What is fluid? How are fluids classified?

Fluid : Fluid is a liquid or gas or any material that does not possess any definite shape or size and it can easily gets yield to any external pressure.

At rest, fluid is unable to withstand a shearing or tangential forces acting on it.

When the forces are applied to the fluid, it goes through a continuous change in shape.

Fluids have zero shear modulus i.e. they cannot resist the shear forces applied to them.

Classification of fluid :

Mainly fluid can be classified into 4 classes ;

• Steady fluid or Unsteady fluid

• Compressible fluid or Incompressible fluid

• Viscous fluid or Non – Viscous fluid

• Rotational fluid or Irrotational fluid

• Steady fluid : It is type of fluid whose velocity remains constant at any point while flowing.

• Unsteady fluid : It is the type of fluid whose velocity differs at any two points while flowing.

• Compressible fluid : It is the type of fluid which have mach number between 0.3-1.

• Incompressible fluid: It is the type of fluid which have mach number less than 0.3.

• Viscous fluid: It is the type of fluid which are thick in nature, e.g. oil, shampoo, etc.

• Non – Viscous fluid: It is the type of fluid which are compatible less thicker than viscous fluids , e.g. Liquid helium, etc.

• Rotational fluid: When the angle between the two intersecting lines, in which the fluid is flowing, changes during the flow , it is known as Rotational fluid.

• Irrotational fluid : When the angle between the two intersecting lines, in which the fluid is flowing , does not changes during the entire flow then the fluid is called Irrotational fluid.