This document contains 5 questions regarding fluid mechanics. Question 1 involves calculating the torque and power required to overcome viscous resistance in a rotating shaft. Question 2 involves calculating pressure drop, head loss, and power required for a given water flow rate through a pipe and orifice system. Question 3 determines the necessary counterweight to balance a water gate. Question 4 calculates the water level in a tank given pump specifications and a triangular weir. Question 5 determines if a hydraulic machine is a pump or turbine and calculates its power output or input.
2. Question No. 1 β6 Marksβ
The figure shows a bearing, in which a vertical shaft is rotating. An oil film
between the bottom surface of the shaft and a bearing is provided to
rotate. The viscous resistance is offered by the oil to the shaft. Let:
N = speed of the shaft, & R = radius of the shaft,
y = thickness of oil film & the area of the elementary ring = 2ο° r. dr
Estimate:
a) Total torque required to overcome
the viscous resistance,
a) The power βPβ absorbed.
4. Question No. 2 β16 Marksβ
Water is pumped at a rate of 20 cfs through the system shown in the figure.
a) What differential pressure will occur across the orifice?
b) What power must the pump supply to the flow for the given conditions?
c) Also, draw the HGL and the EGL for the system. Assume f = 0.015 for the
pipe.
9. Question No. 3 β10 Marksβ
The counterweight pivot gate shown in the figure, controls the flow from a
tank. The gate is rectangular and is 3m x 2 m.
Determine the value of the counterweight βWβ such that the upstream
water can be 1.5 m.
1.5 m
w
Pivot
10. ο΅ The hydrostatic force on the gate
πΉπΊ = π π βtβ π΄ = 1000 Γ 9.81 Γ 1.50 2 Γ 3 Γ 2 =
44145 π =044145 π 103 = 44.145 ππ
ο΅ tIt acts at βπ.π
= β +
πΌπ.π
π΄ β β
=
2
3
Γ 3.0 = 2.0 π ππππ£π π‘βπ βππππ ππ π‘βπ ππππππππ πππ‘π
w
1.5 m
π = πππ β1
1.5
3
= 60π
π
hinge
W sin 60
11. ο΅ Taking moments about the hinge
πΉπΊ Γ 3 β βπ.π = π sin 60π Γ 0.60
β΄ π =
πΉπΊ Γ 3 β βπ.π
sin 60π Γ 0.60
=
44.145 Γ 3 β 2
sin 60π Γ 0.60
= 84.96 πΎπ
ο΅ The value of the counterweight βWβ such that the upstream water can
be 1.5 m is 84.96 KN. (10 Marks)
12. Question No. 4 β12 Marksβ
A pump is used to deliver water from a well to a tank. The bottom of the tank is
2 m above the water surface in the well. The pipe is commercial steel 2.5 m
long with a diameter of 5 cm and f = 0.02. The pump develops a head of 20 m.
A triangular weir with an included angle of 60Β° βCd = 0.58β is located in a wall
of the tank with the bottom of the weir 1 m above the tank floor. Find the level
of the water in the tank above the floor of the tank.
13. Solution:
ο΅ The discharge through a triangular
Weir is expressed as
ο΅ ππ€=
8
15
πΆπ 2 π tan
π
2
β π»π€
5 2
β΄ For π = 60π & πΆπ = 0.58
ππ€ =
8
15
Γ 0.58 Γ 2 Γ 9.81 Γ tan
60
2
β π»π€
5 2
= 0.79 π»π€
5 2
ο΅ From the given data
ππ€ = 0.79 β β β 1 5 2
ο΅ To have a constant water level,
πππ’ππ = ππ€πππ
1
2
14. ο΅ Applying Bernoulli equation between points β1β and β2β gives,
π +
π
π π
+
π2
2 π 1
+ βπ = π +
π
π π
+
π2
2 π 2
+ βπΏ 1 β2
ο΅ Taking water surface at the lower tank as a reference datum
π +
π
π π
+
π2
2 π 1
+ βπ = π +
π
π π
+
π2
2 π 2
+ βπΏ 1 β2
ο΅ The head loss in the given system βπΏ 1 β2 =
8 π πΏ
π π2 π·5 Γ π2
=
8Γ0.02Γ2.5
9.81Γ π2 Γ0.055 Γ π2
= 13220.3 π2
0 0 0 2 + h 0 0
20 m
15. β΄ 20 = (2 + β) + 13220.3 π2
or
18 = β + 13220.3 π2
ο΅ But, πππ’ππ = 0.79 β β β 1 5 2
β΄ 18 = β + 13220.3 Γ 0.79 β β β 1 5 2 2
π. π. 18 = β + 8250 β β β 1 5
β΄ π΅π¦ π‘ππππ πππ πππππ "β" = 1.28 π
ο΅ The level of the water in the tank above the floor of the tank = 1.28 π.
