bluuebunny/crossref_metadata_2025 huggingface dataset:
A porous-wavemaker theory
Authors: Chwang Allen T.
Published: April 2006

A porous-wavemaker theory is developed to analyse small-amplitude surface waves on water of finite depth, produced by horizontal oscillations of a porous vertical plate. Analytical solutions in closed forms are obtained for the surface-wave profile, the hydrodynamic-pressure distribution and the total force on the wavemaker. The influence of the wave-effect parameter C and the porous-effect parameter G, both being dimensionless, on the surface waves and on the hydrodynamic pressures is discussed in detail.
Time-dependent density profiles in a filling box
Authors: Worster M. Grae , Huppert Herbert E.
Published: April 2006

An approximate analytic expression for the time-dependent density profile formed by a turbulent buoyant plume in a confined region is presented. The analysis is based on the approximation that the density of the fluid behind the first front changes at a rate which is virtually independent of position. The approximate expression is shown to be in excellent agreement with a full numerical integration of the governing equations.
Wave evolution over a gradual slope with turbulent friction
Authors: Miles John W.
Published: April 2006

The evolution of a weakly nonlinear, weakly dispersive gravity wave in water of depth d over a bottom of gradual slope δ and Chezy friction coefficient Cf is studied. It is found that an initially sinusoidal wave evolves into a periodic sequence of solitary waves with relative amplitude a/d = α1 = 15δ/4Cf if α1 < αb, where αb is the relative amplitude above which breaking occurs. This prediction is supported by observations (Wells 1978) of the evolution of swell over mudflats.
Mean flow in round bubble plumes
Authors: Milgram J. H.
Published: April 2006

Previous experimental studies are reviewed and those whose data are deemed reliable are identified. New experiments at larger scale are described and the results are reported. These are combined with the reliable previous studies to form a data set covering heights from 3.66 to 50 m and gasflow rates from 0.0002 to 0.59 normal m3/s. These wide-ranging data are combined with an integral theory for bubble plumes to determine functional relationships between local plume properties and the entrainment coefficient and the fraction of the momentum flux that is carried in the turbulent velocity fluctuations. These relationships together with the integral theory provide a set of equations that are suitable for numerical solution for the mean flow properties of any round bubble plume. Examples of the numerical solutions are presented and a comparison of one of these with existing experimental data is given. The relationships between the local plume properties and the entrainment coefficient and the momentum flux carried by the turbulence are interpreted to provide a qualitative understanding of the parameters involved and their influences on the plume.
Double-Diffusive Convection: A report on an Engineering Foundation Conference
Authors: Chen C. F. , Johnson D. H.
Published: April 2006

Under the auspices of the Engineering Foundation, financial support of the National Science Foundation, and the cochairmanship of the authors, a conference on ‘Double-Diffusive Convection’ was held from 14–18 March 1983 in Santa Barbara. The conference attracted more than seventy scientists and engineers working in various disciplines, and 45 talks were presented. There was an ad hoc film session which ended with a demonstration of a laboratory experiment on crystallization in a double-diffusive system.
One-dimensional self-similar solution of the dynamics of axisymmetric slender liquid bridges
Authors: Rivas D. , Meseguer J.
Published: April 2006

Liquid bridges appear in a large variety of industrial processes such as the so-called floating-zone technique, used in recent years in crystal growth and in purification of high-melting-point materials.

In this paper the dynamics of axisymmetric, slender, viscous liquid bridges having volume close to the cylindrical one, and subjected to a small gravitational field parallel to the axis of the liquid bridge, is considered within the context of one-dimensional theories. Although the dynamics of liquid bridges has been treated through a numerical analysis in the inviscid case, numerical methods become inappropriate to study configurations close to the static stability limit because the evolution time, and thence the computing time, increases excessively. To avoid this difficulty, the problem of the evolution of these liquid bridges has been attacked through a nonlinear analysis based on the singular perturbation method and, whenever possible, the results obtained are compared with the numerical ones.
The boundary layer induced by a convected two-dimensional vortex
Authors: Doligalski T. L. , Walker J. D. A.
Published: April 2006

The response of a wall boundary layer to the motion of a convected vortex is investigated. The principal cases considered are for a rectilinear filament of strength –κ located a distance a above a plane wall and convected to the right in a uniform flow of speed U∞*. The inviscid solution predicts that such a vortex will remain at constant height a above the wall and be convected with constant speed αU∞*. Here α is termed the fractional convection rate of the vortex, and cases in the parameter range 0 [les ] α < 1 are considered. The motion is initiated at time t* = 0 and numerical calculations of the developing boundary-layer flow are carried out for α = 0, 0.2, 0.4, 0.55, 0.7, 0.75 and 0.8. For α < 0.75, a rapid lift-up of the boundary-layer streamlines and strong boundary-layer growth occurs in the region behind the vortex; in addition an unusual separation phenomenon is observed for α [les ] 0.55. For α [ges ] 0.75, the boundary-layer development is more gradual, but ultimately substantial localized boundary-layer growth also occurs. In all cases, it is argued that a strong inviscid–viscous interaction will take place in the form of an eruption of the boundary-layer flow. The generalization of these results to two-dimensional vortices with cores of finite dimension is discussed.
A simple turbulence closure hypothesis for the triple-velocity correlation functions in homogeneous isotropic turbulence
Authors: Domaradzki J. A. , Mellor G. L.
Published: April 2006

