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

Time step too small?

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 6, 2010, 03:28
Default Time step too small?
  #1
Member
 
Derwin Parkin
Join Date: Feb 2010
Posts: 35
Rep Power: 16
derz is on a distinguished road
Hi guys,

I'm running a transient simulation of a little truck model we have in the wind tunnel. Width Re is around 640,000 for this and I'm using DES.

They have been getting a Cd value of around 0.29. I was always getting around 0.33 but ever since I reduced the size of the time steps I have gotten it to around 0.30, which is great.

However, I noticed that when I reduce the size of the time step to around 0.002 s, the vortex shedding I had observed with time step 0.005 s disappears. So my drag coefficient has improved, but I feel there should be some shedding at the rear, as that's what is happening in the wind tunnel.

Is my time step somehow too small to resolve the shedding somehow? I always thought going as small as possible was the goal.

* It should be noted I made a small geometric change before lowering the time step...it is possible that small change has somehow eradicated the shedding, but I highly doubt it.
derz is offline   Reply With Quote

Old   September 6, 2010, 08:39
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,850
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
The smaller timestep will usually mean increased temporal resolution and therefore more shedding, not less. If it results in less shedding then you may be approaching numerical round-off and that is limiting convergence. Have you tried running double precision?
ghorrocks is offline   Reply With Quote

Old   September 8, 2010, 01:03
Default
  #3
Member
 
Derwin Parkin
Join Date: Feb 2010
Posts: 35
Rep Power: 16
derz is on a distinguished road
Thanks, Glen. I'll see what happens when I run it in double precision mode...
derz is offline   Reply With Quote

Reply


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
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 06:36
How to write k and epsilon before the abnormal end xiuying OpenFOAM Running, Solving & CFD 8 August 27, 2013 16:33
Differences between serial and parallel runs carsten OpenFOAM Bugs 11 September 12, 2008 12:16
AMG versus ICCG msrinath80 OpenFOAM Running, Solving & CFD 2 November 7, 2006 16:15
VOF özgür FLUENT 8 January 6, 2004 09:23


All times are GMT -4. The time now is 00:00.