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1 Mechanics of Real Fluids WITPRESS WIT Press publishes leading books in Science and Technology. Visit our website for the current list of titles. WITeLibrary Home of the Transactions of the Wessex Institute, the WIT electronic-library provides the international scientific community with immediate and permanent access to individual papers presented at WIT conferences. Visit the WIT elibrary athttp://library.witpress.com

3 Mechanics of Real Fluids M. Rahman Halifax, Nova Scotia, Canada

4 M. Rahman Halifax, Nova Scotia, Canada Published by WIT Press Ashurst Lodge, Ashurst, Southampton, SO40 7AA, UK Tel: 44 (0) ; Fax: 44 (0) For USA, Canada and Mexico WIT Press 25 Bridge Street, Billerica, MA 01821, USA Tel: ; Fax: British Library Cataloguing-in-Publication Data A Catalogue record for this book is available from the British Library ISBN: Library of Congress Catalog Card Number: The texts of the papers in this volume were set individually by the authors or under their supervision. No responsibility is assumed by the Publisher, the Editors and Authors for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. The Publisher does not necessarily endorse the ideas held, or views expressed by the Editors or Authors of the material contained in its publications. WIT Press 2011 Printed in Great Britain by Martins the Printers. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the Publisher.

5 This book is dedicated to the loving memory of Sir James Lighthill, F.R.S. who gave the author tremendous inspiration and a love for fluid mechanics science.

7 A fluid mechanics memoir Fluid mechanics is an important branch of applied mathematics. It has enormous applications in our real world problems. Since it originated about three centuries ago it is considered old, but because of the new development in new diverse directions it is modern. On the basis of the classical theories with solid foundation, researchers in this field have advanced the subject tremendously. Many excellent treaties on fluid mechanics are available in the literature. There are many books in this area that were written by many pioneers in the subject. We can name a few books which are written during the last two centuries and which are still useful as reference texts for the scientists, engineers and applied mathematicians. The Modern Development in Fluid Dynamics, published by the Clarendon Press, Oxford in 1938, was an outstanding treaty under the editorship of Sydney Goldstein. Laminar Boundary Layers, the fluid motion memoir, published by the Clarendon Press in 1963, under the editorship of L. Rosenhead was another excellent book authored by some pioneers in fluid mechanics. High-speed Flow edited by Leslie Howarth is another excellent book. The Hydrodynamics authored by Sir Horace Lamb is considered a masterpiece of fluid mechanics. The Introduction to the Homogeneous Turbulence published by Cambridge University Press in 1953 was written by G.K. Batchelor and it is considered as a very good book in the fluid mechanics field. Recently Sir James Lighthill wrote an excellent book on Informal Introduction to Theoretical Fluid Mechanics published by the Clarendon Press in This book describes the fundamental theoretical development of fluid flow problems in the real world and is considered an outstanding masterpiece for the young scientists and applied mathematicians. The Dynamics of Upper Ocean written by O.M. Phillips and published by Cambridge University Press is another outstanding book in the water related science. Modern Fluid Dynamics published by Van Nostrand Company, London in 1968 written by N.J. Curle and H.J. Davies is one of the best

8 students paperback editions. G.B. Whitham s book on Linear and Nonlinear Waves is a very good book on water wave problems. This fluid mechanics memoir is a compendium of works by many pioneering authors and research works of the author since he started publishing scientific papers in reputed journals. I have borrowed some physical concepts from my book Water Waves: Relating Modern Theory to Engineering Applications published by the Clarendon Press in This book, I hope, will be suitable for the young scientists, graduate students, applied mathematicians and professional engineers. Theory is explained clearly and some applications are manifested in the book. The theory part is heavily borrowed from the standard textbooks but the applications part is completely new and hopefully the reader will appreciate my effort. M. Rahman

9 Contents Preface Acknowledgments xiii xvii Chapter 1 Introduction Preliminary background Real and ideal fluids Specification of the motion Outline of the book... 7 Chapter 2 The equations of fluid motion Introduction The equations of motion The mechanical energy equation The Boussinesq approximation The Bernoulli equation The Reynolds stresses Derivations of equations of motion Conservation of mass Euler s equation of motion Bernoulli s equation revisited The existence of irrotational motion Two-dimensional flow Physical interpretation of velocity potential Physical interpretation of stream function Complex potential Flow along a stream tube Vortex kinematics Vortexlines and vortextubes Circulation Vortex dynamics The persistence of circulation Line vortices and vortex sheets Navier-Stokes equations of motion Cartesian coordinates Cylindrical polar coordinates... 39

10 Spherical polar coordinates Exercises Chapter 3 Mechanics of viscous fluids Introduction Motion of a liquid in two-dimensions Pressure distribution The drag force on the cylinder Motion in an axially symmetric 3D-body Pressure distribution The drag force on the sphere Distinction between ideal and real fluids Drag forces in a real fluid Secondary flows Some exact solutions of Navier-Stokes equations Steady flow between two-dimensional channel c z c Steady flow through a circular section of radius c Steady flow through the annular region b r c Steady flow through an elliptic cylinder Steady flow in a rectangular section Steady Couette flow between rotating cylinders Steady flow between parallel planes Reynolds theory of lubrication Steady flow due to a rotating circular disc Some solutions of Navier-Stokes equations for unsteady flows Flow due to motion of an infinite plate Flow due to constant pressure gradient and motion of the plate Flow due to oscillation of the plate Very slow motion Stokes s flow using tensor calculus Stokes flow using vector calculus Oseen flow Exercises Chapter 4 Laminar boundary layers Introduction The concept of the boundary layer Mathematical expression of the boundary-layer thickness δ(x) Boundary layer separation Derivation of the boundary layer equations for flow along a flat plate Boundary conditions for steady flow

11 4.4 Boundary layer equations for flow along a curved surface Boundary-layer thicknesses, skin friction, and energy dissipation Momentum and energy equations Momentum integral Energy integral The von Mises transformation for steady flow Analytical solutions of boundary layer equations Flow along a flat plate at zero incidence in a uniform stream Method of solution Steady flow in the boundary layer along a cylinder near the forward stagnation point Steady flow along a wedge: the Falkner-Skan solutions Pohlhausen s method Flow in laminar wakes and jets Exercises Chapter 5 Similarity analysis in fluid flow Introduction Concept and definition of heat and mass diffusion General statement of the problem Similarity analysis of the basic equations Use of the similarity variable 1 γ β k2 ( yx ) η( x, y) = k1e Use of the similarity variable 1 η( x, y) = k2 ( yx γ ) Natural convection flow along a vertical plate Mathematical formulation Method of numerical solution Numerical results Exercises Chapter 6 Turbulence Introduction The mechanism of transition to turbulence The essential characteristics of turbulence Reynolds equations for turbulent motion Turbulent flow between parallel planes Mixing-length theories of turbulence Turbulent boundary layers Correlation theory of homogeneous turbulence Theoretical development of correlation theory Isotropic turbulence

12 6.9 Spectral theory of homogeneous turbulence Probability distribution of u(x) Calculation of the pressure covariance in isotropic turbulence Exercises Subject index 227

13 Preface Fluid mechanics is one of the most major areas of successful applications of mathematics. It can be considered as one of the branches of applied mathematics. The idea of writing this important book springs from Lighthill s An Informal Introduction to Theoretical Fluid Mechanics published by the Clarendon Press in Although fluid motion is concerned in both gas and liquids, as both of them are fluids, this book mainly deals with the motion of liquids in general, and water in particular. The theory of fluid mechanics has grown so considerably in recent years that study of the mechanics of fluids is important in many aspects of our real life. In our real world all creatures live immersed in fluids (air or water) and their capability of motion through it is of crucial importance for their life style. As we know, systems of circulating fluid offer important means for distributing things where they are needed. As for example, the blood circulation in our body is vital. Similarly, the ocean is another great circulation system practically equally important to man. Energy stored as potential energy, chemical energy or heat energy becomes converted into kinetic energy in a water turbine, a gas turbine or a steam turbine, in each case by means of fluid flow action on rotating blades. Such flow is studied in order to improve the efficiency of turbines, which may also, in many cases, depend upon effective fluid motion for transferring heat quickly from one part to another in such an engine. Electric power generated by tides is the application of motion of water through the turbine. The design of structures intended to resist strong winds, river erosion, or violent sea motions requires an understanding of the forces exerted by winds, currents or waves upon stationary structures. So, comprehensive knowledge of flow of fluids is very important in all these cases. These are very complex problems and can be tackled only by advanced knowledge of boundary layer flow and turbulence because the fluid motion is usually propagated in a random fashion. For complex fluid flow problem we must take recourse to the laboratory experiment or field experiment in association with the theory. Most fluid

14 motions are much too complex and a computer is essential to find solutions, but still the problem is so complex that even if the largest and fastest of the modern computers may fail to obtain the correct result. Great progress with the effective study, and the effective computations have been made, however, it is realized that such progress required creative input on a continuing basis both from theory and experiment. The study of fluid flow, especially the theory of water waves, has been the subject of intense scientific research since the days of Airy in As we have described above, it is of great practical importance to scientists and engineers from many disciplines in gaining insight into the complex systems of fluid motions in oceans. Chapter 1 briefly outlines the content of the book and gives an overview of the specification of the fluid motion. Described in Chapter 2 are the basic equations of fluid motion from the view point of general fluid dynamics. Developments of Euler equations of motion for inviscid fluids have been described from a mathematical view point due to the fact that Euler equations form the backbone for the study of water wave motion. This chapter also considers two important concepts with regard to vortex kinematics and vortex dynamics. Some examples of practical interest are solved using the theory developed in this chapter. Navier-Stokes equations, only, are cited for examples as equations for viscous fluid motion. The philosophy behind the source, sink, singularities and circulation of water particles is explained accompanied by some examples. Physical interpretations of velocity potentials and stream functions are clearly explained. Chapter 3 contains the concept of mechanics of real fluids. We describe the motion in axially symmetric 3-D bodies; pressure distribution and drag forces on a sphere are evaluated. Some exact solutions of Navier-Stokes equations are considered in this chapter. Very slow motions of fluids as manifested by Stokes and Oseen are explained with examples. Chapters 4 deals with the two-dimensional fluid motion in laminar boundary layers. Boundary layer equations for a variety of problems are discussed. The Von Misses transformations and Pohlhausen s method are discussed. Concepts of momentum and energy integrals are clearly explained. Boundary layer thicknesses such as displacement ä 1, momentum ä 2 and energy ä 3 are defined in integral forms. Flow in laminar wakes and jets are also considered. Chapter 5 is devoted to the development of similarity technique and the perturbation method in fluid mechanics. Natural convection flow along a vertical plate is considered and its solution technique discussed by similarity analysis. The book concludes with Chapter 6 which is devoted to the theoretical development of turbulent flow. Some interesting theoretical examples are solved for the benefit of the graduate students who are working in the field of turbulence. Some knowledge of vector calculus including the integral theorems such as Green s theorem, Stokes s theorem and divergence theorem is assumed

15 on the part of the reader. Isotropic tensor calculus is used sparingly in some chapters. A familiarity with the Bessel functions, Legendre polynomials and hypergeometric functions is also expected. Matiur Rahman, 2011 Halifax, Canada

17 Acknowledgments The author is grateful to Natural Sciences and Engineering Research Council (NSERC) of Canada for its financial support. Thanks are extended to Professor Carlos Brebbia, Director of Wessex Institute of Technology, Southampton UK for his constant encouragement to write this book. Dr Seyed Hossein Mousavizadegan deserves my special thanks for his assistance in drafting and designing some figures for this book. I also appreciate the assistance from Dr Adhi Susilo, Mr Rezaul Abid, Ms Rhonda Sutherland and Mrs Karen Conrod for helping me in drafting the figures contained in this book. It is my great pleasure to acknowledge the kind permission granted by Professor Hubert J. Davies to reproduce Figures 5.4, 5.16, 6.5 and 7.2 from Modern Fluid Dynamics, Vol. 1 by N.J. Curle and H.J. Davies, published by Van Nostrand, London in 1968 into my book Mechanics of Real Fluids. I am extremely thankful to Terry Edwards for sending me the formal letter of granting me permission to reproduce the figures 22, 23 and 29 from the book An Informal Introduction to Theoretical Fluid Mechanics by James Lighthill, Clarendon Press, Oxford, 1986 into my new book, Mechanics of Real Fluids by Matiur Rahman. It is my great pleasure to acknowledge the IMA and its Executive Director, Dr David Youdan for granting me permission to use the illustrations from Lighthill s book. The Oxford University Press (the Clarendon Press) deserves my grateful appreciation for allowing me free permission to use some of the materials and illustrations contained in this book. Special thanks are given to Ms Shelagh Phillips, permission assistant at the Oxford University Press for taking troubles to verify the material included in this book and for her kind permission to use them in my new book. I am also grateful to the referees for their kind reviews which help improve the contents of the book considerably, and to WIT Press, Southampton UK for their efforts in producing such a superb book.

WITPRESS WIT Press publishes leading books in Science and Technology. Visit our website for the current list of titles.

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