A simple two-point closure scheme for homogeneous axisymmetric turbulence is developed. For the isotropic case it is essentially an eddy-viscosity assumption in real space for the Kármán-Howarth equation. The eddy-viscosity function for large internal Reynolds numbers is derived from Kolmogoroff's 1941 theory. For moderate Reynold's numbers of order 102, approximately the same expression for the eddy-viscosity function is determined from experimental data. The resulting closed equation for the double-correlation function is solved numerically for both large and moderate Reynolds numbers, and the results are compared with experimental data. Self-similar solutions of the basic equation predict turbulent energy decay inversely proportional to time. It is shown that the departure from this ‘initial-period decay law’ observed in laboratory data is due to the behaviour of grid-produced correlation functions for large separation distances.
High-order velocity structure functions in turbulent shear flows
Authors: Anselmet F. , Gagne Y. , Hopfinger E. J. , Antonia R. A.
Published: April 2006

Measurements are presented of the velocity structure function on the axis of a turbulent jet at Reynolds numbersRλ≤ 852 and in a turbulent duct flow atRλ= 515. Moments of the structure function up to the eighteenth order were calculated, primarily with a view to establish accurately the dependence on the order of the inertial range power-law exponent and to draw conclusions about the distribution of energy transfer in the inertial range. Adequate definition of the probability density of the structure function was achieved only for moments of ordern≤ 10. It is shown, however, that, although the values of moments ofn> 10 diverges from their true values, the dependence of the moment of the structure function on the separationris still given to a fair accuracy for moments up ton≈ 18. The results demonstrate that the inertial-range power-law exponent is closely approximated by a quadratic dependence on the power which for lower-order moments (n[lsim ] 12) would be consistent with a lognormal distribution. Higher-order moments diverge, however, from a lognormal distribution, which gives weight to Mandelbrot's (1971) conjecture that ‘Kolmogorov's third hypothesis’ is untenable in the strict sense. The intermittency parameter μ, appearing in the power-law exponent, has been determined from sixth-order moments 〈(δμ)6〉 ∼r2−μto be μ = 0.2 ± 0.05. This value coincides with that determined from non-centred dissipation correlations measured in identical conditions.
Behaviour of a converging-channel breakwater; theory and experiment
Authors: Pullin D. I. , Joubert P. N.
Published: April 2006

The energy-transmission characteristics of an array of open convergent-channel-type wavebreakers to a normally incident wavetrain is studied experimentally and theoretically. A model of the wavebreaker transmission response to a sinusoidal incident wavetrain on a constant-depth ocean is developed as a boundary-value problem within the framework of linearized water-wave theory. This results in an integral equation based on a Green-function solution to the Helmholtz equation, which is solved numerically for each of several wavebreaker geometries over a range of incident wavelengths. Experiments on fixed model wavebreakers performed in an open wave tank are described, and results are compared with the numerical predictions. Theory and experiment indicate that the convergent-channel design is effective in attenuating the transmitted wave at resonant values of the incident wavelength λ that scale on the channel length L. Forces experienced by each half-wavebreaker are largely transverse; the calculated longitudinal forces for the fixed breakwater are small compared to those on a solid wall except at one resonance near L/λ = 0.5. Experiments indicate that this last condition does not occur for a floating breakwater, where the corresponding transmission of wave energy is high.
Volume-cycled oscillatory flow in a tapered channel
Authors: Grotberg James B.
Published: April 2006

Oscillatory viscous flow in a tapered channel is analysed under conditions of fixed stroke volume. A lubrication theory is developed for small taper and the general result is steady bidirectional drift and an induced steady pressure gradient. The characteristics of the drift profiles change significantly as the Womersley parameter is increased. For large values difficulties arise in the matched asymptotics method which are resolved by introducing a steady drift layer that is much thicker than the Stokes layer. This double boundary layer does not arise in pressure-cycled oscillations. Both Eulerian and Lagrangian drift are examined. The results are compared qualitatively to experimental observations which primarily focus on the application to ventilation in the lung.
Instability of the flow of two immiscible liquids with different viscosities in a pipe
Authors: Joseph Daniel D. , Renardy Michael , Renardy Yuriko
Published: April 2006

We study the flow of two immiscible fluids of different viscosities and equal density through a pipe under a pressure gradient. This problem has a continuum of solutions corresponding to arbitrarily prescribed interface shapes. The question therefore arises as to which of these solutions are stable and thus observable. Experiments have shown a tendency for the thinner fluid to encapsulate the thicker one. This has been ‘explained’ by the viscous-dissipation principle, which postulates that the amount of viscous dissipation is minimized for a given flow rate. For a circular pipe, this predicts a concentric configuration with the more viscous fluid located at the core. A linear stability analysis, which is carried out numerically, shows that while this configuration is stable when the more viscous fluid occupies most of the pipe, it is not stable when there is more of the thin fluid. Therefore the dissipation principle does not always hold, and the volume ratio is a crucial factor.
Wave speeds in wavy Taylor-vortex flow
Authors: King Gregory P. , Lee Y. Li, W. , Swinney Harry L. , Marcus Philip S.
Published: April 2006

