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Page 1: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg
Page 2: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

A HYBRID ELEMENT METHOD FOR CALCULATINGTHREE - DIMENSIONAL WATER WAVE SCATTERING

BY

DICK K. P. YUE

H. S. CHEN

AND

CHIANG C. MEI

Report No. MITSG 76-10Index No. 76-310-Cbs

Page 3: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

ABSTRACT

A HYBRID ELEMENT METHOD FOR CALCULATING

THREE-DIMENSIONAL SCATTERING OF WATER WAVES

DICK K. P. YVZ, H. S. CHEN and C. C. MEI

Under the assumption of a harmonic, small-amplitude incident wave,an effective hybrid finite element method for calculating wave scatteringby and rest.lting forces on a general three-di.mensional body is presented.

The method is applied to four different geometries. Analytic orindependently obtained soI.utions are available for two of the geometries andcomparison indicates perfect agreement. No other solutions are known forthe other two geometric

All numerical results are checked using Optical Theorem, snd convergencetests are performed. A user's manual for the computer program as well assuggested extensions to the program are also included.

While only the scattering probI em is formulated and solved in thepresent work, extension to the solution cf the mathematically similar radia-tion problem is quite straightforward.

The three-dimension-l hybrid element method is a practical one forengineering applicati.or.s in water wave problems, and offers a good alternateto the Green function method, especially for complex geometries.

Page 4: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

AC KNCiWLEDGENENT

This is the final repcrt of the work sponsored jointly by the Public

Service Electric and Gas Company, Newark, New Jersey, as part of a study of

a proposed of fshore floating nuclear power facility, and by the National

Oceanic and Atmo pheric Administration as part of the M.I.T. Sea Grant

Program.

The entire work was carried out at the Ralph N. Parsons Laboratory for

Water Resources and Hydrodynamics of the Department. of Civil Engineering at

N.I.T. The material in thi.s report is submitted by Nr. Yue in partial

fulfillment of the recuirements for the degree of Naster of Scierce in Civil

Engineering at N.I.T.

The authors are grateful to Ns. Louise Ketchen and Nary Maison for the

excellent typing.

Page 5: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

TABLE CF CONTENTS

~Pa e

TITLE PAGE

ABSTRACT

ACKNOWLEL3GEFENT

TABLE OF CONTENTS

1. INTRODUCTION

1.1 ~Sco s of the Present ~Stud 10

1.2 Mathematical Statement of the. Problem 12

1.3 ~Hbrid Finite Element FormuIation � The Variational 14

Pr iIBc ical

2. THE FINITE ELKMF.NT FORl'fULATION

2.1 The Volume~lute ral I

18

20

262.2 The -urface I~etc ral 12

282. 3 ~Inte rale on S, I ~to I

282. 3.1

2. 3. 2 lot~eral I

32

332.3.4 Integral I<

352.4 Quadrature

2.5 Sv32m3a and th» Total Global Natrix37

403. NUMERIC'. E3 AMPLES

413.1 Vertical Circular C linder in Conata

3.2 Circular~CIIndrical Platform in Constant Wat~er De th

Con..tant ~De th3. 3 Rectan~ul853.4 ~Elli tic Island with a Circular Base

Page 6: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

~Pa e

1064. ERROR ESTIMATES AND CONVERGENCE TESTS

1064.1 Th

1074.2 ~Conver ence of the. Finite Element A roximation

4.3 Conv~er ence of Series Terse in tahe Ansi t~ic Pe resentstion 114

1175. COMP1.TATXONAL ASPECTS AN1! THE PROGRAM USAGE

1175.1 Fr~pram ~lm lementati~on ~Esca and Time Re nirements

1195.2 ~The H hrid Finite E1ement P~ro ram

1195.2.1 ~Pro ra~mLo ic

129

52.30t tF 129

1315.2.4 Illost t. E le: ~Pxo ram List~in

5. 2. 4.1 Variable h'.arne Table

5.2.4.2 Sample Input5.2.4.3 Program Listing5e2.4.4 Sample Output

131

134

134

180

1976. CONCLUDING REMARKS

6. 1 SLI~es ted I~mrovements 197

2026.2 Recommendations and Cost Estimates for the Atlantic

Generatin Station Problem

6. 3 Concl usi one 203

5.2.1.1

5.2.1.2

5.2.1.3

5,2.1.4

5.2.1.5

5.2.1..6

5.2.1.7

5.2.1.8

5.2.1.9

5.2.1.10

5.2.1.11

5.2.1.12

5.2.1..13

MAIN

INPUT

ME SHO, ME SH1KIGVALHANKEI,

ASEMKV and EI.KV

FMAP2

SURFA S

GROS SK, PLOAD, CLOAD, DIACKDENSF. and COEFFSOLV and BKSUB'1'

FORCE

OUTPUT

123

123

123

124

124127

1.2 7

127127

128

129129

129

Page 7: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

L IS T OF S KF ERENCES

LIST OF FIGURES

LIST OF TABLES

LIST OF SYl%GLS

APPENDIX A: A PROOF OF THE OPTICAI, THFORE1f

~Pa e

205

207

212

213

2I8

Page 8: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

1. INTRODUCTION

Along with the technological advances in the design and construction

of large ships and off � shore structures, the need for an efficient and

versatile method for calculating wave forces and other hydrodynamical effects

to supplement laboratory testing is now widely recognized. The present

work is also motivated by such needs in an imaginative project of. ocean

engineering, i.e., the Atlantic Generating Station project. To meet the

increasing demand of electric power and to minimize environmental hazards,

the Public Service Electric and Gas Company of New Jersey proposes to

construct a nuclear power plant at a site three miles offshore from Atlantic

City. Two nuclear power plants of ll50 MeW capacity will rest on two

square platforms floating in a basin protected by two giant breakwaters. To

ensure sufficient supply of cooling water at a low intake flow rate, two

openings of 130 ft. width will be made in the breakwater system which is

roughly 2000 ft. in diameter. For the proper design of mooring systems,

prediction of possible basin and platform oscillation, and the wave forces,

due to incoming sea is needed. This work deals with a modern numerical

method which is potentially useful for the Atlantic Generating station as

well as for other large ocean structures.

In reality, the waves in a severe storm are highly complicated, being

large in amplitude and of a random nature. For theoretical work, it is

often necessary to assume the incident wave amplitude to be small so that

one can work with a linearized approximation. In this limited framework.

useful general relationships among the hydrodynamical quantities can be

deduced; see for example Haskind, 1957, Wehausen, 1971 and Newman, 1976.

Page 9: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

For a few very specialized geometries, analytic solutions are also possible

for example, MacGamy and Fuchs, 1952! . For general geometries, however,

a largely numerical method is necessary, the semi-analytic Green function

method being the most popular. By using a Green function of some sort, this

method transforms the boundary value problem into an. integral equation along

a line for two-dimensional, or over a surface for three-dimensional problems,

which usually includes the boundary of the solid structure� By dividing

the intervaI into smalI segments and making piece-vise approximation, the

integral equation is discretized to give an approximate set of algebraic

equations. Solving the algebraic system, the unknown in the interval is

found and the flow quantities in the entire fieId can be subsequently

computed. TIris method is conceptually simple but the Green function or the

kernel of the integral equation is singular, and numerical evaluation of

its integral or derivative is laborious and complicated. The resulting

algebraic equation has a fully popul.ated coefficient matrix with very involved

elements. Furthermore, the computation of flow quantities after the solution

of the integral eouation is found can a]so be quite time consurrring.

The method is consequently expensive, both computationally and in terms of

the analytic preparation required. Nevertheless, this method has been

successfully used by several authors; see for ex rmple Garrison, 3974.

There are also situations where the Green functions are either too

complicated or cannot be easily constructed. Alternative, more efficient

methods are desirable.

We present here a more direct method involving finite elements, The

general. idea is to use discrete finite elements in the region near

Page 10: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

complicated bodies and/or bottom surfaces, while some formal analytical

representation is assumed in a region away from them. Similar ideas

have been used long ago in conj unction with finite difference schemes

where there is a singularity in the region of computatio». It is based on

a plausible argument that an analytical solution of reasonably elementary

form which satisfies the differential equation and some of the boundary

conditions, In particular the singularity condition, must provide a more

efficient representation than a simple interpolation function which

satisfies few or none of these conditions. Near solid surfaces of arbitrary

geometry, analytic representation is of ten too cumbersome or impossible;

it is here, however, that the. power of finite elements can be most fully

utilized.

In this hybrid approach, the boundary value problem is recast as a

variational principle which incorporates the matching of the solutions in

the finite element interior region and the analytic exterior region as

natural boundary conditions so that ail the unknown parameters associated

with both regions can be solved simultaneously after discretization. This

is the most important feature of the present hybrid method. Under this

scheme, the finite element region can be greatly confined; the radiation

condition is satisfied exactly; local and global matchi»g are implicit, and

no iteration is required. The total number of unknowns is greatly reduced,

and the coefficient. matrix involved is narroW-banded with quite simple

components.

For two � dimensional problems, such as plane waves incident on an

infinitely long cylinder, a body with rotational symmetry about the vertical

Page 11: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

axis, or long waves in shallow water where the vertical dependence isomitted, this method has been applied with success by Chen and Mei, 1974,and Bai and Yeung, 1974, where computational experiments show that the

hybrid element scheme is an attractive alternative to the integral equation

method, especial ly for complex geometries.

For strictly three � dimensional bodies, the only numerical method

that has been successful is the Green function method, and it is valuableto extend the hybrid element method to general three-dirrensional problemsto study its feasibil.ity and ef ficiency. This is the task undertaken inthe present study. Because of the significant increase in computation timeand storage associated with an additional space dimension Black, 1975estimates a typical increase in computation time of the order 100 times forsame body dimensions and wavelength!, it is not clear a ~riori whetherthis method offers a viable alternative ro the Green function method.

'1.1 Scop«of the Present Study

The linerrrized boundary value problem for general three-dimensional

scattering of water waves is stated. By constructing an artificial surfaceto enclose all complex geometries, an equivalent variational statement,

which states that a functional made up of certain integrals involvirrg theunknown flow field is stationary, is given and proved. Based on this

variational principle, and using a vertical circular cylindrical surface. ahybrid element method is formulated, The fluid region inside is discretizedinto a number of hexahedral six-sided block ! isoparametric quadratic

finite elements, while a truncated eigenfunction series solution with

� 10

Page 12: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

unknown coefficients is used in the exterior which is assumed to be of

constant depth. Imposing the stationarity of the discretized functional,

a system of linear algebraic equations is obtained, which includes as

unknowns the finite element nodal parameters and the coefficients of the

exterior analytic solution. This resultant system is solved by direct

Gauss elimination.

For illustration, the three-dimensional wave scattering problem is

solved numerically. We point out that the radiation problem~ X e- waves

due to oscillating bodies, is mathematical so close to the scattering problem

that the present method can be adapted with trivial modification. The

following geometries are considered:

�! a vertical circular cylinder in constant water depth,

�! a circular cylindrical platform in water of constant depth,

�! a square platform in constant depth, and

�! an elliptical island with a circular toe.

Results for a range of' wave periods and incidence angles for the cases are

presented in Chapter 3. While all the four geometries involve one or

more vertical planes of symmetry and therefore simpler methods of computation

are available, we have taken no advantage of symmetry and each example is

treated as a general three � dimensional problem. In particular, the first

two geometries possess axisymmetry and can be solved in a two � dimensional

manner, and analytic MacCamy and Fuchs, 1952 for the vertical cylinder! and

semi-analytic Black, Mei and Bray, 197l and Garrett, 1971 for the circular

platform! solutions are available for comparison. To our knowledge,

no other solution exists for the latLer two examples.

Page 13: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

1.2 Mathematical Statement of the Problem

The general three-dimensional scattering of water waves in an ocean

is formulated under the following assumptions:

a! incompressible, inviscid, irrotational flow;

b! small wave amplitude;

c! simple harmonic incident waves;

d! impermeable, perfectly reflective solid boundaries.� i'd%

If we define the velocity potential as the real part of 4> = $ e

where a is the angular frequency of the oscill.ations, then $ must satisfy

see Fig. 1.1!:

�.F 1!in V

3Q dBz g

�.2.2!on z = 0 in F!

on all solid boundaries in B! �.2.3!

The scattered wave defined by $ = $ � $ where $ is the incident wave,s

must satisfy the radiation boundary condition:

� 12�

The mathematical background � the boundary value problem and the

variational principle is given in the remainder of this chapter while

Chapter 2 summarizes the hybrid element formulation. Convergence tests

and error estimates are presented in Chapter 4, computational aspects and

program usage are discussed in Chapter 5 where sample computer listingsare also included. Conclusions and suggestions for improving and extending

the program are given in the last chapter.

