bluuebunny/crossref_metadata_2025 huggingface dataset:
Field measurements of wave-induced pressure over wind-sea and swell
Authors: Hasselmann Dieter , Bösenberg Jens
Published: April 2006

Results of two field experiments in the North Sea are presented. Pressure was measured at two fixed heights above the mean water level and correlated with simultaneous wave height measurements. Roughly 90 hours of data have been analysed and the results are in agreement with earlier results obtained by Snyder et al. (1981). Measurements over swell give no indication of wave decay or growth for waves travelling faster than the wind or against the wind.
Capillary–gravity waves produced by a wavemaker
Authors: Hocking L. M. , Mahdmina D.
Published: April 2006

Surface waves in a channel can be produced by the horizontal motion of a plane wavemaker at one end of the channel. The amplitude and the frequency of the waves depend on both surface tension and gravity, as well as on the condition imposed at the contact line between the free surface and the wavemaker. Some of the previous work on the generation of capillary–gravity waves has been based on the unjustified assumption that the slope of the free surface at the contact line can be prescribed. A more acceptable condition is one that relates the slope to the motion of the contact line relative to the wavemaker; in this way the dynamic properties of the contact angle can be incorporated. The waves generated by a plane wavemaker in fluid of infinite depth and in fluid of a depth equal to that of the wavemaker are determined. An important reason for including surface tension is that in its absence the transient motion initiated by an impulsive start is singular; when surface tension is included this singularity is removed.
Energy growth of three-dimensional disturbances in plane Poiseuille flow
Authors: Gustavsson L. Hårkan
Published: April 2006

The development of a small three-dimensional disturbance in plane Poiseuille flow is considered. Its kinetic energy is expressed in terms of the velocity and vorticity components normal to the wall. The normal vorticity develops according to the mechanism of vortex stretching and is described by an inhomogeneous equation, where the spanwise variation of the normal velocity acts as forcing. To study specifically the effect of the forcing, the initial normal vorticity is set to zero and the energy density in the wavenumber plane, induced by the normal velocity, is determined. In particular, the response from individual (and damped) Orr–Sommerfeld modes is calculated, on the basis of a formal solution to the initial-value problem. The relevant timescale for the development of the perturbation is identified as a viscous one. Even so, the induced energy density can greatly exceed that associated with the initial normal velocity, before decay sets in. Initial conditions corresponding to the least-damped Orr–Sommerfeld mode induce the largest energy density and a maximum is obtained for structures infinitely elongated in the streamwise direction. In this limit, the asymptotic solution is derived and it shows that the spanwise wavenumbers at which the largest amplification occurs are 2.60 and 1.98, for symmetric and antisymmetric normal vorticity, respectively. The asymptotic analysis also shows that the propagation speed for induced symmetric vorticity is confined to a narrower range than that for antisymmetric vorticity. From a consideration of the neglected nonlinear terms it is found that the normal velocity component cannot be nonlinearly affected by the normal vorticity growth for structures with no streamwise dependence.
Electrophoresis of a colloidal sphere along the axis of a circular orifice or a circular disk
Authors: Keh Huan J. , Lien Liang C.
Published: April 2006

The axisymmetric electrophoretic motion of a dielectric sphere along the axis of an orifice in a large conducting plane or of a conducting disk is considered. The radius of the orifice or the disk may be either larger or smaller than that of the sphere. The assumption of thin electrical double layers at the solid surfaces is employed. To solve the electrostatic and hydrodynamic governing equations both the electric and the flow fields are partitioned at the plane of the orifice or the disk. For each field, separate solutions are developed on both sides of the plane that satisfy the boundary conditions in each region and the unknown functions for the field at the fluid interface. The continuities of the electric current flux and the fluid stress tensor at the matching interface lead to sets of dual integral equations which are solved analytically to determine the unknown functions for the fields at the matching interface. Then, a boundary–collocation technique is used to satisfy the boundary conditions on the surface of the sphere.

The numerical solutions for the electrophoretic velocity of the colloidal sphere are presented for various values ofa/banda/d, whereais the particle radius,bis the radius of the orifice or the disk, anddis the distance of the particle centre from the plane of the wall. For the limiting case of electrophoresis of a sphere perpendicular to an infinite plane wall, our results for the boundary effects agree very well with the exact calculations using spherical bipolar coordinates. Interestingly, the electrophoretic velocity of a sphere approaching an orifice of a larger radius increases when the sphere is close to the orifice, and this velocity can be even larger than that for an identical sphere undergoing electrophoresis in an unbounded fluid. If the sphere has a radius larger than that of the orifice, or if the sphere has a smaller radius and is located sufficiently far from the orifice, its electrophoretic mobility will decrease with the increase of the spacing parametera/d. For the electrophoretic motion of a sphere along the axis of and close to a disk of finite radius, the resistance to the particle movement can be stronger than that for an equal sphere undergoing electrophoresis normal to an infinite plane wall at the same value ofa/d. As the particle approaches the disk wall, its mobility decreases steadily and vanishes at the limita/d→ 1. The boundary effects on the particle mobility and the fluid flow pattern in electrophoresis differ significantly from those of the corresponding sedimentation problem with which a comparison is made.
Higher-order modulation effects on solitary wave envelopes in deep water Part 2. Multi-soliton envelopes
Authors: Akylas T. R.
Published: April 2006

