Practical Ship Hydrodynamics Episode 1 ppt

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 623.8 0 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 623.8 0 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 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 S ¨ oding. 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 S ¨ oding 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 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 [...]... described by a coefficient c as follows: FDcÐ 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 is introduced in analogy to stagnation pressure 2 q D 1 Ð V2 Combining the above equations then yields: 2 cs Ð 1 Fs 2 D Fm cm Ð 1 2 s Ð V2 Ð As s m Ð V2 Ð Am m D s m Ð As Ð Am Vs Vm 2 6 Practical Ship Hydrodynamics This results in cs D cm ,... forces do not scale down for constant viscosity Ships operating at the free surface are subject to gravity forces (waves) and frictional forces Thus in model tests both Froude’s and Reynolds’ laws should be fulfilled This would require: Rns D Rnm m s Ð 3 Ls D 3 Lm m s Ð 1. 5 D1 10 Practical Ship Hydrodynamics v + vy dy y + dy u + ux dx u v y x + dx x Figure 1. 1 Control volume to derive continuity equation... 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... 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 Introduction 5 there... propeller operates in a different self-propulsion point than the full-scale ship propeller Despite these concerns, the manoeuvring 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... scale effects such as surface roughness, flow separation etc.) The kinematic viscosity of seawater [m/s2 ] can be estimated as a function of temperature t[° C] and salinity s [%]: D 10 6 Ð 0. 014 Ð s C 0.000645 Ð t 0.0503 Ð t C 1. 75 Sometimes slightly different values for the kinematic viscosity of water may be found The reason is that water is not perfectly pure, containing small organic and inorganic... 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 Ä: Fs D Ä Ð Fm Forces acting on the ship encompass inertial forces, gravity forces, and frictional 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... 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... the velocity component in z direction The Navier–Stokes equations together with the continuity equation suffice to describe all real flow physics for ship flows The Navier–Stokes equations describe conservation of momentum in the flow: ut C uux C vuy C wuz D f1 px C uxx C uyy C uzz vt C uvx C vvy C wvz D f2 py C vxx C vyy C vzz wt C uwx C vwy C wwz D f3 pz C wxx C wyy C wzz fi is an acceleration due to a... a volumetric force, p the pressure, the viscosity and t the time Often the volumetric forces are neglected, but gravity can be included by setting f3 D g D9. 81 m/s2 or the propeller action can be modelled by a distribution of volumetric forces f1 The l.h.s of the Navier–Stokes equations without the time derivative describes convection, the time derivative describes the rate of change (‘source term’), . 7506 48 51 1 1. Ships – Hydrodynamics I. Title. VM156 .B457 2000 623.8 0 12 –dc 21 00-034269 ISBN 0 7506 48 51 1 Typeset by Laser Words, Madras, India Printed in Great Britain by Preface ix 1 Introduction. 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. seakeeping 14 6 Discourse: hydrodynamic mass 14 8 5 Ship manoeuvring 15 1 5 .1 Introduction 15 1 5.2 Simulation of manoeuvring with known coefficients 15 2 5.2 .1 Introduction and definitions 15 2 5.2.2