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Variable Time Steps in transient CFX simulation

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Old   November 15, 2019, 06:09
Default Variable Time Steps in transient CFX simulation
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Dear all,

Iam developing a pipe model a steam flow.

For my Simulation runs, i plan a transient Simulation over a 3600 s period.

For the inital Startup it is necessary to start with very short time steps (0.1-0.5s) but after the first minutes this solution wont be necessary.

So, my Question:
Iam Looking for a solution to Change the time steps in the Simulation or Setting a timestep profile.

So from second 0 - 120 Timestep size = 0.5s
From 120 to end Timestep size = 20s

Is there any possibility to do that (i have seen i can add functions to the timestep but iam not sure About the Syntax)

Thanks and best regards
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Old   November 15, 2019, 08:45
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There are dozen threads about achieving something like this, so instead of doing all the work for you, here are some keywords to look for:
cfx expression language, cfx step function, cfx step syntax


Good luck
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Old   November 18, 2019, 01:19
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This can be done in two ways:

First one is a simple expression like:
if(t<120[s], 0.5[s], 20[s])

The other way is a table input under user functions:
slide 12 here...
https://www.slideshare.net/al11429964/cfx12-11-cel
create a table of values which will map [s] to [s]
folowed by
expresionuserfunction(t)
t stands for time or time dependant


The first way is far simpler for what you want to achieve just be careful when jumping from 0,5 to 20 as it is better to ramp the value unless you know that this jump won't cause any failure of the solver
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Old   November 19, 2019, 01:59
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Thank you very much,

it worked
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