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254

9 Fluid Mechanics

and Stevino’s law is obtained: The pressure linearly increases with depth by an

amount equal to the weight of the liquid column acting on the unit surface.

4. The simplest constitutive equation (9.2) is given by a linear function relating

the pressure to the mass density. By assuming that the proportionality factor is

a linear function of the absolute temperature Â, the corresponding constitutive

equation defines a perfect gas

p D R Â;

(9.10)

where R is the universal gas constant. Let the gas be at equilibrium at constant

and uniform temperature when subjected to its weight action. Assuming the axis

Oz is oriented upward, we get U.z/ D gzCconst and, taking into account (9.10),

Eq. (9.7) becomes

Z

p

RÂ

p0

dp

D

p

gz;

so that

gz Á

:

RÂ

p.z/ D p0 exp

(9.11)

5. Archimedes’ principle is a further consequence of (9.8): The buoyant force on

a body submerged in a liquid is equal to the weight of the liquid displaced by

the body.

To prove this statement, consider a body S submerged in a liquid, as shown

in Fig. 9.2.

If p0 is the atmospheric pressure, then the force acting on S is given by

Z

Z

FD

.p0 C gz/N d ;

p0 N d

e

i

Fig. 9.2 Archimedes principle

(9.12)

9.2 Stevino’s Law and Archimedes’ Principle

255

where N is the outward unit vector normal to the body surface , i is the

submerged portion of , and e is the portion above the waterline. By adding

and subtracting on the right-hand side of (9.12) the integral of p0 N over (see

Fig. 9.2), (9.12) becomes

Z

Z

FD

.p0 C gz/N d ;

p0 N d

@Ce

(9.13)

@Ci

so that the knowledge of F requires the computation of the integrals in (9.13).

To do this, we define a virtual pressure field (continuous on ) in the interior of

the body:

on Ce ;

p D p0

on Ci :

p D p0 C gz

Applying Gauss’s theorem to the integrals in (9.13), we obtain

Z

Z

Z

p0 N d D

@Ce

Z

rp0 d V D 0;

Ce

p0 N d D

@Ci

Z

Z

gzN d D g rz d V:

@Ci

rp0 d V D 0;

Ci

Ci

Finally,

FD

gVi k;

(9.14)

where k the unit vector associated with Oz.

Equation (9.14) gives the resultant force acting on the body S . To complete

the equilibrium analysis, the momentum MO of pressure forces with respect to an

!

arbitrary pole O has to be explored. If r D OP , then

Z

Z

MO D

p0

r

Nd

r

e

.p0 C gz/N d :

(9.15)

i

Again, by adding and subtracting on the right-hand side of (9.15) the integral of

p0 r N over , (9.15) becomes

Z

Z

MO D

p0

r

@Ce

Nd

r

@Ci

.p0 C gz/N d :

256

9 Fluid Mechanics

By applying Gauss’s theorem, we get

Z

p0

Ce

Z

@xj

dV D

ij l

@xl

Z

p0

ij l ıj l

d V D 0;

Ce

@

@xl

Z

Ci

ij l xj .p0

D

ij l ıj l

C gz/ d V

.p0 C gz/ C

ij l xj

gı3l d V

Ci

Z

D

g

ij 3 xj

d V:

Ci

Finally,

M0 D

g Œx2C i

x1C j Vi ;

(9.16)

where i and j are orthonormal base vectors on the horizontal plane and

Z

Z

x1C Vi D

x2C Vi D

x1 d V;

Ci

x2 d V:

(9.17)

Ci

Expression (9.16) shows that the momentum of the pressure forces vanishes if

the line of action of the buoyant force passes through the centroid of the body. The

centroid of the displaced liquid volume is called center of buoyancy.

In summary: A body floating in a liquid is at equilibrium if the buoyant force is

equal to its weight and the line of action of the buoyant force passes through the

centroid of the body. It can be proved that the equilibrium is stable if the center of

buoyancy is above the centroid and it is unstable if the center of buoyancy is below

the centroid.

An extension of Archimedes’ principle to perfect gases is discussed in the

Exercise 3.

9.3 Fundamental Theorems of Fluid Dynamics

The momentum balance equation (5.30), when applied to a perfect fluid subjected

to conservative body forces, is written as

vP D

rp

rU I

(9.18)

with the additional introduction of (9.5), it holds that

vP D

r.h.p/ C U /:

(9.19)

9.3 Fundamental Theorems of Fluid Dynamics

257

By recalling the above definitions, the following theorems can be proved.