The speed of travelling azimuthal waves on Taylor vortices in a circular Couette system (with the inner cylinder rotating and the outer cylinder at rest) has been determined in laboratory experiments conducted as a function of Reynolds number R, radius ratio of the cylinders η, average axial wavelength $\overline{\lambda}$, number of waves m1 and the aspect ratio Γ (the ratio of the fluid height to the gap between the cylinders). Wave speeds have also been determined numerically for axially periodic flows in infinite-length cylinders by solving the Navier-Stokes equation with a pseudospectral technique where each Taylor-vortex pair is represented with 32 axial modes, 32 azimuthal modes (in an azimuthal angle of 2π/m1) and 33 radial modes. Above the onset of wavy-vortex flow the wave speed for a given η decreases with increasing R until it reaches a plateau that persists for some range in R. In the radius-ratio range examined in our experiments we find that the wave speed in the plateau region increases monotonically from 0.14Ω at η = 0.630 to 0.45Ω at η = 0.950 (where the wave speed is expressed in terms of the rotation frequency Ω of the inner cylinder). There is a much weaker dependence of the wave speed on $\overline{\lambda}$, m1 and Γ. For three sets of parameter values (R, $\overline{\lambda}$, η and m1) the wave speeds have been measured, extrapolated to infinite aspect ratio, and compared with the numerically computed values. For each of these three cases the agreement is within 0.1 %.
Mixing layer produced by a screen and its dependence on initial conditions
Authors: Oguchi Hakuro , Inoue Osamu
Published: April 2006

This paper aims to elucidate the structure of the turbulent mixing layers, especially, its dependence on initial disturbances. The mixing layers are produced by setting a woven-wire screen perpendicular to the freestream in the test section of a wind tunnel to obstruct part of the flow. Three kinds of model geometry are treated; these model screens produced mixing layers which may be regarded as the equivalents of the plane mixing layer and of two-dimensional and axisymmetric wakes issuing into ambient streams of higher velocity. The initial disturbances are imposed by installing thin rods of various sizes along the edge of the screen or at the origin of the mixing layer. Flow features are visualized by the smoke-wire method. Statistical quantities are measured by a laser-Doppler velocimeter. In all cases large-scale transverse vortices seem to persist, although comparatively small-scale vortices are superimposed on the flow field in the mixing layer. The mixing layers are in self-preserving state at least up to third-order moments, but the self-preserving state is different in each case. The growth rates of the mixing layer are shown to depend strongly on the initial disturbance imposed.
The structures and energetics of fully nonlinear symmetric baroclinic waves
Authors: Miller Timothy L.
Published: April 2006

A finite-difference Navier-Stokes model has been used to study rotating baroclinic flow for Richardson number [lsim ] 1, assuming no variations except in the vertical plane wholly containing the density-gradient vector. A section of a horizontally infinite channel has been studied, assuming periodic boundary conditions at the vertical computational boundaries and no-slip conducting horizontal boundaries. Two configurations were studied, both of which have an analytic basic solution with no horizontal variations in the velocities or density gradients. Symmetric baroclinic waves developed in the flows, as long as the Richardson number was not too large and the thermal Rossby number was large enough (for fixed diffusion parameters), consistent with linear theory. The structures and energetics of the fully developed waves were found to be especially dependent upon the Prandtl number Pr. Potential energy was the ultimate wave-energy source in all cases, and the average zonal flow was never much affected by the waves. For Pr > 1 the conversion from potential energy to wave kinetic energy was direct, via temperature and vertical-motion correlation. For Pr < 1 the conversion was from potential energy, to average kinetic energy by virtue of an induced meridional flow, to wave kinetic energy. For Pr = 1 the energy conversion was by either or both of the above, depending upon the other parameters.
Unsteady measurements in a separated and reattaching flow
Authors: Cherry N. J. , Hillier R. , Latour M. E. M. P.
Published: April 2006

Measurements of fluctuating pressure and velocity, together with instantaneous smoke-flow visualizations, are presented in order to reveal the unsteady structure of a separated and reattaching flow. It is shown that throughout the separation bubble a low-frequency motion can be detected which appears to be similar to that found in other studies of separation. This effect is most significant close to separation, where it leads to a weak flapping of the shear layer. Lateral correlation scales of this low-frequency motion are less than the reattachment length, however; it appears that its timescale is about equal to the characteristic timescale for the shear layer and bubble to change between various shedding phases. These phases were defined by the following observations: shedding of pseudoperiodic trains of vortical structures from the reattachment zone, with a characteristic spacing between structures of typically 60% to 80% of the bubble length; a large-scale but irregular shedding of vorticity; and a relatively quiescent phase with the absence of any large-scale shedding structures and a significant ‘necking’ of the shear layer downstream of reattachment.

