Chapter 13 Fluids

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Chapter 13 - Fluids

• Pressure variation with height or depth
• Pascals Principle
• Archimedes Principle
• Fluid Flow
• Bernoullis Equation

2
Fluid

• Matter that cannot maintain its own shape and
  therefore flows readily under the influence of
  forces
• Gases Do not maintain their size (volume)
• Liquids Do maintain their size
  incompressible
• Solids maintain both their size and shape under
  the influence of forces

3
Density

• Which is heavier, wood or iron?
• Amount of mass per unit volume
• Compressible - Density easily altered
• Incompressible - Density varies only slightly or
  not at all.
• Specific Gravity - ratio of the density of a
  material to that of water

4
Pressure

• The normal force per unit area that a fluid
  exerts on the walls of its container, adjacent
  fluids or other boundaries.
• Units N/m2 1 Pascal
• 1 Bar 105 Pa
• 1 atm 1.013 x 105 Pa
• 1 atm 14.7 lb/in2
• 1 atm 760 Torr (mm Hg)

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Variation of pressure with height (depth)
6
Variation of pressure with height (depth)
7
Problem 1 What is the pressure at the faucet?
What do you do if you want more pressure at the
faucet?
8
Absolute vs. Gauge Pressure

• Gauge Pressure That read on a gauge which
  compares it to atmospheric pressure
• Absolute Pressure Sum of the gauge pressure and
  atmospheric pressure

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Problem 2

• What is the pressure at a depth of 1300 feet
  (approximately 400 m)
• rw 1000 kg/m3

10
Problem 3

• What is the height of a mercury column if the
  pressure at the bottom is 101.325 kPa and the
  pressure at the top is zero?
• rHg 13.595 x 103 kg/m3

h
11
Pascals Principle

• An increase in the pressure at any point in a
  confined fluid is transmitted undiminished
  throughout the fluid volume and to the walls of
  the container.

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Problem 4

• In the system below, a 1.0 N force is applied to
  the piston at the left. The piston is moved 5.0
  cm
• What is the force on the large mass on the right.
• How far can the large block move?

A 0.10 m2
A 1.0 m2
13
Archimedes Principle

• The buoyancy force on an object is equal to the
  weight of the fluid it displaces.
• Pressure at the top of an object is less than at
  the bottom

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Problem 5

• What volume of water must be displaced for a 6900
  Ton submarine to hover?

15
Fluid dynamics - equation of continuity

• Laminar flow
• Mass flow rate is constant

16
Problem 4

• Water leaves the nozzle of a firehose at 50 m/s.
  What is the velocity of water in the hose.
• Hose inner diameter 10 cm
• Nozzle inner diameter 3 cm
• Using the rocket equation, how much thrust does a
  fireman feel from this exiting fluid?

17
Bernoullis principal

• Where the velocity of a fluid is high, the
  pressure of a fluid is low, and where the
  velocity is low, the pressure is high.

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Bernoullis equation
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Bernoullis equation
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Problem 5

• Find the pressure in the fire hose with exhaust
  velocity 50 m/s

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Problem 6

• What is the velocity of the fluid leaving the
  pipe at point 2.
• A small pipe directs the water upward. What is
  the height of the plume?

1
h
2
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Problem 7 (47)

• What gauge pressure is necessary in a fire main
  if a firehose is to spray water to a height of 15
  m?

23
Problem 8 (50)

• If the wind blows at 25 m/s over your house, what
  is the net force on the roof if its area is 240
  m2?