Doctoral theses at NTNU, 2010:215 Yan-Lin Shao Numerical ... · Trondheim, August 2010 Norwegian...

Doctoral theses at NTNU, 2010:215

Yan-Lin ShaoNumerical Potential-Flow Studies onWeakly-Nonlinear Wave-BodyInteractions with/without SmallForward Speeds

ISBN 978-82-471-2415-4 (printed ver.)ISBN 978-82-471-2416-1 (electronic ver.)

ISSN 1503-8181

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Doctoral theses at N

TNU

, 2010:215Yan-Lin Shao

Yan-Lin Shao

Numerical Potential-Flow Studieson Weakly-Nonlinear Wave-BodyInteractions with/without SmallForward Speeds

Thesis for the degree of doctor philosophiae

Trondheim, August 2010

Norwegian University ofScience and TechnologyFaculty of Engineering Science and TechnologyDepartment of Marine Technology

Yan-Lin Shao





Yan-Lin Shao





Yan-Lin Shao





NTNUNorwegian University of Science and Technology


Faculty of Engineering Science and TechnologyDepartment of Marine Technology

©Yan-Lin Shao

ISBN 978-82-471-2415-4 (printed ver.)ISBN 978-82-471-2416-1 (electronic ver.)ISSN 1503-8181

Doctoral Theses at NTNU, 2010:215

Printed by Tapir Uttrykk




©Yan-Lin Shao







©Yan-Lin Shao







©Yan-Lin Shao




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Abstract

A two-dimensional Quadratic Boundary Element Method (QBEM) and a three-dimensional cubic Higher-order Boundary Element Method (HOBEM) are developed to study respectively the two-dimensional and three-dimensional weakly-nonlinear wave-body interactions with/without forward speed within potential flow theory of an incompressible liquid.

A direct method based on a triangular polar-coordinate system transformation for the evaluation of the Cauchy Principle Value (CPV) integrals for the diagonal terms of the influence matrix in the 3D HOBEM is presented.

A numerical module based on the Fast Multipole Method (FMM) is developed, which can be used as an option to speed up the present 3D HOBEM solver. Both the operation count and the required memory of a FMM accelerated BEM is asymptotically O(N), where N is the total number of the unknowns. Suggestion on the selection of a proper matrix solver for a specific problem is given.

A new approach based on domain decomposition using body-fixed coordinate system in the inner domain and the inertial reference frame in the outer domain is proposed for the weakly-nonlinear wave-body analysis. Consistent theoretical description of the new method based on second-order theory is presented. The new method does not require any derivatives on the right-hand sides of the body boundary conditions and thus avoid the mj-like terms and their derivatives. Furthermore, because the body boundary condition is formulated on the instantaneous position of the body, the resulting integral equations are valid for both smooth bodies and bodies with sharp corners. In order to improve the convergence of the second-order forces/moments on a body with sharp corners in the near-field approach, a re-formulation of the quadratic force is suggested. This re-formulation transfers the integrals on the body into the sum of two groups of integrals. The first group contains integrals on body surface with integrands whose singularities are weaker than that of the velocity square. The second group consists of regular integrals on the inner free surface and the control surface in the inner domain.

A two-dimensional third-order numerical wave tank (NWT) is developed. The effect of the Stokes drift in the second-order solution is discussed. A two-time scale approach is proposed as a secularity (solvability) condition in order to avoid unphysical third-order results. The numerical results for the second-order diffraction/radiation of a horizontal semi-submerged circular cylinder are verified by some other analytical and numerical results. Comparisons with the experimental results are also made.

The second-order wave-body interaction with/without the presence of a small forward speed for a three-dimensional floating body is studied by both the traditional method (if applicable) with a formulation in the inertial coordinate system and the new method with a formulation in the body-fixed coordinate system near the body. Both bodies without sharp corners and a truncated vertical circular cylinder with sharp corners are studied. Comparisons between the present numerical results with some

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other analytical and numerical results (if any) show good agreement. The influences of a small forward speed on the second-order wave loads on floating bodies are investigated.

The complete third-order wave diffraction of a stationary three-dimensional body is studied by the time-domain HOBEM, which means that the solution contains not only the triple-harmonic effect but also the third-order contribution with fundamental frequencies of the incident waves. Careful convergence studies and alternative way of calculating the force have been made with very satisfactory results.

