# Fluid mechanics Lecture notes

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FLUID MECHANICS FOR CIVIL ENGINEERS Bruce Hunt Department of Civil Engineering University Of Canterbury Christchurch, New Zealand  Bruce Hunt, 1995Table of Contents Chapter 1 – Introduction ................................................... 1.1 Fluid Properties ........................................... 1.2 Flow Properties ............................................ 1.4 Review of Vector Calculus ................................... 1.9 Chapter 2 – The Equations of Fluid Motion .................................... 2.1 Continuity Equations ....................................... 2.1 Momentum Equations ...................................... 2.4 References ............................................... 2.9 Chapter 3 – Fluid Statics ................................................... 3.1 Pressure Variation.......................................... 3.1 Area Centroids ............................................ 3.6 Moments and Product of Inertia ............................... 3.8 Forces and Moments on Plane Areas ........................... 3.8 Forces and Moments on Curved Surfaces ...................... 3.14 Buoyancy Forces .......................................... 3.19 Stability of Floating Bodies ................................. 3.23 Rigid Body Fluid Acceleration ............................... 3.30 References .............................................. 3.36 Chapter 4 – Control Volume Methods ......................................... 4.1 Extensions for Control Volume Applications ................... 4.21 References .............................................. 4.27 Chapter 5 – Differential Equation Methods ..................................... 5.1 Chapter 6 – Irrotational Flow ................................................ 6.1 Circulation and the Velocity Potential Function .................. 6.1 Simplification of the Governing Equations ...................... 6.4 Basic Irrotational Flow Solutions .............................. 6.7 Stream Functions ......................................... 6.15 Flow Net Solutions ........................................ 6.20 Free Streamline Problems ................................... 6.28 Chapter 7 – Laminar and Turbulent Flow ...................................... 7.1 Laminar Flow Solutions ..................................... 7.1 Turbulence .............................................. 7.13 Turbulence Solutions ...................................... 7.18 References .............................................. 7.24 iPreface Fluid mechanics is a traditional cornerstone in the education of civil engineers. As numerous books on this subject suggest, it is possible to introduce fluid mechanics to students in many ways. This text is an outgrowth of lectures I have given to civil engineering students at the University of Canterbury during the past 24 years. It contains a blend of what most teachers would call basic fluid mechanics and applied hydraulics. Chapter 1 contains an introduction to fluid and flow properties together with a review of vector calculus in preparation for chapter 2, which contains a derivation of the governing equations of fluid motion. Chapter 3 covers the usual topics in fluid statics – pressure distributions, forces on plane and curved surfaces, stability of floating bodies and rigid body acceleration of fluids. Chapter 4 introduces the use of control volume equations for one-dimensional flow calculations. Chapter 5 gives an overview for the problem of solving partial differential equations for velocity and pressure distributions throughout a moving fluid and chapters 6–9 fill in the details of carrying out these calculations for irrotational flows, laminar and turbulent flows, boundary-layer flows, secondary flows and flows requiring the calculation of lift and drag forces. Chapter 10, which introduces dimensional analysis and model similitude, requires a solid grasp of chapters 1–9 if students are to understand and use effectively this very important tool for experimental work. Chapters 11–14 cover some traditionally important application areas in hydraulic engineering. Chapter 11 covers steady pipe flow, chapter 12 covers steady open channel flow, chapter 13 introduces the method of characteristics for solving waterhammer problems in unsteady pipe flow, and chapter 14 builds upon material in chapter 13 by using characteristics to attack the more difficult problem of unsteady flow in open channels. Throughout, I have tried to use mathematics, experimental evidence and worked examples to describe and explain the elements of fluid motion in some of the many different contexts encountered by civil engineers. The study