Showing papers by "Cranfield University published in 1979"

Transition in the Infinite Swept Attachment Line Boundary Layer

Abstract: The transition behaviour of the boundary layer which is formed along an infinite swept attachment line has been studied experimentally. Circular trip wires and turbulent flat plate boundary layers have been used as sources of disturbance and the range of parameters covered has been such that the results are directly applicable to full scale flight conditions. Simple criteria have been deduced which allow the state of the boundary layer to be determined for given geometric and free stream properties. Sample calculations for typical swept wing configurations suggest that the majority of civil aircraft will have turbulent attachment lines in the cruise and that subsequent relaminarisation in regions of favourable pressure gradient is unlikely.

The effect of fibre length on the overall moduli of composite materials

Abstract: A n application of the self-consistent method (s.c.m.) is given to the problem of determining overall moduli for short fibre reinforced composites. It is assumed that the fibres can be considered to be spheroids. For fully-aligned fibres, the numerical results are presented in graphical form and show the dependence of the compliances on aspect ratio and volume fraction. It is shown that an aspect ratio of 100 is essentially infinite. By making use of some (albeit tentative) ideas on how to handle the misalignment of fibres the s.c.m. results are shown to compare favourably with experiment.

A computer-aided analysis of one-dimensional thermal transients in n-p-n power transistors

Abstract: An algorithm is presented which permits the one-dimensional simulation of thermal transients in n - p - n transistors. Band distortion effects, temperature dependent mobilities and thermal conductivity are incorporated into the model. Accurate expressions are derived for the heat generated at any point within the semiconductor and the finite difference forms of the current continuity equations. Results showing the internal device behaviour under various bias conditions are presented.

The structure of a premixed turbulent flame

Abstract: An experimental study of the structure of a premixed turbulent flame propagating in a duct-confined, stoichiometric propane-air mixture has been carried out. Care was taken to reduce any effects of axial pressure gradients. By suitable changes in grid geometry, the turbulence intensity and scale of the approach flow were varied independently over a wide range. The results of these experiments show a strong link between the mechanisms of turbulent flame propagation and the flame-generated turbulence. Thus, three distinct regions, each having different structural characteristics in regards to the effects of turbulence scale on flame-generated turbulence, may be identified. The physical processes for each region, namely a 'wrinkled-pulsating' behaviour of region 1, the 'jet-like' characteristics of region 2 and the 'eddy entrainment - combustion in depth' mechanism of region 3 are described. A comparison between the mean and the turbulence properties of a flame and of a coaxial jet of lighter fluid (hydrogen) spreading into a turbulent, co-flowing air stream has been made. Based upon this, the contribution of shear-generated turbulence to total flame turbulence is calculated. A definition of flame-generated turbulence is then proposed. It is shown that in some cases the flame damps the turbulence whereas in most other instances it generates additional turbulence. The total magnitude of the relative flame-generated turbulence intensity does not exceed about 12 %. Finally, it is shown that the flame damps the unburnt stream fluctuating vorticity in region 1, has no effect in region 2 and augments the vorticity in region 3.

Shewhart Control Charts

Abstract: The concept of a control chart was evolved by Dr. Shewhart in America in 1924 — the initial development in Britain being mainly by Dr. Dudding and W. Jennett. Dr. Shewhart suggested that the control chart should have three main objectives.

Laminar Film Cooling Experiments in Hypersonic Flow

Abstract: The paper describes an experimental study of the thermal protection of a surface, in hypersonic flow, by tangential injection of a coolant through a slot with laminar flow prevailing throughout. Heat-transfer measurements were taken on a flat plate in a gun tunnel under isothermal wall conditions and the effect of slot height, streamwise position of slot, flow conditions, and differing coolant gases were examined. A simple discrete layer theory was found to give fair agreement with the experiments. Optimizations of cooling effectiveness using this theory showed that hydrogen was likely to be the most efficient gas for constant mass injection rate.

An Airborne Radar Technique for the Investigation and Control of Migrating Pest Insects

Abstract: A new airborne radar technique has been developed and used during two seasons on three aircraft in Canada, to investigate the source areas, flight behaviour and areas of deposition of migrating insects. The radar measures, records and analyses more than 100 high resolution profiles of insect orientation and absolute density per second, at a spacing of less than one metre of aircraft track. During some 11000 km flown with this equipment in New Brunswick and neighbouring areas, synoptic and small-scale meteorological systems have been traversed and their effects on the flying insects (spruce budworm moths) have been measured.

Epoxy granite: a structural material for precision machines

Abstract: All precision machine tools and measuring machines rely on the stiffness, long term stability and damping properties of a structural material for their static and dynamic performance and, of course, the manufacturing cost of the machine depends on the structural materials chosen This article outlines the main properties and relative merits of the more common structural materials and describes in more detail epoxy granite, a new material which has been blended to overcome the inherent disadvantages of more traditional materials

On the evolution of compression pulses in an exploding atmosphere: initial behaviour

Abstract: The development in space and time of a plane initial disturbance to a spatially uniform exploding atmosphere is analysed on the assumption that the disturbance amplitude is comparable in magnitude with the inverse (dimensionless) activation energy of the explosion reaction. Particular attention is focused on the shock-fitting problem, which has features that distinguish it from its inert-atmosphere counterpart.Using the positive half of a sine wave to typify an isolated compression perturbation, it is found that the amplifying effect of the ambient reaction leads to very rapid shock wave development, which depends significantly on the spatial extent of the disturbance. The latter also influences the question of whether local explosion (local explosion is recognized here as a logarithmically unbounded growth of the disturbance amplitude; in other words as a local breakdown of the present approximations) occurs at the shock wave or some distance behind it. The subsequent evolution of these two states will no doubt be significantly different, but the answer to this speculation must await extension of the present theory to encompass the rapid events that ensue near the local explosion regions.

