# Time step reduction

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

 September 19, 2012, 19:52 Time step reduction #1 Member   navid Join Date: Jan 2010 Posts: 98 Rep Power: 8 Hi, I have a weird problem in my unsteady VOF simulation. when I use variable time stepping and use CFL=0.5 my simulation is OK. But when I change CFL to 0.1 to have smaller time steps, I see oscillations in my solution which is the pressure. Does anyone know reducing CFL (or Time step) further than a specific amount can inversely affect the solution? Thank you,

 September 21, 2012, 16:02 #2 Member   navid Join Date: Jan 2010 Posts: 98 Rep Power: 8 any ideas? has anybody had such a problem before? I mean reducing time step causes convergence issues

 September 22, 2012, 07:23 #3 Senior Member   Join Date: Mar 2011 Location: Germany Posts: 416 Rep Power: 11 strange situation never came across it but i hope you must have reduced the time step size limits once you reduced the CFL number? are oscillations too large?

September 23, 2012, 21:55
#4
Member

navid
Join Date: Jan 2010
Posts: 98
Rep Power: 8
Hi and thanks for your reply. yes I know its strange. I have reduced all the limits accordingly. the minimum pressures in domain sometimes oscilate strong. for example at time step "n" minimum pressure is 70000 pascal and in next time step it is 1 pascal! below are the figures of minimum pressure variation during simulation. first picture is for the case when courant number is 0.1 and minimum time step is 1e-09. the second figure is for the case when courant number is 0.01 and minimum time step is 2e-10.
Attached Images
 courant 01.jpg (21.0 KB, 12 views) courant 001.jpg (25.3 KB, 11 views)

 September 23, 2012, 21:56 #5 Member   navid Join Date: Jan 2010 Posts: 98 Rep Power: 8 in the second figure in post above, all those white lines are times when minimum pressure in domain is oscillating very strong.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post miliante OpenFOAM Running, Solving & CFD 132 October 7, 2012 22:50 Artex85 OpenFOAM Running, Solving & CFD 9 January 3, 2012 09:06 rosh OpenFOAM Running, Solving & CFD 1 December 14, 2011 18:52 Forrest_Lei OpenFOAM 3 July 19, 2011 06:00 Burn OpenFOAM 0 July 16, 2009 10:01

All times are GMT -4. The time now is 21:10.