What is a fluid?
Matter around us exists in three phases (excluding plasma)
From the above three phases liquid and gas are combinedly known as fluids.
The main difference between fluids and solid lies in their ability to resist shear stresses.
When a constant shear force is applied, a solid eventually stops deforming, whereas a fluid never stops deforming and approaches a constant rate of strain
(ref. fluid mechanics by Cengel & Cimbala)
Types of fluids
On the basis of shear and velocity gradient relationship, fluids can broadly be classified into following five types.
Image Source: NPTEL
Ideal fluid is incompressible and has no viscosity. It is an imaginary fluid and does not exists in reality.
Real fluid possesses viscosity. In practice all fluids are real.
A real fluid which obeys Newton’s law of viscosity is known as Newtonian fluid.
Note: According to Newton’s law of viscosity shear stress is directly proportional to strain rate.
A real fluid which does not obeys Newton’s law of viscosity is known as Non-Newtonian fluid.
Ideal plastic fluid
A fluid, in which shear stress is more than the yield value and shear stress to proportional to the rate of shear strain, is known as real plastic fluid.
Properties of fluids
There are some basis properties associated with fluids which help us to understand them in a better way.
It is the ratio of applied shear stress to the obtained shear strain rate in fluids. Viscosity offers resistance to the movement of fluid layers.
It is the term used in fluids which is analogues to the term stress used in solids. Both are the ratio of applied force and area of surface (on which the force is applied)
Specific gravity of fluids is defined as the ratio of density of a particular fluid to the density of standard fluid (generally water).
Examples of fluids
- Any other gas or liquid