Chapter 2: Fluid Mechanics - Notes, Mechanical, Engineering, Semester Mechanical Engineering Notes | EduRev

Mechanical Engineering : Chapter 2: Fluid Mechanics - Notes, Mechanical, Engineering, Semester Mechanical Engineering Notes | EduRev

 Page 1


Kreith K., Berger S.A., Churchill S. W., Tullis J. P., White F. M., etal....“ Fluid Mechanics.” 
The CRC Handbook of Thermal Engineering.  
Ed. Frank Kreith 
Boca Raton: CRC Press LLC, 2000 
Page 2


Kreith K., Berger S.A., Churchill S. W., Tullis J. P., White F. M., etal....“ Fluid Mechanics.” 
The CRC Handbook of Thermal Engineering.  
Ed. Frank Kreith 
Boca Raton: CRC Press LLC, 2000 
 
2
 
-1
 
© 2000 by CRC Press LLC
 
2
 
Fluid Mechanics
 
2.1 Fluid Statics
 
Equilibrium of a Fluid Element • Hydrostatic Pressure • 
Manometry • Hydrostatic Forces on Submerged Objects • 
Hydrostatic Forces in Layered Fluids • Buoyancy • Stability 
of Submerged and Floating Bodies • Pressure Variation in 
Rigid-Body Motion of a Fluid
 
2.2 Equations of Motion and Potential Flow
 
Integral Relations for a Control Volume • Reynolds Transport 
Theorem • Conservation of Mass • Conservation of Momentum • 
Conservation of Energy • Differential Relations for Fluid 
Motion • Mass Conservation–Continuity Equation • 
Momentum Conservation • Analysis of Rate of Deformation • 
Relationship between Forces and Rate of Deformation • The 
Navier–Stokes Equations • Energy Conservation — The 
Mechanical and Thermal Energy Equations • Boundary 
Conditions • Vorticity in Incompressible Flow • Stream 
Function • Inviscid Irrotational Flow: Potential Flow
 
2.3 Similitude: Dimensional Analysis and 
Data Correlation
 
Dimensional Analysis • Correlation of Experimental Data and 
Theoretical Values
 
2.4 Hydraulics of Pipe Systems
 
Basic Computations • Pipe Design • Valve Selection • Pump 
Selection • Other Considerations
 
2.5 Open Channel Flow 
 
De?nition • Uniform Flow • Critical Flow • Hydraulic Jump • 
Weirs • Gradually Varied Flow
 
2.6 External Incompressible Flows
 
Introduction and Scope • Boundary Layers • Drag • Lift • 
Boundary Layer Control • Computation vs. Experiment
 
2.7 Compressible Flow
 
Introduction • One-Dimensional Flow • Normal Shock Wave • 
One-Dimensional Flow with Heat Addition • Quasi-One-
Dimensional Flow • Two-Dimensional Supersonic Flow
 
2.8 Multiphase Flow
 
Introduction • Fundamentals • Gas–Liquid Two-Phase Flow • 
Gas–Solid, Liquid–Solid Two-Phase Flows
 
Frank Kreith, Editor
 
Engineering Consultant
University of Colorado
 
Stanley A. Berger
 
University of California, Berkeley
 
Stuart W. Churchill
 
University of Pennsylvania
 
J. Paul Tullis
 
Utah State University
 
Frank M. White
 
University of Rhode Island
 
Alan T. McDonald
 
Purdue University
 
Ajay Kumar
 
NASA Langley Research Center
 
John C. Chen
 
Lehigh University
 
Thomas F. Irvine, Jr.
 
