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-   -   BOUNDARY CONDITION THAT CHANGES WITH TIME (http://www.cfd-online.com/Forums/cfx/19857-boundary-condition-changes-time.html)

George August 29, 2003 08:42

BOUNDARY CONDITION THAT CHANGES WITH TIME
 
Hi

I really appreciate if someone could help me out.

How can I set BC's that aren't constant in time, like an inlet that work for 15 seconds in a 3 minutes transient simulation.

How can I do that using fluid sub-domains as energy sources that also vary on time.

Thanks.

cfd guy August 29, 2003 10:12

Re: BOUNDARY CONDITION THAT CHANGES WITH TIME
 
It's the same approach for the both problems.

Create an expression in CEL and combine it with the step function. Suppose that you have a B.C. that follows the equation:
Vel = A * t Where A is a constant and t is time according to CFX-5 VARIABLES file (located on ~CFXROOT/etc). If you set this you'll get a linear variation until the solver ends. But if you want to get it just for the first 15 seconds, use the STEP function. Example:
step(any negative dimensionless number) returns 0. step(0 dimensionless) returns 0.5 step(any positive dimensionless number) returns 1.
In your case(assuming 15 seconds):


Vel = step( (15 [s] - t)/ 1[s] ) * A * t


Note that is divided by 1[s] to keep the argument dimensionless. Supposing that your time step is 1s, in the 15th iteration you'll get 0.5*A*t, 16th A*t, 17th A*t, and so on. If you want to get rid of 0.5*A*t, just do this:


Vel = step( (15.0001 [s] - t)/ 1[s] ) * A * t


This way you'll never get 0.0 and you'll really get a stepwise behavior on the activation of your function.


Hope this helps, cfd guy


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