Tài liệu Practical Ship Hydrodynamics doc

Practical Ship Hydrodynamics

Practical Ship Hydrodynamics

Volker Bertram

Butterworth-Heinemann

Linacre House, Jordan Hill, Oxford OX2 8DP

225 Wildwood Avenue, Woburn, MA 01801-2041

A division of Reed Educational and Professional Publishing Ltd

First published 2000

 Volker Bertram 2000

All rights reserved. No part of this publication

may be reproduced in any material form (including

photocopying or storing in any medium by electronic

means and whether or not transiently or incidentally

to some other use of this publication) without the

written permission of the copyright holder except

in accordance with the provisions of the Copyright,

Designs and Patents Act 1988 or under the terms of a

licence issued by the Copyright Licensing Agency Ltd,

90 Tottenham Court Road, London, England W1P 9HE.

Applications for the copyright holder’s written permission

to reproduce any part of this publication should be

addressed to the publishers

British Library Cataloguing in Publication Data

Bertram, Volker

Practical ship hydrodynamics

1. Ships – Hydrodynamics

I. Title

623.80

12

Library of Congress Cataloguing in Publication Data

Bertram, Volker.

Practical ship hydrodynamics / Volker Bertram.

p. cm.

Includes bibliographical references and index.

ISBN 0 7506 4851 1

1. Ships – Hydrodynamics I. Title.

VM156 .B457 2000

623.80

12–dc21 00-034269

ISBN 0 7506 4851 1

Typeset by Laser Words, Madras, India

Printed in Great Britain by

Preface ............................................. ix

1 Introduction .................................. 1

1.1 Overview of problems and

approaches ............................................ 1

1.2 Model tests … similarity laws .............. 4

1.3 Full-scale trials ................................. 8

1.4 Numerical approaches

(computational fluid dynamics)............... 9

1.4.1 Basic equations ............................. 9

1.4.2 Basic CFD techniques................... 14

1.4.3 Applications................................... 15

1.4.4 Cost and value aspects of CFD .... 19

1.5 Viscous flow computations ............... 22

1.5.1 Turbulence models........................ 23

1.5.2 Boundary conditions...................... 26

1.5.3 Free-surface treatment.................. 28

1.5.4 Further details ............................... 29

1.5.5 Multigrid methods.......................... 31

1.5.6 Numerical approximations............. 32

1.5.7 Grid generation ............................. 34

2 Propellers...................................... 37

2.1 Introduction ...................................... 37

2.2 Propeller curves ............................... 39

2.3 Analysis of propeller flows ................ 42

2.3.1 Overview of methods .................... 42

2.3.2 Momentum theory ......................... 44

2.3.3 Lifting-line methods ....................... 45

2.3.4 Lifting-surface methods................. 46

2.3.5 Boundary element methods .......... 49

2.3.6 Field methods................................ 50

2.4 Cavitation ......................................... 51

2.5 Experimental approach .................... 54

2.5.1 Cavitation tunnels.......................... 54

2.5.2 Open-water tests........................... 55

2.5.3 Cavitation tests.............................. 56

2.6 Propeller design procedure .............. 56

2.7 Propeller-induced pressures ............ 60

3 Resistance and propulsion ......... 62

3.1 Resistance and propulsion

concepts ................................................. 62

3.1.1 Interaction between ship and

propeller ................................................. 62

3.1.2 Decomposition of resistance......... 65

3.2 Experimental approach .................... 68

3.2.1 Towing tanks and experimental

set-up ..................................................... 68

3.2.2 Resistance test.............................. 69

3.2.3 Method ITTC 1957 ........................ 71

3.2.4 Method of Hughes… Prohaska........ 73

3.2.5 Method of ITTC 1978 .................... 74

3.2.6 Geosim method of Telfer............... 75

3.2.7 Propulsion test .............................. 75

3.2.8 ITTC 1978 performance

prediction method................................... 76

3.3 Additional resistance under

service conditions................................... 80

3.4 Simple design approaches ............... 83

3.5 CFD approaches for steady flow...... 83

3.5.1 Wave resistance computations ..... 83

3.5.2 Viscous flow computations............ 90

3.6 Problems for fast and

unconventional ships.............................. 91

3.7 Exercises: resistance and

propulsion............................................... 95

4 Ship seakeeping ........................... 98

4.1 Introduction ...................................... 98

4.2 Experimental approaches (model

and full scale) ......................................... 99

