Skip to main content Accessibility help
×
  1. Home
  2. Browse subjects
  3. Engineering
  4. Thermal-fluids engineering
  5. Content listing

Refine search

Refine search

Access:
Content type:
Publication date:
Apply   From and To filters
Subject: Show more
Journals Show more
Publishers: Show more
Societies Show more
Series: Show more
Collections: Show more

Actions for selected content:

Select all | Deselect all
  • View selected items
  • Export citations
  • Download PDF (zip)
  • Save to Kindle
  • Save to Dropbox
  • Save to Google Drive

Save Search

You can save your searches here and later view and run them again in "My saved searches".

Please provide a title, maximum of 40 characters.
Cancel
×

5517 results in Thermal-fluids engineering

Page 235 of 276

  • Epilogue

  • Mohamed Gad-el-Hak, University of Notre Dame, Indiana
  • Book:
    Flow Control
    Published online:
    23 December 2009
    Print publication:
    15 August 2000, pp 359-362

  • Summary

    To be conscious that we are perceiving or thinking is to be conscious of our own existence.

    (Aristotle, 384–322 B.C.)

    Minds are like parachutes. They only function when they are open.

    (Sir James Dewar, 1842–1923)

    The field of flow control is broad, practically very important, and rich in scientific and technological challenges. Though the field is as old as man himself, its potential for improving our lives will keep it going strong for yet another millennium. In this book, I have made a modest attempt to place the field in a unifying framework and to categorize the different control strategies properly. At a minimum, I hope to have provided a useful navigation tool through the colossal literature in the field of flow control and its intricately related subfields such as control theory, chaos, microelectromechanical systems, soft computing, and so forth. Review articles were favored for citation because they themselves usually contain many useful citations.

    As is clear from all the previous chapters, there is no lack of flow-control methods to achieve a particular goal for free-shear as well as wall-bounded flows across the entire range of Mach and Reynolds numbers. Ranging from simple to complex, from inexpensive to expensive, from passive to active to reactive, and from market-ready to futuristic, a great variety of control devices are at the disposal of the fluids engineer. Flow control is most effective when applied near the transition or separation points; in other words, near the critical-flow regimes where flow instabilities magnify quickly.

  • 14 - Frontiers of Flow Control

  • Mohamed Gad-el-Hak, University of Notre Dame, Indiana
  • Book:
    Flow Control
    Published online:
    23 December 2009
    Print publication:
    15 August 2000, pp 318-358

  • Summary

    There is always an easy solution to every human problem-neat, plausible and wrong.

    (Henry Louis Mencken, 1880–1956)

    As for the future, your task is not to foresee, but to enable it.

    (Antoine-Marie-Roger de Saint-Exupéry, 1900–1944, in Citadelle (The Wisdom of the Sands)

    PROLOGUE

    In contrast to the early chapters, in the present chapter I will emphasize the frontiers of the field of flow control, pondering mostly the control of turbulent flows. I will review the important advances in the field that have taken place during the past few years and are anticipated to dominate progress in the future, essentially covering the fifth era outlined in Section 1.2. By comparison with laminar flow control or separation prevention, the control of turbulent flow remains a very challenging problem. Flow instabilities quickly magnify near critical flow regimes, and therefore delaying transition or separation is a relatively easier task. In contrast, classical control strategies are often ineffective for fully turbulent flows. Newer ideas for turbulent flow control to achieve skin-friction drag reduction, for example, focus on the direct onslaught on coherent structures. Spurred by the recent developments in chaos control, microfabrication, and soft computing tools, reactive control of turbulent flows through which sensors detect oncoming coherent structures and actuators attempt to modulate these quasi-periodic events favorably is now in the realm of the possible for future practical devices.

  • Preface

  • Norbert Peters
  • Book:
    Turbulent Combustion
    Published online:
    06 January 2010
    Print publication:
    15 August 2000, pp xi-xvi

  • Summary

    Fossil fuels remain the main source of energy for domestic heating, power generation, and transportation. Other energy sources such as solar and wind energy or nuclear energy still account for less than 20% of total energy consumption. Therefore combustion of fossil fuels, being humanity's oldest technology, remains a key technology today and for the foreseeable future. It is well known that combustion not only generates heat, which can be converted into power, but also produces pollutants such as oxides of nitrogen (NOx), soot, and unburnt hydrocarbons (HC). Ever more stringent regulations are forcing manufacturers of automotives and power plants to reduce pollutant emissions, for the sake of our environment. In addition, unavoidable emissions of CO2 are believed to contribute to global warming. These emissions will be reduced by improving the efficiency of the combustion process, thereby increasing fuel economy.

