Fluid mechanics notes

1. Lecture 2
03.07.2020
12.30 pm to 1.30 pm
Basic Properties of Fluids

2. Density
It is a ratio of mass to volume.
It is denoted by ρ
Its unit is kg/m3 (Commonly used unit is gm/cc)
Density of water = 1000 kg/m3 or 1 gm /cc

3. Significance of density
Requirement of volume for given mass or calculation of mass
for given volume
Quality of material

4. Unit weight
It is a ratio of weight to volume
It is denoted by ϒ
Its SI unit is N/m3 (Commonly used is kN/m3)
Unit weight of water, ϒw = kN/m3
It is derived quantity from density.
ϒ = ρ.g (g = 9.81 m/s2)
Mass is basic quantity and weight is derived quantity.

5. Significance of Unit weight
Requirement of volume for given weight or calculation of
weight for given volume
Quality of material
It is related to density by,

6. Specific gravity
It is defined as, ratio of density of a substance to the density
of a given reference material.
Reference material is water in this course and in most
practical cases.
It is denoted by S.
w





water
of
Density
material
a
of
Density
S
gravity
Specific
w





water
of
t
Unit weigh
material
a
t
Unit weigh
S
gravity
Specific
w
w
w g
g
S









.
.

7. Specific Surface
Specific surface area is a property defined as the total surface
area of a material per unit of mass.
Its unit is m2/kg.
It is inverse of density.
Usually adopted or gases, as their density is low.
Symbol = ?

8. Viscosity
The viscosity of a fluid is a measure of its resistance to
deformation at a given rate.
Viscosity is related to internal frictional force that arises between
adjacent layers of fluid that are in motion.
Example:Water, honey, ediable oil
Typical application: is in engine oil

9. Newton’s Law of Viscosity
Used to measure/quantify viscosity.
y
dy
du


Shear Stress is proportional to
rate of change of displacement
with respect to distance.

10. Newton’s Law of Viscosity
dy
du
dy
du


 


μ is known as coefficient of viscosity or viscosity.
It is known as dynamic viscosity
Unit of dynamic viscosity is Ns/m2

11. Types of Viscosity
Dynamic viscosity, denoted by μ
Kinematic viscosity, denoted by ν


 
• Unit of kinematic viscosity,
s
m
kg
m
m
s
s
kg
kg
m
kg
m
Ns 2
3
2
2
3
2 .
/
/
density
of
Unit
viscosity
dynamic
of
Unit














12. Other units of viscosity
Dynamic viscosity has SI unit of N.s/m2 .
But, it is commonly expressed as, poise
Kinematic viscosity has SI unit of m2/s.
It is commonly expressed as, stoke.
stokes
s
m
poise
m
N
1000
/
1
10
/
1
2
2


Home work
Find relation between
1) SI unit of dynamic viscosity and poise
2) SI unit of kinematic viscosity and stoke
3) Relation between centi poise and poise
4) Relation between stokes and centistokes

13. Viscosity
Dynamic viscosity
Denoted by μ
Involves consideration of
force
Unit is N.s/m2
Poise
• Kinematic viscosity
• Denoted by ν
• Does not involve
consideration of
force
• Unit is m2/s
• Stokes

14. Effect of temperature on viscosity
Liquids show reduction in viscosity with increase in
temperature.
Gases show increase in viscosity with increase in
temperature.
Example of viscosity in automobile

15. Summary
• Density
•Unit weight
•Specific surface
•Viscosity
•Newton’s law of viscosity
Next Lecture:
1.SurfaceTension
2.Capillary Action
3.Vapour Pressure
4.Numericals

16. Thank you !