of fluid mechanics requires a subtle blend of mathematics and physics that many students find difficult to master. Classes at Canterbury tend to be large and sometimes have as many as a hundred or more students. Mathematical skills among these students vary greatly, from the very able to mediocre to less than competent. As any teacher knows, this mixture of student backgrounds and skills presents a formidable challenge if students with both stronger and weaker backgrounds are all to obtain something of value from a course. My admittedly less than perfect approach to this dilemma has been to emphasize both physics and problem solving techniques. For this reason, mathematical development of the governing equations, which is started in Chapter 1 and completed in Chapter 2, is covered at the beginning of our first course without requiring the deeper understanding that would be expected of more advanced students. A companion volume containing a set of carefully chosen homework problems, together with corresponding solutions, is an important part of courses taught from this text. Most students can learn problem solving skills only by solving problems themselves, and I have a strongly held belief that this practice is greatly helped when students have access to problem solutions for checking their work and for obtaining help at difficult points in the solution process. A series of laboratory experiments is also helpful. However, courses at Canterbury do not have time to include a large amount of experimental work. For this reason, I usually supplement material in this text with several of Hunter Rouse's beautifully made fluid-mechanics films. vThis book could not have been written without the direct and indirect contributions of a great many people. Most of these people are part of the historical development of our present-day knowledge of fluid mechanics and are too numerous to name. Others have been my teachers, students and colleagues over a period of more than 30 years of studying and teaching fluid mechanics. Undoubtedly the most influential of these people has been my former teacher, Hunter Rouse. However, more immediate debts of gratitude are owed to Mrs Pat Roberts, who not only encouraged me to write the book but who also typed the final result, to Mrs Val Grey, who drew the large number of figures, and to Dr R H Spigel, whose constructive criticism improved the first draft in a number of places. Finally, I would like to dedicate this book to the memory of my son, Steve. Bruce Hunt Christchurch New Zealand viChapter 1 Introduction A fluid is usually defined as a material in which movement occurs continuously under the application of a tangential shear stress. A simple example is shown in Figure 1.1, in which a timber board floats on a reservoir of water. Figure 1.1 Use of a floating board to apply shear stress to a reservoir surface. If a force, F, is applied to one end of the board, then the board transmits a tangential shear stress, , to the reservoir surface. The board and the water beneath will continue to move as long as F and  are nonzero, which means that water satisfies the definition of a fluid. Air is another fluid that is commonly encountered in civil engineering applications, but many liquids and gases are obviously included in this definition as well. You are studying fluid mechanics because fluids are an important part of many problems that a civil engineer considers. Examples include water resource engineering, in which water must be delivered to consumers and disposed of after use, water power engineering, in which water is used to generate electric power, flood control and drainage, in which flooding and excess water are controlled to protect lives and property, structural engineering, in which wind and water create forces on structures, and environmental engineering, in which an understanding of fluid motion is a prerequisite for the control and solution of water and air pollution problems. Any serious study of fluid motion uses mathematics to model the fluid. Invariably there are numerous approximations that are made in this process. One of the most fundamental of these approximations is the assumption of a continuum. We will let fluid and flow properties such as mass density, pressure and velocity be continuous functions of the spacial coordinates. This makes it possible for us to differentiate and integrate these functions. However an actual fluid is composed of discrete molecules and, therefore, is not a continuum. Thus, we can only expect good agreement between theory and experiment when the experiment has linear dimensions that are very large compared to the spacing between molecules. In upper portions of the atmosphere, where air molecules