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

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

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

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

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

Venturimeter is an instrument used to measure the discharge of liquid flowing in a pipe. Venturimeter consists of three parts
1. Convergent cone
2. Throat
3. Divergent cone
The length of divergent cone is made about three to four times longer than that of the divergent cone in order to avoid tendency of breaking away the stream of liquid and to minimise frictional losses.
For measuring flow by a venturimeter, it can be installed in any direction and in any location.