I found from the help files that "For planar 2D geometries, do not use free slip walls; doing so will hurt accuracy because control volume gradients will not be computed." I do not understand exactly why it reduce accuracy, could anyone please explain this to me in deep detail? Thanks.

Atit

If you have a 2D geometry which is one element thick, make sure you use symmetry conditions rather than free slip walls on the transverse planes. If you use slip walls, you will have only first order accuracy; for example, the advection scheme will be first order upwind because the gradients used for second order corrections are zero.