Problem with a periodic boundary condition
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!!!!! |
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. |
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? |
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 |
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? |
are you trying to do this in POST or while the simulation is running?
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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? |
Yes discretize it. Look up trans_loop for details on how to delay update of additional variable.
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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? |
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. |
Thanks a lot!
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