4.3 Waves and seaway .......................... 101

4.3.1 Airy waves (harmonic waves of

small amplitude) ..................................... 101

4.3.2 Natural seaway ............................. 106

4.3.3 Wind and seaway.......................... 109

4.3.4 Wave climate................................. 4.2

4.4 Numerical prediction of ship

seakeeping ............................................. 117

4.4.1 Overview of computational

methods ................................................. 117

4.4.2 Strip method.................................. 121

4.4.3 Rankine singularity methods ......... 127

4.4.4 Problems for fast and

unconventional ships.............................. 130

4.4.5 Further quantities in regular

waves ..................................................... 132

4.4.6 Ship responses in stationary

seaway ................................................... 132

4.4.7 Simulation methods....................... 134

4.4.8 Long-term distributions.................. 136

4.5 Slamming ......................................... 138

4.6 Exercises: seakeeping ..................... 146

Discourse: hydrodynamic mass ............. 148

5 Ship manoeuvring ........................ 151

5.1 Introduction ...................................... 151

5.2 Simulation of manoeuvring with

known coefficients .................................. 152

5.2.1 Introduction and definitions ........... 152

5.2.2 Force coefficients .......................... 153

5.2.3 Physical explanation and force

estimation............................................... 158

5.2.4 Influence of heel............................ 163

5.2.5 Shallow water and other

influences ............................................... 164

5.2.6 Stopping........................................ 164

5.2.7 Jet thrusters .................................. 165

5.2.8 CFD for ship manoeuvring ............ 166

5.3 Experimental approaches ................ 169

5.3.1 Manoeuvring tests for full-scale

ships in sea trials.................................... 169

5.3.2 Model tests.................................... 175

5.4 Rudders............................................ 177

5.4.1 General remarks and definitions ... 177

5.4.2 Fundamental hydrodynamic

aspects of rudders and simple

estimates................................................ 181

5.4.3 Rudder types ................................. 188

5.4.4 Interaction of rudder and

propeller ................................................. 190

5.4.5 Interaction of rudder and ship

hull.......................................................... 193

5.4.6 Rudder cavitation .......................... 195

5.4.7 Rudder design............................... 200

5.4.8 CFD for rudder flows and

conclusions for rudder design ................ 201

5.5 Exercise: manoeuvring..................... 203

6 Boundary element methods ........ 207

6.1 Introduction ...................................... 207

6.2 Source elements .............................. 209

6.2.1 Point source .................................. 209

6.2.2 Regular first-order panel ............... 211

6.2.3 Jensen panel................................. 215

6.2.4 Higher-order panel ........................ 218

6.3 Vortex elements ............................... 223

6.4 Dipole elements ............................... 226

6.4.1 Point dipole ................................... 226

6.4.2 Thiart element............................... 227

6.5 Special techniques ........................... 229

6.5.1 Desingularization........................... 229

6.5.2 Patch method ................................ 230

7 Numerical example for BEM ........ 236

7.1 Two-dimensional flow around a

body in infinite fluid................................. 236

7.1.1 Theory ........................................... 236

7.1.2 Numerical implementation............. 237

7.2 Two-dimensional wave resistance

problem .................................................. 238

7.2.1 Theory ........................................... 238

7.2.2 Numerical implementation............. 241

7.3 Three-dimensional wave

resistance problem ................................. 242

7.3.1 Theory ........................................... 242

7.3.2 Numerical implementation............. 247

7.4 Strip method module (two

dimensional)........................................... 250

7.5 Rankine panel method in the

frequency domain................................... 253

7.5.1 Theory ........................................... 253

7.5.2 Numerical implementation............. 261

References ....................................... 265

Index ................................................. 269

Preface

The first five chapters give an introduction to ship hydrodynamics, which is

in my opinion suitable for teaching at a senior undergraduate level or even at

a postgraduate level. It is thus also suitable for engineers working in industry.

