When we are analysing the symmetry condition and considering as unknowns the u and v Cartesian components of the velocity vector we notice that the momentum equation in x direction the h axis must be parallel to y axis, where the condition is applied and the velocity u is perpendicular to the boundary of symmetry. In an analogous way, for the momentum equation in y direction the x must be parallel to the x-axis, where the boundary condition is being applied (by definition the velocity v must be perpendicular to the symmetry boundary).

Now, if we look at (8.2.3.1), and defining a normal derivative equal to zero at a h surface, we have: