The Significance of Reynolds Number, Re

THE SIGNIFICANCE OF REYNOLDS NUMBER, Re

In the Reynolds’ experiment, described previously, progressive increase in velocity caused a change from laminar to turbulent flow. This change would also have occurred if the diameter of the tube was increased while maintaining the velocity or if the fluid was replaced by one of higher density. On the other hand, an increase in viscosity could promote a change in the opposite direction. Obviously, all these factors are simultaneously determining the nature of flow. These factors, which alone determine the character of flow, combine to give some value of Re. This indicates that the forces acting on some fluid element have a particular pattern. If some other geometrically similar system has the same Re, the fluid will be subject to the same force pattern.

More specifically, the Reynolds number describes the ratio of the inertia to viscous or frictional forces. The higher the Reynolds number, the greater is the relative contribution of inertial effects. At very low Re, viscous effects pre-dominate and the contribution of inertial forces can be ignored. A clear example of the changing contribution of viscous and inertial or momentum effects and the resulting changes in the flow pattern is given in Figure 2.11. The Reynolds number can also characterize flow in this quite different system.