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Old   May 7, 2012, 13:26
Default Problem with a periodic boundary condition
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Hi,

I have a problem with a periodic boundary condition and i really don't find how to solve it.

In fact I'm trying to make an iteration on my BC at each time step.For instance, my boundary condition could be:

Pstatic(t+1)=Pstatic(t)+DeltaP ,
with deltaP=constant value, and t=time.

To recuperate the Pstatique at the boundary, i use a CEL function:
Pstatique(t)=areaAve(pressure)@outlet.

The problem is that the Pstatic(t+1) isn't equal to Pstatic(t)+DeltaP, and it is always less.

That's why a try to change my CEL fundtion to:

Pstatic(t+1)=Pstatic(t)

And surprise: the average pressure is decreasing at each time step....(whereas is suppose to be constant)

I think the probleme come from the difference between the Hybrid and conservative value that are always different, but i don't find how to solve it....

is it possible to recuperate the hybrid value in CFX PRE using areaAve(pressure)@outlet?or to directly impose a conservative value?
Does somebody know how to solve this problem?

Thank a lot!!!!!
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Old   May 7, 2012, 15:18
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Dear chuck,

Maybe you can use function to do this. You can import a chart that contains pressure changes according to time change. and you can take this function to your boundary condition.
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Old   May 7, 2012, 16:47
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Hi!
Thanks for your answer.
Unfortunately, my CEL is more complicated than the one I give you.I really need to recuperate the previously value (Pressure) to find the next value.

Do you think using a routine function could help me?For this, do I need to use Fortran?
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Old   May 7, 2012, 17:00
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Edmund Singer P.E.
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Cant do that within CEL normally but depending on what you want to do, you might be able to delay the update of a variable and get what you want.

Do a search on TRANS_LOOP
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Old   May 8, 2012, 16:32
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thank you. I'm trying to do it with de AV.
But for instance, if I would like to simulate a derivative term like dP/dt, may have other choice than doing:
(P(t+1)-P(t))/deltat
I have to discretize, is'nt it?
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Old   May 8, 2012, 22:38
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are you trying to do this in POST or while the simulation is running?
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Old   May 9, 2012, 15:12
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I'm trying to do it when the simulation it's running.
In fact i need to change the BC (static pressure) at the outlet and for that, i have a function:

dP/dt=f(P,axialvelocity)

And i need to use previously value for P because i'm using the following discretisation:

(P(n+1)-P(n))/deltat=f(p(n),axial velocity(n))
Do you know what i mean?
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Old   May 9, 2012, 15:21
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Yes discretize it. Look up trans_loop for details on how to delay update of additional variable.
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Old   May 9, 2012, 16:06
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Ok!
I just have a last question: how can i be sure than the delay has been correctly apply?
I mean is it possible to calculate for each step the variable with the delay and the variable once the formula compute?
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Old   May 9, 2012, 16:09
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You should most certainly do this. Set up a monitor point for the pressure, and another monitor point for the Additional Variable that is delayed.

You should be able to see the lag in the solver manager.
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Old   May 9, 2012, 17:15
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Thanks a lot!
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