    In technical processes, combustion nearly always takes place within a turbulent rather than a laminar flow field. The reason for this is twofold: First, turbulence increases the mixing processes and thereby enhances combustion. Second, combustion releases heat and thereby generates flow instability by buoyancy and gas expansion, which then enhances the transition to turbulence.

    This book addresses gaseous turbulent flows only. Although two-phase turbulent flows such as fuel sprays are also of much practical interest, they are omitted here, because their fundamentals are even less well understood than those of turbulent combustion. We also restrict ourselves to low Mach number flows, because high speed turbulent combustion is an area of its own, with practical applications in supersonic and hypersonic aviation only.

  • 2 - Premixed turbulent combustion

  • Norbert Peters
  • Book:
    Turbulent Combustion
    Published online:
    06 January 2010
    Print publication:
    15 August 2000, pp 66-169

  • Summary

    Introduction

    Premixed combustion requires that fuel and oxidizer be completely mixed before combustion is allowed to take place. Examples of practical applications are spark-ignition engines, lean-burn gas turbines, and household burners. In all three cases fuel and air are mixed before they enter into the combustion chamber. Such a premixing is only possible at sufficiently low temperatures where the chain-branching mechanism that drives the reaction chain in hydrogen and hydrocarbon oxidation is unable to compete with the effect of three-body chain-breaking reactions. Under such low temperature conditions combustion reactions are said to be “frozen.” At ambient pressures the crossover from chain-branching to chain-breaking happens when the temperature decreases to values lower than approximately 1,000K for hydrogen flames or lower than approximately 1,300K for hydrocarbon flames (cf. Peters, 1997). The frozen state is metastable, because a sufficiently strong heat source, a spark for example, can raise the temperature beyond the crossover temperature and initiate combustion.

    Once fuel and oxidizer have homogeneously been mixed and a heat source is supplied it becomes possible for a flame front to propagate through the mixture. This will happen if the fuel-to-air ratio lies between the flammability limits: Flammable mixtures range typically from approximately ϕ = 0.5 to ϕ = 1.5, where ϕ is the fuel-air-equivalence ratio defined by (3.12) in Chapter 3. Owing to the temperature sensitivity of the reaction rates the gas behind the flame front rapidly approaches the burnt gas state close to chemical equilibrium, while the mixture in front of the flame typically remains in the unburnt state. Therefore, the combustion system on the whole contains two stable states, the unburnt (index u) and the burnt gas state (index b).

  • 9 - Low-Reynolds-Number Aerodynamics

  • Mohamed Gad-el-Hak, University of Notre Dame, Indiana
  • Book:
    Flow Control
    Published online:
    23 December 2009
    Print publication:
    15 August 2000, pp 189-204

  • Summary

    Oh, how much is today hidden by science! Oh, how much it is expected to hide!

    (Friedrich Wilhelm Nietzsche, 1844–1900)

    My philosophy of life is work. Bringing out the secrets of nature and applying them for the happiness of man. I know of no better service to render during the short time we are in this world.

    (Thomas Alva Edison, 1847–1931)

    PROLOGUE

    Among the goals of external flow control are separation postponement, lift enhancement, transition delay or advancement, and drag reduction. These objectives are not necessarily mutually exclusive. For low-Reynolds-number lifting surfaces, where the formation of a laminar separation bubble may have a dominant effect on the flow- field, the interrelation between the preceding goals is particularly salient, presenting an additional degree of complexity when flow control is attempted to achieve, say, maximum lift-to-drag ratio. This chapter discusses the aerodynamics of low- Reynolds-number lifting surfaces—particularly the formation and control of separation bubbles.

    Introduction

    Insects, birds and bats have perfected the art of flight through millions of years of evolution. Man's dream of flying dates back to the early Greek myth of Daedalus and his son Icarus, but the first successful heavier-than-air flight took place less than a century ago. Today, the Reynolds numbers for natural and man-made fliers span the amazing range from 102 to 109, insects being at the low end of this spectrum and huge airships occupying the high end (Carmichael 1981).

