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PHILADELPHIA UNIVERSITY
Department of Civil Engineering
Hydraulics
(670441)
CHAPTER 1
Fundamental Properties
of Water
Instructor:
Eng. Abdallah Odeibat
Civil Engineer, Structures , M.Sc.
1

 The word hydraulic comes from two Greek words:
"hydor" (meaning water) and "aulos" (meaning pipe).
Through the years, the definition of hydraulics has
broadened beyond merely pipe flow. Hydraulic systems
are designed to accommodate water at rest and in
motion.
 The fundamentals of hydraulic engineering systems,
therefore, involve the application of engineering
principles and methods to the planning, control,
transportation, conservation, and utilization of water.
2

1.1 THE EARTH'S ATMOSPHERE AND
ATMOSPHERIC PRESSURE
 The earth’s atmosphere is a thick layer (approximately
1,500 km) of mixed gases. Nitrogen makes up
approximately 78 percent of the atmosphere, oxygen
makes up approximately 21 percent, and the remaining
1 percent consists mainly of water vapor, argon, and
trace amounts of other gases.
 Each gas possesses a certain mass and consequently
has a weight. The total weight of the atmospheric
column exerts a pressure on every surface it contacts.
At sea level and under normal conditions, the
atmospheric pressure is approximately equal to 1.014
*105 N/m2, or approximately 1 bar. 3

 The pressure unit 1 N/m2 is also known as 1 pascal,
named after French mathematician Blaise Pascal
(1623 to 1662).
 Water surfaces that come in contact with the
atmosphere are subjected to atmospheric pressure. In
the atmosphere, each gas exerts a partial pressure
independently of the other gases. The partial pressure
exerted by the water vapor in the atmosphere is called
the vapor pressure.
4

1.2 THE THREE PHASES OF WATER
 The water molecule is a stable chemical bond of oxygen and
hydrogen atoms. The amount of energy holding the
molecules together depends on the temperature and
pressure present. Depending on its energy content, water
may appear in solid, liquid, or gaseous form.
 Snow and ice are the solid forms of water; liquid is its most
commonly recognized form; and moisture, water vapor in
air, is water in its gaseous form. The three different forms of
water are called its three phases.
 To change water from one phase to another phase, energy
must either be added or taken away from the water. The
amount of energy required to change water from one phase
to another is known as a latent energy. This amount of
energy may be in the form of heat or pressure. One of the
most common units of heat energy is the calorie (cal). 5

 One calorie is the energy required to increase the
temperature of 1 gram (g) of water, in liquid phase, by
1°C. The amount of energy required to raise the
temperature of a substance by 1°C is known as the
specific heat of that substance.
 Under standard atmospheric pressure, the specific heat
of water and ice are, respectively, 1.0 and 0.465
cal/g.°C. For water vapor, the specific heat under
constant pressure is 0.432 cal/g.°C, and at constant
volume it is 0.322 cal/g.°C.
 These values may vary slightly depending on the
purity of the water. To melt 1 g of ice, changing water
from its solid to liquid phase, requires a latent heat
(heat of fusion) of 79.7 cal. To freeze water, the same
amount of heat energy must be taken out of each of
water, thus the process is reversed.
6

 Evaporation, the changing of liquid-phase water into
its gaseous phase, requires a latent heat (heat of
vaporization) of 597 cal/g.
 Evaporation is a rather complex process. Under
standard atmospheric pressure, water boils at 100°C.
At higher elevations, where the atmospheric pressure
is less, water boils at temperatures lower than 100°C,
This phenomenon may be explained best from a
molecular-exchange viewpoint.
 At the gas—liquid interface, there is continual
interchange of molecules leaving the liquid to the gas
and molecules entering the liquid from the gas.
7

 Net evaporation occurs when more molecules are
leaving than are entering the liquid; net condensation
occurs when more molecules are entering than are
leaving the liquid. Equilibrium exists when the
molecular exchange at the gas-liquid interface are
equal over a time interval.
 Vapor molecules in the air exert a partial pressure on
any contact surface that is known as the vapor
pressure. This partial pressure combined with the
partial pressures created by other gases in the
atmosphere makes up the total atmospheric pressure.
8

 If the temperature of a liquid is increased, the
molecular energy is raised, causing a large number of
molecules to leave the liquid. This, in turn, increases
the vapor pressure.
 When the temperature reaches a point at which the
vapor pressure is equal to the ambient atmospheric
pressure, evaporation increases significantly, and
boiling of the liquid takes place. The temperature at
which a liquid boils is commonly known as the liquid’s
boiling point. For water at sea level, the boiling point is
100°C.
9

