When is the solution gravity-dependant?
 

Hello everybody,

I'm interested in knowing when the gravity plays a role in a VOF simulation. Is there some rule of thumb related to this?

I know that when buoyancy effects are foreseen due to different temperatures that lead to different densities the gravity plays an important role.

However, if we simulate the filling of a tank, the gravity plays a role even though there is no buoyancy effect here.

Thanks a lot!

Hi Jordi,

Best practice would be to relate the strength of the gravity force to the strength of the other forces in the domain. For example, in an interfacial application, the Bond number (gravity vs. surface tension) will probably play a role; and so you can look for other forces as well.

By the way, there definitly is a buoyancy effect in tank filling, basically archimedes; a volume of air displaced by a volume of water due to the significant density difference between the different phases (the difference is, in temperature-driven flows the density differences are within one phase).

Hope this helps!
Cheers,
Cees

Dear cessH,

related to the gravity question, thanks a lot!

Related to the buoyancy effect, you left me thinking... I mean, I know that the fluid filling the tank have some buoyancy effect due to the displaced air, but due to the low density of the air, the buoyancy effect might be nearly insignificant. Is buoyancy taken into account with this kinds os simulation?

If we simulate a fluid going through a pipe previously full of air, is there some buoyancy effect too?

Hi,

I think you are focusing a bit too much on buoyancy in terms of intra-phase forces; what I meant was buoyancy as an inter-phase force.

In essence, phase separation in a channel flow has everything to do with buoyancy. Say you have a perfectly mixed flow coming in, under the influence of gravity they will separate into two distinct, co-flowing phases. In the reason where the flows are segregating, gravity, via the density difference between the phases, leads to buoyancy - causing the gas to rise with respect to the fluid. So I would say yes, if you have a pipe full of air and you pump water in, there is buoyancy. The density difference between gas and liquid will push the gas up, not just forward.

Dear CeesH,

Imagine an horizontal pipe full of air where water comes from left to right. Since water moves the air, the water has an insignificant (due to the low density of air) buoyancy force upwards. We are agree.

However, I don’t understand your sentence “if you have a pipe full of air and you pump water in, there is buoyancy. The density difference between gas and liquid will push the gas up, not just forward.”

Thanks a lot!

What I mean is that, if there is no gravity difference, the water would simply push out the air as a plug.

However, due to the density difference, the water will first occupy the bottom of the pipe, and air in the top section will flow out later than air in the bottom. The will be some axial profile, rather than a total plug.

I guess you mean density, don't you?

Thanks for the explanation :)

Eh yeah indeed ;)