Is This a Shockwave? - Physics Forums

[thefortyseven]

Hi all,

Long time reader of Physics Forums but first time posting. Anyway, I work for a national magazine, and we are running an article on "Shockwaves". For the picture in the article we are using the attached image (or link here http://commons.wikimedia.org/wiki/File:Uss_iowa_bb-61_pr.jpg).

However, from my understanding of shockwaves, this isn't one; it's merely a concussion wave resulting from the explosion from the barrel of the gun. I've been doing a lot of research on the image and several places have identified it as a shockwave, but I don't think it is. Could anyone confirm or deny whether it is or not, and if it's not a shockwave, what is the wave being produced on the water? Does it have any relevance to a shockwave?

Thanks!

 

[Kalrag]

According to a direct definition: Yes it does seem that there is a shockwave visible from the right cannon blast. But it does interest me that you have labeled it as a concussion wave.

Heres the definintion:http://www.britannica.com/EBchecked/topic/541339/shock-wave

Hope This Helps, Kalrag

 

[thefortyseven]

Thanks for the reply. I'd read that definition before but I had some issue with it. It seemed oddly explained and the following statement,

"In particular, shock waves travel faster than sound, and their speed increases as the amplitude is raised"

, isn't true (I think), as the waves themselves travel at the speed of sound but the source exceeds it.

Anyway, sorry, that's moving away from the point. Are you sure it's a shockwave? If it is, it should be being produced by a source moving faster than the speed of sound. What source is this? The explosion itself? This seems to be to indicate it's not a shockwave, but merely a wave as a result of the cannon blast.

I'm confused.

 

[rcgldr]

In my opinion, from what I can see in the picture, I'm not sure how you can tell if the image shows concussion or a shock wave.

At 24 seconds into this video, you can see the effect the shock wave has on the ground:



In the second half of this video you can see the effect the shock wave has on the water:

http://rcgldr.net/real/f14flyby.wmv

 

[thefortyseven]

Thanks rcgldr, those videos help clarify the effects of a shockwave. I agree, though, that it's difficult to tell in the USS Iowa image what sort of wave is being produced, hence my confusion as various places have identified it as a shockwave (although the official US Navy caption for the image claims it is a concussion wave!). I think it brings somewhat into question what exactly a shockwave is. Is it a wave, quite literally, produced by a shock? Or is the use of the word "shock" arbitrary and in fact its more to do with the motion of the source producing the wave? With the USS Iowa image I'm leaning slightly towards more that it's not a shockwave, although I want it to be because we really want to use the image for our article in the magazine.

So, in summary, what's the thoughts people? Is it a shockwave? One thing to note is the ripple effects inside the wave in the USS Iowa image. Perhaps these can help indicate one way or the other.

I've researched the speed of the projectile produced by the cannon, and that itself travels faster than the speed of sound. However, it's evident that the projectile isn't the source of the waves on the water, as waves produced by the projectile would be traveling in the opposite direction.

Again, I'm still confused. Help!

 

[rcgldr]

Wiki gives several definitions for shock wave:

http://en.wikipedia.org/wiki/Shock_wave

For the common usage of a shock wave, it's the result of some object traveling faster than the sound. As shown near the bottom of this wiki page:

http://en.wikipedia.org/wiki/Sound_pressure

A shock wave is clipped at zero miminum pressure, with greater than 1 atm peak pressure.

From the flyby video shown above, a shock wave will sound like a crack. Over distance and time, the shock wave transitions into a normal sound wave with a deep sound, which is the sonic boom you'd normally hear from supersonic jet if it's far enough away.

Regarding the cannon photo, it shows a round shape impression on the water. A shock wave from a single moving object will normally look like a wide angle v shaped line or cone. A shock wave from an explosion could look like a sphere, so it's possible that the explosive gasses expanded at greater than the speed of sound to create that round pattern on the water, but the shell itself would not make such a pattern.

 

[thefortyseven]

Interesting. So, perhaps in that USS Iowa image, the initial wave produced by the cannon blast is a shockwave, but as it loses energy and dissipates over the water it becomes a regular (or concussion) wave. Would you say that's about right?

 

[rcgldr]

Interesting. So, perhaps in that USS Iowa image, the initial wave produced by the cannon blast is a shockwave, but as it loses energy and dissipates over the water it becomes a regular (or concussion) wave.

I don't know if the explosive wave has transitioned from shock wave to normal sound wave at the time of impact with the water. As seen in the youtube video, that shock wave travels quite a distance, beyond the field range of the camera. All I'm sure of if that it's not the shock wave of the fired shell.

 

[thefortyseven]

Yep, I think we can agree it's not the result of the projectile fired from the cannon. See, my problem is that a shockwave should be in a sort of cone protruding back, as seen here:

http://dev.physicslab.org/Document.aspx?doctype=3&filename=WavesSound_BarrierWaves.xml

How can a relatively stationary source of waves produce a shockwave, then? Are people using the term "shockwave" loosely and, in reality, it's not really a shockwave, even that first video you posted?