Page 14: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

~=on

, Dqbdn

I ~ I ' -- � v I1

I/ l~ 8e'

Figure 1.2 Introduction of the artificial circular cylindrical boundary,

13�

Figure 1.1 The boundary value pxoblem

yg2$ p !gm

II

I

!

Page 15: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

lim Px' � � ik ! I! = 0a

k~ Br o s0

�.2.4!on S

wherek is the wave number associated with t!!. The solid boundary B includes0

all floating or submerged bodies and the sea bottom.

1.3 H brid Finite Element Formulation � the Variational Princi leAssuming that all body and depth irregularities are reasonably cogfj n-

ed, say, within a radius r , we introduce an artificial vertical circular8

cylindrical surface S of that radius so Chat all solid bodies and depthvariations are just enclosed see Fig. 1.2! .

We now show the equivalence of the boundary value problem and thestationarity of the following functional Chen and Mei, 1974!:

2 ~s 3 h' s ~E ] dSJ t!! = I � v ! ' dv � � $' dF + [ � � $ !2 2g 2 s 3r 2 8rU S �.3.1.!

� 14-

For clar ity, we use the ! ' notation to denote quantities outside of Swhile the non-primed variables denote now only quantities associated withthe fluid region inside of S. The first integral is hence a volume integralover the fini.te element fluid volume, V; while the integrals over F and Sare surface integrals, the former only over the free surface in the interiorof S. The total wave potential outside of S, t!' and the scattered potential

are assumed to satisfy Eqts. �.2.1! � �.2. 3! and Eqt. �.2.4!I

respectively. Clearly, if $ also satisfies Eqs. �.2.1! � �.2.3! in V, andif Q and t!' are matched on S such that

Page 16: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

on S

aycIr Br

l. 3. 3!

then $ and $' form the complete solution in V and V'

By assuming constant depth in V', it is easily shown that the scattered

potential g ' can be represented by a double series of eigeniunctions:S

Ix cos n0 + p sin n0! cosh k z + h!H k r!�!on on 0 n 0

n=Q

CO CO+ ! <x cos n6 + 8 sin n0! cos K z + h!K K' r! �,3.4!

mn IBn m n III

k tanh k h =�0 0 g

VJtanl<h=

m Ill g�. 3.5!2, 3.

The proof of equivalence goes as follows. Taking the. first variation,

we have

W2hJ g! = V$ 76$!dV � � !ALII dF

g

~s I+ ]ds

2 s dr 2 s cjr ~ 2 3r

15

where h is the constant depth in V', and k, tc, m = 1, 2, 3,

respectively the positive real roots of the equivalent dispersion relations

Page 17: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

where use is made of the fact that $ is prescribed i.e., 6$ = 0! . Now

by Green's theorem:

t V'064 dF + ~ 6y dB+ ~ 6g dF + -~ 6y dsJ Bn F S Sr

where n is the unit normal pointing outward from the fluid, so

6J !! = � 7 $6$ dV + � � $�$ dF + 6$ dBRd> 3Bz g Bn

+ I >,� > �g- g- y'! > ]asay', aea'

S

+ -- f64' � �' � 4 ! � �' � 4 ! ]dS1, 3 36 b '2 Br I I 3r

S

The last integral

a6y[6$ ' � � t! ' ]dS

2 s Br 's 3rS

r g26y t 6~ r y2 y t !dye

S U-16-

Page 18: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

vanishes by virtue of Eqs. �.2.1! � �.2.4!, whence

5J�! = � V'g 5$ dV + � � y�$ dF + � 6g dB3d> d 3k

Qz g 3n

on account of Eqs. l.2.1! � �.2.3! and the matching conditions �.3.2!,

l. 3. 3! on S. Thus 3 $! is s tationary if and only if $ is a solution to the

boundary value problem in V, $' a solution to the boundary value problem in

V', and the coef ficients of $ ' in Eqt. l. 3.4! are such that f, $' ares

matched on S according to Eqts. �.3.2! and �.3.3! .

It should be noted that only $ must be prescribed and a11 other match�I

ing and boundary conditions are satisfied as natural boundary conditions.

Before we conclude this chapter, a few comments are due. First, it

should be noted that while we have chosen a circular cylindrical surface for

S for simplicity of evaluating the integrals over it, S can in practice take

on quite general say elliptical, spherical etc.! shapes. Secondly, later

investigation see Sections 4.3! indicates that it is almost always more

efficient to choose S as small as possible at the cost of having to use more

series terms in the analytic expression for the solution in V'.

must still enclose all floating or submerged bodies and bottom changes, and

for numerical accuracy, S must allow finite element aspect ratio not to

deviate significantly from unity.

Ihirdly, the analytic expression in V' Eqt. �.3.4!! may in general be

chosen in a number of other ways, say, using a Green function representation.

The latter choice, however, would lead to an integral equation on S, and hence

possesses much of the undesirability of the direct integral equation approach,

� 17

Page 19: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

2. THE FINITE ELEMENT FORMJLATION

We rewrite the functional �.3.1! as

> y! = 2 ~4! dV

Q 2-I � p ds'2g

F

I2

+ � <I> � $ asl 2 s 3r s

ds8 fBr s

+ g � $' asI Br s

3r II6

Here the incident wave $ can also be expressed as partial waves

iga ik r cos e � e ! cosh k z + h!0 0 I 0

e cosh k h0

�.1. a!

iga cosh k z + h! Z a i J k r!cos n e � el! �.1.1!cosh k h n n o

o n=O

n 0K ~ 1n

n = l, 2, 3,

where a and 9 are respectively the amplitude and incidence angle of theo I

incident wave, and the Jacobi symbol

Page 20: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

is introduced. The scattered wave in the outer region V' is as before

[ x cos n6 + 5 sin n6] cosh k z + h!H k r!�!on on 0 n 0

$ts

n=0

+ Z Z cI cos n6 + 5 sin n6! cos K z + h!K � r! �.2.4!mzl mn Ill n m

m 1 n=0

where k , K , m = 1, 2, ... are defined as in Eq. �.2.5!. For simpli-0 m

city we define

�. 2!m = 1, 2, 3,k m Ill

and k are now the roots of

k tanh k h - � = 0m m g

m=0, l, 2, 3, ~ ~ ~

that for m = 1, 2, 3,

cosh k z + h! = cos Ic z + h!m m

H l! k ! K g ! Z +~!n m n m

l! ' k !,, ! 2 --Pi II+1!n m n m II

Ve can now rewrite, by redefining cx andmn mn

Z Z a cos n6 + 8 sin n6! cosh k z + h!H k r!�!mxl IBIl ll1 n

� 3!

The evaluation of each of the integrals denoted as T., I , ... I6!

are presented in the following sections.

which are real for m = 0 and pure imaginary for m = l, 2, 3, .... Note

Page 21: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

2.1 The Volume Inte ral I

I = � V$! dV1 2

If we subdivide the domain V into M finite elements and choose NV

nodal points x in V, i = 1, 2, ...,N» such that the potential in eachelement can be approximated as a linear function of the potential values

eon nodes within or on that element, using superscript ! to denote

e

quantities associated with element e, we have, for an element with Mnodes

�. 1.1. a!

or if we let

1 + M

M

1xM

then

�.1. 1. b!

eHere the interpolation functions N. are in general functions of coordi~i

nates, whether global or local to the element e.

For properly chosen elements and associated interpolations, and. for

sufficiently large M , we can write

20�

Page 22: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

"vI 1 Z 2 Vf ! dV

e=l e

e<V

vZ � VtN'! <4 !!' dV'

e=l e

ecV

Z 2 f]I] ! f VfN ! V N ! dV ] $ !e=l e

ecV

z � , {e'! [K'�] f0'!e=l

eeV

1 y!T [K,j f0! �. 1. 2!

where

[K ] = [ J V N ] ]' M ] dV ]V

H x M

aNi aN'. aN. SN'. aNi aN'.v ij

'~+ '~+ � i � 1! dx' dy dz'ax ax ay ay az az

e

is the union of all the j,]t }'s which in general are not mutually exclu-

sive.

21�

ei, j = 1toM �.13!

and in the last equality of Eq. �.1.2! an assemblage is implicit where

Page 23: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

e eClearly each [K j is symmetric and real N are real!, and hence

the globally assembled result [K ] is also real and symmetric. Further-

more, with shrewd node-numbering, [K ] can usually be made banded.

In this study, we have chosen isoparametric hexahedral elements of

the "Serendipity" family containing 20 exterior nodes and quadratic

interpolation functions, N , i = 1 to 20 Zienkiewicz et al, 1969! see

Fig. 2. 1!:

i lto8

,, q., . = +1 corner nodes!i' i' i

� E ! � + qq ! � + !i 4 i

i 9 to 12

E = 0- q = +1i i

N = � � + g ! � � 'q ! � + g4.!4 i i

i = 13 to 16

q = 0; F., 4. = +1 midside nodesi i

i = 17 to 20

0; ., q. =+1i ' i' i

wa get

22

�.1.4!

where , rl, form the local coordinates and ,, g., g = 0, +1 are thei i

nodal coordinate values.

By choosing the same functions N to map a local point from the4 space into the global x, y, z space via the nodal coordinates,

Page 24: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

20

l2

Figure 2.1 The 'i-node hexahedral element with exterior nodes only in

local coordinates!

Figure 2.2 The caughter arbitrarily oriented, auadratic � sided hexahedral

element in global coordinates after isoparametric mapping

� 23-

Page 25: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

ex

[X ! N�.1.5. a!

where

1 2 ' 20

yl y2 ''' y20

1 2 20

lx ! =�.1.5,b!

is th» 3 x 20 matrix of the gLobal coordinates of the nodes and

[N! F -I «! ' N20 Z 0 0! ] �.1. 5. c!

Arbitrarily shaped and oriented hexahedral daughter elements with quadratic

sides can be adopted in region V see Fig. 2.2! . Note that aside from

aspec.t ratio considerations for good numerical convergence, the sides ofthe daughter element need not be of equal length nor the "midside" nodes

be at mid-distance, under this isoparametric mapping.

The choice of the element is made for several reasons. The block shape

allows for convenience of element arrangement and of node numbering. The

element is quadratic so that a much smaller number of them would be requiredto achieve the same accuracy; furthermore, isoparametric mapping results in

much better approximation of boundary surfaces, for example, for quadratic

surfaces, the representation is exact.

Returning to Eq. �.1, 3!, we note that since Eq. �.1.4! cannot be

readily inverted, the integration must be performed in local coordinates.

BN BNi e e e

! dx dy dzB. B,'aN'i BN'. BN'. BN'.

K 1V ij Bx Bx By By

V

�.1.e!

i, j = 1 to 20

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wr"l ting

1 1 1

dx dy dz = ~J[ d dn dCeV -1 -1 � l

a

ac

= [J]

-1where I J~ is the determinant and [J] the inverse of the Jacobean matrix

~3an 3F

3z

an az[x, y, z][J] =

a

ar.

a+an ar.

ax

az

Substituting from Eq. �.1.5!

3

an[N![X][J] =

ewhich can be evaluated in terms of E, n, I. and [x ] since N. is expliciti

in E�rr, g as given in Fq. �.1.4! . Since the integrand in Eq. �.1.6! isin general a function of the nodal coordinates of the clement, numerical

quadrature is necessary in the integration.

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2.2 The Surface Inte ral 12

I = � - � $ ! dF1 hl

2 2 g

Substituting from Eq. �. l.l.b!, we obtain

~2z~=~ !' - � !I <!!!�!! d!'eE;F

Fe

2Z ~ ! [< � � " > N'! N ! dF ]� !

eEFg

F

z ~! z ]� !eCF

�.2.1!

e eponding only to the nodes on the free surface, i.e., $ c F

For simplicity, we require elements on F to have exactly one face

on F, and N !, <f> ! are 8 x 1 so that

2K ! : � - � ! N ! N !T dF ]g

8x8

F

2K, ~ � � NN dF

ijg ij1 to 8 �. 2. 2!

� 26�

e e e ewhere N !, $ !are the subvectors of N ?, $ ! containing entries corres-

Page 28: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

eE F are involved. Note also that as in I , [K ] and [K ] are real and

e esymmetric, and since I.g ! C k$ ], [K ] always has a bandwidth less thanF

or equal to that of [K ].V

eTo evaluate I and in particular K , we perform the integration

l ij

in local coordinates

l 12

F ij g NN. ~J I! dEdq

i j p =1�. 2. 3!

-I -1

where the Jacobian is given by

which is readily evaluated in terms of local coordinates and [X ] by using

Eqs. �.1.5!.

The resulting integral is again evaluated by quadrature.