  • 5 - Classical Models of Jets, Wakes, and Boundary Layers

  • Jean Mathieu, Ecole Centrale de Lyon, Julian Scott, Ecole Centrale de Lyon
  • Book:
    An Introduction to Turbulent Flow
    Published online:
    28 May 2018
    Print publication:
    26 June 2000, pp 138-238

  • Summary

    Three important classes of statistically steady turbulent flows will be examined in some detail in this chapter, namely jets, wakes, and boundary layers. These flows are of considerable practical interest and are illustrated in Figure 5.1, together with a shear layer, which is not specifically considered here, but whose treatment is not essentially different from that of jets and wakes. Much is known about these flows from numerous experimental and theoretical studies, stretching back over many decades, and the resulting theoretical models are semiempirical, being based on the experimentally observed properties of the flows, such as the small angles of divergence of jets and wakes and the existence of self-similar behavior far downstream of the nozzle, in the case of jets, or body, in the case of wakes. We attempt to give a unified presentation of these models, emphasizing their common origins in the boundary-layer approximation and various types of mean-flow self-similarity.

    The basis of the theories developed in this chapter is the mean-flow equations, (4.9), to which simplifying assumptions, similar to those of laminar boundary-layer theory, are applied, leading to neglect of some of the terms in the equations. This turbulent boundary-layer approximation, which applies to jets and wakes as well, is justified by slowness of mean-flow development with streamwise distance, as witnessed by the small angles of divergence of turbulent jets and wakes. Since the approximations used are similar to Prandtl's theory of laminar boundary layers, with which we assume the reader is familiar, and since, despite their quite different mechanisms of growth, the behavior of turbulent boundary layers in many ways resembles that of laminar ones, we devote Section 5.1 to reminding the reader of the principal characteristics and theory of laminar boundary layers, before developing the turbulent analog of Prandtl's theory in Section 5.2.

    Having applied the boundary-layer approximation to simplify the mean-flow equations, one is still faced with their lack of closure, apparent from the Reynolds stress terms. Here, empirical information is introduced, whose details depend on the class of flows considered. In the case of jets and wakes, we use the observed fact that the mean flow becomes self-similar sufficiently far downstream, together with an eddy-viscosity approximation and an assumed eddy viscosity that does not vary across the flow, but only with streamwise distance.

  • 7 - Kolmogorov's and Other Theories Based on Spectral Analysis

  • Jean Mathieu, Ecole Centrale de Lyon, Julian Scott, Ecole Centrale de Lyon
  • Book:
    An Introduction to Turbulent Flow
    Published online:
    28 May 2018
    Print publication:
    26 June 2000, pp 283-326

  • Summary

    This chapter covers a variety of different subjects, of which the most important is the Kolmogorov theory of inertial and dissipative range scales in its original (Kolmogorov 1941a, b, c) version. The Kolmogorov theory is discussed in Section 7.3 and is one of the foundation stones of the theory of turbulence, leading to the famous E α k -5/3 inertial-range energy spectrum, which is now a benchmark for measurements and theoretical models alike. However, the original theory is not without its problems, as we will see in Section 7.5, where it is reformulated to avoid Landau's objection to the original version, and the possibility of intermittency corrections is introduced. To illustrate intermittency effects, we use the y6-model here, not because it gives a fully satisfactory account of intermittency, but because it is relatively simple to describe, understand, and analyze.

    Aside from the theory of the small scales (high wavenumbers), we also discuss spectral behavior at small wavenumbers (in the course of Section 7.1), the final phase of viscous decay (Section 7.2), in which the turbulent Reynolds number is small, and the consequences of large-scale self-similarity for high-Reynolds-number gridlike turbulence (Section 7.4). Throughout the chapter, the case of high-Reynolds-number turbulence will form our main preoccupation, and we will suppose developed turbulence, that is, that the cascade has been allowed the time to reach equilibrium over the full range of scales from the large, energy-containing scales, down to the small, dissipative ones. We further assume homogeneous, isotropic turbulence without mean flow1 in the analyses of this chapter, although these assumptions are not really essential to all the conclusions reached, in particular Kolmogorov's theory of the small scales is observed to be quite robust, as we will discuss later.

    Properties of the Energy Spectrum and Velocity Correlations

    We first want to recall some relevant results from the spectral analysis of homogeneous, isotropic turbulence, which were derived in the previous chapter.

