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ME 330 Fluid Mechanics
Spring 2007
Department of Engineering
Western Kentucky University
CHAPTER 3B HW  Homework Solutions
Problem #’s 328, 3 36, 338E, 345, 346 and 363E, Due February 9, 2007
328
Solution
The air pressure in a tank is measured by
an oil manometer. For a given oillevel difference
between the two columns, the absolute pressure in the
tank is to be determined.
Properties
The density of oil is given to be
ρ
= 850 kg/m
3
.
Analysis
The absolute pressure in the tank is determined from
32
2
1 kPa
(98 kPa)
(850 kg/m )(9.81 m/s )(0.45m)
1000 N/m
101 75 kPa
atm
PP
g
h
.
=+
⎛⎞
⎜⎟
⎝⎠
=≅
102 kPa
Discussion
If a heavier liquid, such as water, were used for the manometer fluid, the
column height would be smaller, and thus the reading would be less precise (lower
resolution).
336
Solution
Fresh and seawater flowing in parallel horizontal pipelines are connected to
each other by a double Utube manometer. The pressure difference between the two
pipelines is to be determined.
Assumptions
1
All the liquids are
incompressible.
2
The effect of air
column on pressure is negligible.
Properties
The densities of seawater and
mercury are given to be
sea
= 1035 kg/m
3
and
Hg
= 13,600 kg/m
3
. We take the
density of water to be
w
=1000 kg/m
3
.
Analysis
Starting with the pressure in
the fresh water pipe (point 1) and moving
along the tube by adding (as we go down)
or subtracting (as we go up) the
gh
terms
until we reach the sea water pipe (point 2),
and setting the result equal to
P
2
gives
AIR
P
atm
= 98 kPa
0.45 m
Fresh
water
h
se
h
air
Sea
water
Mercury
Air
h
Hg
h
w
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View Full Document ME 330 Fluid Mechanics
Spring 2007
Department of Engineering
Western Kentucky University
2
sea
sea
air
air
Hg
Hg
w
1
P
gh
gh
gh
gh
P
w
=
+
−
−
+
ρ
Rearranging and neglecting the effect of air
column on pressure,
)
(
sea
sea
w
Hg
Hg
sea
sea
Hg
Hg
w
2
1
h
h
h
g
gh
gh
gh
P
P
w
w
−
−
=
−
+
−
=
−
Substituting,
kPa
3.39
=
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
−
−
=
−
2
2
3
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This homework help was uploaded on 04/19/2008 for the course ME 330 taught by Professor An during the Spring '08 term at Western Kentucky University.
 Spring '08
 an

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