# 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

** ****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

**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** [τ**yx du / dy **]=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.** 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 gradient**)**is 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. |

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