Showing papers by "Imperial College London published in 1975"

Progress in the development of a Reynolds-stress turbulence closure

Abstract: The paper develops proposals for a model of turbulence in which the Reynolds stresses are determined from the solution of transport equations for these variables and for the turbulence energy dissipation rate E. Particular attention is given to the approximation of the pressure-strain correlations; the forms adopted appear to give reasonably satisfactory partitioning of the stresses both near walls and in free shear flows. Numerical solutions of the model equations are presented for a selection of strained homogeneous shear flows and for two-dimensional inhomogeneous shear flows including the jet, the wake, the mixing layer and plane channel flow. In addition, it is shown that the closure does predict a very strong influence of secondary strain terms for flow over curved surfaces.

Identification of two related pentapeptides from the brain with potent opiate agonist activity

Abstract: Enkephalin, a natural ligand for opiate receptors is composed of the pentapepides H-Tyr-Gly-Gly-Phe-Met-OH and H-Tyr-Gly-Gly-Phe-Leu-OH. The evidence is based on the determination of the amino acid sequence of natural enkephalin by the dansyl-Edman procedure and by mass spectrometry followed by synthesis and comparison of the natural and synthetic peptides.

Fundamentals of Queueing Theory

Abstract: Praise for the Third Edition: "This is one of the best books available. Its excellent organizational structure allows quick reference to specific models and its clear presentation . . . solidifies the understanding of the concepts being presented."IIE Transactions on Operations EngineeringThoroughly revised and expanded to reflect the latest developments in the field, Fundamentals of Queueing Theory, Fourth Edition continues to present the basic statistical principles that are necessary to analyze the probabilistic nature of queues. Rather than presenting a narrow focus on the subject, this update illustrates the wide-reaching, fundamental concepts in queueing theory and its applications to diverse areas such as computer science, engineering, business, and operations research.This update takes a numerical approach to understanding and making probable estimations relating to queues, with a comprehensive outline of simple and more advanced queueing models. Newly featured topics of the Fourth Edition include:Retrial queuesApproximations for queueing networksNumerical inversion of transformsDetermining the appropriate number of servers to balance quality and cost of serviceEach chapter provides a self-contained presentation of key concepts and formulae, allowing readers to work with each section independently, while a summary table at the end of the book outlines the types of queues that have been discussed and their results. In addition, two new appendices have been added, discussing transforms and generating functions as well as the fundamentals of differential and difference equations. New examples are now included along with problems that incorporate QtsPlus software, which is freely available via the book's related Web site.With its accessible style and wealth of real-world examples, Fundamentals of Queueing Theory, Fourth Edition is an ideal book for courses on queueing theory at the upper-undergraduate and graduate levels. It is also a valuable resource for researchers and practitioners who analyze congestion in the fields of telecommunications, transportation, aviation, and management science.

A more general effective-viscosity hypothesis

Abstract: A discussion of the applicability of an effective-viscosity approach to turbulent flow suggests that there are flow situations where the approach is valid and yet present hypotheses fail. The general form of an effective-viscosity formulation is shown to be a finite tensor polynomial. For two-dimensional flows, the coefficients of this polynomial are evaluated from the modelled Reynolds-stress equations of Launder, Reece & Rodi (1975). The advantage of the proposed effective-viscosity formulation, equation (4.3), over isotropie-viscosity hypotheses is that the whole velocity-gradient tensor affects the predicted Reynolds stresses. Two notable consequences of this are that (i) the complete Reynolds-stress tensor is realistically modelled and (ii) the influence of streamline curvature on the Reynolds stresses is incorporated.

Missing Values in Multivariate Analysis

Abstract: SUMMARY This paper presents computational results for some alternative methods of analysing multivariate data with missing values. We recommend an algorithm due to Orchard and Woodbury (1972), which gives an estimator that is maximum likelihood when the data come from a multivariate normal population. We include a derivation of the estimator that does not assume a multivariate normal population, as an iterated form of Buck's (1960) method. We derive an approximate method of assigning standard errors to regression coefficients estimated from incomplete observations, and quote supporting evidence from simulation studies. A brief account is given of the application of these methods to some school examinations data.

