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Old   September 19, 2012, 19:52
Default Time step reduction
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navid
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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?

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Old   September 21, 2012, 16:02
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any ideas? has anybody had such a problem before? I mean reducing time step causes convergence issues
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Old   September 22, 2012, 07:23
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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?
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Old   September 23, 2012, 21:55
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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.
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File Type: jpg courant 01.jpg (21.0 KB, 12 views)
File Type: jpg courant 001.jpg (25.3 KB, 11 views)
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Old   September 23, 2012, 21:56
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in the second figure in post above, all those white lines are times when minimum pressure in domain is oscillating very strong.
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