Physics
Fluids
Fluids are substances that flow and take the shape of their container. They include liquids and gases, and their behavior is described by fluid dynamics. Key properties of fluids include density, viscosity, and pressure. Fluids play a crucial role in various physical phenomena, such as hydrodynamics, aerodynamics, and buoyancy.
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9 Key excerpts on "Fluids"
- Michael Clifford, Kathy Simmons, Philip Shipway(Authors)
- 2009(Publication Date)
- CRC Press(Publisher)
Fluids (the generic name given to both liquids and gases) are all around us – the air we breathe, the water in lakes, rivers and the sea, the liquids we drink. They are a familiar part of everyday life and the way in which Fluids behave is understood by everyday experiences. For instance, liquids have a defined volume but take the shape of the container they are in; a kilogram of water generally has a fixed volume but it can be confined inside a bottle or spread out over a large area if spilled on the floor. Some Fluids flow more easily than others: water is easy to pour, but syrup moves more slowly. Fluids that are moving create forces on solid objects: the umbrella blows inside out on a windy day! In engineering it is important to understand the behaviour of Fluids in a scientific way, to be able to quantify the effects so that calculations can be undertaken to design systems involving Fluids and to predict their performance. For instance, if water needs to be pumped 25 metres uphill at a rate of 10 litres per second, how big does the pump have to be and how much energy is needed to pump the water? Could the system be designed to be more sustainable and use less energy? If an oil storage tank needs to be built, how thick will the walls need to be? How much force does the oil exert on the walls? In this chapter the behaviour of Fluids will be explained in a mathematical way and tools will be introduced to enable design calculations to be performed. These will be related to some of the everyday experiences that we have of Fluids to help understanding. The chapter is divided into five main sections. Following the introduction in Section 3.1, Section 3.2 looks at fluid statics. The concept of pressure is described, showing how to calculate the forces created by the pressure of a fluid on a solid surface and how pressure can be measured using a manometer. The concept of buoyancy and why some things float and others sink is explained.
- Hozefa Ebrahim, David Ashton-Cleary(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
Fluids possess no rigidity and change their shape to fill the container into which they are poured. Fluids are conventionally classified as either liquids or gases. The most important difference between these two types of fluid lies in their relative compressibility: gases can be compressed much more easily than liquids. Consequently, any intervention that involves significant pressure variations is generally accompan- ied by much larger changes in density in the case of a gas than in the case of a liquid. The similar physical behaviours of gases and liquids in motion have led to the development of the science of fluid mechanics. Fluid mechanics is premised on three major assumptions: 1. Fluids are isotropic media: i.e. the physical properties are independent of direction through the fluid. 2. Fluids are regarded as Newtonian: i.e. there is a linear relationship between the local rate of change of the shape of a body and the force applied to it, as first postulated by Newton. In reality, many of the common ‘Fluids’ we use on a daily basis do not follow these assumptions and are described as non-Newtonian. 3. The macroscopic motion of ordinary Fluids is well described by Newtonian dynamics; the effects dictated by quantum mechanics and general relativity can be safely ignored. 53 Are powders Fluids? A powder is a substance that can flow, made up of microscopic grains. A traditional fluid can also be considered to have (sub-)microscopic grains. However, the larger the grain, the greater the propensity to flow in a non-linear manner, thus not behaving like an ideal fluid. Viscosity Viscosity is the property of a fluid that causes it to resist flow; it refers to the stickiness of a fluid. When comparing the flow of water through a tube with a more viscous substance such as honey, water flows faster. The velocities of the adjacent layers of the fluid differ, and a ‘slip’ occurs between parallel layers as a result of the shear forces acting between them.
- Jonathan Wickert, Jonathan Wickert, Kemper Lewis(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 6.2 Properties of Fluids 223 design golf balls capable of longer flight and to help ski jumpers, racing cyclists, marathon runners, and other athletes improve their performances. ▸ ▸ 6.2 Properties of Fluids A lthough you already have some intuition as to the behavior and properties of Fluids in everyday situations, we begin this chapter with a seemingly simple question: From an engineering standpoint, what exactly is a fluid? Scientists categorize compositions of matter in different ways. A chemist classifies materials according to their atomic and chemical structures in the context of the periodic table of elements. An electrical engineer might group materials according to the manner in which they respond to electricity— as conductors, insulators, or semiconductors. Mechanical engineers often categorize substances as being either solids or Fluids. The technical distinction between the two centers on how they behave when forces are applied to them. In Chapter 5, we saw how the behavior of a solid material is described by a stress–strain curve. A rod that is made of an elastic solid will satisfy Hooke’s law, Equation (5.4), and its elongation is proportional to the force acting on it. When a tension, compression, or shear force is applied to a solid object, it usually deforms by a small amount. As long as the yield stress has not been reached, a solid material springs back to its original shape once the force has been removed.
