Fluid mechanics - Density, Specific Gravity, Viscosity, Vapour pressure etc
Blog Contents -
- Fluid mechanics [Fluid properties]
- Classification of fluids
- Density of fluid.
- Weight density
- Specific volume
- Specific Gravity of fluid.
- Viscosity of fluid.
- Vapour pressure of fluid.
Interview Questions from this blog
- What is definition of fluid?
- Classifications of fluid
- What are Newtonian fluids and Non Newtonian fluids?
- Why density is depend on Temperature & Pressure?
- What is classification of Non Newtonian fluid?
- What is relation between mass density & mass density?
- What is relation between Vapour Pressure & Temperature?
- How vapour change with molecular forces?
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Fluid definitions
Fluid is a substance which under goes continuous deformation (flows) when subjected to external force or gravity.
Other definitions
- Fluid is a substance which one’s particles easily move and change their respective position with other particles when substance is free or under pressure or under gravity
- Fluid is capable to flow when allowed to flow freely. or Fluid is a substance tends to conform to the containers outline or free to flow
- Fluid has no definite shape but when it fill in any Container it follow container's shape called Fluid
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Classification of fluid
Fluid classification is as per below
Classification of fluid |
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Definitions of Fluids of each type:
Ideal Fluid -- Ideal fluid is a fluid which does not offer any resistance in flow / deformation / shape change.
- Fluid doesn't has viscosity
- It is friction-less & In compressible.
- Ideal fluid is not exists It is imaginary fluid.
Real Fluid
- Fluid found in nature and reality called real fluid.
- Real fluid offers resistance when it is in motion.
- Fluids which's density depends on Temperature & Pressure called Compressible fluid.
- Fluid of which the density is sensible to change in temperature or pressure called Compressible fluid
- Generally gases are compressible fluids.
- If density of fluid is not affected by change in temperature or pressure applied in temperature that fluid is called Incompressible fluid.
- Generally liquids are incompressible
- Fluids which follows Newton's law of viscosity are called Newtonian fluids.
- Newton's law of viscosity : The fluids for which the ratio of the shear stress to the rate of shear is constant
- Fluids which are not following newton's law of viscosity calling Non Newtonian fluid
Density of fluid [Mass density]
- Density ⍴ or mass density of fluid is ratio of Mass of fluid m per unit Volume V
- SI unit of Density is Kg/m3
- Density of water at 4℃ [277K] is 1000 kg/m3
Density ⍴ = Mass (m) / Unit Volume (V)
- Density is dependent on Temperature and Pressure.
- As per Ideal Gas Law
PV = nRT
- When Temperature is increases Volume of fluids increases.
- When Pressure is Increases Volume of fluid decreases.
- Generally Density of fluid is not much change in slight change in Temperature.
- Gas density is change higher than liquid in raise temperature.
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Example of Density -
Copper cube sink in water, but in mercury it float.
Case 1 : Copper Cube in Mercury
Why? -
Density of Mercury : 13600 kg/m3
Density of Copper : 9000 kg/m3
Copper is light in comparison.
This is the reason Copper cube if floating
Case 1 : Copper Cube in Water
Why? -
Density of Water : 999 kg/m3
Density of Copper : 9000 kg/m3
Water is light in comparison.
This is the reason Copper cube if floating
Density Chart - Download now
Weight density
- Weight of fluid per unit volume.
- Weight density of Water at 4°C : 9810 N/m3
- Relation between Mass density & Weight density
w = ⍴ g
- g = gravitational force 9.81 m/s2
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Specific Volume
- It is volume in unit mass
- unit : m3/kg
Specific Gravity [ Sp.Gr., Relative Density]
Its ratio of Density of fluid to Density of standard fluid.Standard fluids
Liquid - Water at 4℃, 1 Atmospheric pressure.
Air - Air at 20℃, 1 Atmospheric
Unit : Unit less [You can consider kg/m3]
Unit Calculation -
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Viscosity
Fluid is a substance able to flow but there is resistance in flow is called Viscosity- Honey has more viscosity so it is not flow freely.
