Fluid Flow

 Fluid:

Fluid is one which cannot remain at rest under the action of shear force or fluid is the one which can flow and deforms under the action of shear stress or external force.

Fluid can be categorized based on following properties:

• Viscosity
• Conductivity
• Density
• Compressibility
• Temperature
• Pressure etc.
• Fluid Flow

 Fluid flow  is a part of fluid mechanics and deals with fluid dynamics. It involves the motion of a fluid subjected to unbalanced forces. This motion continues as long as unbalanced forces are applied.

For example, if you are pouring water from a mug, the velocity of water is very high over the lip, moderately high approaching the lip, and very low at the bottom of the mug. The unbalanced force is gravity, and the flow continues as long as the water is available and the mug is tilted.

Types of Fluids

Fluid can be classified as:

• Ideal Fluid
• Real Fluid
• Compressible Fluid
• Non-Compressible Fluid
• Newtonian Fluid
• Non-Newtonian Fluid

Ideal fluid

A fluid is said to be ideal when it cannot be compressed and the viscosity doesn’t fall in the category of an ideal fluid. It is an imaginary fluid which doesn’t exist in reality.

e.g.: This fluid does not exist

Real fluid

All the fluids are real as all the fluids possess viscosity.

e.g. : Water, Air etc.

Newtonian fluid

When the fluid obeys Newton’s law of viscosity, it is known as a Newtonian fluid.

e.g. : Water, Benzene etc.

Non-Newtonian fluid

When the fluid doesn’t obey Newton’s law of viscosity, it is known as Non-Newtonian fluid.

e.g.: Plaster, Honey, Ketchup etc

There are three categories of Non-Newtonian Fluid:

·         Time Dependent Fluid.

·         Time Independent Fluid.

·         Visco-Plastic Fluid.

1. Time Dependent Fluid:

The fluids whose apparent viscosity changes with time as the fluid is continuously sheared are time dependent fluids.

      a) Thixotropic Fluids

·         b) Rheopectic Fluids

Thixotropic Fluids:

The fluids whose apparent viscosity decreases with time when sheared continuously.

These fluids are thick under normal condition but when shaken, agitated, sheared or stressed they will start to flow. It will take some time to become viscous again.

e.g.: The best example of thixotropic fluids is tomato ketchup.

Rheopectic Fluids:

 The fluids whose apparent viscosity increases with time when continuously stress is applied.

e.g.: The examples of such fluids are gypsum paste and printer ink.

2. Time Independent Fluid:

The fluids whose apparent viscosity does not change with time as the fluid is continuously sheared are time dependent fluids.

·         a) Dilatant Fluid.

·         b) Pseudo plastic Fluid.

Dilatant Fluid.

The type of fluid whose viscosity increases when shear stress is applied on it independent of time which mean that in no time the fluid will become viscous when stress is applied and in no time, it will return to its non-viscous condition when stress is removed.

e.g.: The best example of dilatant fluids is Corn Flour-Water Solution.

Pseudo Plastic Fluids:

The type of fluid whose viscosity decreases when stress is applied irrespective to time are known as pseudo plastic fluids.

e.g.: The best example of pseudo plastic fluids is Paint.

3. Visco-Plastic Fluid:

a) Bingham Plastic Fluid.

Bingham Plastic Fluid.

For some fluid, when the stress is moderate or low it acts as solid but on applying stress more than its yield stress the viscosity decreases and the fluid started to flow.

e.g.: The best example of Bingham Plastic Fluid is Toothpaste

Ideal plastic fluid

When the shear stress is proportional to the velocity gradient and shear stress is more than the yield value, it is known as ideal plastic fluid.

Incompressible fluid

When the density of the fluid doesn’t change with the application of external force, it is known as an incompressible fluid.

e.g.: All the liquids

Compressible fluid

When the density of the fluid changes with the application of external force, it is known as compressible fluid.

e.g.: All the gases.

                  The table below represents the density and viscosity of different types of fluids

Classification of flows on the basis of Mach number

• Incompressible flow has M<0.3.
• Compressible subsonic flow has M between 0.3 to 1.

• Types of Fluid Flow

  Fluid flow has all kinds of aspects — steady or unsteady, compressible or incompressible, viscous or non-viscous, and rotational or irrotational, to name a few. Some of these characteristics reflect the properties of the liquid itself, and others focus on how the fluid is moving.

  Steady or Unsteady Flow

  Fluid flow can be steady or unsteady, depending on the fluid’s velocity:

  • Steady: In steady fluid flow, the velocity of the fluid is constant at any point.
  • Unsteady: When the flow is unsteady, the fluid’s velocity can differ between any two points.

  Viscous or Non-viscous Flow

  Liquid flow can be viscous or non-viscous.

  Viscosity is a measure of the thickness of a fluid, and very gloppy fluids such as motor oil or shampoo called viscous fluids.     

  
  
  

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•                                        The different types of fluid flow are represented in the graph