Principles of Control

Abstract: The concepts of statistical control theory as they are used in the design and operation of control charts will now be discussed. It is important to note that these basic principles of control apply to any control problem, e.g. stock control, accidents, overheads, etc., and examples of the use of the control theory to these wider management areas will be given later in the book.

The influence of laminar burning velocity on the structure and propagation of turbulent flames

Abstract: An experimental study of the influence of laminar burning velocity on the structure and propagation of duct-confined premixed turbulent flames has been carried out. Propane, acetylene and hydrogen were used as fuels to vary the laminar burning velocity in the range from 20 to 280 cm/s. These experiments fully verify the three region model (region 1: u 9 S L , η > δ L ; region 2: u 9 ≈ 2 S L , η ≈ δ L to η ≫ δ L ; region 3: u 9 > 2 S L , η δ L ) of turbulent flames proposed earlier by Ballal & Lefebvre. Since a large increase in the laminar burning velocity has a stabilizing influence it is possible to suppress the ‘instability’ of region 1 and the ‘eddy entrainment’ of region 3. The ‘turbulent diffusion’ mechanism then becomes solely dominant, and the flame shows a ‘jet-like’ behaviour. For such a flame (i) both the burning velocity and flame turbulence intensity are independent of scale, (ii) the equations developed by Karlovitz and Ballal for regions of stable combustion accurately predict all the experimental data on turbulent burning velocity and flame turbulence, respectively, and (iii) the laminar burning velocity remains an important parameter of flame propagation even at very high turbulence intensity. Finally the important role of shear-generated turbulence and the ability of the flame either to dampen or to generate additional turbulence has been fully confirmed.

Working and Learning — Identifying Your Preferred Ways of Doing Things

Abstract: It was John Dewey who clearly argued that we should integrate learning with work and work with learning. His view was that experience could be a great teacher, providing we had opportunities to learn from action. He emphasised that there was ‘An intimate and necessary relation between the processes of actual experience and education’. However, he also emphasised ‘it is not enough to insist upon the necessity of experience, nor even of activity in experience. Everything depends upon the quality of experience which is had.’

Some developments in Grating Transducer Systems

Abstract: In 1874 Lord Rayleigh 1 outlined some of the scientific possibilities of patterns formed by overlaid diffraction gratings. The patterns were named 'moire' after the techniques used by French silk weavers to produce decorative designs from overlapped layers of fine fabric. Although Righi 2 and later Giambiasi each contributed to the sum of knowledge en route, i t was to take the simult,~neous arrival in the early 1950's of the scientific definition of the technique by Roberts 4, the development of practical means for producing accurate gratings by NPL (based on a method suggested by Merton=), and the commercial requirement for optical grating transducers within a radically new concept of machine tool control by Williamson, Sheppard and Walker 6 , to end the gestation period and herald the birth of modern linear and rotary grating transducers. The technique relies on a 'magnification' of the spacing of ruled lines on two transparent gratings such that the spacing can be readily perceived by the eye or by optical instrumentation. Many methods exist to produce moir6 fringe patterns. For a simple explanation of the technique, consider two identical gratings with equi-spaced vertical rulings superimposed at a slight angle. Ignoring for the moment the effects of diffraction, it will be seen that the lines of one grating will cross the other at a vertical separation equal to the spacing of the lines on the grating, divided by the sine of the angle between them. The total effect of all the lines crossing at the same pitch height is to produce horizontal bands of light and dark, or transparency and opacity, along the grating representing a magnified horizontal version of the vertical grating (Fig 1). Horizontal movement of one grating relative to the other causes these bands to move in the vertical direction, where one vertical pitch movement is e.quivalent to a horizontal movement of one pitch of the grating. Diffraction effects can be utilized as a means of artificially improving contrast and sharpness by adjustment of the spacing between the gratings ~ . By interposing the gratings between a lamp and a photocell detector, the periodic variations in light intensity caused by movement of one grating relative to the other can be measured and counted. For gratings v~ith one hundred lines per millimeter the technique provides a direct measurement resolution of ten #m. Since the bands are produced by the effects of many lines on the gratings, small imperfections, blemishes, or random pitch errors in the rulings do not affect the accuracy of the measurements: indeed this improvement, by averaging, makes measurement by band-counting more accurate than the grating itself. Pressures induced by a need for greater accuracy and resolution prompted developments in electronic sub-division of the wave form out of the photocell to divide-byfour, by ten and then divide-by-twenty levels, the latter

Effect of Surface Distortions upon the Thermal Resistance of Pressed Contacts

Abstract: The thermal resistances of two pressed contacts, which under isothermal conditions were respectively between (i) a flat and a convex surface and (ii) nominally flat surfaces, have been predicted and the deductions compared with experimental measurements. The analysis evolved is useful in the design of thermal rectifiers.

Production of spring steel camera shutter blades by photoetching

Abstract: The feasibility of producing spring steel shutter blades to exacting specifications by photofabrication has been studied. In comparison with the current production method, stamping, some advantages of using photofabrication are apparent and it is possible that economic advantages might accrue with an ‘in-house’ photofabrication facility capable of producing a range of components