Theorem 9.1 (W. Thomson, Lord Kelvin). In a barotropic flow under conservative body forces, the circulation around any closed material curve is preserved;

i.e., it is independent of time:

d

dt

Z

v d s D 0:

(9.20)

in C such

Proof. If is a material closed curve, then there exists a closed curve

that is the image of

under the motion equation D x. ; t /. It follows that

d

dt

Z

Â

Ã

Z

Z

@xi

d

@xi

d

vi

dXj

vi dxi D

vi

dXj D

dt

@Xj

dt

@Xj

Ã

Ã

Z Â

Z Â

@xi

@xP i

@vi

vP i

dXj D

vP i C vi

dxj ;

D

C vi

@Xj

@Xj

@xj

and, taking into account (9.19), it is proved that

d

dt

Z Â

Z

v ds D

1

vP C rv2

2

Ã

Z

ds D

Â

r h.p/ C U

1 2

v

2

Ã

d s D 0:

(9.21)

Theorem 9.2 (Lagrange). If at a given instant t0 the motion is irrotational, then it

continues to be irrotational at any t > t0 , or equivalently, vortices cannot form.

Proof. This can be regarded as a special case of Thomson’s theorem. Suppose that

in the region C0 occupied by the fluid at the instant t0 the condition ! D 0 holds.

Stokes’s theorem requires that

Z

0 D

v ds D 0

0

for any material closed curve 0 . But Thomson’s theorem states that D 0 for all

t > t0 , so that also !.t / D 0 holds at any instant.

t

u

Theorem 9.3 (Bernoulli). In a steady flow, along any particle path, i.e., along the

trajectory of an individual element of fluid, the quantity

H D

1 2

v C h.p/ C U;

2

(9.22)

is constant. In general, the constant H changes from one streamline to another, but

if the motion is irrotational, then H is constant in time and over the whole space of

the flow field.

258

9 Fluid Mechanics

Proof. By recalling (4.18) and the time independence of the flow, (9.19) can be

written as

vP D .r

1

v C rv2 D

2

v/

r.h.p/ C U /:

(9.23)

A scalar multiplication by v gives the relation

Â

Ã

1 2

vP r

v C h.p/ C U D 0;

2

which proves that (9.22) is constant along any particle path. If the steady flow is

irrotational, then (9.23) implies H D const through the flow field at any time. u

t

In particular, if the fluid is incompressible, then Bernoulli’s theorem states that

in a steady flow along any particle path (or through the flow field if the flow is

irrotational) the quantity H is preserved; i.e.,

1 2 p

v C C U D const:

2

H D

(9.24)

The Bernoulli equation is often used in another form, obtained by dividing (9.24)

by the gravitational acceleration

hz C hp C hv D const;

where hz D U=g is the gravity head or potential head, hp D p= g is the pressure

head and hv D v2 =2g is the velocity head.

In a steady flow, a stream tube is a tubular region † within the fluid bounded

by streamlines. We note that streamlines cannot intersect each other. Because

@ =@t D 0, the balance equation (5.22) gives r . v/ D 0, and by integrating over a

volume V defined by the sections 1 and 2 of a stream tube (see Fig. 9.3), we obtain

Z

Z

v Nd D

v Nd :

(9.25)

QD

1

2

Fig. 9.3 Stream tube

9.3 Fundamental Theorems of Fluid Dynamics

259

This relation proves that the flux is constant across any section of the stream tube.

If the fluid is incompressible, then (9.25) reduces to

Z

Z

v Nd D

1

v Nd :

(9.26)

2

The local angular speed ! is also called the vortex vector and the related integral

curves are vortex lines; furthermore, a vortex tube is a surface represented by all

vortex lines passing through the points of a (nonvortex) closed curve. By recalling

that a vector field w satisfying the condition r w D 0 is termed solenoidal, and that

2r ! D r r v D 0, we conclude that the field ! is solenoidal. Therefore vortex

lines are closed if they are limited, and they are open if unconfined. We observe that

Fig. 9.3 can also be used to represent a vortex tube if the vector v is replaced by !.

The following examples illustrate some relevant applications of Bernoulli’s

equation.

1. Consider an open vessel with an orifice at depth h from the free surface of

the fluid. Suppose that fluid is added on the top, in order to keep constant the

height h. Under these circumstances, it can be proved that the velocity of the fluid

leaving the vessel through the orifice is equal to that of a body falling from the

elevation h with initial velocity equal to zero (this result is known as Torricelli’s

theorem, because it was found long before Bernoulli’s work). Assuming that at

the free surface we have v D 0 and z D 0, it follows that H D p0 = and, by

applying (9.24), we derive the relation

H D

p0

D

v2

2

gh C

p0

;

p

so that v D 2gh.

