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Fluid Mechanics
Dr. Mohammed Zakria Salih Xoshnaw
57:020 Fluid Mechanics 2
History
Faces of Fluid Mechanics
Archimedes(C. 287-212 BC)
Newton(1642-1727)
Leibniz(1646-1716)
Euler(1707-1783)
Navier(1785-1836)
Stokes(1819-1903)
Reynolds(1842-1912)
Prandtl(1875-1953)
Bernoulli(1667-1748)
Taylor(1886-1975)
57:020 Fluid Mechanics 3
Weather & Climate
Tornadoes
HurricanesGlobal Climate
Thunderstorm
57:020 Fluid Mechanics 4
Vehicles
Aircraft
SubmarinesHigh-speed rail
Surface ships
57:020 Fluid Mechanics 5
Environment
Air pollution River hydraulics
57:020 Fluid Mechanics 6
Physiology and Medicine
Blood pump Ventricular assist device
57:020 Fluid Mechanics 7
Sports & Recreation
Water sports
Auto racing
Offshore racingCycling
Surfing
Background and introduction
• Physical Characteristics of Fluids
• Distinction between Solids, Liquids, and gases
• Flow Classification
• Significance of Fluid Mechanics
• Trends in Fluid mechanics
Physical Characteristics of Fluids
Statics Dynamics
Rigid Bodies
(Things that do not change shape)
Deformable Bodies
(Things that do change shape)
Incompressible Compressible
Fluids
Mechanics
Branch of Mechanics
Physical Characteristics of Fluids
• Fluid mechanics is the science that deals with the action of forces on fluids.
. Fluid is a substance
• The particles of which easily move and change position
• That will continuously deform
• A fluid can be either gas or liquid.
• Solid molecules are arranged in a specific lattice formation and their movement is restricted.
• Liquid molecules can move with respect to each other when a shearing force is applied.
• The spacing of the molecules of gases is much wider than that of either solids or liquids and it is also variable.
Distinction Between Solids, Liquids & Gases
Flow ClassificationThe subject of Fluid Mechanics
• Hydrodynamics deal with the flow of fluid with no density change, hydraulics, the study of fluid force on bodies immersed in flowing liquids or in low speed gas flows.
• Gas Dynamics deals with fluids that undergo significant density change
• Turning on our kitchen faucets
• Flicking on a light switch
• Driving cars
• The flow of bloods through our veins
• Coastal cities discharge their waste
• Air pollution
• And so on so forth …
Significance of Fluid Mechanics
Trends in Fluid Mechanics
• The science of fluid mechanics is developing at a rapid rate.
Fluid Mechanics
FLUID PROPERTIES
Today’s subject:
Objectives of this section• Work with two types of units.
• Define the nature of a fluid.
• Show where fluid mechanics concepts are common with those of solid mechanics and indicate some fundamental areas of difference.
• Introduce viscosity and show what are Newtonian and non-Newtonian fluids
• Define the appropriate physical properties and show how these allow differentiation between solids and fluids as well as between liquids and gases.
UNIT SYSTEMS
• SI UNITS
In the SI system, the unit of force, the Newton,
is derived unit. The meter, second and
kilogram are base units.
• U.S. CUSTOMORY
In the US Customary system, the unit of mass,
the slug, is a derived unit. The foot, second
and pound are base unit.
• We will work with two unit systems in FLUID MECHANICS:
• International System (SI)
• U.S. Customary (USCS)
Basic Unit System & Units
Derived Units
There are many derived units all obtained from combination of the above
primary units. Those most used are shown in the table below:
The SI system consists of six primary units, from which all
quantities may be described but in fluid mechanics we are generally
only interested in the top four units from this table.
Derived Units
Table summarizes these unit systems.
SI System of Units• The corresponding unit of force derived from Newton’s
second law:
“ the force required to accelerate a kilogram at one meter per second per second is defined as the Newton (N)”
The acceleration due to gravity at the earth’s surface: 9.81 m/s2.
Thus, the weight of one kilogram at the earth’s surface:
W = m g
= (1) (9.81) kg m / s2
= 9.81 N
Traditional Units• The system of units that preceded SI units in several countries is the so-called English system.
Length = foot (ft) = 30.48 cm
Mass = slug = 14.59 kg
The force required to accelerate a mass of one slug at one foot per second per second is one pound force (lbf).