eBook - PDF- Joseph Katz(Author)
- 2010(Publication Date)
- Cambridge University Press(Publisher)
1.5 Properties of Fluids Fluids, in general, may have many properties related to thermodynamics, mechan-ics, or other fields of science. In the following subsections, only a few, which are used in introductory fluid mechanics, are mentioned. 1.5.1 Density Density, by definition, is mass per unit volume. In the case of Fluids, we can define the density (with the aid of Fig. 1.3) as the limit of this ratio when a measuring volume V shrinks to zero. We need to use this definition because density can change from one point to the other. Also in this picture, we can relate to a volume element in space that we can call “control volume,” which moves with the fluid or can be stationary (in any case it is better to place this control volume in inertial frames of reference). Therefore the definition of density at a point is ρ = lim V → 0 m V . (1.5) Typical units are kilograms per cubic meter (kg/m 3 ) or grams per cubic centimeter (g/cm 3 ). m V Control volume Figure 1.3. Mass m in a control volume V . Density is the ratio of m / V . 8 Basic Concepts and Fluid Properties dS n Figure 1.4. Pressure acts normal to the surface dS ( n is the unit vector normal to the surface). 1.5.2 Pressure We can describe the pressure p as the normal force F per unit area acting on a surface S . Again, we use the limit process to define pressure at a point, as it may vary on a surface: p = lim S → 0 F S . (1.6) Bernoulli pictured the pressure as being a result of molecules impinging on a surface (so this force per area is a result of the continuous bombardment of the molecules). Therefore, the fluid pressure acting on a solid surface is normal to the surface, as shown in Fig. 1.4. Consequently the force direction is obtained by multi-plying with the unit vector n normal to the surface. Because the pressure acts normal to a surface the resulting F force is F = − p n ds .
eBook - PDF- Michael Tammaro(Author)
- 2019(Publication Date)
- Wiley(Publisher)
310 Properties of Fluids and Solids 12 This sailing yacht is competing in a round-the-world race. Sailing—perhaps better than any other activity—captures the physical principles discussed here in Chapter 12: buoyant forces keep the yacht afloat; air flow across the sails generates the power; extreme forces create stresses and strains in the rigging and hull; the viscosity of the ocean water con- tributes to drag forces on the hull; and in high-tech yachts, like the Volvo Open 70 shown here, ballast in the form of water is pumped from one side to the other through a series of hoses and pipes. © Bluegreen Pictures/Alamy States of Matter and Density | 311 12.1 Solve problems dealing with the concept of density. A sample of matter may be characterized by its mass, which may be measured in units of kilograms (kg). This sample of matter occupies a certain volume, which may be meas- ured in units of cubic meters (m 3 ). The mass per unit volume (whose SI units are kg m 3 / ) is called density. Ice and water are made of the same molecules (H 2 O), so 1 kg of water contains the same number of molecules as 1 kg of ice. Ice and water, however, are different states of matter. As it turns out, they have different densities, too. Much of Chapter 12 is devoted to the study of liquids, in which density is a central theme. States of Matter Matter is made of atoms and molecules (these two terms have different meanings, but they are sometimes used interchangeably when describing matter in general terms). The forces between the atoms, as well as their physical arrangement, determine which of the three familiar states of matter—solid, liquid, or gas—is formed under the prevailing conditions of temperature and pressure. Here we briefly describe each of the three states of matter. Solids: In solids, the atoms and molecules are closely packed and the bonds between them are strong enough so that they cannot move freely, but vibrate instead about their equilibrium positions.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Here are four assumptions that we make about our ideal fluid; they all are concerned with flow: 1. Steady flow In steady (or laminar) flow, the velocity of the moving fluid at any fixed point does not change with time. The gentle flow of water near the center of a quiet stream is steady; the flow in a chain of rapids is not. Figure 14.6.1 shows a transition from steady flow to nonsteady (or nonlaminar or turbulent) flow for a rising stream of smoke. The speed of the smoke par- ticles increases as they rise and, at a certain critical speed, the flow changes from steady to nonsteady. 2. Incompressible flow We assume, as for Fluids at rest, that our ideal fluid is incompressible; that is, its density has a constant, uniform value. Figure 14.6.1 At a certain point, the rising flow of smoke and heated gas changes from steady to turbulent. Will McIntyre/Science Source 3. Nonviscous flow Roughly speaking, the viscosity of a fluid is a measure of how resistive the fluid is to flow. For example, thick honey is more resistive to flow than water, and so honey is said to be more viscous than water. Viscosity is the fluid analog of friction between solids; both are mechanisms by which the kinetic energy of moving objects can be transferred to thermal energy. In the absence of friction, a block could glide at constant speed along a horizontal surface. In the same way, an object moving through a nonviscous fluid would experience no viscous drag force—that is, no resistive force due to viscosity; it could move at constant speed through the fluid. The British scientist Lord Rayleigh noted that in an ideal fluid a ship’s propeller would not work, but, on the other hand, in an ideal fluid a ship (once set into motion) would not need a propeller! 4. Irrotational flow Although it need not concern us further, we also assume that the flow is irrotational. To test for this property, let a tiny grain of dust move with the fluid.
