Axisymmetric stagnation strain
= Axisymmetric stagnation strain
{title2=$\phi=W(x^2+y^2-2z^2)$}
A regular axisymmetric <irrotational flow> near a stagnation point on an impermeable tangent plane has equal tangential strain rates. The <incompressibility condition> forces the normal rate to be their negative double. Thus the leading <velocity potential> has the displayed quadratic form. Its streamline invariants are the azimuth and $z(x^2+y^2)$; for incoming flow from positive $z$, $W>0$.