 In a closed system (e.g., pipelines or pumps), water
vaporizes rapidly in regions where the pressure drops
below the vapor pressure. This phenomenon is known
as cavitation. The vapor bubbles formed in cavitation
usually collapse in a violent manner when they move
into higher pressure regions. This may cause
considerable damage to a system.
 Cavitation in a closed hydraulic system can be avoided
by maintaining the pressure above the vapor pressure
everywhere in the system.
10

1.3 MASS (DENSITY) AND WEIGHT (SPECIFIC
WEIGHT)
 In the International System of Units (SI),* the unit of
measurement for mass is either gram or kilogram (kg).
 The density of a substance is defined as the mass per
unit volume.
 It is a property inherent in the molecular structure of
the substance. This means that density depends not
only on the size and weight of the molecules but also on
the mechanisms by which the molecules are bonded
together. The latter usually varies as a function of
temperature and pressure.
11

 Because of its peculiar molecular structure, water is
one of the few substances that expands when it freezes.
 The expansion of freezing water when contained in a
closed container causes stresses on the container walls.
These stresses are responsible for the bursting of
frozen water pipes, the creation of cracks and holes in
road pavement, and the weathering of rocks in nature.
 Water reaches a maximum density at 4°C. It becomes
less dense when further chilled or heated.
 Seawater (or ocean water) contains dissolved salt. The
molecules that make up the salt have more mass than
the molecules they displace. Therefore, the density of
seawater is about 4 percent more than that of
freshwater.
12

 In the SI system, the weight of an object is defined by
the product of its mass (m, in grams, kilograms, etc.)
and the gravitational acceleration (g = 9.81 /sec2 on
Earth). The relationship may be written as
 Weight in the SI system is usually expressed in the
force units of newtons (N). One newton is defined as
the force required to accelerate 1kg of mass at a rate of
1m/sec2.
 The specific weight (weight per unit volume) of water
(ˠ) can be determined by the product of the density (ρ)
and the gravitational acceleration (g).
14

 The ratio of the specific weight of any liquid at a given
temperature to that of water at 4°C is called the
specific gravity of that liquid.
 The unit of mass in the British system is the slug. One
slug is defined as the mass of an object that requires
1lb of force to reach an acceleration of 1ft/sec2.
 In the British (Imperial) system, the mass of an object
is defined by its weight (ounce or pound) and the
gravitational acceleration (g = 32.2 ft/sec2 on Earth).
The relationship is written as
15

1.4 VISCOSITY OF WATER
 Water responds to shear stress by continuously
yielding in angular deformation in the direction of the
shear as shown
 This leads to the concept of viscosity.
17

 Consider that water fills the space between two
parallel plates (lightweight plastic) that are a distance
y apart. A horizontal force T is applied to the upper
plate and moves it to the right at velocity v while the
lower plate remains stationary. The shear force T is
applied to overcome the water resistance R, and it
must be equal to R because there is no acceleration
involved in the process.
 The resistance per unit area of the upper plate (shear
stress, τ = R/A = T/A) has been found to be proportional
to the rate of angular deformation in the fluid, dθ/dt.
The relationship may be expressed as
18

 The proportionally constant,µ, is the absolute viscosity
of the fluid. Previous Equation is known as Newton's
law of viscosity.
 Most liquids abide by this relationship are called
Newtonian fluids. Liquids that do not abide by this
linear relationship are known as Non- Newtonian
fluids. These include most house paints and human
blood.
 Absolute viscosity has the dimension of force per unit
area (stress) multiplied by the time interval
considered. It is usually measured in the unit of poise
(named after French engineer-physiologist J.L.M.
Poiseuille).
 The Absolute viscosity of water at room temperature
(20.2°C) is equal to 1 centipoise (cP), which is one one-
hundredth (1/100) of a poise:
19

 The absolute viscosity of air is approximately 0.018 cP
(roughly 2 percent of water).
 In engineering practice, it is often convenient to
introduce the term kinematic viscosity, ν, which is
obtained by dividing the absolute viscosity by the mass
density of the fluid at the same temperature: 𝜈 =
𝜇
𝜌
 The kinematic viscosity carries the unit of cm2/sec
(with the unit of stokes, named after British
mathematician G. G. Stoke).
21

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