27

Again assemblage is implied in the last equality of Eq. �.2.1! where only

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2.3 Inta rale on 3 I ~eo I

For computation the series �. leb! and �.3! are truncated as

follows:

Niga cosh k z + h! o

Z c i J k r! cos n B � 6 ! �.3.1!I 0! cosh k h n n o E0 n=0

and

M Nm l!Z Z u cos nG + 8 sin nB! cosh k z+ h! H k r!s mn mn m n m

m=0 n=0�. 3. 2!

where N,, i = 0, l, ..., M, and M are chosen depending on wave period andi

geometry.

2.3.l

T = � $ � $ dS1, 82 s 3r s

S0 2m

dz dB r

0

M Nm

K Z �, cos n6+ P sinmn mn

m=0 n=0

M N

Z Z � cos nB + 8 sinmn mn

m=O n=0

nB!cosh k z + h!H ' k r !k ! l!,

28�

r

K2 R=O

M N

Z Z � x u +kn mn

m On=0 n

nB! cosh k z + h!H k r !! l!m n m S

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0

r cosh k< z + h! cosh k z + h! dz� h

Mr m 27rk sin.h 2k h

2 E mn mn n m S n m S 4kZ Z G +8 !H kr!H ' kr! + !

2m=0 n=O n m

lf we define a coefficient vector

= I:00' 01' 01' 02' 02' ON ' ON

0 0

IO' ll' l1' 12' 12' 1N , 1N1.' 1

MO' Ml' Ml' M2' M2' ' ' ' ' MN ' MNM

1 " N say! �.3.3..1!

= 1 Iej~ [V ]<V! �.3.1.2!

where [K ] N ~ N is diagonal and

K = 2~ H k r !H ' k rS! sinh 2k h+ 2k h!�! �!,n mS n mS m m

i n

i= 1 toN

where the i-th element of �! is < or 8 . Clearly by our notationmn mn

in Eq. �.3!, K are complex for < ~ 2N � 1, and real otherwise.

29

noting that all the terms associated with B. , i = 0 to M are zero and soi0'

are excluded, then we can writeiO

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2.3.2

Nm

N ! {f ! Z Z Q cos n6 + 5 sin n6!cosh k z+ h!eES m=0 n=0

S

� >, ...��.n mS' m

where we have substituted from Zqs. �.l.l.b! and �.3.2! and N !, $ !are subvectors of N !, ff ! where only entries corresponding to $ E Se e e

are included.

Again in this formulation, we require, for simplicity, elements on Se etG have exactly 1 face on S and {N !, $ ! are vectors of dimension 8.

Me now write, using the def inition �. 3. l. 1!

ehere [K ] is defined as the 8 < N matrix withC

K .. = � N. cosh k z + h!H ' k r !k ! dSij i m n m S msinn6S

To perform the integration in local coordinates, we write

30

where the j-th element of

is indicated depending on

gf s

g! is ~ !, and a choice of cos n6 or sin n6j, since ip! is defined to include both 0's and

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Page 33: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

2.3. 3

Ij' dS5 I br s

0 2m iga cosh k z + h! oN

d6 -! hkh Zc i J k Sc«n BI d cosh k h n n o So n=0� h

M

cos n6 + p sin nB!cosh k z + h!H ' k rs!k ! I! IIILn m n m S m Z

m=O

iga0 2 tt cosh k z+h! o

N

- � !r dz i d60 0 .nS J coshkh On n o S

Z c i J k r ! cos n6 cos neI

-h 0

+ sin nB sin n6 !! ~

M N

. g g ~ cos n6 + g sin n6!cosh k z + h!H ' k rS!k ! I!,mn mn m n m S m

m=O n=O

21' Niga 0 !r p F i J k rS! cos nB cos nBI + sin n6 sin nBI!!

s n n o S0

n=O

N

~ u cos n6 + g sin n6!cosh k z + h!H ' k rS!! I!,on on 0 n o S

n=0

sinh 2k h + 2k h!0 0

4 cosh k h0

iga

!

sinh 2k h + 2k h!0 0

2 coshkh0

32�

0

N

K umn

N0

Z a coson

n=O

nB + 8 sin nBI!i J k r !H ' k rs!vrn I!,I on I n o S n o S

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Niga o

- � ! w r ! Z Q cos n8 + 8 sin n8 ! d S on on I

n=0

i J k rS H ' k rS,n �!,

sinh 2k h. + 2k h!2 cosh k h 0 0

0

l> = � jq ! >!; �.3. 3. l!

here Q ! is defined as the �N � l! by l vector with0

iga. sinh 2k h + 2k h! -" 'S! " ' '" 'S'"'k 'S' 2

i 0

cas n8

s~ ne ' �. 3. 3. 2!

0onwhere the i-th element of p! is ~ !-on

2. 3.4

3

6 ar

Substituting $ from Eq. �. l.a!,

iga ik rS cos {8 � 8 !j $ ! - ! ik cos 8 � 8 !e

6 e e m o IecS

S

cosh k z + h!ds

cosh k h0

33�

where the orthogonality properties in z and 8 are invoked. Now we write

Page 35: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

K {q ! <> jeCS

�. 3. 4. 1!

where we have defined the 8 x 1 vector {q! ao thate

e e 0ik r cos� � 9 !

N. - !ik cos� � 8 !e0 I

i ld o Ii

S

cosh k z + h!0

ds

cosh k h0

ik x cos 6 + y sin. 6 !e cosh k z + h! IJSI! dg d4 �.3.4 2!

0 S q=l

which after expressing x, y, z in terms of local coordinates can be

integrated numerically. Assembling {Q !'s for all e E S, we obtain

6 = � {q j {4! �. 3. 4. 3!

where now {Q 3 is an N >< 1 complex vector.y S

34�

cosh k h0

iga - �!ik

1 1

N � cos 8 +~sin 6 !i r I r I

-1 � 1 S

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2.4

Three different Gauss-Legendre quadrature formulas Kopal, 1955;

Abramowitz and Stegun, 1964! are used in the evaluation of the integrals

Il' I2' 14 and I6

�! for I a 21 point formula is used for the cubic volume:

f C, V, C!d dV dC

[-496f +128Z f +BZf +5Z f ]+0 R!m r f v

�.4. 1!

where f = f�, 0, 0!

Z f = sum of values of f at the 6 points midway from the center tor

the 6 faces.

Z f = sum of values of f at the 6 centers of the faces.

Z f = sum of values of f at the 8 vertices.v

r 1 1 3 3

f E, q!dg dq = Z Z .. f g.,q!i=1 j=l-1 -1

�.4.2!

where

W.i

.8888889

.5555556+ 0. 77459671,3

35

�! for I and I , the evaluation over the square surface is approximated

at 9 integration points:

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�! for I, we note the presence of rapidly varying functions sin nB and

cos na in the. integrand and adopted a more accurate 16 point formula

for the square surface area:

r 1 1 4 4

f g!dF dr! = 2 Z w w. f E, ~ !i=1 g 1

-1 -1

�.4. 3!

where

E,qi

1,4

2 $3

-36-

+. 3478548

+.6521452

~ 7745967

.3399810

Page 38: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

2. 5 Summar and the Total Global Matrix

becomes

J {�!, P!! = 2 �] [K ]�]

+ � �] [K ] y!

+ � u] [K ] p]

�!

�!

+ � p! [K ] �]- + � �] [Kc] 'p! �!

{5!

�. 5. 1!

For J to be stationary, we require

i 1 I ~ ~ N

and

�.5.2.a,b!i ly ~ ~ ~ ! NT

which gives

[K ] C! + [K,]�! + [K,] W] = < ! �.5 3.a!

and

[K j p! + [K<j�! = <Q<> �. 5. 3. b!

37�

Summarizing and treating P! as unknown also, the functional {2.0!

Page 39: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

H we define

y}' = [ ~}', >}'] lx N +N!V T

�.5.4.a!

of g} to all g for nodes on S, i.e.

y.~s} , p} ] �.5.4.b!

and assign the last positions

4]' = «e, ly.~s}', g.l

then iEqs. �.4.3.a,b! combine to give a system of linear equations

[K] q} = Q}

whexe

Q = [ Qp , Q< lx N +N! �.5. 5!

and the total global matrix [K], N + N ! x N + N ! has the following

Nstructure:

N

N

�. 5. 6!

We note that [K ] + [K ] is real symmetric and banded and [K ] is diagonal

and complex while [K ] is full and complex; they are positioned as shown in

~2.5. 6!. [K] is consequently complex and symmetric which is partly a

xesult of the rearrangement in Eq. �.3.2.1!.

Having obtained [K], Q} can then be solved by inversion, or formally�} = [K] q} � 5 >!

38�

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From Eq. �.5.3.b!

{V! = [K,] { , � [ ,] {e! �.5.8!

which when subs t i tuted into Eq. �.5. 3.a! gives

[q] + [KF] � [KG] [KD] [KC ~ {~P! [KC] [KD!

or

[K ] {V! = {q > �. 5.9 .a, b!

where

[K'] : [ ! + [~ ] � [K ] [K ] l G]

and

{a'! = {0,! � [KG] [K ] {q

Clearly, [K' ] is still symmetric and banded, and complex, and only the

symmetric half band of [K'] is stored.

A direct method Gauss elimination! is used for solution.

-39-

and the nodal potentials in V as well as the coefficients of the expansion

in V' are solved. For efficiency the total global matrix is condensed to

eliminate the unknowns {p!, which can be recovered by substitution after

is solved.

Page 41: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

3. NUMERICAL EXAMPLES

Three-dimensional diffraction by four different geometries are treat-

ed and presented in the following sections. For the first case, the analytic

solution is known, while for the second, accurate numerical results are

available by other techniques. They are therefore useful for testing the

pre'sent me tbc~d. The 1 as t twc~ examples have no other known so 1 u t iona, and

are chosen to illustrate the versatility and efficiency of our method,

while for simplicity of grid generation, they are still chosen to have

relatively elementary geometries.

A3 I thc finite. «3 c ment grid systems c>r«gc.nera ted by computer with

automa L ic ally min imiac. d bandwid ths-

Figure 3. I Wave incident on a bottom-seated circular cylinder of radius

� 40�

Page 42: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

3.1 Vertical l'ircvlar Cylinder in Constant Water De th~ Fi . 3.1!

The cylinder has radius a and is centered along the z-axis. The water

depth is everywhere constant h. For the case of h = a, we use a fini te

element grid consisting of 36 elements � two layers of ]8 elements each

stacki d vertically forming a ring surrounding the cylinder. See Fig. 3.2! .

There are a total of 342 nodes, and the average element dimension, L, ise'

about .48a. The incidence angle is taken as B = 'I .I

In the. analytical theory for this two-dimensional geometry, only the

coefficients corresponding to the first eigenvalue k are involved and all

other coefficients vanish. For generality and for an indication of the

accuracy however, 5 eigenvalues are chosen N = 4; N = 12; N = N = Ns o ' 1 2 3

N = 10!, which corresponds to a total of 99 terms in the exterior analy-

tic representation Eq. �.3.2!! .The following quantities are presented'.

�! coefficients of the series solution in the far field:

�! normalized free surface elevation on the cylinder r = a!: p/a 0

where t! a,0! = �.� $ a,0,0!; and,i 4!

g

�! horizltntal force coefficient,

We define dimcnsionless force and moment coefficients by

F.

pgv a Ha,

and �.l.l.a,b!3 = K y Z

Page 43: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

� Z=Q

CIZ=�

2

3G� Z =-4

Z

r =1.5o IOpe

Figure 3,2 Finite Element Structure for a Uniform Cylinder

36 elements, 342 nodal points.42

Page 44: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

F = Re[ F.! e ] , 8 = Re[ M ! ei

For example, in this case, we have

0 2zsos 0 d9 dz

vh igao

FC x

x Ogler aha�.1.2!

� h 0 - � !4>

which is evaluated by numerical integration of the so1 ution

Comparisons are made to known analytic results see, for example,

MacCamy and Fuchs, 1952!

J' k a! cos ne.n n o-c i , - . ! for m = 0n H l!' k cosh k h sin no

k a! 0n o �.1. 3!

form40

3' k a! !i cos n U � 0 ! [J k r! � � � � �,� � H k r! jI n o l!' k n o

k a!n o

tIa n

o n=0

�. l. 4!

and

4 cos 9 tanb khC I o

F �!'H » k a!1 o

�.1. 5!

43�

where H, a are the typical draft and horizontal dimension of the body

respectively. F. refers to the force component in the i direction, and M.i

the moment component about the i-axis, i. e.

Page 45: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

N

NCc0

O

O

N QNO

CO O

O cClO O

00 cA

ChVl

PlO

CCCV

O

00N Ng0Q CCl

O

O

CG Pl

00~N CP

0c

NCV 00NI/l EA

NI I

O N

N'G0

80

IJ

0 0

accl

0ch

I!

