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April 18, 2013, 07:29 

#41 
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Sijal Ahmed Memon (turboenginner@gmail.com)
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This is the post where Re=40 results are posted and mesh file link is also provided. Fluent + flow past cylinder at Re=40
Boundary conditions wre: Dia = 1 m (necessary for Reynolds number) Density = 40 Kg/m3 Viscosity = 1 Velocity = 1 m/sec You can check your self how fast is coupled solver. Yes you are right memory requirements are high, approximately twice of SIMPLE 

April 18, 2013, 07:56 

#42 
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OJ
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I am not suspecting the idea. Personally, am a big fan of CFX's multigrid coupled solver. In one of the cases involving porous interface, for same mesh/physics, CFX converged within 80 iterations while FLUENT had to go till 1000!
I just like to observe things in totality Cheers OJ 

April 18, 2013, 07:59 

#43 
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Sijal Ahmed Memon (turboenginner@gmail.com)
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Are you talking about Fluent 14 + versions. One thing should be noted that in CFX CFL is not provided as option to user and it is decided by solver.


April 18, 2013, 08:27 

#44 
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OJ
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Yes, 14.5. Also, given sudden change of properties across the porous interface, I typically use local timescale option instead of auto/physical timescale, which smartly changes the timescales globally (ie, equivalent to variable CFL).
But of course you can always provide aggressive local timescale factor for faster convergence. Some opine that it is necessary to run last few iterations using Physical/auto timescale, while some think running local timescales "enough" is sufficient, ie when we witness flatter monitors and reduced imbalances. OJ 

April 18, 2013, 16:59 

#45  
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François Grégoire
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Quote:


August 1, 2013, 05:21 

#46  
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April 19, 2014, 08:02 
clarification

#47 
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Dear OJ,
Thanks very much for this post. It is very helpful Could i just ask a few additional questions if possible? According to some documentation i found on the web produced by ANSYS, the Courant No. in the context of the PBCS in Fluent affects the diagonal dominance of the coefficient matrix, and therefore the solution stability  and this is clear in the context of linear solver convergence theory etc. Effectively, this does the same thing as the implicit URF's in the segregated solvers ... However, in one of your posts below, you mention the explicit underrelaxation (relaxation of variables) affects the inner loops of the PBCS? Could i ask you to clarify this further? Do you mean the linear solver loops? i.e. PBiCG solver loops etc? Or where exactly in the algorithm are the explicit URF's applied? I know in OpenFOAM, for instance, in their SIMPLE implmentation, they use explicit underrelaxation for the pressure equation, but implicit (equation) URF's for MOM, TKE and Eps etc. I wonder if you would mind explaining this a little further? Also, in your current (this) post, you use the following nomenclature: I was wondering whether you could just explain your nonemclature? obviously is time ... is cellvolume, i.e. mesh size? Also i am assuming is some sort of global time step?? Quote:
Finally  could you comment on how the explicit URF's affect the simulation behaviour? As per above, i know Courant No. in this context affects the convergence stability  how then do the explicit URFs participate? Are they related somehow to the need to deal with the governing equation nonlinearities? Again, thanks very much for the post, best regards Jonathan Last edited by Jonathan; April 22, 2014 at 11:56. 

Tags 
cfl, coupled, courant, underrelaxation factor 
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