Previous experimental and numerical work indicates that an initially symmetric deep-water wave pulse of uniform frequency and moderately small steepness evolves in an asymmetric manner and eventually separates into distinct wave groups, owing to higher-order modulation effects, not accounted for by the nonlinear Schrödinger equation (NLS). Here perturbation methods are used to provide analytical confirmation of this group splitting on the basis of the more accurate envelope equation of Dysthe (1979). It is demonstrated that an initially symmetric multisoliton wave envelope, consisting of N bound NLS solitons, ultimately breaks up into N separate groups; a procedure is devised for determining the relative speed changes of the individual groups. The case of a bi-soliton (N = 2) is discussed in detail, and the analytical predictions are compared to numerical results.
Nonlinear evolution of subsonic and supersonic disturbances on a compressible free shear layer
Authors: Leib S. J.
Published: April 2006

We consider the effects of a nonlinear–non-equilibrium–viscous critical layer on the spatial evolution of subsonic and supersonic instability modes on a compressible free shear layer. It is shown that the instability wave amplitude is governed by an integro-differential equation with cubic-type nonlinearity. Numerical and asymptotic solutions to this equation show that the amplitude either ends in a singularity at a finite downstream distance or reaches an equilibrium value, depending on the Prandtl number, viscosity law, viscous parameter and a real parameter which is determined by the linear in viscid stability theory. A necessary condition for the existence of the equilibrium solution is derived, and whether or not this condition is met is determined numerically for a wide range of physical parameters including both subsonic and supersonic disturbances. It is found that no equilibrium solution exists for the subsonic modes unless the temperature ratio of the low-to high-speed streams exceeds a critical value, while equilibrium solutions for the most rapidly growing supersonic mode exist over most of the parameter range examined.
Numerical simulations of the nonlinear kink modes in linearly stable supersonic slip surfaces
Authors: Pedelty Jeffrey A. , Woodward Paul R.
Published: April 2006

We have performed high-resolution numerical simulations of supersonic slip surfaces to confirm and illuminate earlier analytic nonlinear stability calculations of such structures. This analytic work was in turn inspired by earlier computer simulations reported in Woodward (1985) and Woodward et al. (1987). In particular Artola & Majda (1987) examined the response of a supersonic slip surface to an incident train of small-amplitude nonlinear sound waves. They found analytic solutions which indicate that nonlinear resonance occurs at three angles of incidence which depend upon the Mach number of the relative motion. The two-dimensional simulations described here numerically solve this problem for a Mach-4 flow using the piecewise-parabolic method (Colella & Woodward 1984; Woodward & Colella 1984). The simulations show that sound waves incident at a predicted resonance angle excite nonlinear behaviour in the slip surface. At these angles the amplitude of the reflected waves is much greater than the incident wave amplitude (i.e. a shock forms). The observed resonance is fairly broad, but the resonance narrows as the strength of the incident waves is reduced.

The nature of the nonlinear kink modes observed in the simulations is similar to that discussed by Artola & Majda. Most of the modes move in either direction with speeds near the predicted value. Speeds of other than this value are observed, but the disagreement is not serious in view of the strongly nonlinear behaviour seen in the simulations but not treated in the analytic work. The stationary modes seen in the analytic results are perhaps observed as transient structures. They may eventually dominate the flow at late times (Woodward et al. 1987).

The role of the kink modes in the stability of slab jets is discussed, and it is argued that the stationary modes are more disruptive than the propagating modes.
Granular flow: physical experiments and their implications for microstructural theories
Authors: Drake Thomas G.
Published: April 2006

