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kbaker June 1, 2012 04:02

Discontinuity at water level in stratified 2 phase flow
3 Attachment(s)
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?

ghorrocks June 2, 2012 06:37

What is the mesh size in this region?

kbaker June 2, 2012 10:51

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.

ghorrocks June 3, 2012 07:12

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.

kbaker June 3, 2012 08:32

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?

ghorrocks June 3, 2012 19:26

The zip file only contains a small section of CCL. Can you post the full CCL?

kbaker June 4, 2012 08:04

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:

kbaker June 6, 2012 01:55

Glenn did you check the cfx-pre file I sent you? You not reply me yet?

ghorrocks June 6, 2012 02:09

I only want to look at your CCL. Please extract the full CCL and post that.

kbaker June 6, 2012 02:50

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

ghorrocks June 6, 2012 03:11

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.

kbaker June 6, 2012 05:08

1 Attachment(s)
Sorry here is it

ghorrocks June 6, 2012 06:20

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.

kbaker June 6, 2012 06:46

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?

ghorrocks June 6, 2012 07:05

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.

kbaker June 6, 2012 07:14

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?

ghorrocks June 6, 2012 07:18

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.

kbaker June 12, 2012 13:52

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?

ghorrocks June 12, 2012 19:47

That sounds pretty straight forward to me - you just need to define an initial condition.

kbaker June 13, 2012 08:03

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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