This document discusses various properties of fluids including density, viscosity, surface tension, and capillary action. It defines key terms like density, viscosity, kinematic viscosity, surface tension, and capillary action. It also provides equations relating these concepts and gives examples of calculating fluid properties based on given values.

1. Course Name: FLUID MECHANICS
Unit: Properties of fluid

2. Properties of Fluids
 Properties of fluids determine how fluids can be used
in engineering and technology. They also determine
the behaviour of fluids in fluid mechanics. They are:
 Density
 Viscosity
 Surface Tension
 Capillary Action
 Specific Weight
 Specific Gravity

3. DENSITY
 Mass Density is the mass per unit volume of a fluid. In other
words, it is the ratio between mass (m) and volume (V) of a fluid.
 Density is denoted by the symbol ‘ρ’. Its unit is
kg/m3.
• Weight Density/ Specific Weight: of a fluid is the ratio of
Weight of a fluid to its Volume. Denoted by ‘w’
• w=
mass of fluid x g
𝑉𝑜𝑙𝑢𝑚𝑒
• w=ρg
• N/m3

4. • Specific Volume:
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑙𝑢𝑖𝑑
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑓𝑙𝑢𝑖𝑑
=1/ρ, m3/kg
• Specific gravity:
Weight density of a liquid= SL X weight density of water
=SL X density of water X g
= SL X 1000 x 9.81 N/m3
Density of a liquid ρL = SL X ρw = 1000ρw
SHg = 13.6 therefore the ρHg = ?
Calculate :
1. density , specific weight and weight of one liter of petrol of
specific gravity = 0.7
2. density , specific weight and specific gravity of one liter of a
liquid which weighs 7 N.

5. Viscosity
 Viscosity is the fluid property that determines the
amount of resistance of the fluid to shear stress.
 It is the property of the fluid due to which the fluid
offers resistance to flow of one layer of the fluid
over another adjacent layer.

6. • Dynamic viscosity : μ
τ α
𝑑𝑢
𝑑𝑦
τ =μ
𝑑𝑢
𝑑𝑦
μ = τ/
𝑑𝑢
𝑑𝑦
, N.s/m2 = Pa.s
CGS: dyne.sec/cm2 = Poise
1 poise = 1/10 N.s/m2
1 centipoise = 1/100 poise

7. KYNEMATIC VISCOSITY
 The kinematic viscosity (also called "momentum
diffusivity") is the ratio of the dynamic viscosity μ to the
density of the fluid ρ.
 ν=μ/ρ, m2/s
 In CGS: cm2/sec = stoke = 10-4 m2/s
 1centistoke = 1/100 stoke
 Newton’s law of viscosity : shear stress on a fluid element
layer is directly proportional to rate of shear strain.
τ α
𝑑𝑢
𝑑𝑦
τ =μ
𝑑𝑢
𝑑𝑦

8. Viscosity and temperature:
For liquids : viscosity decreases with increase in temperature.
For gases: increase with increase in temperature.

9.  Fluids can be classified into five basic types. They
are:
 Ideal Fluid : incompressible and no viscosity
 Real Fluid : possess viscosity
 Ideal-plastic Fluid: shear stress more than yirld
value and newtonian
 Newtonian Fluid: real fluid , obey newton law of
viscosity
 Non-Newtonian Fluid: real fluid , not obey newton
law of viscosity.

10. Solve:
Hint: given:

12. • The velocity distribution for flow over a flat plate is given by
u =
3
4
y - y2 in which u is velocity in meter per second at a distance y
meter above the plate. Determine shear stress at y=0.25m and 0.3 m.
take kinematic viscosity 6 stokes and the density of the liquid
1900kg/m3.
solution:
Shear stress: τ = μ (du/dy)
to find dynamic viscosity : μ
given , ν = 6 stokes = ……… m2/s
ρ = 1900kg/m3
ν = μ/ ρ ; μ = …….. Ns/m2
u =
3
4
y - y2
(du/dy) = (3/4 ) – 2y
(du/dy)y=0.25 = ………. ; τ y=0.25 = μ (du/dy) y=0.25 = …………
(du/dy)y=0.3 = ………. ; τ y=0.3 = μ (du/dy) y=0.3 = …………..

15. • Compressibility and Bulk Modulus
• K = Compressive stress/volumetric strain

16. SURFACE TENSION
 The property of fluids to resist tensile stresses on
their surface is called as Surface Tension.
 Denoted by σ , N/m
 Surface tension on liquid droplet: p=4σ/d
 Hollow bubble: 8σ/d

17. CAPILLARY ACTION
 Capillary action is the property of fluid to flow in a
narrow spaces without assistance of and in
opposition to external forces like gravity.
 The effect can be seen in the drawing up of liquids
between the hairs of a paint-brush, in a thin tube, in
porous materials such as paper and plaster, in
some non-porous materials such as sand or in a
cell.
 It occurs because of intermolecular forces between
the liquid and surrounding solid surfaces.

19. • Capillary rise: h=
4σ
ρ𝑔𝑑
mm
• Capillary fall h=
4σ cos 𝜃
ρ𝑔𝑑
mm
• 𝜃 = 128 0 for Hg and glass tube
• 𝜃 = 0 for water and glass tube

20. Vapour Pressure and Cavitation:
• Change from liquid state to vapour state:
Vaporization.
• Pressure at which liquid is converted into
liquid – vapour pressure.
• Pressure above the liquid surface is reduced
then the boiling temperature will also reduce.
• Pressure equal or less than vapor pressure,
boiling of the liquid will start, though the
temperature of the liquid is 200C.

21. Cavitation: worn out turbine blades.
Phenomenon of formation of vapour bubbles of
a flowing liquid in a region where the pressure
of the liquid falls below the vapour pressure and
sudden collapsing of these vapour bubbles in a
region of higher pressure.

22. • Determine the specific gravity of a fluid having
viscosity 0.05 poise and kinematic viscosity
0.035 stokes.
• Determine the viscosity of a liquid having
kinematic viscosity 6 stokes and specific
gravity 1.9.