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Acknowledgements

Here, first and foremost I would like to express my sincere gratitude to my supervisor, professor Odd M. Faltinsen, for the great guidance, inspiration and supervision he has shown in helping me complete this research. It has been difficult for me with a background on structural mechanics to start a doctoral study on hydrodynamics. It was his patience and encouragement which helped me make through in every way.

I want to thank all the lecturers for their excellent courses that I have learnt during the first year. These courses laid helpful basis for me on marine hydrodynamics in the later stage of the PhD study. I also appreciate the important guidance provided by Prof. Greco Marilena during this work.

I wish to thank Prof. Torgeir Moan, the director of Centre for Ships and Ocean Structures (CeSOS) where my study was carried out, for his successful creation of a scientific and pleasant environment in CeSOS.

I also wish to thank the help from the staff at CeSOS and the Department of Marine Technology, in particular Sigrid Bakken Wold, Marianne Kjølås and Karelle Gilbert.

It was my pleasure to work with all the other members in CeSOS and the Department of Marine Technology. Special thanks to Kota Ravikiran and Linlin Jiao for the fruitful discussions and talks. I also want to mention Dr. Wei Zhu, Dr. Hui Sun, Dr. Trygve Kristiansen and Dr. David Kristiansen for sharing the experiences from their research and providing valuable references. Csaba Pakozdi and Trygve Kristiansen are acknowledged for their help with the computer set up.

My love and gratitude go to my wife, Huirong, whose endless understanding and support made this work possible. Her sacrifices for our small family during the past several years leave debts I can only hope to repay. I am very grateful to my parents. Without their love, constant support and prayer, I could never have come to this far. Through this work, I also wish to express my love to my dear daughter, Tingting.

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Contents

Abstract……………………………………………………….………………………………………i Acknowledgements…………………………………….……………………………………………iii

Contents……………………………………………………………………………………………...v Nomenclature………………………………………………………………………………...……….ix

1 Introduction...………………………………………………………………………………………1 1.1 Scope and objective………………………….…………………………………………………1

1.2 Previous studies………………………………………………………………………………. 2 1.3 Present study …………………………………………………………………………………… 9

1.3.1 Outline of the thesis......…..………………..…………………………………………….10 1.3.2 Major contributions of the present study......…..………...……………………………….12

2 Theoretical Description……………………………………………………………………….…15 2.1 Introduction…………………………………………………………………………………..15 2.2 Coordinate systems…………………………………………………………………………….16 2.3 The definition of the motions…………………………………………………………………..17 2.4 Formulation of the second-order wave-body problem in the inertial coordinate system……...22 2.4.1 General description of the boundary conditions………………………………………...22 2.4.2 Second-order approximations of the boundary conditions……………………………...23 2.4.3 Forces and moments calculation………………………………………………………...25 2.5 Formulation of the third-order diffraction problem in the Earth-fixed coordinate system……28 2.5.1 Free-surface conditions…………………………………………………………………28 2.5.2 Body boundary condition……………………………………………………………….29 2.5.3 Forces and moments calculation…………………………………………...…………...29 2.6 Formulation of the second-order wave-body problem in the body-fixed coordinate system….30

2.6.1 Free-surface conditions…………………………………………………………………30 2.6.2 Body boundary condition……………………………………………………………..33 2.6.3 Forces and moments calculation………………………………………………………..33 2.7 Governing equations of unsteady rigid-body motions………………………………………..35

2.7.1 Rigid-body motion equations in the inertial frame……………………………………..35 2.7.2 Rigid-body motion equations in the body-fixed frame………………………………….38 2.8 Incident wave field……………………………………………………………………………..39 3 Basis of the Time-Domain HOBEM in 2D………………………………………………………..41 3.1 Boundary integral equation…………………………………………………………………….41 3.2 Quadratic boundary element method…………………………………………………………..42 3.3 Time marching of the free-surface conditions……………………………………………….45 3.4 Numerical damping zone and active wave absorber…………………………………………...45

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3.5 Solution of t …………………………………………………………………….…………….47 3.6 Calculation of the higher-order derivatives……………………………………………………49 3.7 Fourier analysis………………………………………………………………………………50

