Fluid Dynamics...

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JacksSmirkingRevenge
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Fluid Dynamics...

Post by JacksSmirkingRevenge » Sun Jan 02, 2011 5:09 am

Would I be correct in thinking that a gas under pressure cannot expand more quickly than it's own speed of sound regardless of how quickly it is released or what pressure it is at?
Anyone...? :think:
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Re: Fluid Dynamics...

Post by mistermack » Mon Jan 03, 2011 1:54 am

Bri wrote:Would I be correct in thinking that a gas under pressure cannot expand more quickly than it's own speed of sound regardless of how quickly it is released or what pressure it is at?
Anyone...? :think:
Sorry, I don't know the answer, but I would imagine it's correct. But what would be relevant is how the pressure of gas affects the speed of sound of that gas.
If you double the pressure, do you double the speed of sound in that gas? Or is the speed of sound related to pressure at all?
The speed of sound in air at 1 bar is about 1000 km per hour ( I think ).
If your gas is compressed at 10 bar, what is the speed of sound in that compressed gas? If it's much higher, then you could have expansion at much more than 1000 kph, until the pressure drops.
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Re: Fluid Dynamics...

Post by Feck » Mon Jan 03, 2011 2:00 am

http://www.engineeringtoolbox.com/speed-sound-d_82.html
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Re: Fluid Dynamics...

Post by charlou » Mon Jan 03, 2011 3:15 am

:shock: gas pressure has a speed of sound? :what:

Well I think I'm about to learn something new ... *clicks on link*
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Re: Fluid Dynamics...

Post by Xamonas Chegwé » Mon Jan 03, 2011 4:53 am

Sound travels faster in denser substances, so it makes sense that it would travel faster in a compressed gas than in the same gas at 1 atmosphere. I can't quite see why the speed of sound should limit the rate of expansion though. That would depend upon the pressure of the gas it was expanding into - ie. a gas at 5atm would expand much faster into a near vacuum than it would into air at 1atm.

I am not saying that your OP is wrong - just that I have never heard of it before and haven't been able to find anything that backs it up online (in about 5 minutes of exhaustive googling!)
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Re: Fluid Dynamics...

Post by epepke » Mon Jan 03, 2011 10:48 am

Yes in the sense of no.

The initial, instantaneous expansion will be no greater than the speed of sound. However, as the gas expands, the speed of sound will drop, but the outer bits of gas will still be going at the same speed, absent resistance from other gas.
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Re: Fluid Dynamics...

Post by Tigger » Mon Jan 03, 2011 11:06 am

What alarms me is "why", Bri? I know, don't I? :demon:
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Re: Fluid Dynamics...

Post by Feck » Mon Jan 03, 2011 11:11 am

Tigger wrote:What alarms me is "why", Bri? I know, don't I? :demon:
Yes, probably :whisper:
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Re: Fluid Dynamics...

Post by FBM » Mon Jan 03, 2011 11:13 am

Feck wrote:
Tigger wrote:What alarms me is "why", Bri? I know, don't I? :demon:
Yes, probably :whisper:
He must've had one tremendous fart to inspire such an in-depth inquiry. I'm impressed.
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Re: Fluid Dynamics...

Post by normal » Mon Jan 03, 2011 11:15 am

Are you making a dangerous gun?
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Re: Fluid Dynamics...

Post by Tigger » Mon Jan 03, 2011 11:30 am

Normal wrote:Are you making a dangerous?
:fix:

Isn't "dangerous gun" a tautology?
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Re: Fluid Dynamics...

Post by normal » Mon Jan 03, 2011 11:31 am

Tigger wrote:
Normal wrote:Are you making a dangerous?
:fix:

Isn't "dangerous gun" a tautology?
More dangerous to the user than factory made guns, I meant :tup:
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Re: Fluid Dynamics...

Post by mistermack » Mon Jan 03, 2011 5:16 pm

Don't guns give a bit of a clue? They shoot bullets at far higher velocities than the speed of sound, so the gasses released must be expanding much faster than the speed of sound in air.
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Re: Fluid Dynamics...

Post by Tigger » Mon Jan 03, 2011 5:21 pm

mistermack wrote:Don't guns give a bit of a clue? They shoot bullets at far higher velocities than the speed of sound, so the gasses released must be expanding much faster than the speed of sound in air.
.
Irrelevant. He's talking about expansion, not explosion.
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Re: Fluid Dynamics...

Post by mistermack » Mon Jan 03, 2011 5:47 pm

Tigger wrote:
mistermack wrote:Don't guns give a bit of a clue? They shoot bullets at far higher velocities than the speed of sound, so the gasses released must be expanding much faster than the speed of sound in air.
.
Irrelevant. He's talking about expansion, not explosion.
Yeh, but the explosion provides the expanding gas that drives the bullet.
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