CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Sudden/random deviation of poisson eqn results

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   December 10, 2006, 01:34
Default Sudden/random deviation of poisson eqn results
  #1
zonexo
Guest
 
Posts: n/a
Hi,

I have been using a few different Krylov linear solver package such as nspcg,sparsekit and now petsc to solve the linear eqns for my NS solver momentum and poisson eqn. For moving bodies simulation, while solving the poisson eqn, sometimes the result suddenly changes drastically, although the iterations seems to have converged e.g. at the previous time step, everything is fine, but at the next time step, the velocity suddenly explodes from 2-3 to 20-30+.

The strange thing is that this deviation happens at different time step for different linear solver package ie for nspcg, it happens at time=20 but for petsc, it happens at time=40. It seems "random" in this sense. Changing the type of solver such as gmres or bcgs does not change the time of deviation usually.

Does anyone know why this is happening? Is it a characteristic of krylov solver to happen once in a while? It does not happen in the stationary body simulation though. I thought that there may be some bug in my code, but the fact that for different solver package the "explosion" time step is different.

Initialization of the guessed value to zero also changes the explosion time

Moreover, suppose the explosion of value happen at t=50. If I store all my relevent values at t=40 into files and restart my simulation from t=40(instead of t=1) by reading the stored values from the files, the explosion will now not happen at t=50.

Anyone experience these before. Btw, my simulation is a 2D FVM NS code.
  Reply With Quote

Old   December 11, 2006, 05:21
Default Re: Sudden/random deviation of poisson eqn results
  #2
Kirikou
Guest
 
Posts: n/a
I had the same problem few weeks ago while triyng to solve pressure equation with iccg (conjuguate gradient with chloesky preconditionner) and it comes from a bad conditionning of the problem. Simply there's no solution to the linear system you're trying to resolve. So review the matrix, the boundary conditions and make sure that everything is alright.
  Reply With Quote

Reply


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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Different Results from Fluent 5.5 and Fluent 6.0 Rajeev Kumar Singh FLUENT 6 December 19, 2010 12:33
validation of CFD results andy FLUENT 0 June 13, 2007 14:55
Recommendation of a good poisson solver Quarkz Main CFD Forum 2 December 2, 2005 10:12
UDFs for Scalar Eqn - Fluid/Solid HT Greg Perkins FLUENT 0 October 14, 2000 00:03
UDFs for Scalar Eqn - Fluid/Solid HT Greg Perkins FLUENT 0 October 11, 2000 04:43


All times are GMT -4. The time now is 22:32.