# Problem with a periodic boundary condition

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 May 7, 2012, 12:26 Problem with a periodic boundary condition #1 New Member   martial Join Date: May 2012 Posts: 7 Rep Power: 6 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!!!!!

 May 7, 2012, 14:18 #2 Member   Ahmet Join Date: Mar 2011 Location: Ankara/ Turkey Posts: 37 Rep Power: 7 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.

 May 7, 2012, 15:47 #3 New Member   martial Join Date: May 2012 Posts: 7 Rep Power: 6 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?

 May 7, 2012, 16:00 #4 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 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

 May 8, 2012, 15:32 #5 New Member   martial Join Date: May 2012 Posts: 7 Rep Power: 6 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?

 May 8, 2012, 21:38 #6 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 are you trying to do this in POST or while the simulation is running?

 May 9, 2012, 14:12 #7 New Member   martial Join Date: May 2012 Posts: 7 Rep Power: 6 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?

 May 9, 2012, 14:21 #8 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 Yes discretize it. Look up trans_loop for details on how to delay update of additional variable.

 May 9, 2012, 15:06 #9 New Member   martial Join Date: May 2012 Posts: 7 Rep Power: 6 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?

 May 9, 2012, 15:09 #10 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 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.

 May 9, 2012, 16:15 #11 New Member   martial Join Date: May 2012 Posts: 7 Rep Power: 6 Thanks a lot!

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