The mass unit in the traditional system is the pound mass (lbm).
FLUID PROPERTIES
Specific Weight
Mass Density
Viscosity
Vapour Pressure
Surface tension
Capillarity
Bulk Modules of Elasticity
Isothermal Conditions
Adiabatic or Isentropic
Conditions
Pressure Disturbances
Every fluid has certain characteristics by which its physical conditions may be
described.
We call such characteristics as the fluid properties.
Properties involving the Mass or Weight of the Fluid
Specific Weight, g
The gravitational force per unit volume of fluid, or simply “weight per unit volume”.
- Water at 20 oC has a specific weight of 9.79 kN/m3.
Mass Density, ρ
The “mass per unit volume” is mass density. Hence it has units of kilograms per cubic meter.
- The mass density of water at 4 oC is 1000 kg/m3
while it is 1.20 kg/m3 for air at 20 oC at standard pressure.
• The ratio of specific weight of a given liquid to the specific weight of water at a standard reference temperature (4 oC)is defined as specific gravity, S.
• The specific weight of water at atmospheric pressure is 9810 N/m3.
• The specific gravity of mercury at 20 oC is
Specific Gravity, S
6.133kN/m 9.81
3kN/m 133S Hg
Ideal Gas Law
• p = absolute pressure [N/m2], 14.7 psi or 101 kpa
• V = volume [m3]
• n = number of moles
• Ru = universal gas constant
• [8.314 kJ/kmol-K; 0.287 kPa·m3/kg ·K]
• T = absolute temperature [K]
• MWgas = molecular weight of gas
A form of the general equation of state, relating pressure, specific volume, and temperature
British Gravitational (BG) System. In the BG system the unit of length is the foot (ft), the time unit is the second (s), the force unit is the pound (lb), and the temperature unit is the degree Fahrenheit (°F) or the absolute temperature unit is the degree Rankine(°R) °R= °F+ 459.67where The mass unit, called the slug, is defined from Newton’s second law (Force x Acceleration ) as1 Ib = (1 Slug). (1 ft/s2)This relationship indicates that a 1-lbforce acting on a mass of 1 slug will give the mass an acceleration of 1 ft/s2
The weight, (which is the force due to gravity, g) of a mass, m, is given by the equation.W= mg and in BG unitsw(lb) = m(slugs) g (ft/s2)g = 32.2 ft/s2it follows that a mass of 1 slug weighs 32.2 lb under standard gravity
VISCOSITY
• What is the definition of “strain”?
“Deformation of a physical body under the action of applied forces”
• Solid:
– shear stress applied is proportional to shear strain
(proportionality factor: shear modulus)
– Solid material ceases to deform when equilibrium is reached
• Liquid:
– Shear stress applied is proportional to the time rate of strain
(proportionality factor: dynamic (absolute) viscosity)
– Liquid continues to deform as long as stress is applied
Example of the effect of viscosity
• Think: resistance to flow.
• V : fluid velocity
• y : distance from solid surface
• Rate of strain, dV/dy
• μ : dynamic viscosity [N.s/m2]
t: shear stress
Shear stress: An applied force per unit area needed to produce deformation in a fluid
t = μ dV/dy
Velocity distribution next to boundary
VISCOSITY µ
• Would it be easier to walk through a 1-m pool of water or oil?
– Water
Why?
– Less friction in the water
• Rate of deformation
– Water moves out of your way at a quick rate when you apply a shear stress (i.e., walk through it)
– Oil moves out of your way more slowly when you apply the same shear stress
t = μ dV/dy
Viscosity is:
• slope of the line shown above
• the ratio between shear stress
applied and rate of deformation
Kinematic Viscosity• Many fluid mechanics equations contain the variables of
- Viscosity, m
- Density, r
So, to simplify these equations sometimes use kinematic viscosity (n)
Terminology
Viscosity, m
Absolute viscosity, m
Dynamic viscosity, m
Kinematic Viscosity, n
smmkg
msN/
/
/. 2
3
2
Other viscosity highlights
• Viscous resistance is independent of the pressure in the fluid.
• Viscosity is a result of molecular forces within a fluid.