4 Basis of the Time-Domain HOBEM in 3D……………………………………………………….51 4.1 Boundary integral equation…………………………………………………………………….51 4.2 HOBEM based on cubic shape functions……………………………………………………...52 4.2.1 Shape functions………………………………………………………………………….52 4.2.2 Solid angle and CPV integrals…………………………………………………………..54 4.3 Time marching of the free-surface conditions…………………………………………………59 4.4 Treatment of t -term and the time integration of body motion equations…………………….60 4.5 Low-pass filter on the free surface……………………………………………………………..62 4.6 Direct calculation of the higher-order derivatives……………………………………………..64 4.7 Types of grid on the free surface……………………………………………………………….69 4.8 Matrix Solver…………………………………………………………………………………..69 4.8.1 Why HOBEM? ………………………………………………………………………….70 4.8.2 Complexity of BEM solvers…………………………………………………………….70 4.8.3 Algorithm of FMM……………………………………………………………………...71 4.8.4 Selection of a proper solver……………………………………………………………..75 5 Use of the Body-Fixed Coordinate System in Weakly-Nonlinear Wave-Body Problems……79 5.1 Comparison of the weakly-nonlinear formulations in inertial and body-fixed coordinate systems…………………………………………………………………………………………79 5.1.1 Free-surface conditions………………………………………………………………….79 5.1.2 Body boundary conditions………………………………………………………………80 5.2 Domain-decomposition approach using body-fixed coordinate system in the near field ……..81 5.3 Generation of incident wave field in body-fixed coordinate system…………………………..87 5.4 The consistency between body-fixed coordinate system and inertial coordinate system……...88 6 Studies on Two-Dimensional Weakly-Nonlinear Problems……………………………………...93 6.1 The steady-state third-order solution of sloshing in a rectangular tank………………………..93 6.2 Free oscillations and forced oscillations in a rectangular tank………………………………..98 6.3 Stokes-drift effect and numerical simulation of the Stokes second-order waves…………….101 6.4 Secularity condition and numerical simulation of the Stokes third-order waves…………….107 6.5 Second-order diffraction of a horizontal semi-submerged circular cylinder………...……….112 6.6 Second-order radiation of a horizontal semi-submerged circular cylinder… …………..……118 7 Three-Dimensional Weakly-Nonlinear Problems with Zero Forward Speed ………………..123 7.1 Second-order and third-order wave diffraction on a fixed body……………………………..123 7.1.1 Second-order diffraction in monochromatic waves……………………………………123 7.1.2 Second-order diffraction in bichromatic waves………………………………………..133 7.1.3 Third-order diffraction in regular waves……………………………………………….135 7.2 Second-order studies of a body under forced oscillations……………………………………142 7.2.1. Linear hydrodynamic coefficients…………………………………………………….142 7.2.2. Second-order loads on forced oscillating bodies……………………………………...144 8 Three-dimensional Weakly-Nonlinear Problems with Small Forward Speeds……………….155 8.1 Second-order wave diffraction………………………………………………………………..156 8.2 Second-order wave radiation…………………………………………………………………162 8.3 Freely-floating body in regular waves………………………………………………………..172

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9 Summary and future perspectives……………………….………………………………………179 9.1 Summary……………………………………………………………………………………...179 9.2 Future perspectives…………………………………………………………………………...183

Appendix…………………………………………………………………………………………….187 Appendix A. The double-body basis flow………………………………………………………...187 A.1 The classical double-body basis flow in the inertial coordinate system………………….187 A.2 The 'double-body' basis flow used in the domain decomposition based method…………188 Appendix B. The second-order analytical solution of a circle under forced surging in an infinite fluid……...……………………………...…………...............................…………..190 B.1 Solution in the Earth-fixed coordinate system……………………………………………190 B.2 Solution in the body-fixed coordinate system…………………………………………….191 Appendix C. The second-order analytical solution for sloshing in a two-dimensional rectangular tank under forced surging…………….……………………………………………192 C.1 Solution in the Earth-fixed coordinate system……………………………………………192 C.2 Solution in the tank-fixed coordinate system……………………………………………..198 Appendix D. Elimination of the secular terms in the third-order free-surface conditions………..201 Appendix E. Indirect method for the evaluation of forces and moments due to the t-term……203 Appendix F. Alternative formulas for the quadratic forces and moments………………………...205 References…………………………………………………………………………………………209

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Nomenclature

General rules

The symbols are defined in the text for the first time they appear Only the most used symbols are declared here The same symbol may have different interpretations in different problems The vectors are represented by an arrow above the symbols The matrices are represented by bold face characters An overdot means time derivative A vector with a prime is described in the body-fixed coordinate system

Abbreviations

2D Two-dimensional 3D Three-dimensional BVP Boundary Value Problem BIE Boundary Integral Equation BEM Boundary Element Method CPV Cauchy Principle Value COG Centre of Gravity FEM Finite Element Method FDM Finite Difference Method HOBEM Higher-Order Boundary Element Method QTF Quadratic Transfer Function RAO Response Amplitude Operator