are relatively far apart, this approximation can place serious limitations on the use of mathematical models. Another example, more relevant to civil engineering, concerns the use of rain gauges for measuring the depth of rain falling on a catchment. A gauge can give an accurate estimate only if its diameter is very large compared to the horizontal spacing between rain droplets. Furthermore, at a much larger scale, the spacing between rain gauges must be small compared to the spacing between rain clouds. Fortunately, the assumption of a continuum is not usually a serious limitation in most civil engineering problems.1.2 Chapter 1 — Introduction Fluid Properties 3 The mass density, , is the fluid mass per unit volume and has units of kg/m . Mass density is a function of both temperature and the particular fluid under consideration. Most applications considered herein will assume that  is constant. However, incompressible fluid motion can occur in which  changes throughout a flow. For example, in a problem involving both fresh and salt water, a fluid element will retain the same constant value for  as it moves with the flow. However, different fluid elements with different proportions of fresh and salt water will have different values for , and  will not have the same constant value throughout the flow. Values of  for some different fluids and temperatures are given in the appendix. 2 The dynamic viscosity, µ, has units of kg/(m s)  N  s/m and is the constant of proportionality between a shear stress and a rate of deformation. In a Newtonian fluid, µ is a function only of the temperature and the particular fluid under consideration. The problem of relating viscous stresses to rates of fluid deformation is relatively difficult, and this is one of the few places where we will substitute a bit of hand waving for mathematical and physical logic. If the fluid velocity, u, depends only upon a single coordinate, y, measured normal to u, as shown in Figure 1.2, then the shear stress acting on a plane normal to the direction of y is given by du   µ (1.1) dy Later in the course it will be shown that the velocity in the water beneath the board in Figure 1.1 varies linearly from a value of zero on the reservoir bottom to the board velocity where the water is in contact with the board. Together with Equation (1.1) these considerations show that the shear stress, , in the fluid (and on the board surface) is a constant that is directly proportional to the board velocity and inversely proportional to the reservoir depth. The constant of proportionality is µ. In many problems it is more convenient to use the definition of kinematic viscosity   µ/ (1.2) Figure 1.2 A velocity field in 2 which u changes only with the in which the kinematic viscosity, , has units of m /s. coordinate measured normal to Values of µ and  for some different fluids and the direction of u. temperatures are given in the appendix. A Newton, N, is a derived unit that is related to a kg through Newton's second law, F ma. 2 Thus, N kg m/s .Chapter 1 — Introduction 1.3 2 Surface tension, , has units of N/m  kg/s and is a force per unit arc length created on an interface between two immiscible fluids as a result of molecular attraction. For example, at an air-water interface the greater mass of water molecules causes water molecules near and on the interface to be attracted toward each other with greater forces than the forces of attraction between water and air molecules. The result is that any curved portion of the interface acts as though it is covered with a thin membrane that has a tensile stress . Surface tension allows a needle to be floated on a free surface of water or an insect to land on a water surface without getting wet. For an example, if we equate horizontal pressure and surface tension forces on half of the spherical rain droplet shown in Figure 1.3, we obtain 2 pr  2r (1.3) in which p = pressure difference across the interface. This gives the following result for the pressure difference: Figure 1.3 Horizontal pressure and 2 p  (1.4) surface tension force acting on half r of a spherical rain droplet. If instead we consider an interface with the shape of a half circular cylinder, which would occur under a needle floating on a free surface or at a meniscus that forms when two parallel plates of glass are inserted into a reservoir of liquid, the corresponding force balance becomes p 2r  2 (1.5) which gives a pressure difference of  p  (1.6) r A more general relationship between p and  is given by 1 1 p    (1.7) r r 1 2 in which r and r are the two principal radii of curvature of the interface. Thus, (1.4) has 1 2 r  r rr while (1.6) has rr and  . From these examples we conclude that (a) 1 2 1 2 pressure