Types of Fluids

Fluids are classified into the following types

Ideal fluid: A fluid said to be ideal if it is both incompressible and have no viscosity. Its bulk modulus is infinite. It has no surface tension. Such fluid is only imaginary and does not exist in nature.

Real fluid: A fluid that possesses viscosity, finite compressibility and surface tension.

Newtonian fluid: A real fluid that obeys the Newton’s law of viscosity. In this type of fluid shear stress is directly proportional to shear strain (or velocity gradient).

Non-Newtonian fluid: A real fluid that doesn't follow the Newton’s law of viscosity.
Ideal plastic fluid: A fluid in which shear stress must overcome a yield value and then shear stress is directly proportional to the rate of shear strain.

Types of non-Newtonian fluids

In this kind of fluid, the relationship between shear stress and velocity gradient is given by the equation
Non-Newtonian fluid classified into
Time independent: (i) Dilatant fluid (ii) Bingham fluid (iii) Pseudoplastic fluid
Time-dependent: (i) Thixotropic fluid (ii) Rheopectic fluid

Dilatant fluids
n > 1, B = 0
Also called shear thickening fluid
Eg. Quicksand, Butter

Bingham fluid
n = 1, B ≠ 0
Act like solid by withstanding a certain amount of stress without a change in strain.
Eg. Toothpaste, water suspension of clay, jellies

Pseudoplastic fluid
n < 1, B = 0
Eg. Paint, slurry, blood, milk

Thixotropic fluids
n < 1, B is a function of time, B = f(t)
Viscosity increases with time.
Eg. Enamels, printer ink

Rheopectic fluids
n > 1, B = f(t)
Viscosity decreases with time.
Eg. Gypsum solution
Read:
- Types of fluid flow
- Flow Lines - Pathlines, Streamlines, Streaklines, Timelines, Stream Tube