Positions, velocities and rotations of individual particles obtained from high-speed motion pictures of essentially two-dimensional flows of plastic spheres in an inclined glass-walled chute were used to test critical assumptions of microstructural theories for the flow of granular materials. The measurements provide a well-defined set of observations for refining and validating computer simulations of granular flows, and point out some important limitations of physical experiments. Two nearly steady, uniform, collisional flows of 6-mm-diameter plastic spheres over a fixed bed of similar spheres inclined at 42.75° were analysed in detail. Particle fluxes were about 2230 particles s−1 and 1280 particles s−1. The nominal depth in both flows was about 18 particle diameters. Profiles of mean downstream velocity and mean rotations, translational temperature and rotational temperature, and bulk density in the flows show slip at the bed of 17 and 26% of the mean flow velocity for the high- and low-flux flows, respectively; mean rotation rates $\overline{\omega}_x$ and $\overline{\omega}_y$ less than 9% of $\overline{\omega}_z$ ($\hat{e}_x$ parallel to the bed, $\hat{e}_x$ normal to the sidewall); translational temperature nearly independent of distance from the bed; rotational temperature decreasing with distance from the bed; and density decreasing almost linearly with distance from the bed. The continuum hypothesis (i.e. small gradients in mean-flow properties) is satisfied throughout the flow except near the fixed bed, where large gradients in the mean rotation $\overline{\omega}_z$ and downstream velocity occur over a few particle diameters. The distributions of velocities and rotations are approximately Maxwellian, except near the fixed bed. Testing microstructural theories with physical experiments is severely hampered by limitations on material properties of particles, flow lengthscale and the spatial and temporal resolution of observations. Only a small volume of the parameter space for collision-dominated flows can reasonably be explored by physical experiment. Extraneous forces due to air drag, sidewall friction and electrical effects are not included in theories but must be addressed in physical experiments. Properly designed experiments are the essential link between computer simulations and theory, because they focus attention on particular features critical to testing the simulations, which in turn provide detailed particle-scale information needed to test theories.
Effect of a plane boundary on oscillatory flow around a circular cylinder
Authors: Sumer B. M. , Jensen B. L. , Fredsøe J.
Published: April 2006

This study deals with the flow around a circular cylinder placed near a plane wall and exposed to an oscillatory flow. The study comprises instantaneous pressure distribution measurements around the cylinder at high Reynolds numbers (mostly atRe∼ 105) and a flow visualization study of vortex motions at relatively smaller Reynolds numbers (Re∼ 103–104). The range of the gap-to-diameter ratio is from 0 to 2 for the pressure measurements and from 0 to 25 for the flow visualization experiments. The range of the Keulegan–Carpenter numberKCis from 4 to 65 for the pressure measurements and from 0 to 60 for the flow visualization tests. The details of vortex motions around the cylinder are identified for specific values of the gap-to-diameter ratio and for theKCregimes known from research on wall-free cylinders. The findings of the flow visualization study are used to interpret the variations in pressure with time around the pipe. The results indicate that the flow pattern and the pressure distribution change significantly because of the close proximity of the boundary where the symmetry in the formation of vortices breaks down, and also the characteristic transverse vortex street observed for wall-free cylinders for 7 <KC< 13 disappears. The results further indicate that the vortex shedding persists for smaller and smaller values of the gap-to-diameter ratio, asKCis decreased. The Strouhal frequency increases with decreasing gap-to-diameter ratio. The increase in the Strouhal frequency with respect to its wall-free-cylinder value can be as much as 50% when the cylinder is placed very close to the wall with a gap-to-diameter ratio ofO(0.1).
Further experiments on vortex formation around an oscillating and translating airfoil at large incidences
Authors: Ohmi Kazuo , Coutanceau Madeleine , Daube Olivier , Loc Ta Phuoc
Published: April 2006

The starting flows past a two-dimensional NACA 0012 airfoil translating and oscillating at large incidences are investigated by visualization experiments and numerical calculations. The airfoil model is set in motion impulsively and subjected simultaneously to a constant translation and harmonic oscillation in pitch. The evolution of the vortex wake is followed in a sequence of streamline visualizations and the wake pattern generated is analysed. The parameters varied in the visualization experiment are the Reynolds number ranging from 1500 to 10000, the reduced frequency from 0.1 to 1.0, the mean incidence 30° or 15° and the angular amplitude 15° or 7°. There are also two additional parameters of special interest: the airfoil cross-section and the pitching axis. The effects of these parameters are discussed in relation to the resultant wake patterns. Some comparison is made with the results of earlier experiments.
The double layer–capillary stability of an annular electrolyte fluid surrounding a dielectric-fluid core in a tube
Authors: Georgiou E. , Papageorgiou D. T. , Maldarelli C. , Rumschitzki D. S.
Published: April 2006

In this paper we examine the linear stability of an annular film surrounding a dielectric-fluid core in a tube in the presence of double layers of charges at the film core and at the film–tube interfaces, when the fluid-fluid interface is of low tension. In the absence of electrostatic forces, the surface tension force arising from the circumferential curvature destabilizes, and that from the axial curvature stabilizes the system. The competition is such that waves larger than the unperturbed interface circumference are unstable and those shorter are stable. For charged layers in the film, two cases are examined: (i) double-layer repulsion where the volume charge density is everywhere of the same sign and (ii) double-layer attraction where the diffusive layers next to the film interfaces are of opposite signs. In the first case, double-layer repulsion and surface tension lowering stabilize the destabilizing action of the circumferential component of the surface tension force, and a window of stability can exist. In the case of double layers of opposite signs, double-layer attraction destabilizes the system, and growth rates larger than those caused by pure capillarity can arise. Finally, for the case of a core bounded by an infinite electrolyte, surface tension lowering stabilizes the destabilizing action of the circumferential component of the surface tension force and destabilizes the longitudinal one, although the magnitudes of these effects may differ. As a result the thread can become unstable to waves shorter than the interface circumference.
Flow around an unsteady thin wing close to curved ground
Authors: Wang Qian-Xi
Published: April 2006

