Fluid is a substance that is capable of flowing.Fluid It has no definite shape of its own. It assumes the shape of its container. Liquids and gases are fluids.

Or, A fluid is a gas or liquid that, unlike a solid, flows to assume the shape of the container in which it is placed. This occurs because a fluid responds to a shear stress, or a force per unit area directed along the face of a cube of fluid, by flowing, rather than by an elastic displacement as in a solid.

**PROPERTIES OF FLUID **

**1.DENSITY**: Density of a fluid is defined as the ratio of the mass of a fluid to its volume.

**2.SPECIFIC VOLUME**: Specific volume of a fluid is defined as the volume of a fluid occupied by a unit mass or volume per unit mass of a fluid.

**3.VISCOSITY OF LIQUID**: Viscosity is defined as the property of a fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of fluid.

**Types of Fluids: Fluids can be classified into four basic types. They are:**

1. Ideal Fluid

2. Real Fluid

3. Newtonian Fluid

4. Non-Newtonian Fluid

**TYPES OF FLUIDS BASED ON VISCOSITY:**

The fluids may be classified into following five types:

1. Ideal fluid

2. Real fluid

3. Newtonian fluid

4. Non-Newtonian fluid

5. Ideal plastic fluid

**Ideal Fluid**: An Ideal Fluid is a fluid that has no viscosity. It is incompressible in nature. Practically, no ideal fluid exists.

Or, An ideal fluid is a fluid that has several properties including the fact that it is:

- Incompressible – the density is constant
- Irrotational – the flow is smooth, no turbulence
- Nonviscous –(Inviscid) fluid has no internal friction ( η = 0)

Ideal fluid do not actually exist in nature, but sometimes used for fluid flow problems.

We use ideal fluid in 2 equations :

1.**Continuity equation**

2.**Bernoulli equation**

**Real Fluid**: Real fluids are compressible in nature. They have some viscosity. Examples: Kerosene, Petrol, Castor oil.

Or, Fluid that have viscosity(μ > 0) and their motion known as viscous flow. All the fluids in actual practice are real fluids.

3. **Newtonian Fluid**:Fluids that obey Newton’s law of viscosity are known as Newtonian Fluids. For a Newtonian fluid, viscosity is entirely dependent upon the temperature and pressure of the fluid. Examples: water, air, emulsions.

Or, A real fluid in which the shear stress is directly proportional to rate of shear strain (or velocity gradient).

4. **Non-Newtonian Fluid**: Fluids that do not obey Newton’s law of viscosity are non-Newtonian fluids. Examples: Flubber, obleck (suspension of starch in water)

Or, A real fluid in which the shear stress is not proportional to the rate of shear strain.

5. **Ideal Plastic Fluid**:A fluid in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain (or velocity gradient).