# Reynolds number - Flow in a pipe with mass flow rate

## Description

For flow in a pipe or tube, the Reynolds number is generally defined as presented here.

For shapes such as squares, rectangular or annular ducts where the height and width are comparable, the characteristic dimension for internal-flow situations is taken to be the hydraulic diameter.

This result is generalized to non-circular channels using the hydraulic diameter, allowing a transition Reynolds number to be calculated for other shapes of channel.

Related formulas## Variables

Re | Reynolds number (dimensionless) |

m | Mass flow rate (kg/s) |

D_{H} | Hydraulic diameter (m) |

μ | Dynamic viscosity of the fluid (Pa*s) |

A | Cross-section (m^{2}) |