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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.

1)**Conduction**: Conduction is a process of heat transfer from one particle of the body to another in the direction of fall of temperature. The particles themselves remain in fixed position relative to each other. The heat transfer in a metal rod is by conduction.

2)**Convection**: Convection is a process of heat transfer from one particle of the body to another by convection current. In this case, the particles of the body move relative to each other. The heat transfer, in case of liquids and gases, takes place according to convection.

3)**Radiation**: Radiation is a process of heat transfer from a hot body to a cold body, in a straight line, without affecting the intervening medium. The heat of sun reaches to us according to radiation.