Chapter13: Fluid Mechanics, Kane

1. Chapter 13
Fluid Mechanics
(
)

2. States of Matter

Solid


Liquid


Has a definite volume and shape
Has a definite volume but not a definite shape
Gas – unconfined

Has neither a definite volume nor shape

3. States of Matter, cont


All of the previous definitions are somewhat
artificial
More generally, the time it takes a particular
substance to change its shape in response to
an external force determines whether the
substance is treated as a solid, liquid or gas

4. Fluids


A fluid is a collection of molecules that are
randomly arranged and held together by weak
cohesive forces and by forces exerted by the
walls of a container
Both liquids and gases are fluids

5. Statics and Dynamics with
Fluids

Fluid Statics


Fluid Dynamics


Describes fluids at rest
Describes fluids in motion
The same physical principles that have applied
to statics and dynamics up to this point will
also apply to fluids

6. Forces in Fluids



Fluids do not sustain shearing stresses or tensile
stresses (
)
The only stress that can be exerted on an object
submerged(
) in a static fluid is one that tends to
compress(
) the object from all sides
The force exerted by a static fluid on an object is always
perpendicular to the surfaces of the object

7. Pressure(

The pressure P of the
fluid at the level to which
the device has been
submerged is the ratio of
the force to the area
P
F
A
)

8. Pressure, cont

Pressure is a scalar quantity



Because it is proportional to the magnitude of the
force
If the pressure varies over an area, evaluate
dF on a surface of area dA as dF = P dA
Unit of pressure is pascal (Pa)
2
1Pa 1 N/m

9. Pressure vs. Force


Pressure is a scalar and force is a vector
The direction of the force producing a
pressure is perpendicular to the area of
interest

10. Measuring Pressure



The spring is calibrated
by a known force
The force due to the fluid
presses on the top of the
piston and compresses
the spring
The force the fluid exerts
on the piston is then
measured

11. Density Notes

12. Density Table

13. Variation of Pressure with Depth



Fluids have pressure that varies with
depth
If a fluid is at rest in a container, all
portions of the fluid must be in static
equilibrium
All points at the same depth must be at
the same pressure

Otherwise, the fluid would not be in equilibrium

14. Pressure and Depth


Examine the darker
region, a sample of liquid
within a cylinder
 It has a cross-sectional
area A
 Extends from depth d
to d + h below the
surface
Three external forces act
on the region

15. Pressure and Depth, cont

The liquid has a density of



Assume the density is the same throughout the
fluid
This means it is an incompressible liquid
The three forces are:



Downward force on the top, P0A
Upward on the bottom, PA
Gravity acting downward, Mg
The mass can be found from the density:
M
V
Ah

16. Atmospheric Pressure
(
)


If the liquid is open to the atmosphere, and P0
is the pressure at the surface of the liquid, then
P0 is atmospheric pressure
P0 = 1.00 atm = 1.013 x 105 Pa
at see level

17. Pascal’s Law (


)
The pressure in a fluid depends on depth
and on the value of P0
Pascal’s law state that an increase in
pressure at the surface must be
transmitted to every other point in the
fluid

18. Pascal’s Principle
• A change in the pressure applied to an
enclosed incompressible fluid is transmitted
undiminished to every portion of the fluid and
to the walls of its container.

19. Pascal’s Law applications

20. Archimedes’ principle

21. Bernoulli’s principle
• If the speed of a fluid element increases
as the element travels along a horizontal
streamline, the pressure of the fluid must
decrease, and conversely.

22. Temperature (




)
We associate the concept of temperature with
how hot or cold an object feels
Our senses provide us with a qualitative
indication of temperature
Our senses are unreliable for this purpose
We need a reliable and reproducible method
for measuring the relative hotness or coldness
of objects

We need a technical definition of temperature

23. Thermal Contact (

)
Two objects are in thermal contact with each
other if energy can be exchanged between
them


The exchanges we will focus on will be in the form
of heat or electromagnetic radiation
The energy is exchanged due to a temperature
difference

24. Thermal Equilibrium(

)
Thermal equilibrium is a situation in
which two objects would not exchange
energy by heat or electromagnetic
radiation if they were placed in thermal
contact

The thermal contact does not have to also
be physical contact

25. Zeroth Law of Thermodynamics

If objects A and B are separately in
thermal equilibrium with a third object C,
then A and B are in thermal equilibrium
with each other


Let object C be the thermometer
Since they are in thermal equilibrium with
each other, there is no energy exchanged
among them

26. Zeroth Law of Thermodynamics,
Example

Object C (thermometer) is placed in contact with A until they achieve
thermal equilibrium


Object C is then placed in contact with object B until they achieve thermal
equilibrium


