**Answer is: **

## Reynolds number

**Explanation: **
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.

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