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Author Topic: Can Someone Explain...  (Read 2563 times)

bikerdude999

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Can Someone Explain...
« on: April 15, 2012, 08:20:51 PM »

Why, when a depth charge explodes, on the surface it creates a dome of water, soon followed by a huge plume of water? Or is this just something that happens in the world of hollywood films?
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Andyn

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Re: Can Someone Explain...
« Reply #1 on: April 15, 2012, 08:23:21 PM »

Shockwave followed by moving water perhaps?
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pugwash

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Re: Can Someone Explain...
« Reply #2 on: April 15, 2012, 08:37:15 PM »

Depth charges or mortar bombs on detonation only give you the huge plume of water if on a shallow setting - set deep
all you will see is a few bubbles or sometimes nothing at all. Don't forget you can't compress water so all the power of
the explosion has to go somewhere and on a shallow setting it goes for where the pressure is less and that is the surface.
Deep explosions have a greater distance to disperse hence little is seen on the surface
Geoff
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Norseman

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Re: Can Someone Explain...
« Reply #3 on: April 15, 2012, 10:20:21 PM »

Deep explosions have a greater distance to disperse hence little is seen on the surface
but I often see the bubbles in the bath  :embarrassed:

Dave
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grasshopper

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Re: Can Someone Explain...
« Reply #4 on: April 15, 2012, 10:26:36 PM »

Unless of course they've destroyed their underwater foe and the big 'dome' is the escaping air from the sub?
Little bubbles created at depth become big bubbles nearer the surface as the pressure decreases.

This may help explain why Dave only sees the small bubbles in the bath - if it was any deeper he would definitely be displaced by them...
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Mad_Mike

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Re: Can Someone Explain...
« Reply #5 on: April 16, 2012, 08:28:18 AM »

but I often see the bubbles in the bath  :embarrassed:

Dave

and occasionally if the tube is loaded you will fire off a couple of torpedoes. %)
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funtimefrankie

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Re: Can Someone Explain...
« Reply #6 on: April 16, 2012, 05:36:04 PM »

and occasionally if the tube is loaded you will fire off a couple of torpedoes. %)
I think you're mudding the water now
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Bryan Young

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Re: Can Someone Explain...
« Reply #7 on: April 16, 2012, 06:18:14 PM »

To follow up on what Pugwash said.
Yonks ago I described what happened to dumped ammo in about 3000ft of water. With "normal" stuff excessive heat will set it off. At 3000ft the compression creates heat and so a lot of the dumped stuff went pop. The sound of a tree falling when no-one is there to hear it.  But we on the surface did hear it....at least the engine-room staff did. All we on deck saw was discolouration on the next "run".
Nuclear stuff is different. Never dumped. But during one of my many "courses" I did watch a very graphic film-cum-art work of what a nuclear depth-charge would do. Assuming an explosin depth of over 2 miles, a small (!) one would create a spherical vacuum about a mile in diameter in the water, then the water rushes in to fill the vacuum. So damage/obliteration from two directions. Very little would be seen on the surface apart from "some disturbance". Nasty. BY.
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Norseman

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Re: Can Someone Explain...
« Reply #8 on: April 16, 2012, 06:48:25 PM »

Hi Guys

Some interesting aerial shots of exploding depth charges towards the end of the film - I've never seen an aerial shots of them before.
http://www.britishpathe.com/video/navy-depth-charge-demonstration Some interesting aerial shots of exploding depth charges towards the end
I couldn't find any film of deepwater explosions - must be some around though.
Wiki have an underwater explosion page http://en.wikipedia.org/wiki/Underwater_explosion

Dave
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Re: Can Someone Explain...
« Reply #9 on: April 17, 2012, 07:59:58 AM »

Not in my tub you dont Dave, Mick B.
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Mick B.

dodgy geezer

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Re: Can Someone Explain...
« Reply #10 on: April 17, 2012, 11:21:43 AM »

Why, when a depth charge explodes, on the surface it creates a dome of water, soon followed by a huge plume of water? Or is this just something that happens in the world of hollywood films?


No one seems to have described just how the differential water pressure causes the narrow plume? Here is a simple sketch I drew which may help....



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Perkasaman2

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Re: Can Someone Explain...
« Reply #11 on: April 17, 2012, 12:42:27 PM »

Larger model boats at speed cause a similar effect  displacing water at the stern - a rooster tail. Far less violent than an explosion but the reaction of the displaced static water is similar or the same principle.  
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bikerdude999

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Re: Can Someone Explain...
« Reply #12 on: April 17, 2012, 04:21:34 PM »


No one seems to have described just how the differential water pressure causes the narrow plume? Here is a simple sketch I drew which may help....





