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Fluid | Fluid Mechanics, Classification, Properties and Difference

Fluid

  • In common usage, ‘ fluid ‘is often used as a synonym for “ liquid “, with no implication that gas could also be present.
  • The fluid may be defined as follows:
  •  A fluid is a substance which is capable of flowing.

                                                   or

  • A fluid is a substance which deforms continuously when subjected to external shearing force

Fluid

 Image. Fluids

 

Behavior of Fluids

  •  The analysis of fluids behavior is based on:

                i. Fundamental laws of mechanics

               ii. Conservation of mass momentum energy and

               iv. Laws of thermodynamics

Fluid Mechanics

  • Mechanics is the oldest physical science that deals with both stationary and moving bodies under the influence of forces.
  • The subcategory fluid mechanics may be defined as that branch of Engineering science which deals with the behaviour of fluids under the conditions of rest and motion.
  • It includes liquids, gases, plasmas and , to some an extent plastics solids.
  • They may be divided into three parts:

          1. Statics                   2. Kinematics                    3. Dynamics

1. Statics

  • The study of incompressible fluids under static conditions is called hydro statics and that dealing with the compressible static gases is termed as aerostatics.

2. Kinematics

  • It deals with the velocities, accelerations and the patterns of flow only.

3. Dynamics

  • It deals with the relations between velocities, accelerations of fluids with the forces or energy causing them.

Application areas of fluid Mechanics

  • Mechanics of fluids is extremely important in many areas of engineering and science. Examples are:

            

     i. Biomechanics

  • Blood flow through arteries and veins
  • Air flow in the lungs
  • Flow of cerebral fluids

     ii. Households

  • Piping systems for cold water, natural gas, and sewage.
  • Piping and ducting network of heating and air – conditioning system.

     iii. Meteorology and Ocean Engineering

  • Movements of air currents and water currents.

     iv. Mechanical Engineering

  • Design of pumps, turbines, air – conditioning equipment, pollution – control equipment, etc.

     v. Civil Engineering

  • Transport of river sediments
  • Pollution of air and water
  • Flood control systems

     vi. Automobile

  • IC engine, air conditioning, fuel flow, external aerodynamics, etc.

 

Classification of Fluid

  •  The fluids can be classified as follows:

    1. Ideal Fluid 

   2. Real Fluid

   3. Newtonian Fluid 

   4. Non -Newtonian Fluid

    5. Ideal Plastic Fluid                                                                                   

Stress- Strain Graphs of Different Types of Fluid

                                                           Stress- Strain Graphs of Different Types of Fluid

1. Ideal Fluid

  • The fluids which are incompressible and is having no viscosity is known as an ideal fluid.
  • An ideal fluid is only an imaginary fluid as all the fluids which exist, have some viscosity.

2. Real Fluid

  • The fluids, which possesses viscosity is known as real fluid.
  • All the fluids, in actual practice, are real fluids.

3. Newtonian Fluids

  •  A real fluids in which the shear stress is directly proportional to the rate of shear strain ( or velocity gradient ) known as a Newtonian Fluids.

4. Non – Newtonian Fluids 

  • A real fluids in which the shear stress is not proportional to the rate of shear strain ( or velocity gradient ) known as a Non – Newtonian Fluids.

 

                                 Ω yx  dudy ]=0

 There are mainly two types of  Non – Newtonian Fluids

  1. Time independent                                                  

    a. Pseudo Plastic                                                                      

    b. Dilatent fluid                                                                       

    c. Ideal Bingham

 2. Time dependent

      a.Thixotropic

    b. Rheopectic 

Graph of Non-Newtonian fluid

                                        Graph. Non- Newtonian Fluid

 1. Time independent   

 a. Pseudo Plastic

  • Pseudo plastic is the higher velocity gradients where the shearing force increases less than proportionally, i.e n < 1 as shown in graph.
  • The phenomena of under the influence of movement, the liquid seems to become thinner. Hence, it is known as “ Shear thinning “.
  • For example: Blood, milk etc.

b. Dilatent fluid

  • In Dilatent, the viscosity of fluids will grow with the rate of shear strain. Here n > 1 and B = 0.
  • It is also term as ” shear thickening “. For example: Butter solution, sugar solution, rice starch solution etc.

c. Ideal Bingham

  • It has some initial strength beyond which deformation starts.
  • For example : Tooth paste, creams, sewage sludge etc.

2. Time dependent

    a.Thixotropic

  • In Thixotropic, fluids which take a finite time to attain equilibrium  viscosity when introduced to a steep change shear rate ( n< 1 & B ≠ 0 ).
  • They are reversible changes from fluid to solid – like elastic gel.
  • Example ; Paints, Printing inks, Gel etc.

  b. Rheopectic

  • μ increases with time for which shearing forces are applied.
  • Here n > 1 & B ≠ 0. Examples – Gypsum paste and Bentonite solution etc.

 

 5. Ideal Plastic Fluids 

  • The fluids in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain ( or velocity gradientis known as ideal plastic fluids.

Also read:

https://mechanicalnotes.com/2019/12/04/lubricant-purposes-properties-classification-selection-methods/

  • The material properties of a fluid, which may vary, sometimes sensitively with temperature, pressure and composition, determine its mechanical behaviour.

 Some of the properties of fluids are given below:

 1. Density

 2. Specific weight                                                       

 3. Specific gravity or Relative Density

2. Specific weight                  

  1. Density

  • The mass per unity volume is known as density.
  • It is usually denoted by rho ( Ρ ). It’s units are kg/m³, i.e.,

                                                       ρ = m / V

 2. Specific weight

  • It is defined as the weight per unity volume.
  • It is usually denoted by w.

3. Specific gravity or Relative Density

  • Specific gravity ( Relative Density )  is the ratio of specific weight of liquid to specific weight of a standard fluids.
  • It is dimensionless and has no units. It is represented by S.

4. Specific volume

  • It is defined as volume per unity mass of liquid.
  • It is denoted by ν.

                                          ν = V / m  or 1 / ρ

Difference between Fluid and Liquid
S.NO. Fluid Liquid
1. It is a condition specific to certain substances or it is a subset of matter. Liquid is  one  of the three phases or state of matter.
2. Fluids flow and has some viscosity (thickness). Liquids also flows and it has volume, but no definite shape.
3. Fluids include liquids. All liquids are fluids.
4. It is a substance that lacks rigidity. it cannot resist force when it is applied to it. Liquids can assume the shape of any container or vessel and they are relatively in-compressible.

10 thoughts on “Fluid | Fluid Mechanics, Classification, Properties and Difference

    • November 17, 2019 at 8:25 am
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      Thanks……

      Reply
  • November 17, 2019 at 8:34 am
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    Very nice notes for all students and thank you so much my dear sir..

    Reply
    • November 17, 2019 at 8:50 am
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      Thanks…

      Reply
  • November 17, 2019 at 8:47 am
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    • November 17, 2019 at 8:49 am
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      Thanks…

      Reply
  • November 17, 2019 at 9:14 am
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    WoW very useful article 👍👍

    Reply
    • November 17, 2019 at 9:21 am
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      Thanks…

      Reply
    • November 17, 2019 at 5:07 pm
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      Thanks…

      Reply

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