A U

EO

II II

CIcd

I I I

O

O j ON

00

Q4J4JVV!

O cd4J

cdQI

dc6

dJ

'cdC cd

V cdC C dcdI

dICCI

'4 cdc0

cd

C C OJJcd'0dI

dcV dI

dI

C dI

V 0 CLI4 0

� 44�

Page 46: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

IO IO O O'K

0r

X

«ir

ICr IOrri0O

0 E

0 0 «I~ I0

CVCr

PlIO IO

Cr

E 0'riC

IOIO cr

rrJ0SSC0JJrg0

I Cl

0 O '4 0W~ J

4I

0C'«4«0

«J0«

0IOI CI I Cr

4 0r«J

Cl«0«' i S

0

IO I ClIa«0IO «S«J

0 4 00rO

O I SlJJ0zrCl

45

plI I w 0a a o

I Io aX ««r« IIO Pl«0

lI IO CI

IJrJ

IIil0 0

0 IS

0 0 4 rS«l.E 0«J

0 0~ J

I 0 IrSr0

Page 47: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

For 9 = m s the comparison for a range of wave1.engths k a for C and0 X

are presented in Table 3.1. It is evident that the results are quiten

0

satisfactory <1%! for up to k a ~ 4- This implies a maximum k L ! of0 oe max

about 1.9, A . > 3.3 L ! . In Table 3,2, a sample of the coefficientsItin e

calculated for the case k a = l is listed for comparison, Note that their0

contribution to f' decreases rapidly though the magnitude of the coeff'is

cients themselves are not necessarily small! due to the magnitude of the

Besse1 f unc t iona.

See also Section 4.2 for summary of present and other results.

3. 2 Circular C~tinctric >1 Platters in Constant Water~De thWe consider a fixed dock in the torm of a circular cylinder with its

axis vertical. The radius of the cylinder is designated by a and thedraf t by H. The water depth h is constant. Specifically we have chosenH = . 5a and h = . 75a see Fig. 3. 3!,

The f inite element grid system now has a total of 56 elements tworings of 16 elements each stacked vertically outside the dock and 24 underit! and 435 nodes see 1'ig. 3.4!, Note in particular the use of the hexa-hedral elements in the center core, and the choice of greater node densitynear sharp edges. The average element dimension for this example is L 4.36a.e

Again, 5 eigenvalues and a total of 99 terms N = 4; N = 12, N = N = N0

10! are used in the outer series solution Rq. �.3.2!!, and we chooseThe vertical and horizontal force coefficients are now respectively

I

Page 48: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3. 3 Nave incident on a floating cylindrical dock of radius

a e = ~; l = .75 a!

z=0

� z =-,30

z =-,5Q

� z = .625a

� z =-75a

r/a =0 .25 .5,8 1 1.15 1.3

36 eler:.ents,i;ure .4 Finite element structure for a floating dock:435 nodal points

Page 49: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

a 27t ~ ! dO dr

0 0

FC, z

z pgvraHa0

�.2 l!

and

0 2m

! cos 0 d6 dzmH iga

0� H 0

FC x

x jglTaHa0

� 2.2!

A table of these coefficients for a range of k a and their plot in0

comparison to those obtained by Garrett, l971, is presented in Table 3.3

and Fig. 3.5.a,b. Tlute results are clearly satisfactory and discrepancies

are imperceptib1 e on th» graphs for the entire range we considered. A

complet» 1 ist of nodal potential and series coefficient solutions for k a = 10

is also given as a part of the sample computer output in Section 5. 2.4.4.

Page 50: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Tabl» 3.3 Computed horizontal and vertical force coefficients

for a semi-immersed circular cylinder

-- = .7S, � = .S!h Ha ' a

Page 51: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

1.0

0.8

0.

O.

3

koa

Figure 3.5 a! Horizontal force coefficient for a semi-immersed cylinderh, H � =,75; � = .5 B = ml.a ' ' a I

+, amplitude; X, phase: f inite element solution

amplitude: � � � phase. Garrett, 1971 results

50

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2.0

1,6

14

1.2

O.

0,

Fig~re 3. 5 b! Vertical force coefficient for a semi-iauoereed cylinder See description in Figure 3.5 a!!

51

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3. 3 Rect~an nla~rBar e in Constan~t. De th

We now consider di ffraction by a fixed semi-immersed draft H! rectan--

gular box in constant water depth h. Unlik» the previous two examples, this

problem must be treated zs a fully three-dimensional one. We choose a

special case. of a square barge, length 2a, H = .5a and h = a, and consider

two dif ferent incidence angles 6 = vr and 5m/4,I

The finite clement grid employed is similar to the one we use for the

circular dock and has 56 elements and 435 nodes. The previouts choice of

the number of series terms is still kept.

Tfte force and mooient coefficients for both angles of incidencc over a

range of wave number k a are plotted in Figs. 3.6.a,b,c and Figs. 3.7.a,b,c.

In general, far a given grid, the firiitc element approximation becomes

more accurate as the w-velength relative to it increases k a decreases! .0

On acccunt of the comparisons in the last section for a cylindrical dock

where a similar grid system i.s used, we expect the present results to be

rea.onably reliable at least for k a up to ~ 6, and certainly far beyond the0

regions of peak values around k a = l.0

In Pigs. 3.8.a,b,c,d and Figs. 3.9.a,b,c,d, the run-up, q/a, on the0

sides of this sqtare barge for k a = l,2,3,4 and both incidence angles are0

presented. Note that this local value becomes quite highIy varying ss the

wavelength decreases to about the body length k a = 4!, and the interpola-

tion between the nodal values in those instances are somewhat unreliable.

Furthermore, die to the finite element smoothing, both the value of ~rI/a0

which is proportional to the potential! and its derivative are finite at

the sharp edges, while according to two-dimensional potential flow around

52

Page 54: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

-iso present.

For a measzre of the argular intensity cf scattered wave energy, we

define the differential scattering cross-section IA H!I , where A 8! is th»

angular variation of the faz-field scattered wave, i.e.

� iga ikor� e cosh k z+h! A 8!

s 4Q v 'lT ovk r

0

�.3.1! k r»l!0

and if we substitute in this asymptotic form of $ into Eq. �.3.4!, wes

have

CO igaA 8! = 2 -i! u cos n8 + P sin n8! / � !

n=0 on on�-3 2!

The differential scattering cross-section, ~A 8! ~, as a function of 8 for

k a = 1,2,3,4 and the two inciderce angles are given in Figs. 3.10.a,b,c,d0

ard Figs. 3.11.a,b,c,d; ar d in polar coordinates in Figs. 3.12.a,b,c,d, i!,

d. ii! and in Figs. 3.13.a,b,c,d for a better physical picture.

53

-1/3a 90 wedge, the gradient of the. potential exhibits a r type singularity.

This local discrepency can in principle be corrected see Chen and Hei, 1974!

by constructing special analytic elements similar to S around all sharp

edges and corners, Such a procedure is however, unnece sary if one is only

interested in averaged g1oba] ! results such as forces and moments, or in

local values in the major region not close to singularities. This is

evidenced by comparisons for the circular dock where a bottom sharp edge is

Page 55: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

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Figure 3.12 a! Polar Plot of Differential Scattering Cross-section,IA�! I, for a Square Barge, Normal Incidence: 8

a.ka=l0

76�

Page 78: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.12 b! Polar Plot of Differential Scattering Cross-section,

~A 8! !, for a Square Barge. Normal Incideace: 6b. ka=2

o

77

Page 79: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.12 c! Polar Plot of Differential Scattering Cross- ection,iA 8! !, for a Square Barge. Normal Incidence: 8

c.ka=30

� 78�

Page 80: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3. I2 d! . i! Polar Plot of Di f ferential Scattering Cross-section,

!A�! ~, for a Square Barge. Normal Incidence: 8Id.i k a = 4

0

79

Page 81: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.12 d! . ii! Polar Plot of Differential Scattering Cross-section, A 8! !, for a Square Barge. Normal Incidence: 8

d.ii ka= 40

7'; 3Tr 5m 71TDetails for � ' < 8 < � and � < 8 < �,4 4 4 4

Page 82: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.13 a! Polar Plot of Differential Scattering Cross-section,IA 8! I, for a Square Barge. Oblique Incidence: 8 = 5'/4.

a. ka=lo

Page 83: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Pigure ..13 b! Polar Plot of Differential Scattering Cross-section,~4�! I, for a Square Barge. Oblique Incidence: 6 = 5w/4.

b.%a=20

82

Page 84: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.l3 c! Polar Plot of Differential Scattering Cross-section,

IA 9! I, for a Square Barge. Oblique Ivciden-.e: 9 = Sm/4.c, k a 3

0

Page 85: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.13 d! Polar Plot of Differential Scattering Cross-section,~A 9!I , for a Square Barge. Oblique Incidence: 8 = 5x/4.

d. ka=40

84�

Page 86: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

3.4 Elliptic Island with a Circular Base

The scattering of a plare wave by an island has been of theoretical

and practic;1 interest in recent years. Lonpuet-Higgins, 1967 showed the

existence oF trapped wave . around circular islands, while Vastano ard Reid,

1966, 1967, Lautenbacher, 1970, Smith and Sprinks, 1975 and 3orsscn,

Skovgaard, Kjaer �975! studied the tsunami run-up os various islands of

somewhat specialized geometries. All existing treatments require either

an assumption allowing linear shallow water long wave! theory vhich ignores

vertical variations, or that the ocean bottom i s slowly v"rying mild sj ope

theory! . It is of interest to pcint out that owing to both bottom refrac-

tion and posribi e resonance of the. trapped medea, the maximum run-up on an

island can often be very 1 arge.

In this last example, we ccnsider the diffraction by ar: island th-t

extends from the tree surface in the form cf a vertical elliptic cy1inder

up to a depth H, then slope out "ccnically" to form a circular toe on an

ocean bottom of constant depth h . The geom try and finite element grid

�6 elements � two rirjgs of 18 each, and 342 nodes! are shown in Fig. 3.14.

For an «pper e}liptic cylinder of m-.jor anc'. minor axes A, B anc' length

stbmergenc e! H, i.e. from z = 0 to � 8!, the lover "conical" portion is

generated ly the parameters ! and 8:

x = t XA + � � A!R] cos 8

�.4.l.a,b,c!

y = f XB + � � A!R] sin 0

and

z+ h = A h � H!

-S5-

Page 87: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

where R is the radius of the. circular toe at the sea bc ttom z = � h! . Cut

planes normal to the z-axis - h < z < � H! yield elliptic cut sections

given by

2 2

[X A-R! + R]z [A B-R! + P.]'�.4. 2!

with eccentricity. e, varying fram J B e to 0:s I v-ries from iA

z= � H! to 0 z= � h!:

A- B!!A+ L-X!R

�.4. 3!

As a particular example, we choose A = a, B = .5a, H = .5a, h = a and

R = 1,5a as in Fig. 3.14! . The "conical" beach now varies from a width c f

a wi th a 1 on 2 slope at 8 = m/2, 3Ti/2 tn a 1 on 1 beach of width G.5a at

8 = 0,1!. Three incider t wave-numbers k a = l,2,3! at two angles cf

incidence 8 = vr and 3m/2! are considered.

The run-up, q/a, on the island z = C. x = A ccs8, y = B sin 8! are

comparisons

-86-

plotted versus azimuth 8! in Figs. 3.15.a,b,c and Figs. 3.16.a,b,c.

Due to the rather shcrrt and steep beach, the run-up dces not exceed 2 for

8 = m'or ~2 5 for 8 = 3z/2.I I

Again, plots of !A 8! ~ as a function of 8 are given in Figs.

3.li.a,b,c anc' Figs. 3.18.a,b,c, and in polar coordinates in Figs. 3.19.a,b,c

and Figs. 3.20,a,b,c,

Unfortunately, our preser t geometry does nc t resemble suf ficiently

any of tho.e studied by other investigators to justify quantitative

Page 88: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.14 Geometry an2 Finite-element Structure for an Elliptical

Island with a Circular Base. Major and N.nor axes: A = a, B = .5a;

"draft" of elliptic cylinder: H = .5a, radius of circular toe:

R = 1.5a; water depth: h = a.

Page 89: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

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Figure 3.19 a! Polar Plot of Differential Scattering Cross-section,IA�! ~, for an Elliptic Island with a Circular Base. WaveIncident from z ~ + ~. e = T!.I

a. ka=10

� 100�

Page 102: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.19 b! Polar Plot of Differential Scattering Cross-section,

~A 9! !, for sn Elliptic Island with a Circular Base. Wave2

Incident from x ~ + ~. 9 = vr! .

b.ka=20

101

Page 103: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.19 c! Polar Plot of Differential Scattering Cross-section,IA�! I, for an Elliptic Island with a Circular Base. Wave Incidentfrom x %+'-. � = 7I!.