Spanwise correlations of velocity in the shear layer show on average an almost linear growth of spanwise scale up to reattachment. It appears that the shear layer reaches a fully three-dimensional state soon after separation. The reattachment process does not itself appear to impose an immediate extra three-dimensionalizing effect upon the large-scale structures.
Laminar boundary-layer transition on a heated underwater body
Authors: Lauchle Gerald C. , Gurney G. B.
Published: April 2006

A large (3.05 m long × 0.32 m diameter) heated-surface, axisymmetric body, designed for transition research in a 1.22 m diameter water tunnel is described. Boundary-layer transition data are presented as functions of the heating power supplied to the body and the total concentration of free-stream particulate matter in the water. Body surface temperatures range from 0 to 25°C over the ambient water temperature, and the total heat supplied ranges from 0 to 93.3 kW. Transition-arclength Reynolds numbers are found to vary from 4.5 × 106 for the body operating cold to 3.64 × 107 for the maximum heat level considered. The concentration of free-stream particles is shown to affect the transition Reynolds number. These particles range in diameter from 10 to 70 μm and their concentration ranges from less than 5 to 198 particles per cm3. The decrease in transition Reynolds number due to to the higher concentration of particles is of order 30%.
Surface-layer similarity in turbulent circular Couette flow
Authors: Claussen Martin
Published: April 2006

Smith & Townsend's (1982) experimental data on circular Couette flow are re-examined in the framework of surface-layer similarity theory. Surface-layer similarity of horizontally stratified shear flow is shown to have its counterpart in a narrow-gap Couette flow between concentric cylinders. Smith & Townsend's data of mean angular momentum and mean-velocity profiles in a region near a cylinder lend support to the applicability of Monin–Obukhov similarity to circular Couette flow. Only for flows of very high Reynolds numbers is a region of logarithmic variation of mean profiles found close to the cylinder wall. Because of curvature effects on the flow, the mean profiles deviate from the logarithmic profile as distance from the cylinder wall increases. For flows of sufficiently low Reynolds number, but still very high Taylor number, no logarithmic profile seems to exist; instead, profiles in the viscous region and in the outer region are connected to each other by a ‘free-convection (rotation)’ profile. From Smith & Townsend's data the velocity field is not observed to follow the prediction of ‘free-convection’ similarity; however, the ‘free-convection’ profile is found in the distribution of mean angular momentum.
Pattern selection in single-component systems coupling Bénard convection and solidification
Authors: Davis S. H. , Müller U. , Dietsche C.
Published: April 2006

A horizontal layer is heated from below and cooled from above so that the enclosed single-component liquid is frozen in the upper part of the layer. When the imposed temperature difference is such that the Rayleigh number across the liquid is supercritical, there is Bénard convection coupled with the dynamics of the solidification interface. An experiment is presented which shows that the interfacial corrugations that result are two-dimensional when this solid is thin but hexagonal when the solid is thick. A weakly nonlinear convective instability theory is presented which explains this behaviour, and isolates this ‘purely thermal’ mechanism of pattern selection. Jump behaviour is seen in the liquid-layer thickness at the onset of hexagonal convection.
Two-dimensional impervious sails: experimental results compared with theory
Authors: Newman B. G. , Low H. T.
Published: April 2006

Experiments have been made on quasi two-dimensional sails of small camber and at small incidence. Four excess-length ratios have been tested at a Reynolds number of 1.2 x 105. The results for lift, tension, centre of lift, maximum camber and its position, and leading- and trailing-edge membrane angles have been compared with existing inviscid theories and show poor agreement in general. This is attributed to leading- and trailing-edge flow separations as indicated by supplementary flow-visualization experiments. The optimum incidences in particular are much greater than the theoretical value of 0°. Luffing occurs at slightly negative incidences and appears to be a dynamic instability. The highest lift-to-drag ratio obtained was 16.5 on a membrane with an excess-length ratio of 0.03.
On the stability of the unsteady boundary layer on a cylinder oscillating transversely in a viscous fluid
Authors: Hall Philip
Published: April 2006

The stability of the two-dimensional flow induced by the transverse oscillation of a cylinder in a viscous fluid is investigated in both the linear and weakly nonlinear regimes. The major assumption that is made to simplify the problem is that the oscillation frequency is large, in which case an unsteady boundary layer is set up on the cylinder. The basic flow induced by the motion of the cylinder depends on two spatial variables, and is periodic in time. The stability analysis of this flow to axially periodic disturbances therefore leads to a partial differential system dependent on three variables. In the high-frequency limit the linear stability problem can be reduced to a system dependent only on a radial variable and time. Furthermore, the coefficients of the differential operators in this system are periodic in time, so that Floquet theory can be used to reduce this system further to a coupled infinite system of ordinary differential equations together with uncoupled homogeneous boundary conditions. The eigenvalues of this system are found numerically and predict instability entirely consistent with the experiments with circular cylinders performed by Honji (1981). Results are given for cylinders of elliptic cross-section, and it is found that for any given eccentricity the most dangerous configuration is when the cylinder oscillates parallel to its minor axis. Some discussion of nonlinear effects is also given, and for the circular cylinder it is shown that the steady-streaming boundary layer of the basic flow is significantly altered by the instability.
A turbulent bore on a beach
Authors: Svendsen I. A. , Madsen P. A.
Published: April 2006

