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

Adaptive Timestep and Residuals

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 22, 2012, 06:44
Default Adaptive Timestep and Residuals
  #1
siw
Senior Member
 
Stuart
Join Date: Jul 2009
Location: Portsmouth, England
Posts: 732
Rep Power: 25
siw will become famous soon enough
Hi,

I'm running a transient external aero simulation around some bluff bodies (ICEM hybrid mesh ~20 million elements - on my 8 core PC takes ages to run). Re = 6 million.

I did the usuall steady state simulations (SST k-omega) to determine mesh and physcial time scale sensitivity in order to get a flowfield to initialise the transient run.

I expect a turbulent flowfield behind the bluff bodies with a range of scales - not a periodic shedding of vortices. So I don't know what transient time set-up to use. Therefore, following Glenn's reply (http://www.cfd-online.com/Forums/cfx...e-problem.html) I'm using the adaptive timestepping to CFX can work it out it self.

But I've found the RMS residuals have increased greatly (see image of the run in progress). In using the SAS-SST with Bounded CD advection scheme following the ANSYS SRS Best Practice document. I could not get the residuals any lower in the steady state run as the mesh size was getting far too large (however, the steady state monitor points became constant and the imbalances were all 0%).

Should I let this simulation to continue (it'll take ages) or does anyone known how to improve things. I'm also watching the monitor points which are starting to oscillate (which I expect was they are points in the turbulent wake).

Thanks
Attached Images
File Type: jpg Capture.JPG (65.9 KB, 81 views)
siw is online now   Reply With Quote

Old   August 22, 2012, 07:42
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
You cannot do time scale sensitivity on a steady state run. It has to be done on a transient run. Doing mesh sensitivity on a steady state run sounds OK. But if you use adaptive time stepping that avoid the problem - but you still need to define a convergence criteria and that requires a sensitivity analysis.

You cannot directly compare residuals from steady state and transient runs. They are normalised differently.

So do a sensitivity analysis on your convergence criteria on the transient run and if that shows it is OK then you are fine.
ghorrocks is offline   Reply With Quote

Old   August 22, 2012, 08:05
Default
  #3
siw
Senior Member
 
Stuart
Join Date: Jul 2009
Location: Portsmouth, England
Posts: 732
Rep Power: 25
siw will become famous soon enough
Thanks Glenn,

Since a timescale (in my case a physical timescale) is needed for a steady state simulation I always thought it a good idea to do a sensitivty test on that parameter. Do you consider that pointless?

With regards to a sensitivity analysis on the convergence criteria. The images shows from the CFX-Solver Manager Out File (the run is still going) the Timestep and Solver Controls I set. BTW the 1 second limit was arbitary and I'll end up stopping the run before then.

So which input parameter would I need to change? I guess it must be the Coeff Loops because since the RMS residuals (mass and momemtum) are not even getting close to 1e-06 then any change to that would, surely, not make any difference.

Regards
Attached Images
File Type: jpg Capture.JPG (37.9 KB, 50 views)
File Type: jpg Capture2.JPG (32.1 KB, 56 views)
siw is online now   Reply With Quote

Old   August 22, 2012, 08:14
Default
  #4
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
Sensitivity analysis on physical time step size for a steady state run is pointless. The only thing which matters is how tight the convergence is. The time step is just a means to achieve convergence.

You have specified a very tight convergence. Are you sure you need to be that tight?

Why are you using a central difference advection scheme (very accurate), second order time differencing (very accurate) but only first order turbulence numerics? This is not necessarily wrong, just that it is unusual.
ghorrocks is offline   Reply With Quote

Old   August 22, 2012, 08:27
Default
  #5
siw
Senior Member
 
Stuart
Join Date: Jul 2009
Location: Portsmouth, England
Posts: 732
Rep Power: 25
siw will become famous soon enough
Hi Glenn,

My reason for setting a target residual = 1e-06 was because I thought the solver would never get to that level and so the solver would not stop running until it got the the specified number of iterations - which I set to 200 to ensure all the monitor points became flat for a good handful of iterations. I'm looking through the CFX guides about the transient residuals are normalized differently but I'm not seeing anything to say it's a different method to steady state residuals.

I was under the impression from reading through the CFX guides that because I'm not using LES (or a RANS-LES) model I should leave the turbulence numerics to 1st-order as with a RANS simulation. Do you recommend changing it for SAS-SST? Again the ANSYS SRS Best Practice gude says nothing with that respect.

So for a sensitivty test on the convergence, what parameter(s) do you suggest I change?

Regards
siw is online now   Reply With Quote

Old   August 22, 2012, 08:37
Default
  #6
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
OK, you are just running it until it converges as far as it can.

The transient residual contains a transient term which obviously the steady state one does not.

The SAS-SST model is a transient model. You cannot run it steady state.

For sensitivity on convergence, the main parameter is the residual tolerance, but for some models the imbalance is important.
ghorrocks is offline   Reply With Quote

Old   August 23, 2012, 03:21
Default
  #7
siw
Senior Member
 
Stuart
Join Date: Jul 2009
Location: Portsmouth, England
Posts: 732
Rep Power: 25
siw will become famous soon enough
I know SAS-SST is transient only so cannot be run in steady state - I'm not sure where in the topic I gave the impression that I was trying to run it as steady state.

Anyway, the simulation crashed last night. Not 100% why but it did have a high Mach number notice (M ~ 8) which is completely none physical for this simulation. So I've set a fixed timestep and try again. If that fails I'll try SST-URANS instead of SAS-SST although that's not the preferred model.
siw is online now   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
How does openFoam calculate residuals? sahm OpenFOAM 3 October 26, 2017 18:24
Adaptive Timestepping: Possible Glitch? Josh CFX 9 August 25, 2010 22:52
Monitor residuals Juan Catelén CFX 4 March 26, 2007 09:25
Use of Timestep in obtaining solution. hagupta CFX 7 February 28, 2006 14:14
Convergence - scaled vs unscaled residuals HS FLUENT 1 November 7, 2005 06:45


All times are GMT -4. The time now is 07:13.