|
[Sponsors] |
March 29, 2007, 01:23 |
1. Why is there a limit of 501
|
#1 |
Senior Member
Srinath Madhavan (a.k.a pUl|)
Join Date: Mar 2009
Location: Edmonton, AB, Canada
Posts: 703
Rep Power: 21 |
1. Why is there a limit of 501 iterations on the AMG and 5001 on the ICCG solvers? Is this is a known and fixed constraint?
2. For non-conformal structured block meshes, does increasing the number of non-orthogonal correctors really help? What is the ballpark percentage limit (as reported by checkMesh) beyond which non-orthogonal correctors are absolutely necessary. Is it 25%, 50%, 75% ?? 3. In the lid-driven cavity case, dirichlet boundary conditions are prescribed for the stationary walls. Quoting from F & P: "At a wall the no-slip boundary condition applies, i.e. the velocity of the fluid is equal to the wall velocity, a Dirichlet boundary condition. However, there is another condition that can be directly imposed in a FV method; the normal viscous stress is zero at a wall." Is this implicitly done in the OF solvers? If that is the case, then the zero value dirichlet boundary conditions specified in the subdirectory 0/U should apply to the continuity equation? Please excuse my naiveness. Any thoughts/corrections are much appreciated. Thanks! |
|
April 13, 2007, 00:44 |
Answering my own question. I'm
|
#2 |
Senior Member
Srinath Madhavan (a.k.a pUl|)
Join Date: Mar 2009
Location: Edmonton, AB, Canada
Posts: 703
Rep Power: 21 |
Answering my own question. I'm sure it will be useful for other n00bs like myself:
1. Possibly because a well-posed problem (proper BC and/or discretization) should definitely not require more than that many iterations. How do I know this? After fooling around with all kinds of meshes for the flow past a bluff body, I have concluded that the time invested on proper meshing is well rewarded. Case in point, when I tried to reduce the mesh size by introducing non-conformal blocks into my domain giving a mesh size of approximately 1 million cells, I also introduced more problems for the solvers. As a result, most of the iterations topped around 450 to 500 for the AMG solver and around 1500-2000 for the ICCG solver. Not only that, I also had to reduce the time-step to a very very low value (sometimes even 0.00025) to keep the Courant number from blowing up (stability requirement). After properly creating the mesh (and by that I mean not exceeding an aspect ratio of 1:5 on any cell in the domain and keeping it strictly orthogonal), the multigrid solver took only 100-120 iterations despite the new mesh being 4 times as big as what it was before! To sum up crappy discretization is identically equal to crappy performance. |
|
April 13, 2007, 00:51 |
Update: On the proper mesh, th
|
#3 |
Senior Member
Srinath Madhavan (a.k.a pUl|)
Join Date: Mar 2009
Location: Edmonton, AB, Canada
Posts: 703
Rep Power: 21 |
Update: On the proper mesh, the number of iterations for the AMG solver has reduced to around 40 now. Only the first 5-10 iterations topped around 100-150.
|
|
April 13, 2007, 06:40 |
This is interesting.
By sayin
|
#4 |
Senior Member
Join Date: Mar 2009
Posts: 225
Rep Power: 18 |
This is interesting.
By saying 'the new mesh being 4 times as big as what it was before' you mean around 4 million cells? BTW. just because of my curiosity, what kind of hardware/machine are you using? |
|
April 13, 2007, 10:20 |
Exactly. 4 million cells with
|
#5 |
Senior Member
Srinath Madhavan (a.k.a pUl|)
Join Date: Mar 2009
Location: Edmonton, AB, Canada
Posts: 703
Rep Power: 21 |
Exactly. 4 million cells with optimal discretization solves faster than 1 million cells with crappy discretization.
Check one of the earlier messages on this forum concerning Super-linear speedup. The machine info is listed there. |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Turbomachinery queries | Manas Psul | FLUENT | 0 | March 11, 2008 08:30 |
queries about turbulence | Micheal | Main CFD Forum | 0 | August 21, 2006 02:51 |
QUERIES ABOUT DIFFERENT PDEs | Shivashankar.K | Main CFD Forum | 1 | February 9, 2005 05:39 |
some queries | Shaoming | FLUENT | 2 | August 25, 2000 09:16 |
some queries regarding y+ value | Abhijit Tilak | Main CFD Forum | 3 | June 12, 2000 07:13 |