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5486 results in Thermal-fluids engineering

Page 74 of 275

  • 6 - What's Under the Skin?: Airframe Systems

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 83-100

  • Summary

    SECONDARY POWER SYSTEMS

    We have looked at an aircraft's primary power system, its propulsion, and will now examine its secondary power systems (SPS). Secondary power is usually provided by an aircraft's main engines when they are operating, but can be supplied by other sources such as an auxiliary power unit (APU) in the aircraft or an external source on the ground.

    Almost all present aircraft SPS are of three types, shown in Fig. 6.1.

    Power is extracted from turbo-fan powerplants in two forms. Bleed air is tapped from one or more points along the engine compressor to provide pneumatic power. Drive shafts, from the engine's high-pressure shaft, drive an accessory gear box on which are mounted generators and hydraulic pumps, as well as the engine's own accessories. A typical large turbo-fan is shown in Fig. 5.13.

    Turbo-jet powerplants can also supply bleed air and shaft power for systems, but turboprop powerplants in general have difficulty in supplying sufficient bleed air for airframe requirements. In such a case, extraction of shaft power to drive separate compressors may be considered.

    SPSs are often linked with each other, but it is convenient to separate them into the types described below.

    Air-Conditioning and Pressurization

    On many civil transports the largest amounts of secondary power are extracted in the form of bleed air and the largest continual user of this power is the aircraft's air-conditioning and pressurization system.

    It is usual to maintain an aircraft cabin at a pressure equivalent to that at 8000 ft or less, and this requires a constant supply of pressurized air. In addition to this, oxygen used by the occupants must be replaced and the temperature and contaminant levels controlled, requiring a significant throughput of air, although some of it is recycled through the cabin, after filtering.

    It is the control of temperature that usually determines the rate at which air must be supplied and, with the type of air-conditioning system installed on most aircraft, this also requires the air be supplied at pressures significantly above those in the cabin.

    Figure 6.2 shows the major elements of an air-conditioning system. There are extremely remote events where the pressurization system fails. In this case, on-board emergency oxygen systems are used to preserve life. The combination of air-conditioning, pressurization and oxygen systems are usually termed the environmental control system (ECS).

  • 1 - Introduction

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 1-5

  • Summary

    WHY ANOTHER AIRCRAFT DESIGN BOOK?

    Aircraft design is a complex and fascinating business and many books have been written about it. The very complexity and dynamic nature of the subject means that no one book can do it justice.

    This book, therefore, will primarily act as an introduction to the whole field of aircraft design leading towards the subjects summarized in Fig. 1.1. It will not attempt to duplicate material found in existing design books, but will give information about the whole aircraft design environment, together with descriptions of aircraft and component design. It also presents otherwise unpublished data and design methods that are suitable for aircraft conceptual, preliminary and detail design activities.

    TOPICS

    The following chapters are arranged as a series of questions about aircraft design, the answers to which give largely descriptive overviews of all aspects of aircraft design. This will provide an introduction to the conflicting requirements of aircraft design specialists in a design team, with a view to improving understanding and the integration of a sound overall design.

    The book is divided into chapters that answer a number of significant design questions.

    The question, ‘Why design a new aircraft?’ is answered in Chapter 2, which shows the derivation of aircraft requirements for civil and military aircraft from market surveys, and gives examples of operator-derived specifications.

    Chapters 3 and 4 answer the question, ‘Why is it that shape?’ with an initial discussion of aircraft wing and tail shapes, followed by descriptions of the configurations of a wide range of civil and military aircraft types.

    The question, ‘What's under the skin?’ is answered in Chapters 5, 6 and 7, which deal with structures and propulsion, airframe systems, avionics, flight controls and weapons, respectively. These chapters describe the interiors of aircraft, ranging from structures to weapons systems via airframe systems, avionic systems and landing gears.

    In Chapter 8 the crucial areas of acquisition and operating costs are discussed, some prediction methods are described and the importance of good reliability and maintainability are stressed in order to answer the question, ‘Why do aircraft cost so much?’

    The answer to the question, ‘What help can I get?’ is provided in Chapter 9, which contains a bibliography of the most important current aircraft design books.

  • 10 - The Shape of Things to Come: Should the Project Continue?

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 165-179

  • Summary

    INTRODUCTION

    This book has attempted to give an introduction to most aspects of aircraft design, with emphasis on giving the reasons for the shapes of aircraft, and descriptions of their constituent parts. The preface clearly stated that it was not intended to produce a book about aircraft conceptual design, as many good texts are currently available. Some of these are referred to in the previous chapter, and they should be used, with help, to produce aircraft conceptual designs. Conceptual designs, however, are not usually end-products, but merely important steps in the whole design–manufacture–operation cycle.

