## Fluids

#### How to consider the effect of gravity and buoyancy ?

• Long Yun
Subscriber

I'm doing a caculation about the two phase flow in floatation tank by fluent. I use Mixture model ,and enable the gravity. The mass transfer between two phase and surface tension have been ignored. The air volume fration is set as 0.1 in velocity inlet boundary. The air volume fraction shloud be a much high value at the top and low value at the bottom, while my result show that air volume fraction in almost the whole domain has the same value. I think  the  effect of gravity and buoyancy should be included in this caculation, while I can't ontain the right results  following the guideline of ANSYS help "Natural Convection and Buoyancy-Driven Flows . "

So how to consider the effect of  gravity and buoyancy when using Mixture model to calculate the water-air two phase flow?

• DrAmine
Ansys Employee

You need to activate Gravity under the operating conditions and give a sensitive operating density here. This would affect your input on the pressurized boundaries. You can five the operating density as the air density as first.

• Long Yun
Subscriber

Thank you. If I enable the specified the operating density, the velocity vector  is wrong.

Correct velocity vector should be as follow,

• Long Yun
Subscriber

I don't know why this happen.

• DrAmine
Ansys Employee

You need to attach a picture of your geometry where you indicate the boundary conditions which you used.

• Long Yun
Subscriber

• Long Yun
Subscriber

This  is  the simplified geometry.

• Rob
Ansys Employee

OK, that looks sensible.

For a steady state system how does the vapour escape?  Otherwise what'll happen is the flow will enter the domain, the liquid will overflow the weir and then run down the other side. Having an outlet smaller than the inlet may cause water to back up and flood the system, if this doesn't occur the main chamber will be full of vapour.

What are you wanting to see in the results: ie what is the question you're trying to answer using CFD?

• Long Yun
Subscriber

I have read some literatures. The experimental result show that due to the gravity, vapor will concentrate on the up location, and water will concentrate on the bottom. The up surface is the free surface, and few vapor will escape from the up surface in experiment. In calculation, they usually treat it as symmetry. I'm sorry for the wrong sketch map. This is the setup in my calculation.

• Long Yun
Subscriber

This the result I want to obtain

• Long Yun
Subscriber

while my calcauted resutl is always like this

• DrAmine
Ansys Employee

So you have two phase system and you are using the mixture model. Now you are comparing with some other results where perhaps mass transfer has been accounted for.

1/Which operating density are you now using?

2/Which specification have you given at outlet?

3/Moreover try to make the symmetry boundary condition a little bit far from the weir

4/Are you accounting for slip between the phases?

• Long Yun
Subscriber

Thank you. Up surface is a water-air interface in real flow, while it is simplified as a wall boundary when calculation. I found that the air will totally escape from the up surface if I set the up surface as outlet boundary. The real flow is that some of air escapes from the up surface, and some of air stay in the water and is influenced by the gravity. In addition, I use the fluent.

• Long Yun
Subscriber

Thank you. I found this is only need to consider the effect of gravity. The problem is the set of up surface.  Up surface is a water-air interface in real flow, while it is simplified as a wall boundary when calculation. I found that the air will totally escape from the up surface if I set the up surface as outlet boundary. The real flow is that some of air escapes from the up surface, and some of air stay in the water and is influenced by the gravity.