Showing papers by "University of Cambridge published in 1988"

Cellular Solids: Structure and Properties

Abstract: 1. Introduction 2. The structure of cellular solids 3. Material properties 4. The mechanics of honeycombs 5. The mechanics of foams: basic results 6. The mechanics of foams refinements 7. Thermal, electrical and acoustic properties of foams 8. Energy absorption in cellular materials 9. The design of sandwich panels with foam cores 10. Wood 11. Cancellous bone 12. Cork 13. Sources, suppliers and property data Appendix: the linear-elasticity of anisotropic cellular solids.

Eddies, streams, and convergence zones in turbulent flows

Abstract: Recent studies of turbulent shear flows have shown that many of their important kinematical and dynamical properties can be more clearly understood by describing the flows in terms of individual events or streamline patterns These events or flow regions are studied because they are associated with relatively large contributions to certain average properties of the flow, for example kinetic energy, Reynolds stress, or to particular processes in the flow, such as mixing and chemical reactions, which may be concentrated at locations where streamlines converge for fast chemical reactions (referred to as convergence or C regions), or in recirculating eddying regions for slow chemical reactions The aim of this project was to use the numerical simulations to develop suitable criteria for defining these eddying or vortical zones The C and streaming (S) zones were defined in order to define the whole flow field It is concluded that homogeneous and sheared turbulent flow fields are made up of characteristic flow zones: eddy, C, and S zones A set of objective criteria were found which describe regions in which the streamlines circulate, converge or diverge, and form high streams of high velocity flow

One-dimensional transport and the quantisation of the ballistic resistance

Abstract: The authors present experimental results, and a supporting theory, showing that a one-dimensional system in which transport is ballistic possesses a quantised resistance, h/2ie2, where i is the number of occupied 1D sub-bands and the spin degeneracy is two. A short narrow channel is defined in the 2DEG of a GaAs-AlGaAs heterojunction and as the width of the system is changed, the sub-bands pass through the Fermi energy and the resistance jumps between quantised values. The value of the quantised resistance is derived and the accuracy of the quantisation is discussed. The effect can be strong at temperatures approximately 0.1 K, with up to 17 sub-bands being observed. The action of a transverse magnetic field is to depopulate the sub-bands and form hybrid levels; a parallel field lifts the spin degeneracy and brings about a further quantisation of resistance at values h/2(i+1/2)e2.

Industrial Microwave Heating

Abstract: This book offers a broad coverage of the theory and practice of industrial microwave heating.

Tidal disruption of stars by black holes of 10 6 –10 8 solar masses in nearby galaxies

Abstract: Stars in galactic nuclei can be captured or tidally disrupted by a central black hole. Some debris would be ejected at high speed', the remainder would be swallowed by the hole, causing a bright flare lasting at most a few years. Such phenomena are compatible with the presence of 106–108 M⊙ holes in the nuclei of many nearby galaxies. Stellar disruption may have interesting consequences in our own Galactic Centre if a ∼106 M⊙ hole lurks there.

An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program

Abstract: This paper provides methods and a description of a Pascal computer program, thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput from thermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end-members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset. thermocalc is available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per version.

Human gene expression first occurs between the four- and eight-cell stages of preimplantation development

Abstract: The earliest stages of development in most animals, including the few mammalian species that have been investigated, are regulated by maternally inherited information. Dependence on expression of the embryonic genome cannot be detected until the mid two-cell stage in the mouse, the four-cell stage in the pig (J. Osborn & C. Polge, personal communication), and the eight-cell stage in the sheep. Information about the timing of activation of the embryonic genome in the human is of relevance not only to the therapeutic practice of in vitro fertilization and embryo transfer (IVF), but more importantly for the successful development of techniques for the preimplantation diagnosis of certain inherited genetic diseases. We describe here changes in the pattern of polypeptides synthesized during the pre-implantation stages of human development, and demonstrate that some of the major qualitative changes which occur between the four- and eight-cell stages are dependent on transcription. In addition, it appears that cleavage is not sensitive to transcriptional inhibition until after the four-cell stage.

The CpG dinucleotide and human genetic disease

Abstract: Reports of single base-pair mutations within gene coding regions causing human genetic disease were collated. Thirty-five per cent of mutations were found to have occurred within CpG dinucleotides. Over 90% of these mutations were C → T or G → A transitions, which thus occur within coding regions at a frequency 42-fold higher than that predicted from random mutation. These findings are consistent with methylation-induced deamination of 5-methyl cytosine and suggest that methylation of DNA within coding regions may contribute significantly to the incidence of human genetic disease.

Cytosolic calcium oscillators.

Abstract: Many cells display oscillations in intracellular calcium resulting from the periodic release of calcium from intracellular reservoirs. Frequencies are varied, but most oscillations have periods ranging from 5 to 60 s. For any given cell, frequency can vary depending on external conditions, particularly the concentration of natural stimuli or calcium. This cytosolic calcium oscillator is particularly sensitive to those stimuli (neurotransmitters, hormones, growth factors) that hydrolyze phosphoinositides to give diacylglycerol and inositol 1,4,5-trisphosphate (Ins1,4,5P3). The ability of Ins1,4,5P3 to mobilize intracellular calcium is a significant feature of many of the proposed models that are used to explain oscillatory activity. Receptor-controlled oscillator models propose that there are complex feedback mechanisms that generate oscillations in the level of Ins1,4,5P3. Second messenger-controlled oscillator models demonstrate that the oscillator is a component of the calcium reservoir, which is induced to release calcium by a constant input of either Ins1,4,5P3 or calcium itself. In the latter case, the process of calcium-induced calcium release might be the basis of oscillatory activity in many cell types. The function of calcium oscillations is still unknown. Because oscillator frequency can vary with agonist concentration, calcium transients might be part of a frequency-encoded signaling system. When an external stimulus arrives at the cell surface the information is translated into a train of calcium spikes, i.e., the signal is digitized. Certain cells may then convey information by varying the frequency of this digital signal.