The method of matched asymptotic expansions is applied to the flow analysis of a three-dimensional thin wing, moving uniformly in very close proximity to a curved ground surface. Four flow regions, i.e. exterior, bow, gap, and wake, are analysed and matched in an appropriate sequence. The solutions in expansions up to third order are given both in nonlinear and linear cases. It is shown that the flow above the wing is reduced to a direct problem, and the flow beneath it appears to be a two-dimensional channel flow. The wake assumes a vortex-sheet structure close to the curved ground, undulating with the amplitude of the ground curvature, and the flow beneath it is also two-dimensional channel flow. As a consequence, an equivalence is found between the extreme curved-ground effect and the corresponding flat-ground effect, which can be treated by the image method.
Inviscid free-surface flow over a periodic wall
Authors: Bontozoglou V. , Kalliadasis S. , Karabelas A. J.
Published: April 2006

A numerical method is described, based on the hodograph formulation, for analysing in viscid, free-surface flows over a periodic wall. An efficient implementation of the wall boundary condition results in a straightforward method, accurate for a wide range of bottom undulation heights and flow parameters. It is demonstrated that a series of resonances is possible between the bottom undulations and the free surface. The steady, free-surface profiles are accurately calculated for a wide range of current velocities and are shown to be significantly dimpled by higher harmonics. A study of the flow field indicates that the free-surface shape strongly affects the velocities close to the wall, leading to distributions which change dramatically with current velocity. Some implications of the new results on the phenomena of wall dissolution or material deposition, Bragg scattering of surface waves and sediment transport in rivers, are discussed.
Trapped internal waves over undular topography
Authors: Kranenburg C. , Pietrzak J. D. , Abraham G.
Published: April 2006

We describe observations of slowly decelerating stratified flow over undular bottom topography in an estuary. The flow, which initially was supercritical with respect to the first internal wave mode, approached a resonance after it had become subcritical. A series of acoustic images showed large-amplitude first-mode trapped waves during this phase of the tide. We derive a criterion for quasi-steady response, and present an extension of Yih's class II linear finite-amplitude solutions that accounts for the waves observed.
An optical interferometric study of the wake of a bluff body
Authors: Green R. B. , Gerrard J. H.
Published: April 2006

A Fizeau optical interferometer has been used to visualize the wake behind a circular cylinder at low Reynolds numbers Re. As well as showing the vortex shedding mechanism and development of the far wake in a new light, the shed vortex strength and age were derived from the results. The vortex velocity distributions, at downstream distances of 5 to 16 diameters, were found to be those of convected Oseen vortices. These measurements confirmed the existence of a transition at Re ≈ 100, which reflects the emergence of convection as the dominant process in the near wake.
Vorticity dynamics in an oscillatory flow over a rippled bed
Authors: Blondeaux P. , Vittori G.
Published: April 2006

In the present paper we determine the oscillatory flow generated by surface gravity waves near a sea bottom covered with large-amplitude ripples. The vorticity equation and Poisson equation for the stream function are solved by means of a numerical approach based on spectral methods and finite-difference approximations. In order to test the numerical algorithm and in particular the numerical scheme used to generate vorticity along the ripple profile, we also perform an asymptotic analysis, which holds as the timettends to zero. The main features of the time development of vorticity are analysed and particular attention is paid to the dynamics of the large vortices generated by flow separation at the ripple crests and along the ripple profile. Some of the results obtained by Longuet-Higgins (1981) are recovered; in particular, the present results show a vortex pair shed from the ripple crest every half-cycle. The determination of flow separation along the ripple profile induced by the pressure gradient and the inclusion of viscous effects allows us to obtain accurate quantitative results and detect some important phenomena never observed before.

In particular it is shown that: (i) Whenever a vortex structure moves towards the bottom, a secondary vortex is generated near the ripple profile, which interacts with the primary vortex and causes it to move away from the bottom, (ii) Depending on the values of the parameters, the time development of the free shear layer shed from the ripple crest may produce two or even more vortex structures, (iii) Occasionally vortices generated previously may coalesce with the free shear layer shed from the ripple crest, generating a unique vortex structure.
Onset of convection in an anisotropic porous medium with oblique principal axes
Authors: Tyvand Peder A. , Storesletten Leiv
Published: April 2006

