Dimensionless numbers in fluid mechanics
Dimensionless numbers (or characteristic numbers) have an important role in analyzing the behavior of fluids and their flow as well as in other transport phenomena. They include the Reynolds and the Mach numbers, which describe as ratios the relative magnitude of fluid and physical system characteristics, such as density, viscosity, speed of sound, and flow speed.
Source: Wikipedia — Dimensionless numbers in fluid mechanics (CC BY-SA 4.0)