State University of New York, 
Stony Brook
 
Massimo Capobianchi
 
Gonzaga University
Page 3


Kreith K., Berger S.A., Churchill S. W., Tullis J. P., White F. M., etal....“ Fluid Mechanics.” 
The CRC Handbook of Thermal Engineering.  
Ed. Frank Kreith 
Boca Raton: CRC Press LLC, 2000 
 
2
 
-1
 
© 2000 by CRC Press LLC
 
2
 
Fluid Mechanics
 
2.1 Fluid Statics
 
Equilibrium of a Fluid Element • Hydrostatic Pressure • 
Manometry • Hydrostatic Forces on Submerged Objects • 
Hydrostatic Forces in Layered Fluids • Buoyancy • Stability 
of Submerged and Floating Bodies • Pressure Variation in 
Rigid-Body Motion of a Fluid
 
2.2 Equations of Motion and Potential Flow
 
Integral Relations for a Control Volume • Reynolds Transport 
Theorem • Conservation of Mass • Conservation of Momentum • 
Conservation of Energy • Differential Relations for Fluid 
Motion • Mass Conservation–Continuity Equation • 
Momentum Conservation • Analysis of Rate of Deformation • 
Relationship between Forces and Rate of Deformation • The 
Navier–Stokes Equations • Energy Conservation — The 
Mechanical and Thermal Energy Equations • Boundary 
Conditions • Vorticity in Incompressible Flow • Stream 
Function • Inviscid Irrotational Flow: Potential Flow
 
2.3 Similitude: Dimensional Analysis and 
Data Correlation
 
Dimensional Analysis • Correlation of Experimental Data and 
Theoretical Values
 
2.4 Hydraulics of Pipe Systems
 
Basic Computations • Pipe Design • Valve Selection • Pump 
Selection • Other Considerations
 
2.5 Open Channel Flow 
 
De?nition • Uniform Flow • Critical Flow • Hydraulic Jump • 
Weirs • Gradually Varied Flow
 
2.6 External Incompressible Flows
 
Introduction and Scope • Boundary Layers • Drag • Lift • 
Boundary Layer Control • Computation vs. Experiment
 
2.7 Compressible Flow
 
Introduction • One-Dimensional Flow • Normal Shock Wave • 
One-Dimensional Flow with Heat Addition • Quasi-One-
Dimensional Flow • Two-Dimensional Supersonic Flow
 
2.8 Multiphase Flow
 
Introduction • Fundamentals • Gas–Liquid Two-Phase Flow • 
Gas–Solid, Liquid–Solid Two-Phase Flows
 
Frank Kreith, Editor
 
Engineering Consultant
University of Colorado
 
Stanley A. Berger
 
University of California, Berkeley
 
Stuart W. Churchill
 
University of Pennsylvania
 
J. Paul Tullis
 
Utah State University
 
Frank M. White
 
University of Rhode Island
 
Alan T. McDonald
 
Purdue University
 
Ajay Kumar
 
NASA Langley Research Center
 
John C. Chen
 
Lehigh University
 
Thomas F. Irvine, Jr.
 
State University of New York, 
Stony Brook
 
Massimo Capobianchi
 
Gonzaga University
 
2
 
-2
 
© 2000 by CRC Press LLC
 
2.9 Non-Newtonian Flows
 
Introduction • Classi?cation of Non-Newtonian Fluids • 
Apparent Viscosity • Constitutive Equations • Rheological 
Property Measurements • Fully Developed Laminar Pressure 
Drops for Time-Independent Non-Newtonian Fluids • Fully 
Developed Turbulent Flow Pressure Drops • Viscoelastic Fluids
 
2.1 Fluid Statics
 
Stanley A. Berger
 
Equilibrium of a Fluid Element
 
If the sum of the external forces acting on a ?uid element is zero, the ?uid will be either at rest or
moving as a solid body — in either case, we say the ?uid element is in equilibrium. In this section we
consider ?uids in such an equilibrium state. For ?uids in equilibrium the only internal stresses acting
will be normal forces, since the shear stresses depend on velocity gradients, and all such gradients, by
the de?nition of equilibrium, are zero. If one then carries out a balance between the normal surface
stresses and the body forces, assumed proportional to volume or mass, such as gravity, acting on an
elementary prismatic ?uid volume, the resulting equilibrium equations, after shrinking the volume to
zero, show that the normal stresses at a point are the same in all directions, and since they are known
to be negative, this common value is denoted by –
 
p
 
, 
 
p
 
 being the pressure.
 
