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Pressure question

A place to ask general spud cannon related questions.
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Unread postAuthor: Gun Freak » Mon Feb 15, 2010 10:24 pm

Thanks for trying :roll:
So is it ok if it tells me that?

BTW... Is that a barrett 50 cal. in your picture?
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Unread postAuthor: D_Hall » Tue Feb 16, 2010 12:16 am

Gun Freak wrote:So is it ok if it tells me that?

Yes.

BTW... Is that a barrett 50 cal. in your picture?

Yes.
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Unread postAuthor: Davidvaini » Tue Feb 16, 2010 3:00 am

I might be butchernig this trying to explain it in easier terms.. so here goes nothing:

when air at a certain temperature and pressure flows though something smaller, or a restriction of some kind like a valve, into a lower presure chamber, the fluid velocity must increase intially "upstream" factors, as it flows though the valve.

while this happens, the venturi effect casues the overall static pressure to decrease.

you might want to read up on the conservation of mass and how it effects fluids and gases, I remember learning about it at the same time we were talking about conservation of mass.
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Unread postAuthor: btrettel » Tue Feb 16, 2010 8:27 am

The flow velocity through the restriction is at the speed of sound, which is the speed waves (sound/pressure/they're all the same) propagate at. This means that pressure waves can't travel back through the restriction and the downstream end is unaware of the pressure at the upstream end and vice-versa, so further decreases in pressure on the downstream end won't affect the flow.

Here's an old video showing some choking phenomena.

http://modular.mit.edu:8080/ramgen/iflu ... n_Fluid.rm

For incompressible flow, velocity must increase for a fluid to flow through a restriction (in compressible fluids the density could change such that the velocity is constant or slows down... but let's ignore that for now). So if the increased velocity through the valve (a restriction) doesn't reach the speed of sound but the flow reaches the speed of sound in the barrel, then the barrel is the choke point.

As has been said, it's not necessarily a bad thing. It just means that upgrading your valve won't increase performance.
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Last edited by btrettel on Tue Feb 16, 2010 8:33 pm, edited 2 times in total.
All spud gun related projects are currently on hold.

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Unread postAuthor: Ragnarok » Tue Feb 16, 2010 10:54 am

D_Hall wrote:Anybody got a super-basic explanation for the concept of choked flow?

Super basic...

*Mutters*

...roadworks. To the flow of road traffic, these represent a choke point. If you've only got a few cars, they can get through without a problem - flow rate isn't hindered (notably).
However, eventually, you'll hit a limit on the rate at which cars can get through the roadworks, and this will be lower than the rate on an open road - there isn't the space to get the cars through at the same rate.

In the workings of your launcher, the breech is somewhat like a set of road works - it's a point where the flow of air particles has reached the limit, despite the fact that the rest of the system could support higher flow.
This doesn't mean the launcher will blow up or anything - it just limits airflow to a maximum of whatever the choked section can support (thus limiting power to some degree).

The actual science behind it is more complex (as btrettel's post loosely explains), but the roadworks explanation is a close enough analogy.
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Unread postAuthor: Gun Freak » Wed Feb 17, 2010 5:19 pm

Sort of makes sense now... not that important though.

Back to the piston disscusion, you could pressurize the sides poroportionally according to the area of each side of the piston, but that would be really hard to do.
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