We investigate the onset of Rayleigh–Bénard convection in a horizontal porous layer with anisotropic permeability. The permeability is transversely isotropic, whereas the orientation of the longitudinal principal axes is arbitrary. This is sufficient to achieve qualitatively new flow patterns with a tilted plane of motion or tilted lateral cell walls. The critical Rayleigh number and wavenumber at marginal stability are calculated. There are two different types of convection cells (rolls): (i) the plane of motion is tilted, whereas the lateral cell walls are vertical; (ii) the plane of motion is vertical, whereas the lateral cell walls are tilted as well as curved. It turns out that type (i) occurs when the transverse permeability is larger than the longitudinal permeability, and for the converse case type (ii) is preferred.
On the interaction of a collapsing cavity and a compliant wall
Authors: Duncan J. H. , Zhang S.
Published: April 2006

The collapse of a spherical vapour cavity in the vicinity of a compliant boundary is examined numerically. The fluid is treated as a potential flow and a boundary-element method is used to solve Laplace's equation for the velocity potential. Full nonlinear boundary conditions are applied on the surface of the cavity. The compliant wall is modelled as a membrane with a spring foundation. At the interface between the fluid and the membrane, the pressure and vertical velocity in the flow are matched to the pressure and vertical velocity of the membrane using linearized conditions. The results of calculations are presented which show the effect of the parameters describing the flow (the initial cavity size and position, the fluid density and the pressure driving the collapse) and the parameters describing the compliant wall (the mass per unit area, membrane tension, spring constant and coating radius) on the interaction between the two. When the wall is rigid, the collapse of the cavity is characterized by the formation of a re-entrant jet that is directed toward the wall. However, if the properties of the compliant wall are chosen properly, the collapse can be made to occur spherically, as if the cavity were in an infinite fluid, or with the reentrant jet directed away from the wall, as if the cavity were adjacent to a free surface. This behaviour is in qualitative agreement with the experiments of Gibson & Blake (1982) and Shima, et al. (1989). Calculations of the transfer of energy between the flow and the coating are also presented.
Chaos in a model of forced quasi-geostrophic flow over topography: an application of Melnikov's method
Authors: Allen J. S. , Samelson R. M. , Newberger P. A.
Published: April 2006

We demonstrate the existence of a chaotic invariant set of solutions of an idealized model for wind-forced quasi-geostrophic flow over a continental margin with variable topography. The model (originally formulated to investigate mean flow generation by topographic wave drag) has bottom topography that slopes linearly offshore and varies sinusoidally alongshore. The alongshore topographic scales are taken to be short compared to the cross-shelf scale, allowing Hart's (1979) quasi-two-dimensional approximation, and the governing equations reduce to a non-autonomous system of three coupled nonlinear ordinary differential equations. For weak (constant plus time-periodic) forcing and weak friction, we apply a recent extension (Wiggins & Holmes 1987) of the method of Melnikov (1963) to test for the existence of transverse homoclinic orbits in the model. The inviscid unforced equations have two constants of motion, corresponding to energy E and enstrophy M, and reduce to a one-degree-of-freedom Hamiltonian system which, for a range of values of the constant G = E − M, has a pair of homoclinic orbits to a hyperbolic saddle point. Weak forcing and friction cause slow variations in G, but for a range of parameter values one saddle point is shown to persist as a hyperbolic periodic orbit and Melnikov's method may be applied to study the perturbations of the associated homoclinic orbits. In the absence of time-periodic forcing, the hyperbolic periodic orbit reduces to the unstable fixed point that occurs with steady forcing and friction. The method yields analytical expressions for the parameter values for which sets of chaotic solutions exist for sufficiently weak time-dependent forcing and friction. The predictions of the perturbation analysis are verified numerically with computations of Poincaré sections for solutions in the stable and unstable manifolds of the hyperbolic periodic orbit and with computations of solutions for general initial-value problems. In the presence of constant positive wind stress τ0 (equatorward on eastern ocean boundaries), chaotic solutions exist when the ratio of the oscillatory wind stress τ1 to the bottom friction parameter r is above a critical value that depends on τ0/r and the bottom topographic height. The analysis complements a previous study of this model (Samelson & Allen 1987), in which chaotic solutions were observed numerically for weak near-resonant forcing and weak friction.
Experimental and theoretical investigation of large-amplitude oscillations of liquid droplets
Authors: Becker E. , Hiller W. J. , Kowalewski T. A.
Published: April 2006

Finite-amplitude, axially symmetric oscillations of small (0.2 mm) liquid droplets in a gaseous environment are studied, both experimentally and theoretically. When the amplitude of natural oscillations of the fundamental mode exceeds approximately 10% of the droplet radius, typical nonlinear effects like the dependence of the oscillation frequency on the amplitude, the asymmetry of the oscillation amplitude, and the interaction between modes are observed. As the amplitude decreases due to viscous damping, the oscillation frequency and the amplitude decay factor reach their asymptotical values predicted by linear theory. The initial behaviour of the droplet is described quite satisfactorily by a proposed nonlinear inviscid theoretical model.
The stability of filamentary vorticity in two-dimensional geophysical vortex-dynamics models
Authors: Waugh D. W. , Dritschel D. G.
Published: April 2006

The linear stability of filaments or strips of ‘potential’ vorticity in a background shear flow is investigated for a class of two-dimensional, inviscid, non-divergent models having a linear inversion relation between stream function and potential vorticity. In general, the potential vorticity is not simply the Laplacian of the stream function – the case which has received the greatest attention historically. More general inversion relationships between stream function and potential vorticity are geophysically motivated and give an impression of how certain classic results, such as the stability of strips of vorticity, hold under more general circumstances.

