different boundary conditions for flat plate
 

dear all,

so far I have seen three approaches for the boundary conditions of flow over a flat plate (for the top one):
1. free stream surface with velocity perpendicular to flat plate = 0
2. inflow bc on the top with positive slope of the mesh on the top
3. outflow boundary condition on the top

which is the best to choose ?
can I expect any differences in the results ?