Ic. ka=3

o

102-

Page 104: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

Figure 3.2G a! Polar Plot of Differential Scattering Cross � section,

~A�! , for an Eliiptic Island vith a Circu].ar Base. Wave Incidentfrom y ~ + ~. B = 3'/2! .

I

a. ka=l0

103�

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Figure 3.20 b! Polar Plot of Differential Scattering Cross-section,~A O!~ , for an Flliptic Island with a Circular Base. Wave Incidentfrom y s + ~. � = 3'/2! .

!b.ka=2

o

104

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Figure 3.20 c! Polar Plot of Differential Scattering Cross � section,

~A B! ~, for an Elliptic Island with a Circular Base. Nave Incidentfrom y '4 + ~. � = 3vr/2! .

Ic.ka=3

o

105

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4. ERROR ESTIMATES AND CONVERGENCE TESTS

4. 1 Th e ~0 tie a! Theo rem

Whi le a number of general relationships governing the scattering andradiat. ion of ocean structures Haskind, 19 57; Mehausen, 1971; Newman, 19 76!are availabl e, there is just one relationship that relates the far-field

1t is a statement of global conservation of wavescattered wave only.

energy «nd is well known in quantum physics as the Optical Theorem. Forscattering of. a plane wave by a two-dimensional body, it is the iamiliar

�, 1. 1!

For thr~ e-dimensional scattering, it can be stated, as

277 ! A�! f d0 = � � � � -- RetA� ! ]2TI cosh k h

�. 1. 2!

0

where h is the cons tan t depth at inf inity and A 0! is the angular variationof the far-f ie. ld s< at tered wave, as give n in Eqs. �. 3. 1! and �. 3. 2! ~

106

where R and T are respectively the reflection and transmission coeffi ients.

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The integral on the left-h;.nd � side is proportiona3 to what is common]y

known as the total scattering cross-sectior in physics, while th~ expression

on the. right of the equality is a measure of the wave energy in the forward

direction, i.e. the- direction of the incident wave. There are several

slightly different proofs cf Eq. �.3..2! all based or~ Green's theorem, a

rather direct. one using the method of statiorary phase is given in Appendix

We define a norma]ized error measure based on this theorem;

L.H.S. � R.H.S

OPT L.H ~ S ~

where L.H.S. and F.H.S. refer to the left and right � hand side respectively

of Eq. �.1.2! .

-6 -21'or all our numerical examples, E ranged from ~ 10 tc ~ 10

depending on wave-number, fini.te element grid fineness ard number oi serie,

terms used in the "nalytic expression in the exterior. As an example,

in Fig. 4.1, we plot EOPT over a range of k a for the particular case of0

the square dock in Section 3.3 under obliqueiy incident. waves � = 5%/4! .

While E > is useful as a global error estimate, its expression involves

only the propagating modes directly, and it is possible for one to make

some small error in an evanescent mode without causing an equal discrepancy

in E . Hence, it is also necessary and useful to study some localOPT '

quantities sr ch as integrated forces on a body or a local pressure to

estimate the z c teal error.

4. 2 Conver ence of the Finite Element A rozimation

Once a geometry is given, an appropriate choice of finite element grid

size and nlunber of series terms must be made. Iv general, elements should

107

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IO

Figure 4.1

Obliquely Incident Waves 9> = 5'll'/4!.

108

IO

8 6

IO

8 6

IO'86

EopT 4

IO

8 64

0-586

4

Relative Error in Optical Theorem for the Square Dock under

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-210 8 6

IQ

IFI io 0koa

Figure 4.2 Relative Error in the Magnitude of the HorizontalForce on a Uniform Cylinder.

110�

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IO 86

IO8

EIqI84

IO 8 6 Eigure 4. 3 Relative Error in the liagnitude of the Run � up on aUniform Cylinder at; + : 0 = 0; X.: I3 = -/2; o: 9 = Tt. � = Tt!

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k I ~ 2.9 A. < 2.2 L ! for an integrated quantityo 'e min ' e

ma xdimension L

emax

such as forces and k L < 1.9 X , % 3.3 L ! for a local quantity sucho e ' min ' e

as pressure or wave height.

In Tab3e 4.1, we tabuIate the results for a given k a = 1 for several0

diff erert grid systems note that p x q means a finite element structure of

p vertica3. layers of elements arid q radial elements per layer! . Here we

define relative error in the total scattering cross-section by

2Tt 27r

I<«! I' «!,��,0 0

S

0

k a = 1! even though for the present geometry, only the first verticaln

cosh k z + h!mode is present g x, y, z! = 4 x, y, o! 0

cosh k h0

using a sufficient. number of elements in the vertical dimension should not

The importance of

be overlooked for a general prob3.em,

112

The results are quite satisfactory even when the number of radial elements

are reduced by about haI f � x 10!, but a few percent error is introduced

when arly 1 layer of elements is used � x 18! . This is somewhat surprising

since. the average element dimension in the radial direction is about .8a for

� x 30! grid whi.ch is not much smaller than a vertic" 1 dimension of a for

the � x 18! grid. The implication is that the vertical length scale of the

velocity is smaller or at best equal to the horizontal scale for this c.ase

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O

JIQl

Q

n Q

O, O

Y QQ'Q QQ Q C

I I I IC C I C C

Xr rvO a

I

C C; C

j

4IrL IIII

QlJ Q

'J

8 4 Q

Q

! rlc CQ Q

I

r

v:JI

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4.3 of Series Terms in t esentation

� !

!o

and

where ! refers to a quantity computed with the original choice of numberD

of series terms and ! refers to that computed with the present set.P

114

For numerical computations, it is necessary to truncate the doubly

infinite series in Eq. �. 3.4! to the finite sum in Eq. �.3.2! . Unlike

many other "hybrid" finite element methods, the present formulation reducesmatching conditions to ratural boundary conditions and an arbitrary numberof vertical eigenfunctions M! as well as the angular eigenfunctions N !for each vertical mode is permitted. The number of such terms that is

required for a giver. accuracy depends on the incident wavelength, the: bodyand bottom geometry and depth, and more particularly on the size of theartificial boundary S and the finite element grid structure inside S. To

provide some guidelines on the number of series terms that is necessary fora given problem, we solve the particular example of the square barge inSection 3.3 length 2a, draft .5a, depth a! under a normally incident

9 = 'tt! wave. of unit. waienimLber k a = 1! for severaI sets of series terms.I 0

We start from the origin 1 choice M = 4 N =- 12 N = N = N = N = 10,o ' 1 2 3 4

total 99 terms! and reduce, first M,keeping N fixed, then reduce N. keepingi

8 fixed. The results cf a set of such convergence tests are presented in

Table 4.2. Note that

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~ ItlIII

~ IIt4tt

C

It40CI

QIltI0

C

IIII

II:I

U:II

ll.'I4I IICI tJ 115

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While the present results are not unrelated to the incident wave, the

particular geometry and element grid, they do indicate that the original

set using 99 series terms is extremely conservative with error in both local

and integrated quantities less than % lQ ' . Et is quite amazing to ncte for5

example that the solution converged to within much less than IX even when

only a total of l2 ternis Y. = 1, N = 4, N = 3! are used.

This confirms the earlier observation that it is almost always

numerically advantageous to use a smaller S, and hence reduce the number

of fini te element nodes, even at the cost of having to include more

unkncwns in the exterior series representation, since the total number of

series terms required is much smaller than the semi-bandwidth of the finite

element. grid for most problems.

It should be noted that a complete study of ccnvergence of the series

terms would recuire a systematic set of tests similar to the one performed,

for a range of incident. wave lengths, depth., ance body dimensions or

geometry!, as well as the size of S and of the fini.te elements. While

such extensive tests are beyond the scope of this work, this and especially

the last section on convergence of the fivite elements should already

provide the basic guidelines to the choice of number of series terms, and

the element size to use for a given problem.

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5. COMPUTATIONAL ASPECTS AND THE PROGRAM USAGE

S.i ~Pro ram T~mlementatt'o~n, S ate and Time Se nirementa

The program is impEemented on an IBM 370/168 computer running in an

OS/MVT environment at the M.I. T. Information Processing Center IPC!-

The program is written in IBM FORTRAN Gl language extensive testing also

done using a WATFIV compi]er! . Three routines BJS, BYS, BKS! to calculate

Bessel and related functions are called from the IBM SL � MATH library and an

assembly language program to set consecutive array elements to zero, ERASE

not WATFIV compatible! supported and provided for unrestricted use by M.I.T.

IPC! is also called.

The FORTRAN program including comment statements! is 1308 cards long

and requires about 78 K � K = 1024 s torage locations bytes! of 8 bits each!

ob jec t code.

The total primary core storage requirements object code and arrays!

and the CPU processing time required for each of the examples in Chapter 3

and 4 are summari zed in Tab E e 5. l. Single precision accuracy � bytes,

equivalent to approximately 7.2 decimal digits accuracy! is adopted through-

out, and no s< condary stv>rage is used. The timing, which includes mesh

generation, but not input/output operations or program compilation, is

calculated from cperating system cost summaries and not from dirert timing

routines. It is hence onEy an approximate estimate, and in light of the

findings in Chapter 4, likely to bc on the conservative side since an

extravagent numher of coefficients and con..ervative finite element grids

are always used.

EE7

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N

W 0

'Cd0 0

O 0Vl

ll8

4JCd0 4J

OJdd

Cd

C 0 F-CdCJ

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5.2 The Hybrid Fini te Element P~ro ram

5 . 2 . 1 P r a~ram L otic

Thr total program consists of a main program and l9 subroutines.

The program is general with regard to geomet ry, < ttoice of grids or

co« fficients, or in ident wave period or angle, with the exception of

the fol lowing routines

l! MAIN, the main program which must be slightly modified to reflect

corre~ t array sizes;

�! 1NPU'1', the input p~ogram whi «h mus t be changed for any changes in

geometry or grid st.rue ture;

:3! FORC'E, this routine to calculate force and moment. coef ficients

is dependent on the partic.ular body geometry; and

minor modi fications for oth~ r geometries are veryexamp1es Lr~ ated.

advisablu.

The overal1 program logic is best illustrated in Fig. 5.1 hy the

flow of < on tro1 among the subprograms

119

�! HESHO, HESH1. While t hese mesh generation routines are quite genera1,

they are particularly suitable for cylindrical geometric as in the

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initialize constants and I/O device numbers

set up and print geometrical and grid

data

loop over each frequency point

input and print physical data

assemble [K�] corresponding to the volume integral

assemble into [K ] the matrix [K ] resulting from

the integral over the free surface

calculate all required eigenvalues k

output summary of eigenvalues etc.

calculate all required noramlized Eankel and re-

lated functions

output summary of Hankel functions, normalization

factors, etc.

120�

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assemble matrices / vectors due to

artificial boundary S

static condensation to eliminate coefficient

unknowns {p!

Gauss elimination to reduce equation matrix to

upper triangular

back substitution for solution {g!

print dimensionless nodal potential solution

substitute to recover

unnormalize and output all dimensionless

coefficient solutions

<,i I «<1;< I. e and output upi i < «] th«:.>r<. z results

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calculate and output farce

coefficients

end of loop for one wavelength

Figure 5.1 Flow chart for the progra~

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The following is a description of each of the programs used:

5. 2. 1, 1 MAIN

This main program is responsible for the transfer of control indi-

cated in Fig. 5.1. It also initiates universal constants,

input/output device numbers and performs basic conversions, input and

output, and error calculation Optical Thenrem! .

5.2.1.2 INPUT

This subroutine sets up and prints all geometric and grid data.

It must be rmxitten for any ohanqe in geo+et>~~. For all the

examples studied, no data is input and the grids are generated via

calls to mesh generation programs: MESHO and MESHl.

5. 2. l. 3 MESHO, MESHl

These routines generate vertical cylindrical rings of 20-node

hexahedral elements: the node coordinates and the element node connecti-

vities. The number of vertical, angular and radial layers of elements

and the positions of the nodal points, as well as the initial values for

node and element numbering can be arbitrarily specified. Node numbering

is radially, from top to bottom and sprials out. By using a suitable

set of calling parameters, the resulting bandwidth of the system can be

minimized. This is achieved in all the examples treated,

The element node cones <-t ivi ties aze set-up in the uzdez assumed

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5.2.1.4 ETGVAL

This routine obtains the specified number of eigenvalues k of

Eq. �. 3! by using fixed-point iteration.