The book assumes that the reader has a solid knowledge of general fluid

dynamics. In teaching, general fluid dynamics and specific ship hydrodynamics

are often mixed but I believe that universities should first teach a course

in general fluid dynamics which should be mandatory to most engineering

students. There are many good textbooks on the market for this purpose. Naval

architects should then concentrate on the particular aspects of their field and

cover material more suited to their needs. This book is organized to support

such a strategy in teaching.

The first chapter is an introduction to computational fluid dynamics, and

Chapters 2 to 5 cover the four main areas of propeller flows, resistance and

propulsion, ship seakeeping and manoeuvring. It is recommended that this

sequence be followed in teaching. The first five chapters try to find a suitable

balance for practical engineers between facts and minimizing formula work.

However, there are still formulae. These are intended to help those tasked

with computations or programming. Readers with a practical interest may

simply skip these passages. The final two chapters involve more extensive

formula work and are more specialized. They may be reserved for graduate and

post-graduate teaching and will help understanding and developing boundary

element codes. Field methods are not covered in depth here, as my colleague

Milovan Peric has already co-authored an excellent book on this particular

topic. I tried in vain to find a similar suitable textbook for boundary element

methods which would be both easy to understand and address the typical

problems encountered in ship flows. As I could not find such a book, I wrote

two chapters intended to support me in my teaching and to be of use for many

colleagues.

The book is supplemented by some public domain software written

in Fortran which is available for downloading in source code on

www.bh.com/companions/0750648511. The software consists of small

programs or subroutines which may help in developing own codes. Some of the

programs have been written by myself, some by Professor Soding, and some ¨

by colleagues. Feel free to download the software, but there is no additional

documentation available except for the in-program comments. I will not answer

questions about the software, but you can comment on which programs you

ix

x Preface

felt difficult to understand. We may then either update the documentation or

take the software off the website. There is no guarantee that the programs are

completely debugged and of course neither I nor the publisher will take any

responsibility for what happens if you use these programs. Furthermore, the

software is public domain and you may not sell it to third parties.

Despite all this, I have worked with most of the software myself without

any problems. The website will be updated more often than the book, and

there will be a short read.me file on the web with some information on the

available software.

This book is based largely on lectures for German students. The nucleus of

the book was formed by lectures on ship seakeeping and ship manoeuvring,

which I have taught for several years with Professor Heinrich Soding. I always ¨

felt that we should have a comprehensive textbook that would also cover resis￾tance and propulsion, as ship seakeeping and manoeuvring are both interwoven

strongly with the steady base flow. Many colleagues helped with providing

material, allowing me to pick the best from their teaching approaches. A lot

of material was written and compiled in a new way, inspired by these sources,

but the chapters on ship seakeeping and manoeuvring use extensive existing

material.

Thanks are due to Seehafen-Verlag Hamburg for permission to reprint text

and figures from the Manoeuvring Technical Manual, an excellent book unfor￾tunately no longer in print. Thanks are due to Hansa-Verlag Hamburg for

permission to reprint text and figures from German contributions in Handbuch

der Werften XXIV.

Countless colleagues supported the endeavour of writing this book by

supplying material, proof-reading, making comments or just discussing

engineering or didactic matters. Among these are (in alphabetical order)

Poul Andersen, Kai Graf, Mike Hughes, Hidetsugu Iwashita, Gerhard Jensen,

Meinolf Kloppenburg, Jochen Laudan, Maurizio Landrini, Friedrich Mewis,

Katsuji Tanizawa, Gerhard Thiart, Michel Visonneau, and Hironori Yasukawa.

Most of all, Professor Heinrich Soding has supported this book to an extent that ¨

he should have been named as co-author, but, typically for him, he declined

the offer. He even refused to allow me to dedicate this book to him.

