Introduction

At the very beginning of this book, the concept of wave–mode duality was emphasised. Its importance to engineering noise and vibration analysis will be illustrated in this chapter via a specific case study relating to pipe flow noise.

The general subject of flow-induced noise and vibrations is a large and complex one. The subject includes: (i) internal axial pipe flows – the transmission of large volume flows of gases, liquids or two phase mixtures across high pressure drops through complex piping systems comprising bends, valves, tee-junctions, orifice plates, expansions, contractions, etc.; (ii) internal cross-flows in heat exchangers, etc. with the associated vortex shedding, acoustic resonances and fluid-elastic instabilities; (iii) external axial and cross-flows – e.g. vortex shedding from chimney stacks; (iv) cavitation; and (v) structure-borne sound associated with some initial aerodynamic type excitation. The reader is referred to Naudascher and Rockwell, a BHRA (British Hydromechanics Research Association) conference publication and Blake for discussions on a wide range of practical experiences with flow-induced noise and vibrations.

This chapter is, in the main, only concerned with the study of noise and vibration from steel pipelines with internal gas flows – this noise and vibration is flow-induced and is of considerable interest to the process industries. There are many instances of situations where flow-induced noise and vibration in cylindrical shells have caused catastrophic failures. The mechanisms of the generation of the vibrational response of and the external sound radiation from pipes due to internal flow disturbances are discussed in this chapter.

Introduction

Sound is a pressure wave that propagates through an elastic medium at some characteristic speed. It is the molecular transfer of motional energy and cannot therefore pass through a vacuum. For this wave motion to exist, the medium has to possess inertia and elasticity. Whilst vibration relates to such wave motion in structural elements, noise relates to such wave motion in fluids (gases and liquids). Two fundamental mechanisms are responsible for sound generation. They are:

1. the vibration of solid bodies resulting in the generation and radiation of sound energy – these sound waves are generally referred to as structure-borne sound;

2. flow-induced noise resulting from pressure fluctuations induced by turbulence and unsteady flows – these sound waves are generally referred to as aerodynamic sound.

With structure-borne sound, the regions of interest are generally in a fluid (usually air) at some distance from the vibrating structure. Here, the sound waves propagate through the stationary fluid (the fluid has a finite particle velocity due to the sound wave, but a zero mean velocity) from a readily identifiable source to the receiver. The region of interest does not therefore contain any sources of sound energy – i.e. the sources which generated the acoustic disturbance are external to it. A simple example is a vibrating electric motor. Classical acoustical theory (analysis of the homogeneous wave equation) can be used for the analysis of sound waves generated by these types of sources.

Introduction

Statistical energy analysis (S.E.A.) is a modelling procedure for the theoretical estimation of the dynamic characteristics of, the vibrational response levels of, and the noise radiation from complex, resonant, built-up structures using energy flow relationships. These energy flow relationships between the various coupled subsystems (e.g. plates, shells, etc.) that comprise the built-up structure have a simple thermal analogy, as will be seen shortly. S.E.A. is also used to predict interactions between resonant structures and reverberant sound fields in acoustic volumes. Many random noise and vibration problems cannot be practically solved by classical methods and S.E.A. therefore provides a basis for the prediction of average noise and vibration levels particularly in high frequency regions where modal densities are high. S.E.A. has evolved over the past two decades and it has its origins in the aero-space industry. It has also been successfully applied to the ship building industry, and it is now being used (i) as a prediction model for a wide range of industrial noise and vibration problems, and (ii) for the subsequent optimisation of industrial noise and vibration control.

Lyon's book on the general applicability of S.E.A. to dynamical systems was the first serious attempt to bring the various aspects of S.E.A. into a single volume. It is a useful starting point for anyone with a special interest in the topic.

Introduction

Wave–mode duality concepts were introduced and discussed in some detail in chapter 1. It was pointed out that, whilst the lumped-parameter approach to mechanical vibrations is adequate to describe mode shapes and natural frequencies, it is not suitable for relating vibrations to radiated noise. One therefore has to use the fundamental wave approach to obtain an understanding of the essential features of mechanical vibrations as they relate to sound radiation and sound transmission. These interactions between sound waves and the mechanical vibrations of solid structures form a very important part of engineering noise and vibration control.

