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CHAPTER 1.0: Principles of Fluid
1.1 Introduction to Fluid Mechanics
1.2 Thermodynamic Properties of a Fluid:
Density, specific weight, specific gravity ,
viscocity (kelikatan)berat tentu,
compressibility (kemampatan), Bulk
modulus (modulus pukal), dynamic &
kinematic viscosity (kelikatan dinamik dan
kinematik) ,surface tension
(ketegangan permukaan) and capillarity
(kererambutan).
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CHAPTER 1.0: Principles of Fluid
1.1 Introduction to Fluid Mechanics
Mechanics is the oldest physical science that deals with
both stationary and moving bodies under the influence of
forces. The branch of mechanics that deals with bodies at
rest is called statics, while the branch that deals with
bodies in motion is called dynamics.
The subcategory fluid mechanics is
defined as the science that deals with the behavior of
fluids at rest (fluid statics)or in motion (fluid dynamics),
and the interaction of fluids with solids
or other fluids at the boundaries.
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1.1 Introduction to Fluid Mechanics
- Fluid engineering applications is enormous:
breathing, blood flow, swimming, pumps, fans,
turbines, airplanes, ships, rivers, windmills, pipes,
icebergs, engines, filters, jets, and sprinklers, to
name a few…..
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1.1 Introduction to Fluid Mechanics (Cont’d)
- From the point of view of fluid mechanics, all matter consists of only two states, fluid and solid. -Distinction between a solid and a fluid is made on the basis of the substance’s ability to resist an applied shear (or tangential) stress that tends to change its shape. - Any shear stress applied to a fluid, no matter how small, will result in motion of that fluid. The fluid moves and deforms continuously as long as the shear stress is applied.
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Figure 1 illustrates a solid
block resting on a rigid
plane and stressed by its
own weight. The solid
sags into a static
deflection, shown as a
highly exaggerated
dashed line, resisting
shear without flow. Figure 1
1.1 Introduction to Fluid Mechanics (Cont’d)
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Figure 2
- The liquid and
gas at rest in
Figure 2 require
the supporting
walls to eliminate
shear stress.
1.1 Introduction to Fluid Mechanics (Cont’d)
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- The liquid retains its volume and forms a free
surface in the container.
- If the walls are removed, shear develops in the
liquid and a big splash results.
- If the container is tilted, shear again develops,
waves form, and the free surface seeks a horizontal
configuration, pouring out over the lip if necessary.
1.1 Introduction to Fluid Mechanics
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1.2 Thermodynamic Properties of a Fluid:
a) Density, (ketumpatan)
- Density is highly variable in gases and
increases nearly proportionally to the pressure
level.
)/( 3mkgV
m
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1.2 Thermodynamic Properties of a Fluid: (Cont’d)
- Density in liquids is nearly constant; the density
of water (about 1000 kg/m3) increases only 1
percent if the pressure is increased by a factor of
220. Thus most liquid flows are treated
analytically as nearly “incompressible.”
- Compare their densities at 20°C and 1 atm:
Mercury: = 13,580 kg/m3
Hydrogen: = 0.0838 kg/m3
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1.2 Thermodynamic Properties of a Fluid( Cont’d):
The density of liquids and solids
depends more strongly on
temperature than it does on
pressure. At 1 atm, for example,
the density of water changes
from 998 kg/m3 at 20°C to 975
kg/m3 at 75°C, a change of 2.3
percent, which can still be
neglected in many engineering
analyses.
0C Density (kg/m3)
0
10
20
30
40
50
100
999.8
999.2
998.2
995.7
992.3
988.0
958.4
Nota : 1000 kg/m3 = 1.94 slugs/ft3
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b) Specific volume, (Isipadu tentu)
1
m
V
c) Relative density (ketumpatan relatif)
and is defined as the ratio of the density of a
substance to the density ofsome standard substance
at a specified temperature (usually water at 4°C,
for which H2O 1000 kg/m3).
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d) Specific gravity, SG (graviti tentu)
the density of a substance is
given relative to the density of a
well-known substance.
COatH
SG0
2 4
Substances SG
Water
Blood
Seawater
Gasoline
Mercury
Wood
Gold
Ice
Air
1
1.05
1.025
0.7
13.6
0.3-0.9
19.2
0.92
0.0013
e) specific weight, s (berat tentu)
s = g ( N/m3)
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Example 1:
Given Specific Gravity of Mercury is 13.55 (20 0C). Calculate mercury’s density.
Solution:
3/100055.13
mkg
merkuri
33 /106.13 mkgxmerkuri
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Example 2:
Calculate the gasoline’s density at 200C. The mass
and volume are 60 kg dan 0.5 m3 respectively
Solution:
)/( 3mkgV
m
3/1205.0
60mkg
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f) Compressibility, (kebolehmampatan) refers to the
change in volume (V) of a substance that is subjected
to a change in pressure on it.
Added
Volume
Bar
0
0.5
1.0
1.5
0
25
50
75
-the usual quantity used to measure this phenomenon is the bulk modulus of elasticity or simply bulk modulus, E
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g) Bulk Modulus, E (Modulas Pukal)
Pressure changes needed for changing the volume
Initial Volume
Volume changes Showing the pressure increment with volume reduction
E , harder to compress
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h) Viscosity, (Kelikatan)
is a property that represents the internal resistance
of a fluid to motion or the “fluidity,” and that property
is the viscosity.
The force a flowing fluid
exerts on a body in the
flow direction is called the
drag force, and the
magnitude of this force
depends, in part, on
viscosity
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To obtain a relation for viscosity, consider a fluid layer
between two very large parallel plates (or equivalently, two
parallel plates immersed in a large body of a fluid)
separated by a distance l .