• For liquid, cohesive forces decrease with increasing temperature → decreasing μ
• For gas, increasing temperature → increased
molecular activity & shear stress: increasing μ
Kinematic viscosity for air & crude oilIncreasing temp → increasing
viscosity
Increasing temp → decreasing
viscosity
Newtonian vs. Non-Newtonian Fluids
• Newtonian fluid: shear stress is proportional to shear strain
– Slope of line is dynamic viscosity
• Shear thinning: ratio of shear stress to shear strain decreases as shear strain increases (toothpaste, catsup, paint, etc.)
• Shear thickening: viscosity increases with shear rate (glass particles in water, gypsum-water mixtures).
Surface tension
• What’s happening here?
– Bug is walking on water
• Why is this possible?
– It doesn’t weigh much
– It’s spreading its weight out
– The downward forces are less than the effects of surface tension
Surface Tension
• A molecules in the interior of a liquid is under attractive force in all direction.
• However, a molecule at the surface of a liquid is acted on by a net inward cohesive force that is perpendicular to the surface.
• Hence it requires work to move molecules to the surface against this opposing force and surface molecules have more energy than interior ones
• Higher forces of attraction at surface
• Creates a “stretched membrane effect”
Surface Tension• Surface tension, σs: the force resulting from
molecular attraction at liquid surface [N/m]
• surface tension varies with temperature
Fs= σs L
Fs= surface tension force [N]
σs = surface tension [N/m]
L = length over which the surface tension acts [m]
CapillarityRise and fall of liquid in a capillary tube is caused by surface tension.
Capillarity depends on the relative magnitudes of the cohesion of the liquid
to walls of the containing vessel.
When the adhesive forces between liquid and solid are larger than the
liquid's cohesive forces, the meniscus in a small diameter tube will tend to
be concave
If adhesive forces are smaller than cohesive forces the meniscus will tend
to be convex, for example mercury in glass.
water mercury
concaveconvex
Differences between adhesive & Cohesive
A distinction is usually made between an adhesive force,
which acts to hold two separate bodies together (or to stick
one body to another)
and
a cohesive force, which acts to hold together the like or unlike
atoms, ions, or molecules of a single body.
h=height of capillary rise (or depression)
s=surface tension
q=wetting angle
G=specific weight
R=radius of tube
If the tube is clean, qo is 0 for water
Capillary EffectFor a glass tube in a liquid…
0, WF z
hRCosR 22
r
Cosh
2
Vapor PressureVapor pressure: the pressure at which
a liquid will boil.
Vapor pressure ↑ when
temperature increases
At atmospheric pressure,
water at 100 °C will boil
Water can boil at lower
temperatures if the
pressure is lower
When vapor pressure > the
liquid’s actual pressure
Fundamentals of Fluid Mechanics 41
Coefficient of Compressibility
• How does fluid volume change with P and T?
• Fluids expand as T ↑ or P ↓
• Fluids contract as T ↓ or P ↑
Fundamentals of Fluid Mechanics 42
Coefficient of Compressibility
• Need fluid properties that relate volume changes to changes in P and T.
– Coefficient of compressibility
– k must have the dimension of pressure (Pa or psi).
– What is the coefficient of compressibility of a truly incompressible substance ?(v=constant).
T T
P Pv
v
09:10
(or bulk modulus of compressibility
or bulk modulus of elasticity)
is infinity
Fundamentals of Fluid Mechanics 43
Coefficient of Compressibility
• A large implies incompressible.
• This is typical for liquids considered to be incompressible.
• For example, the pressure of water at normal atmospheric conditions must be raised to 210 atmto compress it 1 percent, corresponding to a coefficient of compressibility value of = 21,000 atm.
09:10
Fundamentals of Fluid Mechanics 44
Coefficient of Compressibility
• Small density changes in liquids can still cause interesting phenomena in piping systems such as the water hammer—characterized by a sound that resembles the sound produced when a pipe is “hammered.” This occurs when a liquid in a piping network encounters an abrupt flow restriction (such as a closing valve) and is locally compressed. The acoustic waves produced strike the pipe surfaces, bends, and valves as they propagate and reflect along the pipe, causing the pipe to vibrate and produce the familiar sound.
