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

Local Timescale Factor

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 11, 2007, 06:35
Default Local Timescale Factor
  #1
Adam
Guest
 
Posts: n/a
Can anyone explain why using the local timescale factor option is not advisable for obtaining final results? I know it has been mentioned somewhere but cannot find the reference.

Regards

Adam
  Reply With Quote

Old   May 11, 2007, 12:21
Default Re: Local Timescale Factor
  #2
Robin
Guest
 
Posts: n/a
The local timescale factor will advance the solution within each control volume at a multiple of the local timescale. This means that as a whole, the solution is advancing at different rates throughout the solution and can make the RMS and MAX residuals appear to be low when a solution may not actaully be acheived.

If the solution is truly converged, changing to a Physical Timescale will have no effect, whereas if it hasn't, the residuals will jump back up. It is best to think of this as a sanity check.

Personally, I never use the Local Timescale Factor. It used to be necessary in some cases to get things going, but the solver is much more robust now. Generally, if the solver blows with a Physical Timescale and you need to resort to this, it is only masking other problems such a grid or setup issues.

Regards, Robin
  Reply With Quote

Old   May 11, 2007, 17:54
Default Re: Local Timescale Factor
  #3
Bak_Flow
Guest
 
Posts: n/a
Hi Adam,

this techique is useful when the problem has a wide range of time scales but since the local time step can be very small in some cells it is possible that you run what seems to be a long time but by the global scale is not enough to bring it to steady state.

If you can running a physical time step is advisable since a steady solution should run for all time steps...that is the theory anyhow. In practice especially with complex physics this is not so. If you run problems with all one characteristic time scale like a blade passage then the physical time step usually works fine...that is why CFX makes this strong recommendation.

Hope this helps,

Regards,

Bak_Flow
  Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to write k and epsilon before the abnormal end xiuying OpenFOAM Running, Solving & CFD 8 August 27, 2013 15:33
Upgraded from Karmic Koala 9.10 to Lucid Lynx10.04.3 bookie56 OpenFOAM Installation 8 August 13, 2011 04:03
BlockMeshmergePatchPairs hjasak OpenFOAM Native Meshers: blockMesh 11 August 15, 2008 07:36
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58
Could anybody help me see this error and give help liugx212 OpenFOAM Running, Solving & CFD 3 January 4, 2006 19:07


All times are GMT -4. The time now is 05:37.