On the effects of a gravitational field on the turbulent transport of heat and momentum

Abstract: This paper suggests a simple way of including gravitational effects in the pres-sure-containing correlations that appear in the equations for the transport of Reynolds stress and heat flux. The predicted changes in structure due to the gravitational field are shown to agree closely with Webster's (1964) measurements in a stably stratified shear flow.

The determination of the fracture parameters for polymers in impact

Abstract: A method of analysis is given by which the critical strain energy release rate Gc for impact tests may be deduced for both Charpy and Izod tests from normal energy measurements Suitable calibration factors are determined and the method is applied to a range of polymers Very close agreement is achieved between the Charpy and Izod results except for highly ductile materials for which it was necessary to use a fully plastic analysis The method is extended to blunt notches and it is shown that the use of a strain energy per unit volume to yielding, together with a blunt notch stress analysis, gives a good description of the results

The design of experiments for discriminating between two rival models

Abstract: where the design points xi are known and the random variables Cik are independently normally distributed with zero mean and constant variance 0y2. In the theoretical development, but not in the numerical examples, we shall, without loss of generality, take 0-2 to be unity. The function st(x) is one of two known functions 81(x, 01) and 82(x, 02), where 01 and 02 are sets of unknown parameters of dimension m1 and in2. The purpose of the experiment is to determine which of the two models is true. The design of experiments for discriminating between any number of models has been investigated by several authors. References to this work and some comments on general aspects of the problem are given in the recent paper of Atkinson & Cox (1974) and in the ensuing discussion. Designs specific for discriminating between only two models have been derived by Fedorov & Malyutov (1972) and Fedorov & Uspensky (1975). In the present paper we collect, exemplify and generalize these results on designs for two models and describe the relationship with the designs of Atkinson & Cox, which are based on an extension of D-optimum design theory. In ? 2 we describe nonsequential designs which are the limits to which the sequential designs of ? 3 converge as the number of trials increases. In ? 4 we discuss the relationship between the two approaches when both models are linear. The least squares estimates of the parameters in the two models, which in general need not be linear, are the solutions of the equations

The instability of a straight vortex filament in a strain field

Abstract: A straight infinite vortex of finite cross section is deformed by the action of weak irrotational plane strain. The deformed vortex is shown, in the absence of axial flow, to be unstable to disturbances whose axial wavelengths lie in a narrow band, whose width is proportional to the imposed strain. The band is centred on the wavelength of the helical wave which does not propagate on the unstrained circular vortex. Thus support is given to the instability mechanism proposed recently by Widnall, Bliss & Tsai (1974). The argument depends, however, on the mirror image of the helical wave also being a possible non-propagating disturbance on the unstrained vortex.

Optimal design : Experiments for discriminating between several models

Abstract: In this paper we consider experimental designs for discriminating between three or more rival regression models. In an earlier paper (Atkinson & Fedorov, 1975) we described optimal designs for discriminating between two models. Although the approach and nomenclature of the present paper are similar to those of the earlier one, the results are not, except in some simple cases, a straightforward generalization of those when there are only two models. We now suppose that there are v rival regression models

Superfields and Fermi-Bose symmetry

Abstract: The realization of supergauge transformations on fields defined over an 8-dimensional space whose points are labeled by ${x}_{\ensuremath{\mu}}$ and the anticommuting Majorana spinor ${\ensuremath{\theta}}_{\ensuremath{\alpha}}$ is described. The covariant derivative is defined and applied to the problem of decomposing superfields into irreducible (chiral) parts and to the problem of constructing "supersymmetric" Lagrangians. Further, it is shown how to build internal symmetries (both global and local) into these Lagrangians. An example is discussed in which the internal (global) symmetry is spontaneously violated, giving rise to a supermultiplet of Goldstone particles (including fermions). When a local symmetry is broken the Higgs mechanism (for bosons and fermions) is shown to be operative. A possible solution to the problem of defining a conserved fermion number is indicated.

