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

convergence in tetramesh

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 27, 2010, 21:30
Default convergence in tetramesh
  #1
New Member
 
KaySee
Join Date: Apr 2010
Posts: 5
Rep Power: 16
khaiching is on a distinguished road
Hi colleagues:

I would like to gather comments from you all regarding the convergence performance using tetrahedral mesh.

I build up my 3D tetrahedral mesh using Delaunay method and find that the convergence is kinda irregular. For a simple incompressible flow condition in a flow distributor at a certain tetrahedral mesh setup, I could not achieve convergence at all. The pressure correction residual error is getting unstable ( residual error goes down to a certain level and it spikes up again).

Subsequently, I extract the cells (problematic cell) that contain the largest residual error and try to remesh the region of those problematic cells. In this case, the convergence can be attained.

Dear colleagues, I would like to know whether you have come across this phenomenon before? Something come across my mind: for the matrix solver of pressure correction equation, I utilise the Bicgstab method with preconditioner. Could this problem due to the characteristic of the Bicgstab method itself, whereby the error cannot be pushed out from the domain as efficiently as the AMG method, particularly in the unstructured tetrahedral environment? I have not tried the AMG yet.

Or, could it be due to any other reasons, which is unknown to me?

Thanks.
khaiching is offline   Reply With Quote

Old   October 27, 2010, 23:01
Default
  #2
New Member
 
eric planting
Join Date: Oct 2010
Posts: 3
Rep Power: 15
eplanting is on a distinguished road
Variable dt or fixed dt? Also, are you using upwinding or midpoint, midpoint is a bit more accurate.
eplanting is offline   Reply With Quote

Old   October 28, 2010, 01:57
Default
  #3
New Member
 
KaySee
Join Date: Apr 2010
Posts: 5
Rep Power: 16
khaiching is on a distinguished road
Hi:

I am using steady-state algorithm to solve the equations. SIMPLE algorithm in fact.

I am using 1st order upwind scheme for all the convective terms. Therefore, I think the instability occured is not due to my convection scheme.
khaiching is offline   Reply With Quote

Old   October 28, 2010, 05:10
Default
  #4
New Member
 
eric planting
Join Date: Oct 2010
Posts: 3
Rep Power: 15
eplanting is on a distinguished road
ick, while upwinding is powerful, it tends to give more pretty pictures than accurate ones. Anyways, I still think it's probably something to do with a value of dt occuring that's too high for the solution, that's a pretty common problem around the graduate level at least, one gradient is pretty steep, and it looks like the rest of the solution is converging, but one area is developing an error, anyway, I'd recheck ur code for the calculation of dt, or you could post it.
eplanting is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
Problems with convergence with an easy system franzdrs Main CFD Forum 0 June 15, 2009 18:17
increasing mesh quality is leading to poor convergence tippo CFX 2 May 5, 2009 10:55
Defect correction and convergence ganesh Main CFD Forum 4 June 30, 2006 14:20
Convergence problems Chetan FLUENT 3 April 15, 2004 19:13


All times are GMT -4. The time now is 02:34.