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maryam March 9, 2009 10:31

define the differen fluids in a assembly

I want to simulate a circular channel with the cylindrical tube that placed in the channel. Water flows in the channel and He gas is confined in the tube. The inlet and outlet of the tube is wall. I can't define different fluids in the tube and channel. Cfx Takes He in the channel, as the same as the fluid in the tube, however it defined as water. How can I solve the problem? Thank you

Rajee Assudani March 9, 2009 11:40

Re: define the differen fluids in a assembly
Make sure that the Channel and Tube are created and defined as different domains. Also, make sure to select both water and He under Fluids list. Let me know if you have already done these and still doesn't work.

Rikio March 10, 2009 04:00

Re: define the differen fluids in a assembly
I think Rajee may misunderstand the point... Maryam, you want to simulate the model with two fluids in two separated domains, right? If so, you can enable the beta feature in the option window, and uncheck the "Constant Domain Physics" by right-clicking on Simulation branch. All of this are done in CFX 11.0. You can refer to CFX-Pre/Domains/Using Multiple Domains for details.

Good luck...


steventay January 2, 2010 08:35

Defining two fluid domains
Hi, I have a similar application. One fluid is passing thru a tube and another fluid is surrounding the same tube. Defining the fluid domain for the first fluid will automatically set the same fluid for the second fluid domain. :confused: Can anyone advise me on how to set two fluid domains on one analysis? I tried changing the options suggested by Rikio but it's still not working. I am using Ansys 12. Many thanks.

Abou ali January 2, 2010 12:00

Look this thread

steventay January 3, 2010 05:44

It works now. Thanks Ali. :)

cosine January 4, 2010 15:21

I make a similar simulation. However interaction occurs between the domains using an interface. I have two questions:
1) How can i use the wall shear stress calculated in the first domain to an expression in the other domain?
2) At the post-processing of the results i want to make an analysis of some variables calulated on the one side of the interface with the other side of the interface. The problem is that i use GGI mesh connection, while i don't have one-to-one node mesh. Does anyone have an idea?
Thanks a lot

Abou ali January 4, 2010 17:35

1- In finite volume method all quantities are calculated in faces and nods of an elements and CFX uses this quantities to calculate the additional variable in the same element that means you can't use the shear stress of a domain in other one. But is this possible by using a FORTRAN subroutine?!!!
2- What I know that you can export the result from CFX Post and use a FORTRAN program calculate what you need.

cosine January 4, 2010 17:47

Ok, thanks. I will try it.
Do you know what boundary condition i need to use at the interface if i want to model the flux of an additional variable across the interface?
I need flux in or transfer coefficient?

Thanks again

Abou ali January 4, 2010 18:15

If I understand, you can do that in the Source panel of one side of the Interface window.

cosine January 4, 2010 18:26

You are right
Thanks a lot:)

steventay January 30, 2010 09:44

Solver error

I am having the below error when I tried to run the solver. Has anyone seen this type of error before? Please advise me what I should do to resolve this error. Thanks!

"A negative ELEMENT volume has been detected. This is a fatal
error and execution will be terminated. The location of the first
negative volume is reported below.
Volume : -0.2770E-08
Location : ( -0.42463E-01, 0.21956E+00, -0.54754E-01) "

ghorrocks January 30, 2010 16:45

It means you are running a moving mesh simulation and the mesh has moved so far that some elements have turned inside out. You need to change the mesh smoothing option or put a remeshing step in there. Sometimes in FSI simulations this happens when the motion is diverging and the body has moved to a ridiculous position.

steventay January 31, 2010 01:48

Solver Error
Hi Glenn,

Thanks for your advise. What do you mean by remeshing step?

ghorrocks January 31, 2010 04:55

CFX V12 now supports remeshing and multiple configurations in a def file which can be very handy for this type of stuff.

steventay February 15, 2010 21:54

Aspect ratio exceeding warning limits
Hello All,

I got this warning when doing meshing. I tried to make the mesh finer hoping to improve the aspect ratio but this warning kept coming up even the mesh is the finest I could go... Any one has any advise?

If the catelogue size is too big, what does it means? I am using a research license, so there should not be any limit...

ghorrocks February 16, 2010 18:35

The catalogue size is not a licensing issue. You can increase the catalogue size but in general it is the result of poor convergence. Try to improve the stability of the numerics (improve mesh quality, smaller timesteps, better initial guess or lower order differencing) and the problem should disappear.

steventay February 16, 2010 20:11

Hi Glen,

Thanks for your prompt respond and appreciate your advice.

One question on meshing, I am getting the max skewness of 0.99958 eventhough I have set the mesh with fine span angle and fine relevance. What can I do to reduce the skewness ratio?


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