(12 Marks)
16. Question No. 5 β16 Marksβ
i. For a hydraulic machine shown in the figure, the following data are available:
Flow : From βAβ to βBβ
Discharge: 200 L/s of water
Diameters: at βAβ 20 cm, and at βBβ 30 cm
Elevation (m): at βAβ 105 m, and at βBβ 100 m
Pressures : at βAβ 100 kpa, and at βBβ 200 kpa.
a) Is this machine a pump or a turbine?
b) Calculate the power input or output depending on whether it is pump or a
turbine.
17. For a hydraulic machine shown in the figure, the following data are available:
Flow : From βAβ to βBβ
Discharge : 200 L/s of water
Diameters : at βAβ 20 cm, and at βBβ 30 cm
Elevation (m) : at βAβ 105 m, and at βBβ 100 m
Pressures : at βAβ 100 KPa, and at βBβ 200 KPa.
a) Is this machine a pump or a turbine?
b) Calculate the power input or output depending on whether it is pump or a
turbine.
Solution:
Using the continuity equation
π1 = π π΄1 =
(200 1000)
1000 Γ π .0.20 2 4
= 6.37π π
and π2 = π π΄2 =
(200 1000)
1000Γ π .0.30 2 4
= 2.83 π π
18. Applying Bernoulliβs equation to point βAβ, gives the (TEL)A
π +
π
π π
+
π2
2 π π΄
= 105 +
100Γ103
103Γ9.81
+
6.372
2Γ9.81
= 117.26π
and that at point βBβ, (TEL)B
π +
π
π π
+
π2
2 π π΅
= 100 +
200 Γ 103
103 Γ 9.81
+
2.832
2 Γ 9.81
= 120.80π
ο΅ Since the TEL at βBβ is larger than that at βAβ (the friction losses is
assumed to be zero), this machine a pump with a head = 120.8 β
117.26 = 3.54 π
ο΅ To estimate the power
πππ€ππ = π π Γ π Γ βπ=
103Γ9.81Γ(200 103)Γ3.54
103 = 6.95 ππ. π π = 6.95 ππ€
(8 Marks)
19. 19
ii- A vertical circular tank of 600mm
diameter and 2.5m height is full of
water. It contains two orifices each of
1300 mm2 area, one at the bottom of
the tank and the other at a height of
1.25m above the bottom as shown in
figure.
ο Determine the time required to empty
the tank. Take coefficient of discharge
for both of the orifices as 0.62.
20. 20
Given:
β’ Diameter of the tank =600 = 0.60 m
β’ Height of the tank = 2.50 m
β’ Area of each orifice = 1300 mm2 = 1300x 10-6 m2
β’ C
d
= 0.60
ο For the sake of simplicity, let use divide the tank into two parts, i.e.,
ο first up to the center of the top orifice, and the to the bottom orifice.
ο First of all, consider first part of the example. In this case, the water
is flowing through both orifices.
ο Now consider an instant, when the height of water above center of
the top orifice be βhβ meters and the height of the water above the
bottom orifice will be βh +1.25β.
21. 21
The surface area of the tank is constant, π΄ =
πΓ0.62
4
= 0.283 π2
ο£ Now let us use the general equation for the time to empty the tank,
ππ‘ = β
π΄βπβ
πΆπβ ππππππππ 2π β
β (1)
or for this case (two identical orifices)
ππ‘ = β
π΄ β πβ
πΆπ β ππππππππ 2π β + πΆπ β ππππππππ 2π β + 1.25
=β
π΄βπβ
2. πΆπ Γ ππππππππ 2π β+ β+1.25
23. 23
Now consider the flow of water below the center of the top orifice. A
little consideration will show that now the water will be flowing through
the bottom orifice only. We know that the time required to empty the
tank,
π2 =
2 π΄ β
πΆπβ ππππππππ 2π
=
2 Γ 0.283 Γ 1.25
0.6 Γ 0.0013 Γ 2 Γ 9.81
= 183.16 sec
The total time,
π1+ π2= 55.82 + 183.16 = 239 sec β 4 min
(8 Marks)
24. 24
Question No. 6 β10 Marksβ
A sluice gate is used to control the water flow rate over a dam. The
gate is 20 ft wide, and the depth of the water above the bottom of the
sluice gate is 16 ft. The depth of the water upstream of the gate is 20 ft,
and the depth downstream is 3 ft.
i. Estimate the flow rate under the gate and the force on the gate.
πΉπ₯
25. Applying Bernoulliβs equation to point β1β and point β1β, gives,
(Neglecting minor losses and taking the bed of the channel as a
reference datum)
π +
π
π π
+
π2
2 π 1
= π +
π
π π
+
π2
2 π 2
For unit discharge
π +
π
π π
+
π2
2 π 1
= π +
π
π π
+
π2
2 π 2
=2
0
ο 0 π2
2 π π¦2
= 3 ο 0 π2
2 π π¦2
20 + 0 +
π2
2 π π¦1
2 = = 3 + 0 +
π2
2 π π¦1
2
29. ο State the assumptions upon which simple tank drainage problems
are based.