Thanks, I was wondering if anyone had the actual answer lol it's just something that I've always wondered about, so thank you.
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dodgy geezer

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Re: Can Someone Explain...
« Reply #13 on: April 17, 2012, 05:00:11 PM »

I wouldn't call it the 'actual' answer - it's just my interpretation of the what the physics is likely to be. I haven't looked it up anywhere. It could well be wrong...

In practice, I suspect that the towering column of water is greatest if the gas bubble breaches the surface while it is still holding the sides of the hole apart - when that happens the water will rush up from the bottom very rapidly. I guess that the bubble might become quite elongated as it rises, since the upper part of the gas-filled void will be under an increasingly lower pressure. If the bubble collapses completely before it reaches the upper levels of the ocean, there will be a bit of turbulence on the surface, but no real spout. And the turbulence would be quite delayed, if it has to travel from depth.

Note that I have not included the shock wave in the drawing - this is what does the damage, and it will travel more equidistantly from the explosion (it might actually travel downwards faster in the higher pressure. The shock wave will hit the surface either with, or before the dome, but I would not expect to see very much resulting from this....
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Norseman

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Re: Can Someone Explain...
« Reply #14 on: April 17, 2012, 09:50:28 PM »

Not in my tub you dont Dave, Mick B.
Ah, you're back then Mick  :} :-))

Hi Dodgy
Thanks, and I understand the diagram but what about Pugwash's point in reply2 re deeper explosions - any thoughts?

Dave
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dodgy geezer

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Re: Can Someone Explain...
« Reply #15 on: April 18, 2012, 12:00:05 AM »

Well, it seems fairly obvious that at a very shallow setting, where the gas bubble vents to the surface as it expands initially, you will just get a lot of water thrown up and scattered widely (and not so much shock wave transmitted through the water). Deeper down, the bubble collapses completely before it surfaces (into a set of small bubbles), and you only get a bit of turbulence and bubbles coming up.

I've had a quick look around for papers on underwater explosion dynamics - they are actually rather interesting and I think the wiki is surprisingly poor on this. Here http://www.abdn.ac.uk/~eng907/teaching/explosion-engineering/Underwater-I.pdf is a little undergrad paper which explains things a bit better. It points out that my simplistic initial assumption that the gas bubble just expands is wrong - in fact the gas bubble expands and contracts rhythmically as it rises. Snay (1956) seems to be the authority here: here is a preview of some of his work:

http://books.google.co.uk/books?id=4IEXG6GRxOUC&pg=PA3&lpg=PA3&dq=snay+1956+bubble&source=bl&ots=3LeIv0jNyy&sig=anM5zm-A44iVGkFKUVf5xFO_X9E&hl=en&sa=X&ei=WuuNT-a0IMul8gPLhojBCw&ved=0CCoQ6AEwAQ#v=onepage&q=snay%201956%20bubble&f=false

So my revised view is that an explosion creates a gas bubble and a shock wave. The shock wave takes some energy from the bubble, but by no means all. A typical initial size for a torpedo explosion seems to be a gas bubble 10-20 mt diameter, which then collapses as hydrodynamic pressure reasserts itself.

This collapse continues until the bubble is small and highly compressed (since gas is elastic). At that point it begins to re-expand, and a second shock wave is generated, losing some more energy. At each expansion pulse the bubble moves upwards; step-like rather than continuously, as I has assumed. This oscillation cycle may continue for 10 or more times, with the gas bubble adopting a toroidal rather than elongated shape, as I had pictured. For a small explosion the cycle duration may be 1/10th of a second - for a larger one it may be of the order of several seconds. These rhythmic pulses can cause considerable damage to an underwater structure, especially if they coincide with a natural vibration frequency.

The above describes an explosion in open water. Other documents I have glanced at suggest that this oscillating bubble will tend to move towards a fixed structure close to it, and, if it strikes it, it will collapse onto that structure with a jet of water coming from the opposite wall of the bubble striking the structure - rather like an 'explosively formed penetrator' for anti-tank use. That, at least, seems to match what I had assumed in my diagram! This will cause local damage if it hits a structural surface - and this jet is probably what is seen when the bubble surfaces and collapses into the air/water interface.

Comments from underwater explosives experts will be gratefully received...  O0




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