CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

transient converging, but not steady

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 21, 2011, 19:21
Default transient converging, but not steady
  #1
New Member
 
Join Date: Jul 2011
Posts: 9
Rep Power: 15
PHS- is on a distinguished road
I everybody!

I am currently working on a CFD-simulation of a Solar Tower Plant (that produce renewable solar electricity: the sun heats up the air which rises in the chimney through turbines)

The transient simulation is converging (temperature, pressure, and velocity remains stable after a few time step)
A steady simulation should also been converging but it's not! (no matter what turbulence model I try)

Has anyone already seen such a problem, and could give me some advice?
PHS- is offline   Reply With Quote

Old   July 22, 2011, 15:14
Default
  #2
New Member
 
Nik
Join Date: Mar 2010
Location: Michigan
Posts: 25
Rep Power: 16
nikhil is on a distinguished road
Are you trying to run the steady state with the initial/boundary conditions that are equivalent to the last flow time from your transient simulation?

If so, you should be getting the same results. If you are running the steady simulation with different BC/IC, then you may get different solution
nikhil is offline   Reply With Quote

Old   July 22, 2011, 15:29
Default
  #3
New Member
 
Join Date: Jul 2011
Posts: 9
Rep Power: 15
PHS- is on a distinguished road
Thanks for your answer!

I tried to run the transient simulation until I get convergence, and then to turn on the steady calculation without initialising (since the solution has already converged, steady calculation should work).

But when I do that, it's diverging, for example the temperature was stable for 2000 time step at 322K, and when I turn on the steady calculation, it's oscillating very fast between 300K and 360k, and is not converging at all (even after 30000 iterations...)
PHS- is offline   Reply With Quote

Old   July 24, 2011, 13:58
Default
  #4
Senior Member
 
Micael
Join Date: Mar 2009
Location: Canada
Posts: 157
Rep Power: 18
Micael is on a distinguished road
Did you try to decrease the under-relaxation factor of energy equation? From 1 to 0.8 as an example?
Micael is offline   Reply With Quote

Old   July 24, 2011, 21:59
Default
  #5
New Member
 
Join Date: Jul 2011
Posts: 9
Rep Power: 15
PHS- is on a distinguished road
yep, but that leads to high discontinuities in temperatures: the more I decrease it, the more convergence I get, but more discontinuities appears...

(by discontinuity I mean high change in temperature on the elbow, I can "see" the meshing through the temperature contours, instead of having a smooth transition.)
PHS- is offline   Reply With Quote

Old   July 25, 2011, 15:25
Default
  #6
jsm
Senior Member
 
JSM
Join Date: Mar 2009
Location: India
Posts: 192
Rep Power: 20
jsm is on a distinguished road
Hi,

I think this phenomenon is transient in nature. If you have good results in transient solver, then why are you switching back to steady state solver?

If you want quick results, then increase the time step size without decreasing convergence level.
__________________
With regards,
JSM
jsm 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
Steady solution from Transient simulations wawa FLUENT 2 November 9, 2010 18:44
About using transient method to solve steady problems doctor evil Main CFD Forum 1 April 15, 2010 00:46
Mass Diffusion: Transient and Steady State BC rval CFX 3 November 19, 2008 01:52
Transient vs Steady State Adam CFX 1 April 12, 2007 12:34
Difference between steady and transient problems Jianping Zhang Main CFD Forum 9 April 14, 2003 07:17


All times are GMT -4. The time now is 14:26.