There are aerodynamic forces created by an object moving through
a fluid, which depend on the velocity of the object (or fluid),
the mass and shape of the object, and the viscosity of the fluid.
The Reynolds number is a dimensionless number that expresses the
ratio of inertial (resistant to change or motion) forces to the
viscous (heavy and gluey) forces in the fluid. Thus this equation
can be used to analyze different vortices throughout this site.
"Where NR is the Reynolds number, u is the velocity
of the vortex ring, dh is a characteristic length,
and ν is the kinematic viscosity of air."