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time step in CFX pre

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Old   April 11, 2021, 22:07
Default time step in CFX pre
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Hi, there

May I confirm two things regarding to the time step set-up in CFX pre?

1, in the steady simulation, Id like to set up my time scale depending on the number of iterations. When the number of iterations is smaller than 200, I will use 2e-6 [s] as the physical time scale. Then the time scale slides gradually to 2e-4 [s] in the following 100 iterations. After that, the time scale keeps constant as 2e-4 [s] in the last 700 iterations.

can I use the following CEL:
step(200-citern)*2e-6 +
step(citern-200)* step(300-citern)*((citern-200)*1.98e-6+2e-6) +
step(citern-300)* 2e-4
to achieve my goal? Is there any other better way of doing that?

Also what is the difference between aitern and citern? Does aitern also count the iteration from the initial guess?

2, In the transient simulation, what is the difference between citern and ctstep? Does citern refer to the number of iteration in the inner loop and ctstep refer to the number of time step in the outer loop?

If Id love like to have a sinusoid inlet velocity depending on the time, could I use the following CEL
11.07[m/s]+1.63[m/s]*sin(41.9[s^-1]*t )
?

Thank you very much!
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Old   April 12, 2021, 03:00
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Gert-Jan
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Search the CFX forum to find the difference between aitern and citern, e.g. aitern vs citern

Regarding the functions, I think these are completely valid for the purpose.
I would recommend to plot the CEL expressions in CFX-Pre to see if it provides what your expect. If so, then you can trust that CFX will use the time step increase or inlet velocity as you proposed.
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Old   April 12, 2021, 05:46
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Glenn Horrocks
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And don't forget all variables are defined in the CFX documentation, in the reference guide.

A further comment: While your function with lots of step() functions is valid, if you use a 1D interpolation function it is much easier to set up and understand. You just plot out the points to define the function and CFX interpolates along it.
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Old   April 12, 2021, 16:53
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Hi, Gert-Jan, Thank you very much for your reply. Drawing the CEL expression is really a good idea.
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Old   April 12, 2021, 16:55
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Quote:
Originally Posted by ghorrocks View Post
And don't forget all variables are defined in the CFX documentation, in the reference guide.

A further comment: While your function with lots of step() functions is valid, if you use a 1D interpolation function it is much easier to set up and understand. You just plot out the points to define the function and CFX interpolates along it.
Thanks for your always helpful answer, ghorrocks. I've changed my step function to 1d interpolation function.
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