water shielding: Pressure?


by Berner
Tags: flow, force, hydraulics, math
Berner
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Feb28-12, 02:24 PM
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I just read a short story about a post apocalyptic world in which humanity was on the brink of destruction due to the loss of protection from the planetary magnetic field (pole reversal in progress, process turns out to take some 100 years to finish). Electronics go haywire and large amounts of radiation starts cooking crops and such.

One part of the story got me to thinking on how you could shield existing housing from those levels of radiation and that lead me to water shielding (to let sunlight through). But I got to thinking that that wouldn't work well in a highrise due to static pressure (later I came up with the simple solution of simply dividing the shielding into segments a few meters in height) of the water acting on the glass or plastic holding it in. I came up with a solution (complicated and expensive but it was the first thing to come to mind), let the water flow down between two sheets of transparent material and make sure that nothing restricted the outflow at the bottom, problem solved. :)

Now for the actual question: Would water flowing freely between two plates exert any sort of pressure against those plates or does this occur only when flow is restricted? And if there is no pressure what would a graph of pressure vs. restriction look like? That is to say, if I restricted the opening at the bottom to be 5% narrower than the top how large a percentage of the maximal static head would I experience?

I thought these to be interesting questions that really aren't that critical to anything but that have been fun pondering, I just wanted to share... :)
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Simon Bridge
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Feb28-12, 05:57 PM
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Welcome to LQ.
Congratulations, you seem to have noticed that if the water exerts no pressure on the walls then you run into problems modelling the case where the restriction varies.
Flowing water does exert pressure on the pipe (or whatever) it flows through. This would include two plates. You know about Bernoullis principle right?

Of course the scheme suggested has to balance the cost saved in support for the mass of water against the ongoing cost of continually pumping the water to the top. (Against other solutions, like moving everyone underground and/or blocking up all the windows.)
Logan Fife
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Feb28-12, 06:06 PM
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if you had a falling wall of water, (like some fancy fountains you see around), thats putting pressure on the air around it. you could build 2 sheets of glass on either side, but no pressure means a vacuum. how does water respond to flowing through a vacuum?

Q_Goest
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Feb28-12, 06:52 PM
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water shielding: Pressure?


Hi Berner. Welcome to the board. Yes, you could design a pipe or a set of flat plates (glass windows) such that the irreversible pressure loss exactly equals the head pressure. Are you familiar with the Darcy Weisbach equation?


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