Density problem in fluent

Zaktatir · December 10, 2013, 15:57

Node values are explicitly defined or obtained by weighted averaging of the cell data. Various boundary conditions impose values of field variables at the domain boundaries, so grid node values on these boundary zones are obtained by simple averaging of the adjacent boundary face data. In addition, for several variables (e.g., node coordinates) explicit node values are available at all nodes.

By the way we do not have any entrainment of air or tearing off of liquid. It would be better if you would like to sharpen the interphase to geo-reconstruct. The free surface is still at 0,5.

ghost82 · December 10, 2013, 16:09

You should see more definite water and air phases. Can you post here the 2 contour plots with and without node values?

flow_CH · December 11, 2013, 09:22

Quote:

Originally Posted by ghost82

You should see more definite water and air phases. Can you post here the 2 contour plots with and without node values?

Please see attached image

flow_CH · December 14, 2013, 08:41

Let me say this problem another way.

My flow depth in experiments is 1.5 meters. I have flow depth=1.5 meters in fluent too (from volume fraction 1 to 0.5). Then OK.
But from volume fraction 0.5 to 0, the distance is about 1 meter!
I mean the difference between VF=0.5 to VF=0 is very large. (about 1 meter)
What is reason of this problem?

Thanks

Zaktatir · December 14, 2013, 08:57

Do you mean with your flow Depth the film thickness?

flow_CH · December 14, 2013, 09:14

Quote:

Originally Posted by Zaktatir

Do you mean with your flow Depth the film thickness?

I am sorry i don't understand. If i decrease the size of mesh, then the distance between VF=0.5 and VF=0 decrease too?

flow_CH · December 14, 2013, 10:20

Please see attached image

flotus1 · December 14, 2013, 11:03

Quote:

Originally Posted by flow_CH

Please see attached image
vf.jpg

From the contour without interpolated "node values" we can see that the cell size is quite large and definitely affects the width of the interface between the two phases.
Decreasing the cell size should also decrease the distance between VF=0.5 and VF=0

flow_CH · December 14, 2013, 11:23

Thank you flutus1

I have very large issue. I modeled this simulation for 5 times. I have just 2 month for my thesis and i can not decrease mesh. For sketching air concentration (void fraction), can i equivalent VF=0 with VF=0.5 since this modeling is true and problem is just from meshing?

Zaktatir · December 14, 2013, 11:48

Try to make a dynamic adaption starting from this solution and use compressive scheme (which is more quicker and more stable than geo-recon). The adaption should occur by means of the gradient of VOF.

Or you can refine your mesh only in the vicinity of the free surface since you now approximately when it sets in. Interpolate the old results in the new one.

I do not like to comment your intention to assume VOF= 0,5 to be zero...

flow_CH · December 14, 2013, 16:02

Thanks.
1- What is dynamic adaption?
2- With increasing mesh by refining, The time for iterating will be too long. In the present (without refining), the time for iterating of my model is about 26 hours (number of elements is: 1 600 000 elements). Then if i refine the mesh and with 5 simulation, i think i need 2 weeks for iterating (just for a mesh).

3-Why you do not like to comment about assuming VF=0 to VF=0.5? Because i thing my problem is just about meshing and there is no other issue.

flotus1 · December 14, 2013, 18:32

Is there more to this simulation than you have shown by now? To me it looks like quite a simple flow and I dont see where you put those 1.6 million cells.

flow_CH · December 15, 2013, 01:24

Quote:

Originally Posted by flotus1

Is there more to this simulation than you have shown by now? To me it looks like quite a simple flow and I dont see where you put those 1.6 million cells.

Model is 3d and the length is 270 meters in prototype.
Why you do not like to comment about assuming VF=0 to VF=0.5? Because i thing my problem is just about meshing and there is no other issue.

flotus1 · December 15, 2013, 07:49

I think he doesnt want to comment on this because the assumption is, lets say daring.
You would not define velocity=1 $\equiv$ velocity=0.5 just to get a different thickness of the boundary layer that better suits your expectations.

Since there were a lot of comments that were not particularly helpful, I suggest you re-express the problem you are facing in detail.
Then I suggest the experts here have another look on the topic and try to comment from a more professional perspective

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December 10, 2013, 15:57		#21
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	Node values are explicitly defined or obtained by weighted averaging of the cell data. Various boundary conditions impose values of field variables at the domain boundaries, so grid node values on these boundary zones are obtained by simple averaging of the adjacent boundary face data. In addition, for several variables (e.g., node coordinates) explicit node values are available at all nodes. By the way we do not have any entrainment of air or tearing off of liquid. It would be better if you would like to sharpen the interphase to geo-reconstruct. The free surface is still at 0,5.

December 10, 2013, 16:09		#22
ghost82 Senior Member Rick Join Date: Oct 2010 Posts: 1,016 Rep Power: 27	You should see more definite water and air phases. Can you post here the 2 contour plots with and without node values?

December 14, 2013, 08:41		#24
flow_CH Senior Member FHydro Join Date: Jan 2013 Posts: 197 Rep Power: 13	Let me say this problem another way. My flow depth in experiments is 1.5 meters. I have flow depth=1.5 meters in fluent too (from volume fraction 1 to 0.5). Then OK. But from volume fraction 0.5 to 0, the distance is about 1 meter! I mean the difference between VF=0.5 to VF=0 is very large. (about 1 meter) What is reason of this problem? Thanks

December 14, 2013, 08:57		#25
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	Do you mean with your flow Depth the film thickness?

December 14, 2013, 11:23		#29
flow_CH Senior Member FHydro Join Date: Jan 2013 Posts: 197 Rep Power: 13	Thank you flutus1 I have very large issue. I modeled this simulation for 5 times. I have just 2 month for my thesis and i can not decrease mesh. For sketching air concentration (void fraction), can i equivalent VF=0 with VF=0.5 since this modeling is true and problem is just from meshing?

December 14, 2013, 11:48		#30
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	Try to make a dynamic adaption starting from this solution and use compressive scheme (which is more quicker and more stable than geo-recon). The adaption should occur by means of the gradient of VOF. Or you can refine your mesh only in the vicinity of the free surface since you now approximately when it sets in. Interpolate the old results in the new one. I do not like to comment your intention to assume VOF= 0,5 to be zero...

December 14, 2013, 16:02		#31
flow_CH Senior Member FHydro Join Date: Jan 2013 Posts: 197 Rep Power: 13	Thanks. 1- What is dynamic adaption? 2- With increasing mesh by refining, The time for iterating will be too long. In the present (without refining), the time for iterating of my model is about 26 hours (number of elements is: 1 600 000 elements). Then if i refine the mesh and with 5 simulation, i think i need 2 weeks for iterating (just for a mesh). 3-Why you do not like to comment about assuming VF=0 to VF=0.5? Because i thing my problem is just about meshing and there is no other issue.

December 14, 2013, 18:32		#32
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,427 Rep Power: 49	Is there more to this simulation than you have shown by now? To me it looks like quite a simple flow and I dont see where you put those 1.6 million cells.

December 15, 2013, 07:49		#34
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,427 Rep Power: 49	I think he doesnt want to comment on this because the assumption is, lets say daring. You would not define velocity=1 $\equiv$ velocity=0.5 just to get a different thickness of the boundary layer that better suits your expectations. Since there were a lot of comments that were not particularly helpful, I suggest you re-express the problem you are facing in detail. Then I suggest the experts here have another look on the topic and try to comment from a more professional perspective