For infinite plates, there would be no variation of in the z- direction i.e. . With these conditions, the Navier-Stokes equation c an be written as,
                                    (19a-c)
Eqs. (19-b and c) can be integrated to yield,
                                    (20)
At , i.e. pressure varies hydrostatically in y- direction only. Thus, Eq. (19-a) can be written as,
                                    (21)
Integrating Eq. (21) two times,
                                    (22)