# time step

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 August 11, 2008, 06:24 time step #1 MAB Guest   Posts: n/a Sponsored Links 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 ???

 August 11, 2008, 07:11 Re: time step #2 Rogerio Fernandes Brito Guest   Posts: n/a Change the scheme!

 August 12, 2008, 14:21 Re: time step #3 latslosh Guest   Posts: n/a Also depends on what flow physics you're trying to model. What kind of Courant number are you getting?

 August 12, 2008, 22:34 Re: time step #4 Glenn Horrocks Guest   Posts: n/a 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

 August 14, 2008, 10:01 Re: time step #5 John S. Guest   Posts: n/a 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).

 August 15, 2008, 14:42 Re: time step #6 Rajee Assudani Guest   Posts: n/a Try using adaptive time step.

 August 15, 2008, 19:08 Re: time step #7 Rogerio Fernandes Brito Guest   Posts: n/a "Try using adaptive time step." Cool!

 August 17, 2008, 17:00 Re: time step #8 latslosh Guest   Posts: n/a If it is a transient simulation, be careful with the adaptive time stepping scheme - especially if the time step undergoes large changes

 August 18, 2008, 06:41 Re: time step #9 MAB Guest   Posts: n/a Courant number is changing in each timesept...where should i check for a Courant number...???

 August 18, 2008, 14:59 Re: time step #10 latslosh Guest   Posts: n/a The Courant Number (Co in CFX, often Cn) is in the .out file of your run, both RMS Co and Max Co are shown.

 August 19, 2008, 15:02 Re: time step #11 Rajee Assudani Guest   Posts: n/a You can reduce the factor by which the time step will change.

 August 26, 2008, 10:01 Re: time step #12 MAB Guest   Posts: n/a 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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