In all models, a strip of potential vorticity is unstable in the absence of a background shear flow. Imposing a shear flow that reverses the total shear across the strip, however, brings about stability, independent of the Green-function inversion operator that links the stream function to the potential vorticity. But, if the Green-function inversion operator has a sufficiently short interaction range, the strip can also be stabilized by shear having the same sense as the shear of the strip. Such stabilization by ‘co-operative’ shear does not occur when the inversion operator is the inverse Laplacian. Nonlinear calculations presented show that there is only slight disruption to the strip for substantially less adverse shear than necessary for linear stability, while for co-operative shear, there is major disruption to the strip. It is significant that the potential vorticity of the imposed flow necessary to create shear of a given value increases dramatically as the interaction range of the inversion operator decreases, making shear stabilization increasingly less likely. This implies an increased propensity for filaments to ‘roll-up’ into small vortices as the interaction range decreases, a finding consistent with many numerical calculations performed using the quasi-geostrophic model.
From UBGs to CFGs A practical corpus-driven approach
Authors: KRIEGER HANS-ULRICH
Published: April 2006

We present a simple and intuitive unsound corpus-driven approximation method for turning unification-based grammars, such as HPSG, CLE, or PATR-II into context-free grammars (CFGs). Our research is motivated by the idea that we can exploit (large-scale), hand-written unification grammars not only for the purpose of describing natural language and obtaining a syntactic structure (and perhaps a semantic form), but also to address several other very practical topics. Firstly, to speed up deep parsing by having a cheap recognition pre-flter (the approximated CFG). Secondly, to obtain an indirect stochastic parsing model for the unification grammar through a trained PCFG, obtained from the approximated CFG. This gives us an efficient disambiguation model for the unification-based grammar. Thirdly, to generate domain-specific subgrammars for application areas such as information extraction or question answering. And finally, to compile context-free language models which assist the acoustic model of a speech recognizer. The approximation method is unsound in that it does not generate a CFG whose language is a true superset of the language accepted by the original unification-based grammar. It is a corpus-driven method in that it relies on a corpus of parsed sentences and generates broader CFGs when given more input samples. Our open approach can be fine-tuned in different directions, allowing us to monotonically come close to the original parse trees by shifting more information into the context-free symbols. The approach has been fully implemented inJAVA.
Developing Children and Young People's Participation in Strategic Processes: The Experience of the Children's Fund Initiative
Authors: Spicer Neil , Evans Ruth
Published: April 2006

The extent of children's and young people's participation activities has increased considerably among statutory, voluntary and community sector organisations across the UK in recent years. The Children's Fund, a major government initiative launched in 2000, represents a systematic drive towards promoting children and young people's participation in planning, implementing and evaluating preventative services within all 149 local authority areas in England. Based on research carried out by the National Evaluation of the Children's Fund, this paper explores the experience of Children's Fund partnerships of engaging children and young people in strategic processes.
Die Aufmerksamkeitsdefizit-/Hyperaktivitätsstörung (ADHS) im Erwachsenenalter
Authors: Rösler Michael , Retz Wolfgang
Published: April 2006

Zusammenfassung: Die ADHS ist eine häufige chronische Erkrankung des Kindes-, Jugend- und Erwachsenenalters. Die psychopathologische Kernsymptomatik besteht in allen Lebensaltern aus den Syndromen Aufmerksamkeitsstörung, Impulsivität und Hyperaktivität. Während Impulsivität und Hyperaktivität im Verlauf eine rückläufige Tendenz erkennen lassen oder einem Symptomwandel unterliegen, bleiben die Aufmerksamkeitsstörungen bei Erwachsenen oft unverändert und sind meist mit desorganisiertem Verhalten vergesellschaftet. Es sind spezielle diagnostische Kriterien für die Anwendung im Erwachsenenalter entwickelt worden. Sie ermöglichen zusammen mit den obligatorischen Diagnosekriterien nach DSM-IV oder ICD-10 eine ausreichend differenzierte Erfassung der Psychopathologie. Der Verlauf der ADHS ins Erwachsenenalter ist mit einer Fülle von gesundheitlichen Risiken, komorbiden Leiden und sozialen Gefährdungen belastet. Die Erforschung der Ursachen und pathogenetischen Abläufe hat gezeigt, dass es sich bei ADHS um ein genetisch besonders nachhaltig verankertes Störungsmuster handelt, das mit strukturellen und funktionellen zerebralen Auffälligkeiten und mit Dysfunktionen in zerebralen Transmitterfunktionen einhergeht.
Diagnostik der Aufmerksamkeitsdefizit-/Hyperaktivitätsstörung (ADHS) im Erwachsenenalter
Authors: Stieglitz Rolf-Dieter , Rösler Michael
Published: April 2006