I then dedicate this book to the best mentor I ever had, a role model as a

scientist and a man, so much better than I will ever be. You know who.

Volker Bertram

Models now in tanks we tow.

All of that to Froude we owe.

Will computers, fast and new,

Make us alter Euler’s view?

Marshall Tulin

1

Introduction

1.1 Overview of problems and approaches

The prediction of ship hydrodynamic performance can be broken down into

the general areas of

ž resistance and propulsion

ž seakeeping

ž manoeuvring

Propeller flows and propeller design can be seen as a subtopic of resistance

and propulsion, but it is so important and features special techniques that it

is treated as a separate topic in its own right. Morgan and Lin (1998) give a

good short introduction to the historical development of these techniques to

the state of the art in the late 1990s.

The basic approaches can be roughly classified into:

ž Empirical/statistical approaches

Design engineers need simple and reasonably accurate estimates, e.g. of the

power requirements of a ship. Common approaches combine a rather simple

physical model and regression analysis to determine required coefficients

either from one parent ship or from a set of ships. The coefficients may be

given in the form of constants, formulae, or curves.

Because of the success with model testing, experimental series of hull

forms have been developed for varying hull parameters. Extensive series

were tested in the 1940s and the subsequent two decades. These series were

created around a ‘good’ hull form as the parent form. The effect of essential

hull parameters, e.g. block coefficient, was determined by systematic varia￾tions of these parameters. Because of the expense of model construction and

testing, there are no recent comparable series tested of modern hull forms

and the traditional ship series must be considered as outdated by now.

Although empirical and statistical approaches are still popular in design

practice, we will not treat them in detail here, because they are of little rele￾vance to the ship hydrodynamicist. Ship designers are referred to Schneek￾luth and Bertram (1998) for a review of these empirical approaches.

ž Experimental approaches, either in model tests or in full-scale trials

The basic idea of model testing is to experiment with a scale model to

extract information that can be scaled (transformed) to the full-scale ship.

1

2 Practical Ship Hydrodynamics

Despite continuing research and standardization efforts, a certain degree of

empiricism is still necessary, particularly in the model-to-ship correlation

which is a method to enhance the prediction accuracy of ship resistance

by empirical means. The total resistance can be decomposed in various

ways. Traditionally, model basins tend to adopt approaches that seem most

appropriate to their respective organization’s corporate experience and accu￾mulated databases. Unfortunately, this makes various approaches and related

aggregated empirical data incompatible.

Although there has been little change in the basic methodology of

ship resistance since the days of Froude (1874), various aspects of the

techniques have progressed. We now understand better the flow around

three-dimensional, appended ships, especially the boundary layer effects.

Also non-intrusive experimental techniques like laser-Doppler velocimetry

(LDV) allow the measurement of the velocity field in the ship wake to

improve propeller design. Another more recent experimental technique is

wave pattern analysis to determine the wave-making resistance.

In propulsion tests, measurements include towing speed and propeller

quantities such as thrust, torque, and rpm. Normally, open-water tests on the

propeller alone are run to aid the analysis process as certain coefficients are

necessary for the propeller design. Strictly, open-water tests are not essential

for power prediction alone. The model propeller is usually a stock propeller

(taken from a large selection/stock of propellers) that approximates the actual

design propeller. Propulsion tests determine important input parameters for

the actual detailed propeller design, e.g. wake fraction and thrust deduction.

The wake distribution, also needed for propeller design, is measured

behind the ship model using pitot tubes or laser-Doppler velocimetry

(LDV). For propeller design, measured nominal wakes (for the ship without

propeller) for the model must be transformed to effective wakes (for the

ship with working propeller) for the full-scale ship. While semi-empirical

methods for this transformation work apparently well for most hull forms,

for those with considerable flow separation at the stern, i.e. typically full

hulls, there are significant scale effects on the wake between model and

full scale. To some extent, computational fluid dynamics can help here in

estimating the scale effects.