The reading on C is recorded
The reading on C is recorded again
If the two readings are the same, A and B are also in thermal equilibrium

27. Temperature – Definition


Temperature can be thought of as the
property that determines whether an object is
in thermal equilibrium with other objects
Two objects in thermal equilibrium with each
other are at the same temperature

If two objects have different temperatures, they are
not in thermal equilibrium with each other

28. Thermometers


A thermometer is a device that is used
to measure the temperature of a system
Thermometers are based on the principle
that some physical property of a system
changes as the system’s temperature
changes

29. Thermometers, cont

These properties include:







The volume of a liquid
The dimensions of a solid
The pressure of a gas at a constant volume
The volume of a gas at a constant pressure
The electric resistance of a conductor
The color of an object
A temperature scale can be established on the
basis of any of these physical properties

30. Thermometer, Liquid in Glass



A common type of
thermometer is a liquid-inglass
The material in the
capillary tube (
)
expands as it is heated
The liquid is usually
mercury or alcohol

31. Calibrating a Thermometer


A thermometer can be calibrated by placing it
in contact with some natural systems that
remain at constant temperature
Common systems involve water



A mixture of ice and water at atmospheric pressure
Called the ice point of water
A mixture of water and steam in equilibrium
Called the steam point of water
Once these points are established, the length
between them can be divided into a number of
segments

32. Celsius Scale

The ice point of water is defined to be 0o C

The steam point of water is defined to be 100o C

The length of the column between these two
points is divided into 100 increments, called
degrees

33. Problems with Liquid-in-Glass
Thermometers



An alcohol thermometer and a mercury thermometer may
agree only at the calibration points
The discrepancies between thermometers are especially
large when the temperatures being measured are far
from the calibration points
The thermometers also have a limited range of values
that can be measured


Mercury cannot be used under –39o C
Alcohol cannot be used above 85o C

34. Constant-Volume Gas
Thermometer


The physical change
exploited is the variation of
pressure of a fixed volume
gas as its temperature
changes
The volume of the gas is
kept constant by raising or
lowering the reservoir B to
keep the mercury level at A
constant

35. Constant-Volume Gas
Thermometer, cont



The pressure is indicated by the height
difference between reservoir B and column A
The thermometer is calibrated by using a ice
water bath and a steam water bath
The pressures of the mercury under each
situation are recorded


The volume is kept constant by adjusting A
The information is plotted

36. Constant-Volume Gas
Thermometer, final



To find the temperature of
a substance, the gas flask
is placed in thermal
contact with the
substance
The pressure is found on
the graph
The temperature is read
from the graph

37. Absolute Zero



The thermometer readings are
virtually independent of the
gas used
If the lines for various gases
are extended, the pressure is
always zero when the
temperature is
–273.15o C
This temperature is called
absolute zero

38. Absolute Temperature Scale



Absolute zero is used as the basis of the
absolute temperature scale
The size of the degree on the absolute scale is
the same as the size of the degree on the
Celsius scale
To convert:

TC = T – 273.15

39. Absolute Temperature Scale, 2

The absolute temperature scale is now based
on two new fixed points



Adopted by in 1954 by the International Committee
on Weights and Measures
One point is absolute zero
The other point is the triple point of water
This is the combination of temperature and pressure
where ice, water, and steam can all coexist

40. Absolute Temperature Scale, 3


The triple point of water occurs at
0.01o C and 4.58 mm of mercury
This temperature was set to be 273.16 on the
absolute temperature scale


This made the old absolute scale agree closely
with the new one
The units of the absolute scale are kelvins

41. Absolute Temperature Scale, 4

The absolute scale is also called the Kelvin
scale


The triple point temperature is 273.16 K


Named for William Thomson, Lord Kelvin
No degree symbol is used with kelvins
The kelvin is defined as 1/273.16 of the
difference between absolute zero and the
temperature of the triple point of water

42. Some Examples of Absolute
Temperatures



The figure at right gives
some absolute
temperatures at which
various physical
processes occur
The scale is logarithmic
The temperature of
absolute zero cannot be
achieved

Experiments have come
close

43. Fahrenheit Scale





A common scale in everyday use in the
US
Named for Daniel Fahrenheit
Temperature of the ice point is 32oF
Temperature of the steam point is 212oF
There are 180 divisions (degrees)
between the two reference points

44. viscosity



The term viscosity is commonly used in the
description of fluid flow to characterize the degree
of internal friction in the fluid.
This internal friction, or viscous force, is associated
with the resistance that two adjacent layers of fluid
have to moving relative to each other.
Viscosity causes part of the kinetic energy of a fluid
to be converted to internal energy. This mechanism
is similar to the one by which an object sliding on a
rough horizontal surface loses kinetic energy.