[juste]

hi guys

i am trying to simulate an open channel flow (like a laboratory open channel flume) using the vof model (2 phase) in ansys fluent...using the pressure based steady state solver, gravity enabled...k-epsilon model...

however, after running the iterations, whenever i use the XY plots from the results section - the 'phase' drop down menu has only 1 option - 'mixture'... there is no option to check the results for just 'phase 1' or 'phase 2'.. please look at the image file attached..

i feel there is some very basic mistake i have made, can you give me any
suggestions/idea...thanks a lot..:-)

other details:
vof..implicit scheme..open channel flow..
primary phase is air, and secondary phase is water
inlet bc is mass-flow-inlet
outlet bc is outflow

[`e`]

Are you sure you're modelling two phases?

[juste]

hey, thanks so much for the quick response :-) :-)

well, under 'Multiphase Model' section, i have selected 'Volume of Fluid', then i have entered 2 as 'Number of Eulerian Phases'.

under 'Materials' i have defined 2 materials under 'Fluid'..one is 'water-liquid' and second one is 'air', both from fluent database...

under 'Phases'...i have defined
'Phase 1 - Primary Phase' = 'air'
'Phase 2 - Secondary Phase' = 'water-liquid'

under 'Cell Zone Conditions', only one zone 'Fluid' appears in the box..
and the 'Phase' drop down menu has 3 options
1. mixture
2. phase 1
3. phase 2

i have not entered anything in any of the 3 options above..as i felt that
they are default..please have a look at the pics attached:
maybe the problem is somewhere here...
maybe i need to input something over here, i dont know..

also, in my 3D mesh model i have not created separate zones for air and water; the mesh covers the entire flume model, and in inlet BC of mass-flow inlet, i used 0.0001 kg/s for air, and 17 kg/s for water, and specified free surface level at 0.12m, that's it..

thanks again for the help :-)

[gerardosrez]

Hello

For default Fluent stores a lot of quantities for you to check during post proccessing...Most of them are for the mixture....

Also you can specify in a list another quantities you need to be stored in you results file so you can plot them

[juste]

hi g.s., thanks for the response...

but i am not sure where and how do I specify those other quantities which fluent is not showing by default...can u plz let me know...
thanks again :-)

[juste]

in case, u mean that i need to import additional output parameters from 'File' - 'Data File Quantities', i am attaching a pic of that too...

there are only 3 variables listed separately for phase 1 and phase 2, they are the density, vol fraction and molecular viscosity..that's it..

no option for velocity, pressure, etc etc for phase 1 and 2..
plz have a look...

[`e`]

I'm not very familiar with the VOF model and how Fluent handles the two phases. Try following some tutorials to explore and familiarise yourself with Fluent's VOF model. Perhaps the VOF solver doesn't distinguish the velocity between phases and therefore it's only available for the mixture.

[juste]

oh i see, thanks anyway...
if anyone else has any idea, please do share.

thanks

[juste]

hey, "e" u were right in saying that fluent may be solving the vof just for the mixture; it is indeed the case :-)

so vel of phase 1 = vel of mixture * vol. fraction of phase 1
similarly for phase 2

so i can just make a custom variable from
Define - Custom Field Functions - define the new output variable - save - load - now the XY plot box will show the new cff - done :-)

problem solved...

my first answer on this cfd forum....haha :-) :-)

addendum:
on second thought - wonder why fluent doesn't do it by default - would make it a bit user-friendly at least for beginners like me, would save enormous time too.
on third thought - i am still having many other issues with my model and convergence, but i guess i problem is solved today, step by step ;-)

[`e`]

Your equation for determining the phase velocities puzzled me (why would a small volume fraction phase travel, by such definition, slower than the other phase(s)?). I've had a quick read of the theory guide on the VOF section in the multiphase flows chapter. You're mistaken that the phases have different velocities; a single momentum equation is solved which yields a shared velocity field between the phases.

Returning to your original query, there are quantities/variables that Fluent solves with the VOF model which are only available for the mixture (e.g. velocity), and other values that are only available for individual phases (e.g. volume fraction).

[juste]

hey, "e" thanks again for your reply...

well i am not sure if i completely understand your last post #10. are u saying whether the 2 phases should move at the same speed or at two different speeds...

if this is what u mean to ask; then since i am modeling vof - open channel flow (a river), don't you think the water (secondary phase) and the air (primary phase) above it will move at different speeds..in fact the air is just there and not much moving, just sitting/blowing over the water, again just like over a river..

i am attaching a pic of the model, in case it helps..plz have a look..

and i am more interested in the velocity of the water rather than the air, in my case..

so i guess the phases have different velocities...
its just like in the water portion, vol fraction of water = 1.0 so velocity of water = velocity of mixture * 1.0 till water level...and 0 above it
and above water level, vol fraction of water = 0 and vol fraction of air = 1.0, so velocity of air = velocity of mixture * 1.0... above water level
so velocity of the 2 phases are the same as that of the mixture..isnt it...but only in the water and air region..
it is only in the air-water interface that the vof will not be 0 or 1 but something in between, and hence air vof * mixture velocity + water vof * mixture velocity = mixture velocity displayed by fluent

i guess the confusion is arising because fluent is describing it as a mixture, but air and water here are not not mixing like 2 liquids water and milk mix together in a cup..

i hope i am able to make my problem clearer..
if not, please let me know...

thanks for giving your thoughts on my question..
thanks so much...

[azna]

In VOF, Is it possible to export values for water and air separately? I working on a case, where dispersed air bubbles are injected into the water.

Thanks

[openAC]

@ajna and @juste

Were you able to figure out how to get velocity of an individual phase from the VOF simulations? If so, please share some tips.

Thanks in advance.

[rku]

Originally Posted by openAC

@ajna and @juste

Were you able to figure out how to get velocity of an individual phase from the VOF simulations? If so, please share some tips.

Thanks in advance.

As the op mentioned in their 'answer' post:

vel of phase 1 = vel of mixture * vol. fraction of phase 1

My question to anyone who is still looking at this is if we can plot vectors of only the water phase in CFD Post?

[openAC]

Originally Posted by rku

As the op mentioned in their 'answer' post:

vel of phase 1 = vel of mixture * vol. fraction of phase 1

My question to anyone who is still looking at this is if we can plot vectors of only the water phase in CFD Post?

You may be interested in the discussion here:
What is the "phase velocity" in VOF simulations?

[manq]

Juste..please let me know how you given two different velocities in your case??