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# Define Fluid and Types of Fluids in Fluid Mechanics

In this post, you’ll learn what is fluid and different types of fluids.

## Fluid and its types

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The substance that has a tendency to flow is called as fluid. Generally, fluid is defined as a substance which is capable of spreading and changing its shape, according to is surroundings, without offering internal resistance.

In fluid mechanics, fluid is defined on the basis of its behaviour under the application of external forces. A solid, when subjected to a shearing force, deforms until the internal shear resistance equals the externally applied stress.

Within elastic limits or removal of the external force, the formation disappears and solid comes to its original shape. However, this is not so in the of fluids. When a small shearing force is applied externally to the fluid it starts deforming and continues to do as long as applied force exists.

On removal of the external force, the fluid does not come to its original position. On the basis of this property, fluid can be defined as a substance which deforms continuously when subjected to external shearing force. This continuous deformation of fluid is known as flow.

## Types of Fluids

Based on how the property of viscosity of fluid changes in various fluids, they are divided into 5 types.

These have been described in details as follows:

1. Ideal fluid
2. Real fluid
3. Newtonian fluid
4. Non-Newtonian fluid
5. Ideal plastic fluid
6. Incompressible fluid
7. Compressible fluid

### Ideal Fluid

The fluid, which is incompressible and has no viscosity or no friction, is known as an ideal fluid. Ideal fluid is only an imaginary fluid.

### Real Fluid

A fluid, which possesses viscosity or friction, is known as real fluid.

### Newtonian Fluid

A real fluid, in which the shear stress is directly proportional to the rate of the shear strain, is known as a Newtonian fluid. Fluids obeying Newton’s law where the value of viscosity (μ) is constant are known as Newtonian fluids.

### Non-newtonian Fluid

A real fluid, in which the shear stress is not proportional to the rate of the shear strain, is known as a Non-newtonian fluid. Fluids in which the value of viscosity (μ) is not constant are known as non-newtonian fluids.

### Ideal Plastic Fluid

A fluid, in which the shear stress is more than the yield value and shear stress is proportional to the rate of shear strain, is known as ideal plastic fluid.

### Incompressible Fluid

A fluid, in which the density of the fluid does not change which changes to an external force or pressure, is known as an incompressible fluid.

### Compressible Fluid

A fluid, in which the density of fluid changes while the change in external pressure or force, is known as compressible fluid.

## Graphical Representation of Fluids 