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Boundary conditions: inlet and outlet

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Old   May 25, 2010, 09:19
Default Boundary conditions: inlet and outlet
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Hello,


Does anyone know where I can find how to use the boundary conditions?
I've looked at the user guide which tells me that a range of BCs are available at src/finiteVolume/fields/fvPatchFields/derived. But I don't know how to use them. I don't understand what goes with what (for example, a specific BC goes with mixed or calculated?).

I'm studying a flow with an inlet (with a definite velocity) and an outlet. I don't know which BC I have to put for these 2 surfaces (inlet and outlet). As there is no gravity in my problem, I guess I should take a BC which fixes the pressure to zero for the outlet, but I don't know which one.

Could anyone help me? thanks a lot in advance.
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Old   May 26, 2010, 06:06
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For the velocity, you can put:
Code:
inlet 
     {
      type   fixedValue;
      value  uniform (your inlet value, vector);
      }

outlet
       {
        type zeroGradient;
        }
and for the pressure:
Code:
inlet 
     {
      type   zeroGradient;
      }

outlet
       {
       type   fixedValue;
       value  uniform 0;
       }
These conditions works with laminar regime.

Regards

David
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Old   May 26, 2010, 06:07
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Repeated post

Last edited by David_010; May 26, 2010 at 06:08. Reason: Repeated post
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Old   June 25, 2010, 01:56
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Hey David,

So what happens when you switch to turbulent? Can you keep the same boundary fields??

T
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Old   June 28, 2010, 05:56
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Hi Foamers,

I have the same question : are you sure it works only in a laminar case, one told me to use such conditions in my turbulent case.
If it does not work in a turbulent case, do you know what kind of boudnary conditions would work so ?

Cheers

PS : Finally those conditions did not work in my DNS case, it gives me a field without any variation ... Help ^^

Last edited by NorbertB; June 28, 2010 at 07:01.
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Old   June 28, 2010, 18:00
Default using outlet velocity profile as inlet
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Hi All,

Does anyone know how to specify (in COMSOL) an inlet velocity profile from a previously solved model's outlet velocity profile?

I've been trying to create a Fourier Series representation of the profile over time, but I am having trouble doing this with a space and time dimension together.

Any help greatly appreciated. Thank you!
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Old   June 29, 2010, 03:22
Default To “using outlet velocity profile as inlet”
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Quote:
Originally Posted by labarc35 View Post
Hi All,

Does anyone know how to specify (in COMSOL) an inlet velocity profile from a previously solved model's outlet velocity profile?

I've been trying to create a Fourier Series representation of the profile over time, but I am having trouble doing this with a space and time dimension together.

Any help greatly appreciated. Thank you!
For this, the form provides two ways, one is mapFields, and the other is directMappedPatch. With the former, I mapped the whole field, but not outlet to inlet. Maybe I take a wrong understanding, you can have a try. I mapped the outlet velocity to the inlet using the directMappedPatch boundary. You should change two files:
1. in constant/polyMesh/boundary
inlet
{
type directMappedPatch;
nFaces #;
startFace #;
offset (0 0 1); // offset (0 0 1) is the distance between the inlet and the outlet

sampleMode nearestPatchFace;
samplePatch outlet;
}


2. in 0/U
inlet
{
type directMappedFixedValue;
value (0 0 0);
setAverage true;

average (0 0 2);
}

Then you can check whether you want results.
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Old   June 29, 2010, 03:45
Default problem with boundary
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I also have problems on defining the bounary. For pressure, when using zeroGradient for inlet, everything goes well. After changing zeroGradient to calculated, I get this message:
valueInternalCoeffs cannot be called for a calculatedFvPatchField
on patch inlet of field p in file "/home/flow/OpenFOAM/flow-1.6/run/pipe/tphase/ke/0/p"
You are probably trying to solve for a field with a default boundary condition.

From function calculatedFvPatchField<Type>::valueInternalCoeffs( const tmp<scalarField>&) const
in file fields/fvPatchFields/basic/calculated/calculatedFvPatchField.C at line 145.


can anyone tell us how to use the boundary conditions?
Thanks

beauty
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