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part1.doc(440k) - Physics

    STUDY OF FLUID DYNAMICS WITH EMPHASIS ON

    COUETTE FLOW AND TAYLOR VORTICES

    Project Report submitted to the

    FERGUSSON COLLEGE

    In Partial Fulfillment for the Degree of

    BACHELOR OF SCIENCE

    IN

    PHYSICS

    BY

    ABHAY MADHUSUDAN KARNATAKI

    (Exam No. 01038)

    T. Y. B. Sc.

    DEPARTMENT OF PHYSICS

    FERGUSSON COLLEGE,

     PUNE.

    FERGUSSON COLLEGE, PUNE 411004.

This is to certify that

    Mr. Abhay Madhusudan Karnataki

    (Exam No. 01038) has satisfactorily completed the project entitled

    STUDY OF FLUID DYNAMICS WITH EMPHASIS ON

    COUETTE FLOW AND TAYLOR VORTICES

    under our guidance in partial fulfillment of Degree of Bachelor of Science

    (B. Sc.) in Physics as per described by University of Pune during the

    academic year 2000-2001.

A. G. Banpurkar Dr. Mrs. R. Joshi

    External Guide Internal Guide

Prof. D. M. Kulkarni Prof. P. T. Purandare

    Project Incharge Head, Department of Physics,

    Fergusson College, Pune.

    To the people who shaped my life,

    my Father, Mother and Brother and

    my beloved teacher Prof. M. Prakash…

    Contents

    Preface Ii Acknowledgements iv 1. What is fluid dynamics? 1 2. Toolkit of fluid dynamics 3 3. Fluid dynamics in daily life 13 4. Canned rolls 21

    4.1 Introduction 22

    4.2 While I was canning the rolls 22

    4.3 Observations 26

    4.4 Theoretical explanations 27

    4.4.1 Argument for inviscid fluids 27

    4.4.2 Plane and circular couette flow 29

    4.4.3 Computer generated trajectory 34

    4.4.4 Linear stability theory 37

    4.5 Other interesting things observed during the project work 38

    4.6 Recent work and further prospects 42

    4.7 Applications 43 5. Playing with soap films 45 6. My hero - Sir G. I. Taylor 58 Conclusions 61 Appendices:

    A. „C‟ program for the particle trajectories 62

    B. Accreting on stars 64

    C. Fascinating BZ reaction 68

    D. More about Navier Stokes equations 73

    E. Some useful properties of common fluids 75 Suggestions for further reading 76

    Acknowledgements

     I express my sincere thanks to my external guide Shri. Arun Banpurkar for taking out time from his busy schedule for Ph.D. thesis and helping me in all possible ways. Many of the ideas in this project report originated and took a better shape in number of exciting discussions with him. Arun sir essentially taught me how to set up an experiment right from the scratch and how to optimize it for the best results with patience and efforts.

    I thank my internal guide Dr. Mrs. R. S. Joshi for always showing me the right direction to proceed and for many important suggestions.

     It gives me a great pleasure to thank Dr. A. V. Limaye for helpful discussions at several stages, which enabled me to analyze various situations in minute details.

     I take this opportunity to express my thanks to Prof. S. B. Ogale for providing me a strong impetus for hard work and future studies. I thank Dr. K. P. Adhi for raising many important questions and for providing many suggestions. I thank all the members of CLAMP; Dr. S. I. Patil, Dr. Bathe, Narhe, Khandkar, Mandar and Sadakale for making the working environment so wonderful and for their help while performing the experiments.

     I thank Dr. Tapas K. Das from IUCAA for explaining me in detail the analysis of the process of accretion of matter on stars and related things.

     I thank the Heads of Departments of physics in Pune University and Fergusson College for providing me the required infrastructure facilities. I thank the Center for Non-Linear Dynamics, Texas, for sending me the reprints of some research papers on Taylor vortices. I thank our elderly lab assistant Haribhau for his numerous suggestions while making the apparatus.

     I thank my friend Sandeep for many exciting discussions on mathematics and especially for his book “What is mathematics?”, which provided me lot of things on soap

    films. I thank all my friends from chemistry discipline, Ashutosh, Devayani, Sarita and Mrunalini and teachers from department of Chemistry, Pune University, Dr. Avinash Kumbhar and Dr. P. K. Choudhary for their patient help in flow visualization methods and the fascinating BZ reaction. Special thanks to Ashutosh for always sharing the excitement in science.

     Because this project work marks the end of my undergraduate studies and as this work was motivated from my previous studies in Physics, I would like to express my thanks to all those who helped me in the last three years. First of all I would like to thank Desai sir from Exploratory for giving me a free access to the lab facilities there, in these

    years. I thank all the staff members of Physics department in Fergusson College for making the studies of physics so joyful and exciting. Special thanks to Ogale madam, Alawani madam and Dabhade madam for always encouraging me for extra curricular activities. I also thank my Mathematics teachers, especially Acharya sir, Kulkarni sir for always promoting my interest in Mathematics. I also thank my teachers from Electronics department, especially Bhide sir and Khedkar sir for introducing me to the excitement in the world of Electronics. I would like to thank the Professors in Pune University Physics Department Prof.. P. V. Panat, Prof. A.W. Joshi, Dr. C. V. Dharmadhikari, Dr. Mrs. A. Kshirsagar, Dr. R. K. Pathak for their ever-welcoming nature towards my Physics queries.

     My special thanks to Dr. A. D. Gangal from Pune University Physics Department for his guidance for reading extra curricular books and for resolving many mysteries of fluid dynamics as well as other topics in “Feynman Lectures on Physics” in a delightful

    manner. He introduced me the joy of theoretical physics in the simplest ways.

     My special thanks also to Dr. S. V. Dhurandhar for teaching me the concepts of special and general theory of relativity in an elegant manner; which has given me a deep satisfaction of studying physics.

    I thank all my seniors who have become my close friends, Deepti, Harshad, Prasad, Anuradha, Aditi, Subhangi, Anamica, Aparajita, Deepanjan and Devraj. They have always shown me the opportunities lying ahead and have provided the glimpses of advanced physics. I thank all my friends in colleges for sharing the joy of doing physics. Special thanks to Priya, Vidyut, Sonal, Shashank, Sourabh, Abhijit, Maitreyi, Sheetal, Sulbha and Rahul for many exciting discussions. I thank my chess friends Nivedita, Ashwini and Udayan among others and my chess teacher Mr. Joseph D‟ Souza for keeping the chess player in me alive, which was essential for my good studies after leaving chess as a profession. My special thanks to my friend Sneha for her constant support and encouragement, and to Abhijit and Virendra for always being with me in my adventures.

     Last but not the least; I thank my friend Vaibhav for helping me in typing this project report and also for being a “test student”! Some names might have got omitted in

    this acknowledgement by mistake, and the people I have mentioned here have helped me in ways more than I can describe in words. I thank them all with all my sincerity.

    Abhay Karnataki.

    Chapter 1

    What Is Fluid Dynamics?

    We think of fluids as liquids or gases only. But the subject of fluid dynamics deals with a large class of systems, than just liquids and gases e.g. Certain types of glasses can be thought of as a fluid. Why? Because they „yield‟ under the force of gravity. Glasses of windows in old houses are found to be thicker at bottom than at the top. Because glass can flow! So fluids are those substances which cannot stand the

    shearing forces.

     If you push the upper end of a thick book resting on a table, then its pages tilt its

    pages slide upon each other. This is the action of shearing stress. Similarly, layers of fluid slide over each other. Now, sliding involves friction, and this sideways force between two layers is called the Viscous Force. The measure of amount of viscous force is the property of fluids called Viscosity. More the viscosity of the fluid, more is the viscous force. And this viscous force stops the fluid layers from moving indefinitely on each other. But, if the external force is applied for sufficiently long time, the fluid layers have to move.

     Thus, we may define fluid as, “A Fluid is matter in a readily distortable form,

    so that the smallest external unbalanced force on it causes an infinite change of shape, if applied for a time long enough” [1].

     Fluid dynamics can be used to model a variety of physical situations. E.g. it can be used to study the rate of accretion of stellar dust on a star moving through dust clouds in a galaxy. Now, such dust particles may be a few kilometers apart, but on astronomical scales, we can still treat the dust cloud as a „Continuous Fluid‟. Fluid dynamics may also be used to model traffic on Mumbai Pune Highway. Here the particles would be of different types, corresponding to various vehicles. But as a whole, they can be treated as a fluid.

     But for all purposes, we may study fluid dynamics with the working fluid as water, to start with. Water can be used as a standard example of fluid while studying fluid dynamics.

     Having understood what a fluid is, we should try to see what are the various physical parameters associated with the flows of the fluid. It is the wide diversity of these parameters that makes the subject of fluid dynamics so challenging and difficult to understand.

     Simplest parameters we can think of are, density of fluid and velocity of fluid at each point inside the fluid. Also, from Bernoulli‟s Equation and Hydrostatistics we are familiar with pressure at a point in fluid, which is another important parameter. In addition there can be external forces imposed by the experimenter using pumps, or the forces like gravity, which are omnipresent. On a free surface of a fluid, e.g. on the surface of flow in river, surface tension will be an important factor. To complicate the matter, there may be charges present in the fluid e.g. ions of some chemicals. Moving charges constitute current. So if external electric and magnetic fields are applied, the charges will interact with them. Also, internally the charges will interact with each other. This will modify the flow of the fluid. These branches of study are called „Magnetohydrodynamics‟, and „Electrohydrodynamics‟. To further complicate the things, there may be temperature gradient inside the fluid, which will in turn induce density variations in the fluid. There may be density variations produced due to impinging sound waves [2].

     Fluid Dynamics is the study of fluid under these various conditions put separately or together. It also deals with interaction of fluid with solid boundaries near it, e.g. body of a fish swimming in a pool of water.

     This may give an impression that Fluid Dynamics is a hopelessly complicated subject. But in many real life situations only few parameters are of prime importance. And over the centuries, physicists, with their unique insight into natural phenomena, have discovered many rules that regulate the fluids.

    References:

    1. A textbook of fluid dynamics- Fransis.

    2. The Feyman Lectures on Physics -Vol. II

    Chapter 2

    Toolkit of Fluid Dynamics

    So, how do we formulate a theory of such a nasty fluid, flowing with its own will - so to say, interacting with so many physical parameters?

    The central and starting assumption of fluid dynamics is that we can describe the fluid under the consideration as a continuous medium. This is the so-called “Continuum

    Hypothesis ”.

    But we know that there is even in case of water, a lot of void space between two molecules of water. So how can we possibly treat water or any other practical fluid as continuous?

    The point is that we will be observing phenomena, which are sufficiently macroscopic in nature. And „sufficiently macroscopic‟ means that the characteristic length scales over which the flow is examined, are larger than the mean free path of the molecular collisions. E.g. we might be interested in knowing how the presence of an obstacle like an airplane causes change in the flow velocity of the air near it. Now, these changes occur with typical length scales of a few meters, and mean free path of air molecules may be of a few millimeters. Or we may put some obstacle like cylinder in flowing water. Here flow patterns change over a few millimeters, but the mean free path of water molecules is a few micrometers. So, the fluid in consideration can be treated as continuous. Similarly, the dust particles accreting on stars, which are a few kilometers apart, as in case of upper atmosphere of earth, can be treated as continuous medium over the length scales of tens of lakhs of kilometers.

    So, when I say that "consider a fluid element of volume dV", I mean a cube of length greater than the mean free path of fluid under consideration, although mathematically we treat dV as an infinitesimal element.

    When I say that "a fluid particle" has moved from A to B, I mean the ensemble of particles in a fluid element of volume dV situated at A has moved, on an average, to B. Or the " fluid particle" has a velocity V at (x, y, z), I mean the ensemble of particles in a

    fluid element of volume dV at (x, y, z) has, on an average, the velocityV.

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