In liquids the cohesive forces predominates the
molecular
momentum
transfer due to closely packed
molecules and
with the increase in temperature, the
cohesive forces
decreases with the result of decreasing
viscosity.
40

Properties of Fluids
But in the case of gases the cohesive force are small and
molecular momentum transfer predominates. With the
increase in temperature, molecular momentum transfer
increases and hence viscosity increases. The relation between
viscosity and temperature for liquids and gases are:
42
o
2
(ii) For a
gas,
1
where for air
o
0.000017,
0.000000056,
0.1189x 10
-9
,
are constants for theliquid
For water,
μ
o
1.79 x 10
poise,
0.03368and
0.000221
-3
(i) For liquids,
o
t
t
2
where
Viscosity of liquid at t
o
C, in poise
o
Viscosity of liquid at 0
C
, in poise
10
m
2
1 poise
1
Ns
o
1
t
t

Types of Fluids
1.
Ideal Fluid.
A fluid, which is incompressible and is
having no viscosity, is known as an ideal fluid.
Ideal
fluid is only an imaginary fluid as all the fluids, which
exist, have some viscosity.
2.
Real fluid
. A fluid, which possesses viscosity, is known as
real fluid. All the fluids: in actual practice, are real fluids.
3.
Newtonian Fluid.
A real fluid, in which the shear stress is
directly, proportional to the rate of shear strain (or
velocity gradient), is known as a Newtonian fluid.
4.
Non-Newtonian fluid.
A real fluid, in which shear stress
is not proportional to the rate of shear strain (or velocity
gradient), known as a Non-Newtonian fluid.
42

Types of Fluids
5.
Ideal Plastic Fluid.
A fluid, 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
fluid.
43

Example 3
If the velocity distribution over a plate is given
by
metre above the plate, determine the shear stress at y = 0
and y= 0.15 m. Take dynamic viscosity of fluid as 8.63
poises.
3
in which u is velocity in metre per second at a
distance y
44
u
2
y
y
2

45

Example 4
Calculate the dynamic viscosity of an oil, which is used for
lubrication between a square plate of size 0.8 m x 0.8 m and an
inclined plane with angle of inclination 30
o
as shown in Fig. 1.4.
The weight of the square plate is 300 N and it slides down the
inclined plane with a uniform velocity of 0.3 m/s. The thickness
of oil film is 1.5 mm.
Fig.1.4
46

47

Example 5
The space between two square flat parallel plates is filled with
oil. Each side of the plate is 60 cm. The thickness of the oil
film is 12.5 mm. The upper plate, which moves at 2.5 metre per
sec requires a force of 98.1 N to maintain the speed.
Determine : ·
i.
the dynamic viscosity of the oil, and
ii.
the kinematic viscosity of the oil if the specific gravity of the
oil is 0.95.
m

48

49

Thermodynamic Properties
Fluids consist of liquids or gases. But gases are compressible
fluids and hence thermodynamic properties play an important
role.