 

[I like Serena]

I'm speculating here, but I would expect the wave to be caused by the air disturbance when the projectile leaves the cannon.
This would produce high/low pressures in the air that expands like a wave.
You can see that the center of the water wave is the cannon.

Same thing for rcgldr's video of the plane passing over water.
You can see how the disturbance begins in front of the plane, where the air would be compressed.

Btw, I'm not really clear on what a "concussion wave" is.
Wiki does not have an article on it.
Is there a definition somewhere?

 

[thefortyseven]

I'll be honest, I'm not entirely sure what a concussion wave is either... I'd presumed it was merely a disturbance caused by a violent explosion or event, causing vibrations in a medium (but not traveling fast enough to be classed a shockwave).

I also agree that the wave is being produced by the air disturbance of the projectile being fired. This is what had me initially confused though: is this actually a shockwave, or just a sort of "mirror image"?

 

[I like Serena]

As rcgldr already said, wiki gives several definitions for shock wave.
Or rather one definition with a lot of different forms.

He also said:

For the common usage of a shock wave, it's the result of some object traveling faster than the sound. As shown near the bottom of this wiki page:

http://en.wikipedia.org/wiki/Sound_pressure

However, I cannot find this reference.

So I think a shock wave is a concept that covers all the different types, including a concussion wave, whatever that is exactly.

I also agree that the wave is being produced by the air disturbance of the projectile being fired. This is what had me initially confused though: is this actually a shockwave, or just a sort of "mirror image"?

What do you mean by "mirror image"?

 

[thefortyseven]

I think rcgldr was referring to the table at the bottom of the Wiki Sound Pressure page.

By mirror image I mean that in the USS Iowa image and rcgldr's second video, the effect produced on the water, is that the shockwave coming into contact with the water? Or is it the result of another source, such as the movement of the cannon/plane? Basically, in the USS Iowa image, is the shockwave being pushed out at 360 degrees, but what we see is the effect the shockwave has on the water?

Another point I've just thought of. If we've established that the wave on the water is produced by the cannon's explosion, but the projectile is traveling faster than the speed of sound, then where is the wave produced by the projectile? Why isn't the projectile producing a shockwave that then appears on the water?

 

[I like Serena]

I think rcgldr was referring to the table at the bottom of the Wiki Sound Pressure page.

I believe you're right. Still, IMHO this is only one specific form of a shock wave.

By mirror image I mean that in the USS Iowa image and rcgldr's second video, the effect produced on the water, is that the shockwave coming into contact with the water? Or is it the result of another source, such as the movement of the cannon/plane? Basically, in the USS Iowa image, is the shockwave being pushed out at 360 degrees, but what we see is the effect the shockwave has on the water?

What do you mean with the movement of the cannon/plane?

I believe we see the effect of a compressed air wave which drives the water away.

Another point I've just thought of. If we've established that the wave on the water is produced by the cannon's explosion, but the projectile is traveling faster than the speed of sound, then where is the wave produced by the projectile? Why isn't the projectile producing a shockwave that then appears on the water?

Well, I do see an inverted V shape...
Can't say if that is the supersonic shock wave of the projectile though, although at the top perhaps we see the missile itself, just on the edge of the water wave.
That just might fit, considering the projectile probably has the same speed as the compressed air...

 

[boneh3ad]

A shock wave is, in essence, a bunch of sound waves that get bunched up together when either a fluid is moving supersonically or an object is moving supersonically through a fluid.

You don't need a moving projectile to get a shock wave, as evident by the shock wave visualized in high-speed footage of many explosions. In the case of the image here, the wave is not a result of the projectile, but the rapid expansion of hot, pressurized gases as they leave the barrel of the gun. It is difficult to tell whether the image shows an actual shock wave though. The problem is we have no idea how fast the gas is moving at that point. It could just be the result of enormous sound pressure.

 

[A_Paradox]

The Fortyseven,

1. The expanding gas which has driven the projectile out of the gun barrel, is extremely hot and, by definition if you like, the 'speed of sound' within that gas is much greater than that of the cool atmosphere. This means that the projectile is probably still within the vicinity of the gas. IE it is invisible within the photograph.
2. The barrels of the guns will be elevated so the projectiles are rising away from the surface which means that when their shockwaves becomes discernible beyond the "concussion wave" as the other guy called it, the disturbance of the water will be decreasing as the shells fly away from the ship

Cheers,
Mark

 

[cjl]

As boneh3ad said, a lot of people here haven't really understood what a shock wave is. A shock wave is, quite simply, a large amplitude pressure wave. It can be caused by an object moving at greater than the speed of sound, it can be caused by an explosion, or it can be caused by something like a piston moving in a tube (even if the piston speed is subsonic). Shock wave speed is dependent on amplitude, and for a larger amplitude shock wave, the speed is higher. I've never heard a distinction between a shock wave and a "concussion wave" - explosions create a spherical shock. Far from the explosion, the shock becomes basically an ordinary sound wave.

As for the picture? It looks like a shock wave to me. It's almost definitely from the expanding gases though, and not from the projectile itself (based on the shape).

 

[thefortyseven]

All sorted now, thanks for the help and information everyone, much appreciated.