Zusammenfassung: Die Diagnose einer Aufmerksamkeitsdefizit-/Hyperaktivitätsstörung (ADHS) im Erwachsenenalter ist ein komplexer Prozess, vor allem wenn die Störung im Kindesalter nicht diagnostiziert wurde. Verschiedene Datenebenen und -quellen sind dabei zu berücksichtigen. Ziel des Beitrages ist es, einen Überblick über allgemeine Probleme der Diagnostik der ADHS, die verschiedenen Stufen des diagnostischen Prozesses sowie den diagnostischen Instrumenten zu geben, die hilfreich eingesetzt werden können, um die Diagnose sowie komorbide Störungen zuverlässig zu erfassen. Zur Verfügung stehen vor allem Interviews sowie Selbst- und Fremdbeurteilungsverfahren. Abschließend werden offene Fragen und bestehende Probleme in der Diagnostik der ADHS diskutiert.
Cholinesterase Inhibitor Blockade and its Prevention by Statins of Sympathetic α7-nAChR-Mediated Cerebral Nitrergic Neurogenic Vasodilation
Authors: Mozayan Mansoor , Chen Mei-Fang , Si Minliang , Chen Po Yi , Premkumar Louis S , Lee Tony JF
Published: April 2006

Cholinesterase inhibitors (ChEIs) have been used to treat Alzheimer's disease (AD). The efficacy of these drugs, however, is less than satisfactory. The possibility that ChEIs may have effects unrelated to ChE activity, such as negatively modulate neuronal nicotinic acetylcholine receptors (nAChRs) was evaluated. Since α7-nAChRs on cerebral perivascular sympathetic neurons mediate cerebral parasympathetic-nitrergic vasodilation, effects of physostigmine, neostigmine, and galantamine on α7-nAChR-mediated dilation in isolated porcine basilar arterial rings denuded of endothelium was examined using in vitro tissue bath technique. The results indicated that these ChEIs blocked vasodilation induced by choline (0.3 mmol/L), nicotine (0.1 mmol/L), and transmural nerve stimulation (TNS). The ChEl inhibition of dilation induced by TNS but not by choline or nicotine was prevented by atropine (0.1 μmol/L) pretreatment. Furthermore, using confocal microscopy, significant calcium influx induced by choline and nicotine in cultured porcine superior cervical ganglion (SCG) cells was attenuated by ChEIs. In α7-nAChR-expressed Xenopus oocytes, nicotine-induced inward currents were attenuated by α-bungarotoxin and ChEIs. Moreover, ChEI inhibition of nicotine- and choline-induced dilation was prevented by pretreatment with mevastatin and lovastatin (10 μmol/L), which did not affect ChEI inhibition of TNS-induced relaxation. These findings suggest that ChEIs inhibit the α7-nAChRs located on postganglionic sympathetic nerve terminals of SCG origin, causing a decreased release of nitric oxide in the neighboring nitrergic nerves and cerebral vasodilation. Inhibition of α7-nAChRs leading to a potential cerebral hypoperfusion may contribute to the limitation of ChEIs and question the validity of using a ChEI alone in treating AD. The efficacy of ChEIs may be improved by concurrent use of statins.
VEGF Protein Associates to Neurons in Remote Regions following Cortical Infarct
Authors: Stowe Ann M , Plautz Erik J , Eisner-Janowicz Ines , Frost Shawn B , Barbay Scott , Zoubina Elena V , Dancause Numa , Taylor Michael D , Nudo Randolph J
Published: April 2006

Vascular endothelial growth factor (VEGF) is thought to contribute to both neuroprotection and angiogenesis after stroke. While increased expression of VEGF has been demonstrated in animal models after experimental ischemia, these studies have focused almost exclusively on the infarct and peri-infarct regions. The present study investigated the association of VEGF to neurons in remote cortical areas at three days after an infarct in primary motor cortex (M1). Although these remote areas are outside of the direct influence of the ischemic injury, remote plasticity has been implicated in recovery of function. For this study, intracortical microstimulation techniques identified primary and premotor cortical areas in a non-human primate. A focal ischemic infarct was induced in the M1 hand representation, and neurons and VEGF protein were identified using immunohistochemical procedures. Stereological techniques quantitatively assessed neuronal-VEGF association in the infarct and peri-infarct regions, M1 hindlimb, M1 orofacial, and ventral premotor hand representations, as well as non-motor control regions. The results indicate that VEGF protein significantly increased association to neurons in specific remote cortical areas outside of the infarct and peri-infarct regions. The increased association of VEGF to neurons was restricted to cortical areas that are functionally and/or behaviorally related to the area of infarct. There was no significant increase in M1 orofacial region or in non-motor control regions. We hypothesize that enhancement of neuronal VEGF in these functionally related remote cortical areas may be involved in recovery of function after stroke, through either neuroprotection or the induction of remote angiogenesis.
Microphase separation in two-dimensional systems with competing interactions
Authors: Imperio A. , Reatto L.
Published: April 2006