differences increase as the interface radius of curvature decreases and (b) pressures are always greatest on the concave side of the curved interface. Thus, since water in a capillary tube has the concave side facing upward, water pressures beneath the meniscus are below atmospheric pressure. Values of  for some different liquids are given in the appendix. Finally, although it is not a fluid property, we will mention the “gravitational constant” or 2 g, . Both these terms are misnomers because “gravitational acceleration”, which has units of m/s1.4 Chapter 1 — Introduction g is not a constant and it is a particle acceleration only if gravitational attraction is the sole force acting on the particle. (Add a drag force, for example, and the particle acceleration is no longer g.) The definition of gM states that it is the proportionality factor between the mass, , and weight, W, of an object in the earth's gravitational field. W  Mg (1.8) Since the mass remains constant and W decreases as distance between the object and the centre of the earth increases, we see from (1.8) that g must also decrease with increasing distance from the earth's centre. At sea level g is given approximately by 2 g  9.81 m/s (1.9) which is sufficiently accurate for most civil engineering applications. Flow Properties Pressure, p, is a normal stress or force per unit area. If fluid is at rest or moves as a rigid body with no relative motion between fluid particles, then pressure is the only normal stress that exists in the fluid. If fluid particles move relative to each other, then the total normal stress is the sum of the pressure and normal viscous stresses. In this case pressure is the normal stress that would exist in the flow if the fluid had a zero viscosity. If the fluid motion is incompressible, the pressure is also the average value of the normal stresses on the three coordinate planes. 2 Pressure has units of N/m  Pa, and in fluid mechanics a positive pressure is defined to be a compressive stress. This sign convention is opposite to the one used in solid mechanics, where a tensile stress is defined to be positive. The reason for this convention is that most fluid pressures are compressive. However it is important to realize that tensile pressures can and do occur in fluids. For example, tensile stresses occur in a water column within a small diameter capillary tube as a result of surface tension. There is, however, a limit to the magnitude of negative pressure that a liquid can support without vaporizing. The vaporization pressure of a given liquid depends upon temperature, a fact that becomes apparent when it is realized that water vaporizes at atmospheric pressure when its temperature is raised to the boiling point. Pressure are always measured relative to some fixed datum. For example, absolute pressures are measured relative to the lowest pressure that can exist in a gas, which is the pressure in a perfect vacuum. Gage pressures are measured relative to atmospheric pressure at the location under consideration, a process which is implemented by setting atmospheric pressure equal to zero. Civil engineering problems almost always deal with pressure differences. In these cases, since adding or subtracting the same constant value to pressures does not change a pressure difference, the particular reference value that is used for pressure becomes immaterial. For this reason we will almost always use gage pressures. One exception occurs in the appendix, where water vapour pressures are given in kPa absolute. They could, however, be referenced to atmospheric pressure at sea level simply by subtracting from each pressure the vapour pressure for a temperature of 100(C (101.3 kPa).Chapter 1 — Introduction 1.5 If no shear stresses occur in a fluid, either because the fluid has no relative motion between particles or because the viscosity is zero, then it is a simple exercise to show that the normal stress acting on a surface does not change as the orientation of the surface changes. Consider, for example, an application of Newton's second law to the two-dimensional triangular element of fluid shown in Figure 1.4, in which the normal stresses  ,  and  have all been assumed x y n to have different magnitudes. Thus F  ma gives x x  2 2  y   x y cos  xya (1.10) x n x 2 in which a  acceleration component in the x direction. Since the triangle geometry gives x y cos  (1.11) 2 2 x y we obtain after inserting (1.11) in (1.10) for cos and dividing by y     xa  (1.12) x n x 2 Thus, letting x  0 gives    (1.13) x n A similar application of Newton's law in the y direction gives Figure 1.4 Normal stress forces    acting on a two-dimensional (1.14) y n triangular fluid element. Therefore, if no shear stresses