with the boundary face normal Â, the length d b of the vector between cell centroid and boundary face centroid, and its normalization êb . Evaluating the difference of the scalar stored in the cell and face centroid not along the direction of the boundary face normal results in an additional flux aligned tangentially to the boundary face. The secondary gradient or cross diffusion term Ṡκ describes the impact of this tangential flux on the boundary flux ωb orientated in the direction of Â. The method of is applied to explicitly compute Ṡκ :