A theoretical model is developed giving a moderately detailed description of the flow in a turbulent bore, the velocity profiles, the shear stresses, the energy dissipation, etc. An analysis of the flow conditions at the toe of the turbulent front indicates significant differences from the usual description based on the finite-amplitude shallow-water equations, and it is shown that the present model gives a closer description of the actual physical conditions. Finally, numerical results are presented that illustrate how the model works, and test its validity on an example with known properties.
Waves at the interface between a clear liquid and a mixture in a two-phase flow
Authors: Prasad Phoolan
Published: April 2006

In the two-dimensional sedimentation process beneath an inclined wall, the mixture of the participate and liquid phases is separated from the wall by a boundary layer of the clear liquid. This paper contains a simple mathematical model giving waves on the interface between the clear liquid and the mixture. These waves are caused by a discontinuity in the gradient of the tangential velocity of the clear liquid, across the interface. In the limiting case of small concentration of the particulate phase in the mixture, the model gives a dispersive wave running upward along the interface in the direction of the flow in the boundary layer. The effect of finite concentration is to introduce a damping.
Convective motions in a spherical shell
Authors: Zebib Abdelfattah , Goyal Atul K. , Schubert Gerald
Published: April 2006

We compute the axisymmetric convective motions that exist in a spherical shell heated from below with inner to outer radius ratio equal to 0.5. The boundaries are stress-free and gravity is directly proportional to radius. Accurate solutions at large Rayleigh numbers (O(105)) are made feasible by a spectral method that employs diagonal-mode truncation. By examining the stability of axisymmetric motions we conclude that the preferred form of convection varies dramatically according to the value of the Rayleigh number. While axisymmetric motions with different patterns may exist for modestly nonlinear convection, only a single motion persists at sufficiently large values of the Rayleigh number. This circulation is symmetric about the equator and has two meridional cells with rising motion at the poles. Instability of this single axisymmetric motion determines that the preferred pattern of three-dimensional convection has one azimuthal wave.
Attached cavitation and the boundary layer: experimental investigation and numerical treatment
Authors: Franc J. P. , Michel J. M.
Published: April 2006

Attached cavitation on a wall with continuous curvature is investigated on the basis of experiments carried out on various bodies (circular and elliptic cylinders, NACA 16 012 foil). Visualization of the boundary layer by dye injection at the leading edge shows that a strong interaction exists between attached cavitation and the boundary layer. In particular, it is shown that the cavity does not detach from the body at the minimum pressure point, but behind a laminar separation, even in largely developed cavitating flow. A detachment criterion which takes into account this link between attached cavitation and boundary layer is proposed. It consists of connecting a cavitating potential-flow calculation and a boundary-layer calculation. Among all the theoretically possible detachment points, the actual detachment point is chosen to be the one for which the complete calculation predicts a laminar separation just upstream. This criterion, applied to the NACA foil, leads to a prediction which is in good agreement with experimental results.
Axisymmetric compressible flow in a rotating cylinder with axial convection
Authors: Ungarish Marius , Israeli Moshe
Published: April 2006

The steady compressible flow of an ideal gas in a rotating annulus with thermally conducting walls is considered for small Rossby number ε and Ekman number E and moderate rotational Mach numbers M. Attention is focused on nonlinear effects which show up when σ and εM2 are not small (σ = ε/HE½, H is the dimensionless height of the container). These effects are not properly predicted by the classical linear perturbation analysis, and are treated here by quasi-linear extensions.

The extra work required by these extensions is only the numerical solution of one ordinary differential equation for the pressure.

Numerical solutions of the full Navier–Stokes equations in the nonlinear range are presented, and the validity of the present approach is confirmed.
Topological constraints associated with fast dynamo action
Authors: Moffatt H. K. , Proctor M. R. E.
Published: April 2006

The conjecture of Vainshtein & Zel'dovich (1972) concerning the existence of a fast dynamo (i.e. one whose growth rate is independent of magnetic diffusivity η in the limit η → 0) is discussed with particular reference to (i) the stretch–twist–fold cycle which can double the strength of a magnetic flux tube, and (ii) the space-periodic Beltrami flow of maximal helicity, which has been shown to be capable of space-periodic dynamo action with the same period as the velocity field, by Arnold & Korkina (1983) and by Galloway & Frisch (1984). The topological constraint associated with conservation of magnetic helicity is shown to preclude fast dynamo action unless the scale of the magnetic field is almost everywhere of order η½ as η → 0; in this case, the field structure is severely singular in the limit. A steady incompressible velocity field, quadratic in the space variables, is shown to mimic the action of the stretch–twist–fold cycle, and is proposed as a plausible candidate for fast dynamo action.
Wave propagation in bubbly liquids at finite volume fraction
Authors: Caflisch Russel E. , Miksis Michael J. , Papanicolaou George C. , Ting Lu
Published: April 2006

We derive effective equations for wave propagation in a bubbly liquid in a linearized low-frequency regime by a multiple-scale method. The effective equations are valid for finite volume fraction. For periodic bubble configurations, effective equations uniformly valid for small volume fraction are obtained. We compare the results to the ones obtained in a previous paper (Caflisch et al. 1985) for a nonlinear theory at small volume fraction.
Stability of finite-amplitude interfacial waves. Part 2. Numerical results
Authors: Pullin D. I. , Grimshaw R. H. J.
Published: April 2006

