# The influence of gravity on liquid

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June 30, 2014, 16:22
The influence of gravity on liquid
#1
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Ye Bai
Join Date: Mar 2014
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Hi,
I am tring to simulate single-phase flow in a vertical pipe. The geometry is shown below.
Inlet: Velocity inlet
Outlet: Pressure outlet
The result is fine when ignoring the gravity. But when the gravity is taken account, the velocity becomes weird. I think the velocity should be accelerated due to the gravity. Could anyone tell me the reason?
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 July 1, 2014, 00:45 #2 Member   Thiagu Join Date: Oct 2012 Location: India Posts: 60 Rep Power: 13 What about axis directions .

July 1, 2014, 09:55
#3
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Ye Bai
Join Date: Mar 2014
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Quote:
 Originally Posted by jthiakz What about axis directions .
The axis is along the x-axis. The direction of gravity is also along x-axis. I choose the gravity acceleration as (+)9.81m/s2.

 July 3, 2014, 10:59 #4 Senior Member   Blanco Join Date: Mar 2009 Location: Torino, Italy Posts: 193 Rep Power: 17 how many cells do you have in the transverse direction? have you considered the wall boundary layer growth in you mesh? if you look at the velocity magnitude it increases a lot when gravity is on, I suggest you to perform the same simulations without the wall in order to avoid problems related to the wall boundary layer. Just put a symmetry plane where you have the axis and the wall, it should work then.

July 3, 2014, 11:24
#5
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Ye Bai
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Quote:
 Originally Posted by Blanco how many cells do you have in the transverse direction? have you considered the wall boundary layer growth in you mesh? if you look at the velocity magnitude it increases a lot when gravity is on, I suggest you to perform the same simulations without the wall in order to avoid problems related to the wall boundary layer. Just put a symmetry plane where you have the axis and the wall, it should work then.
What did you mean by putting a symmetry plan? I just changed the wall to axis in boundary condition, but unfortunately, it did not work. Maybe I did not catch up what you mean....

 July 3, 2014, 11:44 #6 Senior Member   Blanco Join Date: Mar 2009 Location: Torino, Italy Posts: 193 Rep Power: 17 Yes sorry, I mean "symmetry plane". In that way the boundary acts like a mirror and it will let the fuild to slip on the boundary. Another option is to model both axis and wall as "slip walls",with this option you need to setup those two boundaries to be "free slip wall" (under boundary detail tab in cfx). Btw are you using fluent or cfx?

July 3, 2014, 11:56
#7
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Ye Bai
Join Date: Mar 2014
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Quote:
 Originally Posted by Blanco Yes sorry, I mean "symmetry plane". In that way the boundary acts like a mirror and it will let the fuild to slip on the boundary. Another option is to model both axis and wall as "slip walls",with this option you need to setup those two boundaries to be "free slip wall" (under boundary detail tab in cfx). Btw are you using fluent or cfx?
It's fluent.

 July 3, 2014, 13:37 #8 Senior Member   Blanco Join Date: Mar 2009 Location: Torino, Italy Posts: 193 Rep Power: 17 Ok it's Fluent, then you can use a symmetry plane because "They can also be used to model zero-shear slip walls in viscous flows" (taken from the fluent manual, symmetry boundary conditions page)

 July 3, 2014, 23:37 #9 Member   Thiagu Join Date: Oct 2012 Location: India Posts: 60 Rep Power: 13 Outlet BC pressure is it same ?. Let me know domain dia, length .inlet velocity. Make a 1D calculation. For both condition pressure may not be same.