CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Time step independence?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   October 19, 2012, 09:35
Default Time step independence?
  #1
Senior Member
 
David Gaden
Join Date: Apr 2009
Location: Winnipeg, Canada
Posts: 407
Rep Power: 13
marupio is on a distinguished road
Basic stuff here.

I cannot get consistent results from pisoFoam. I have a simple cavity with a cross flow inlet / outlet. I vary the time step from 0.001 to 0.0000001 s and the resulting pressure fields vary by 20-60%.

Does anyone have any ideas?

So far I've observed:
  • Changing the cavity size has no effect (between 0.1m and 10m)
  • Changing the mesh density has no effect (10x10 vs 100x100)
  • Turbulence model has no effect (kEplison vs disabled)

Yeah, I'm an OpenFOAM developer... not a user.

-Dave
__________________
~~~
Follow me on twitter @DavidGaden
marupio is offline   Reply With Quote

Old   October 20, 2012, 21:54
Default
  #2
Senior Member
 
Awais Ali
Join Date: Feb 2010
Location: Germany
Posts: 128
Rep Power: 8
owayz is on a distinguished road
Send a message via MSN to owayz
Hi David,
are you comparing averaged pressure or just pressure at some time ?

Regards,
Awais
owayz is offline   Reply With Quote

Old   October 22, 2012, 06:49
Default
  #3
Senior Member
 
chegdan's Avatar
 
Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 560
Rep Power: 18
chegdan will become famous soon enough
Dave,

Are you sure that there is convergence between all of your fields at each timestep? If not, then this will have a big affect on your pressure field. Adding outer-correction will ensure convergence at each timestep. Have you looked at using pimpleFoam instead of pisoFoam? This is suggested on interFoam: timestep / mesh / PISO nCorrector dependency
__________________
Dan

Find me on twitter @dancombest and LinkedIn

Last edited by chegdan; October 22, 2012 at 07:01. Reason: edit: bad grammar
chegdan is offline   Reply With Quote

Old   November 5, 2012, 23:50
Default
  #4
Senior Member
 
David Gaden
Join Date: Apr 2009
Location: Winnipeg, Canada
Posts: 407
Rep Power: 13
marupio is on a distinguished road
@Awais

I am calculating an RMS error across the entire pressure field. Average pressure doesn't vary nearly as much.

@Dan

Thanks for the suggestions, I will try implementing pimpleFoam into the solver.

I've attached some pictures of the pressure and flow field. I'm getting a pressure fluctuation at the inlet, and discontinuities at the corners. I'm sure this is a rookie problem, but it mystifies me. These images are for velocity driven flow. When I do pressure driven flow, though, it smooths out, but I still can't get timestep independence.

Thank you all for your suggestions. I will continue troubleshooting this between my full-time job and parenthood!

-Dave
Attached Images
File Type: jpg velocityDrivenPFull.jpg (10.4 KB, 11 views)
File Type: jpg velocityDrivenUInlet.jpg (64.2 KB, 12 views)
__________________
~~~
Follow me on twitter @DavidGaden
marupio 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
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 06:36
Hydrostatic Pressure and Gravity miliante OpenFOAM Running, Solving & CFD 132 October 7, 2012 22:50
High Courant Number @ icoFoam Artex85 OpenFOAM Running, Solving & CFD 9 January 3, 2012 09:06
Extrusion with OpenFoam problem No. Iterations 0 Lord Kelvin OpenFOAM 6 April 12, 2011 11:24
Full pipe 3D using icoFoam cyberbrain OpenFOAM 4 March 16, 2011 10:20


All times are GMT -4. The time now is 01:08.