Consider a steady plane emerging into the same fluid, a type of submerged jets from a narrow slit, which is supposed to be very small (such that the fluid loses memory of the shape and size of the slit far away from the origin, it remembers only the net momentum flux). Let the velocity be  in Cartesian coordinate and the axis of the jet be  axis with origin at the orifice. The flow is self-similar for large Reynolds number (the jet is so thin that  varies much more rapidly in the transverse  direction than the streamwise  direction) and can be approximated with boundary layer equations.
where  is the kinematic viscosity and the pressure is everywhere equal to the outside fluid pressure.
Since the fluid is at rest far away from the center of the jet
: as ,
and because the flow is symmetric about  axis
: at ,
and also since there is no solid boundary and the pressure is constant, the momentum flux  across any plane normal to the  axis must be the same
is a constant, where  which also constant for incompressible flow.