    Conceptual designs must be objectively assessed to see if they warrant the significantly increased expenditure that would be required to perform preliminary and detailed design. A vital precursor to this decision-making process is a clear definition of the characteristics of the conceptual design, and this is discussed in Section 10.2. Section 10.3 shows the options that are available following a conceptual design process and Section 10.4 describes simple decision-making techniques. The final section of this chapter gives an example of a conceptual-design definition, and describes the decisions that were made about its future.

    CONCEPTUAL DESIGN DEFINITION

    It is important to summarize clearly the results of the conceptual design process by providing the following types of information:

  • (i) Configuration description – This will be defined by means of conventional engineering drawings, tabular data, computer models, physical models or a combination of these methods, together with a brief description of the rationale of the design.

  • (ii) Mass estimates – These will include gross and empty masses, together with component mass estimates to be used as targets for subsequent work.

  • (iii) Performance estimates – Drag and lift predictions will be produced, together with powerplant data to give payload-range curves, field-performance, and direct operating cost targets for commercial aircraft. Military aircraft will also require predictions to be made of sortie performance, and point performance criteria such as attained turn rate, sustained turn rate and specific excess power.

  • (iv) Reliability and maintainability targets – The direct operating cost targets, mentioned above, imply estimates of acquisition costs as well as reliability and maintainability (R and M) characteristics. These may be used to set R and M targets for subsequent design stages (Appendix C).

Foundations of Gas Dynamics
  • Ruey-Hung Chen
  • Published online:
    31 March 2017
    Print publication:
    09 March 2017
  • Foundations of Gas Dynamics covers supersonic and subsonic flow phenomena where compressibility of the fluid cannot be ignored. It finds application in jet and rocket propulsion systems as well as handling industrial gas flow at high speeds. Students and engineers in the mechanical, aerospace, and chemical disciplines will find it useful. It begins with basic concepts such as isentropic flows, shock, and supersonic expansion waves in one dimension. These are followed by one-dimensional flows with friction and heat exchange. Two-dimensional theory with small perturbations is presented, with its applications illustrated by supersonic airfoils. Method of characteristics is used for flows with two independent variables, either with two spatial coordinates or with time variations in one dimension. In later chapters, acoustic wave propagation, supersonic flow combustion, and unsteady shock formation are treated thoroughly. The book ends with a chapter on basic hypersonic flow, with a discussion of similarity rules.

  • Preface

  • Iain D. Boyd, University of Michigan, Ann Arbor, Thomas E. Schwartzentruber, University of Minnesota
  • Book:
    Nonequilibrium Gas Dynamics and Molecular Simulation
    Published online:
    13 April 2017
    Print publication:
    23 March 2017, pp xvii-xx

  • Summary

    There are two top-level goals that we aim to address in this book: (1) to provide a description of a gas by considering its most basic constituents, i.e., atoms and molecules, and (2) to introduce readers to computer simulation techniques that are available to analyze a gas at this fundamental level.

    The first question we must ask ourselves is, Why should we consider a gas at the molecular level? After all, there are well-established equations and ideas that provide accurate descriptions of gas flow at the macroscopic level that employ variables such as density, flow velocity, temperature, and pressure, and these represent properties that take into account the molecules in an aggregate sense. Certainly, the molecular approach will provide us with a deeper understanding of all gas flows. However, more than that, under certain conditions, the aggregate, or sometimes called fluid, approach is not able to provide a physically accurate picture of the gas.We will find that these conditions arise when there is either not enough time or physical space for a sufficient number of intermolecular collisions to occur to maintain the gas in the well-understood equilibrium state. We refer to such conditions as nonequilibrium. To describe nonequilibium flows accurately, we need to study the molecular nature of the gas.

    There are a number of important application areas in aerospace engineering where nonequilibrium gas flows arise. In general, we will find that nonequilibrium occurs when the gas flow is at low density and/or involves very small length scales. One important application area for nonequilibrium is the flight of high-speed vehicles at very high altitude in the Earth's atmosphere. Examples include spacecraft returning from orbit, such as the space shuttle, or hypersonic cruise vehicles. These vehicles have a length scale of several meters, and move at very high speed so that the flow field surrounding them involves very high temperatures. However, it is their operation in the low-density environment of near-space that leads to nonequilibrium gas flow phenomena. We focus on low-density, high-temperature air in many of the examples and analyses presented throughout the book.

