The Reynolds Measurement

In 1883, Osborne Reynolds conducted an experiment where dye was injected into a glass tube with water flowing due to gravity.

He studied the conditions of transition from laminar flow to turbulent flow, indicated by the flow patterns in the tube. He analyzed that the flow patterns differed as the flow velocity was changed. At low speed, the dye showed regular straight lines of a laminar flow pattern. When the flow velocity increased, the dye became random (or chaotic) with eddies filling the entire tube, and mixed with water. This random flow pattern is a turbulent flow. He further noticed that the appearance of turbulent flow highly depended on a dimensionless parameter of velocity and length scale, as well as viscosity. This parameter is now known as the Reynolds number.
Figure 1. The Dye Experiment Conducted by Osborne Reynolds