Conserved momentum flux of a laminar round jet
= Conserved momentum flux of a laminar round jet
For axial velocity $w(r,z)$ in the boundary-layer approximation,
$$
M=\int_0^\infty r w^2\,dr
$$
is independent of $z$. If $W$ and $\delta$ are characteristic centre-line speed and width, then $W\delta\sim M^{1/2}$, while the momentum balance gives $W\delta^2\sim\nu z$. Thus
$$
\delta\sim\frac{\nu z}{M^{1/2}},
\qquad
W\sim\frac{M}{\nu z}.
$$