Regulation of synthesis of cell wall degrading enzymes by Veticillium albo-atrum and Fusarium oxysporum f. sp. lycopersici☆

Abstract: Induction and catabolite repression of synthesis of cell wall degrading enzymes by the vascular wilt fungi Verticillium albo-atrum and Fusarium oxysporum f. sp. lycopersici have been studied. In cultures containing inorganic salts and tomato stem cell walls each fungus produced a range of extracellular, polysaccharide degrading enzymes. In V. albo-atrum cultures, the enzymes appeared sequentially over 2 to 9 days in the order endopolygalacturonase (endo-PG), exo-arabinanase, endo-pectin-trans-eliminase (endo-PTE), endoxylanase and cellulase; β- d -galactosidase, galactanase and β- d -glucosidase were also produced. Under similar conditions F. oxysporum produced endo-PG, endo-PTE, arabinanase, β- d -galactosidase, xylanase and cellulase. Synthesis of each enzyme was almost always induced specifically by the sugar or uronic acid unit predominant in the specific polymeric substrate for the enzyme. This became evident only when inducers were supplied at rates which prevented their accumulation in cultures. Exceptionally, cellulase was induced by cellobiose but not glucose, and β-galactosidase a and galactanase were induced both by d -galactose and the structurally related l -arabinase. Synthesis was repressed when inducers were present in slight excess of requirements for growth. Synthesis by V. albo-atrum of endo-PG and endo-PTE increased with inducer supply until inducer began to accumulate, then decreased sharply and was almost completely repressed at supply rates three times the optimal value. Synthesis of exo-arabinanase and β-galactosidase decreased as supply of arabinose increased above the optimum but remained at much higher levels relative to those for endo-PG and endo-PTE. Constitutive synthesis of enzymes by V. albo-atrum was also catabolite repressed but to a lesser degree. The significance of induction and catabolite repression in pathogenesis in vascular wilts is discussed.

Components and patterns in the behaviour of the nematode Caenorhabditis elegans

Abstract: The behavioural activities during movement, feeding and defaecation have been recorded and measured in adult females of Caenorhabditis elegans. The postures and components of recognizable wave forms are described. Stress has been laid on the mechanism of antagonistic interaction of backward and forward movement, and the rates and characteristics of “spontaneous” and “induced” reversal periods. During feeding, rapid rates of pharyngeal activity are invariably related to low rates of somatic muscle wave propagation. Head oscillations are considered to be separate events not directly linked with feeding or foraging. The combination of certain wave forms, together with other measurements have been used to develop a hypothesis to describe a co-ordinating mechanism applicable to the nematode level of organization.

Incidence and significance of hyperprolactinaemia in women with amenorrhoea

Abstract: Serum prolactin and gonadotrophin concentrations were measured by radioimmunoassay in 106 women with amenorrhoea. Prolactin was normal in those with weight related disorders, primary ovarian failure, those with a variety of systemic diseases and in those in whom amenorrhoea followed treatment with the oral contraceptive and in unexplained primary amenorrhoea. Gonadotrophin concentrations in the above patients were normal except in those with primary ovarian failure. Prolactin was elevated in eight of forty patients (20%) with functional secondary amenorrhoea and was greatly raised in all but one of the thirteen women in this series with pituitary tumours (five of whom were studied only after treatment). Only three patients in each of the last two groups had galactorrhoea. Gonadotrophin levels were normal or slightly raised in all of the hyperprolactinaemic patients apart from those studied after hypophysectomy. Four hyperprolactinaemic patients (three with pituitary tumours and one with functional amenorrhoea) who did not have galactorrhoea have been treated with bromocriptine. Prolactin secretion was reduced in all patients and, in the two with normal gonadotrophins, ovulatory menstruation was resumed. One became pregnant in the second ovulation cycle after starting treatment. We conclude that, despite the rarity of galactorrhoea, hyperprolactinaemia is common in patients with functional amenorrhoea and in those with pituitary tumours. Treatment with bromocriptine in patients with normal gonadotrophins restores ovulation when the infertility is due to prolactin excess.

