The influence of gravity on liquid
 

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?
Thanks in advance.

What about axis directions .

Thanks for your reply, jthiakz.
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.

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.

Hi, Blanco. Thanks for replying.
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....

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.

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)

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.