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July 7, 2010, 09:31 

#121  
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maddalena
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Quote:
Thanks for the detailed answer to my second question. This will be useful for sure on a following step investigation. mad Last edited by maddalena; July 7, 2010 at 09:44. Reason: added link 

July 8, 2010, 05:17 
cht and cyclic?

#122  
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maddalena
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Hi Pawel,
I need your help one more time... Some posts ago you suggested to Michea to change her cylindrical geometry: Quote:
At the moment I have some difficulties to run cyclic in chtMultiRegionFoam: the geometry and patches are managed correctly, but the simulation always blows up when using cyclic. However, I could run the same case using symmetry in place of cyclic, without changing geometry, mesh or settings. Are there, in your experience, some incompatibilities between cht and cyclic? Have you ever run a case using that? Some more details on what I am running are posted here. Thanks for your helpfulness! mad 

July 8, 2010, 05:38 

#123 
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Pawel Sosnowski
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Hello Maddalena,
unfortunately I have not yet run a chtMRFoam case with cyclic BC. But I looked at your geometry and after first glance I had the following thought. You have a very sharp, thin edge in the center of your problem. At the same time, we are expecting some radialdirection movements of the fluid due to presence of heat. In my opinion, the fluid simply does not have enough "space" to develop proper flow near that corner. I would try to widen the case to quarter of a circle and then apply cyclic BC. If this does not work, then you probably will be forced to work on full circle (which works and gives acceptable results I did a case like that). Best, Pawel 

July 8, 2010, 05:51 

#124  
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maddalena
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Hi Pawel,
Quote:
I am going to test another solution: cut the tip of the slice and apply a waveTransmissive bc on the cutting. And see what happens... I'll keep you informed. thanks mad NB: the uniform (0 0 0) velocity was indeed a bug. It has been fixed in 1.7.x. 

July 12, 2010, 14:51 
Need help setting up very simple model

#125 
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Ben K
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Hi all, I'm trying to use the chtMultiRegionFoam solver in OF1.6.
At the moment I'm just trying a very very simple test case: 1D steady state diffusion between two regions with fixedValue boundary conditions at both ends. This is just to make sure I understand how everything works. I've basically set up my solver in a standard way and then used the solidWallMizedTemperatureCoupled boundary condition to handle the coupling between the two regions. My problem is that my model just isn't solving correctly when I try to couple the boundaries of the two regions using the solidWallMixedTemperatureCoupled boundary condition. Everything compiles and runs fine but for the results I'm getting values over 10E15 when they should be bounded between 100 and 0 (which are the fixedValue boundary conditions that I'm applying). If I replace the solidWallMixedTemperatureCoupled BC with fixedValue in my changeDictionaryDict file, the solver works (except this isn't making use of the coupled boundary condition which I need). Can anybody help me troubleshoot this? My entire model can be found right here: http://dl.dropbox.com/u/12812/OF/tes...ultiRegion.tgz and all you need to do to get it running is: cd solver;wmake;cd ../case;./Allrun 

July 13, 2010, 03:35 
cht can not reach steady state

#126 
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maddalena
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Hi Pawel,
some update before one more question: cutting the tip of the slice and apply a waveTransmissive BC for pressure did not worked, neither simulating a 90° slice + cyclic. The problem is always the same: h or U on the fluid region gets high suddenly, and the simulation crashes. Using a symmetry bc did not raise the problem, so I think I will lay on this solution. Since the slice is thin, the introduced error should be low. With these results, I simulated my real geometry finally. The set up was fine, and I could manage the coupling of 16 different regions. However, as you did, The temperature in my case raise only by heat generation, and I know that the final temperature of the reference point is around 400K, with a time constant of 20000 s. However, after 10000 simulated seconds, that temperature is already 450K, and the steady state is not reached (thus it is going to get higher...) I am thinking to your example of some post ago: may this be due to a time step that is too high? In my case, I have U (0 0 0) and the time step is driven by the solid part (maxDi) mainly. In order to speed up the calculation and have a first insight if everything is fine or not, I set maxDi = 1E3, while the diffusivity of my solid materials is included between 1E4 and 1E8. If I want to see a proper transitory of the solid, I should set maxDi = 1E8, I guess. However, using 1E3 should affect only the transitory and have no effect on the final steady state temperature. What you think? Am I right? Thanks for any suggestions, cheers, mad 

