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Fluid Mechanics-61341
An-Najah National UniversityCollege of Engineering
Chapter [2]
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics Dr. Sameer Shadeed1
Dr. Sameer Shadeed
Chapter [2]
Fluid Statics
Fluid Statics Problems
Fluid statics refers to the study of fluids at rest ormoving in such a manner that no shearingstresses exist in the fluid
These are relatively simple problems since no
Dr. Sameer Shadeed2
These are relatively simple problems since novelocity gradients exist. Thus, viscosity does notplay a role
Applications include the hydraulic pressure,manometry, dams, and fluid containment (tanks)
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
Pressure (P) is the force per unit area, where the force isperpendicular to the area
P (N/m2 or Pa) =A (m2)
F (N)
Pressure
1 kN/m2 = 1 kPa
1 kPa = 0.145 psi
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics Dr. Sameer Shadeed3
Pressure in a fluid acts equally in all directions
Pressure in a static liquid increases linearly with depth
increase in depth (m)
pressure increase
p= γ h
Pressure at a Point
Pressure is a scalar quantity that is defined atevery point within a fluid
Dr. Sameer Shadeed4 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
Force balance in the x-direction:
Pressure at a Point
Force balance in the z-direction:
Vertical force on DA Vertical force on lower boundary
Total weight of wedge element
= specific weight
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics5 Dr. Sameer Shadeed
From last slide:
Divide through by to get
Pressure at a Point
Now shrink the element to a point:
This can be done for any orientation, so
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics6 Dr. Sameer Shadeed
The result shows that pressure at any pointin a fluid at rest has a single value,independent of direction as long as thereare no shearing stresses (or velocitygradients) present in the fluid
Pressure at a Point
Dr. Sameer Shadeed7
gradients) present in the fluid
For fluids in motion with shearing stresses,this result is not exactly true, but is still avery good approximation for most flows
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
Pressure Transmission
In a closed system, pressure changes from onepoint are transmitted throughout the entire system(Pascal’sPascal’s LawLaw).
Hydraulic LiftFluid Mechanics-2nd Semester 2010- [2] Fluid Statics8 Dr. Sameer Shadeed
outout in
in
AF F
A
Pressure Variation with Elevation
Static fluid: All forces must balance as there are no accelerations
Look at force balance in direction of D ll
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics9 Dr. Sameer Shadeed
From the previous figure, note that
Shrink cylinder to zero length:
Pressure Variation with Elevation
From the previous slide:
or
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics10 Dr. Sameer Shadeed
The pressure-elevation relation derived on the previous slide,
is perfectly general (applies also to variable gg)
But if gg is constant, the above equation is easy to integrate:
Pressure Variation with Elevation
The quantity is known as the piezometric pressure
and is called the piezometric head
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics11 Dr. Sameer Shadeed
For an incompressible fluid (gg is constant), pressure andelevation at one point can thus be related to pressure andelevation at another point as:
Pressure Variation with Elevation
Constant22
11 z
pz
p
or
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics12 Dr. Sameer Shadeed
21
12
pphzz
hzzppp )( 1221
or
or
Absolute and Gage Pressure
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics13 Dr. Sameer Shadeed
Absolute and Gage Pressure
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What is the pressure at the faucet?
What do you do if you want more pressure at the faucet?Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics15 Dr. Sameer Shadeed
Blood Pressure
26.8 k Pa
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Example 1
Solution:
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics17 Dr. Sameer Shadeed
What is the water pressure at a depth of 35 ft?
Solution: With the information given, all we can calculate is the pressure difference between points 1 and 2
Example 2
and 2
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics18 Dr. Sameer Shadeed
What is the gage pressure at point 3?
