Ever wondered from where does water come into the taps? Why the aircraft or the birds don’t fall on to the ground? How does the fuel system work in your vehicles? How do pebbles reach the bottom of the cylindrical tube? The list goes on and to all these questions the answer remains the same: fluid mechanics. It is a branch of Physics which explains the mechanism of fluids and the forces acting on them. Liquids (water) and gases (air) are the fluid. Fluid mechanics can be divided as:

- Fluid statics: Also known as hydrostatics is the study of fluid when at rest and the pressure exerted or experienced by the body that is immersed in the fluid.
- Fluid dynamics: The study of fluid when in motion and the forces acting on the object. It is further divided as aerodynamics and hydrodynamics.

**Terminologies in fluid mechanics**

- Viscosity of a fluid: It is defined as the fluid’s resistance to flow. Honey is an example of a high viscosity liquid whereas water is an example of low viscosity liquid.
- Deborah number: It is defined as the ratio of the relaxation time of the material to time scale of deformation and is also referred to as a De number. Deborah number can be divided as:
- Low Deborah number: When there is relaxation time for the material. Corn starch is an example of low De number.
- High Deborah number: When the material behaves elastically. Water mixture is an example of high De number.

**Classification of flows**

- Laminar: Laminar flow is a part of fluid dynamics where the flow occurs in parallel layers. It is also known as streamline flow and fluid properties like velocity, pressure, direction remains constant. Flow is said to be laminar if the Reynolds number is less than 2000.
- Turbulent: Turbulent flow is a part of fluid dynamics where the flow is having irregular fluctuations and the flow properties are not constant. Reynolds number for turbulent flow is greater than 4000.
- Newtonian fluid: When the viscous stress is proportional to the local strain rate. It is known as Newton’s law of viscosity. Examples of Newtonian fluid are water, air, alcohol and glycerol.
- Non-Newtonian fluid: When the fluid violates Newton’s law of viscosity, it is known as Non-Newtonian fluid. Curd, honey, toothpaste, ketchup are examples of Non-Newtonian fluid.

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