kuliah-5 mekflu1
TRANSCRIPT
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FLUID STATICS
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Outline of Chapter 3
The Basic Equation of Fluid Statics
Types of Pressures
Pressure Variation in a Static Fluid
Example Problem
Hydrostatic Force on Submerged Surfaces
Example Problems
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Hydrostatic forces
Energy conversion
Bernoulli equation
Turbine
Hydrostatic uplift
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What does Static Fluid mean?
Statics means that the fluid is not moving, i.e., its
velocity =0; and its acceleration = 0.
Fluid velocity = 0 means that it does not flow.
If a static fluid does not flow, how much shear
stress the fluid is exposed to?
The word “statics” is derived from Greek word“statikos”= motionless
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What does Static Fluid mean?
In this case, fluid can be exposed to only
normal forces and behaves as
“a rigid body” – no deformation
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The Basic Equations
of Fluid Statics
Newton’s 2nd law:
Divide both sides d∀ by gives:
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The Basic Equations of Fluid Statics
• Surface Force
x
y
z
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The Basic Equations of Fluid Statics
• Body Force
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• Surface Force
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• Total Force
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• Newton’s Second Law
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Fluid statics Chee 223 2.22
Pascal’s Laws
Pascals’ laws:
– Pressure acts uniformly in all directions on a small
volume (point) of a fluid – In a fluid confined by solid boundaries, pressure
acts perpendicular to the boundary – it is a normal
force.
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Direction of fluid pressure on boundaries
Furnace duct Pipe or tube
Heat exchanger
Dam
Pressure is a Normal Force
(acts perpendicular to
surfaces)
It is also called a Surface
Force
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Absolute and Gauge Pressure
• Absolute pressure: The pressure of a fluid is expressed relative
to that of vacuum (=0)
• Gauge pressure: Pressure expressed as the difference
between the pressure of the fluid and that of the surroundingatmosphere.
Usual pressure guages record guage pressure. To calculate
absolute pressure:
gaugeatmabs P P P
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Units for Pressure
Unit Definition orRelationship
1 pascal (Pa) 1 kg m-1 s-2
1 bar 1 x 105 Pa
1 atmosphere (atm) 101,325 Pa
1 torr 1 / 760 atm
760 mm Hg 1 atm
14.696 pounds per
sq. in. (psi)
1 atm
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Pressure Variation in a Static Fluid
Reference
level and
pressure
Location and
pressure of
interest
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Measurement of Pressure
Manometers are devices in which one or more
columns of a liquid are used to determine the
pressure difference between two points.
– U-tube manometer
– Inclined-tube manometer
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Measurement of Pressure Differences
mambb
mmba
gR Z g P P
R Z g P P
)(
)(
3
2
Apply the basic equation of static fluids to
both legs of manometer, realizing that
P2=P3.
)( bamba gR P P
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Inclined Manometer
•To measure small pressure differences need to magnify Rm some way.
sin)(1 baba gR P P
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Hydrostatic Force on Submerged Surfaces
In order to fully determine the force on a surface
submerged in a liquid, we must determine the
following:
1. The magnitude of the force;
2. The direction of the force; and
3. The line of action of the force.
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1. Direction of the Force on a
Plane Submerged Surface Since fluid is not moving (static), there is no
shear, i.e., only normal forces might exist.
Since this force is caused by pressure of fluid,
it will always be normal to the surface.
This determines the direction of the force.