Solution: Two step solution:1) Calculate2) Calculate
Example 3
s.g.=0.8
(relative to atmospheric pressure at point 1)
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics19 Dr. Sameer Shadeed
Pressure Measurement
MechanicalMechanical PressurePressure GagesGages
The BourdonBourdon pressure and AneroidAneroid barometerare typical mechanical devices for measuring gageand absolute pressures, respectively
Dr. Sameer Shadeed20 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
Pressure Measurement
LiquidLiquid PressurePressure GagesGages
ManometerManometer:: gravimetric device based upon liquid leveldeflection in a tube
Mercury barometer: evacuated glass tube with openend submerged in mercury
Dr. Sameer Shadeed21 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
The Manometer
Simple, accurate device for measuring small tomoderate pressure differences
Rules of ManometryManometry:
pressure change across a fluid column ofgg
Dr. Sameer Shadeed22
height h is ggh
pressure increases in the direction of gravity,decreases in the direction opposing gravity
two points at the same elevation in acontinuous static fluid have the samepressure
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
The Manometer
Dr. Sameer Shadeed23 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
hlp
pp
x 1
21
0
The Manometer
Dr. Sameer Shadeed24 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
hlplp
pp
yx 32211
54
)(01 gagepp atm
The Manometer
lpp
plhp
pphp
m
m
34
41
321
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics25 Dr. Sameer Shadeed
Find the location of the surface in the manometer
Solution: The distance Dhis the height of the liquid in the manometer above the heavier liquid in the tank
A
B C
D
Example 4
heavier liquid in the tank
cmh
h
pp
m
w
wm
BC
33.33
110
10
10
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics26 Dr. Sameer Shadeed
A
C
Find the gage pressure at the pipe center
Example 5
Solution: Manometer equation from the pipe center to the open end of the manometer
0
)4.62)(1(5.0)4.62)(2(1)4.62)(1(5.2
)(0
C
CA
A
p
pp
gagep
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics27 Dr. Sameer Shadeed
Example 6
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Example 6 (Solution)
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2 cm3
hA
B C
D
Find the specific weight of the fluid which filled part CD of the tube
cmhcmh
hdV
186.102)5.0(4
4
32
2
Solution:
Example 7
B C4 Manometer Equation
DliqA phhp )05.0(
3/4995)9810(10186.0
)05.010186.0(
)05.0(
mN
h
h
liq
liq
)(gageopp DA
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics30 Dr. Sameer Shadeed
Differential Manometer
Used for measuring pressure differences betweenpoints along a pipe
ggww
l
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics31 Dr. Sameer Shadeed
21 )( phlhlp wmw
hppp mw )(12
ggmm
Find the change in piezometric pressure and in piezometrichead between points 1 and 2.
In general
Solution: Manometer equation from point 2 to point 1
Example 8
In general
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics32 Dr. Sameer Shadeed
(piezometric pressure)
(piezometric head)
Example 9
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Example 9 (Solution)
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Example 9 (Solution)
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics35 Dr. Sameer Shadeed
Pressure Forces on Plane Surfaces Surfaces exposed to fluids experience a force due to thepressure distribution in the fluid The resultant force on vertical, rectangular surfaces canbe found using a graphical interpretation known as thepressurepressure prismprism
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics36 Dr. Sameer Shadeed
The differential force is
Integrating to get the total force on the gate yields:
dAldAhpdAdF sin
dAlF sin
Pressure Forces on Plane Surfaces
37
From basic mechanics, we recall for first moment of area,
So,
Recognizing that
AldAl c
αsinAlγF c
AhFhαsinl ccc Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics Dr. Sameer Shadeed
In general, the resultant force on an area equals the pressure atthe centroid of the area (ghc) times the area (A)
To complete the analysis, we must compute the location of thecenter of pressure where the resultant force F can be assumedto act
Pressure Forces on Plane Surfaces
dAldAllldFFlp
sin)sin( 2
38
Al
Ill
lAl
IFl
Al
IAlFl
AlAlIAlIFl
AlIIdAl
c
ccp
cc
cc
c
ccp
cccccp
cco
sin
/bygmultiplyin)(sin
thatgRecognizin
2
22
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics Dr. Sameer Shadeed
Example 10
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Example 10 (Solution)
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Example 10 (Solution)
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Example 10 (Solution)
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Example 10 (Solution)
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Example 10 (Solution)
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Example 11
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Example 11 (Solution)
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Example 11 (Solution)
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Example 11 (Solution)
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Find the normal force required to open the rectangular gateif it is hinged at the top. The gate is 5 m wide and θ =30o
Example 12
Solution: First find the total hydrostatic force acting on the plate
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics49 Dr. Sameer Shadeed
MNmmmmN
AhF c
45.25510/9810 3
mAl
Ill o
c
ccp 92.11
2574.11
12/55)5.2)30cos/8((
3
MNFMNmF
oMo
hinge
31.1))30cos/8(92.11((45.25
Given: Gate AB is 4 ft wide, hinged at A and Gage Greads -2.17 psi. Find the horizontal force at B to hold gate.
