Reynolds number is a dimensionless number formed from different flow properties. Essentially, it expresses the ratio of inertial to resistive effects of the flow.

Reynolds number is most important in giving a criterion for the transition from laminar to turbulent flow. For a value smaller than the crtitical Reynolds number, the flow is laminar.

rho: density of liquid

eta: viscosity of liquid

v: flow speed

D: characterisitc length (in the case of a pipe, this is the diameter)

laminar flow law, turbulent flow laws, viscosity,

If the Reynolds number is smaller than a critical value (which depends upon the fluid and the pipe or duct or channel) the flow will be laminar. Above that there is a transition to turbulence.

Reynoldszahl