Because solids can store energy in shear and compression, all types of waves can be sustained in structures – i.e. compressional (longitudinal) waves, flexural (transverse or bending) waves, shear waves and torsional waves. On the other hand, since fluids can only store energy in compression, they can only sustain compressional (longitudinal) waves. For reasons which will become evident later on in this chapter, flexural (bending) waves are the only type of structural wave that plays a direct part in sound radiation and transmission. At this stage it is sufficient to note that the primary reason for this is that the bending wave particle velocities are perpendicular to the direction of wave propagation (see Figure 1.1b) resulting in an effective exchange of energy between the structure and the fluid.

The study of noise and vibration and the interactions between the two is now fast becoming an integral part of mechanical engineering courses at various universities and institutes of technology around the world. There are many undergraduate text books available on the subject of mechanical vibrations and there are also a relatively large number of books available on applied noise control. There are also several text books available on fundamental acoustics and its physical principles. The books on mechanical vibrations are inevitably only concerned with the details of vibration theory and do not cover the relationships between noise and vibration. The books on applied noise control are primarily designed for the practitioner and not for the engineering student. The books on fundamental acoustics generally concentrate on physical acoustics rather than on engineering noise and vibration and are therefore not particularly well suited to the needs of engineers. There are also several excellent specialist texts available on structural vibrations, noise radiation and the interactions between the two. These texts do not, however, cover the overall area of engineering noise and vibration, and are generally aimed at the postgraduate research student or the practitioner. There are also a few specialist reference handbooks available on shock and vibration and noise control – these books are also aimed at the practitioner rather than the engineering student.

A list of several international journals that publish research and development articles related to various aspects of engineering noise and vibration control is presented below.

1. Acustica – S. Hirzel Verlag

2. Applied Acoustics – Elsevier Applied Science

3. Current Awareness Abstracts – Vibration Institute

4. Journal of the Acoustical Society of America – Acoustical Society of America

5. Journal of Fluid Mechanics – Cambridge University Press

6. Journal of Fluids and Structures – Academic Press

7. Journal of Sound and Vibration – Academic Press

8. Journal of Vibration, Acoustics, Stress, and Reliability in Design – American Society of Mechanical Engineers

9. Mechanical Systems and Signal Processing – Academic Press

10. Noise and Vibration in Industry – Multi-Science

11. Noise Control Engineering Journal – Institute of Noise Control Engineers

12. Shock and Vibration Digest – Vibration Institute

13. Sound and Vibration – Acoustical Publications, Inc.

Introduction

A vast amount of applied technology relating to noise and vibration control has emerged over the last twenty years or so. It would be an impossible task to attempt to cover all this material in a text book aimed at providing the reader with a fundamental basis for noise and vibration analysis, let alone in a single chapter! This chapter is therefore only concerned with some of the more important fundamental considerations required for a systematic approach to engineering noise and vibration control, the main emphasis being the industrial environment. The reader is referred to Harris4.1 for a detailed engineering-handbook-type coverage of existing noise control procedures, and to Harris and Crede4.2 for a detailed engineering-handbook-type coverage of existing shock and vibration control procedures. Beranek4.3 also covers a wide range of practical noise and vibration control procedures. Some of the more recent advances relating to specific areas of noise and vibration control are obviously not available in the handbook-type literature, and one has to refer to specialist research journals. A list of major international journals that publish research and development articles in noise and vibration control is presented in Appendix 1.

This chapter commences with a discussion on noise and vibration measurement units. The emphasis is on the fundamental principles involved with the selection of objective and subjective sound measurement scales, vibration measurement scales, frequency analysis bandwidths, and the addition and subtraction of decibels.

INTRODUCTION

The gas turbine has many important applications but it is most widely used as the jet engine. In the last few years, since the regulations changed to permit natural gas to be burned for electricity generation, gas turbines have become important prime movers for this too. Many of the gas turbines used in land-based and ship-based applications are derived directly from aircraft engines; other gas turbines are designed specifically for land or marine use but based on technology derived for aircraft propulsion.

The attraction of the gas turbine for aircraft propulsion is the large power output in relation to the engine weight and size – it was this which led the pre-Second World War pioneers to work on the gas turbine. Most of the pioneers then had in mind a gas turbine driving a propeller, but Whittle and later von Ohain realised that the exhaust from the turbine could be accelerated to form the propulsive jet.

This chapter looks at the operation of simple gas turbines and outlines the method of calculating the power output and efficiency. The treatment is simplified by treating the working fluid as a perfect gas with the properties of air, but later some examples are discussed to assess the effect of adopting more realistic assumptions. It is assumed throughout that there is a working familiarity with thermodynamics – this is not the place to give a thorough treatment of the first and second laws (something covered very fully in many excellent text books, for example Van Wylen and Sonntag, 1985).