2. In a horizontal pipe of variable cross section, the pressure of an incompressible

fluid in steady motion decreases in the converging section.

First, the mass balance equation (9.26) requires that

v1

1

D v2 2 ;

(9.27)

so that the fluid velocity increases in the converging section and decreases in the

diverging section.

Furthermore, since U D gz D const along the stream tube, (9.24)

implies that

v2

v21

p1

p2

C

D 2C ;

2

2

and this proves that the pressure decreases in a converging section. This result

is applied in Venturi’s tube, where a converging section acts as a nozzle, by

increasing the fluid velocity and decreasing its pressure.

260

9 Fluid Mechanics

Theorem 9.4 (First Helmoltz’s Theorem). The flux of the vortex vector across

any section of a vortex tube is constant.

Proof. Let 1 and 2 be two sections of a vortex tube T and consider the closed

surface † defined by 1 , 2 and the lateral surface of T . By applying Gauss’s

theorem, we have

Z

Z

Z

1

! Nd D

r ! dV D

r .r v/ d V D 0;

2 V

†

V

where N is the unit outward vector normal to †. The definition of a vortex tube

implies that ! is tangent to † at any point, so that the theorem is proved since

Z

Z

Z

! Nd D

! N1 d C

†

1

! N2 d D 0;

2

where N1 is the unit vector normal to 1 , pointing toward the interior of the tube,

and N2 is the outward unit vector normal to 2 .

t

u

From this theorem it also follows that the particle vorticity increases if the vortex

curves are converging.

Theorem 9.5 (Second Helmoltz’s Theorem). Vortex lines are material lines.

Proof. At the instant t0 D 0, the vector ! is supposed to be tangent to the surface 0 .

Denote by .t / the material surface defined by the particles lying upon 0 at the

instant t0 . We have to prove that .t / is a vortex surface at any arbitrary instant.

First, we verify that the circulation along any closed line 0 on 0 vanishes. In fact,

if A is the portion of 0 contained in 0 , it holds that

Z

Z

v ds D

D

r

A

0

Z

v Nd D 2

! N d D 0;

A

since ! is tangent to A. According to Thomson’s theorem, the circulation is

preserved along any material curve, so that, if .t / is the image of 0 , it follows

that

Z

Z

Z

v ds D

r v Nd D 2

! Nd D 0:

.t/

A.t/

A.t/

Since A.t / is arbitrary, ! N D 0 and the theorem is proved.

t

u

The theorem can also be stated by saying that the vortex lines are constituted by

the same fluid particles and are transported during the motion. Examples include the

smoke rings, whirlwinds, and so on.

9.4 Boundary Value Problems for a Perfect Fluid

261

9.4 Boundary Value Problems for a Perfect Fluid

The motion of a perfect compressible fluid S subjected to body forces b is governed

by the momentum equation (see (9.18))

1

vP D

rp. / C b

(9.28)

and the mass conservation

P C r v D 0:

(9.29)

Equations (9.28) and (9.29) are a first-order system for the unknowns v.x; t / and

.x; t / and, to find a unique solution, both initial and boundary conditions have to

be specified.

If we consider the motion in a fixed and compact region C of the space, (e.g., a

liquid in a container with rigid walls), the initial conditions are

v.x; 0/ D v0 .x/;

.x; 0/ D

8x 2 C;

0 .x/

(9.30)

and the boundary condition is

v ND0

8x 2 @C;

t > 0:

(9.31)

This boundary condition states that the fluid can perform any tangential motion

on the fixed surface, whose unit normal is N.

The problem is then to find in C Œ0; t the fields v.x; t / and .x; t / that satisfy

the balance equations (9.28), (9.29), the initial conditions (9.30), and the boundary

condition (9.31).

If the fluid is incompressible . D const /, then the Eq. (9.28) becomes

vP D

1

rp C b;

(9.32)

while the mass conservation (9.29) leads us to the condition

r v D 0:

(9.33)

The unknowns of the system (9.32) and (9.33) are given by the fields v.x; t / and

p.x; t /, and the appropriate initial and boundary conditions are

v.x; 0/ D v0 .x/

v ND0

8x 2 C;

8x 2 @C; t > 0:

(9.34)

262

9 Fluid Mechanics

A more complex problem arises when a part of the boundary is represented by

a moving or free surface f .x; t / D 0. In this case, finding the function f is a

part of the boundary value problem. The moving boundary @C 0 , represented by

f .x; t / D 0, is a material surface, since a material particle located on it has to

remain on this surface during the motion. This means that its velocity cN along the

unit normal N to the free surface is equal to v N ; that is, f .x; t / has to satisfy the

condition (see (4.33))

@

f .x; t / C v.x; t / rf .x; t / D 0:

@t

In addition, on the free surface it is possible to prescribe the value of the pressure,

so that the dynamic boundary conditions are

@

f .x; t / C v.x; t / rf .x; t / D 0;

@t

p D pe

8x 2 @C 0 ; t > 0;

(9.35)

where pe is the prescribed external pressure.

