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

davide_c · September 12, 2011, 10:38

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

davide_c · September 13, 2011, 04:41

sorry for bumping....

swetkyz · September 13, 2011, 09:33

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?

davide_c · September 13, 2011, 10:03

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...

September 12, 2011, 10:38	inlet/outlet boundary conditions for a forced flow in a channel	#1
davide_c New Member Davide Cagnoni Join Date: Jul 2011 Posts: 14 Rep Power: 15	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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September 13, 2011, 04:41		#2
davide_c New Member Davide Cagnoni Join Date: Jul 2011 Posts: 14 Rep Power: 15	sorry for bumping....

September 13, 2011, 09:33		#3
swetkyz Member Zachariah Swetky Join Date: Aug 2011 Posts: 36 Rep Power: 15	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?

September 13, 2011, 10:03		#4
davide_c New Member Davide Cagnoni Join Date: Jul 2011 Posts: 14 Rep Power: 15	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...