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

[Mykola Tasinkevych]

      Dear Stéphane,

  thank you for your advise! 

  As I understand, your penalty term is forcing the  flow field "u" only to be parallel to the surface of obstacle, but 
 does not prescribe a specific slip,  "u = slip on [ref] ", where slip  is some  function.

  One, in principle, can  also penalize the variational formula in order to impose
         u = slip 
  on the boundary.   Is it also possible to  impose this condition rigidly, as a boundary
  condition?

   Thank you in advance, and
   the best regards,
   Mykola.