On the fixed boundary part, the previous impenetrability condition (9.32) applies,

and if the boundary C extends to infinity, then conditions related to the asymptotic

behavior of the solution at infinity have to be added.

9.5 2D Steady Flow of a Perfect Fluid

The following two conditions define an irrotational steady motion of an incompressible fluid S :

r

v D 0;

r v D 0;

(9.36)

where v D v.x/. The first condition allows us to deduce the existence of a velocity

or kinetic potential '.x/ such that

v D r';

(9.37)

where ' is a single- or a multiple-valued function, depending on whether the motion

region C is connected or not.1

R

If C is not a simply connected region, then the condition r v D 0 does not imply that v d s D

0 on any closed curve , since this curve could not be the boundary of a surface contained in C . In

this case, Stokes’s theorem cannot be applied.

1

9.5 2D Steady Flow of a Perfect Fluid

263

In addition, taking into account (9.36)2 , it holds that

' D r r' D 0:

(9.38)

Equation (9.38) is known as Laplace’s equation and its solution is a harmonic

function.

Finally, it is worthwhile to note that, in dealing with a two-dimensional (2D) flow,

the velocity vector v at any point is parallel to a plane and it is independent of

the coordinate normal to this plane. In this case, if a system Oxyz is introduced,

where the axes x and y are parallel to and the z axis is normal to this plane, then

we have

v D u.x; y/i C v.x; y/j;

where u, v are the components of v on x and y, and i, j are the unit vectors of these

axes.

If now C is a simply connected region of the plane Oxy, conditions (9.36)

become

@v

@u

C

D 0;

@y

@x

@u

@v

C

D 0:

@x

@y

(9.39)

These conditions allow us to state that the two differential forms !1 D udx C vdy

and !2 D vdx C udy are integrable, i.e., there is a function ', called the velocity

potential or the kinetic potential, and a function , called the stream potential or

the Stokes potential, such that

d' D udx C vdy;

d

D

vdx C udy:

(9.40)

From (9.37) it follows that the curves ' D const are at any point normal to the

velocity field. Furthermore, since r' r D 0, the curves

D const are flow

lines.

It is relevant to observe that (9.40) suggest that the functions ' and satisfy the

Cauchy–Riemann conditions

@

@'

D

;

@x

@y

@'

D

@y

@

;

@x

(9.41)

so that the complex function

F .z/ D '.x; y/ C i .x; y/

(9.42)

is holomorphic and represents a complex potential. Then the complex potential

can be defined as the holomorphic function whose real and imaginary parts are the

velocity potential ' and the stream potential , respectively. The two functions '

and are harmonic and the derivative of F .z/,

264

9 Fluid Mechanics

V Á F 0 .z/ D

@

@'

Ci

Du

@x

@x

i v D jV j e

iÂ

;

(9.43)

represents the complex velocity, with jV j being the modulus of the velocity vector

and Â the angle that this vector makes with the x axis.

Within the context of considerations developed in the following discussion, it

is relevant to remember that the line integral of a holomorphic function vanishes

around any arbitrary closed path in a simply connected region, since the Cauchy–

Riemann equations are necessary and sufficient conditions for the integral to be

independent of the path (and therefore it vanishes for a closed path).

The above remarks lead to the conclusion that a 2D irrotational flow of an

incompressible fluid is completely defined if a harmonic function '.x; y/ or a

complex potential F .z/ is prescribed, as it is shown in the examples below.

Example 9.1. Uniform motion. Given the complex potential

F .z/ D U0 .x C iy/ D U0 z;

(9.44)

it follows that V D U0 , and the 2D motion

v D U0 i

(9.45)

is defined, where i is the unit vector of the axis Ox. The kinetic and Stokes potentials

are ' D U0 x, D U0 y, and the curves ' D const and D const are parallel

to Oy and Ox (see Fig. 9.4), respectively. This example shows that the complex

potential (9.44) can be introduced in order to describe a 2D uniform flow, parallel

to the wall y D 0.

Example 9.2. Vortex potential. Let a 2D flow be defined by the complex potential

F .z/ D

i

ln z D

2

i

ln re iÂ D

Â

2

2

i

y

v

x

Fig. 9.4 Uniform motion

ln r;

2

(9.46)

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