  • 2 - Statistical Tools

  • Jean Mathieu, Ecole Centrale de Lyon, Julian Scott, Ecole Centrale de Lyon
  • Book:
    An Introduction to Turbulent Flow
    Published online:
    28 May 2018
    Print publication:
    26 June 2000, pp 31-56

  • Summary

    The apparently random character of turbulent flows strongly suggests that statistical methods will be fruitful. In this chapter, we discuss some statistical techniques, in preparation for later chapters. The reader is presumed to have a modest background in probability theory and this chapter summarizes the main elements, while placing statistical ideas in the context of turbulent flows (those whose knowledge of the basic theory is rusty or nonexistent should perhaps keep one of the many textbooks on the mathematics of probability and statistics to hand; Lumley (1970) and Monin and Yaglom (G 1971) describe the theory as applied to turbulence).

    As discussed in Chapter 1, despite being generally considered as deterministic, turbulent flows are highly nonunique in practice. Thus, if an experiment is repeatedly carried out, a different velocity field is obtained in each realization, even if the experimental conditions are nominally the same (i.e., the experimenter endeavors to reproduce the same flow). This is because the detailed behavior of the flow in any one realization is extremely sensitive to small changes in the initial or boundary conditions, which the experimenter cannot control to infinite precision. This type of problem is ideally suited to statistical methods. Indeed, it was the nonrepeatability of an experiment such as tossing a coin that led to the idea of probabilities in the first place. By looking at the statistical properties of an ensemble of different flow realizations, all obtained using the same nominal conditions, one hopes to extract useful quantities, namely probabilities and averages, which depend only on parameters that the experimenter controls. To take an example, the detailed behavior of the turbulent wake of a sphere placed in a uniform flow may vary with tiny perturbations in the incoming stream and small vibrations of the sphere, to name just two possible extraneous factors, but one hopes and expects that, for instance, the average velocity is well defined. Of course, the flow statistics depend on the gross experimental conditions used, for example, a turbulent boundary layer can have quite different thickness and drag with suction than without, but we expect them not to change significantly with small variations in those conditions.

  • 1 - An Introduction to Turbulence

  • Jean Mathieu, Ecole Centrale de Lyon, Julian Scott, Ecole Centrale de Lyon
  • Book:
    An Introduction to Turbulent Flow
    Published online:
    28 May 2018
    Print publication:
    26 June 2000, pp 1-30

  • Summary

    This book, intended as an introductory course in turbulence, presumes the reader to be well acquainted with basic fluid mechanics and to have a reasonable level of mathematical sophistication (the General References following the Preface include some textbooks on fluid dynamics to which the reader may want to refer if necessary). In this chapter we take a critical look at the concepts underlying a description of turbulence and lay the foundations for the chapters to follow.

    Most flows occurring in nature and in industrial applications are said to be turbulent. Everyday life gives us an intuitive understanding of turbulence in fluids and thus the concept is often accepted without further discussion. Among other properties, typical turbulent flows have apparently random velocity fluctuations with a wide range of different length and time scales. Thus, for instance, a graph of turbulent velocity as a function of time at a fixed point in space shows random variations. The graph has a “furry” appearance and successively higher magnifications of the “fur” show smaller fluctuations of shorter time scales, until finally the shortest is reached and the velocity is revealed as a smooth function. The same is true if the velocity is plotted as a function of one of the spatial coordinates at fixed time. However, these characteristics of turbulence do not provide a rigorous definition allowing one to objectively distinguish turbulence from a complicated laminar flow. In fact, it is difficult to give a watertight definition of turbulence, although it is easy to give specific examples in which turbulence is present in at least part of the flow:

  • • the rapid flow of fluid around a bluff body or airfoil,

  • • the majority of terrestrial atmospheric and oceanic currents,

  • • the motion of the atmospheres of the sun and planets,

  • • the flow inside most industrial plant.

    • It would certainly help in the systematic study of turbulence if there were a complete and succinct definition of a turbulent flow, but no fully satisfactory one has been found to date and so, rather than attempt a strict definition, we will describe specific examples and some general properties of turbulence in the course of this chapter. We begin by considering how turbulence arises in the first place, via the instability of laminar flows.

