For an orthogonal computational cell, Â and êb are equal resulting in Â• Â Â•êb = 1, so that Ṡκ turns to zero and can be neglected as suggested by . However, in a non-orthogonal computational cell, Â can differ from êb and the secondary gradient must be considered in the calculation of the boundary fluxes. In a turbulent flow regime modeled with the RANS approach, the impact of the turbulence on the boundary fluxes can not be resolved by the mesh. Therefore, the transfer of the scalar from the cell centroid to the boundary face centroid is described by Newton's law leading to an identical formulation of the boundary flux as given in Equation for laminar flow, but without the secondary gradient. In contrast to the laminar case, the transfer parameter κ depends not only on the transport parameters Γ c and the geometry of the computational cell, but also on the local flow regime. The transfer parameter is computed as a function of the velocity scale u * and the dimensionless scalar ϕ + . These values are calculated with the blended wall function for velocity according to and for scalar transport suggested by . The applied wall functions are comprehensively presented in the supplementary information (SI Section S3). Table summarizes the physical quantities used in the calculation of boundary heat and species mass fluxes according to the described general formulations of the scalar boundary flux.