
[Sponsors] 
March 20, 2013, 15:44 
parallel decomposition method in openfoam

#1 
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Hi All,
In openfoam, the hierarchical method to split the computational domain gives the options to specify the order, e.g. xyz, does anybody know how this order affects the simulation results? Thanks. 

March 20, 2013, 16:29 

#2 
Senior Member
Lieven
Join Date: Dec 2011
Location: Leuven, Belgium
Posts: 297
Rep Power: 14 
Hi Hz283,
It could influence the simulation time, but that's basically it. It certainly does not affect the simulation results. At least it should not, else there is something wrong with the code. Cheers, L 

March 20, 2013, 17:57 

#3 
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Hi Lieven,
Thank you very much. Could you please say more about how it can affect the simulation time? Thanks. 

March 21, 2013, 04:27 

#4 
Senior Member
Lieven
Join Date: Dec 2011
Location: Leuven, Belgium
Posts: 297
Rep Power: 14 
Sorry, not an expert in that field so I can't give you an exact answer to that.
Cheers, L 

March 25, 2013, 05:52 

#5 
Senior Member
Join Date: Mar 2010
Location: Germany
Posts: 140
Rep Power: 8 
Hi,
as far as I know, a general answer cannot be given. This substantially depends on your computational grid. Try to spread the work evenly on all processors/ cores. At the same time try to minimize the the interfaces of the mesh parts to reduce the communication between the nodes. cutter 

October 7, 2013, 16:16 

#6  
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Thank you very much for your suggestions. About the method hierarchical, I found that we can change the order of xyz to yzx or zyx and so on. I was wondering in which case I need to do this. Is this helpful to improve the computational efficiency?
Maybe the best way for me is to directly test the case with different partition method. But my case is a little large. So can I try get some suggestions from you before using this method? Thank you in advance! h Quote:


October 7, 2013, 17:07 

#7 
Senior Member
Mohsen KiaMansouri
Join Date: Jan 2010
Location: CFD Lab
Posts: 118
Rep Power: 8 
Dear hz283
1) In my experience, one of the parameters that can affect your parallel simulation is the delta parameter which is equal to 0.001 by default. But if you have a mesh with faces of highly skewed cells you should increase this value to e.g. 0.01. you can find your cell skewness by running the following command in Terminal: Code:
checkMesh allGeometry see the following post for more info: Cell skew Factor 2) Also there is another thing that you should notice. use the complex methods for decomposePar only if you have a complex mesh. but if you have a simple mesh, use the simpler methods. I have a experience in this field that by using complex methods (such as scotch) for my complex mesh (film cooling of turbine blades), my run time reduced to 1/3. but by using scotch method for a simple mesh (flow around a cube) neither my run time nor the results did not change significantly in comparison with the simple method. Summary: use complex methods for decomposePar only for complex meshes. Also see the following post: scotch or ptscotch? 3) And finally about your 1st question: I have not worked with hierarchical method and I don't know how to choose the order of xyz to yzx or zyx. but it definitely depends on your geometry. Please share your experiences with hierarchical method in here.
__________________
“If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.” 

October 7, 2013, 17:41 

#8  
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Thank you so much for your so detailed reply. Besides, I found that the metis and scotch methods are intensively discussed in this forum. Most people think scotch is good. But I use OF211 and found that there is not such method. What kind of version are you using?
Thanks. Quote:


October 7, 2013, 18:09 

#9 
Senior Member
Mohsen KiaMansouri
Join Date: Jan 2010
Location: CFD Lab
Posts: 118
Rep Power: 8 
Are you sure?
https://github.com/OpenFOAM/OpenFOAM...llel/decompose
__________________
“If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.” 

October 7, 2013, 18:11 

#10  
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Ah, Thanks!!
I only saw the option in the file system/decompose, in which only three options. Could you please share me one of this file in your case? Thank you a million! Quote:


October 7, 2013, 18:53 

#11  
Senior Member
Join Date: Nov 2012
Posts: 168
Rep Power: 5 
Hi cfdonline2mohsen,
About the metis coefficients, if all the cores are totally the same, the coefficients for all the cores are only needed to be set to 1? Any comments about this point? Thank you very much! Quote:


October 8, 2013, 04:35 

#12 
Senior Member
Mohsen KiaMansouri
Join Date: Jan 2010
Location: CFD Lab
Posts: 118
Rep Power: 8 
I don't recommend the metis method because if you review the cfdonline posts, most of them stated that metis has some restrictions about licensing problems.
It was just a recommendation. Search more about this matter. I've used the scotch method with the same coefficients that exist in OpenFOAM tutorials.
__________________
“If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.” 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Problem with RenumberMesh in parallel in OpenFOAM 2.1.1  srini_esi  OpenFOAM Mesh Utilities  1  November 8, 2013 03:48 
OpenFOAM 2.0.0. and 2.0.1 doesn't work in parallel mode  rv82  OpenFOAM Running, Solving & CFD  3  October 3, 2011 10:47 
OpenFoam parallel crashes at random  prapanj  OpenFOAM Running, Solving & CFD  3  April 22, 2009 07:49 
Parallel performance OpenFoam Vs Fluent  prapanj  Main CFD Forum  0  March 26, 2009 06:43 
IcoFoam parallel woes  msrinath80  OpenFOAM Running, Solving & CFD  9  July 22, 2007 02:58 