- Water has less viscosity so it flows more freely.
- It is not depend on Pressure.
- Viscosity decrease when temperature increase. [ When temperature rises molecular vibration also increases which makes fluid free and reduce it's viscosity].
- Viscosity governed by Cohesive forces[Attractive forces].
Viscosity calculation
Force is proportional to velocity and area and inversely proportional to DistanceF ∝ u A / YF = Force required to move fluid from two parallel layers 1 & 2.
F = μ u A / Y
μ = Viscosity of fluid
Coefficient of Viscosity, Dynamic Viscosity
F / A = μ u / Y
𝛕 = F / A𝛕 = μ * u / Y
We need velocity gradient to measure viscosity.μ = 𝛕 / (du/dY)
Velocity gradient is velocity of fluid with respect to distance traveled.
This is Newton's law of viscosity
- Viscosity μ is Shear stress required to produce unit flow [Shear deformation].
- Unit : N.s/m^2 [SI Unit]
- Kinetic Viscosity of fluid is ration of viscosity of fluid to density. 𝝂 = μ/⍴
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Newtonian & Non Newtonian fluid
Newtonian fluid definitions
- Shear stress to Rate of shear Ratio of is constant
- Fluids which has velocity gradient equal to Shear stress applied.
- Fluid which has 45° angle graph in Shear stress vs Velocity gradient du/dy
- Examples - All gases, Air; Liquids - Kerosene, alcohol, glycerin.

Non Newtonian fluid definitions
- Fluid for which Share stress vs Velocity gradient is not constant
- Fluids not follow Newton's law of Viscosity.
Examples - There are three types of Newtonian fluids
- Bingham fluids
- Pseudo plastic
- Dilatent fluid
1. Bingham fluids
- When Shear stress is small it resist to flow.
- When Shell stress is larger it follow linear shear stress vs flow called Bingham fluids of Plastics.
- Examples - Toothpaste, Jellies, Paints, Slurries.
- 𝜏 = 𝜏0; du/dy=0; 𝜏>𝜏0 𝜏=𝜏0 + η du/dy. 𝜏0 = yield stress, η = rigidity coefficient
2. Pseudo plastic or fluids [shear rate thinning fluid]
- Viscosity decrease with increase in velocity.
- Examples - Blood, High molecular weight solution, paper pulp, rubber latex.
3. Dilatent fluids [shear rate ticking fluid]
- Viscosity increases when velocity increases.
- Examples - Suspensions of starch, pulp in water, emulsions.
Experimental curve of Shear stress vs Velocity given by Ostwald-de-waele equation
𝜏 = k (du/dy)n
Where k & n are arbitrary
n = 1 k = μ : Newtonian fluid
n < 1 : Pseudo plastic
n > 1 : Dilatant fluids
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Vapour Pressure
Pressure formed by vapour of liquid over the surface of liquid in closed container which is in thermodynamic equilibrium state at a given temperature.- Pure water's Vapour pressure is 1 atmosphere at 100℃
- All liquids have tendency to vaporize.
- Molecules of liquid escaping through free surface of liquid to space.
- Vapour accumulation develop pressure on liquid surface and towards container vessel.
PMavg = ρRT
Vapour pressure vs Temperature
- Vapour pressure increase with increase in temperature
- Water Vapour pressure Vs Temperature Graph
- Vapour pressure is not affected by surface area.
- Water vapour pressure is same on small container & large vessels.
- Vapour pressure is depends on dipole-dipole forces & London dispersion forces.
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Wikipedia -
Fluid
SI Unit
Density
Specific Gravity
Viscosity
Vapour Pressure
Fluid
Read on merriam-webster
Read on Dictionary
Viscosity
Read more on Pipeflow.com
Read more on BRITANNICA
Read on Physics.info
Read on rheosense
Vapour Pressure
Read on Chem.Pardue
Buy related instruments
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Photo credits
Specific Gravity
Viscosity example
Vapour Pressure Graph