What is the classification of fluid

Classification of fluid

There are various criteria for the classification of fluid

1.     Based on the behavior of fluids under the action of shear stress

2.     Based on the behavior of fluids (i. e. variation in density)under the action of externally applied pressure and temperature

Based on the behavior of fluids under the action of shear stress

• A fluid possesses a definite density at a given temperature and pressure.       Hence based upon the behavior of fluids (i. e. variation in density)under the action of externally applied pressure and temperature, the fluids are classified as 

1. Compressible fluids

If the density of the fluid is affected appreciable by changes in temperature and pressure, the fluid is called as compressible fluid.

• Example: Gases

2. Incompressible Fluids

If the density of the fluid is not appreciably affected by moderate changes in temperature and pressure, the fluid is called as the incompressible fluid.

• Example: Liquid

Based on the behavior of fluids (i. e. variation in density)under the action of externally applied pressure and temperature

• Fluids obey Newton's law of viscosity which gives the relation between shear stress and shear rate or velocity gradient. Hence based upon the behavior of fluids under the action of shear stress, the fluids are classified as 

1. Newtonian Fluids

Fluids which follows Newton's law of viscosity are called as  Newtonian fluids.

• Hence the ratio of the shear stress to the rate of shear or shear rate is constant.
• The viscosity of Newtonian fluid independent of shear rate.
• Example:- All gases, air, liquids, such as kerosene, alcohol, glycerin, benzene, hexane either, etc solutions of inorganic salts and sugar in water

2. Non- Newtonian Fluids

Fluids which do not follow Newton's law of viscosity is called as  non - Newtonian fluids.

• Hence the ratio of the shear stress to the rate of shear or shear rate is not constant and is considered as a function of the rate of shear.
• The viscosity of Non - Newtonian fluid varies with shear rate.
• Example:- Toothpaste, paints, gels, jellies, slurries and polymer solution

Types of Non- Newtonian fluids based on time-independent behaviors (Those which are independent of the duration of shear).

• The mathematical model for describing the mechanistic behavior of a variety of commonly used non-Newtonian fluids is the Power-Law model which is also known as the Ostwald-de Waele model (the name is after the scientist who proposed it). 
• According to Ostwald-de Waele model

• When n = 1, m equals, the model identically satisfies Newtonian model as a special case. 
• When n < 1, the model is valid for pseudoplastic fluids.
• When n > 1, the model is valid for dilatant fluids.
• There are some substances which require yield stress for the deformation rate (i.e. the flow) to be established, and hence their constitutive equations do not pass through the origin thus violating the basic definition of a fluid. They are termed as Bingham plastic. For an ideal Bingham plastic, the shear stress- deformation rate relationship is linear.

1. Bingham fluids or Bingham plastics

These fluids resist a small shear stress indefinitely but flow linearly under the action of larger shear stress.

• These fluids do not deform that does not flow unless a threshold shear stress value (τ₀ ) is not exceeded
• This fluid can be represented by
• when τ = τ_0, du/dy = 0

                              and τ > τ_0, τ = τ₀ + η.du/dy

                                     where 

                                        τ_{0 =} the yield stress/ threshold shear stress

                                         η = coefficient of rigidity

• Example: - Toothpaste, jellies, paints, sewage, and some more slurries.

2. Pseudoplastic Fluids

The viscosity of these fluids decreases with increase in velocity gradient, i. e. shear rate.

• These fluids also called as shear rate thinning fluids.
• At low shear rates(du/dy) the shear-thinning fluid is more viscous than the Newtonian fluid, and at high shear rates, it is less viscous.
• Example:- blood, the solution of high molecular weight polymers, paper pulp, muds, most slurries, and rubber latex

3. Dilatant fluids

The viscosity of these fluids increases with an increase in velocity gradient.

• Dilatant fluids are also called as shear thickening fluids.
• Example:- suspensions of starch in water, pulp in water, and sand-filled emulsions

Types of non-Newtonian fluid based on time-independent behavior( Those which are dependent upon the duration of shear).

1. Thixotropic fluids:

The dynamic viscosity of these fluids decrease with the time for which shearing forces are applied.

• Example:- thixotropic jelly paints.

2. Rheopectic fluids:

The dynamic viscosity of these fluids increases with the time for which shearing forces are applied.

• Example:- gypsum suspension in water.

3. Visco-elastic fluids:

These fluids have elastic properties, which allow them to spring back when a shear force is released.

• Example:- egg white, flour dough