The Reynolds number characterizes the flow regime (laminar vs. turbulent) of the wind passing over the pipe or insulation jacket. It is used to determine the forced-convection Nusselt number.

\begin{equation}\label{eq:insulated-pipe-reynolds} \mathrm{Re} = \frac{d \cdot v}{\nu}\end{equation}

Symbols

\(\mathrm{Re}\)	Reynolds number \([\unit{ \unitless}]\)
\(d\)	Outer diameter of the surface (pipe or insulation jacket) \([\unit{ \meter}]\)
\(v\)	Free-stream wind velocity \([\unit{ \meter\per\second}]\)
\(\nu\)	Kinematic viscosity of air at the film temperature \([\unit{ \meter\squared\per\second}]\)