Hydrostatic Pressure
 
If we carry out an equilibrium of forces on an elementary volume element 
 
dxdydz
 
, the forces being
pressures acting on the faces of the element and gravity acting in the –
 
z
 
 direction, we obtain
(2.1.1)
The ?rst two of these imply that the pressure is the same in all directions at the same vertical height in
a gravitational ?eld. The third, where 
 
?
 
 is the speci?c weight, shows that the pressure increases with
depth in a gravitational ?eld, the variation depending on 
 
?
 
(
 
z
 
). For homogeneous ?uids, for which 
 
?
 
 =
constant, this last equation can be integrated immediately, yielding
(2.1.2)
or
(2.1.3)
where 
 
h
 
 denotes the elevation. These are the equations for the hydrostatic pressure distribution.
When applied to problems where a liquid, such as the ocean, lies below the atmosphere, with a
constant pressure 
 
p
 
atm
 
, 
 
h
 
 is usually measured (positive) downward from the ocean/atmosphere interface
and 
 
p
 
 at any distance 
 
h
 
 below this interface differs from 
 
p
 
atm
 
 by an amount
(2.1.4)
Pressures may be given either as 
 
absolute pressure,
 
 pressure measured relative to absolute vacuum,
or 
 
gauge pressure,
 
 pressure measured relative to atmospheric pressure.
?
?
?
?
?
?
??
p
x
p
y
p
z
g = = =- =- 0, and
pp gz z gh h
21 2 1 2 1
-=- -
()
=- -
()
??
pgh p gh
22 1 1
+=+ = ?? constant
pp gh -=
atm
?
Page 4


Kreith K., Berger S.A., Churchill S. W., Tullis J. P., White F. M., etal....“ Fluid Mechanics.” 
The CRC Handbook of Thermal Engineering.  
Ed. Frank Kreith 
Boca Raton: CRC Press LLC, 2000 
 
2
 
-1
 
© 2000 by CRC Press LLC
 
2
 
Fluid Mechanics
 
2.1 Fluid Statics
 
Equilibrium of a Fluid Element • Hydrostatic Pressure • 
Manometry • Hydrostatic Forces on Submerged Objects • 
Hydrostatic Forces in Layered Fluids • Buoyancy • Stability 
of Submerged and Floating Bodies • Pressure Variation in 
Rigid-Body Motion of a Fluid
 
2.2 Equations of Motion and Potential Flow
 
Integral Relations for a Control Volume • Reynolds Transport 
Theorem • Conservation of Mass • Conservation of Momentum • 
Conservation of Energy • Differential Relations for Fluid 
Motion • Mass Conservation–Continuity Equation • 
Momentum Conservation • Analysis of Rate of Deformation • 
Relationship between Forces and Rate of Deformation • The 
Navier–Stokes Equations • Energy Conservation — The 
Mechanical and Thermal Energy Equations • Boundary 
Conditions • Vorticity in Incompressible Flow • Stream 
Function • Inviscid Irrotational Flow: Potential Flow
 
2.3 Similitude: Dimensional Analysis and 
Data Correlation
 
Dimensional Analysis • Correlation of Experimental Data and 
Theoretical Values
 
2.4 Hydraulics of Pipe Systems
 
Basic Computations • Pipe Design • Valve Selection • Pump 
Selection • Other Considerations
 
2.5 Open Channel Flow 
 
De?nition • Uniform Flow • Critical Flow • Hydraulic Jump • 
Weirs • Gradually Varied Flow
 
2.6 External Incompressible Flows
 
Introduction and Scope • Boundary Layers • Drag • Lift • 
Boundary Layer Control • Computation vs. Experiment
 
2.7 Compressible Flow
 
Introduction • One-Dimensional Flow • Normal Shock Wave • 
One-Dimensional Flow with Heat Addition • Quasi-One-
Dimensional Flow • Two-Dimensional Supersonic Flow
 
2.8 Multiphase Flow
 
Introduction • Fundamentals • Gas–Liquid Two-Phase Flow • 
Gas–Solid, Liquid–Solid Two-Phase Flows
 
Frank Kreith, Editor
 
Engineering Consultant
University of Colorado
 
Stanley A. Berger
 
University of California, Berkeley
 
Stuart W. Churchill
 
University of Pennsylvania
 
J. Paul Tullis
 
Utah State University
 
Frank M. White
 
University of Rhode Island
 
Alan T. McDonald
 
Purdue University
 
Ajay Kumar
 
NASA Langley Research Center
 
John C. Chen
 
Lehigh University
 
Thomas F. Irvine, Jr.
 
State University of New York, 
Stony Brook
 
Massimo Capobianchi
 
Gonzaga University
 
2
 
-2
 
© 2000 by CRC Press LLC
 
2.9 Non-Newtonian Flows
 
Introduction • Classi?cation of Non-Newtonian Fluids • 
Apparent Viscosity • Constitutive Equations • Rheological 
Property Measurements • Fully Developed Laminar Pressure 
Drops for Time-Independent Non-Newtonian Fluids • Fully 
Developed Turbulent Flow Pressure Drops • Viscoelastic Fluids
 
2.1 Fluid Statics
 
Stanley A. Berger
 
Equilibrium of a Fluid Element
 
If the sum of the external forces acting on a ?uid element is zero, the ?uid will be either at rest or
moving as a solid body — in either case, we say the ?uid element is in equilibrium. In this section we
consider ?uids in such an equilibrium state. For ?uids in equilibrium the only internal stresses acting
will be normal forces, since the shear stresses depend on velocity gradients, and all such gradients, by
the de?nition of equilibrium, are zero. If one then carries out a balance between the normal surface
stresses and the body forces, assumed proportional to volume or mass, such as gravity, acting on an
elementary prismatic ?uid volume, the resulting equilibrium equations, after shrinking the volume to
zero, show that the normal stresses at a point are the same in all directions, and since they are known
to be negative, this common value is denoted by –
 
p
 
, 
 
p
 
 being the pressure.
 
Hydrostatic Pressure
 
If we carry out an equilibrium of forces on an elementary volume element 
 
dxdydz
 
, the forces being
pressures acting on the faces of the element and gravity acting in the –
 
z
 
 direction, we obtain
(2.1.1)
The ?rst two of these imply that the pressure is the same in all directions at the same vertical height in
a gravitational ?eld. The third, where 
 
?
 
 is the speci?c weight, shows that the pressure increases with
depth in a gravitational ?eld, the variation depending on 
 
?
 
(
 
z
 
). For homogeneous ?uids, for which 
 
?
 
 =
constant, this last equation can be integrated immediately, yielding
(2.1.2)
or
(2.1.3)
where 
 
h
 
 denotes the elevation. These are the equations for the hydrostatic pressure distribution.
When applied to problems where a liquid, such as the ocean, lies below the atmosphere, with a
constant pressure 
 
p
 
atm
 
, 
 
h
 
 is usually measured (positive) downward from the ocean/atmosphere interface
and 
 
p
 
 at any distance 
 
h
 
 below this interface differs from 
 
p
 
atm
 
 by an amount
(2.1.4)
Pressures may be given either as 
 
absolute pressure,
 
 pressure measured relative to absolute vacuum,
or 
 
gauge pressure,
 
 pressure measured relative to atmospheric pressure.
?
?
?
?
?
?
??
p
x
p
y
p
z
g = = =- =- 0, and
pp gz z gh h
21 2 1 2 1
-=- -
()
=- -
()
??
pgh p gh
22 1 1
+=+ = ?? constant
pp gh -=
atm
?
 
2
 
-3
 
© 2000 by CRC Press LLC
 
Manometry
 
The hydrostatic pressure variation may be employed to measure pressure differences in terms of heights
of liquid columns — such devices are called manometers and are commonly used in wind tunnels and
a host of other applications and devices. Consider, for example the U-tube manometer shown in Figure
2.1.1 ?lled with liquid of speci?c weight 
 
?
 
, the left leg open to the atmosphere and the right to the region
whose pressure 
 
p
 
 is to be determined. In terms of the quantities shown in the ?gure, in the left leg
(2.1.5a)
and in the right leg
(2.1.5b)
the difference being
(2.1.6)
which determines 
 
p
 
 in terms of the height difference 
 
d
 
 = 
 
h
 
1
 
 – 
 
h
 
2
 
 between the levels of the ?uid in the
two legs of the manometer.
 
Hydrostatic Forces on Submerged Objects
 
The force acting on a submerged object due to the hydrostatic pressure is given by
(2.1.7)
where 
 
h
 
 is the variable vertical depth of the element 
 
dA
 
 and 
 
p
 
0
 
 is the pressure at the surface. In turn we
consider plane and nonplanar surfaces.
 
Forces on Plane Surfaces
 
Consider the planar surface 
 
A
 
 at an angle 
 
?
 
 to a free surface shown in Figure 2.1.2. The force on one
side of the planar surface, from Equation (2.1.7), is
(2.1.8)
 
FIGURE 2.1.1
 
U-tube manometer.
pgh p
02
-= ?
atm
pgh p
01
-= ?
pp gh h gd d -=- -
()
=- =-
atm
???
12
F dA n dA dA == · = +
?? ?? ?? ??
p p dA gh p ?
0
Fn n =+
??
?ghdA pA
A
0
Page 5


Kreith K., Berger S.A., Churchill S. W., Tullis J. P., White F. M., etal....“ Fluid Mechanics.” 
The CRC Handbook of Thermal Engineering.  
Ed. Frank Kreith 
Boca Raton: CRC Press LLC, 2000 
 
2
 
-1
 
© 2000 by CRC Press LLC
 
2
 
Fluid Mechanics
 
2.1 Fluid Statics
 
Equilibrium of a Fluid Element • Hydrostatic Pressure • 
Manometry • Hydrostatic Forces on Submerged Objects • 
Hydrostatic Forces in Layered Fluids • Buoyancy • Stability 
of Submerged and Floating Bodies • Pressure Variation in 
Rigid-Body Motion of a Fluid
 
2.2 Equations of Motion and Potential Flow
 
Integral Relations for a Control Volume • Reynolds Transport 
Theorem • Conservation of Mass • Conservation of Momentum • 
Conservation of Energy • Differential Relations for Fluid 
Motion • Mass Conservation–Continuity Equation • 
Momentum Conservation • Analysis of Rate of Deformation • 
Relationship between Forces and Rate of Deformation • The 
Navier–Stokes Equations • Energy Conservation — The 
Mechanical and Thermal Energy Equations • Boundary 
Conditions • Vorticity in Incompressible Flow • Stream 
Function • Inviscid Irrotational Flow: Potential Flow
 
2.3 Similitude: Dimensional Analysis and 
Data Correlation
 
Dimensional Analysis • Correlation of Experimental Data and 
Theoretical Values
 
2.4 Hydraulics of Pipe Systems
 
Basic Computations • Pipe Design • Valve Selection • Pump 
Selection • Other Considerations
 
2.5 Open Channel Flow 
 
De?nition • Uniform Flow • Critical Flow • Hydraulic Jump • 
Weirs • Gradually Varied Flow
 
2.6 External Incompressible Flows
 
Introduction and Scope • Boundary Layers • Drag • Lift • 
Boundary Layer Control • Computation vs. Experiment
 
2.7 Compressible Flow
 
Introduction • One-Dimensional Flow • Normal Shock Wave • 
One-Dimensional Flow with Heat Addition • Quasi-One-
Dimensional Flow • Two-Dimensional Supersonic Flow
 
2.8 Multiphase Flow
 
Introduction • Fundamentals • Gas–Liquid Two-Phase Flow • 
Gas–Solid, Liquid–Solid Two-Phase Flows
 
Frank Kreith, Editor
 
Engineering Consultant
University of Colorado
 
Stanley A. Berger
 
University of California, Berkeley
 
Stuart W. Churchill
 
University of Pennsylvania
 
J. Paul Tullis
 
Utah State University
 
Frank M. White
 
University of Rhode Island
 
Alan T. McDonald
 
Purdue University
 
Ajay Kumar
 
NASA Langley Research Center
 
John C. Chen
 
Lehigh University
 
Thomas F. Irvine, Jr.
 
State University of New York, 
Stony Brook
 
Massimo Capobianchi
 
Gonzaga University
 
2
 
-2
 
© 2000 by CRC Press LLC
 
2.9 Non-Newtonian Flows
 
Introduction • Classi?cation of Non-Newtonian Fluids • 
Apparent Viscosity • Constitutive Equations • Rheological 
Property Measurements • Fully Developed Laminar Pressure 
Drops for Time-Independent Non-Newtonian Fluids • Fully 
Developed Turbulent Flow Pressure Drops • Viscoelastic Fluids
 
2.1 Fluid Statics
 
Stanley A. Berger
 
Equilibrium of a Fluid Element
 
If the sum of the external forces acting on a ?uid element is zero, the ?uid will be either at rest or
moving as a solid body — in either case, we say the ?uid element is in equilibrium. In this section we
consider ?uids in such an equilibrium state. For ?uids in equilibrium the only internal stresses acting
will be normal forces, since the shear stresses depend on velocity gradients, and all such gradients, by
the de?nition of equilibrium, are zero. If one then carries out a balance between the normal surface
stresses and the body forces, assumed proportional to volume or mass, such as gravity, acting on an
elementary prismatic ?uid volume, the resulting equilibrium equations, after shrinking the volume to
zero, show that the normal stresses at a point are the same in all directions, and since they are known
to be negative, this common value is denoted by –
 
p
 
, 
 
p
 
 being the pressure.
 
Hydrostatic Pressure
 
If we carry out an equilibrium of forces on an elementary volume element 
 
dxdydz
 
, the forces being
pressures acting on the faces of the element and gravity acting in the –
 
z
 
 direction, we obtain
(2.1.1)
The ?rst two of these imply that the pressure is the same in all directions at the same vertical height in
a gravitational ?eld. The third, where 
 
?
 
 is the speci?c weight, shows that the pressure increases with
depth in a gravitational ?eld, the variation depending on 
 
?
 
(
 
z
 
). For homogeneous ?uids, for which 
 
?
 
 =
constant, this last equation can be integrated immediately, yielding
(2.1.2)
or
(2.1.3)
where 
 
h
 
 denotes the elevation. These are the equations for the hydrostatic pressure distribution.
When applied to problems where a liquid, such as the ocean, lies below the atmosphere, with a
constant pressure 
 
p
 
atm
 
, 
 
h
 
 is usually measured (positive) downward from the ocean/atmosphere interface
and 
 
p
 
 at any distance 
 
h
 
 below this interface differs from 
 
p
 
atm
 
 by an amount
(2.1.4)
Pressures may be given either as 
 
absolute pressure,
 
 pressure measured relative to absolute vacuum,
or 
 
gauge pressure,
 
 pressure measured relative to atmospheric pressure.
?
?
?
?
?
?
??
p
x
p
y
p
z
g = = =- =- 0, and
pp gz z gh h
21 2 1 2 1
-=- -
()
=- -
()
??
pgh p gh
22 1 1
+=+ = ?? constant
pp gh -=
atm
?
 
