Reynolds number Re

• Reynolds gave us a bridge between the different fluids and different scales

Reynolds number is a dimensionless number linking viscosity, density, and a reference length. The reference length can be :

• The inside diameter of the duct (for flows in pipes)
• For the study of the geometric body drag, unshaped, this reference length is the width of the frontal area (perpendicular to flow)
• For the study of the lift, the drag of profiled bodies, this length is measured parallel to the flow.
• For the Study of the drag friction flat surface, the reference length is the length of the wetted area, taken parallel to the flow.

Re=V.L/u

Re = ((average speed) x (reference length)) / (kinematic viscosity of the fluid)

or also

Re = ((mass per units volume) x (average speed) x (reference length)) / (dynamic viscosity of the fluid)

The Reynolds number is used to determine the flow regime, Laminar, Transitional and Turbulent and equations to use.