  • 8 - Numerical Simulation of Turbulent Flows

  • Jean Mathieu, Ecole Centrale de Lyon, Julian Scott, Ecole Centrale de Lyon
  • Book:
    An Introduction to Turbulent Flow
    Published online:
    28 May 2018
    Print publication:
    26 June 2000, pp 327-368

  • Summary

    The ability of modern computers to store and rapidly manipulate large quantities of data has allowed the development of flow simulations that complement, and sometimes even replace, traditional laboratory experiments. In industry, the considerable expense of constructing and exploiting physical models motivates the search for reliable simulation techniques to reduce the cost of the design and development phases. Full, detailed coverage of numerical methods for turbulent flows goes well beyond the scope of this book and would require a volume to itself. This chapter is therefore limited to a relatively brief overview of the three main types of numerical techniques, represented by the following three sections, in which the reader desiring further details of numerical methods will find references to more specialized works.

    Direct Numerical Simulation (DNS)

    The first, and most obvious, approach to the numerical simulation of turbulent flows treats them like any others, solving the unsteady, three-dimensional Navier-Stokes equations numerically without further ado. Such methods are referred to as direct simulations, in contrast with those based on the statistically averaged equations considered in Section 8.3, which involve closure approximations, but are much less computationally costly than DNS at high Reynolds numbers. Direct simulation allows numerical experiments to be carried out that are often difficult or impossible to realize in the laboratory, and yields detailed information concerning the flow field in individual realizations and its statistical properties that again often surpasses what can be measured in real flows. Furthermore, unlike other simulation techniques for turbulent flows, DNS does not involve approximations, other than those due to discretization, which are inherent in any numerical solution of differential equations, and so provides benchmarks against which less costly simulation methods may be evaluated and parameterized, just as they can by using the results of physical experiments. However, the continuum of different spatial (and temporal) scales intrinsic to turbulence, a continuum whose width increases with the turbulent Reynolds number, ReL, leads to computer time and storage requirements that increase rapidly with ReL. In consequence, DNS is limited to relatively low Reynolds numbers in practice and it is currently very expensive to conduct a DNS calculation at even moderately high Reynolds number.

  • 3 - Space and Time Scales of Turbulence

  • Jean Mathieu, Ecole Centrale de Lyon, Julian Scott, Ecole Centrale de Lyon
  • Book:
    An Introduction to Turbulent Flow
    Published online:
    28 May 2018
    Print publication:
    26 June 2000, pp 57-72

  • Summary

    Besides its apparent randomness, one of the important characteristics of turbulent flows at high Reynolds number (which we presume throughout this chapter) is the wide range of different space and time scales which they contain. As illustrated in Figure 3.1a, the velocity measured at a given point within a turbulent flow fluctuates randomly and does not yield a smooth function of time, but rather one with finescale “fur.” If part of such a graph is examined at successively higher magnifications, instead of approaching a straight line, as would a smooth function, the fur is revealed as being itself “furry,” and so on, through a succession of smaller and smaller time scales, until eventually the smallest of all is reached. At this scale, the velocity finally appears as a smooth function of time, with no further structure revealed by additional magnification. On the other hand, the largest time scales of turbulence are characterized by the time interval required for statistical decorrelation of the turbulent velocity fluctuations. At high Reynolds number, the separation of scales between the largest and smallest is wide, and gets wider as the Reynolds number increases.

    The continuum of time scales described above is also reflected in the spatial structure of the flow. The order of size of the largest spatial scales of turbulence is measured by the velocity correlation length L, which characterizes the spatial separation required for significant decorrelation of the velocity fluctuations. Successively higher magnification of the spatial structure of the velocity field shows smaller and smaller scales down to the smallest, known as the Kolmogorov scale, at which it appears smooth. Thus, turbulence contains a continuum of scales, ranging from the large ones, whose size is of order the correlation length, to those of order the Kolmogorov scale. Both the Kolmogorov scale, r\, and correlation scale, L, are best considered as orders of magnitude: one talks of scales comparable with, much smaller than, or much larger than these quantities. The spatial and temporal “furriness” of the velocity field are related; indeed, one can think of the small time scales as resulting from convection of fine spatial structures past the measurement probe, as we will see later.

    If one attempts to take the derivative of the velocity field, either with respect to time or one of the spatial coordinates, its furry nature becomes markedly more apparent.

Page 235 of 276