Re: [ff3d-users] Extraction of the unit normal vector, or normal/tangential derivatives.

[Stephane Del Pino]

Dear Mykola.

A way that I already used for slip condition consists in penalizing the 
variational formula in order to impose
        u.n = 0
on the boundary.

On the variational point of view, we added 
        1/\epsilon*\int_{boundary}( u.n * v.n)
to the bilinear part.

in ff3d's language, it means something like that:
        +int[ref](u*nx/eps*U*nx + v*ny/eps*U*nx + w*nz/eps*U*nx)
        +int[ref](u*nx/eps*V*ny + v*ny/eps*V*ny + w*nz/eps*V*ny)
        +int[ref](u*nx/eps*W*nz + v*ny/eps*W*nz + w*nz/eps*W*nz)

where 'ref' is the reference of the boundary, eps is to be taken small and 
U,V,W are the test functions.

Best regards,
Stéphane.