Consider an axisymmetric jet emerging from an orifice, located at the origin of a cylindrical polar coordinates , with  being the jet axis and  being the radial distance from the axis of symmetry. Since the jet is in constant pressure, the momentum flux in the  direction is constant and equal to the momentum flux at the origin,
where  is the constant density,  are the velocity components in  and  direction, respectively and  is the known momentum flux at the origin. The quantity  is called as the kinematic momentum flux. The boundary layer equations are
where  is the kinematic viscosity. The boundary conditions are
The Reynolds number of the jet,
is a large number for the Schlichting jet.