Historical Biogeography of the Drosophila melanogaster Species Subgroup

Abstract: Whereas there has been increasing interest in the eight members of the melanogaster species subgroup of Drosophila, no comprehensive survey exists of the biogeography and ecology of these species in the Afrotropical region.

Naturally occurring auxin transport regulators.

Abstract: The process of polar auxin transport, central to a plant's auxin relations, can be inhibited by a group of synthetic compounds that apparently act by binding to a plasma membrane protein known as the naphthylphthalamic acid (NPA) receptor No endogenous ligand to the NPA receptor, capable of affecting polar auxin transport in plants, has yet been found It is now shown that a group of flavonoids-including quercetin, apigenin, and kaempferol-can specifically compete with [(3)H]NPA for binding to its receptor and can perturb auxin transport in a variety of plant tissues and transport systems in a manner closely paralleling the action of synthetic transport inhibitors Because the active flavonoids are widely distributed in the plant kingdom and exert their effects at micromolar concentrations approximating likely endogenous levels, they may act as natural auxin transport regulators in plants

The oceanic chemistry of the rare-earth elements

Abstract: Because of the unique chemical characteristics of the rare-earth elements (rees), they provide information as oceanic tracers of element source and reactivity. The absolute and relative concentrations of the rees in ocean waters reflect their input from rivers, by aeolian transport and from hydrothermal vents; their interaction with the biogeochemical cycle involving removal from surface waters by adsorption and oxidation at particle surfaces (probably with organic coatings) and deeper regeneration; and the effects of advective transport. Within the ree group, Ce anomalies occur in response to oceanic redox conditions; Eu anomalies in response to aeolian and hydrothermal input; and Nd isotopic variations reflect element sources. Both ree concentration patterns and Nd isotopic variations allow the effects of the vertical particle flux and of lateral advection to be separated.

The product of the Drosophila gene vasa is very similar to eukaryotic initiation factor-4A

Abstract: The vasa gene product of Drosophila melanogaster is required only in the female germ line. Progeny of females homozygous for vasa mutations lack posterior structures and pole cells. Isolation and characterization of vasa genomic and complementary DNA clones show that the transcript is abundant in the female germ line and early embryos only. The predicted amino acid sequence is very similar to those of the translation initiation factor eIF-4A and the human nuclear antigen p68.

Analysis of a complete galaxy redshift survey – II. The field-galaxy luminosity function

Abstract: Analyse statistique de la fonction de luminosite des galaxies de champ de plusieurs catalogues. Les differentes methodes d'estimation sont testees. On montre que la fonction de luminosite de champ peut etre decrite par une fonction de Schechter dont on calcule les differents parametres

The force exerted on a body in inviscid unsteady non-uniform rotational flow

Abstract: A general expression is derived for the fluid force on a body of simple shape moving with a velocity v through inviscid fluid in which there is an unsteady non-uniform rotational velocity field uo(x,t) in two or three dimensions. It is assumed that the radius is small compared with the scale over which the strain rate changes, though for the sphere it is also assumed that the changes in the ambient velocity field over the scale of the sphere are small compared with the velocity of the body relative to the flow. Given these approximations it is shown that the effects of the rate of change of the vorticity of the ambient flow is of second order and can be neglected. However the rate of change of the irrotational straining motion is included in the analysis. It is shown that the inertial forces derived by many authors for irrotational flow can be simply added to a generalization of the lift force derived by Auton (1987) in a companion paper. It is shown how this lift force is made up of a rotational and an inertial or added-mass component. For three-dimensional bluff bodies the latter is generally larger (by a factor of three for a sphere), and can be simply calculated from the added-mass coefficient. For illustration, the general expression is used to derive formulae for (i) the motion of a spherical bubble in a steady non-uniform flow to contrast with the motion in an unsteady flow, and (ii) the motion of rigid volumes of neutral density across an inviscid shear flow. These results show how added-mass (and lift) forces lead to different motions for a sphere and a cylinder. The general expression is useful in two-phase flow calculations, and for indicating the forces and motions of 'lumps of fluid' in turbulent flows.

Criminal career research: its value for criminology*

Abstract: In a recent paper published in this journal, Gottfredson and Hirschi (1986)1 argue that the concepts of criminal careers, career criminals, selective incapacitation, prevalence, and incidence, and longitudinal studies all have little value for criminology In our view their paper misrepresents these concepts and our research on these topics We are pleased to have the opportunity in this paper to develop these concepts more clearly and to show their relevance for criminology

HOL: A Proof Generating System for Higher-Order Logic

Abstract: HOL is a version of Robin Milner’s LCF theorem proving system for higher-order logic. It is currently being used to investigate (1) how various levels of hardware behaviour can be rigorously modelled and (2) how the resulting behavioral representations can be the basis for verification by mechanized formal proof. This paper starts with a tutorial introduction to the meta-language ML. The version of higher-order logic implemented in the HOL system is then described. This is followed by an introduction to goal-directed proof with tactics and tacticals. Finally, there is a little example of the system in action which illustrates how HOL can be used for hardware verification.