[sasanghomi]

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.

[sasanghomi]

Any ideas?

[wyldckat]

Greetings Sasan,

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

Best regards,
Bruno

[sasanghomi]

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.

[wyldckat]

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:
   

   cd $FOAM_SRC

2. Searched for the boundary condition in question:
   

   find . -iname "*componentMixed*"

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

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

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

       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:
      

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

      For your example:
      

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

      It does this:
      

      (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:
   

   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:
   

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

7. Looks like the file:

   OpenFOAM/fields/PointPatchFields/PointPatchField/PointPatchField.H

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

   OpenFOAM/matrices/blockLduMatrix/BlockLduMatrix/BlockLduMatrix.H

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

   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:
   

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

   What we're looking for is this constructor:
   

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

   The meaning of each variable:
   

           //- 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:
    

            // 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:

I don't want to look at C++ for the rest of the day.

Anyway, I believe that the answer is given on #4.a.

Best regards,
Bruno

[sasanghomi]

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.