# boundary condition: alpha*dp/dt = u&n

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 July 22, 2013, 05:39 boundary condition: alpha*dp/dt = u&n #1 New Member   Andreas Ruopp Join Date: Aug 2009 Location: Stuttgart / Germany Posts: 29 Rep Power: 8 Dear Foamers, normally, on wall boundaries for example, we must guarantee: Code: `0 = U&n` where U is the velocity vector and n is the patch normal surface vector, and the inner product has to be zero. On a specific patch, I impose a diriclet boundary condition for the pressure. For the velocity, I want to have: Code: `alpha*dp/dt = U*n` I'm thinking about treating this patch as a modified free slip wall, but I'm wondering, how I can impose that mentioned boundary condition for U. A slip wall uses in the end the basicSymmetryFvPatchField method, resulting in 0 = u&n Code: ```tmp nHat = this->patch().nf(); 00116 00117 const Field iF(this->patchInternalField()); 00118 00119 Field::operator= 00120 ( 00121 (iF + transform(I - 2.0*sqr(nHat), iF))/2.0 00122 ); 00123 00124 transformFvPatchField::evaluate();``` For example if the cell value next to patch face is in 2D: if=(1 1) and nf is normal patch face vector pointing outward with nf=(0 1) Then evaluate returns a vector (1 0) resulting from, ((1 1) + transform([1 0;0 1] - [0 0 ;0 2],(1 1)])/2 ok. And then n&U is zero, hence 0=1*0+0*0 and 0 = U&n is fullfilled. But how can Code: `alpha*dp/dt = U*n` be achieved? Any help will be appreciated! Best regards, Andy

 July 23, 2013, 05:59 After a long night... direction mixed boundary condition approach, maybe?! #2 New Member   Andreas Ruopp Join Date: Aug 2009 Location: Stuttgart / Germany Posts: 29 Rep Power: 8 Hello, here my update, maybe things become clearer: since I want to impose Code: `alpha*dp/dt = U*n` where alpha*dp/dt represents a velocity term for that boundary in normal direction to that face, I was digging in under .../src/finiteVolume/fields/fvPatchFields/basic/ where I found directionMixedFvPatchField.C If I understand it right: For a vector (like in my case it is velocity U), this boundary condition is decomposed into the normal and tangential component. I have to set three reference values for that boundary condition (good description under DirectionMixed mixed bc: 1) refValue = which will be the value on face in normal direction 2) refGradient = will be enforced in the patch tangential direction 3) valueFraction = which is the symmetrical Tensor, which I have to define for transformation purposes I think this b.c. is a good starting point for manipulating my boundary condition for U... Best regards, Andy

 Tags boundary, condition, slip, treament, wall

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