Single Phase water jet Problem against gravity

beyonder1 · May 18, 2017, 17:14

Hi,
I am trying to simulate a waterjet(like a fountain) coming out a hole using pressure difference against gravity in the vertical direction. The problem is water jet should stop after some height but it's not happening. It is hitting the boundary of the domain. I am guessing somehow gravity(buoyancy) is not working.
Any Suggestions.

P.S.- I went through earlier threads.

ghorrocks · May 18, 2017, 19:57

Have you turned gravity on?

Do you have a density difference from something (probably different phases) for gravity to have some effect on? Gravity has no effect if the fluid is constant density.

beyonder1 · May 19, 2017, 02:40

In Buoyancy option, I have specified z-direction as '-9.81m/s2'.
Why do I need multiphase for gravity to work? Can it work like decrease/increase by 'rho*g*h' when I move through the water. I am hoping using only water(single phase) with atmospheric pressure as 1 atm and the relative pressure at opening be around 100 Pa.

ghorrocks · May 19, 2017, 03:35

The buoyancy force (rho*g*h) in a constant density fluid just creates the hydrostatic pressure. As all the fluid at that height has the same density it all sees the same buoyancy force, hence nothing has any force than anything else and nothing happens.

For buoyancy to effect the flow you need a density difference, so some heights of fluid see different buoyancy forces to others. This means some fluid goes up and some goes down, and that generates secondary flows and so on.

So buoyancy force in a single phase flow with constant density does nothing, other than generate a fixed hydrostatic pressure.

beyonder1 · May 19, 2017, 04:44

Can you suggest how should I proceed? I want to model a simulation of water(fountain) flow for some height(due to gravity) dictated by inlet relative pressure.
del(p)=rho*g*h
Do I have to simulate a multiphase flow with air and water? And how should I apply gravity so as to balance the relative pressure at inlet with water height(due to gravity)?

ghorrocks · May 19, 2017, 07:14

The last fountain I saw was a two phase flow with water jetting into air. So it sounds like a multiphase model to me.

A model like this would appear easiest modelled as Lagrangian particles. You give the droplets and initial velocity and the model tracks them from there. If you model a fountain type of flow the particles should follow a roughly parabolic path.

May 18, 2017, 17:14	Single Phase water jet Problem against gravity	#1
beyonder1 Member beyonder Join Date: May 2015 Posts: 67 Rep Power: 11	Hi, I am trying to simulate a waterjet(like a fountain) coming out a hole using pressure difference against gravity in the vertical direction. The problem is water jet should stop after some height but it's not happening. It is hitting the boundary of the domain. I am guessing somehow gravity(buoyancy) is not working. Any Suggestions. P.S.- I went through earlier threads.

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May 18, 2017, 19:57		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Have you turned gravity on? Do you have a density difference from something (probably different phases) for gravity to have some effect on? Gravity has no effect if the fluid is constant density.

May 19, 2017, 02:40		#3
beyonder1 Member beyonder Join Date: May 2015 Posts: 67 Rep Power: 11	In Buoyancy option, I have specified z-direction as '-9.81m/s2'. Why do I need multiphase for gravity to work? Can it work like decrease/increase by 'rhogh' when I move through the water. I am hoping using only water(single phase) with atmospheric pressure as 1 atm and the relative pressure at opening be around 100 Pa.

May 19, 2017, 03:35		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The buoyancy force (rhogh) in a constant density fluid just creates the hydrostatic pressure. As all the fluid at that height has the same density it all sees the same buoyancy force, hence nothing has any force than anything else and nothing happens. For buoyancy to effect the flow you need a density difference, so some heights of fluid see different buoyancy forces to others. This means some fluid goes up and some goes down, and that generates secondary flows and so on. So buoyancy force in a single phase flow with constant density does nothing, other than generate a fixed hydrostatic pressure.

May 19, 2017, 04:44		#5
beyonder1 Member beyonder Join Date: May 2015 Posts: 67 Rep Power: 11	Can you suggest how should I proceed? I want to model a simulation of water(fountain) flow for some height(due to gravity) dictated by inlet relative pressure. del(p)=rhogh Do I have to simulate a multiphase flow with air and water? And how should I apply gravity so as to balance the relative pressure at inlet with water height(due to gravity)?

May 19, 2017, 07:14		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	The last fountain I saw was a two phase flow with water jetting into air. So it sounds like a multiphase model to me. A model like this would appear easiest modelled as Lagrangian particles. You give the droplets and initial velocity and the model tracks them from there. If you model a fountain type of flow the particles should follow a roughly parabolic path.