componentMixed BC in the motionU file
 

Hi Foamers ,

I want to know about this BC in the motionU file:
type componentMixed;
refValue uniform (0 0 0);
valueFraction uniform (1 0 1);

How does it work? I know it is a combination of Neuman and Dirichlet BCs. Actually I want to know the meaning of refValue uniform (0 0 0); and valueFraction uniform (1 0 1); exactly !!
Could anyone explain it mathematically ?

I appreciate your help.
Thanks and best regards
Sasan.

Any ideas? :(

Greetings Sasan,

1. Which OpenFOAM version are you referring to?
2. Which tutorial case?

Best regards,
Bruno

Hi Dear Bruno,

Thanks for your reply.I am using OP 1.6-ext and about tutorial case , I should say Prof.Jasak has attached a test case about engine simulation in this thread : http://www.cfd-online.com/Forums/ope...changes-4.html
honestly, I need to know this BC in the motionU file in the zero (0) directory. Also I attached this test case here. Please help me to gain a lot of information about this BC.

I appreciate your help,
Thanks and best regards.
Sasan.

Hi Sasan,

When in doubt, look at the source code. Here's what I've done:

1. Went into the folder for the main source code libraries:
   
   Code:

   ---

   cd $FOAM_SRC

   ---

2. Searched for the boundary condition in question:
   
   Code:

   ---

   find . -iname "*componentMixed*"

   ---

3. The first one seems the most likely, namely the files at:
   
   Code:

   ---

   OpenFOAM/fields/PointPatchFields/derived/componentMixed/ComponentMixedPointPatchVectorField.*

   ---

4. The class description in the ".H" file, says this:
   
   Quote:

   Code:     ComponentMixedPointPatchVectorField. The boundary condition is a mix     of a fixedValue and a zeroGradient boundary condition, where a     fixedValue/zeroGradient mix may be different for each direction.     I am still not sure how to do the fixedValue-fixedGradient     combination.  HJ, date deleted

5. Looking at the ".C" file, it's a buit tricky to figure out what-is-what:
   1. The method that has the description "Evaluate patch field" is the one responsible for the actual boundary condition field calculations. The specific code is this one:
      
      Code:

      ---

          vectorField values =
      cmptMultiply(refValue_, valueFraction_)
      + cmptMultiply(internalValues, vector::one - valueFraction_);

      ---

      For your example:
      
      Quote:

      refValue uniform (0 0 0); valueFraction uniform (1 0 1);

      It does this:
      
      Code:

      ---

      (0 0 0)*(1 0 1) + U * ((1 1 1) - (1 0 1)) =
(0 0 0) + U * (0 1 0) =
(0 Uy 0)

      ---

   2. The method that has the description "Set boundary condition to matrix", seems to do a manipulation to the equation matrix, by hacking into the mesh points for the patch which was assigned this boundary condition, by setting a constraint for each point on the patch. This requires us to look for what the class "MatrixType<vector>::ConstraintType" actually does.
6. Another find command:
   
   Code:

   ---

   find . -iname "*MatrixType*"

   ---

   Gets me nothing... Ooops, that's because "MatrixType" is a object name place-holder, given that this is a template class. OK, let's do a broader search, this time searching inside certain files:
   
   Code:

   ---

   find . -name "*.[CH]" -type f | xargs grep -sl "ConstraintType"

   ---

7. Looks like the file:
   Code:

   ---

   OpenFOAM/fields/PointPatchFields/PointPatchField/PointPatchField.H

   ---

   is a good candidate. But it's not it. It should be:
   Code:

   ---

   OpenFOAM/matrices/blockLduMatrix/BlockLduMatrix/BlockLduMatrix.H

   ---

8. Ouch... this isn't good:
   
   Code:

   ---

   template<class Type>
class BlockConstraint;
//...
typedef BlockConstraint<Type> ConstraintType;

   ---

   Now I gotta go look for where "BlockConstraint" is properly defined...
9. Another search with find, leads me to these files:
   
   Code:

   ---

   OpenFOAM/matrices/blockLduMatrix/BlockLduMatrix/BlockConstraint/BlockConstraint.*

   ---

   What we're looking for is this constructor:
   
   Code:

   ---

           //- Construct from components
        BlockConstraint
        (
            const label row,
            const Type value,
            const Type& fixedCmpts = pTraits<Type>::one
        );

   ---

   The meaning of each variable:
   
   Code:

   ---

           //- Matrix row ID
        label rowID_;

        //- Fixed value
        Type value_;

        //- Fixed components (0-1) 1 = fixed, 0 = free
        Type fixedComponents_;

   ---

10. All of this searching because of #5.b:
    
    Code:

    ---

            // create a constraint
        typename MatrixType<vector>::ConstraintType bc
        (
            curPoint,
            refValue_[pointI],
            cmptMag(valueFraction_[pointI])
        );

    ---

I hope you've understood all of that, because my brain just went: Anyway, I believe that the answer is given on #4.a.

Best regards,
Bruno

Dear Bruno,

I really appreciated your help. I don't know how to thank you, honestly.
Thank you very much for your attention. :)

Best regards,
Sasan.