occur, the normal stress acting on a surface does not change as the surface orientation changes. This result is not true for a viscous fluid motion that has finite tangential stresses. In this case, as stated previously, the pressure in an incompressible fluid equals the average value of the normal stresses on the three coordinate planes.1.6 Chapter 1 — Introduction Figure 1.5 The position vector, r, and pathline of a fluid particle. Let t = time and r (t)  x(t)i  y(t)j  x(t)k be the position vector of a moving fluid particle, as shown in Figure 1.5. Then the particle velocity is dr dx dy dz V   i  j  k (1.15) dt dt dt dt If we define the velocity components to be V  u i  v j  w k (1.16) then (1.15) and (1.16) give dx u  dt dy v  (1.17 a, b, c) dt dz w  dt If e = unit tangent to the particle pathline, then the geometry shown in Figure 1.6 allows us to t calculate s e dr r (tt ) r (t) t V     V e (1.18) t dt t t in which sV = arc length along the pathline and  ds /dt   V  particle speed. Thus, the velocity vector is tangent to the pathline as the particle moves through space.Chapter 1 — Introduction 1.7 Figure 1.6 Relationship between the position vector, arc length and unit tangent along a pathline. It is frequently helpful to view, at a particular value of t, the velocity vector field for a collection of fluid particles, as shown in Figure 1.7. Figure 1.7 The velocity field for a collection of fluid particles at one instant in time. In Figure 1.7 the lengths of the directed line segments are proportional to  V  V, and the line segments are tangent to the pathlines of each fluid particle at the instant shown. A streamline is a continuous curved line that, at each point, is tangent to the velocity vector V at a fixed value of t. The dashed line AB is a streamline, and, if dr = incremental displacement vector along AB, then V  dr (1.19) in which dr  dx i  dy j  dz k and is the scalar proportionality factor between  V and  dr. Multiplying the vector dr by the scalar does not change the direction of dr, and (1.19) merely requires that V and dr have the same direction. Thus, will generally be a function of position along the streamline. Equating corresponding vector components in (1.19) gives a set of differential equations that can be integrated to calculate streamlines.1.8 Chapter 1 — Introduction dx dy dz 1    (1.20) u v w There is no reason to calculate the parameter in applications of (1.20). Time, t, is treated as a constant in the integrations. Steady flow is flow in which the entire vector velocity field does not change with time. Then the streamline pattern will not change with time, and the pathline of any fluid particle coincides with the streamline passing through the particle. In other words, streamlines and pathlines coincide in steady flow. This will not be true for unsteady flow. The acceleration of a fluid particle is the first derivative of the velocity vector. dV a  (1.21) dt When V changes both its magnitude and direction along a curved path, it will have components both tangential and normal to the pathline. This result is easily seen by differentiating (1.18) to obtain de dV t a  e  V (1.22) t dt dt The geometry in Figure 1.8 shows that de e tt  e (t ) 1 s V t t t   e  e (1.23) n n dt t R t R in which R  radius of curvature of the pathline and e  unit normal to the pathline (directed n through the centre of curvature). Thus, (1.22) and (1.23) give 2 dV V a  e  e (1.24) t n dt R Equation (1.24) shows that a has a tangential component with a magnitude equal to dV /dt and 2 a normal component, V /R, that is directed through the centre of pathline curvature.Chapter 1 — Introduction 1.9 Figure 1.8 Unit tangent geometry along a pathline. e (tt )  e (t )  1 so that  e tt  e (t) 1 t t t t Review of Vector Calculus When a scalar or vector function depends upon only one independent variable, say t, then a derivative has the following definition: dF (t ) F (tt )  F (t )  as t  0 (1.25) dt t However, in almost all fluid mechanics problems p and V depend upon more than one independent variable, say x, y, z and t.x, y, z and t are independent if we can change the value of any one of these variables without affecting the value of the remaining variables. In this case, the limiting process can involve only one independent variable, and the remaining independent variables are treated as constants. This process is shown by using the following notation and definition for a partial derivative: F (x, y, z, t ) F (x, y  y, z, t )  F (x, y, z, t )  as y  0 (1.26) y y In practice, this means that we calculate a partial derivative with respect to y by differentiating with respect toyx while treating , z and t as constants. The above definition has at least two important implications. First, the order in which two partial derivatives are calculated will not matter. 