July 13, 2010, 08:20 

#127 
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Pawel Sosnowski
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Hello Ben, Maddalena!
regarding Ben's case I compiled the solver, unpacked the case. You modified it quite a bit (fields names c1, D1, S1 do not tell me much, sorry). And the test case does not have any field connected with solidWallMixedTemperatureCoupled. If you support the solver, a final test case and some readme file with description what is what (plus maybe with a log of what went wrong), then I maybe will be able to help you regarding Maddalena's post you say that you support energy by a heat generation. I understand that you added a source term to solved energy equations and additional source field to the problem am I right? (if not how do you supply energy?) If you do support energy by a source term, consider the ways the energy is transported in a real case you have advection (in fluids), diffusion (both solids and fluids) plus radiation (in transparent solids and fluids). Take to account that the chtMRFoam does not consider radiation. This may result in reduced energy transport. The other possible answer to your problem is a bit more devastating maybe the coupling does not work the way it should... I mentioned it several times earlier, that I do not fully trust the coupling method used in chtMRFoam (I found no literature that would confirm its credibility, and the ones that I found show different approaches!). If this is the case then the only way would be to improve the coupling BC condition... And as the final word I still hold my statements regarding time step control. If chtMRFoam has some builtin mechanisms that control the time step do NOT give any boundaries to it (as by setting minimum time step). And always make your own calculations (on paper) to determine the smallest possbile time step: dTime=size_of_smallest_cell^2 / diffusion_coeff. Hope it helps a bit. Best, Pawel 

July 13, 2010, 09:07 

#128  
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maddalena
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Hi Pawel,
Quote:
Quote:
Quote:
Quote:
... In any case, you forget one more possible answer to my problem: temperatures are higher than I thought. In that case, CFD can help me to understand what is wrong on my experimental setup... Yes, as usual! thanks, mad 

July 13, 2010, 12:02 

#129  
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Ben K
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Quote:
laplacian(D1,c1) = 0 with boundary conditions c1=100 on the left and c1=0 on the right. D1 is the diffusivity. I included a field S1 because later on I'll be adding a source term but that's just equal to 0 for the moment. In the dictionary file for c1 in left and right regions, you'll find: Code:
c1 { internalField uniform 100; boundaryField { left_to_right { type solidWallMixedTemperatureCoupled; neighbourFieldName c1; K D1; value uniform 0; } } } The problem with the solver is that when I run it, I get the following output: Code:
... DICPCG: Solving for c1, Initial residual = 0.97919, Final residual = 7.327857e29, No Iterations 2 DICPCG: Solving for c1, Initial residual = 0.333287, Final residual = 1.480727e29, No Iterations 2 Time = 9 DICPCG: Solving for c1, Initial residual = 0.333287, Final residual = 5.874262e31, No Iterations 2 DICPCG: Solving for c1, Initial residual = 0.97919, Final residual = 6.189845e28, No Iterations 2 Time = 10 DICPCG: Solving for c1, Initial residual = 0.97919, Final residual = 5.618467e28, No Iterations 2 DICPCG: Solving for c1, Initial residual = 0.333287, Final residual = 1.147113e29, No Iterations 2 Time = 11 DICPCG: Solving for c1, Initial residual = 0.333287, Final residual = 5.0275e30, No Iterations 2 DICPCG: Solving for c1, Initial residual = 0.97919, Final residual = 3.03365e28, No Iterations 2 ExecutionTime = 0.03 s ClockTime = 0 s 

July 13, 2010, 13:03 

#130 
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Pawel Sosnowski
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Hello Ben,
I took another glance at your case, here are some thoughts. I would still include Code:
fvm::ddt(c1_left_or_right) Code:
ddtSchemes { default steadyState; } Most important result is if you decouple the cases and set the left_to_right and right_to_left as fixedValue. Run the solver, and... obtain nice gradient results after first iteration... This simply means that there has to be some major problem in the coupling condition. At least in your case (with changed equations and dimensions of the problem from temperature to quantity) it does not work. The only way will be to get "into" the coupling condition, and try to understand what is not ticking well for your simulation. In the end it may be necessary to modify or write your own coupling BC. Best, Pawel 

July 16, 2010, 11:10 
interface coupling & heat source

#131  
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maddalena
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Hi Pawel, hi everyone,
in order to find an explanation to the hard time I am getting to reach steady state for my simulation, I performed some tests on a simple case: a flat plane, with or without insulation, heated by an external heat source (fixedValue temperature in one side), or by inner heat generation. I compared these results with the analytical solution, that is known in these cases. Meshes and setting are equal in all the cases. In detail, I declared every coupling as: Code:
cube1_to_cube2 { type directMappedWall; nFaces 10; startFace 390; sampleMode nearestPatchFace; sampleRegion cube2; samplePatch cube2_to_cube1; offset (0 0 0); } Code:
cube1_to_cube2 { type solidWallMixedTemperatureCoupled; value uniform 300; neighbourFieldName T; K K; } Code:
cube1_to_cube2 { type zeroGradient; }
What I can conclude is that: Quote:
Has anyone experienced similar problems? If you have already found a solution to the problem, I would be grateful if you can share the code, or at least point out where I can look for a coupling improvement. Suggestions are really welcome. Regards, mad Last edited by maddalena; July 16, 2010 at 11:24. Reason: added bc 