09:10
Fundamentals of Fluid Mechanics 45
Coefficient of Compressibility
• Differentiating = 1/v gives d = - dv/v2; therefore, d/ = -dv/v
• For an ideal gas, P = RT and (∂P/∂)T = RT = P/, and thus
ideal gas = P (Pa)
• The inverse of the coefficient of compressibility is called the isothermal compressibility a and is expressed as
09:10
Fundamentals of Fluid Mechanics 46
09:10
Coefficient of Volume Expansion
The density of a fluid depends
more strongly on temperature
than it does on pressure.
To represent the variation of
the density of a fluid with
temperature at constant
pressure. The Coefficient of
volume expansion (or volume
expansivity) is defined as
1 1
P P
v
v T T
(1/K)
Fundamentals of Fluid Mechanics 47
Coefficient of Volume Expansion
• For an ideal gas, ideal gas = 1/T (1/K)
• In the study of natural convection currents, the condition of the main fluid body that surrounds the finite hot or cold regions is indicated by the subscript “infinity” to serve as a reminder that this is the value at a distance where the presence of the hot or cold region is not felt. In such cases, the volume expansion coefficient can be expressed approximately as
• where is the density and T is the temperature of the quiescent fluid away from the confined hot or cold fluid pocket.
09:10
Fundamentals of Fluid Mechanics 4809:10
Coefficient of Compressibility
The combined effects of pressure and temperature
changes on the volume change of a fluid can be
determined by taking the specific volume to be a
function of T and P. Differentiating v = v(T, P) and
using the definitions of the compression and expansion
coefficients a and give
P T
v vdv dT dP
T P
= (dT - adP)v
• What is the weight of a pound mass on the earth’s surface, where the acceleration due to gravity is 32.2 ft/s2, and on the moon’s surface, where the acceleration is 5.31 ft/s2.
• Solution by Newton’s second law
W=Mg (lbf = slug*ft/s2)
Example 2.1:
2.32
1
/2.32
11 slugs
sluglbm
lbm
g
lbmM
c
Example 2.1: Cont…….
Therefore, the weight on the earth’s surface is
And on the moon’s surface is
lbf1s
ft32.2x
32.2
1slugsW
2
lbf165.0s
ft5.31x
32.2
1slugsW
2
Example 2.2: Capillary Rise Problem
• How high will water rise in a glass tube if the inside diameter is 1.6 mm and the water temperature is 20°C?
Answer: 18.6 mm
• Hint: for water against glass is so small it can be assumed to be 0.N/m 073.0
Example 1• A) calculate the density , specific weight and specific volume of Oxygen at 100 °F and 15 Psi.
• B) what would be the temperature and or pressure of this gas if it were compressed isentropically to 40 percent of its origin volume.
• C) if the process described in (b) had been isothermal , what would the temperature and pressure have been.
Solution
a) ρ= P/RT = 15*144/(1552)*(100+460) = 0.00248 slug/ft3
γ= ρ g= 0.00248(32.2)= 0.0799 Ib/ ft3
Vs= 1/ ρ = 1/0.00248 = 403 ft3/slug
b) P1(Vs1)K = P2(Vs2)
k =P2= 54.1 Psi
P2 = ρ2 RT2 (54.1)*144= (0.00218/0.4)*1552*(T2+460)
T2 = 350 °F
c) If its isothermal , T2=T1= 100 °F
15*144*403= P2(144*0.4*403) =
• P2= 37.5 Psi
Example 2• What is specificweight of air at 70 psi and 70°F , R= 53.3 ft/°R
• γ = 70(144) /(53.3)*(70+460)= 0.357 Ib/ft3
Example 3• A cylinder contains 12.5 ft3 of air at 120 °F and 40 Pisa, The air compressed to 2.5 ft3
A) assuming isothermal condition what a pressure at the new volume and bulk modules of elasticity
B) assuming adiabatic conditions, what are the final pressure and temperature and the bulk modules of elasticity for isothermal condition
Solution
A) P1V1= p2V2 for isothermal P2 = 200 Pisa
K = ( ∆P/∆V/V)= ( 40- 200)/(12.5-2.5)/12.5= 200 psi
b) P1 (V1)k = P2 (V2)
k, k= 1.4 ,
P2 381 Pisa
T2/T1= (P2/P1) k-1/k,
T2 = 1104 °R or 644 °F ,
K = bulk modules= k* P2 = 583 Ps
Example 4
Example 5
Example 6
Example 7
Example 8