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Discontinuity at water level in stratified 2 phase flow

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Old   June 1, 2012, 04:02
Default Discontinuity at water level in stratified 2 phase flow
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Khalid Baker
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Hi I am trying simulating stratified two phase air water flow in a pipe (its D=98.5 mm and length = 25 m while water level fixed at 8 mm) with CFX from couple of a days ago. I set the case as 2D I divide the inlet into two parts (one for water and the second for air) to set the velocities of the individual phases separately (all details mentioned at the attached report). I used the homogeneous model because the phases never mixed and with steady state analysis type. Also I made a CEL expression to some variables similar to that mentioned in free surface flow over a Bump tutorial (see attachment for ccl file too).
The problem I face currently the water is formulated near the lower inlet in a very small portion of pipe length then disappeared (see first image attached) then re-formulated after nearly 2 meters of length (which full length is 25 m) and still fixed at this level (as I want it) to the outlet (see the other image for vof attached). I am wondered why this discontinuity appeared in water at the results? what happened? Any suggestions please?
Attached Images
File Type: jpg vof-inlet.JPG (95.7 KB, 27 views)
File Type: jpg vof-outlet.JPG (94.9 KB, 19 views)
Attached Files
File Type: zip problem settings.zip (2.7 KB, 14 views)

Last edited by kbaker; June 1, 2012 at 04:25.
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Old   June 2, 2012, 06:37
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What is the mesh size in this region?
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Old   June 2, 2012, 10:51
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Hi Glenn the mesh is equally spaced over the entire length there is 600 elements distributed over the 25 m length of the pipe (I draw the mesh in Gambit) while for the vertical dimension there is 40 elements for the 100 mm diameter as I said the mesh distributed equally over the length so there is no different in mesh quality at inlet and outlet as I expect.
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Old   June 3, 2012, 07:12
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Your mesh across the diameter is a bit coarse, you are resolving the water in only 4 nodes or so.

But the problem you are seeing is going to be something about your set up. Either it is not converged properly or not set up properly.
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Old   June 3, 2012, 08:32
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There is 10 nodes for water and 30 nodes for air? The convergence is very well and I post my problem setup at the previous zip file in my first post you can have a look? Even if you need my cfx-pre file I can post it here?

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Old   June 3, 2012, 19:26
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The zip file only contains a small section of CCL. Can you post the full CCL?
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Old   June 4, 2012, 08:04
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Hi Glenn I check the CCL file on the attached zip file it full and nothing missing with it you can have a look at the Report.html file which contain the problem settings too or you can download my cfx-pre file from the link below I uploaded it:

http://www.4shared.com/file/snuV21Oh/two_inlets.html

Last edited by kbaker; June 4, 2012 at 11:49.
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Old   June 6, 2012, 01:55
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Glenn did you check the cfx-pre file I sent you? You not reply me yet?
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Old   June 6, 2012, 02:09
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I only want to look at your CCL. Please extract the full CCL and post that.
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Old   June 6, 2012, 02:50
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CEL:
EXPRESSIONS:
DenH = (DenWater - DenRef)
DenRef = 1.225 [kg m^-3]
DenWater = 1000 [kg m^-3]
DownH = 0.008 [m]
DownPres = DenH*g*DownVFWater*(DownH-y)
DownVFAir = step((y-DownH)/1[m])
DownVFWater = 1-DownVFAir
UpH = 0.008 [m]
UpPres = DenH*g*UpVFWater*(UpH-y)
UpVFAir = step((y-UpH)/1[m])
UpVFWater = 1-UpVFAir
END
END
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Old   June 6, 2012, 03:11
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That is just the section specifying the CEL. The full CCL includes the bondary condition setup, convergence parameters, output file specification, physical models, material properties and lots more.
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Old   June 6, 2012, 05:08
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Sorry here is it
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File Type: zip two inlets.zip (4.0 KB, 10 views)
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Old   June 6, 2012, 06:20
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Some comments:

1) Free surface modelling is tricky in steady state. Very hard to get convergence. Try doing it as a transient model. This is especially the case as this is a surface tension model.
2) Are you sure surface tension is significant on these length scales? Surface tension modelling increases the difficulty of the mode a lot.
3) Your convergence tolerance is loose.
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Old   June 6, 2012, 06:46
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Thanks a lot
1) I will take your advice about shifting to transient analysis.
2) I activate surface tension option because I am interested in generating waves at the interface furthermore I took the values of velocities, liquid level , pipe diameter and pipe length depending on an experimental case mentioned in a paper I want to validate CFX results with it.
3) How my convergence tolerance not specified? I set it as 1E-04?
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Old   June 6, 2012, 07:05
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2) You do not need surface tension to generate waves. I think you will find ST is not having a significant effect on the final results, but will make convergence MUCH harder to achieve.
3) Yes, and 1E-4 is quite loose for a steady state model.
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Old   June 6, 2012, 07:14
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2) You advice me still working with steady state simulations but without activating surface tension?
3) How much you think the better tolerance value for steady-state modeling?
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Old   June 6, 2012, 07:18
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2) Definitely turn ST off unless you know you need it. Try that first, and if you still have problems try transient.
3) Do a sensitivity check to work out the convergence residual you need for this model for the accuracy you want to achieve.
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Old   June 12, 2012, 13:52
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Glenn I attempt to change my problem to transient but the following message appeared to me:


In Analysis 'Flow Analysis 1' - Domain 'Default Domain': Transient analyses require that initial conditions are specified unless an Initial Values file is specified at run-time.

I tried several options but failed to fix it? may you tell me with details how to resolve it?
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Old   June 12, 2012, 19:47
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That sounds pretty straight forward to me - you just need to define an initial condition.
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Old   June 13, 2012, 08:03
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I need to make the whole problem unsteady because I still face convergence difficulties with steady state solution you advice me to go through transient at your last posts if you remember (see the above posts pls)?
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