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CFX-10 - Adaptive Time Stepping

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Old   September 28, 2005, 06:22
Default CFX-10 - Adaptive Time Stepping
  #1
James Date
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Can anyone explain how the adaptive time stepping control values work. I've had a play but they don't seem to be doing what i expect! I assume the relaxation factors are meant to relax the time steps from one iteration to another? This doesn't seem to be happening in my simulation by the factors specified! Any answers guys?

SIMULATION TYPE:

Option = Transient

INITIAL TIME:

Option = Automatic with Value

Time = 0 [s]

END

TIME DURATION:

Option = Total Time

Total Time = 1.25 [s]

END

TIME STEPS:

First Update Time = 0.0 [s]

Initial Timestep = 0.005 [s]

Option = Adaptive

Timestep Update Frequency = 1

TIMESTEP ADAPTION:

Courant Number = 0.75

Decreasing Relaxation Factor = 1.0

Increasing Relaxation Factor = 0.5

Maximum Timestep = 0.01 [s]

Minimum Timestep = 0.00001 [s]

Option = MAX Courant Number

END

END

END

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Old   September 28, 2005, 08:52
Default Re: CFX-10 - Adaptive Time Stepping
  #2
Ribeiro
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Hi James,

These relaxation factors work like other relaxation factors. The solver will try to reach the timestep which satisfy the Courant Number condition (in your case, 0.75 MAX) but the solver will do this finding a increasing (or decreasing) factor. This factor is multiplied by the relaxation factor in order to obtain a more conservative factor. That's the idea.

Regards,
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Old   September 29, 2005, 05:16
Default Re: CFX-10 - Adaptive Time Stepping *NM*
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banjara kotresha
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Old   October 17, 2005, 22:15
Default Re: CFX-10 - Adaptive Time Stepping
  #4
Neale
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With this setup the solver will reduce the timestep without under realaxtion, but increase it with under relaxation of 0.5.

The raw deltat (for the next timestep) is obtained by multiplying the old deltat by the the factor (target courant number)/(last courant number) to force the deltat used for the next timestep to give your specified target.

Neale
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