INTRODUCTION

This part of the book begins the consideration of the engine requirements of a new fighter aircraft. In parallel with the treatment in earlier chapters for the engines of the new large civil aircraft, the approach chosen is to address the design of engines for a possible new aircraft so that the text and exercises can be numerically based with realistic values. The specifications for the New Fighter Aircraft used here have a marked similarity to those available for the new Eurofighter.

The topic of the present chapter is the nature of the combat missions and the type of aircraft involved. Figure 13.1 shows the different regions in which aircraft operate in terms of altitude versus Mach number, with the lines of constant inlet stagnation temperature overlaid. We are concerned here with what are referred to in the figure as fighters, a major class of combat aircraft. Figure 13.1 shows the various boundaries for normal operation. Even high-speed planes do not normally fly at more than M = 1.2 at sea level because in the high density air the structural loads on the aircraft and the physiological effects on the crew become too large. At high altitude high-speed aircraft do not normally exceed M ≈ 2.3, largely because the very high stagnation temperatures preclude the use of aluminium alloys without cooling.

The book has been well received and Cambridge University Press approached me with the invitation to bring out a second edition. This was attractive because of the big events in aerospace, most significantly the decision by Airbus Industrie at the end of 2000 to launch their new large aircraft, the A380. This meant that some changes in the first ten chapters were needed. Another major development is the decision to develop an American Joint Strike Fighter, the F-35.

Another more personal change took place when I left academia to become Chief Technologist of Rolls-Royce from the beginning of 2000. It should be noted, however, that the character and ideas of this second edition remain those of the university professor who wrote the first edition and do not reflect my change of role.

The aim and style of the book is unchanged. The primary goal of creating understanding and the emphasis remains on simplicity, so far as this is possible, with the extensive use of relevant numerical exercises. In a second edition I have taken the opportunity to update a number of sections and to include some explanatory background on noise; noise has become a far more pressing issue over the last four or five years. The book remains, however, very similar to the first edition and, in particular, numerical values have been kept the same and the exercises have not been changed.

INTRODUCTION

This chapter looks at the layout of some jet engines, using cross-sectional drawings, beginning with relatively simple ones and leading up to the large engines for one of the most recent aircraft, the Boeing 777. Two concepts are introduced. One is the multi-shaft engine with separate low-pressure and high-pressure spools. The other is the bypass engine in which some, very often most, of the air compressed by the fan bypasses the combustor and turbines.

Any consideration of practical engines must address the temperature limitations on the turbine. The chapter ends with some discussion of cooling technology and of the concept of cooling effectiveness.

THE TURBOJET AND THE TURBOFAN

Figure 5.1 shows a cut-away drawing of a Rolls-Royce Viper engine. This is typical of the simplest form of turbojet engine, which were the norm about 40 years ago, with an axial compressor coupled to an axial turbine, all on the same shaft. (The shaft, the compressor on one end and turbine on the other are sometimes referred to together as a spool.) Even for this very simple engine, which was originally designed to be expendable as a power source for target drones, the drawing is complicated and for more advanced engines such drawings become unhelpful at this small scale. Simplified cross-sections are therefore more satisfactory and these will be shown for more advanced engines. A simplified cross-section is also shown for the Viper in Fig. 5.1, as well as a cartoon showing the major components.

INTRODUCTION

In this chapter we will consider three separate engine designs corresponding to distinct operating conditions. For convenience here the three design points are at the tropopause (altitude 11 km; standard atmosphere temperature 216.65 K and pressure 22.7 kPa) for Mach numbers of 0.9, 1.5 and 2.0. The thrusts required for these conditions were determined in Exercise 14.4. At each condition a separate engine is designed – this is quite different from designing the engine for one condition and then considering its operation at different conditions, which is the topic of Chapter 17.

For this exercise all design points will correspond to the engine being required to produce maximum thrust, even though the ultimate suitability of an engine for its mission may depend on performance, particularly fuel consumption, at conditions for which the thrust is very much less than maximum. The designs will first be for engines without an afterburner (operation ‘dry’) and then with an afterburner; the afterburner will be assumed to raise the temperature of the exhaust without altering the operating condition of the remainder of the engine so the stagnation pressure entering the nozzle is unchanged.

The engines considered will all be of the mixed turbofan type – such an engine was shown in Fig. 15.1 with a sketch showing the station numbering system adopted. Note that the numbering shows station 13 downstream of the fan in the bypass and station 23 downstream of the fan for the core flow; in the present simplified treatment it will be assumed that p023 = p013 and that T023 = T013.