## Reynolds number $\mathrm{Re}=\frac{uL}{\nu}=\frac{\rho uL}{\mu}=\frac{\textrm{inertial force}}{\textrm{viscous force}}$ It is the ratio of inertial forces and viscous forces, therefore helps to indicate the flow pattern -- low $\rm Re$ means laminar flow while high $\rm Re$ corresponds to turbulent flow. Here $\rho$ is density, $u$ is velocity, $L$ is characteristic length, $\nu$ is [kinematic viscosity](https://en.wikipedia.org/wiki/Kinematic_viscosity), $\mu$ is [dynamic viscosity](https://en.wikipedia.org/wiki/Dynamic_viscosity), $\nu = \frac{\mu}{\rho}$. One typical criteria for laminar and turbulent flow is whether $\rm Re$ less than 2000 (laminar) or greater than 4000 (turbulent), between 2000 and 4000, the flow is considered as transit. >[!Info] >See more on wiki: https://en.wikipedia.org/wiki/Reynolds_number.