    A second important technology area involving nonequilibrium gas flow is micro- and nanoelectromechanical systems (MEMS/NEMS) that involve fabrication and operation of very small machines based on microfabrication technology.

  • 6 - Direct Simulation Monte Carlo

  • from Part II - Numerical Simulation
  • Iain D. Boyd, University of Michigan, Ann Arbor, Thomas E. Schwartzentruber, University of Minnesota
  • Book:
    Nonequilibrium Gas Dynamics and Molecular Simulation
    Published online:
    13 April 2017
    Print publication:
    23 March 2017, pp 183-251

  • Summary

    Introduction

    The direct simulation Monte Carlo (DSMC) method was created by Graeme Bird, a professor of aeronautical engineering at the University of Sydney, Australia. The first publication, appearing in Physics of Fluids in 1963, demonstrated the DSMC method as an alternative to molecular dynamics (MD) for a hard-sphere gas (Bird 1963). Since that time, Bird has written two textbooks on the DSMC method (Bird 1994, 2013) and an entire field of research has emerged with hundreds of publications on the DSMC method and its application to scientific and engineering problems. The DSMC method has become a powerful and widespread technique for the simulation of nonequilibrium gases where the molecular nature of the gas must be accounted for.

    To begin this chapter, it is important to identify the range of flow conditions and engineering applications for which the DSMC method is most appropriate. As described by Bird (1994), dilute gas conditions can be separated into various regimes through nondimensional quantities.

    First, the transition between a dilute gas and a dense gas occurs when the size (d) of the molecules themselves (the extent of their interatomic forces) becomes comparable to the mean separation distance (δ) between molecules in the gas. As the molecular size becomes comparable to the molecular separation distance, the mean-free-path and mean-collision-time scales vanish. Indeed, the molecules in a condensed phase (a solid or liquid) are in a constant collisional state, experiencing continual forces due to neighboring molecules. Although the molecules in a dense gas may have nonzero meanfree- path and mean-collision-time scales, these scales begin to approach the Angstrom and femtosecond scales used in molecular dynamics simulations. Furthermore, in dense gases, multimolecule interactions are frequent and must be accounted for. In contrast, in a dilute gas, in which the mean molecular spacing is large compared to molecular size, collisions are predominately binary in nature. Such binary collisions occur over time scales that are much shorter than the mean collision time and can be thought to occur instantaneously. As described in Chapter 1, this leads to the ideal gas equation of state.

  • 1 - Kinetic Theory

  • from Part I - Theory
  • Iain D. Boyd, University of Michigan, Ann Arbor, Thomas E. Schwartzentruber, University of Minnesota
  • Book:
    Nonequilibrium Gas Dynamics and Molecular Simulation
    Published online:
    13 April 2017
    Print publication:
    23 March 2017, pp 3-53

  • Summary

    Introduction

    The primary aim of kinetic theory is to relate molecular level behavior to macroscopic gas dynamics. This is achieved by consideration of the behavior of individual particles, and integrating their collective properties up to the macroscopic level. Consider the simple case of a gas at rest as illustrated in Fig. 1.1. At the macroscopic level, this is an uninteresting situation because all the gas properties, such as density (ρ), pressure (p), and temperature (T), are constants. However, at the molecular level, there is a great deal of activity with particles traveling individually at relatively high speed, and undergoing collisions with other particles. When one considers the behavior of particles at the molecular level, they really only undergo two processes: translational motion in space due to their velocity, and intermolecular collisions with other particles in the gas. While kinetic theory analysis has to consider these two physical phenomena, we will see that it is a complex process. For example, the motions of particles will be divided into consideration of bulk, directed motion, and random, thermalmotion. Collisions of particles involve a nonlinear process that includes elastic events where only the particle velocities change, and inelastic processes involving energy exchange with internal modes and even chemical reactions.

    Fundamental Concepts

    In this section, we first provide an introduction to some basic concepts and definitions that will be needed to achieve our goal of relating molecular behavior to macroscopic gas dynamics. We then employ these concepts later in the chapter to analyze a number of different gas flow situations.

    Particle Model

    The particle is the fundamental unit in kinetic theory and we will use this term generically to refer to atoms and molecules. Each particle has the following properties:

    1. • Mass (typically around 10-26 to 10-25 kg)

    2. • Size (typically a few 10-10 m)

    3. • Position, velocity, and internal energy

    The mass of a particle is simply the sum of the masses of its constituent atoms. Position is the center of mass location of the constituent atoms and velocity is the center-of-mass velocity of those atoms. For molecules, atomic motion relative to the center of mass (i.e., rotation and vibration) contributes to the internal energy of the particle.

Page 74 of 275