2 2  F  F  (1.27) xy yx1.10 Chapter 1 — Introduction Second, integration of a partial derivative F (x, y, z, t )  G (x, y, z, t ) (1.28) y in which Gy is a specified or given function is carried out by integrating with respect to while treating x, z andtC as constants. However, the integration “constant”, , may be a function of the variables that are held constant in the integration process. For example, integration of (1.28) would give F (x, y, z, t )  G (x, y, z, t ) dy  C (x, z, t ) (1.29)  in which integration of the known function Gx is carried out by holding , z and t constant, and C (x, z, t ) is an unknown function that must be determined from additional equations. del There is a very useful definition of a differential operator known as :      i  j  k (1.30) x y z Despite the notation, del  is not a vector because it fails to satisfy all of the rules for vector algebra. Thus, operations such as dot and cross products cannot be derived from (1.30) but must be defined for each case. The operation known as the gradient is defined as      i  j  k (1.31) x y z in which is any scalar function. The gradient has several very useful properties that are easily proved with use of one form of a very general theorem known as the divergence theorem  d  e dS n (1.32)   S in which  is a volume enclosed by the surface S with an outward normal e . nChapter 1 — Introduction 1.11 Figure 1.9 Sketch used for derivation of Equation (1.32). A derivation of (1.32) is easily carried out for the rectangular prism shown in Figure 1.9. x 2   i d  i dx dy dz  i x , y,zdydz 2     x x x S 1 2 (1.33)  i x , y,zdydz 1  S 1 Since i is the outward normal on S and  i on S , (1.33) becomes 2 1  i d  e dS  e dS n n (1.34)    x S S 2 1 Similar results are obtained for the components of (1.31) in the jk and directions, and adding the three resulting equations together gives 6  d  e dS  e dS  n n (1.35)    i 1 S S i in which S is the sum of the six plane surfaces that bound . Finally, if a more general shape for  is subdivided into many small rectangular prisms, and if the equations for each prism are added together, then (1.32) results in which S is the external boundary of . (Contributions from the adjacent internal surfaces S and S cancel in the sum since but . )  e e i j i j n n i j One easy application of (1.32) is the calculation of the pressure force, F , on a tiny fluid p element. Since p = normal stress per unit area and is positive for compression, we calculate F  p e dS p n (1.36)  S1.12 Chapter 1 — Introduction However, use of (1.32) with p substituted for gives F  pd  p p (1.37)  Thus, p is the pressure force per unit volume acting on a tiny fluid element. Figure 1.10 A volume chosen for an application of (1.32) in which all surfaces are either parallel or normal to surfaces of constant . Further progress in the interpretation of  can be made by applying (1.32) to a tiny volume whose surfaces are all either parallel or normal to surfaces of constant , as shown in Figure 1.10. Since has the same distribution on S and S but e e , contributions from 3 4 n n 3 4 S and S cancel and we obtain 3 4   S e  S e 1 1 n 2 2 n (1.38) 1 2 But S  S and e e so that (1.38) becomes 1 2 n n 2 1   S e  1 n 1 2 (1.39) 1 Since   S n in which n = thickness of  in the direction perpendicular to surfaces of 1 constant , division of (1.39) by  gives  d 1 2   e  e (1.40) n n 1 1 n dnChapter 1 — Introduction 1.13 Thus,  has a magnitude equal to the maximum spacial derivative of and is perpendicular to surfaces of constant in the direction of increasing . Application of the preceding result to (1.37) shows that the pressure force per unit volume, p, has a magnitude equal to the maximum spacial derivative of p. (The derivative of p in the direction normal to surfaces of constant pressure.) Furthermore, because of the negative sign on the right side of (1.37), this pressure force is perpendicular to the surfaces of constant pressure and is in the direction of decreasing pressure. Finally, a simple application of (1.40) using the geometry shown in Figure 1.11 will be used to derive a relationship known as the directional derivative. Equation (1.40) applied to Figure 1.11 gives the result  d 1 3 ¯   e ˆ  e ˆ (1.41) n n dn n Figure 1.11 Geometry used for the calculation of the directional derivative. If e is a unit vector that makes an angle  with e , then dotting both sides of (1.41) with e t n t gives   1 3 1 3 e   e  e  cos (1.42) t t n n n However n  s cos, and (1.42) gives the result  d 1 3 e    (1.43) t s ds1.14 Chapter 1 — Introduction In words, (1.43) states that the derivative of with respect to arc length in any direction is