Although the procedures for predicting full-scale resistance from model

tests are well accepted, full-scale data available for validation purposes

are extremely limited and difficult to obtain. The powering performance

of a ship is validated by actual ship trials, ideally conducted in calm seas.

The parameters usually measured are torque, rpm, and speed. Thrust is

measured only as a special requirement because of the difficulty and extra

expense involved in obtaining accurate thrust data. Whenever possible and

appropriate, corrections are made for the effects of waves, current, wind, and

shallow water. Since the 1990s, the Global Positioning System (GPS) and

computer-based data acquisition systems have considerably increased the

accuracy and economy of full-scale trials. The GPS has eliminated the need

for ‘measured miles’ trials near the shore with the possible contamination

of data due to shallow-water effects. Today trials are usually conducted far

away from the shore.

Model tests for seakeeping are often used only for validation purposes.

However, for open-top containerships and ro-ro ships model tests are often

performed as part of the regular design process, as IMO regulations require

Introduction 3

certain investigations for ship safety which may be documented using

model tests.

Most large model basins have a manoeuvring model basin. The favoured

method to determine the coefficients for the equations of motion is through a

planar motion mechanism and rotating arm model tests. However, scaling the

model test results to full scale using the coefficients derived in this manner

is problematic, because vortex shedding and flow separation are not similar

between model and full scale. Appendages generally make scaling more

difficult. Also, manoeuvring tests have been carried out with radio-controlled

models in lakes and large reservoirs. These tests introduce additional scale

effects, since the model propeller operates in a different self-propulsion

point than the full-scale ship propeller. Despite these concerns, the manoeu￾vring characteristics of ships seem generally to be predicted with sufficient

accuracy by experimental approaches.

ž Numerical approaches, either rather analytical or using computational fluid

dynamics (CFD)

For ship resistance and powering, CFD has become increasingly important

and is now an indispensable part of the design process. Typically inviscid

free-surface methods based on the boundary element approach are used to

analyse the forebody, especially the interaction of bulbous bow and forward

shoulder. Viscous flow codes often neglect wave making and focus on the

aftbody or appendages. Flow codes modelling both viscosity and the wave￾making are at the threshold of practical applicability. CFD is still considered

by industry as too inaccurate for resistance or power predictions. Instead, it

is used to gain insight into local flow details and derive recommendation on

how to improve a given design or select a most promising candidate design

for model testing.

For seakeeping, simple strip methods are used to analyse the seakeeping

properties. These usually employ boundary element methods to solve a

succession of two-dimensional problems and integrate the results into a

quasi-three-dimensional result with usually good accuracy.

A commonly used method to predict the turning and steering of a ship is

to use equations of motions with experimentally determined coefficients.

Once these coefficients are determined for a specific ship design – by

model tests or estimated from similar ships or by empirically enhanced

strip methods – the equations of motions are used to simulate the dynamic

behaviour of the ship. The form of the equations of motions is fairly standard

for most hull designs. The predictions can be used, e.g., to select rudder size

and steering control systems, or to predict the turning characteristics of ships.

As viscous CFD codes become more robust and efficient to use, the reliance

on experimentally derived coefficients in the equations of motions may be

reduced. In an intermediate stage, CFD may help in reducing the scaling

errors between model tests and full scale.

Although a model of the final ship design is still tested in a towing tank,

the testing sequence and content have changed significantly over the last few

years. Traditionally, unless the new ship design was close to an experimental

series or a known parent ship, the design process incorporated many model

tests. The process has been one of design, test, redesign, test etc. sometimes

involving more than 10 models each with slight variations. This is no longer

feasible due to time-to-market requirements from shipowners and no longer

4 Practical Ship Hydrodynamics

necessary thanks to CFD developments. Combining CAD (computer-aided

design) to generate new hull shapes in concert with CFD to analyse these

hull shapes allows for rapid design explorations without model testing. CFD

allows the preselection of the most promising design. Then often only one or

two models are actually tested to validate the intended performance features in

the design and to get a power prediction accepted in practice as highly accurate.