In the preceding paper (Grimshaw & Pullin 1985) we discussed the long-wavelength modulational instability of interfacial progressive waves in a two-layer fluid. In this paper we complement our analytical results by numerical results for the linearized stability of finite-amplitude waves. We restrict attention to the case when the lower layer is infinitely deep, and use the Boussinesq approximation. For this case the basic wave profile has been calculated by Pullin & Grimshaw (1983a, b). The linearized stability problem for perturbations to the basic wave is solved numerically by seeking solutions in the form of truncated Fourier series, and solving the resulting eigenvalue problem for the growth rate as a function of the perturbation wavenumber. For small or moderate basic wave amplitudes we show that the instabilities are determined by a set of low-order resonances. The lowest resonance, which contains the modulational instability, is found to be dominant for all cases considered. For higher wave amplitudes, the resonance instabilities are swamped by a local wave-induced Kelvin–Helmholtz instability.
A dopamine transporter polymorphism is a risk factor for borderline personality disorder in depressed patients
Authors: JOYCE PETER R. , McHUGH PATRICK C. , McKENZIE JANICE M. , SULLIVAN PATRICK F. , MULDER ROGER T. , LUTY SUZANNE E. , CARTER JANET D. , FRAMPTON CHRISTOPHER M. A. , ROBERT CLONINGER C. , MILLER ALLISON M. , KENNEDY MARTIN A.
Published: April 2006

Background. Borderline personality disorder (BPD) is often co-morbid with major depression and may complicate its treatment. We were interested in differences in genetic and developmental risk factors between depressed patients with or without a co-morbid BPD.

Method. Out-patients with major depressive disorder were recruited for two treatment trials. Assessment of depressed patients included the assessment of personality disorders, developmental risk factors and DNA samples for genetic analyses.

Results. In each study there was a significant association between the 9-repeat allele of the dopamine transporter (DAT1) and BPD, with odds ratios (OR) >3 and p[les ]0·02. This association remained significant when developmental risk factors for BPD (childhood abuse and neglect and borderline temperament) were also included in the analyses. The OR was even larger in the depressed patients aged [ges ]35 years (OR 9·31, p=0·005).

Conclusion. This replicated association in depressed patients between the 9-repeat allele of DAT1 and BPD may provide clues to understanding the neurobiology of BPD. The finding that the association is larger in the older depressed patients, suggests that the 9-repeat allele may be associated with a poorer prognosis BPD, rather than a young adult limited variant of BPD.
A recurring outbreak of Shigella sonnei among traditionally observant Jewish children in New York City: the risks of daycare and household transmission
Authors: GARRETT V. , BORNSCHLEGEL K. , LANGE D. , REDDY V. , KORNSTEIN L. , KORNBLUM J. , AGASAN A. , HOEKSTRA M. , LAYTON M. , SOBEL J.
Published: April 2006

Dispersed community outbreaks of Shigella sonnei have occurred cyclically among traditionally observant Jews in the United States. In February 2000, we investigated a S. sonnei outbreak in one Jewish community in New York City. To determine risk factors for introduction of infection into households, we conducted a cohort study of households to compare risk factors for illness among primary subjects within households and age-matched well siblings. Isolates were subtyped by pulsed-field gel electrophoresis (PFGE). We used a random effects model to assess extra-household vs. intra-household transmission in households with multiple ill household members. Daycare or pre-school attendance [matched odds ratio (mOR) 16·1, P<0·001] and age <60 months (mOR 6·3, P<0·001) were independently associated with index subject illness. Outbreak isolates were closely related by PFGE analysis to the strain previously observed in Jewish community outbreaks. The random effects model strongly indicated that multiple illnesses in a single household are due to secondary transmission. Disease containment efforts should focus on reducing Shigella transmission in childcare settings and within homes.
Risk factors for hospital-acquired methicillin-resistant Staphylococcus aureus bacteraemia: a case-control study
Authors: CARNICER-PONT D. , BAILEY K. A. , MASON B. W. , WALKER A. M. , EVANS M. R. , SALMON R. L.
Published: April 2006

A case-control study was undertaken in an acute district general hospital to identify risk factors for hospital-acquired bacteraemia caused by methicillin-resistant Staphylococcus aureus (MRSA). Cases of hospital-acquired MRSA bacteraemia were defined as consecutive patients from whom MRSA was isolated from a blood sample taken on the third or subsequent day after admission. Controls were randomly selected from patients admitted to the hospital over the same time period with a length of stay of more than 2 days who did not have bacteraemia. Data on 42 of the 46 cases of hospital-acquired bacteraemia and 90 of the 92 controls were available for analysis. There were no significant differences in the age or sex of cases and controls. After adjusting for confounding factors, insertion of a central line [adjusted odds ratio (aOR) 35·3, 95% confidence interval (CI) 3·8–325·5] or urinary catheter (aOR 37·1, 95% CI 7·1–193·2) during the admission, and surgical site infection (aOR 4·3, 95% CI 1·2–14·6) all remained independent risk factors for MRSA bacteraemia. The adjusted population attributable fraction, showed that 51% of hospital-acquired MRSA bacteraemia cases were attributable to a urinary catheter, 39% to a central line, and 16% to a surgical site infection. In the United Kingdom, measures to reduce the incidence of hospital-acquired MRSA bacteraemia in acute general hospitals should focus on improving infection control procedures for the insertion and, most importantly, care of central lines and urinary catheters.
How do microorganisms reach the stratosphere?
Authors: Wainwright M. , Alharbi S. , Wickramasinghe N.C.
Published: April 2006

