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

Convergence parameter CFX Solver

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 16, 2014, 12:34
Question Convergence parameter CFX Solver
  #1
New Member
 
chiragsvnit's Avatar
 
Chirag Trivedi
Join Date: Sep 2009
Location: Norway
Posts: 26
Rep Power: 16
chiragsvnit is on a distinguished road
Send a message via Skype™ to chiragsvnit
Hi,
Few Days back I have started a steady state simulation (Hexahedral mesh, node = 35 million) of a Penstock using ANSYS CFX Solver. Boundary condition are mass flow inlet (34000 kg/s) and pressure outlet (225 kPa absolute). Convergence criteria was 0.001 MAX. Residual for all
I have observed following points during the simulation.
(1) First I set automatic timescale, the solution was stabilized at 0.01 MAX Residual and automatic time scale was 0.7 s.
(2) Then I selected Physical time scale of 0.1 s the solution converged below than the automatic timescale residual value but not converged.
(3) Further I was continuously changed the physical timescale to 1E-9 s, then immediately the maximum residual reached to 0.001 and the solution converged.

The point is selected physical timescale was far below than the actual residence time in the domain. The residence time was 151 s (Checked in CFX Post streamline Timescale Min=0 and Max=151 s).

Mass imbalance <0.01%.

Is there something wrong with the solution since residence time (i.e 151 s)is far away from the selected timescale (i.e. 1E-9 s)?
chiragsvnit is offline   Reply With Quote

Old   March 16, 2014, 18:00
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Yes, I would be cautious of this result. When you need to use a microscopic time scale to get it to converge you can get into positions where the residuals appear tight but convergence has not been achieved.

If you have to use such a small time scale for convergence I would look at your simulation and improve the numerical stability - that means things like better mesh quality, double precision numerics or better initial condition.
ghorrocks is offline   Reply With Quote

Old   March 17, 2014, 01:45
Default
  #3
New Member
 
chiragsvnit's Avatar
 
Chirag Trivedi
Join Date: Sep 2009
Location: Norway
Posts: 26
Rep Power: 16
chiragsvnit is on a distinguished road
Send a message via Skype™ to chiragsvnit
Thank You Glenn for suggestions.

(1) Quality of the hexahedral mesh is above 0.7 in ICEM CFD.
(2) For initial guess, I used result file which was obtained based on automatic timescale.
(3) It is always double precision, high resolution for all solving equations including turbulence numeric (k-epsilon).

More importantly, When I run this simulation as transient using time step of 0.001 s, it converges fast with just three coefficient loop iterations. I do have limitations of RAM I could not continue with transient simulation and switched to steady state.

(1) Will another turbulence model make difference in this case, i.e. k-omega
(2) Will I try another boundary condition, i.e. Pressure inlet and pressure outlet
chiragsvnit is offline   Reply With Quote

Reply

Tags
ansys, cfx 14

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 Centurion2011 FLUENT 48 June 15, 2022 00:29
How to check the convergence of the CFX solver? Amoul CFX 10 May 30, 2013 06:57
Error in CFX Solver Leuchte CFX 5 November 6, 2010 07:12
Convergence Definition of Segregated Solver Amir FLUENT 0 April 18, 2006 07:21
Convergence with coupled implicit solver Henrik Ström FLUENT 1 October 29, 2005 04:57


All times are GMT -4. The time now is 17:42.