Reynolds number is the dimensionless variable, providing information of flow regime of fluid. It is a ratio of inertial and viscous forces. Up to certain (critical) value of  a flow is laminar and after its overrun a flow becomes turbulent. Reynolds number  is defined:

(23.1)

In a pipe-like systems the flow regime is laminar for   . The flow regime is transient for     . And the flow regime is fully turbulent for   .