Segment V7.1: Reynolds Number

(Related to Textbook Section 7.6 - Common Dimensionless Groups in Fluid Mechanics)

Important dimensionless groups can frequently be given useful physical interpretations. For example, the Reynolds number is an index of the ratio of inertia forces to viscous forces in a moving fluid.

For a rotating tank containing a very viscous fluid, which gives a small Reynolds number, viscous forces are dominant. Thus, when the tank is suddenly stopped fluid particles also suddenly stop due to the dominance of viscous forces over inertia forces. Correspondingly, when a low viscosity fluid is in the tank, which gives a much higher Reynolds number, inertia forces are dominant. When the tank suddenly stops the fluid particles continue to move.