The full gradient of a property \(Q\) at a face can can be interpolated from the cell-based gradient. The surface-normal contribution is then represented by:

\[\snGrad Q = \vec{n} \dprod \left( \grad Q \right)_f\]

where \(\vec{n}\) is the face unit normal. The stencil for the gradient calculation at the face, \(f\), between cells P and N is described by the following figure: Surface-normal gradient schematic where the vector \(\vec{d}\) joins the two cell centres. A variety of schemes are available that differ in their application based on the angle, \(\theta\), between the \(\vec{d}\) and \(\vec{n}\) vectors, representing the degree of non-orthogonality.


Surface-normal gradient schemes are specified in the fvSchemesfile under the snGradSchemes sub-dictionary using the syntax:

    default         none;
    snGrad(Q)       <snGrad scheme>;



Further information

Source code: