1
Annex
1
Basic equations
of
motion in fluid mechanics
1.1
Introduction
It is assumed that the reader of this book is familiar with the basic laws of fluid mecha
nics. Nevertheless some of these laws will be discussed in this annex to summarise
material and to emphasize certain subjects which are important in the context of dis
charge measurement structures in open channels.
1.2
Equation
of
motion Euler
In fluid mechanics we consider the motion
of
a fluid under the influence of forces
acting upon it. Since these forces produce an unsteady motion, their study is essentially
one of dynamics and must be based on Newton’s second law of motion
F
=
ma (Al.l)
where F is the force required to accelerate a certain mass (m) at a certain rate (a).
If we consider the motion of an elementary fluid particle (dx dy dz) with a constant
massdensity
(p),
its mass
(m)
equals
m
=
pdxdydz (Al
.2)
The following forces may act on this particle:
a. The normal pressures (P) exerted on the lateral faces of the elementary volume
by the bordering fluid particles;
b. The mass forces, which include in the first place the gravitational force and in the
second the power of attraction of the moon and the sun and the Coriolis force.
These forces, acting on the mass
(p
dx dy dz) of the fluid particle, are represented
together by their components
in
the X, Y, and Zdirection. It is common practice
to express these components per unit of mass, and therefore as accelerations; for
example, the gravitational force is expressed as the downward acceleration g;
c. Friction. There are forces in a fluid which, due to friction, act as shear forces
on
the lateral faces of the elementary particle (dx dy dz). To prevent complications
unnecessary in this context, the shear force is regarded as a mass force.
Gravitation and friction are the only mass forces we shall consider. If the fluid
is in motion, these two forces acting on the particle (dx dy dz) do not have to
be in equilibrium, but may result in an accelerating or decelerating force (pos. or
neg.). This net force is named:
d. Net impressed force. This force equals the product of the mass of the particle and
the acceleration due to the forces of pressure and mass not being in equilibrium.
The net impressed force may be resolved in the
X, Y,
and Zdirection.
If we assume that the pressure at a point is the same in all directions even when the
fluid is in motion, and that the change of pressure intensity from point to point is
345
continuous over the elementary lengths dx, dy, and dz, we may define the normal
pressures acting, at time t, on the elementary particle as indicated in Figure A
I. I.
Acting on the lefthand lateral face (Xdirection) is a force
k
(P'!,xdx
ap
1
dydz
while on the righthand face is a force
ap
'(P
+
'/2
ax
dx) dy dz
The resulting normal pressure on the elementary fluid particle
in
the Xdirection equals
(A 1.3)
The resultant
of
the combined mass forces in the Xdirection equals
p
dx dydz
k,
where
k,
is
the acceleration due to gravitation and friction in the Xdirection. Hence
in the Xdirection, normal pressure and the combined mass forces on the elementary
particle result in a total force
(A
1.4)
ap
ax
F,
=
dXdydz
+
k,pdxdydz
I
1
b P
b Y
P'T
dY
Figure
Al.
I
Pressure
distribution
on
an elementary fluid particle
346
1
L P
p 
dZ
*
b z
t +dt )
Figure
A
1.2
The velocity
as
a function
of
time and position
Similarly, for the forces acting on the mass (p dx dy dz) in the
Y
and Zdirection,
we may write
and
(A1.5)
(A
1.6)
The reader should note that in the above equations k,, k,, and k, have the dimension
of an acceleration.
In
a moving liquid the velocity varies with both position and time (Figure
AI
.2).
Hence:
(A 1.7)
v
=
f(x, Y,
2,
t)
and as such
vx
=
fx(x, Y, z, t)
vy
=
f,(x, Y,
2,
t)
vz
=
fz
(x,
Y,
z,
t>
and
If we consider the Xdirection first, we may write that at the time (t
+
dt) and at
‘
the point (x
+
dx, y
+
dy,
z
+
dz) there is a velocity component in the Xdirection
which equals v,
+
dv,.
347
The total differential of
v,
is equal to
av, av
av
av
at
ax
a Y
aZ
dv,
=
dt
+
L d x
+
L d y
+
"dz (A 1.8)
In Figure A1.3 we follow a moving fluid particle over a time dt, and see it moving
along
a
pathline from point (x, y, z) towards point
(x
+
dx, y
+
dy, z
+
dz) where
it arrives with another velocity component (v,
+
dv,). The acceleration of the fluid
particle in the Xdirection consequently equals
dv,
dt
a
=
while the elementary variations in time and space equal
dx
=
v,dt
dy
=
v,dt
dz
=
V,
dt
(A 1.9)
(A1.lO)
(Al.l I )
(A 1
.I
2)
Equation A1.8, which is valid for a general flow pattern, also applies to a moving
fluid particle as shown in Figure A1.3,
so
that Equations A1.10 to A1.12 may be
substituted into Equation A1.8, giving
av,
av,
av
av
at ax
a Y
a Z
dv,
=
 dt + V,dt
+
L v,d t
+
L v,d t
and after substitution of Equation AI
.9
(A1.13)
(A 1
.I
4)
Figure
A
I
.3
The flow path
of
a fluid particle
348
and similarly
Substitution of Equations
A 1.2, A 1.4,
and
A 1.14
into Equation
A
I.
1
gives
av
a Z
1
 Ed x d y d z
+
k,pdxdydz
=
pdxdydz
+
2 ~ y
av
+
>v,
ax
aY
or
(A
I.
15)
(A1.16)
In the same manner we find for the
Y
and Zdirection
av av av
av
I
ap
av, av, av,
1
aP
$
+
$vx
+
Y v y
+
Yv,
=
+
k
aY
az Pay
y
av,
+
vx
+
v
+
v,
=
+
k,
at ax ay
y
aZ
P
(A1.17)
(A
1.1
8)
(A
1
.I 9)
These are the Euler equations of motion, which have been derived for the general
case of unsteady nonuniform flow and for an arbitrary Cartesian coordinate system.
An
important simplification of these equations may be obtained by selecting a coordi
nate system whose origin coincides with the observed moving fluid particle (point
P). The directions of the three axes are chosen as follows:

sdirection: the direction of the velocity vector at point P, at time t.
As
defined,
this vector coincides with the tangent to the streamline at
P
at time t (vs
=
v).

ndirection: the principal normal direction towards the centre of curvature of the
streamline at point P at time t.
As
defined, both the
s
and ndirection lie in the
osculating plane.

mdirection: the binormal direction perpendicular to the osculating plane at
P
at
time t (see also Chapter
1).
If we assume that a fluid particle is passing through point P at time t with a velocity
v,
the Eulerian equations of motion can be written
as:
av,
avs
av av
I
ap
+vs
+v,
+
v,
=
at as
an
am
P
as
avn avn av
av

+vs
+ 1v,
+!v,
=
at as an am
P
an
av,
av,
av, av,
I
ap
at
+
vS
+
v,
+
v,
=
as an am
P
am
+
ks

'
+
k,
+
k,
( Al
.20)
(A1.21)
(A 1.22)
Due to the selection of the coordinate system, there is no velocity perpendicular to
the sdirection; thus
v,
=
O
and
v,
=
O
(A 1.23)
349
Therefore the equations of motion may be simplified to
(A 1.24)
(Al
.25)
( AI .26)
Since the streamline at both sides
of P
is situated over an elementary length in the
osculating plane, the variation of v, in the sdirection equals zero. Hence, in Equation
A1.26
(A 1.27)
In Figure
AI
.4
an elementary section
of
the streamline at point
P
at time t
is
shown
in the osculating plane. It can be seen that
%
ds
ds
av
r
 _

as
t andp
=
v,
+
A d s
as
or
(A
1.28)
( AI
.29)
av
In the latter equation, however, S d s is infinitely small compared with v,; thus we
may
rewrite
Equation Al
.29
as
(A 1.30)
a v n

3

as r
av,
v2
or
=
2
r
Substitution of Equations
A
1.27
and
A 1.3
1
into Equation
vely gives Eder’s equations of motion as follows
av, av
i
ap
at
as pas
.
+LV
=
   +k
av,
v2

+
k,
x++

P
as
350
( Al .31)
AI .26
and
A 1.25
respecti
(A
1.32)
( AI .33)
M
elementary
xciion
(12)
of a
streamline at ti me
i
o.
& A d s
/’
in
the osculating plane
(osculating
plane)
__
J
\
\
,/A’
I
vs
+
‘r
ds
‘.
Figure
A
I
.4
Elementary section of a streamline
’
av,
at
(Al .34)
These equations of motion are valid for both unsteady and nonuniform flow. Here
after, we shall confine our attention to steady flow, in which case all terms 8. ./at
equal zero.
Equations
A
1.32, A 1.33, and AI .34 are of little use in direct applications, and they
tend to repel engineers by the presence of partial derivative signs; however, they help
one’s understanding
of
certain basic equations, which will be dealt with below.
1.3
Equation
of
motion in the sdirection
If
we follow a streamline (in the sdirection)?we may write
v,
=
v, and the partial
derivatives can be replaced by normal derivatives because s is the only dependent vari
able. (Thus
a
changes into d). Accordingly, Equation
Al
.32 reads for steady flow
dv
1
dP
ds
p
ds
V
=+
k
(Al
.35)
where
k,
is the acceleration due to gravity and friction. We now define the negative
Zdirection as the direction of gravity, The weight of the fluid particle is

p
g ds dn
dm
of
which the component in the sdirection is
dz
 p
g ds dn dm
ds
and per unit of mass
dz
p
g ds dn dm

ds dz

p
ds dn dm

 g z
(Al .36)
351
d
2
pg ds dndmz ds
Idtrectlon
Sdirection
;I
/
W
(due
t o
friction)
Sdirection
\
\
\
\
\
\
\
pgdsdndm
V
Figure
A1.5
Forces due to gravitation and friction acting
on
an elementary fluid particle
The force due to friction acting on the fluid particle in the negative sdirection equals
per unit of mass
W
p
ds
dn
dm
w
=
(Al
.37)
;The acceleration due to the combined massforces
(k,)
acting in the sdirection accor
dingly equals
k
= w g dz (A1.38)
ds
Substitution of this equation into Equation A1.35 gives
1
dP dz dv
ds
p
ds ds
gw
v
=
or
dv dP dz
ds ds ds
pv+
 +
pg
=
 pw
(Al .39)
(A
1.40)
or
d
(I/*
p
v2
+
P
+
pgz)
=

p
w
ds
(A1.41)
The latter equation indicates the dissipation of energy per unit of volume due to local
352
friction. If, however, the decelerating effect of friction is neglected, Equation Al .41
becomes
(A 1.42)
d
ds
 ( y 2
pv2
+
P
+
pgz)
=
o
Hence
'/2
pv2
+
P
+
pgz
=
constant
(A 1.43)
where
p
v2
=
kinetic energy per unit of volume
p
g z
P
=
potential energy per unit of volume
=
pressure energy per unit of volume
If Equation A1.43
is
divided by pg, an equation in terms of head is obtained, which
reads
v2 P

+

+
z
=
constant
=
H
2g Pg
(A 1.44)
where
v2/2g
=
the velocity head
P/pg
=
the pressure head
Z
=
the elevation head
P/pg+z
=
the piezometric head
H
=
the total energy head
The last three heads all refer to the same reference level (see Figure 1.3, Chapter
I).
The Equations A1.43 and A1.44 are alternative forms of the wellknown Bernoulli
equation, and are valid only if we consider the movement of an elementary fluid parti
cle along a streamline under steady flow conditions (pathline) with the massdensity
(p)
constant, and that energy losses can be neglected.
1.4
Piezometric gradient in the ndirection
The equation
of
motion in the ndirection reads for steady flow (see Equation Al
.33)
v2
1
dP
r pdn
+
kn
 

(Al .45)
Above, the
a
has been replaced by d since n is the only independent variable. The
term v2/r equals the force per unit of mass acting on a fluid particle which follows
a curved path with radius! at a velocity! (centripetal acceleration). In Equation A 1.45,
k,
is the acceleration due to gravity and friction in the ndirection. Since v,
=
O,
there
is no friction component. Analogous to its component in the direction of flow here
the component due to gravitation can be shown to be
dz
dn
k =g
(AI .46)
353
Substitution into Equation A1.45 yields
v2 1 dP dz
r pdn
gdn



which, after division by g, may be written as
(A 1.47)
(Al .48)
After integration of this equation from point 1 to point 2 in the ndirection we obtain
the following equation for the change of piezometric head in the ndirection
(i
+
z),

(i
+
z ) ~
=

1
J
Tdn
v2
g,
(Al .49)
where (P/pg
+
z) equals the piezometric head at point 1 and 2 respectively and
1
2 v *
 j  d n
g,
r
is the loss of piezometric head due to curvature of the streamlines.
1.5
Hydrostatic pressure distribution in the mdirection
Perpendicular to the osculating plane, the equation of motion, according to Euler,
reads for steady flow
I
O
U
0 L
._
.
?
Figure
A1.6
The principal normal direction
3
54
(A1.50)
4Since there is no velocity component perpendicular to the osculating plane (v,
=
O),
there is no friction either. The component of the acceleration due to gravity in the
mdirection is obtained as before,
so
that
(A1.51)
Substitution of this acceleration in the equation
of
motion (Equation A 1.50) gives
1
dP dz
pdm dm
g =o
which may be written as
&
(5
+
z)
=
o
It follows from this equation that the piezometric head in the mdirection is
P

+
z
=
constant
Pg
(A1.52)
(A1.53)
(A1.54)
irrespective
of
the curvature of the streamlines. In other words, perpendicular to the
osculating plane, there is a hydrostatic pressure distribution.
1
355
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