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boundary conditions for a pipe underwater

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Old   November 16, 2012, 02:32
Default boundary conditions for a pipe underwater
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jpc
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i have a tube that is completely submerged in water
it is oriented horizontally ("parallel" to the water surface)
it is open on both ends
it is ok to assume that the hydrostatic pressure effect (pressure difference from top to bottom can be ignored) and that the depth is unimportant (ie, atmospheric pressure is ok)

in an internal analysis, what type of BCs would you apply on each end?
static pressure?
total pressure?

in this specific case i'm using solidworks flow simulation and inside the tube is a prop rotating at a specific RPM that i'm interested in understanding its thrust.

thrust results with static pressure of 101325 on each side are different than using flow simulation's environmental pressure that sets a total pressure of 101325 on the inlet and static pressure of 101325 on the other end. with static pressure the static pressure difference is about 5Pa, the environmental pressure case about 4000Pa. total thrust is about 1.5x larger in the environmental pressure case but average velocity at the oulet and mass flow/volume flow rates are similar for both cases.

the obvious answer seems to be go with an external analysis instead of prescribing the boundary conditions on the ends of the tube and that better matches the environmental pressure case but i'm curious to know if anyone has an opinion on the setup. or whether someone knows where setting static pressures are considered "starting points" or if they should be forced at all times by the software. (ie the pressure drop is the incorrect part)

Thanks.
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Old   November 21, 2012, 15:54
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hi, any ideas?
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Old   November 22, 2012, 03:42
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Rami Ben-Zvi
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Hi JPC,

If you wish to get answers, you should further clarify your problem, e.g.,: is there a flow, and how is the pipe oriented to it, how deep is the pipe, etc.

Rami
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Old   November 28, 2012, 12:20
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Hi Rami, please let me know what I'm missing from my original problem definition.

Regarding your questions, I think there was only one that I missed which was "is there flow". The simple answer is no, consider that the pipe is submerged in completely still water. The actual answer is yes because inside the pipe there is actually a prop that is pulling fluid from one side to another so that will generate flow.

Regarding orientation, the pipe's axis is horizontal with the water surface. If we consider y the direction of gravity, we could consider that the x or z direction.

How deep is the pipe, let's consider it right at the surface and that it's diameter is relatively small such that the hydrostatic pressure difference from the top to the bottom is insignificant and that the actual increase in pressure from being at depth is insiginificant. Also, for the purpose of this analysis we can ignore the effect of the surface.
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Old   December 7, 2012, 13:39
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no body has ideas/suggestions?
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Old   January 9, 2013, 16:57
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cross post

boundary conditions for analysis of a prop (External)
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