5. 2. 1. 5 HANKEL

The required functions J k r !, H k r !, K k r !, H k r !�! �! 'noS'no S�'nmS'noS

and K' k r ! are calculated and normalized in the following manner:n m S

�!

"' !!H x!

n normalized P n,x! H x!!

where

2 2 -1/4 2 2 r x � n ! for x � n > vx

-1/3x

2 n

for Ex � n~~vx2 2

2 -1/4 2 2 1/2 -1 nx ! exp I- n � x ! + n cosh � ! j2 2 7

for n � x > vx

P n,x!

124

in all the other subprograms.

For grid geometries that can be described in cylindrical coordinates

by constant coordinate surfaces, MESHO and MESH1 can be used directly to

generate the complete grid system. When the geometry deviates only

slightly from the former shapes, only those particular regions need to

be input see Pig. 5.2!. For a problem where grid systems of other

analytic such as rectangular, elliptic, etc.! shapes are more efficient,

NESHO, MESHl must be modified. The general program structure, however,

is still valid and required changes can be quite minor.

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Figure 5.2 An example using both automatic generation in region V! and

direct input in region V! to set up finite element grids

125

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andK x}

K x!!n normal ized Q n, x!

where

Q n x! = n + x ! e2 2-1/4 � ~n+x n+v'n +x n

It follows from recursive formulae that

H ! � � 8 !n-l x n l!

n normal iz ed P n x!

and

� K x! � � K x! l! n l!

<' x! !n-I x

n normal ized Q n, x!

J k r ! occurs only in the right-hand side vector and is notn o S

normalized.

The above normalization is clearly not unique but is adequate to

make the special functions involved to be of order unity for the

126�

entire range of m and n. These normalized values are used throughout the

programs. Although the final system of linear equations is well condi-

tioned, this normalization may be important especially when round-off

accumulation may be significant.

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5.2.1. 6 ASKHKV and ELKV

ASkZKV assembles the global matrix [K ] from the element matricesV

e[K ] which are calculated in ELKV using numerical integration. The

mathematical procedure is as described in Section 2.1, which involves

the isoparametric mapping �.1.5!.

5,2.l.7 FMAP2

This mapping routine is called by four other programs SURFAS, CROSSK,

PLOAD and FORCE. It performs the numerical isoparametric mapping from

one face of the parent element in local coordinates onto a face of the

daughter element on the free surface F or the cylindrical surface S in

global x,y,z coordinates. The Jacobian and/or the corresponding global

coordinate values are returned as required for numerical quadrature

see Eqs. �. l. 5!, �. 2.4!, �. 3.2 . 0!! .

5.2.1.8 SURFAS

The matrix [K ] due to the integral over the free surface, I , is

assembled directly into [K ] from each element on the free surface.

Numerical integration i.s performed in local coordinates and mapped into

global coordinate space see Section 2.2!.

5.2.l.9 CROSSK, PLOAD, CLOAD, DIAGK

These four routines construct respectively [K ], {Q ]', [Q ], IK ],C P C D

which are the vectors and matrices resulti.ng from integrals on the

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artificial boundary S I -I !. CROSSK and PLOAD require quadrature and

mapping, while Q ! and fK ! are evaluated directly. The procedures

follow quite directly from Section 2.3.

The load vectors Q !, fQ ! are nondimensionalized by the factorC

iga � !, i.e.,

d

{Q !

P dimensionless iga - � ! 0

and

Q ! Q !> .C dimensionless iga

- �!

The resulting nodal potential and series coefficient solutions are hence

also dimensionless:

!! f4!dimensionless iga

- . '!

and

~.!! .dimensionless iga - � '!

128

5. 2. l. 10 DENSE and COEFF

For computational efficiency in Gauss elimination, DENSE performs

static condensation to eliminate the unknowns p! see Eq. �. 5. 9. a!!. No

matrix inversion is required here as [K j is diagonal. Subroutine COEFF

recovers p! by substitution Eq. �.5.8!! of the solution $!.

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5.2.1.11 SDLV and BKSUBT

We have used two general routines to perform Gauss elimination and

back � substitution respectively for a system of linear algebraic complex

equations. The coefficient matrix of the equations is syrrmretric and

banded, and is therefore stored in a symmetric packed form.

No pivoting partial or complete! is performed or is necessary!.

5. 2. 1. 12 FORCE

This program calculates force and moment coeffi-

cients Eqs. �.1,1 .a,b! from $!. Numerical integration is perFormed

using Eq. �.4.2! .

5.2.1.13 OUTPUT

This routine outputs the real and imaginary parts and the magnitude

and phase of each entry of a complex vector.

5.2.2 ln ut. Re uirements

In addition to the geometric input and the finite element grid as

described in Section 5.2.1.2, we must prescribe physical data regarding

the incident wave, the number of eigenvalues and of the coefficient un-

knowns for each frequency point to be solved. These are described here.

We remark that if the boundary geometry is compLicated so that additional

input cards are needed, they must preceed those described in the present

section. The following input is required:

]29

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CARD NO. DESCRIPTION OF INPUT FORMAT

I51 NCASE: a right-justified integer giving the

number of frequency points to be analyzed in

the current run

I5, 2Fl5. 7NWK, THETAI, WAVET

NWK = right-justified integer representing no.

of eigenvalues k , kl, ... ~ 1 to be used for

first frequency point

THETAI = floating-point number for the wave

incident angle in radians

WAVZT = floating-point no. for the normalized wave

aperiod: WAVET T/v � where T is the wave periodg

20I40' 1' '''' NWK-1

if NWK ! 20, continue on as many cards as necessary!

right � justified integer indicating number of coeffi-

cient terms chosen for eigenvalues k0, k , ..., k.

e.g., if N0 = 3, then cos 00, cos 6, sin 0, cos 20, sin 26,

are included.

Repeat cards 2 and 3 or more cards! for as many points as indiciated in

card 1.

5.2.3

The INPUT program prints a summary of geometry and physical data:

a! far-field depth, radius of cylindrical surface S and other

geometrical data,

130

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b! total number of nodes and elements,

c! elements on S, on F, on B, etc.,

d! a node-coordinate. table,

e! an element.-connectivity table,

f! information on serai-bandwidth of resulting matrix.

For each frequency, the following results are printed:

a! wave period and angle of incidence,

b! table of the eigenvalues and the number of coefficients chosen for

each,

c! table of Bessel functions required,

d! table of normalized modified! Hankel functions and their derivative

and the normalization factors P and Q,

e! complete list of all dimensionless nodal potentials solutions

I0!! .dimensionless'

f! complete list of unnormalized dimensionless coefficient solutions,

g! optical theorem results,

h! force coefCIcients.

5.2.4 Illustrative Errample: program Listing, Sample Input and Gutput

The program listing together with sample input and output are pre-

sented here for the case of the semi � immersed cylinder using

eigenvalues as described in Section 3.2.

5.2.4.1 Variable name table

The following is a list of variable names used in the main program,

these names are preserved as far as possible in the subroutines. The

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Pro ram Variables Ex lanation/Notation in Text

Variables in common:

total number of elements/ M

total number of nodes/ N

number of elements on F

INTEGER*4NELE

NNOD

NB number of elements on S

number of nodes on S/N

number of equations in final

condensed system, equals NNOD

semi-bandwidth of final condensed

NODR

NEQT

system

N + l0

NTOTL

total number of coefficients used/ NT

number of eigenvalues chosen in

g'/M + ls

number of coefficient terms for

NTOTL2

M = 0/NMAX*2 � I

number of elements on circumference

of cylinder of radius a

132

radius of the cylinder a and the incident amplitude a are used as normali�0

zation factors throughout. They are assumed to be unity and do not appear

in the programs.

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INTERGER*4NFZ

REAL*4

WAVFT

HH

important variables not in comm<>n ,'

NCASE

REA1,*4PAT

OPR

OP I.

COHPLEX*SCJ

Arrays

NFI.F.FX NFX!

NELEFZ NF Z!

WK NWK!

WK H NWK!

REAL*4

133�

I READ

IWRITE

THETAI

N .ON 3, NE LE!

NEl,ES NS!

NELEH NB!

NGOF NWK!

number of elements on bottom of.

cylinder of radius a

card input device number

printer output device number

wave incidence angle/01

Tnormalized wave period/-

/a/gradius of S/r

as ymp to t i c dep th /h

depth of immersion/H

number of frequency po i»ts t > solve INTEt,'ER~4

right hand side of Optical Theorem

lef t hand s ide of Optical Theorem

matrix of clem«nt node connectivities INTEGER+4

vector of numbt'rs ol elements on F

vector of numbers of elements on S

vector of number of coe f ficients chosen

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vector of numbers of «1 ements on

cyl inder circumference

vector of number of «1ement s on

cy] inder base

vector ~>f eigenvalues km

vector of k h valuesm

Page 135: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

REAL*4

FNORN NWK, NMAX! matrix of normalization factors for

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5.2.4. 2 Sample Input

For one wavelength solved, only three cards are used:

CARD COLUMN 12345678901234567890123456789012345678901234567890...

Card 1:

5 7.8839178

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workspace required by routine CROSSK

workspace required by routine HANKEL

matrix of node coordinates

matrix of H k r !! . and�!n o S normalized

n m S normalized

matrix of H k rS l' d and�! 'n o S normalized

Page 136: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

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!. 162�!ct!9n,284!Sec ~ Cl'. 1!c2 LG Et qri!. 171715L+n}

294 9! ~ Ocr. 186 lr8!AQAr.5tcqce}t04t, 4556!tC!' . Jeqe 3GEtt! 4',27S665}+06i.71teaert�!.211!!9!et!5'l. I"8353}t04t, 288 369Et'! 4. 1!F I 67!t 0 I

n. 1197Ccit�'!. ll"844!+0!! 65r469EAOJ". 1�2CVAr02

- I, 379! 382.423!ql

-3. 'J�4 I'42 427c02I 57 c612

! I 97 16l. 136''

-!,461777I, 66 986 r

-1. 8407�-i1, 27 8}CQ- I, 75 9579- I. 'I 4 5'! 4 6-". 5F 5149-,. Btt 5'l4 'In. 47qS 2

-l. '! 521 -2. 91 45 'tv

P,!AG ' VA Lu

- 'I, Il;2 �, t 7!9 3'}

� I .. I 5'tet-!, I 'tqn

I � '97"- I !ill F.

i ZG!ert-2.27S'lS'-! c 416 I, 45 Ill�!l

- Z. !i 2874I, 9244 �I, q!ul2. '21 e2, 'll '�471. 140! cq2. �3 42

-l. ' 114

tHA! t VALQEu!91!c

1.7616" I- 2. '15 76 97n. 146!912. 026777

- 'I. 56 Eu 262, 1 131'I

-3. 14113JZ. 26 !$65l. 5771ltt2. 'J�44 rt. �'1791, 2> 23'

-I, u'1121!,634664

-'!. BFSBAB-0.72fi642-1. 22 .1675 382844

~ 92 ~ ~

Page 197: A HYBRID ELEMENT METHOD FOR CALCULATING hybrid element method for calculating three - dimensional water wave scattering by dick k. p. yue h. s. chen and chiang c. mei report no. mitsg

6 ~ 5 5 ~ C 1940646vrCC8 6 5 = r 744898686II~~ 6 ~ PPTEC66 TBBO!er r ~

~ ~ ~ rr ~ ~ rr ~ ~ r ~ ~ r ~ ~ ~ r rr

~ ~ ~ ~ ~ r

6B5. 96187'I . 1 7 6 'I '9 8 F 6 6 I

PPA 6 Pier Ehec. 1'667

--.2812786 CC Q.7841711 ~ IV-CCBVCree.

'Pet PBPT 1865. ~ ~ 91

"..9�7647rCi -C,562266trtr2-ccepl'6567

196�

r ~ ~ 'p C 6 'r 5 ~ ~ ~~ ~ ~ r ~ 92 ~ ~ ~ r

Eee. V6687

r. 1165191rel

P'I!Se Vtl ~ r

76 166 1

VII'. r VBEU-'-. 92 '729

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6. CONCLE DING REMAMS

6.1 ~Su eet.ed

In general, especially when larger or more complex geometries, or

incident waves of higher frequency are to be studied, the following modifica-

tions and extensions to our present program are recomn~ended and may be

necessary!:

A. Second'ary storage: The large core requirements we experienced strongly

suggest the use of additional storage on secondary devices, which

enables many core � aving alternatives:

l! Data storage: When many different wavelengths or modes are to be

analyzed using the same grid structure, a large portion of th»

data such as geometrical information, the global matrix [K ] due

tc the volume integral! can be saved in secondary storage and

need be initiated arly once.

