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Old   August 11, 2008, 06:24
Default time step
  #1
MAB
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when i run it with time step 0.05s it stops saying ' Fatal Overflow '. then i tried with time step 0.005 and it is working and seems creeping from one iteration to the next very very slowly. Is it because the smalles mesh scale (0.005m)is very very small ???
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Old   August 11, 2008, 07:11
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Rogerio Fernandes Brito
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Change the scheme!
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Old   August 12, 2008, 14:21
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latslosh
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Also depends on what flow physics you're trying to model. What kind of Courant number are you getting?
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Old   August 12, 2008, 22:34
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Glenn Horrocks
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Hi,

Generally this error is caused by a overflow error in the linear solver. The fix is to use a smaller timestep, at least when you are starting up.

Glenn Horrocks
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Old   August 14, 2008, 10:01
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John S.
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A good way to estimate timestep is is by using the equation dt = 2U/L, where U is the estimated fluid velocity and L is the length of your domain. This is a good estimation of your advection time. I will usually use this method to estimate an initial timestep for a simulation and after it runs will check the *.out file for the computed advection time. I typically like to use a timestep on the order of 1/3 of that value (small timestep).
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Old   August 15, 2008, 14:42
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Rajee Assudani
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Try using adaptive time step.
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Old   August 15, 2008, 19:08
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Rogerio Fernandes Brito
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"Try using adaptive time step."

Cool!
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Old   August 17, 2008, 17:00
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latslosh
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If it is a transient simulation, be careful with the adaptive time stepping scheme - especially if the time step undergoes large changes
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Old   August 18, 2008, 06:41
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MAB
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Courant number is changing in each timesept...where should i check for a Courant number...???
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Old   August 18, 2008, 14:59
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latslosh
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The Courant Number (Co in CFX, often Cn) is in the .out file of your run, both RMS Co and Max Co are shown.
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Old   August 19, 2008, 15:02
Default Re: time step
  #11
Rajee Assudani
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You can reduce the factor by which the time step will change.
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Old   August 26, 2008, 10:01
Default Re: time step
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MAB
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i still have that problem... the courant number i am getting is Name : Default Domain Modified Water.RMS Courant Number = 5.5075E-01,

Water.Maximum Courant Number = 6.3199E+00

Domain Name : Domain 1 Water.RMS Courant Number = 1.0387E-01,

Water.Maximum Courant Number = 8.8954E-01

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