user defined scalar with periodic B.C.

morihangi · April 17, 2019, 21:45

Hi everyone,
I have faced an issue which is related to periodic boundary condition. Is it possible to use periodic boundary condition when the user defined scalar (UDS) is activated? I have checked the limitations of Periodic Flows in theory guide and nothing is mentioned about UDS. If it is possible, then where is the option related to UDS in periodic condition tab (image attached)? We have the upstream bulk temperature when energy equation is activated, but I cannot see anything related to the upstream value of UDS.
Thanks in advance,

AlexanderZ · April 17, 2019, 22:10

just try it and check if it works or not

best regards

morihangi · April 17, 2019, 22:15

Thanks mate,
I have tried actually; but as you may see in the image attached I am not able to give the solver the upstream value of UDS. I can run a simulation with UDS and periodic BC, but without the upstream value of UDS, I cannot trust the results.

AlexanderZ · April 18, 2019, 00:52

design simplified boundary conditions to check how it works

best regards

morihangi · April 18, 2019, 01:52

I have the simplest case for my simulation, which is a sphere in a cube (2d). I have attached the figure with BCs. However, when I use the periodic boundary condition, I cannot find anywhere to insert the upstream value of UDS.
cheers,

AlexanderZ · April 18, 2019, 02:00

in this configuration periodic and symmetry will give the same distribution, isnt it?

best regards

morihangi · April 18, 2019, 03:58

Quote:

Originally Posted by AlexanderZ

in this configuration periodic and symmetry will give the same distribution, isnt it?

best regards

In fact it does, and the values of the distribution depends on the value of UDS applied on the wall, which totally makes sense. But what is the upstream value of UDS? I'm so confused.

LuckyTran · April 18, 2019, 10:26

There's two kinds of periodic boundary conditions. 1) the actual periodic boundary condition which you get via a periodic interface and then 2) the streamwise periodic boundary condition which is based on the formulation Patankar. You're looking at the 2nd kind. The streamwise periodic BC is based on reformulating the problem via a source/sink term which makes the periodic part of the variables periodic. The streamwise periodic BC is formulated only for momentum and energy equations and you cannot do it for a UDS. You need a workaround or get very good at programming Fluent.

I notice that you do not have the energy equation enabled. Is it enough to acitvate the energy equation and fudge the material properties such that the temperature solution is equivalent to your scalar?

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April 17, 2019, 22:10		#2
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	just try it and check if it works or not best regards

April 17, 2019, 22:15		#3
morihangi New Member Mori Join Date: Oct 2012 Posts: 9 Rep Power: 13	Thanks mate, I have tried actually; but as you may see in the image attached I am not able to give the solver the upstream value of UDS. I can run a simulation with UDS and periodic BC, but without the upstream value of UDS, I cannot trust the results.

April 18, 2019, 00:52		#4
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	design simplified boundary conditions to check how it works best regards

April 18, 2019, 02:00		#6
AlexanderZ Senior Member Alexander Join Date: Apr 2013 Posts: 2,363 Rep Power: 34	in this configuration periodic and symmetry will give the same distribution, isnt it? best regards

April 18, 2019, 10:26		#8
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	There's two kinds of periodic boundary conditions. 1) the actual periodic boundary condition which you get via a periodic interface and then 2) the streamwise periodic boundary condition which is based on the formulation Patankar. You're looking at the 2nd kind. The streamwise periodic BC is based on reformulating the problem via a source/sink term which makes the periodic part of the variables periodic. The streamwise periodic BC is formulated only for momentum and energy equations and you cannot do it for a UDS. You need a workaround or get very good at programming Fluent. I notice that you do not have the energy equation enabled. Is it enough to acitvate the energy equation and fudge the material properties such that the temperature solution is equivalent to your scalar?