Classification of Fluids

CLASSIFICATION OF FLUIDS

a) Ideal fluid:

This is a fluid which has no viscosity and incompressible. Ideal fluid is an imaginary fluid and do not exist in nature. Ideal fluid is used to compare the properties and behavior with the real fluids.

b) Real fluids:

These are fluid which has viscosity and compressible. All the fluids, in actual practice are real fluids.

c) Newtonian fluid:

A fluid which obeys the Newton's law of viscosity is called Newtonian fluid. This is a real fluid "in which the shear stress is directly proportional to the velocity gradient”. 

d) Non Newtonian fluid:

A fluid which does not obey the Newton's law of viscosity is called Non Newtonian fluid. This is a real fluid in which the shear stress is not directly proportional to the velocity gradient. The Non-Newtonian fluid can be further classified as follows,

1) Pseudo plastic fluid or shear thinning fluid:

A fluid in which the apparent viscosity (i.e initial viscosity) decreases with the increasing rate of deformation is called pseudo plastic fluid or shear thinning fluid. 

Ex: Milk, Gum, blood, Slurries, etc.

2) Dilatant fluid or shear thickening fluid:

A fluid in which the apparent viscosity increases with increasing rate of deformation is called dilatant fluid or shear thickening fluid. Ex: Butter, starch, sugar solution and suspension of sand, etc.

3) Bingham plastics or ideal plastic fluid:

A fluid, which act like solid by with standing a definite shear stress without strain change and later rate of shear strain increases with shear stress after yielding.

Ex: Oil, Paints, Jellies, drilling sanded, etc.

4) Thixotropic fluid or plastic fluid:

A fluid, which possess a definite yield, shear stress and later their shear stress varies non-linear with the rate of strain is called thixotropic fluid or plastic fluid. Ex: Lipstick and printer ink, etc. Figure 1.2 shows the rheological diagram that shows the relationship between the shear stress and velocity gradient for various fluids.