# PrimitiveMesh intersection point computation

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 August 9, 2005, 00:33 Mr Henry, I have a question a #1 Member   Leosding Join Date: Mar 2009 Posts: 51 Rep Power: 8 Mr Henry, I have a question about primitiveMesh class. In primitiveMeshCheck.C the private member function checkFacesSkewness(const bool report, labelHashSet* setPtr) const, to compute the intersection point between the vector PN and the face, the code in FOAM as below: scalar dOwn = mag(faceCtrs[faceI] - cellCtrs[own[faceI]]); scalar dNei = mag(faceCtrs[faceI] - cellCtrs[nei[faceI]]); point faceIntersection = cellCtrs[own[faceI]]*dNei/(dOwn+dNei) + cellCtrs[nei[faceI]]*dOwn/(dOwn+dNei); following the code, the point isn't the point wanted, it is the point of the intersection point of the bisector line of vector dOwn and vector dNei with the vecter PN. Is my view right?

 August 9, 2005, 13:56 It is an approximation of the #2 Super Moderator   Mattijs Janssens Join Date: Mar 2009 Posts: 1,416 Rep Power: 16 It is an approximation of the intersection of the owner-neighbour vector with a plane normal to this vector and through the face centre. It is correct for zero skewness.

 August 9, 2005, 14:44 This is one of mine. The issu #3 Senior Member   Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,758 Rep Power: 21 This is one of mine. The issue is that the face integral is done by using the mid-point rule, and in order for the integral to be second order accurate, the "mid-point" has got to be in the middle of the face, right? At the same time, the face value is calculated y interpolation from the cells around it. Therefore, the face integral will be second order only if the PN line hits the face in the centroid; if it misses, discretisation loses accuracy. The skewness error measures this by evaluating the distance between the intersection and the actual centroid, and normalises it with the PN distance. In principle, you can add skewness correction, but for real cases, this just increases the amount of numerical noise you get (because of the increased molecule), so typically it's not worth having. Enjoy, Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

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