2! Program overlay: The present flow of control among the programs

require only a few programs to be in core at one time while all.

the oth»r subroutines may be stored externally. Such a scheme

results in only a small primary storage saving, however, as the

total object code is only ~ 80K long.

3! Partitioning: This is the most. useful and often necessary extention

to a program using Gauss elimination for solution. tdthi le partition-

ing a Gauss elimination solver is straight forward, efficient

assembling of the equation matrices in partitioned form require

special m-nipulations, Experience with ,imilar procedures in

other problems such as finite element methods in structural

197

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mechanics!, however, can be adapted quite directly.

B. Conversion to double precision: In this type of problem, a sizeable

number of complex unknowns together with a large bandwidth are in-

volved. The great number of operations required to solve such a

system necessitates a careful consideration of possible round-off

accumulation. Our experience in this and other problems indicates

that in some of the examples presented � 500 unknowns, - 200 band-

width, � 10 complex operations!, we are already close to a regime7

where the accuracy af single precision becomes inadequate. By con-

verting to double-precision, round-off accumulation error is essenti-

ally eliminated, While in principle, the conversion is trivial in our

present program, the increased storage need makes other modificationsinvolving use of secondary storage such as partitioning! necessary.

The increase in computational time associated with double precision

is, however, not significant.

C. Substructuring: By treating the overall finite element domain as an

assembly of several smaller regions, the overall problem can be

analyzed separately in terms of each of these substructures. Such amodification is particularly effective when the geometry or expected

physical behavior suggests ready subdivisions of the element domain.In an attempt to extend the original Atlantic Generating Station two-

dimensional finite-element problem Chen and Mei, 1974! to higher

frequencies, a new program using secondary storage capabilities which

involved sub � structuring see Fig. 6.1! but not partitioning! was

198�

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Figure 6.1 Substructuring the AGS into four regions

199

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previously developed by the authors but was not completed for lack

of time. Our experience indicates that it is always important to

include all the elements on the artificial boundary in one substructure

to avoid unnecessary increase in bandwidth.

D. Use of symmetry: When the geometry under consideration possesses one

ar more vertical planes of symmetry, the program can be modified so

that only ore symmetric segment need be studied. The procedure is

illustrated schematically in Fig. 6.2 for the. case with - single

plane of symmetry. WhiIe two separate one symmetric, one antisymmetric!

problems must now be solved, each of them only involves half the

original number of nodal unknowns and bandwidth. The new computational

time required would only be one-fourth and the storage one half of

the non-symmetric problem. Again, a two-dimensional finite element

program using symmetry was previously developed and tested. The

treatment of the symmetric half! problem with a semi-circular outer

boundary and Neumann boundary conditions on the plane of symmetry is

similar to Chen and Mei for their rectangular and semi-circular

harbors. For the antisymmetric half! problem, the Dirichlet boundary

condi tions on the symmetry plane are imposed as essential boundary

conditions lo-dings! af ter the global matrix is assembled.

200

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201

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6.2 Reeommendatfons and Cost Estimates for the At1an tie Cene~ratinStation Problem

For the specific case of the offshore harbor problem for the Atlantic

Generating Station the following extensions to the present program are

recommended:

l! Change all storage variables from single- to double-precision

accuracy;

2! full partitioning in assemb1age and Gauss elimination solution by

using secondary storage; and

3! if an assumption of symmetry is reasonably valid i.e., if the ocean

bottom contours run approximately parallel to the length of the

generating station!, modifications be made to utilize symmetry to

solve only a symmetric! half of the problem.

Assuming a mean radius, a, of the circular breakwater of about 900

feet and a mean storm far-field depth of m 70 feet, we estimate, using

our earlier criterion Section 4.2!, that a total of w 6500 nodes with a

bandwidth of .f 550 is required for a w 8.5 second or higher ka 17!

incident wave. About 6 hours of computational time would be required for

each wavelength or mode!, and if symmetry is present, about 1. 8 hours

per point. The above estimate is arrived at hy a direct extrapolation

of the timing of the present examples which use no secondary storage.

Clearly, an appreciable cost associated with the necessary input/output

operations between primary and secondary devices, which is very much

machine dependent, must also be added.

Finally, the problem of radiation of waves due to forced osci]lations

of one or two platforms in the AGS harbor can be treated by the wave force

202

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program with very little modification required. This is due to the math-

ematical similarities in the two problems. Since there are six degrees

of freedom for each platform, the number of added mass and damping coeffi-

cients involved is of course large and the total computer time needed

depends on the thoroughness of the investigation intended.

6.3 Conclusions

The hybrid element method presented in this work is found to be a

powerful one for three � dimensional water wave scattering problems. While

onIy relatively simpIe geometries have been studied, it is clear that by

using secondary storage and increasing precision of machine representation

to reduce round-off accumulation error, the extension to treating more

complicated structures incI uding depth variations is straightforward� . The

method is hence a practical one For engineering applications.

In our numerical experiments using simple geometries, the hybrid

cI pment method is aire ady quite competitive with known Green function

schemes. I',xpcrience indicates that our computing cost is essent ially

governed by the number of f inite element nodes. For finite element

method, the number of nodes increases with the fluid volume, whi! e for

Green function method the numher of area elements required increases with

the total wetted solid surface. Thus as the number and complexi ty of sol id

components increase in a given fluid region, wc expect the hybrid element

method to be less costly while the opposite is true for Green function

method In light of this, and of the virtual lack of any tedious analytic

preparation required, the hybrid elemene method, is, in our opinion a

203�

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powerful alternative to the Green function method for three � dimensional

water wave problems,

Me emphasize once more that the problem of radiation by an oscillating

body is mathematically very similar to the scattering problem we have

t'reated. The linearized boundary value problem for a radiation problem is

obtained by setting the incident potential f to be zero and replacing

by $ = g radiated potential! in Fqs. 1 .2 .1! � � .2 .4! . and requirings r

further that

= Vd f!Bn n

on the oscillating body S where V is prescribed for a given oscillatingn

mode. The proper functional is now Bai and Yeung, 1974!:

2J f!! = � 'Vg! dV � � $ dF � V <f!dS

2 2g nS

+ � � f!! ~ dSyl g I2 Br

S

The modification of the present program to solve the radiation problem can

be trivially achieved; the forcing comes now from the integral over S and

not S, The total coefficient stiffness! matrix is completely unchanged,

while the forcing integrals 1 , I see Fq. �.0!! are no longer present5

and all the forcing terms come from the integral

V PdS

which can be directly evaluated by quadrature after V is given or computed.

204

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LIST OF REFERENCES

Abramowitz, M. and Stegun, I. A. ed!, Handbook of MathematicalFunctions with Formulas, Graphs, and Mathematical Tables,Applied Math. series 55, National Bureau of Standards, U, S. Dept.of Commerce, June 1964, p, 895,

2. Bai, K. 3. and Yeung, R., "Numerical. Solution of Free-Surface PlowProblems", Proceedi~ns of the Tenth S mposium on Naval H~drod~namics,June 1974 in press! .

3. Black, 3, L. s "Wave Forces on Vertical Axisymmetric Bodies", Journa Iof Fluid Mechanics, Vol. 67, Part 2, 1975, pp. 369-376.

Black, J. L., Mei, C. C. and Bray, M. C. C., "Radiation and Scatteringof Water Waves by Rigid Bodies", Journal of Fluid Mechanics,Vol. 46, Part 1, 1971, pp. 15 I � 164.

4.

5. Chen, H. S. and Mei, C. C,, "Osci11ations and Wave Forces in a Man-

Naval ~g~drod nsmi<:e, June 1974 fn press! .

also

Chen, H. S. and Mei, C. C., "Oscillations and Wave Forces in an OffshoreHarbor", Technical. Report No. 190, Ralph M. Parsons Laboratory for'Water Resources and Hydrodynamics, Dept. of Civi] Engineering, M. l. T.,Cambridge, Mass. August 1974.

6. Garrett, C. 3. R., "Wave Forces on a Circular Dock", Journal of FluidMechanics, Vol. 46, Part 1, 1971, pp. 129-]39.

7. Garrison, C. J., "Hydrodynamics of Large Objects in the Sea; Part IHydrodynamic Analysis", Journal of H~dronautics, Vol. 8, No. 1,January I9 74, pp. 5 � 12.

Gottfried, K., Ijuantum I eel.anics, Volume. I: Fundamentals, W. A,Benjamin, Inc.. New York, kyar<*k 1966, pp. 106-109.

Haskind, M.Ds 9 "The Exci ting Forces and Wet t ing of Ships in Waves", in Russ ian! I zv. Akad. Nauk SSSR, Otd. Tekh. Nauk, 'l 957, Vol. 7,pp. 66-79. English trsnslatinn: David T~alryr Nodal Basin Translation,NA. 307.

10. Jonsson, I. G., Skovgaard, 0. and Brink � Kjaer, Oe 9 "Diffraction andRefraction Calculations for Waves Attacking an Island", submitted tn3ournal of Marine Research, Manuscript--November 1975.

11. Kopal, Z., Nume< ica~lAna 1 ais, Second Fdi t i m, .Iohn Wilef & Sons, InNew Vork, ]961, p. 562.

205

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12. Lautenbacher, C. Ca a "Gravity Wave Refraction by Islands", Journalof Fluid Mechanics, Vol. 41, Part 3, 1970, pp. 655-672.

13. Longuet-Higgins, M. S., "On the Trapping of Wave Energy Round Islands",Journal of Fluid Mechanics, Vol. 29, Part 4, 1967, pp. 781-821.

14. MacCamy, R. C. and Fuchs, R. Aam "Wave Forces on Piles: a DiffractionTheory", Institute of Engineering Research, Waves Investigation I.ab.,Series 3, Issue 334, Berkeley, California, February 1952.

15. Newman, J. N., "The Interaction of Stationary Vessels with RegularWaves", proceed~in s oy the Eleventhde osinm on Naval Hyd~rod namics,April 1976 in press! .

16. Smith, R. and Sprinks, T., "Scattering of Surface Waves by a ConicalIsland", Journal of Fluid Mechanics, Vol. 72, Part 2, 1975, pp.37 3- 384.

I /. Vastano, A. C. and Reid, R. 0., "A Numerical Study of the TsunamiResponse at an Island", Dept. of Oceanography, Texas A 6 M University,A 6 M Project 471, Ref . 66-26T, 1966.

18. Vastano, A. C. and Reid, R. Oa a "Tsunami Response for Islands:Verification of a Numerical Procedure", Journal of Marine Research,Vol. 25, Part 2, 1967, pp. 129-139.

19. Wchausen, J. V., "The Motion of Floating Bodies", Annual Review ofFluid Mechanics, Vol. 3, 1971, pp. 237-268.

20. Zienkiewicz, 0. C., et al., "Iso-Parameter and Associated ElementFamilies for Two � and Three � Dimensional Analysis", Finite Elements

Press, Trondbeim, Norway, 1969, pp. 383-432.

206

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LIST OF FIGURES ~Pa e131.1 The Boundary Val.ue Problem

1.2 Introduction cf the Artificial Circular CylindricalBoundary, S

232.1 The 20-node Hexahedral Element with Exterior Nodes Only in I.ocal Coordinates!

232.2 The Daughter Arbitrarily Oriented, Quadratic � sidedHexahedral Element in Global Coordinates AfterIsoparametric Mapping

403.1 Wave Incident on a Bottom-seated Circular Cylinder ofRadius a � = Tr; h = a!

423.2 Finite Element Structure for a Uniform Cylinder:36 Elements, 342 Nodal Points.

473.3 Wave incident on a Floating Cylindrical Dock of Radiusa �1 = vr; h = .75a!

3.4 Finite Element Structure for a Floating Dock:56 Elements, 435 Nodal Points

3.5 a! Horizontal Force. Coefficiert for a Semi-immersedh HCylinder � = .75; � = .5; 6 = m!.a'a'' I

+, ampli tude; x, phase: f inite element solutionampli.tude; � ---, phase: Garrett, 1971 results

51 b! Vertical Force Coef ficient for a Semi-immersedCyli nder See Description in Figure 3. 5 a! !

3. 6 Forces on a square dock: + magnitude;x � phase. Normal incider.ce: 8 = 7T.'7 I

a! Horizonta] Force Coef f icier.t

b! Vertical Force Coefficient

c! Momert Coefficient About y � axis

207�

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~Pa e

Forces on a Square Dock: +, magni tude� x �, phase. Oblique Incidence: 8 = 5'/4.

3.7

a! Horizontal Force Coefficierit

b! Vertical Force Coefficient

c! Moment Coefficient About Horizontal Axis

57

Run-up, q/a, on the Sides of a Square. Barge: +! �: nodal! nagninnda; a! � �: noda! ! phase.Normal incidence: 0I

3.8

s! k�a = l

b! ka= 2

c! k a = 30

d! ka=4

60

61

62

63

3.9

a! ka=l0

b! ka=20

c! k a = 3o

d! ka=4

66

Differentia1 Scattering Cross-section, ~A e!~z, for aSquare Barge. Normal Incidence: 0 = Tr.