Subscripts

b i indicates the transformation matrix from body-fixed coordinate system to inertial coordinate system

g indicates translatory and rotational motions with respect to a coordinate system with origin at the Centre of Gravity

i i=1, …, 3. The i-th component of a vector i b indicates the transformation matrix from inertial coordinate system to body-

fixed coordinate system in indicates variables for incident waves

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Nomenclature

General rules


Abbreviations


Subscripts





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Nomenclature

General rules


Abbreviations


Subscripts





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Nomenclature

General rules


Abbreviations


Subscripts





x Nomenclature

n indicates normal derivative s indicates variables related to scattered waves t indicates time derivative x,y,z (X,Y,Z) indicate derivatives along x(X), y(Y) and z(Z) directions, respectively

Superscripts:

(i) indicates quantities of the i-th order (i=0, 1, 2, 3) T indicates the transpose of the matrix (2 ) indicates double-harmonic + indicates sum-frequency - indicates difference-frequency

Roman symbols:

A The linear wave amplitude ak k=1,2, 3,4. Coefficients used in Adams-Bashforth-Moulton method b(m) Forcing term in the m-th order body-boundary conditions formulated in the

body-fixed coordinate system B(m) Forcing term in the m-th order body-boundary conditions formulated in the

inertial coordinate system bk k=1,2, 3,4. Coefficients used in Adams-Bashforth-Moulton method c The strength of the low-pass filter C(x) Solid angle coefficient at a position x Ci Solid angle coefficient at the i-th node Pi CW Water line CW0 Mean water line d Draft F Forces vector

( )1

mF , ( )2mF Forcing term in the m-th order free-surface conditions formulated in the

inertial coordinate system. m=1, 2 ( )

1mf , ( )2

mf Forcing term in the m-th order free-surface conditions formulated in the body-fixed coordinate system. m=1, 2

gF External force vector acting on the body described in OXYZ system. G Green function

ikH , ikA Influence coefficients gM External moment vector with respect to COG acting on the body

g gK Gravity acceleration vector in the Earth-fixed coordinate system h Water depth I A 3×3 identity matrix

x Nomenclature


Superscripts:


Roman symbols:




( )1



1mf , ( )2





x Nomenclature


Superscripts:


Roman symbols:




( )1



1mf , ( )2





x Nomenclature


Superscripts:


Roman symbols:




( )1



1mf , ( )2





Nomenclature xi

BI Inertia matrix, its elements being the moments and products of inertia of the body

k Wave number M Moment vector n Normal vector on a surface

kN The k-th shape function NE Total number of elements NOD Total number of nodes OeXeYeZe Earth-fixed coordinate system OXYZ Inertial coordinate system moving with constant forward speed oxyz Body-fixed coordinate system ogxgygzg A body-fixed coordinate system with origin at COG p Pressure r Position vector t Time T Linear wave period R Radius of an axisymmetric body, e.g. hemisphere, circular cylinder

i bR Transformation matrix from inertial coordinate system to body-fixed coordinate system

b iR Transformation matrix from body-fixed coordinate system to inertial coordinate system

SB Wetted body surface SB0 Mean wetted body surface Sbottom Sea bottom SF Free surface SF0 Calm water surface U Forward speed vector in the inertial coordinate system

( )kU The k-th order component of forward speed vector described in the body-fixed coordinate system

kx k-th order displacement of a point, with its components as 1 2,k kx x and 3

kx ku k-th order displacement of a point, with its components as 1 2,

k ku u and 3ku

BU Translatory velocity of the body s=IEP(e, j) A coefficient of the connectivity matrix, which represents the global index of

the j-th node of e-th element

offsetL Offset distance in the desingularized BEM. offset d mL l D dl A constant coefficient in the definition of offsetL

mD Size of the mesh which can be approximated by the square root of the area of the local element.

A constant coefficient in the definition of offsetL

Nomenclature xi

BI Inertia matrix, its elements being the moments and products of inertia of the body

k Wave number M Moment vector n Normal vector on a surface

kN The k-th shape function NE Total number of elements NOD Total number of nodes OeXeYeZe Earth-fixed coordinate system OXYZ Inertial coordinate system moving with constant forward speed oxyz Body-fixed coordinate system ogxgygzg A body-fixed coordinate system with origin at COG p Pre

Doctoral theses at NTNU, 2010:215 Yan-Lin Shao Numerical ... · Trondheim, August 2010 Norwegian...

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