CFD Online Discussion Forums - Diffrence between VOF alone and VOF coupled Level 7 ??

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Diffrence between VOF alone and VOF coupled Level 7 ??

Hello everybody,
I have a problem.I have problem to found the differences between the VOF model alone and the VOF coupled Level7 model ? Fluent help don"t give lot of information on this model...

Do you have an idea ?

I try to simulate the flow of bubble in vertical pipe with diamter ranging from 1 to 10 mm.I succed for the diameter bigger than 5mm with VOF model but for the smaller diameters (D<5mm),the VOF model is not conclusive and i feal that the bubble gets stuck in the pipe...

Someone has an explantion ?

Thanks you very much for your time !

Coupled-set algorithms try to capture the curvatures of the interface correctly, thus keeping the interface sharp. You can consider this coupled method for cases where surface tension plays important role.

OJ

Thank you for your reply !
I understand that VOF coupled to level 7 improved the interface definition.
This is the only interest of level 7 ?

First of all, it is level-set, not level 7. And do not underestimate the importance of interface definition. The whole VOF methodology is designed around it!

If you carefully observe, the benefits of VOF and level-set are complimentary to each other. While VOF is generally very good in ensuring the conservation of volume fractions through out the domains, including at interface, it struggles to exactly define the interface, because there exist very sharp gradients of volume fractions. At the same time level-set method approximates the interface quite correctly but the unevenness of the interface creates errors in mass/momentum calculations and hence the volume fraction conservation is often poor.

But when you couple both of them, you couple the benifits. For instance, to define the interface at a cell level, VOF will accurately give the volumes of the cell occupied by both the phases, while, the level-set algorithms will accurately define the curvature and direction of the interface in the cell - thus providing details as to where exactly the interface intersects the cells. Thus, cell is divided into two phases with right volume fractions and with right shape of interface.

This is particularly beneficial for high surface tensions because the velocity field nearby interface is significantly affected in presence of higher surface tension, influencing the interface.

Now, how would you know if surface tension is important in your case?
Take the following test:

Calculate Capilary number if Re<<1: $Ca = \frac{\mu U} {\sigma}$
Calculate We number if Re>>1: $We = \frac{\rho d U^2} {\sigma}$

where d is characteristic length of your bubble, say diameter and other symbols have usual meaning.

Surface tension is important when We>>1 or Ca<<1 and this is an incentive to use level-set with VOF.

OJ

Hello oj. Bulmer,
Thanks again for your reply and for the details on VOF and VOF coupled Level set model.
I know it's level set and not level 7,this is a shorcut :)

I have already calculate the Capillary and Weber numbers.The results are 0.01 and 0.00001 respectively so the surface tension should not influence the interface.

Have you an idea why i feel that the bubble is stuck in the pipe ? I don't understand

Maybe increase the size of the bubble ?
Or add a low liquid flow ?

I found some publications where they succeed to simulate that type of flow in very small pipes..I don't understand why it doesn't work :(

OJ i have a question? Is U indicates the velocity of gas or liquid?

Quote:

Originally Posted by oj.bulmer (Post 434712)

First of all, it is level-set, not level 7. And do not underestimate the importance of interface definition. The whole VOF methodology is designed around it!
If you carefully observe, the benefits of VOF and level-set are complimentary to each other. While VOF is generally very good in ensuring the conservation of volume fractions through out the domains, including at interface, it struggles to exactly define the interface, because there exist very sharp gradients of volume fractions. At the same time level-set method approximates the interface quite correctly but the unevenness of the interface creates errors in mass/momentum calculations and hence the volume fraction conservation is often poor.

But when you couple both of them, you couple the benifits. For instance, to define the interface at a cell level, VOF will accurately give the volumes of the cell occupied by both the phases, while, the level-set algorithms will accurately define the curvature and direction of the interface in the cell - thus providing details as to where exactly the interface intersects the cells. Thus, cell is divided into two phases with right volume fractions and with right shape of interface.

This is particularly beneficial for high surface tensions because the velocity field nearby interface is significantly affected in presence of higher surface tension, influencing the interface.

Now, how would you know if surface tension is important in your case?
Take the following test:

Calculate Capilary number if Re<<1: $Ca = \frac{\mu U} {\sigma}$
Calculate We number if Re>>1: $We = \frac{\rho d U^2} {\sigma}$

where d is characteristic length of your bubble, say diameter and other symbols have usual meaning.

Surface tension is important when We>>1 or Ca<<1 and this is an incentive to use level-set with VOF.

OJ

Mariam,

Sorry for a late reply, being away for a while from the forum :)

U is the free stream velocity of water, or continuous phase.

Cheers
OJ

Ok thanks Oj.Bulmer !

Have a nice day

Hello

I have a question. how Can I calculate phase velocity and phase position by level set+VOF ?
Thank you so much

Can you not use the CFD-Post to grab the phase velocities? The phase positions can of course be understood by plotting the contour plots.

OJ

Hello mohamad3564 and oj.bulmer !
For the phase position,you can see that like said Oj.bulmer.

For the phase velocity (in my case,velocity of gas bubble is liquid),you can't evaluate it just with CFD-Post.You have to follow the position of the nose of the bubble at each time of the simulation.
In my case,i follow the position of nose of the gas phase on the axis of the tube and i plot the position of the nose as a function of time simulation.
The slope of this curve is the velocity of your gas phase.

I hope this help you

Have a good day