credit to DR ADAM

1. FLUID MECHANICS – 1
Semester 1 2011 - 2012
Week – 1
Class – 1
Properties of Fluid
Compiled and modified
by
Sharma, Adam

2. OBJECTIVES
• Introduction to fluid mechanics
• Applications of fluid mechanics
• Dimensions and Units {CGS, FPS, MKS and SI
system}
• Fluid properties – Density and Viscosity

3. INTRODUCTION - CONCEPTS
Mechanics: The oldest physical science
that deals with both stationary and
moving bodies under the influence of
forces.
Statics: The branch of mechanics that
deals with bodies at rest.
Dynamics: The branch that deals with
bodies in motion.
Fluid mechanics: 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.
Fluid dynamics: Fluid mechanics is also
referred to as fluid dynamics by Fluid mechanics deals
considering fluids at rest as a special with liquids and gases in
case of motion with zero velocity.
motion or at rest. 3

4. INTRODUCTION – SOLIDS AND FLUIDS
Intermolecular bonds are strongest in solids and weakest in gases.
Solid: The molecules in a solid are arranged in a pattern that is repeated
throughout.
Liquid: In liquids, molecules can rotate and translate freely.
Gas: In the gas phase, the molecules are far apart from each other, and
molecular ordering is nonexistent.
The arrangement of atoms in different phases: (a) molecules are at
relatively fixed positions in a solid, (b) groups of molecules move about
each other in the liquid phase, and (c) individual molecules move about at
random in the gas phase. 4

5. INTRODUCTION – LIQUID AND GAS
In a liquid, groups of molecules can move relative to each other, but the
volume remains relatively constant because of the strong cohesive
forces between the molecules. As a result, a liquid takes the shape of
the container it is in, and it forms a free surface in a larger container in a
gravitational field.
A gas expands until it encounters the walls of the container and fills the
entire available space. This is because the gas molecules are widely
spaced, and the cohesive forces between them are very small. Unlike
liquids, a gas in an open container cannot form a free surface.
Unlike a liquid, a gas does not form a free surface, and it expands to
fill the entire available space. 5

6. INTRODUCTION – GAS AND VAPOR
Gas and vapor are often used as synonymous words.
Gas: The vapor phase of a substance is customarily called a gas when
it is above the critical temperature.
Vapor: Usually implies that the current phase is not far from a state of
condensation.
On a microscopic scale, pressure is
determined by the interaction of
individual gas molecules.
However, we can measure the pressure
on a macroscopic scale with a pressure
gage.
6

7. INTRODUCTION - STRESS
Stress: Force per unit area.
Normal stress: The normal
component of a force acting on a
surface per unit area.
Shear stress: The tangential
component of a force acting on a
surface per unit area.
Pressure: The normal stress in a
fluid at rest.
Zero shear stress: A fluid at rest
is at a state of zero shear stress.
When the walls are removed or a
liquid container is tilted, a shear
develops as the liquid moves to The normal stress and shear stress at
re-establish a horizontal free the surface of a fluid element. For
surface. fluids at rest, the shear stress is zero
and pressure is the only normal stress.
7

8. DIFFERENCE BETWEEN SOLID & FLUID
Fluid: A substance in the liquid
or gas phase.
A solid can resist an applied
shear stress by deforming.
A fluid deforms continuously
under the influence of a shear
stress, no matter how small.
In solids, stress is proportional
to strain, but in fluids, stress is Deformation of a rubber block placed
proportional to strain rate. between two parallel plates under the
When a constant shear force is influence of a shear force.
y
applied, a solid eventually stops
deforming at some fixed strain
angle, whereas a fluid never u=U
stops deforming and h u(y)
approaches a constant rate of
strain. x u=0
8

9. AREAS OF APPLICATIONS
Fluid dynamics is used extensively
in the design of artificial hearts.
Shown here is the Penn State
Electric Total Artificial Heart. 9

10. 10

11. 11

12. DIMENSIONS AND UNITS
12

13. DIMENSIONS AND UNITS
• Any physical quantity can be characterized by
dimensions.
• The magnitudes assigned to the dimensions
are called units.
• Some basic dimensions such as mass m,
length L, time t, and temperature T are
selected as primary or fundamental
dimensions, while others such as velocity V,
energy E, and volume V are expressed in
terms of the primary dimensions and are
called secondary dimensions, or derived
dimensions.
• Metric SI system: A simple and logical
system based on a decimal relationship
between the various units.
• English system: It has no apparent
systematic numerical base, and various units
in this system are related to each other rather
13
arbitrarily.

14. Some SI and English Units
Work = Force × Distance
1 J = 1 N∙m
The SI unit prefixes are used in all
branches of engineering.
The definition of the force units. 14

15. W weight
m mass
g gravitational
acceleration
A body weighing 60
kgf on earth will weigh
only 10 kgf on the
moon.
The relative magnitudes of the
force in newton (N) and
kilogram-force (kgf)
The weight of a unit
mass at sea level. See page 21 15

16. Unit conversion constants
16

17. DENSITY
17

18. DENSITY AND SPECIFIC GRAVITY
Density Relative density or Specific gravity:
The ratio of the density of a
substance to the density of some
standard substance at a specified
Specific volume temperature (usually water at 4°C).
Specific weight: The
weight of a unit
volume of a
substance.
Density is mass
per unit volume;
specific volume is
volume per unit
mass.
18

19. VISCOSITY
19

20. VISCOSITY
Viscosity: A property that represents the internal resistance of a fluid to
motion or the “fluidity”.
Drag force: The force a flowing fluid exerts on a body in the flow
direction. The magnitude of this force depends, in part, on viscosity.
The viscosity of a fluid is a
measure of its “resistance to
deformation.”
Viscosity is due to the internal
frictional force that develops
between different layers of fluids
as they are forced to move
relative to each other.
A fluid moving relative to
a body exerts a drag
force on the body, partly
because of friction
caused by viscosity. 20

21. Newtonian fluids: Fluids for
which the rate of deformation
is proportional to the shear
stress.
Shear
stress
The behavior of a fluid in laminar flow Shear force
between two parallel plates when the
upper plate moves with a constant velocity.
µ coefficient of viscosity
Dynamic (absolute) viscosity
Angular displacement or Shear strain
kg/m ⋅ s or N ⋅ s/m2 or Pa ⋅ s
1 poise = 0.1 Pa ⋅ s 21

22. Kinematic viscosity
m2/s or stoke
1 stoke = 1 cm2/s
For liquids, both the dynamic and
kinematic viscosities are practically
independent of pressure, and any small
variation with pressure is usually
disregarded, except at extremely high
pressures.
For gases, this is also the case for
dynamic viscosity (at low to moderate
pressures), but not for kinematic viscosity
since the density of a gas is proportional to
its pressure.
Dynamic viscosity, in general,
Viscosity for gases: does not depend on pressure,
but kinematic viscosity does.
Viscosity for liquids 22

23. L length of the cylinder
number of revolutions per unit time
This equation can be used to calculate the viscosity of a fluid by
measuring torque at a specified angular velocity.
Therefore, two concentric cylinders can be used as a viscometer, a
device that measures viscosity. 23

24. Summary
Introduction to Fluid mechanics
Concepts and definitions
Applications of fluid mechanics
Dimensions and Units
Basic dimensions in FM
Different units of measurement
Properties of fluids
Density and Relative density
Absolute and kinematic viscosity
24