, that I would select, is where the shock front, in "transparent air", first separates from the "opaque combustion products". Before that point the contact surface is accelerating, after that point it is a decelerating shock front.

While I agree with you that the Los Alamos paper makes good reading (reference [1] my post #194 and source of my "Los Alamos" data) and that "The precise origin point of the blast wave… is where the shock front, in 'transparent air', first separates from the 'opaque combustion products'", there are significant divergences in our viewpoints on numerous aspects.
Given the extent of our disagreements, I suggest we respectfully agree to disagree and leave the matter for our peers to review and adjudicate.

 

[Baluncore]

Given the extent of our disagreements, I suggest we respectfully agree to disagree and leave the matter for our peers to review and adjudicate.

Ever since post #7, and it is now more than 200 posts later, you have been insisting that the supersonic shock front is NOT part of the blast wave. That is quite contrary to all the literature, so I must disagree with you on your personal definition of a blast wave. You can disagree, and walk away. I will agree not to follow you.

 

[Squizzie]

I hesitate to use the term 'blast wave' to encompass both the supersonic and the sonic phases due to its potential to mask fundamental differences in the physics at the interface. I refer to Kinney and Graham for this distinction, though similar information can be found in various references.
In the supersonic phase, a thin region exists between the hot, high-pressure gas on the detonation side of the shock and the ambient air through which the shock is travelling, as illustrated in Fig 4-2 below

The interface between a discontinuity in a gas with different state variables is described by the Rankine-Hugoniot condition. This condition is characterised by a set of equations that offer a mathematical model for the process.

The separate sonic phase is characterised by a pressure wave featuring a sharp, almost instantaneous rise in pressure, followed by a smooth reduction back to ambient pressure, as illustrated in Fig 6-3 below:

It also includes a "negative phase"in which the pressure reduces to below ambient pressure.
This shape has been mathematically modelled using the Friedlander equation, and I will refer to it by that name.
As is commonly practised, the depiction of the blast wave often employs a time scale, in contrast to the distance scale used earlier. When both phenomena are graphed on the same set of axes, their distinctions become readily apparent, as illustrated below

While the following table applies to both high explosives such as TNT, C4, and NH4, as well as nuclear explosions, the substantial variation in process intensity may make numerical comparisons somewhat obscure.

Properties​	Phase​	Phase​
	"Hugoniot"​	"Friedlander"​
Propagation speed	Decelerating from hypersonic to sonic.	Steady, Mach 1.0
Conditions on blast side the shock	High pressure, hot.	Atmospheric pressure and temperature
Lifetime, seconds	Very brief:<=10-2 for chemical, <=10 for nuclear	Long: > 102
Propagation distance, metres	Very short: <=102 chemical, <=103 nuclear	Long: > 103
Survivable?	No	Yes

I suggest that the term 'detonation phase' be reserved for the brief, supersonic 'Hugoniot' phase, while 'blast wave' is more appropriately used for the extended sonic 'Friedlander phase' due to these distinctions.