- Reynolds gave us a bridge between the different fluids and different scales
- 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/uRe = ((average speed) x (reference length)) / (kinematic viscosity of the fluid)
or alsoRe = ((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.