The important dimensionless numbers in fluid mechanics are
1. Reynold’s number
2. Froude’s number
3. Weber’s number
4. Euler’s number
5. Mach’s number or Cauchy’s number

Reynold’s number: It is the ratio of inertia force to the viscous force.

Froude’s number: It is the ratio of inertia force to the gravity force.

Weber’s number: It is the ratio of inertia force to the surface tension force.

Euler’s number: It is the ratio of inertia force to the pressure force.

Mach’s number or Cauchy’s number: It is the ratio of inertia force to the elastic force.