- #71
Squizzie
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- 11
@Baluncore I'm not sure that the Rankine–Hugoniot conditions (at least as summarised in Wiki) can be reconciled with the measurements reported in Kinney and Glasston.
But also shock waves propagate with the speed of sound. What's moving with faster-than-sound speed is the source, which leads to the formation of the Mach cone.sophiecentaur said: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?
OK. So you connect a microphone to an oscilloscope. You display the sound of a sonic boom going past. Later, you record the microphone output using a loudspeaker source. Would you expect a different scope trace? Could you call what the loudspeaker produced a shock wave?vanhees71 said:But also shock waves propagate with the speed of sound. What's moving with faster-than-sound speed is the source, which leads to the formation of the Mach cone.
Fair enough. I still have to ask when one should say the sound of an explosion is just a sound and not a shock front?Squizzie said:Could I respectfully request @vanhees71 , @sophiecentaur , that we confine responses on this thread to the discussion of the blast wave/ shock front from an explosion
And that's a very good question.sophiecentaur said:I still have to ask when one should say the sound of an explosion is just a sound and not a shock front?
Squizzie said:From the context of my OPs , where I referred to the Slo Mo Guys' C4 experiment and the Beirut explosion, I was clearly using the "term shock" wave to refer to what I now recognise is more appropriately described in the technical literature as a"blast wave" with a "shock front".
The subject of the shock wave generated by an object like a jet plane or a bullet travelling at a supersonic speed is a fascinating subject, but, as pointed out by @Drakkith at #39, it is a quite different physical phenomenon from that generated by an explosion.
Could I respectfully request @vanhees71 , @sophiecentaur , that we confine responses on this thread to the discussion of the blast wave
They are discussing the same subject.Squizzie said:I'm not sure that the Rankine–Hugoniot conditions (at least as summarised in Wiki) can be reconciled with the measurements reported in Kinney and Glasston.
If you can hear it, it is a sound wave. Feelings do not come into it.Squizzie said:So my feeling is that once it has departed the extreme temperatures of the explosion, it is a sound wave.
You asked to restrict the discussion to detonations and you're back on flying objects. Which do you want?Squizzie said:From rough estimations from the Slo Mo video it seems that the shock front is travelling at about the speed of the bullet (~Mach 1) when
I don't. Our hearing has to deal with many percussive sounds, lasting for a few milliseconds. Do we make a distinction between that and your violin bow playing a minim length G? If I'm picky, I'd say that the speed of an endless note would be very hard to measure without some interruption / modulation as a marker. Kids measure the speed of sound by clapping their hands and counting the time for multiple echos from a wall.Squizzie said:Normally, however, we associate the term sound wave with a sound that persists for a while:
What you seem to be describing is the effect of dispersion. for a large source, a lot of air can be displaced and the resulting wave can travel a long way (across a town) and do damage (windows) at around 300m/s.Squizzie said:So maybe the sound of the explosion is contained in the shock front, but blast wave, immediately behind the front, travelling at roughly the same speed as the shock front, can persist for a few seconds.
Yes, please.berkeman said:Should I edit your thread title from "shockwave" to "blastwave"?
I'm talking about the blast wave from the C4 explosion. I offer the trajectory of the bullet simply as a visualisation of the speed of sound.sophiecentaur said:You asked to restrict the discussion to detonations and you're back on flying objects. Which do you want?
The speed of a bullet is quite irrelevant to the speed of sound.Squizzie said:I offer the trajectory of the bullet simply as a visualisation of the speed of sound.
In the case of the quoted video, the bullet is reported at 8:09 to be travelling atBaluncore said:The speed of a bullet is quite irrelevant to the speed of sound.
Some bullets are supersonic, at Mach 2.5, others are subsonic, at Mack 0.4
May refer you to my earlier post, requesting that discussion be constrained to explosive blast waves on this thread?sophiecentaur said:Alternatively, for very sub sonic speed. Did you ever see / own one of these?
Could I ask you to elaborate on "self-sharpening pressure step" please, and how it applies to blast waves?Baluncore said:The bandwidth of an audio recording system is clearly insufficient to reproduce the self-sharpening pressure step.
The relevance is the possibility of carrying 'destructive' power through the air, slowly.Squizzie said:The air cannon is indeed a fascinating device, and the vortices it generates are an endless topic for discussion, but perhaps on a separate thread?
Squizzie said:May refer you to my earlier post, requesting that discussion be constrained to explosive blast waves on this thread?
The air cannon is indeed a fascinating device, and the vortices it generates are an endless topic for discussion, but perhaps on a separate thread?
sophiecentaur said:The relevance is the possibility of carrying 'destructive' power through the air, slowly.
That is fundamental to the discussion here.Squizzie said:Could I ask you to elaborate on "self-sharpening pressure step" please, and how it applies to blast waves?