GAir
5ft
Example 13
6ft
A
B
OilSG=0.75
Water18ft
gate
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics50 Dr. Sameer Shadeed
First convert negative pressure in tank to ft of water
ftp
h 54.62
14417.2
Example 13 (Solution)
Ibo
AhF coil
33706434.6275.
ftl 4
Total hydrostatic force acting on the gate from oil
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics51 Dr. Sameer Shadeed
Ibo 33706434.6275.
Ib
AhF cwater
1497664)3518(4.62
ftAl
Ill
c
ccwaterp 3.10
2410
12/6410
3
)(
ftl oilp 4)( Total hydrostatic force acting on the gate from water
4ft3.3ft
A
FwFoil
F
Example 13 (Solution)
B
FB
lbF
F
FFF
M
B
B
Boilwater
A
5990
6433703.314976
643.3
0
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics52 Dr. Sameer Shadeed
Resultant pressure forces on curved surfaces are more difficult todeal with because the incremental pressure forces, which are normal tothe surface, vary continually in direction There are two ways to approach the problem. One is to use directintegration and the second method is to utilize the basic mechanicsconcepts of a free body and the equilibrium equations
Pressure Forces on Curved Surfaces
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics53 Dr. Sameer Shadeed
Fig. 2.11
Pressure Forces on Curved Surfaces
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Pressure Forces on Curved Surfaces
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Example 14
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 14 (Solution)
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Example 15
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Example 15 (Solution)
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Example 15 (Solution)
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics68 Dr. Sameer Shadeed
The familiar laws of bouncy (ArchamedesArchamedes’’ principleprinciple)and flotation are usually stated (1) a body immersed in afluid is buoyed up by a force equal to the weight of fluiddisplaced; and (2) a floating body displaces its own weightof the liquid in which it floats
Buoyancy and the Stability of Floating Bodies
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics69 Dr. Sameer Shadeed
The resultant buoyant force on a submerged orpartially submerged object in a static fluid is given byArchimedes’Archimedes’ principleprinciple as:
Buoyancy and the Stability of Floating Bodies
)( objectsubmergedofvolumeFB
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics70 Dr. Sameer Shadeed
)( isplaceddiquidlofvolumeFB
The buoyant force is equal to the weight of thefluid displaced by the object and is in a directionopposite the gravitational force
or
The line of action of the buoyantforce passes through the centroidof the displaced volume, oftencalled the center of buoyancy(COBCOB)
Buoyancy and the Stability of Floating Bodies
The stability of submerged objects(balloon and sub-marine) isdetermined by the center ofgravity (GG):
Stable: GG is below COBCOB
Unstable: GG is above COBCOBDr. Sameer Shadeed71 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
For floating objects, stability is complicated by the fact that theCOBCOB changes with rotation
The stability of floating objects (Ship) is determined by themetacentric (MM) (the point of intersection of the vertical linethrough B’B’ with the centerline of the ship):
Stable: GG is below MM
Buoyancy and the Stability of Floating Bodies
Unstable: GG is above MM
Dr. Sameer Shadeed72 Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics
Example 16
Solution:
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics73 Dr. Sameer Shadeed
Example 17
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Example 17 (Solution)
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Example 17 (Solution)
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Example 18
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Example 18 (Solution)
Fluid Mechanics-2nd Semester 2010- [2] Fluid Statics78 Dr. Sameer Shadeed