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

Adaptive timestep

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

Reply
 
LinkBack Thread Tools Display Modes
Old   November 12, 2012, 05:21
Default Adaptive timestep
  #1
siw
Senior Member
 
Join Date: Jul 2009
Posts: 443
Rep Power: 13
siw will become famous soon enough
Hi,

Glenn posted some information about using the adaptive timestep for a transient simulation when the timestep is not known beforehand ([CFX] Simulation of Flow Separation in a rectangular diffuser - Convergence Problem). I need to do the same.

However, in the other topic it is not clear to me were the mentioned coefficient loops are input (analysis type or solver control - see images) since I've tried and they don't like being the same. So in which should the recommend 3-5 be input?

Also, what's the reason for assessing the influence of convergence tolerance? I always set my simulations to a residual target = 1E-06 or lower and let the solver keep iterating until it reaches whatever values it gets too with enough iterations to observe either constant residual values (steady flow) or oscillating residual values (unsteady flow). Then there's no need to assess this parameter.

Thanks.
Attached Images
File Type: jpg Analysis Type.JPG (72.5 KB, 36 views)
File Type: jpg Solver Control.JPG (63.0 KB, 29 views)
siw is offline   Reply With Quote

Old   November 12, 2012, 19:53
Default
  #2
Senior Member
 
Erik
Join Date: Feb 2011
Location: Earth (Land portion)
Posts: 486
Rep Power: 9
evcelica is on a distinguished road
Analysis type is where you would set up your adaptive time-steps. Solver control is where you would set your convergence criteria and min/max number of time-steps per loop. The timestep will be adapted to try to meet your convergence criteria in 3-5 timesteps, but will never go over 10, or under 1, (or whatever you have set in solver control)

1E-6 might not be possible in some simulations, or may be terribly inefficient for some transient runs. But it sounds like you are talking about a steady state simulation in your last sentence?
evcelica is offline   Reply With Quote

Old   November 13, 2012, 03:19
Default
  #3
siw
Senior Member
 
Join Date: Jul 2009
Posts: 443
Rep Power: 13
siw will become famous soon enough
Hi,

I applied those data from Glenn's post as I thought he was detailing and it has greatly improved all the convergences, compared to the pre-cursor steady-state run (200 iterations) which I used for establishing an initial flowfield. When looking at the *.out file on the last few iterations (which would done for only 3 coeff loops) the solver found at timestep size (2.1074E-08 s). Is it now better practice to continue over the next few thousand iterations using this constant time step?

I was suprised not to see oscillations in the monitor points though. The simulation is a transonic aerodynamic flow behind some bluff bodies, so I'm expecting unsteady behaviour. Next I'm going to keep everything the same but use SAS-SST rather than URANS-SST.

My last sentance was just about why would anyone assess the convergence tolerance when all I do is set it really low.
Attached Images
File Type: jpg Analysis type.JPG (76.6 KB, 28 views)
File Type: jpg Solver control.JPG (55.1 KB, 24 views)
File Type: jpg Mass 7 Mom.JPG (63.9 KB, 30 views)
File Type: jpg Turb.JPG (57.7 KB, 24 views)
File Type: jpg Monitors.JPG (66.8 KB, 21 views)
siw is offline   Reply With Quote

Old   November 13, 2012, 12:28
Default
  #4
Senior Member
 
Erik
Join Date: Feb 2011
Location: Earth (Land portion)
Posts: 486
Rep Power: 9
evcelica is on a distinguished road
You set it "really low" which is causing your timestep to have to get extremely small to meet that convergence criteria. If you were to set your convergence criteria higher you could solve it in much much less time. Then you can change your convergence criteria which would also change your timestep size and you would be checking for convergence and timestep independance.
Your idea of just "setting it really low" is just innefficient.
Also 10^-6 RMS may not be good enough for some types of flows, or may be unattainable for others, meaning your adaptive timestepping would keep on decreasing and decreasing and roundoff errors start to effect the solution.

I would leave the timestepping on adaptive if you are going to run more iterations, there will be times in unsteady flow where a larger timestep can be used and other times when a smaller timestep has to be used; let the solver adapt itself to the flow.
evcelica 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
Adaptive Timestep and Residuals siw CFX 6 August 23, 2012 02:21
questions about sensitivity analysis and adaptive timestep sjtusyc CFX 10 June 21, 2012 09:24
Adaptive Timestepping: Possible Glitch? Josh CFX 9 August 25, 2010 21:52
ANSYS CFX Adaptive Timestep aeroman CFX 8 July 25, 2009 06:37
ANSYS CFX Adaptive Timestep aeroman Main CFD Forum 1 July 23, 2009 19:53


All times are GMT -4. The time now is 21:52.