But don't both Glasstone and Kinney suggest that, if anything, the back of the blast wave travels more slowly than the shock front, causing the back of the blast wave to lag further and further behind the shock front, rather than catch up as the blast wave travels away from the blast?Baluncore said:The back of the shock-front is hotter than the front of the shock-front, so the back of the wave catches up with the front of the wave. That keeps the wave steep. I call that self-sharpening.
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):Squizzie said:So maybe the sound of the explosion is contained in the shock front, but blast wave, immediately behind the front, travelling at roughly the same speed as the shock front, can persist for a few seconds.
Here's a graph of the pressure and velocity of a blast wave from 15 kg of TNT:sophiecentaur said: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?
It seems you insist on confusing the back of the shock-front with the back of the blast-wave. Are you a troll?Squizzie said:But don't both Glasstone and Kinney suggest that, if anything, the back of the blast wave travels more slowly than the shock front, causing the back of the blast wave to lag further and further behind the shock front, rather than catch up as the blast wave travels away from the blast?
I am trying desperately not to be.Baluncore said:It seems you insist on confusing the back of the shock-front with the back of the blast-wave. Are you a troll?
For the 100g to 400g charges in my first reference in post #90 the rise time was less than 1 μs.Squizzie said:The increase in pressure occurs over a very short distance and period of time: it is shown in the images posted at #34 as being vertical, but, as we know, it can't be exactly vertical, but its duration is smaller than the resolution of the plots. Unfortunately the plots have no scale on either axis, so it's hard to tell from the plots.
The front of that shock is in ambient air, while less than ¼ um behind, (one nanosecond later), the back is subjected to full pressure. If it is not vertical, then it will make itself vertical, because the back will overtake the front.Squizzie said:The increase in pressure occurs over a very short distance and period of time: it is shown in the images posted at #34 as being vertical, but, as we know, it can't be exactly vertical, but its duration is smaller than the resolution of the plots.
I noted from the reference that :"The width of the shock front is only very small ",Drakkith said:For the 100g to 400g charges in my first reference in post #90 the rise time was less than 1 μs.
My mistake, I didn't look closely enough. Looking closer now, I can see a step in the vertical line on the 100g charge too. A quick and totally non-rigorous count gives me about 16 steps over about 0.5 ms, or about 30 microseconds per step. Does a 50 microsecond rise time sound more reasonable?Squizzie said:I have no doubt that the shock front has a very short duration, but 1 μs does seem a bit short!
In just one of the plots I can detect a 1 px step in the shock front, but the resolution of the plot is not high enough to ascertain if it indicates a slope in the line, or is an artefact of the image production.
Squizzie said:I have no doubt that the shock front has a very short duration, but 1 μs does seem a bit short!
Over 40 years ago, when I was measuring the velocity of Mach 7+ shock fronts in a reaction tube. I used resistive sensors made from 0.2 mm wide gold leaf, mounted flush against the wall of the shock tube. The gauges were heated by the shock front as it passed, with their resistance rising in proportion to absolute temperature. The speed was measured accurately by the elapsed time between sensors.Drakkith said:Does a 50 microsecond rise time sound more reasonable?
Is that not what the graph in post #90 is showing?Squizzie said:I would expect the high pressure in the blast wave to decay exponentially to atmospheric pressure, but it doesn't, and I have yet to discover why.
In a normal sound wave, the compression and expansion phases have negligible energy transfer and the entire cycle of the wave leaves the medium with virtually no gain or loss of energy. I suspect that a blast wave is somewhat different as some form of energy loss has to occur for the air to cool and moisture to condense. Radiative losses maybe?Squizzie said:Folks, this analysis of the shock front is fascinating, but could I implore you to return to the question of the source of the low pressure in the back of the blast wave?
I deal in provable facts, not negotiated settlements. Everything follows on from the shock front. If you cannot, or will not, accept the shock wave model, then there is no hope for understanding the things that follow.Squizzie said:Folks, this analysis of the shock front is fascinating, but could I implore you to return to the question of the source of the low pressure in the back of the blast wave?
The Friedlander equation is a model for blast damage. It says nothing about any momentary condensation that may appear later. Models come with assumptions. Do you need independent evidence that mountains cause wave clouds?Squizzie said:It has been suggested that it is the expression of a heavily damped oscillation, which is interesting, but there seems to be little independent evidence for that and the absence of an oscillating term in Friedlander equation would not support that view.
Possibly, and that is the piece I'd like to explore more deeplyDrakkith said:In a normal sound wave, the compression and expansion phases have negligible energy transfer and the entire cycle of the wave leaves the medium with virtually no gain or loss of energy. I suspect that a blast wave is somewhat different as some form of energy loss has to occur for the air to cool and moisture to condense. Radiative losses maybe?
Looks interesting, thanks, a little more reading.Drakkith said:On the topic of blast waves, I stumbled across this extremely in depth report on nuclear detonations from Los Alamos from the 1940's. It may or may not be of use but I thought it was interesting and related to the topic. Here's the link: https://apps.dtic.mil/sti/tr/pdf/ADA384954.pdf