Scales of free convection around a vertical cylinder. http://dx.doi.org/10.1088/0143-0807/29/6/017

Abstract

The natural scales of the laminar steady-state free convection flow regime
surrounding an isothermal vertical cylinder are established. It is shown that
nondimensionalizing the momentum and energy equations in terms of the
Rayleigh or Boussinesq numbers allows the use of the Prandtl number as a
criterion to establish whether the motive buoyancy force is mainly balanced
by inertia or by friction. Instead, in the conventional formulation based on the
Grashof number, this distinction is not possible.