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

under relaxation & Periodic Flows

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 17, 2010, 04:24
Red face under relaxation & Periodic Flows
  #1
Member
 
Ardalan
Join Date: Feb 2010
Location: Baku
Posts: 33
Rep Power: 6
Ardalan is on a distinguished road
-What is the difference between under-relaxation and false time step? What are the advantages and disadvantages of each one?

-For problems which have a steady state

we stop calculations when time derivative is zero. What about periodic flows for example flow past a circular cylinder?
Ardalan is offline   Reply With Quote

Old   April 17, 2010, 06:55
Default
  #2
New Member
 
Tanmoy
Join Date: Apr 2010
Posts: 3
Rep Power: 6
tanmoycfd is on a distinguished road
under relaxaation scheme is a power scheme during Iteration.I have used it in FDM ,but widely used in FVM.good for retarded convergence.yet stable.When periodically motions are there no decay solutions with respect to time will observed.dont know about false time step..know about false diffusion!
tanmoycfd is offline   Reply With Quote

Old   April 17, 2010, 11:55
Default
  #3
Senior Member
 
ztdep's Avatar
 
p ding
Join Date: Mar 2009
Posts: 264
Rep Power: 8
ztdep is on a distinguished road
Send a message via Yahoo to ztdep Send a message via Skype™ to ztdep
Dear friends:
For steady problem, when we use the unde-realxation, you can find that the discretized equation is equivalent to the unsteary problem.
ztdep is offline   Reply With Quote

Old   April 17, 2010, 13:13
Default
  #4
Member
 
ganesh
Join Date: Mar 2009
Posts: 40
Rep Power: 7
ganesh is on a distinguished road
Dear Ardalan,

Under relaxation is generally employed when the code shows a tendency to diverge and is therefore more a stabilization tool. False time or pseudo time stepping actually is a convergence acceleration procedure, where ideas from steady--state solvers can be employed for a genuinely unsteady problem. It is possible that solvers might use both these concepts. For periodic problems, as the name suggests we stop the code when a periodic state is acheieved, in other words, the mean of integrated coefficients like Cl and Cd are invariant with time.

Regards,

Ganesh
ganesh is offline   Reply With Quote

Old   April 18, 2010, 00:43
Default
  #5
Member
 
L.Y LIN
Join Date: Mar 2010
Posts: 47
Rep Power: 6
iilw1314 is on a distinguished road
above masters,where could i find some books about these tow concepts?i really want know them,thanks!
iilw1314 is offline   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
Periodic boundary condition Arif FLUENT 2 January 17, 2014 06:48
periodic flows Phil FLUENT 14 May 16, 2013 07:21
Relaxation and convergence sammi Phoenics 0 March 20, 2008 04:32
Question on adjusting relaxation factor CFD Rookie Main CFD Forum 3 January 26, 2004 14:37
periodic flows and heat transfer? scarecrow FLUENT 3 December 2, 2003 00:59


All times are GMT -4. The time now is 18:20.