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.

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