A number of studies have demonstrated that bacteria and fungi are present in the stratosphere. Since the tropopause is generally regarded as a barrier to the upward movement of particles it is difficult to see how such microorganisms can reach heights above 17 km. Volcanoes provide an obvious means by which this could be achieved, but these occur infrequently and any microorganisms entering the stratosphere from this source will rapidly fall out of the stratosphere. Here, we suggest mechanisms by which microorganisms might reach the stratosphere on a more regular basis; such mechanisms are, however, likely only to explain how micrometre to submicrometre particles could be elevated into the stratosphere. Intriguingly, clumps of bacteria of size in excess of 10 μm have been found in stratospheric samples. It is difficult to understand how such clumps could be ejected from the Earth to this height, suggesting that such bacterial masses may be incoming to Earth. We suggest that the stratospheric microflora is made up of two components: (a) a mixed population of bacteria and fungi derived from Earth, which can occasionally be cultured; and (b) a population made up of clumps of, viable but non-culturable, bacteria which are too large to have originated from Earth; these, we suggest, have arrived in the stratosphere from space. Finally, we speculate on the possibility that the transfer of bacteria from the Earth to the highly mutagenic stratosphere may have played a role in bacterial evolution.
Synchronization and time shifts of dynamical patterns for mutually delay-coupled fiber ring lasers
Authors: Shaw Leah B. , Schwartz Ira B. , Rogers Elizabeth A. , Roy Rajarshi
Published: April 2006

A pair of coupled erbium doped fiber ring lasers is used to explore the dynamics of coupled spatiotemporal systems. The lasers are mutually coupled with a coupling delay less than the cavity round-trip time. We study synchronization between the two lasers in the experiment and in a delay differential equation model of the system. Because the lasers are internally perturbed by spontaneous emission, we include a noise source in the model to obtain stochastic realizations of the deterministic equations. Both amplitude synchronization and phase synchronization are considered. We use the Hilbert transform to define the phase variable and compute phase synchronization. We find that synchronization increases with coupling strength in the experiment and the model. When the time series from two lasers are time shifted in either direction by the delay time, approximately equal synchronization is frequently observed, so that a clear leader and follower cannot be identified. We define an algorithm to determine which laser leads the other when the synchronization is sufficiently different with one direction of time shift, and statistics of switches in leader and follower are studied. The frequency of switching between leader and follower increases with coupling strength, as might be expected since the lasers mutually influence each other more effectively with stronger coupling.
A two-component phenomenology for homogeneous magnetohydrodynamic turbulence
Authors: Oughton S. , Dmitruk P. , Matthaeus W. H.
Published: April 2006

A one-point closure model for energy decay in three-dimensional magnetohydrodynamic (MHD) turbulence is developed. The model allows for influence of a large-scale magnetic field that may be of strength sufficient to induce Alfvén wave propagation effects, and takes into account components of turbulence in which either the wave-like character is negligible or is dominant. This two-component model evolves energy and characteristic length scales, and may be useful as a simple description of homogeneous MHD turbulent decay. In concert with spatial transport models, it can form the basis for approximate treatment of low-frequency plasma turbulence in a variety of solar, space, and astrophysical contexts.
Thermionic emission from carbon nanotubes with a thin layer of low work function barium strontium oxide surface coating
Authors: Jin Feng , Liu Yan , Day Christopher M.
Published: April 2006

We have created a thermionic cathode structure that consists of a thin tungsten ribbon, carbon nanotubes (CNTs) on the ribbon surface, and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNT cathode was designed to combine the benefits from the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission and thermionic emission properties of the cathode have been characterized. A field enhancement factor of 266 and a work function of 2.1eV were obtained. At 1437K, a thermionic emission current density of 15mA∕cm2 in an electric field of 0.9V∕μm was obtained, which is 50 times greater than the emission current density from the uncoated CNT cathode at the same temperature.
Site-specific proteolysis of the transcriptional coactivator HCF-1 can regulate its interaction with protein cofactors
Authors: Vogel Jodi L. , Kristie Thomas M.
Published: April 2006