calculated by dotting the gradient of with a unit vector in the given direction. Equation (1.43) has numerous applications in fluid mechanics, and we will use it for both control volume and differential analyses. One simple application will occur in the study of irrotational flow, when we will assume that the fluid velocity can be calculated from the gradient of a velocity potential function, . V   (1.44) Thus, (1.44) and (1.40) show that V is perpendicular to surfaces of constant and is in the direction of increasing . Since streamlines are tangent to V, this means that streamlines are perpendicular to surfaces of constant , as shown in Figure 1.12. If e is a unit vector in any t direction and s is arc length measured in the direction of e , then (1.44) and (1.43) give t d e  V  (1.45) t ds Thus, the component of V in any direction can be calculated by taking the derivative of in that direction. If e is tangent to a streamline, then d /ds is the velocity magnitude, V. If e is t t normal to a streamline, then d /ds  0 along this normal curve (which gives another proof that is constant along a curve perpendicular to the streamlines). If e makes any angle between 0 t and /2 with a streamline, then (1.45) allows us to calculate the component of V in the direction of e . t Figure 1.12 Streamlines and surfaces of constant potential for irrotational flow.Chapter 1 — Introduction 1.15 The divergence of a vector function is defined for (1.30) in a way that is analogous to the definition of the dot product of two vectors. For example, the divergence of V is    u v w  V  i  j  k  ui  vj  wk    (1.46) x y z x y z There is another definition we will make that allows  to be dotted from the left with a vector:       V    u i  vj  w k  i  j  k  u  v  w (1.47) x y z x y z Equations (1.46) and (1.47) are two entirely different results, and, since two vectors A and B must satisfy the law A B  B  A, we now see that  fails to satisfy one of the fundamental laws of vector algebra. Thus, as stated previously, results that hold for vector algebra cannot automatically be applied to manipulations with del. The definition (1.46) can be interpreted physically by making use of a second form of the divergence theorem:   V d  V  e dS n (1.48)   S in which  is a volume bounded externally by the closed surface S, e is the outward normal n on S and VV is any vector function. If is the fluid velocity vector, then V e gives the n component of V normal to S with a sign that is positive when V is out of  and negative when V is into . The product of this normal velocity component with dS gives a volumetric flow rate 3 with units of m /s. Thus, the right side of (1.48) is the net volumetric flow rate out through S since outflows are positive and inflows negative in calculating the sum represented by the surface integral. If (1.48) is applied to a small volume, then the divergence of V is given by 1   V  V  e dS n (1.49)   S Equation (1.49) shows that the divergence of V is the net volumetric outflow per unit volume through a small closed surface surrounding the point where   V is calculated. If the flow is incompressible, this net outflow must be zero and we obtain the “continuity” equation u v w   V     0 (1.50) x y z1.16 Chapter 1 — Introduction A derivation of (1.48) can be obtained by using Figure 1.9 to obtain x 2 u u d  dx dy dz  u x , y,zdydz  u x , y,zdydz (1.51) 2 1     x x x S S 1 2 1 But i e  1on S and i e 1on S since e is the outward normal. Thus, (1.51) n 2 n 1 n becomes u d  ui  e dS  ui e dS  ui  e dS n n n (1.52)     x S S S 2 1 in which use has been made of the fact that i e  0 on every side of the prism except S and S . n 1 2 Similar results can be obtained for v/yd and w /zd, and adding the resulting three   equations together gives u v w   d  ui  vj  wk  e dS n (1.53)   x y z S Equation (1.53) holds for arbitrary functions u,v and w and is clearly identical with (1.48). The extension to a more general volume is made in the same way that was outlined in the derivation of (1.32). In analogy with a cross product of two vectors the curl of a vector is defined in the following way: ij k w v w u   i   j        y z x z      × V    (1.54)   x y z   v u     k       x y u v w   If we let V be the fluid velocity vector, then a physical interpretation of (1.54) can be made with the use of Figure 1.13. Two line segments of length x and y are in a plane parallel to the x, y plane and have their initial locations shown with solid lines. An instant later these lines have rotated in the counterclockwise direction and have their locations shown with dashed lines. x The angular velocity of the line  in the k direction is v  v v 2 1 k  k  k (1.55) z x x

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