FLUENT 6 - Species appear to diffuse across thin solid walls.

Species appear to diffuse across a solid wall with zero thickness. In this specific case no reactions were occurring, there were only two species and the BC at the wall was zero-diffusive flux and coupled heat transfer. The apparent leak occurred where the wall formed an angle, either acute or obtuse. The mesh is axi symmetric with good quality triangular cells everywhere, the solver is the pressure-based solver and the density is computed through the incompressible-ideal-gas law.
The same problem occurred when the BC at the wall was changed to zero-mass-fraction for the leaking species. A third species was added to make sure that it was not a truncation error being accumulated on the third species in the list. Also, it was verified that the problem was not caused by a post-processing error by switching off the "Node Values" contour plotting tab. It was found that the problem was created by the evaluation of the gradients at the cell faces. The "leakage" problem exists only when the gradient is evaluated through the Green-Gauss node-based algorithm. It does not exist when computing the gradient using the Green-Gauss cell-based or the Least-Squares methods.

Solution: If you observe non-physical species diffusion across walls when using Green-Gauss node-based gradients please switch to the default Green-Gauss cell-based or Least Squares cell-based gradient method and turn the "Node Values" switch off.

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