3. 1.0

a! k a 1

b! ka=2

c! k a = 3

d! k a = 40

68

69

70

71

� 208�

Run-up, q/ao, on the Sides o f a Square Barge: +! : nodal magnitude; x! -- � : nodal! phase.Oblique Incidence: 9 = 5T/4.I

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~Fi ure ~Pa e

Differential Scattering Cross-section,~ 4 �!~ , fora Square Barge. Oblique Incidence: 6 = 5v/4.

3. 11

72 a! k a = 1

0

b! ka=2

c! k" = 30

d! ka=-40

I 3

74

75

Polar Plot of Differential Scattering Cross � section,~A 8! ~, fcr a Square Barge. Normal Incidence: 8

3 12

76 -! ka= 1

b! ka=2o

c! ka= 30

77

d!. i! k a = 40

7r 3m 5' 77r d! . ii! k a = 4. Details for � < 8 < � and � < 8

0 4

79

80

3.13

81 a! k a = 1

0

82 b! k a = 2

0

c! k a = 3C

d! k a = 40

84

Geometry and Finite � element Structure for an EllipticalIsland with a Circular Base. major and Minor axes:A = a, B = .5a; "draft" of elliptic cylinder: H = .5a,radius of circu]ar toe: R = 1.5a; water depth: h = a

3.14

Run � up, rr/a, on an Elliptic Island with a Circular Base: +!: ncdal! magni tude; x! � � �: nodal! phase.Wave Incident from x ~ + ~. 9 = rr! .

I

3.15

a! k a = 1

b! ka.= 20

c! k a = 3

89

90

209

Polar Plot of Differential Scattering Cross � section,~A e!~ , for a Square Barge. Ob1ique Incidence: 9 = 5l/4.

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~Fi ute~Pa e

a! k a = 10

91

b' ka=20

c'; ka=30

92

93

3.17 Dif ferential Scattering Cross-section, ~A Q! ~, for anElliptic Island with a Circular Base.Wave Incident from x ~ + ~. � = v! .I

a! ka = 10

b! ka= 2o

c' k a = 30

94

95

3. 18 Differential Scattering Cross-section, ~ A�! !, for anE]liptic Island with a Circular Base,Wave Incident f rom y ~ + ~. � = 3' j 2! .I

a! k a = 10

b! ka=2

c} k a = 30

97

99

3.]9 Polar Plot of Differential Sc-ttering Cross-section, A�!I , for an Elliptic Island with a Circular Base.Wave Incident from x ~ + ~. � = m!.

]00 a! k a = 1

n1.01

b; ka= 2

102 c~ k a 3

0

3. ZG Polar Plot of Differential Scattering Cross-sec ion,~A�! ! ~, for an Elliptic Island with a Circular Base.Wave Incident from y ~ + ~. � = 3>/2! ~I

103 a! k a = 1

0

b! ka=20

c!ka=3

104

].05

2 io

3.16 Run � up, q/a, on an Elliptic Island with a Circular Base: +I �: nodal! magnitude; tx! � --: nodal! phase.Wave Incident from y ~ + gd. � = 3m/2! .

I

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~Pa e

4.1 Relative Error in Optical Theore~ for the Square Dockunder Obliquely Incident Vaves e = 5~/4!.

I

108

110Relative Error in the Yagnitude c f tb e Hc rizontal Forceon a Uni form Cylinder.

4.2

4.3

120 � 1225.1 Flow Chart for the Program

An Example Using Both Automatic Generation in Region V!and Direct Input in Region V! to set up Finite ElementGrids.

1255.2

1996.1 SuLstructuring the AGS into Four Regions

Spli tting a problem with one vertical! Plane o f Symmetry 201into Two Half Problems.

6.2

211

Relative Error in the Magni.tude of the Run-up on a Uniform 111Cylinder at: + : 9 = 0; X: 9 = 7r/2; o: 8 = v. 8 = vr!.

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LIST OF TABLES

TABLE~Pa e

443.1 Comparison Between Analytic and Finite Element Results

for Sc"ttering Off a Uniform Cylinder e = >!:IHorizontal Force Coefficient. CF and Free Surface

Elevation q/a at r = a.0

45Comparison Betweer Analytic and Finite Element Resu1tsfor Scattering off a Uniform Cylinder e = v!:Coefficien.ts u, 5 of the Exterior Solution. k a = l.

mn' mn 0

3.2

49Computed Horizontal and Vertical Force Coef ficients fora Semi-imziersed Circular Cylinder

H � = .75, � = .5!a ' ' a

3.3

113Comparison of Results for Diffraction ly a UniformCylinder, k a = 1, e = tr, Using a Variety of Finite Element

0Grid Structures: pxq p Rings Stacked Vertically of qFinite Elements Each.

4.1

115

1185.1 Summary o f Computational Requirements.

212

4.2 Comparison c f Results for Diffraction by a Square Barge,k a = 1, e = m, Using a Range. of Numbers of CoefficientTermse

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LIST OP SYMBOLS

Typical horizontal dimension cf a body radius fora cylinder, half-length for a square!

Tncident wave amplitude

A�! Angular variation of the far � field scattered potential

All solid surfaces including bottom

Complex cor.jugale of preceding termC.C.

Relative error in the total horizontal force ona uniform cylinder

Relative error defined by the. Optical Theorem

Relative error in the total scattering cross-sectionE

Relative error in the run-up on the body at 6

Total free surfaceFinite element region free surface inside S

Total force vector

Component of total force in i-direction

Acceleration due to gravi.ty

Constant far-field! water depth

Typical vertical dimension of a body water depth fora uniform cylinder, draft for floating docks!

Hankel function of the first kind, order n argument. x

Imaginary unit; = ~l

C ,C ,C ,C ,C ,C Force and moment coefficients respectively iny z x y z x, y, z directions

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Integrals in J $!Il, I2' .. I6

The stationary functional

Bessel function, order n argument x

rea1! wave-number associated with the incident wave

J x!

k0

K x! Modified Bessel function of the second kind, order nargument x

Average dimension of finite elements in a given gridLe

Number of vertical eigenfunctions iv the exterioranalytic representation minus one

Total moment vector

Component of total moment about i-axisM.1

Number of finite elements in V

M Number of ncdes in a finite element

Unit ncrmal vector pointing out of fluid region undercon sidera tion

Number of radial eigenfunction for each vertical modein the exterior analytic representation

N , Nl, N2 ''., N

e

iInterpolation functions for an element

Number of nodes on S

Total number of urknown coefficients in exterioran-lytic representation

Number of finite element nodes in V

Radial coordinate; = ~x2 + y

214

m = l, 2, 3 ... imaginary part of the imaginary rootsof the dispersion relation

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Radius of the artificial cylinder SS

Complex! reflection coefficient in two-dimensionalscattering

Vertical circular cylindrical artificial surfaceseparating finite element and analytic regions

Control surface at infini.ty

Time

Complex! transmission coefficient in two � dimensionalscattering

Total fluid volumeFinite element fluid volume inside S

V

V Finite element region with grids generated by hand! input

Weighting function for quadrature point i

Rectilinear coordinates

Global! nodal coordinate values of a finite element

x, y, z

X. qy,qz

G mn' mnCoe f f icients of the ex terior eigenf unc tion expression

Absolute error in

Relative error in

A small number ; Belongs to

Jacobi symbol; = 1 for n = 0; = 2 for n = 1, 2, 3

Imaginary roots of the dispersion relation

Local coordinates of an element

Quadrature points in local. coordinates

Finite element regio~ with grids generated automatically

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Free surface elevation

-l yPolar angle azimuth!; = tan !

e,

Incident velocity potential

= Re[ t>e ]

Imaginary part of

Real part of

Im !

Re !

d 3Gradient operator; = �, �, � !3x' 3y' 3z

3Bn

= n ~ 7

Absolute value of a real number!Nagnitude of a complex number!Determinant of a matrix

A subset of

Union of

A vector

!* Complex conjugate of

!' Of the analytic region exterior to SDerivative with respect to argument

!' Of an element

Transpose of

Inverse of a matrix!

!'

Incidence angle

Density of fluid

Radian frequency of the incident wave

Total velocity potential

Scattered velocity potential

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of S

Of the free surface

Jacobian matrix of coordinate transformation forvolume, free surface and S

Total global! eruation matrix

Total global! matrix after static! condensation

Pull! matrix due to an integral ia J $!

Diagonal! matrix due to an integral in J $!

Matrix due to integral over free surface

Matrix due to the volume integral

Vector of interpolation func tions

Total load right-hand-side! vector af ter static!conden.,ation

Vectors due to integrals in J $!

Matrix of node coordinate values for an element

Vector of coeffic ient unknowns

Vector containing both FKN and coefficient unknowns

217

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APPENDIX A

A PROOF OF THE OPTICAL THEOREM

Consider an incident wave given as in Eq. �.1.a!

iga cosh k z+h!0 O ik r cos B-01!ocosh k h

0

A. 1 .a!b f ikor cos B 01!o

where we have defined

v2 cosh k z+h!

f z!h+ 4 sinh k h

A. 1.b!

and

iga A + z sinh k ho ld 0

b = - ---!v2 cosh k h

0

A.1. c!a cons tan t

and h is the far-field constant depth.

At infinity, the scattered wave Fq. �.3.4!! can be written aa

ikorc A $! f z!

S A ro

A.2.a! k r -+ ~!0

*The proof here is quite well-known in classical physics and is notdifferent: in essence from that given in Gottfried, 1966.

218�

The Optical Theorem for three-dimensional scattering of water waves

Eq. 4.!.2! can be proved by app1ying Green's Theorem and using the method

of stationary phase.+

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where we define

igaA 8! = � ! Z -i! a, cos nB + g sin n8!

on onn=0

A.2.b!

and

2 -i~/4c=b v' � e cosh k h7r 0

A.2.c!

0 = $V g � /~7'$!dV = g ~> � g* ~~!dS A. 3!

by virtue of Eq. �.2.1! in V, and Eq. �.2.2!, �.2.3! on F and 8

respectively.

It follows that

Im [g > ] dS = 0r Q+* A. 4!

and upon substituting in Eqs. A.l!, A.2!, expanding and performing depth

integration, we have

219

Applying Green's Theorem to 0 = $ + $ and $" over the entire fluid region,I s

V, we have

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2m

0 = d9r Im f-ik b cos 8 � 8 ! ]k ~ 0 I

0

+lim e

k r~0

r 27r ikor [1-cos 8 � 9I! ]d9r Im f-ik bc* 4" 8!

0

0r

ikor [l-cos 8 � 8Z! ]b*c 4 9! ' cos 9 � 9 !]

0 A rI

0

8 A«> I ' A.s!

-it'2where order k r! terms vanish in the limpet.

0

8 � 8 ! ~ 0 k r! = c say!I 0

E I!

and

9 � 8 � m! ~ > 0 k r!I 0

� 220�

Now, the first integrand is periodic in 9 and the integral vanishes

identically. The second integral, I say!, can be evaluated using a

stationary phase argument since k r >! l. In this limit, contribution to0

the integral is neglegible except near stationary phase points 9 � 8 = 0I

and s, in fact, only for

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Expanding cos� � 6 ! in Taylor series in turn about zero and 7r, keeping

only leading contribution in both exponent and coefficient, and evaluating

A at the stationary points, we have

I~Im [-i Arbc*A+�! e-ik «� � 0 !'�

0 Id� - 0 } � C.C.]

I

+ Im [ -i ~r bc* A*� + vr! e o d 8 � 0 � Tr! + C.C.]0 I I

k r ~ ~ ,c « 1!

where C.C. denotes the complex conjugate of the preceding term. Now the

second term vanishes identically upon taking the imaginary part, and the

first term can be. rewritten so that

2 2I~ 2 Im [ -i ~abc*A*� ! e o I d� � 6 !]o I I

kr~~, a<<1!0

Set

ik r � - 6I! '12, i e. 0 - 6I = / k2

0

then on taking the limit

221

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PiI = 2 Im [ � iV'-bc* A+ 9 ! e ds]

i-Pi

I = � ~~> > Re[A R ! ]0

Vhence, Kq. A.5} becomes

27r20 = � ..".» �R.rA<e,» � I* J I<c~! I' «

0

or

21r

A e! I' de =�Re[A� ! ]

cosh k h0

as stated in Eq. �.1.2!.

222

The Fresrel integral is simply vm, and substituting in Eq. A.2.c! for b, we

finally have

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