The alloying behaviour and mechanical properties of polycrystalline Ni3Al (?? phase) with ternary additions

Abstract: A survey of the results from a variety of techniques has shown that the substitution behaviour of alloying additions is primarily determined by electronic considerations. Thus, Si, Ti, V, Mn, Nb, Hf and Ta substitute for aluminium; Co and Cu substitute for nickel and Cr, Fe, W, and Mo substitute for both species. A number of conclusions have been drawn from an analysis of compression test data, the most significant of which was that the mechanical properties of γ′ depended on both the substitution behaviour of the alloying addition and the degree of non-stoichiometry. Considerable strengthening is only obtained when (1) the alloying addition substitutes for aluminium and has a large size misfit parameter, and (2) the alloy is aluminium-rich or stoichiometric.

Prediction of turbulent flow in curved pipes

Abstract: A finite-difference procedure is employed to predict the development of turbulent flow in curved pipes. The turbulence model used involves the solution of two differential equations, one for the kinetic energy of the turbulence and the other for its dissipation rate. The predicted total-velocity contours for the developing flow in a 180° bend are compared with the experimental data. Predictions of fully developed velocity profiles for long helically wound pipes are also presented and compared with experimental measurements.

A note on data-splitting for the evaluation of significance levels

Abstract: SUMMARY It has sometimes been suggested that to overcome difficulties arising in significance tests when the effects tested are selected in the light of the data, the data should be split randomly into two portions. The first portion is used to choose the hypothesis for test and the second portion for the evaluation of significance. After some general criticism of the idea, it is investigated theoretically on a simple problem about normal means. Recommendations are reached about the proportions into which the data should be divided and the theoretical efficiency of the procedure is assessed and found to be quite high.

The density of organized vortices in a turbulent mixing layer

Abstract: It is argued on the basis of exact solutions for uniform vortices in straining fields that vortices of finite cross-section in a row will disintegrate if the spacing is too small. The results are applied to the organized vortex structures observed in turbulent mixing layers. An explanation is provided for the disappearance of these structures as they move downstream and it is deduced that the ratio of average spacing to width should be about 3·5, the width being defined by the maximum slope of the mean velocity. It is shown in an appendix that walls have negligible effect.

A comparison of natriuresis after oral and intravenous sodium loading in sodium-depleted man: evidence for a gastrointestinal or portal monitor of sodium intake.

Abstract: 1. Dietary sodium reduction in man is followed by rapid conservation of sodium by the kidneys. The rapidity of this response suggests that the gastrointestinal tract is involved in early recognition of changes in sodium intake or in mediation of the compensatory response. 2. In order to test the hypothesis, 100 mmol of sodium was given to normal volunteers in balance on a low-sodium diet (5 mmol/24 h): the dose was given either orally or intravenously. 3. Those who received their sodium orally excreted it more rapidly than those who received it intravenously and the difference was most marked in the first 8 h after the dose. 4. This finding is consistent with the presence of an input receptor for sodium in the gastrointestinal tract.

Possible cocarcinogenic effects of coffee constituents

Abstract: ALTHOUGH the carcinogenic properties of secondary N-nitro-samines and related compounds are well established1,2, their causal relationship to human cancer remains an open question because very little is known about the degree of human exposure to them. Carcinogenic N-nitrosamines have been detected in some nitrite-preserved foodstuffs, but only at relatively low levels3,4, so the major uncertainty comes from their formation in vivo from ingested nitrite and amino compounds. Formation of N-nitrosamines in this way in the digestive tract of laboratory animals has been widely demonstrated (for typical examples, see refs 5–7) and has led to the induction of tumours characteristic of N-nitrosamines8,9. We have suggested10 that the interaction between nitrite and phenolic materials should also be considered, because the latter are major dietary constituents which usually react much more rapidly than most amino compounds with nitrous acid. Certain natural phenols may therefore inhibit N-nitrosamine formation both in foodstuffs and in the digestive tract. We report here, however, that readily oxidised phenolic compounds act as catalysts rather than inhibitors for N-nitrosamine formation from nitrite salts and secondary amines at gastric pH. This finding has important implications on the possible cocarcinogenic properties of several foodstuffs and beverages, including coffee.