eBook - PDFBasic Aerodynamics
Incompressible Flow
- Gary A. Flandro, Howard M. McMahon, Robert L. Roach(Authors)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
A proper set of approximations may make unnecessary the use of some laws to produce a practical yet accurate solution for a given problem. This approach is possible only if a clear understanding of the physics of a fluid motion is attained. It is, of course, possible to construct a mathematically correct solution to an incorrectly formulated problem or a solution that is based 16 Physics of Fluids on an inappropriate set of assumptions. In such cases, the results can be confusing or misleading or can even lead to costly mistakes. There is no substitute in aero- dynamics for a sound understanding of the fundamental physical laws on which fluid mechanics is based. It also may be necessary to introduce additional mathematical models or relationships to supplement those in the preceding list. For example, it often is necessary to use equations that characterize a working fluid. These equations of state, or constitutive equations, describe the physical attributes of a fluid. For example, it often is the case in practical problems that the fluid is an ideal or perfect gas, for which the equation of state is: p = ρRT, (2.1) which is a special relationship among the thermodynamic-state variables, pressure p, density ρ, and temperature T, needed to describe the behavior of a gas. R is the gas constant—a constant of proportionality—that is determined by the molecular con- figuration of the gas. This and other equations are discussed in considerable detail as needed throughout this chapter. With regard to details of the molecular structure of a working fluid, we can choose to approach problems from the standpoint of the molecular motion, or a continuum model can be applied that does not attempt to address directly the actual small-scale particle motion. The former is called statistical mechanics, or the kinetic theory of gases.
eBook - PDF- Donald F. Elger, Barbara A. LeBret, Clayton T. Crowe, John A. Robertson(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
FIGURE 2.1 This photo shows engineers observing a flume, which is an artificial channel for conveying water. This flume is used to study sediment transport in rivers. (Photo courtesy of Professor Ralph Budwig of the Center for Ecohydraulics Research, University of Idaho.) Fluid Properties CHAPTER ROAD MAP This chapter introduces ideas for idealizing real-world problems, introduces fluid properties, and presents the viscosity equation. CHAPTERTWO LEARNING OUTCOMES SYSTEM, STATE, AND PROPERTY (§2.1). ● Define system, boundary, surroundings, state, steady state, process, and property. FINDING FLUID PROPERTIES (§2.2). ● Look up appropriate values of fluid properties and document your work. ● Define each of the eight common fluid properties. DENSITY TOPICS (§2.3). ● Know the main ideas about specific gravity. ● Explain the constant density assumption and make decisions about whether or not this assumption is valid. ● Determine changes in the density of water corresponding to a pressure change or a temperature change. STRESS (§2.4). ● Define stress, pressure, and shear stress. ● Explain how to relate stress and force. ● Describe each of the seven common fluid forces. THE VISCOSITY EQUATION (§2.5). ● Define the velocity gradient and find values of the velocity gradient. ● Describe the no-slip condition. ● Explain the main ideas of the viscosity equation. ● Solve problems that involve the viscosity equation. ● Describe a Newtonian and non-Newtonian fluid. SURFACE TENSION (§2.6). ● Know the main ideas about surface tension. ● Solve problems that involve surface tension. VAPOR PRESSURE (§2.7). ● Explain the main ideas of the vapor pressure curve. ● Find the pressure at which water will boil. 32 System, State, and Property 33 2.1 System, State, and Property The vocabulary introduced in this section is useful for solving problems. In particular, these ideas allow engineers to describe problems in ways that are precise and concrete.
eBook - PDFComparative Biomechanics
Life's Physical World - Second Edition
- Steven Vogel(Author)
- 2013(Publication Date)
- Princeton University Press(Publisher)
P A R T T W O We begin with a look at the forms of matter that matter, in particular at the two kinds that flow. We end up recognizing how gases and liquids differ greatly in their behavior when stationary, yet how similar are the rules describing their flow. Along the way we explore a host of physical phenomena whose counterintuitive character is surpassed only by the remarkable ways organisms contend with them. Fluids 53 C H A P T E R 4 if we’re to talk about gases and liquids, we need to distinguish them from solids as well as from each other. One might begin by pointing out an odd peculiarity of this tripartite division of our world. For a large number of situa-tions, gases and liquids are lumped under the heading of “Fluids.” That recognizes lovely similarities in their behavior. Solids are something else again. Intermediates between solids and liquids abound, from ordinary house paint to your inter-vertebral disks. Intermediates between liquids and gases, though, are not house-hold items. (Although sometimes “dissolved” is regarded as a separate state.) States of Matter How might we best encapsulate the differences among the states of matter? One can base an explanation on molecular behavior, but that’s not usefully intuitive. What matters here is how the states of matter differ in behavior—in particular, how they react to applied forces or, more realistically, stresses. Table 4.1 summarizes the distinctions. Solids A solid resists compression, tension, and shear. No matter what you do to a solid, it fights back. As a rule, the more vigorously you torment it, the more it deforms, but the relationship isn’t necessarily linear—doubling the stress needn’t exactly double the strain, whatever you may have heard previously. Liquids A liquid resists compression and tension—squeezing and stretching. It’s very hard to squeeze a liquid—squeeze it in a chamber and it will be forced out of Gases and Liquids: Fluids at Rest Water taken in moderation cannot hurt anybody.
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