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-   -   SSG model (http://www.cfd-online.com/Forums/cfx/72387-ssg-model.html)

Tajnesaie February 4, 2010 04:35

SSG model
 
Hi
I want to use SSG model to free surface modeling in a channel, but i don't know which parameters must be regulated?
When i choose SSG model and start run with default parameters (accept default parameters),program give error.
Who can help me and know which parameters must be regulated?

Regards
Tajnesaie

ghorrocks February 4, 2010 06:03

How can we help you if you don't even post the error text? Or even if the error is in the solver or CFX-Pre?

Tajnesaie February 4, 2010 17:04

This is the error in solver:


Error in subroutine FNDVAR :
Error finding variable VISCEFF
GETVAR originally called by subroutine ASS_STRESS

+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Stopped in routine GV_ERROR |
| |
| |
| |
| |
| |
+--------------------------------------------------------------------+

ghorrocks February 4, 2010 17:28

Did you set the thing up in CFX-Pre? If not I recommend you do. Load it into CFX-Pre and fix up any problems and output a def file again. It will probably work fine.

Tajnesaie February 5, 2010 08:08

Hello
I choose SSG model in cfx-pre and accept default parameter in it.But I don't know the meaning of this error.
Thanks

triple_r February 7, 2010 15:40

Have you tried any other turbulence models? Something like k-e, even laminar :-)

Start with a simpler model, and see if the problem is the turbulence model setup, or from somewhere else. Then if it converges, you can use this "not-so-good-model" solution as the starting point for your SSG model solution.

Tajnesaie February 8, 2010 14:21

Thanks
First I use k-e, then sst and now I want to use ssg model, and test any thing and I sure the problem is from turbulent model setup.
Do you know which parameter need to regulate?

ghorrocks February 8, 2010 18:10

Can you post your CCL?

I suspect you have a boundary or initial condition which was not converted to SSG properly.

Tajnesaie February 9, 2010 14:21

1 Attachment(s)
Hi
This file that attached is my CCL.I follow tutorial 7 but with static pressure for initial condition(height of water in inlet and outlet of channel)
Thanks

triple_r February 9, 2010 17:25

Hi,

I couldn't find anything wrong about the turbulence model setup. My following comments are not about the turbulence model and are probably because of my lack of knowledge of the problem ;).

I can't help wondering what is the driving force for the flow :confused:. It is not a velocity driven flow, and also not a pressure driven one (inlet and outlet pressures are the same). I hope it is a body-force driven flow, which in turn implies that the channel itself is slanted. If that is the case I don't know if just turning on the buoyancy will help or not.

From what I have understood and from my experience, turning on buoyancy only adds a buoyant term to the governing equations and not a body force, thus not resulting in the hydrostatic pressure. In my understanding buoyancy option is only useful when you are concerned about natural convection, and not when hydrostatic pressure plays a major role. If Glenn has a comment on this I will be more than happy to listen to it.

Lastly, as you have a pressure gradient at inlet and outlet, I would guess that might induce some flow locally, but that flow will be unrealistic (If the channel is horizontal, with the type of boundary conditions that you have specified, there should be no flow at all).

I wonder if this modeling combined with RSM models being more sensitive causes the problem that you are encountering.

Sorry for talking too much :) I will be more than happy to hear arguments about my comments, and hope your problem gets solved very soon.

ghorrocks February 9, 2010 17:54

The CFX buoyancy option activates a source term in the momentum equations. The variable "pressure" does not include the static head, but the variable "Absolute Pressure" does. If you are doing a calculation where the static head is important then use the abs. pressure variable. The buoyancy option in CFX is recommended for ALL flows where gravity is important, both natural convection and multiphase. This is all explained in the documentation.

I can't pick any obvious errors in the CCL. Does CFX-Pre give any warnings or errors about it? But Reza's comments about what driving the flow are important - so what is driving the flow?

triple_r February 9, 2010 18:05

Thank you Glenn. I really appreciate that. I must have missed that part of the documentation.

Just another question: In the CCL given here, at inlet and outlet there is a hydrostatic pressure distribution. Now that CFX takes the hydrostatic pressure only into absolute pressure, should we just put a constant pressure at those locations and CFX will take care of the rest by adding the hydrostatic presure? (because at those boundary conditions we are asked for gauge pressure and not absolute)

Thank you again.

AliTr February 9, 2010 19:31

Reza, activating bouyancy and including -g term will create body force to drive the flow. puting a constant pressure over the boundary will result in flow recirculation on an open boundary and boundary blockage on an Inlet/Oulet boundary.
I noticed, CFX uses (ro_water - ro_Air) to calculate hydrostatic pressure in a multiphase case, then applying (ro_water * g *h) over the boundaries will create a residual pressure difference which creates convergence issues in some cases. using (ro_water - ro_Air) * g *h is what I suggest.
I appreciate your comments on this.

zandi February 12, 2010 04:40

salam mohanna
visit Dr Bonakdari in Kermanshah for this problem.

zandi February 12, 2010 05:04

salam mohanna
visit Dr Bonakdari in Kermanshah for this problem.

Tajnesaie February 12, 2010 05:08

Quote:

Originally Posted by ghorrocks (Post 245531)
I can't pick any obvious errors in the CCL. Does CFX-Pre give any warnings or errors about it?

Hi
Thanks a lot ghorroks
CFX-Pre does not give any warnings, But in solver part the solution stoped by that error and this error repeat even with turbulence parameters changing.
Thanks again

feizaghaee February 12, 2010 10:46

i've been with doctor bonakdari today and he didn't have any idea about this problem becaus i have problem same as Tajnesaie

zandi February 12, 2010 13:20

Quote:

Originally Posted by feizaghaee (Post 245852)
i've been with doctor bonakdari today and he didn't have any idea about this problem becaus i have problem same as Tajnesaie

it is just a suggestion gived by someone to me i don't how much it works

ghorrocks February 13, 2010 06:49

Not sure about this but it is worth a try - Try changing from scalable wall functions to standard wall functions. If that does not work try re-entering the turbulence condition on your inlet BC and initial condition.

Tajnesaie February 20, 2010 02:54

Hi
Thanks a lot ghorroks. I examine it.
And dear feizaghaee what did you do for this matter ?I need urgency help. Can you help me? How can I contact with you?


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