ο Water flows at a steady rate of 350 m3/ h into a vertical β sided
tank of area 10 m2. The water discharges continuously from a
sluice gate of area 0.0564 m2.
If the initial level in the tank is 2.50 m above the sluice, determine:
The final depth after 5 minutes assuming a discharge coefficient of
0.60 for the sluice.
( Ans. 0.60 m)
30. A long pipe is installed to carry water from one large reservoir to another (see the
attached figure). The total length of the pipe is of 10 km, its diameter is 0.5 m and
its roughness factor = 0.018. It must climb over a hill, so that the altitude changes
along with distance. The pump must be powerful to push 1 m3/s of water.
What is the pressure drop generated by the water flow?
What is the pumping power required to meet the design requirement,
What would be the power required for the same volume flow if the pipe is
doubled?
Layout of the water pipe (the vertical scale is greatly exaggerated). The
diameter of the pipe is also exaggerated.
31. 25 ft
L = 600 ft, D = ?? , f = 0.02
Main
A
B
Reference datum
The figure shows a pipe delivering water to the putting green on a golf
course. The pressure in the main is 80 p.s.i and it is necessary to
maintain a minimum of 60 p.s.i at point B to adequately supply a
sprinkler system. Specify the required size of steel pipe ( f = 0.02) to
supply 0.50 ft3/s.
Q = 0.5 ft3/s
Pipe Flow and Pipe Systems
32. Solution: Given:
Diameter of the circular disc π· = 200/1000 =0.20 m
β΄ π = D/2 =0.10 m
π¦ = 0.0004 m
π = 1000 r.p.m
π = 1.05 poise = 1.05/10 = 0.105 N.s/m2
Then
The power required to rotate the disc is given as
π =
π π2βπ2π 2β
1800 Γπ¦
=
0.105Γ π2Γ10002Γ0.12β
18000Γ0.0004
= 452.1 π€
Find the power required to rotate a circular disc of diameter 200 mm at
1000 r.p.m. The circular disc has a clearness of 0.4 mm from the bottom
flat plate and the clearness contains oil of viscosity 1.05 poise.
33. For the system shown in figure, calculate the height βHβ of oil
at which the rectangular hinged gate will just begin to rotate
counterclockwise.
Gate 1.5x 0.6 m
Oil
S.G.= 0.80
Air
H
1.5 m
30 KPa
Hinge
F1
F2
Oil
Pressure
Air Pressure
1.5 m
(a)
(b)
49. The Bernoulliβs equation can be considered to be a statement of the
conservation of energy principle appropriate for flowing fluids.
50.
51.
52.
53.
54.
55.
56.
57. A vertical circular tank of 600mm diameter and 2.5m height is full of
water. It contains two orifices each of 1300 mm2 area, one at the bottom
of the tank and the other at a height of 1.25m above the bottom as shown
in figure.
Determine the time required to empty the tank. Take coefficient of
discharge for both of the orifices as 0.62..
1.25 m
2.50 m
600 mm
58. Given,
ο¨ Diameter of the tank = 600 mm = 0.6 m & H = 2.5 m
Area of each orifice, a = 1300 mm2 & Cd = 0.62
For the sake of simplicity, let us divide the problem into two parts, i.e.,
first up to the center of the top orifice, and then up to the bottom orifice.
ο First of all, consider the first part of the problem. In that case, the water
is flowing through both the orifices.
ο Now consider an instant, when the height of water above the center of
the top orifice be h meters. At that instant, the height of water above
the bottom orifice will be (h + 1.25) meters.
ο We know that the surface area of the tank,
59. β΄ π΄π=
π
4
Γ π·π
2
=
π
4
Γ 0.602 = 0.283 π2
ο Now let us use the general equation for the time to empty a tank,
ππ‘ =
β π΄ πβ
πΆπ β π β 2 π β
=
β π΄ πβ
πΆπ β π β 2 π β + 1.25 + πΆπ β π β 2 π β
=
β π΄ πβ
πΆπ β 2 π β 2 π β + 1.25 + β
ο The time βT1β required to bring the water level up to the center of the
top orifice may be found by integrating the above equation between the
limits 1.25m and 0. Therefore
60. π1 =
1.25
0
β π΄ πβ
πΆπ β 2π β 2 π β + 1.25 + β
or π1 =
β π΄
πΆπ β2π β 2 π 1.25
0 πβ
β+1.25 + β
Multiplying the numerator and denominator by
β + 1.25 β β
π1 =
0.283
0.62 Γ 2 Γ 0.0013 Γ 2 Γ 9.81 0
1.25
β + 1.25 β β
1.25
π1 = 39.58 Γ 1.05π = 61.35 π
ο Now consider the flow of water below the center of the top orifice. A
little consideration will show that now the water will be flowing through
the bottom orifice only. We know that the time required to empty the
tank,
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