# Same variable in different domains

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 June 14, 2010, 08:01 Same variable in different domains #1 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 Dear All, I have two fluid domains that are not physically connected to each other. Now, I need to define as boundary condition on both surfaces of such domains the flux of a gas species, which is a function of the molar fractions on both surfaces: flux = a*(x@domain1 - x@domain2) However, I didn't find in CFX manual anything related to how to refer to the same variable (molar fraction, in this case) in different subdomains. How could I solve this problem???

 June 14, 2010, 18:44 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,208 Rep Power: 110 Sounds easy, flux=a*(areaAve(mf)@interface1-areaAve(mf)@interface2) should do it.

 June 15, 2010, 01:06 Same variable in different domains #3 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 Dear Ghorrocks, first of all, thanks a lot for your fast reply. Then, if I 've correctly understood, the expression you provided me calculates the average value of the molar fraction on both surfaces. Therefore, a constant flux is evaluated along the surfaces. However, since such surfaces are quite long, I expect a profile of molar fractions, corresponding to a non-constant profile of flux along the surfaces. Can I change your expression to apply it also to my case?

 June 15, 2010, 07:04 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,208 Rep Power: 110 You can't easily change it to match up node by node. You could split your region into sections and match the sections up and do averages by section, this is easy. Another possibility is to make the pair of connecting faces a periodic pair and define a mass fraction source/sink across it. You might also have to use a momentum sink to stop the flow. This is a little harder but still should be possible.

 June 15, 2010, 21:51 Same variable in different domains #5 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 Actually, I treated the surface as a wall with a kind of "chemical reaction" to express the loss of mass due to flux, so I don't have to worry about the momentum sink. About the node correspondence, I think I have to use a User Fortran Function to match the nodes... I thought there was a simpler way, but evidently this is not the case. I hope future CFX versions will implement this kind of boundary correspondence in order to avoid for the user to use customized external functions. Anyway, thanks again for your advices...

June 15, 2010, 22:21
#6
Super Moderator

Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 14,208
Rep Power: 110
Quote:
 About the node correspondence, I think I have to use a User Fortran Function to match the nodes... I thought there was a simpler way, but evidently this is not the case.
Yes, that's right. Try the periodic boundary approach first, you may be able to get that working. If that does not work you have to go fortran.

Quote:
 I hope future CFX versions will implement this kind of boundary correspondence in order to avoid for the user to use customized external functions.
No chance. Your application is quite specialised so there will not be much demand for it. But put a feature request in, it will never happen unless it is requested.

 June 17, 2010, 22:14 Same variable in different domains #7 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 Dear Ghorrocks, I read in the ANSYS manual that it's possible to couple two solvers. Actually my system can be considered as composed of two completely separated sub-systems, except for the boundary conditions at the interfaces, which are coupled to each other. Now, in your opinion, is it possible to run two different cfx solvers by coupling the boundary conditions? I have the necessary licenses, but I didn't find in the manual an example on how to do that? Thanks in advance

 June 17, 2010, 23:56 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,208 Rep Power: 110 The solver coupling is between a CFD and FEA solver. Not two CFD simulations - there is no point in coupling two CFD simulations, just run them as one. Do not try to couple two solvers, that is a poor way of doing it, it won't work. Have you tried doing it with periodic boundary conditions? I have mentioned it three times now.....

 June 18, 2010, 00:47 Periodic Boundary conditions #9 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 I'm trying just now with setting periodic BCs. However, I have a doubt. My system is composed of two co-axial cylinders (same length), one into the other and separated by certain distance (the external surfaces are distant from each other). I added a "Domain Interface" to link the two surfaces, set up their correspondence in the GGI mode and set up "Additional Interface Model" as "No Slip Wall". However, no other options are available, so: how can I set the flux value of a species at the interface? Actually the periodicity must exist for the species compositions and flux, but not for the absolute pressures, that are different in each side...

 June 18, 2010, 06:02 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,208 Rep Power: 110 Can you explain what you are trying to do? A drawing would be nice.

 June 18, 2010, 06:18 #11 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 OK, sorry for my previous non-clear explanations... My system is a tube-in-tube device, where one multicomponent mixture flows in the annulus, whilst another multicomponent mixture flows in the inner tube (the streams direction is not important). The inner tube is a membrane, through which some species of the mixtures selectively pass with different rates (selectivities) from tube to annulus. All the fluxes can be expressed by the following formula: Flux(Species(i)) = a(i) * (PartialPressureInTube(Species(i)) - PartialPressureInAnnulus(Species(i))), calculated on the corresponding surfaces. My problem is how to express in CFX the fluxes of the species, because I don't know how to pick the required variables from the other side... later I'll send an image of the system...

 June 18, 2010, 06:43 #12 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,208 Rep Power: 110 OK, I see. You may also be able to do this with a thin surface or maybe an interface where you turn momentum transfer off but allow a mass fraction flux. Lots of options.

 June 18, 2010, 06:56 #13 New Member   Join Date: Jun 2010 Posts: 10 Rep Power: 9 I understand what you said, and actually I used a Domain Interface, disabling the momentum transfer and enabling the mass flux. However, the problem about flux unfortunately still remains...

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post jpshulf CFX 3 November 14, 2008 18:46 bhushas Main CFD Forum 1 May 30, 2008 04:35 Krishna Premi CFX 1 October 29, 2007 09:19 gruber2 OpenFOAM Installation 5 December 30, 2005 05:27 lego CFX 3 November 5, 2002 21:09

All times are GMT -4. The time now is 04:57.

 Contact Us - CFD Online - Privacy Statement - Top