CLASSIFICATION OF FLUID

Classification of fluid

Ideal fluid

Ideal fluid is incompressible and possesses no viscosity.

• Such a fluid is only an imaginary fluid.
• All existing fluids have some velocity.

Real fluid : -

A fluid that possesses viscosity is known as a real fluid.

• In actual practice, all fluids are real fluids

Newtonian fluid

A real fluid in which the shear stress is directly proportional to the rate of shear strain (or velocity gradient) is called Newtonian fluid.

Non – Newtonian fluids

A real fluid in which the shear stress is not proportional to the rate of shear strain (or velocity gradient) is called Non -Newtonian fluid.

Ideal plastic fluid

A fluid in which the shear stress is more than the yield value and the shear stress is proportional to the rate of shear strain (or velocity gradient) is called ideal plastic fluid.

VISCOSITY / DYNAMIC VISCOCITY

Viscosity/Dynamics viscosity

It is defined as the property of fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid.

• Two liquid layers at distance y and (y+dy) from the surface have velocity values u and (u+du) 
• The top layer causes a shear stress on the adjacent lower layer while the lower layer causes a shear stress on the adjacent top layer.
• The shear stress is proportional to the rate of change of velocity with respect to y.
• Mathematically

Or

Where,

  = is constant for proportionality and is known as the coefficient of dynamic viscosity
 = is called velocity gradient

• M. K. S. unit : kgf-sec/m3
• C. G. S. unit : dyne-sec/m2 , 1 dyne-sec/m2 is called one poise
• S. I. unit : newton-sec/m2 = Ns/m2

KINEMATIC VISCOSITY

Kinematic viscosity

Kinematic viscosity is the ratio of the two physical properties of the fluid.

It is also defined as the ratio between the dynamic viscosity and density of a fluid.

Mathematically

• M. K. S. and S. I. unit: - m²/s
• G. S. unit: - cm²/s
• One cm²/s is known as one stroke
• one stroke = One cm²/s = (1/100)²/s= 10^-4  m²/s
• The practical unit of kinematic viscosity is centistoke
• 1 centistoke = (1/100) stoke

SPECIFIC GRAVITY

Specific gravity

It is defined as the ratio of weight density of a fluid to the weight of density(or density) of a standard fluid.

• The standard liquid is water and the standard gas is air. It is a dimensionless quantity.
• Mathematically
• For liquids

• For gases

SPECIFIC WEIGHT AND SPECIFIC VOLUME

Specific weight and specific volume

Specific weight

• It is weight of fluid per unit volume.

• M. K. S. unit: kg/m3
• S. I. units: N/ m3
• To convert in SI units multiply (MKS) by 9.81. the specific weight of water is 9810 N/ m3
• Specific weight of a fluid varies due to

1. Change of gravity 
2. Effect of pressure and temperature 

Specific volume

• It is defined as volume per unit mass
• M. K. S. unit: m3/ slug
• S. I. units: m3/ kg

MASS DENSITY

Mass density

• It is mass per unit volume

• Dimensionally

• M. K. S. unit: metric slugs per meter cube (slug/m3)
• S. I. units: kilograms per meter cube (kg/ m3)
• The density of liquid may be considered as constant but the density of gases varies with the variation of pressure and temperature.