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June 21, 2012, 09:30 
Boundary Conditions for Flow Problem

#1 
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Hello,
I´ve some questions about how to implement boundary conditions. I have a domain, where something flows through, in my case I´m using rhoCentralFoam. At the inlet I´ve a fixed velocity, a fixed temperature and a fixed pressure. At the outlet I want to set a fixed pressure and give no defaults to velocity and temperature. These should be calculated from the neighbour cells in the timestep before. I hope, you know what I mean How can I do this in OpenFoam? Is there a possiblity with the standard bc or groovyBC? regards treima 

June 21, 2012, 09:39 

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Bernhard Gschaider
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June 21, 2012, 09:52 

#3 
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Thanks.
I´ve tried fixedValue and zeroGradient. If I set these outlet conditions, U  zeroGradient p  fixedValue T  zeroGradient the case gives a floating exeption and problemes with the courantnumber. If I set U  zeroGradient p  zeroGradient T  zeroGradient the case works, but doesn´t match to my ideas. The best think would be, if it´s possible to set a "new" boundary condition for every timestep. But if this isn´t possible i´ve to setup a new case with other physical relations. I´m just wondering that this case haven´t discussed before regards treima 

June 21, 2012, 10:07 

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June 22, 2012, 07:20 

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My inlet conditions are
p  fixedValue, value uniform 1, T  fixedValue, value uniform 1, U  fixedValue, value uniform (3 0 0). And my outlet conditions just should be p  fixedValue, value uniform 1, T and U should be calculated from the domain in the timestep before. 

June 22, 2012, 15:11 

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June 25, 2012, 02:34 

#7 
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Thank you for your advice.
I´ve tried to fix the pressure at the outlet and set zeroGradient at the Inlet, but I still can´t calculate a solution because of a "floating execption"... Perhaps this is caused by my geomety, which you can see below. My idea was to set at inlet: p  fixedValue, T  fixedValue, U  fixedValue outlet: p  fixedValue Gamma_free and Gamma_fixed: slip condition (u*n=0). I was searching for good choices for the remaing conditions. I want to use these conditions, because they are just the first step in a bigger calculation. The aim is to do a very simple shapeoptimization with adjoint equations for this case. On Gamma_free should be a given pressuredistribution. 

June 25, 2012, 05:34 

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July 2, 2012, 05:21 

#9 
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I´ve solved this problem.
If you take a very low temperature, you have a machnumber > 1, so you have to set just inletconditions and no condition at the outlet. Now I´m taking a appropriate proportion between velocity and temperature, so that Ma = (U^2)/T < 1 for all timesteps and every cell. Now it works very well. Now, in my next step, I want to implement the adjoint equation. I have calculated the adjoint equations and boundary conditions. In the subsonic case, for example, I have just one boundary condition, a function dependent of all adjoint variables. A small example is, with a the adjoint variables, a1 + 2 a2 + 3 a3 + 4 a4 = 0. Is it possible to implement this in OpenFoam? I´ve a underdetermined equation system as a boundary condtion... 

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