Now a constant parallel force F
is applied to the upper plate
while the lower plate is held
fixed. After the initial transients, it
is observed that the upper plate
moves continuously under the
influence of this force at a
constant velocity V.
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The fluid in contact with the upper plate sticks to
the plate surface and moves with it at the same
velocity, and the shear stress acting on this fluid
layer is ( = F/A) where A is the contact area
between the plate and the fluid. Note that the fluid
layer deforms continuously under the influence of
shear stress.
Details : See Cengel( 2005). Fluid Mechanics. Mc Graw Hill
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Fluids for which the rate of deformation is proportional to
the shear stress are called Newtonian fluids
Water, air, gasoline, and oils (Newtonian fluids)
Blood and liquid plastics (non-Newtonian fluids)
In one-dimensional shear flow of Newtonian fluids, shear
stress can be expressed by the linear relationship
Shear stress:
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Example 3:
SAE 30 Oil at 200C of 0.005 in is placed in between two
layer. The bottom layer is fixed while upper layer moves
with acceleration 13 ft/s. Calculate shear stress for the
oil.
Solution:
= (9.20 x 10-3)[ 13/(0.005/2)]
= Ib/ ft2
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Example 4:
Benzene at 200C has a coefficient of viscocity,
0.000651 Pa.s. Calculate the shear stress to deform
this fluid at velocity gradient of 4900s-1?
Solution:
= 0.000651 x 4900 = 3.19Pa
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i) Dynamic Viscosity, (Kelikatan dinamik) defined as shear force per unit area
Units: Ns/m2, kgm-1s-1,Poise P
Typically
Water =1.14 kgm-1s-1, Air =1.78 kgm-1s-1,
Mercury =1.552 kgm-1s-1, Paraffin Oil=1.9 kgm-1s-1,
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Dynamic viscocities of
some fluids at 1 atm
and 200C (unless
otherwise stated)
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j) Kinematic Viscosity, Kelikatan
kinematik, the ratio of dynamic
viscosity to density
Two common units of kinematic
viscosity are m2/s and stoke
(1 stoke 1 cm2/s 0.0001 m2/s).
Dynamic viscosity, in general, does
not depend on pressure, but kinematic
viscosity does.
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Typically
Water =1.14 x 10-6 , m2/s Air =1.46 x 10-5 m2/s ,
Mercury =1.145 x 10-4 m2/s , Paraffin Oil =2.375 x 10-3 m2/s
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a drop of blood forms a hump on a
horizontal glass!
a drop of mercury forms a near-perfect
sphere and can be rolled just like a
steel ball over a smooth surface!
water droplets from rain or dew hang
from branches or leaves of trees!
k) Surface Tension,s (Ketegangan permukaan)
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b
Fs
2
In these and other observances, liquid
droplets behave like small spherical
balloons filled with the liquid, and the
surface of the liquid acts like a
stretched elastic membrane under
tension. The pulling force that causes
this tension acts parallel to the surface
and is due to the attractive forces
Between the molecules of the liquid. The
magnitude of this force per unit
length is called surface tension s and
is usually expressed in the unit N/m
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How surface tension arises??
Let see at microscopic view in
Figure . By considering two
liquid molecules, one at the
surface and one deep within the
liquid body. The attractive forces
applied on the interior
molecule by the surrounding
molecules balance each other
because of symmetry.
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But the attractive forces acting on the surface molecule
are not symmetric,and the attractive forces applied by
the gas molecules above are usually
very small.
Therefore, there is a net attractive force acting on the
molecule at the surface of the liquid, which tends to pull
the molecules on the surface toward the interior of the
liquid.
This force is balanced by the repulsive forces from the
molecules below the surface that are being
compressed.
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The resulting compression effect causes the liquid to
minimize its surface area. This is the reason for the
tendency of the liquid droplets to attain a spherical
shape, which has the minimum surface area for a
given volume.
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That why with amusement,
that some insects can land on
water or even walk on water
and that small steel needles
can float on water. These
phenomena are again made
possible by surface tension
that balances the weights of
these objects.
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l) Capillary effect (Kererambutan)
Another interesting consequence of
surface tension is the capillary
effect,which is the rise or fall of a liquid in
a small-diameter tube inserted into the
liquid.
This effect is usually expressed by saying that
water wets the glass (by sticking to it) while
mercury does not
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The contact angle for wetting and nonwetting fluids.
A liquid is said to
wet the surface
when < 90° and
not to wet the
surface when >
90°.
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The capillary rise of water and
the capillary fall of mercury in a
small-diameter glass tube.
The phenomenon of capillary effect
can be explained microscopically
by considering cohesive forces (the
forces between like molecules,
such as water and water) and
adhesive forces (the forces
between unlike molecules, such as
water and glass). The liquid
molecules at the solid–liquid
interface are subjected to both
cohesive forces by other liquid
molecules and adhesive forces by
the molecules of the solid.
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The magnitude of the capillary
rise in a circular tube can be
determined from a force
balance on the cylindrical liquid
column of height h in the tube
(see figure)
The bottom of the liquid column
is at the same level as the free
surface of the reservoir, and
thus the pressure there must be
atmospheric pressure.
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This balances the atmospheric pressure acting at
the top surface, and thus these two effects cancel
each other. The weight of the liquid column
is approximately
Equating the vertical component of the surface
tension force to the weight gives
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cos
2
gRh s
Solving for h gives the capillary rise to be
R = constant
Example 5:
A 0.6-mm-diameter glass tube is inserted into
water at 20°C in a cup. Determine the capillary
rise of water in the tube
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Solution:
With assumption,
1 There are no impurities
in the water and no
contamination on the
surfaces of the glass tube.
2 The experiment is
conducted in atmospheric
air.
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