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# Definition of fluid and it’s description

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).

## Hydrodynamics| Introduction and Basic concept

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