Test: Properties of Fluids - 2

Real Fluid

• All the fluids are real as all the fluids possess viscosity.
• Real fluid is Compressible and it has some value of shear force.
• Its surface tension value in non-zero.

Ideal fluid

• Ideal fluid is Incompressible and it has zero value of shear force.
• The ideal fluid does not actually exist in nature but is sometimes used for fluid flow problems.
• An ideal fluid is a fluid that has several properties including the fact that it is:
  Incompressible – the density is constant
  Irrotational – the flow is smooth, no turbulence
  Nonviscous – (Inviscid) fluid has no internal friction

A fluid is a substance that deformed continuously when shear stress is applied to it irrespective of the magnitude of the shear stress. Because of the continuous deformation flow of the liquid can be seen. In other words, we can also tell that liquid can not sustain any shear load.

Ideal Fluid: A fluid, which is incompressible and is having no viscosity is known as ideai fluid. Ideal fluid is only an imaginary fluid as all the fluids, which exists have some viscosity.

• Surface tension, in simple words, is defined as the force exerted on the fluid particles on a surface per unit length in the plane of the liquid surface perpendicular to either side of an imaginary line drawn to the surface.
• Mathematically, surface tension is expressed as the ratio of force to length. It is the or type of behavior of the liquid surface to contract into a minimum surface area. The standard unit of surface tension is N m^-1. 

Newton's law of viscosity:

• Viscosity is a property of fluid by virtue of which they offer resistance to shear or angular deformation.
• It is primarily due to cohesion and molecular momentum exchange between fluid layers, and as flow occurs, these effects appears as shearing stresses between the moving layers.
• According to Newton's law of viscosity, shear stress is directly proportional to the rate of angular deformation (shear strain rate) or velocity gradient across the flow.

where, τ = shear stress

μ = absolute or dynamic viscosity

⇒  du/dy = velocity gradient 

⇒ dα/dt = rate of angular deformation (shear strain)

The fluid which obeys the Newton’s law of viscosity is known as Newtonion fluid e.g.: Air, water, Kerosene.

In the expression: 

If n < 1, B = 0
then it is known as pseudo plastic

Surface tension a line force caused by cohesion of fluid particle at the surface, the property of cohesion enables a liquid to resist tensile stress.