# How to define outlet convective BC for the cyclic BC

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 June 18, 2012, 11:30 How to define outlet convective BC for the cyclic BC #1 Senior Member   Jian Zhong Join Date: Feb 2012 Location: Birmingham Posts: 109 Rep Power: 7 I define the inlet and outlet as cyclic BC. But now I want to set the outlet as convective BC for the passive scalar. Is it possible to achieve that?

 July 11, 2012, 09:45 #2 Senior Member   Jian Zhong Join Date: Feb 2012 Location: Birmingham Posts: 109 Rep Power: 7 Or how to define other types of BC for cyclic patches? such as zeroGradient? Does any one have the experience of creating this kind of customer cyclic derived BC?

July 11, 2012, 10:19
#3
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Daniel P. Combest
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Quote:
 Originally Posted by zxj160 Or how to define other types of BC for cyclic patches? such as zeroGradient? Does any one have the experience of creating this kind of customer cyclic derived BC?
The way I got around this easily was to use directMapped for momentum (setting it to an average velocity or flowrate) and that allows you to use fixed and zeroGradient boundary conditions for your passive scalar.

Dan
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July 11, 2012, 10:35
#4
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Jian Zhong
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Quote:
 Originally Posted by chegdan The way I got around this easily was to use directMapped for momentum (setting it to an average velocity or flowrate) and that allows you to use fixed and zeroGradient boundary conditions for your passive scalar. Dan
Dear Dan,

Thanks. Do you mean that I set inlet and outlet as normal 'patch' type and use directMapped to map the cyclic velocity? Then I can set uniform value and zeroGradient for passive scalar?

If yes, I have tried this and also mapped pressure. But the output of pressure is so different (all the other conditions are same)?

for cyclic case:
p
(
0.000572052
0.000389558
6.02371e-05
-0.000262527
-0.000663079
-0.00121091
-0.00193648
-0.00261434
.......

for directMapped case:
p
(
126.072
126.067
126.061
126.055
126.052
126.049
.......

I can not identify the problem.

July 11, 2012, 11:02
#5
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Daniel P. Combest
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Quote:
 Originally Posted by zxj160 Dear Dan, Thanks. Do you mean that I set inlet and outlet as normal 'patch' type and use directMapped to map the cyclic velocity? Then I can set uniform value and zeroGradient for passive scalar? If yes, I have tried this and also mapped pressure. But the output of pressure is so different (all the other conditions are same)? for cyclic case: p ( 0.000572052 0.000389558 6.02371e-05 -0.000262527 -0.000663079 -0.00121091 -0.00193648 -0.00261434 ....... for directMapped case: p ( 126.072 126.067 126.061 126.055 126.052 126.049 ....... I can not identify the problem.
Is your pressure drop through the domain the same? I always never use directMapped for pressure. The values will be different than the cyclic case since its pressure driven in the directMapped case.
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July 11, 2012, 11:12
#6
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Jian Zhong
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Quote:
 Originally Posted by chegdan Is your pressure drop through the domain the same? I always never use directMapped for pressure. The values will be different than the cyclic case since its pressure driven in the directMapped case.
Yes, the pressure drop is same and only the BCs for pressure are different. But the pressure is also cyclic. If we do not use cyclic, then what type of BC can we define for pressure.

Best regards,
Jian

July 11, 2012, 11:20
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Daniel P. Combest
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Quote:
 Originally Posted by zxj160 Yes, the pressure drop is same and only the BCs for pressure are different. But the pressure is also cyclic. If we do not use cyclic, then what type of BC can we define for pressure. Best regards, Jian
0 on the outlet and zeroGradient on the inlet.

See tutorials/incompressible/pisoFoam/les/pitzDailyDirectMapped for inspiration
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July 11, 2012, 11:29
#8
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Jian Zhong
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Quote:
 Originally Posted by chegdan 0 on the outlet and zeroGradient on the inlet. See tutorials/incompressible/pisoFoam/les/pitzDailyDirectMapped for inspiration
Many thanks. I will try it later. By the way, is there any tutorials introducing pressure (p) in the OpenFOAM? Because I cannot understand the meaning of pressure clearly?

 July 21, 2012, 16:27 #9 New Member   April Join Date: Jul 2011 Location: Ann Arbor, MI Posts: 7 Rep Power: 8 Pressure is typically normalized by density in OpenFOAM, hence the units [m^2/s^2]. ScarFace likes this.

 October 9, 2012, 11:28 #10 Senior Member     Mohsen KiaMansouri Join Date: Jan 2010 Location: CFD Lab Posts: 118 Rep Power: 9 This is true only for incompressible flow, but for the compressible flow the dimension of pressure is dimensions [1 -1 -2 0 0 0 0]; check the 0 folder of incompressible & compressible cases in Tutorial

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