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inlet/outlet boundary conditions for a forced flow in a channel

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Old   September 12, 2011, 09:38
Question inlet/outlet boundary conditions for a forced flow in a channel
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Davide Cagnoni
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hi!

I'm trying to run a simulation about an internal flow in a converging channel, and i am in doubt about the boundary conditions to use:

i do not have a fixed mass-flow rate since what i am investigating is the result of a volume force on the fluid flow (therefore i have a source term in the momentum equation...)

this force should (and does) pump the fluid towards the narrower section (which is my "outlet") at which i imposed a 0 pressure - 0 stress boudary condition.
At first i have imposed the same BC on the other channel edge since i thought the inflow should be free from stress and also at the same "external" pressure - being the motion triggered only by the volume force to which the fluid is exposed - but being my results quite different from what i expected i think something in my opinions is wrong...

thanks for every hint anybody would give me
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Old   September 13, 2011, 03:41
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Davide Cagnoni
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sorry for bumping....
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Old   September 13, 2011, 08:33
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Zachariah Swetky
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davide_c,

It seems to me that applying a force to the fluid should yield a pressure gradient from the inlet to the outlet. By applying 0 pressure at both the inlet and the outlet are you not nullifying this gradient? Are you specifying total pressure or static pressure at the inlet and outlet?
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Old   September 13, 2011, 09:03
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Davide Cagnoni
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If i am correct, my pressure object represents static pressure (normalized by the density)

I spent some time thinking about it and realised the same conclusion about the pressure drop, so I switched the pressure condition on inlet to 0-gradient. Still, with this other arrangement i have problems -instabilities- after some time at the outlet.

I am thinking about imposing a 0 gradient condition on the whole boundary - which I know phisically not having much sense together wit a no stress condition for U - just to try out...
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