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3- Line of Action of the Force on a
Plane Submerged Surface
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Hydrostatic Force on Submerged Surfaces
• Plane Submerged Surface
We can find F R , and y ́ and x ́ ,
by integrating, or …
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Hydrostatic Force on Submerged Surfaces
• Plane Submerged Surface
– Algebraic Equations – Total Pressure Force
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Hydrostatic Force on Submerged Surfaces
• Plane Submerged Surface
– Algebraic Equations – Net Pressure Force
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Fluid statics Chee 223 2.43
Centroid Location for Common Shapes
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Hydrostatic Force on Submerged Surfaces
• Curved Submerged Surface
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Hydrostatic Force on Submerged Surfaces
• Curved Submerged Surface
– Horizontal Force = Equivalent Vertical Plane Force
– Vertical Force = Weight of Fluid Directly Above(+ Free Surface Pressure Force)
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Buoyancy
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Buoyancy
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Buoyancy• A body immersed in a fluid experiences a vertical buoyant
force equal to the weight of the fluid it displaces• A floating body displaces its own weight in the fluid in
which it floatsFree liquid surface
The upper surface of
the body is subjected
to a smaller force than
the lower surface
A net force is acting
upwards
F1
F2
h1
h2
H
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Buoyancy
The net force due to pressure in the vertical direction is:
FB = F2- F1 = (Pbottom - Ptop) (DxDy)
The pressure difference is:
Pbottom – Ptop = g (h2-h1) = g H
Combining:
FB = g H (DxDy)
Thus the buoyant force is:
FB = g V
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Compressible fluid
• Gases are compressible i.e. their density varies with
temperature and pressure = P M /RT
– For small elevation changes (as in engineering
applications, tanks, pipes etc) we can neglect the
effect of elevation on pressure – In the general case start from:
o
o
RT
z z M g P P
T for
)(exp
:constT
1212
g
dz
dP
Compressible
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CompressibleLinear Temperature Gradient
)( 00 z z T T
z
z
p
p z z T
dz R M g
pdp
00)( 00
R
M g
T
z z T p z p
0
000
)()(
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Atmospheric Equations
• Assume linear
R
M g
T
z z T p z p
0
000
)()(
• Assume constant
0
0 )(
0)( RT
z z M g
e p z p
Temperature variation with altitude
for the U.S. standard atmosphere
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Compressible Isentropic
v
pC C
P constant
P
1
1 y
P
P
T
T 1
11
D
D
1
12
1
1
121111
RT z gM T T
RT z gM P P
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Vocabulary List
1. Static fluid
2. Manometer.
3. Hydrostatic pressure.
4. Gauge pressure.
5. Vacuum.
6. Hydrostatic force on a submerged surface.
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The nd
Terima kasih
Dam Design
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Dam Design
Design concern: (Hydrostatic Uplift) Hydrostatic pressure
above the heel (upstream edge) of the dam may causeseepage with resultant uplift beneath the dam base (depends
largely on the supporting material of the dam). This reduces the
dams stability to sliding and overturning by effectively reducing
the weight of the dam structure. (Question: What prevents the
dam from sliding?)
Determine the minimum compressive stresses in the base of a
concrete gravity dam as given below. It is important that this
value should be greater than zero because (1) concrete has
poor tensile strength. Damage might occur near the heel of thedam. (2) The lifting of the dam structure will accelerate the
seeping rate of the water underneath the dam and further
increase hydrostatic uplift and generate more instability.
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30 m40 m
20 m
concrete=2.5 water
First, calculate the weight of the dam (per unit width):
W=Vg=(2.5)(1000)(20)(40)(1)(9.8)=19.6106 (N)
The static pressure at a depth of y: P(y)=wgy
y
Free surface
30 22 6
w w
0
The total resultant force acting on the dam by the water pressure is:
R= P(y)dy= (1000)(9.8)(1/ 2)(30) 4.4 10 ( )2
hh
gydy g N
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3
30 3 w2
w w w0
The resultant force, R, is acting at a depth h below the free surface so that
23
Rh= P(y)ydy= ( ) , 20( )3 3
Assume the load distribution under the dam is linear (it mig
hh
g h h
gy ydy g y dy g h m R
max minmin
ht not be linear if the soil
distribution is not uniform)
Therefore, the stress distribution can be written as
(x)=20
x
R
W
20 m x dam,x
20
y , max min
0
In order to reach equilibrium, both the sum of forces and
the sum of moments have to balance to zero
F 0, R=F (frictional force and the air drag force)
F 0, ( ) 10( )
1.96 1
dam yW F x dx
6
max min0 ( ) N min
max
Free surface
Example (cont.)
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20
O
0
20 20
6 6 2max minmax
0 0
6
max min
The sum of moments has to be zero also: Taking moment w.r.t. the heel of the dam
M 0, (10) (10) ( ) 0
(10)(4.4 10 19.6 10 )20
240 10 133.3 66.7
Solve:
R W x xdx
xdx x dx
6 6
max min1.64 10 ( ), 0.32 10 ( )
The minimum compressive stress is significantly lower than the maximum stress
The hydrostatic lift under the dam (as a result of the buoyancy induced by water seeping
un
N N
lift
der the dam structure) can induce as high as one half of the maximum
hydrostatic head at the heel of the dam and gradually decrease to zero at the other end.1
That is ( ) (0.5)(1000)(9.8)(302
w gh 6
6
) 0.147 10 ( )
Therefore, the effective compressive stress will only be 0.173(=0.32-0.147) 10 ( ).
N
N