I want to advect a surface using the fluid velocity using semi lagrangian advection. The fluid is in a box, that is the fluid is surrounded by solid walls in 4 directions. I applied "no slip boundary condition" for fluid and neuman boundary condition for pressure. But the problem is that my surface is going through the wall. At the final frame, only the bottom most row contains part of the surface, other parts got disappeared. I also dont find any condition in semi lagrangian advection to stop moving the surface. Would anyone plz suggest?