As a consequence of this practice, model tests for shipyard customers have

declined considerably since the 1980s. This was partially compensated by more

sophisticated and detailed tests funded from research projects to validate and

calibrate CFD methods.

One of the biggest problems for predicting ship seakeeping is determining

the nature of the sea: how to predict and model it, for both experimental

and computational analyses. Many long-term predictions of the sea require a

Fourier decomposition of the sea and ship responses with an inherent assump￾tion that the sea and the responses are ‘moderately small’, while the physics

of many seakeeping problems is highly non-linear. Nevertheless, seakeeping

predictions are often considered to be less important or covered by empirical

safety factors where losses of ships are shrugged off as ‘acts of God’, until

they occur so often or involve such spectacular losses of life that safety factors

and other regulations are adjusted to a stricter level. Seakeeping is largely not

understood by shipowners and global ‘sea margins’ of, e.g., 15% to finely

tuned (š1%) power predictions irrespective of the individual design are not

uncommon.

1.2 Model tests – similarity laws

Since the purely numerical treatment of ship hydrodynamics has not yet

reached a completely satisfactory stage, model tests are still essential in the

design process and for validation purposes. The model tests must be performed

such that model and full-scale ships exhibit similar behaviour, i.e. the results

for the model can be transferred to full scale by a proportionality factor. We

indicate in the following the full-scale ship by the index s and the model by

the index m.

We distinguish between

ž geometrical similarity

ž kinematical similarity

ž dynamical similarity

Geometrical similarity means that the ratio of a full-scale ‘length’ (length,

width, draft etc.) Ls to a model-scale ‘length’ Lm is constant, namely the

model scale :

Ls D  Ð Lm

Correspondingly we get for areas and volumes: As D 2 Ð Am; rs D 3 Ð rm.

In essence, the model then ‘appears’ to be the same as the full-scale ship.

While this is essential for movie makers, it is not mandatory for naval architects

who want to predict the hydrodynamic performance of a full-scale ship. In fact,

Introduction 5

there have been proposals to deviate from geometrical similarity to achieve

better similarity in the hydrodynamics. However, these proposals were not

accepted in practice and so we always strive at least in macroscopic dimen￾sions for geometrical similarity. In microscopic dimensions, e.g. for surface

roughness, geometrical similarity is not obtained.

Kinematic similarity means that the ratio of full-scale times ts to model-scale

times tm is constant, namely the kinematic model scale :

ts D  Ð tm

Geometrical and kinematical similarity result then in the following scale factors

for velocities and accelerations:

Vs D 

 Ð Vm as D 

2 Ð am

Dynamical similarity means that the ratio of all forces acting on the full-scale

ship to the corresponding forces acting on the model is constant, namely the

dynamical model scale

:

Fs D

Ð Fm

Forces acting on the ship encompass inertial forces, gravity forces, and fric￾tional forces.

Inertial forces follow Newton’s law F D m Ð a, where F denotes force, m

mass, and a acceleration. For displacement ships, m D  Ð r, where  is the

density of water and r the displacement. We then obtain for ratio of the inertial

forces:

D Fs

Fm

D s

m

Ð rs

rm

Ð as

am

D s

m

Ð

4

2

This equation couples all three scale factors. It is called Newton’s law of

similarity. We can rewrite Newton’s law of similarity as:

D Fs

Fm

D s

m

Ð 2 Ð





2

D s

m

Ð As

Am

Ð

Vs

Vm

2

Hydrodynamic forces are often described by a coefficient c as follows:

F D c Ð 1

2 Ð V2 Ð A

V is a reference speed (e.g. ship speed), A a reference area (e.g. wetted surface

in calm water). The factor 1

2 is introduced in analogy to stagnation pressure

q D 1

2 Ð V2. Combining the above equations then yields:

Fs

Fm

D cs Ð 1

2s Ð V2

s Ð As

cm Ð 1

2m Ð V2

m Ð Am

D s

m

Ð As

Am

Ð

Vs

Vm

2