[1] 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.
To compare a real situation (e.g. an aircraft) with a small-scale model it is necessary to keep the important characteristic numbers the same.
As a general example of how dimensionless numbers arise in fluid mechanics, the classical numbers in transport phenomena of mass, momentum, and energy are principally analyzed by the ratio of effective diffusivities in each transport mechanism.
These same quantities may alternatively be expressed as ratios of characteristic time, length, or energy scales.
Droplet formation mostly depends on momentum, viscosity and surface tension.