Limited proteolytic processing is an important transcriptional regulatory mechanism. In various contexts, proteolysis controls the cytoplasmic-to-nuclear transport of important transcription factors or removes domains to produce factors with altered activities. The transcriptional coactivator host cell factor-1 (HCF-1) is proteolytically processed within a unique domain consisting of 20-aa reiterations. Site-specific cleavage within one or more repeats generates a family of amino- and carboxyl-terminal subunits that remain tightly associated. However, the consequences of HCF-1 processing have been undefined. In this study, it was determined that the HCF-1-processing domain interacts with several proteins including the transcriptional coactivator/corepressor four-and-a-half LIM domain-2 (FHL2). Analysis of this interaction has uncovered specificity with both sequence and context determinants within the reiterations of this processing domain. In cells, FHL2 interacts exclusively with the nonprocessed coactivator and costimulates transcription of an HCF-1-dependent target gene. The functional interaction of HCF-1 with FHL2 supports a model in which site-specific proteolysis regulates the interaction of HCF-1 with protein partners and thus can modulate the activity of this coactivator. This paradigm expands the biological significance of limited proteolytic processing as a regulatory mechanism in gene transcription.
Rtn1p Is Involved in Structuring the Cortical Endoplasmic Reticulum
Authors: De Craene Johan-Owen , Coleman Jeff , Estrada de Martin Paula , Pypaert Marc , Anderson Scott , Yates John R. , Ferro-Novick Susan , Novick Peter
Published: April 2006

The endoplasmic reticulum (ER) contains both cisternal and reticular elements in one contiguous structure. We identified rtn1Δ in a systematic screen for yeast mutants with altered ER morphology. The ER in rtn1Δ cells is predominantly cisternal rather than reticular, yet the net surface area of ER is not significantly changed. Rtn1-green fluorescent protein (GFP) associates with the reticular ER at the cell cortex and with the tubules that connect the cortical ER to the nuclear envelope, but not with the nuclear envelope itself. Rtn1p overexpression also results in an altered ER structure. Rtn proteins are found on the ER in a wide range of eukaryotes and are defined by two membrane-spanning domains flanking a conserved hydrophilic loop. Our results suggest that Rtn proteins may direct the formation of reticulated ER. We independently identified Rtn1p in a proteomic screen for proteins associated with the exocyst vesicle tethering complex. The conserved hydophilic loop of Rtn1p binds to the exocyst subunit Sec6p. Overexpression of this loop results in a modest accumulation of secretory vesicles, suggesting impaired exocyst function. The interaction of Rtn1p with the exocyst at the bud tip may trigger the formation of a cortical ER network in yeast buds.
Emticicia oligotrophica gen. nov., sp. nov., a new member of the family ‘Flexibacteraceae’, phylum Bacteroidetes
Authors: Saha P. , Chakrabarti T.
Published: April 2006

An aquatic bacterium, strain GPTSA100-15T, was isolated on nutritionally poor medium TSBA100 (tryptic soy broth diluted 100 times and solidified with 1.5 % agarose) and characterized using a polyphasic approach. The isolate was unable to grow on commonly used nutritionally rich media such as tryptic soy agar, nutrient agar and Luria–Bertani agar. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the isolate was affiliated with the family ‘Flexibacteraceae’ in the phylumBacteroidetes. Phylogenetically, it showed closest similarity (94.0 %) with an uncultured bacterial clone, HP1A92, detected in a sludge microbial community. Among the culturable bacteria, the isolate had highest 16S rRNA gene sequence similarity withLeadbetterella byssophila4M15T(87.8 %). Sequence similarities with other members of the phylumBacteroideteswere less than 85 %. The fatty acid profile of the isolate grown on TSBA100 indicated that the major fatty acid was iso-C15 : 0, which is also present in many members of the family ‘Flexibacteraceae’. Cells of strain GPTSA100-15Tare Gram-negative, strictly aerobic rods. The DNA G+C content of the isolate is 36.9 mol%. Results of phenotypic, chemotaxonomic and phylogenetic analyses clearly indicate that strain GPTSA100-15Trepresents a new genus within the family ‘Flexibacteraceae’; the nameEmticiciagen. nov. is proposed for the genus, withEmticicia oligotrophicasp. nov. as the type species. The type strain ofEmticicia oligotrophicais GPTSA100-15T(=MTCC 6937T=DSM 17448T).
Phylogenetic study and multiplex PCR-based detection of Burkholderia plantarii, Burkholderia glumae and Burkholderia gladioli using gyrB and rpoD sequences
Authors: Maeda Yukiko , Shinohara Hirosuke , Kiba Akinori , Ohnishi Kouhei , Furuya Naruto , Kawamura Yoshiaki , Ezaki Takayuki , Vandamme Peter , Tsushima Seiya , Hikichi Yasufumi
Published: April 2006

In order to develop a detection method for the rice pathogens Burkholderia plantarii, Burkholderia glumae and Burkholderia gladioli, the phylogeny of six plant-pathogenic Burkholderia species was analysed using the combined nucleotide sequences of gyrB and rpoD. B. plantarii, B. glumae and B. gladioli formed tight monophyletic branches supported by high bootstrap probabilities. The high sequence similarity revealed a close phylogenetic relationship between B. glumae and B. plantarii. B. plantarii strains were divided into three subclusters comprising rice strains, whereas the single Vanda strain occupied a unique position in the phylogenetic tree. The gyrB and rpoD sequences of all B. glumae strains examined were highly conserved. In contrast, B. gladioli strains demonstrated a far greater sequence diversity, but this diversity did not correlate with pathovar, host plant or geographical origin of the strains. A multiplex-PCR protocol using specific primers from the gyrB sequences was designed that allowed the specific detection and identification of B. plantarii, B. glumae and B. gladioli in rice seeds infected with these pathogenic species.