2
 
-3
 
© 2000 by CRC Press LLC
 
Manometry
 
The hydrostatic pressure variation may be employed to measure pressure differences in terms of heights
of liquid columns — such devices are called manometers and are commonly used in wind tunnels and
a host of other applications and devices. Consider, for example the U-tube manometer shown in Figure
2.1.1 ?lled with liquid of speci?c weight 
 
?
 
, the left leg open to the atmosphere and the right to the region
whose pressure 
 
p
 
 is to be determined. In terms of the quantities shown in the ?gure, in the left leg
(2.1.5a)
and in the right leg
(2.1.5b)
the difference being
(2.1.6)
which determines 
 
p
 
 in terms of the height difference 
 
d
 
 = 
 
h
 
1
 
 – 
 
h
 
2
 
 between the levels of the ?uid in the
two legs of the manometer.
 
Hydrostatic Forces on Submerged Objects
 
The force acting on a submerged object due to the hydrostatic pressure is given by
(2.1.7)
where 
 
h
 
 is the variable vertical depth of the element 
 
dA
 
 and 
 
p
 
0
 
 is the pressure at the surface. In turn we
consider plane and nonplanar surfaces.
 
Forces on Plane Surfaces
 
Consider the planar surface 
 
A
 
 at an angle 
 
?
 
 to a free surface shown in Figure 2.1.2. The force on one
side of the planar surface, from Equation (2.1.7), is
(2.1.8)
 
FIGURE 2.1.1
 
U-tube manometer.
pgh p
02
-= ?
atm
pgh p
01
-= ?
pp gh h gd d -=- -
()
=- =-
atm
???
12
F dA n dA dA == · = +
?? ?? ?? ??
p p dA gh p ?
0
Fn n =+
??
?ghdA pA
A
0
 
2
 
-4
 
© 2000 by CRC Press LLC
 
but 
 
h
 
 = 
 
y
 
 sin 
 
?
 
, so
(2.1.9)
where the subscript 
 
c
 
 indicates the distance measured to the centroid of the area 
 
A
 
. The total force (on
one side) is then
(2.1.10)
Hence, the magnitude of the force is independent of the angle 
 
?
 
, and is equal to the pressure at the
centroid, 
 
?
 
h
 
c
 
 + 
 
p
 
0
 
, times the area. If we use gauge pressure, the term 
 
p
 
0
 
A
 
 in Equation (2.1.10) is dropped.
Since 
 
p
 
 is not evenly distributed over 
 
A
 
, but varies with depth, 
 
F
 
 does not act through the centroid.
The point of action of 
 
F
 
, called the 
 
center of pressure
 
, can be determined by considering moments in
Figure 2.1.2. The moment of the hydrostatic force acting on the elementary area 
 
dA
 
 about the axis
perpendicular to the page passing through the point 0 on the free surface is
(2.1.11)
so if 
 
y
 
cp
 
 
 
denotes the distance to the center of pressure,
(2.1.12)
where 
 
I
 
x
 
 is the moment of inertia of the plane area with respect to the axis formed by the intersection
of the plane containing the planar surface and the free surface (say 0
 
x
 
). Dividing by 
 
F
 
 = 
 
?
 
h
 
c
 
A
 
 =
 
?
 
y
 
c
 
sin
 
?
 
A
 
 gives
(2.1.13)
 
FIGURE 2.1.2
 
Hydrostatic force on a plane surface.
hdA y dA y A h A
AA
cc
?? ??
== = sin sin ??
FA A =+ ?hp
c 0
ydF y y dA y dA =()= ?? ? ? sin sin
2
yF y dA I
x cp
==
??
?? ?? sin sin
2
y
I
yA
x
c
cp
=
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