July 19, 2010, 10:31 
chtMultiRegionSimpleFoam

#132 
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Hi all,
I would like to simulate conjugate Heattransfer. My treated geometry is a pipe with a porous zone inside. But the porous zone is only one part of the pipe. So I need a combination of chtMultiRegionSimpleFoam and rhoPorousSimpleFoam, in which the darcy law is only uitlized in one part of the fluid region. Therefor I think it would be good to handle the interior pipe flux through two different fluid zones. But if I know correctly there is no possibility to directly couple two fluid regions in chtMultiRegionSimpleFoam. Has anyone an idea how to combine the two solvers chtMultiRegionSimpleFoam and rhoPorousSimpleFoam for my problem. Best ragards Chrisi 

July 19, 2010, 10:55 
interface coupling & heat source

#133 
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maddalena
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... just to say that I moved the conversation here. A new thread seems more appropriate for the problem I am getting.
cheers mad Last edited by maddalena; July 20, 2010 at 04:44. 

July 19, 2010, 11:07 

#134 
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Pawel Sosnowski
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Hello Chrisi,
I would say, that you simply need 3 region types instead of 2 in classical chtMultiRegionFoam. In that case, you would have solid, fluid and porous (in addition). In the end, for solid and fluid, you copy the methodology from chtMultiRegionFoam, and for porous you copy the algorithm from rhoPorousSimpleFoam. Of course there will be a problem with a coupling boundary condition. It is obvious that not only you will have to couple temperature, but also velocity, pressure and mass. Try looking at some solvers which deal with mixing of fluids (I have no experience with them, but directory "OpenFoam1.6/applications/solvers/multiphase" has several solvers, maybe they can be of some use). Good luck! Pawel 

July 20, 2010, 10:29 

#135 
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Hi,
thank you for your hint! I have now added the new region porous in my chtMutltiRegionSimpleFoam solver. Until now this region is treated like the fluid region, bescause for me it seems more difficult to combine the two different synthax of rhoPorousSimpleFoam and chtMultiRegionSimpleFoam in compare adding the darcylaw to the fluid equations. But my problem now is in this file turbulentTemperatureCoupledBaffleFvPatchScalarFiel d.C How can I deal with the two fluid regions to get the neighbouring management in this file work? I also checked the other solvers you told me for coupling the other variables like pressure, velocity etc. , but I did not get an idea. Has anyone dealed with the same problem before? Best regards Chrisi P.S: I am using OF 1.7.0 

July 21, 2010, 11:01 
additional zone in chtMultiRegionSimpleFoam

#136 
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Hi again,
I think I solved the problem with the file turbulentTemperatureCoupledBaffleFvPatchScalarFiel d.C by creating an analogical file for my porous zone. But there is still the problem how to couple the variables of the different meshes fluid and porous. Can the coupling be done only in the boundaries of the files in the 0 folders? I discovered thinks like directMappedPatch, directMappedVelocityFixedValueFVPatch or directMappedFixedValue. But I do not know how to use them and if they realy do what I want to, because they should couple patches of different meshes. Or do I have to prepare the coupling in an other way in my solver? Thanks in advance and best regards Chrisi 

July 21, 2010, 12:37 

#137 
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Pawel Sosnowski
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Hi Chris,
from what I remember, the directMappedPatch (and possibly other directMapped(...)) require that the neighbour meshes match on the boundary. This is the only restriction (so one mesh can be triangular, other square). But the boundaries have to match. If you want to connect notmatching boundaries, you will have to work on your own implementation of the coupling. I remember that somewhere earlier in this thread I explained how to use the directMappedPatch coupling method. Dig up a bit and for sure you will find it Best, Pawel 

July 23, 2010, 08:41 

#138 
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Hi,
Thank you! I tryed directMapped like it is described here http://www.cfdonline.com/Forums/openfoamsolving/59741boundarycondition_mappringinletles.html. But I changed set average to false. I set in one region directMapped as patch at the connection with x_to_y . The BC off the other end y_to_x I think I have to take a standard BC. But so there is no really coupling between the two regions. I only map the calculated field of one region to the other. But the other region is independant of the flow in the neighbour region. How can I really couple the two regions (with back coupling)? Best regards Chrisi Last edited by Chrisi1984; July 23, 2010 at 11:36. 

August 10, 2010, 15:47 

#139 
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Ben K
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Is there any way to specify the number of nodes in each region?
For example, in the model that I'm developing, I have 2 regions that have the same dimensions but I want a higher grid density in one of my regions compared to the other. Is this possible in chtMultiRegionFoam? 

August 12, 2010, 12:45 

#140 
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Aram Amouzandeh
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hi benk,
this can be done e.g. by defining two hexblocks in the blockMeshDict corresponding to your regions. cheers, aram 

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