B. Two-Dimensional Neumann Boundary Term

As usual for the finite element method the assembling is performed element wise. Refer to the triangular element and its corresponding notations from Fig. <4.1>. Only elements which lie on the Neumann boundary (triangles with one or more edges as part of the Neumann boundary) are considered. The three edges in the triangle refer to as $\mathcal{C}^e_{12}$ , $\mathcal{C}^e_{23}$ and $\mathcal{C}^e_{31}$ , where the subscript indexes $1$ , $2$ , and $3$ are the element nodes or to $\mathcal{C}^e_{1}$ , $\mathcal{C}^e_{2}$ and $\mathcal{C}^e_{3}$ , where the subscript indexes $1$ , $2$ , and $3$ are the element edges.