The formation of clusters in condition of thermodynamic equilibrium can be easily observed both in two and three dimensions. In two dimensions relevant cases include pattern formation in Langmuir monolayers and ferrofluids, while in three dimensions cluster phases have been observed in colloids and in protein solutions. We have analyzed the problem within the scenario of competing interactions: typically, a short-range attractive interaction against a long-range repulsive one. This simplified approach is suggested by the fact that the forces, governing self-organization, act on a length scale which is larger than the molecular size; as a consequence many specific details of the molecules of interest are not necessary for studying the general features of microphases. We have tackled the microphase formation by simulations in bidimensional fluids, exploiting the parallel tempering scheme. In particular, we have analyzed the density range in which the particles arrange in circular domains (droplets), and the temperature range in which the system goes from microphases to the homogeneous fluid phase. As the density increases, the droplet size increases as well, but above a certain density the morphology changes and stripes are formed. Moreover at low density, we observe the formation of a liquidlike phase of disordered droplets; at higher densities, instead, the droplets tend to arrange onto a triangular superlattice. Such a change affects the features of the static structure factor, which gives well defined signatures of the microphase morphology. In each case, the specific heat exhibits a peak close to the transition from microphases to the homogeneous fluid phase, which is due to the breaking up of the clusters. The saturation of the height of the specific heat peak, with the increasing of the system size, suggests the possibility of a Kosterlitz-Thouless transition.
Representations of simple finite Lie conformal superalgebras of type W and S
Authors: Boyallian Carina , Kac Victor G. , Liberati Jose I. , Rudakov Alexei
Published: April 2006

We construct all finite irreducible modules over Lie conformal superalgebras of type W and S.
Violet photoluminescence from shell layer of Zn∕ZnO core-shell nanoparticles induced by laser ablation
Authors: Zeng Haibo , Cai Weiping , Hu Jinlian , Duan Guotao , Liu Peisheng , Li Yue
Published: April 2006

A strong violet photoluminescence (PL) band at 425nm (2.92eV) was observed from the ZnO shell layer of the Zn∕ZnO core-shell nanoparticles prepared by laser ablation in liquid media. Such violet PL decreases with increase of the shell thickness or annealing temperature, showing good controllability. Based on the electron paramagnetic resonance measurements, the violet emission is attributed to the electronic transition from the defect level, corresponding to high-concentration zinc interstitials, to the valence band. This study is in favor to clarify the defect-related emissions and to extend the optical and electronic applications of nanostructured ZnO.
Dust confinement and dust-acoustic waves in weakly magnetized anodic plasmas
Authors: Trottenberg Thomas , Block Dietmar , Piel Alexander
Published: April 2006

Experiments on dust-acoustic waves (DAW) in a magnetized anodic plasma are presented for the regime of low collisionality. The dust trapping and the self-excited and synchronized DAW dynamics are studied. Based on Langmuir and emissive probe measurements the dust confinement is found to be well described with respect to size, stability, and position of the dust cloud by an effective potential well formed by ion drag and Coulomb forces. Moreover, the measurements indicate the necessity for a kinetic model for the wave dispersion. By means of singular value decomposition the local wavelengths and growth rates of the waves are measured systematically. It is found that the measured mean wave number is well described by kinetic theory, while the theoretical growth rates overestimate the experiments. A novel observation for the DAW is a systematic variation of the wavelength inside the dust cloud.
Effects of downconversion luminescent film in dye-sensitized solar cells
Authors: Liu Junfeng , Yao Qiaohong , Li Yadong
Published: April 2006

A transparent luminescent film of uniform nanocrystals, which could absorb UV light and downconvert it to visible light, was prepared by coating approach. Application of the as-prepared film in dye-sensitized solar cells was investigated. Comparing with the uncoated solar cell, the introduction of the Dy3+ doped LaVO4 luminescent film onto back of solar cell could greatly enhance the lifetime of the solar cell. As the downconverted light can be reabsorbed by solar cell and generate current, solar cell with luminescent film improved energy conversion efficiency by 23.3% relative to the referenced one coated with an undoped LaVO4 film.
Systematic study of the genus Phorinia Robineau-Desvoidy of the Palearctic, Oriental and Oceanian regions (Diptera:Tachinidae)
Authors: Tachi Takuji , Shima Hiroshi
Published: April 2006

The exoristine genus Phorinia Robineau-Desvoidy of the Palearctic, Oriental and Oceanian regions is revised. Sixteen species are recognised, fifteen of them described as new. Phylogenetic relationships of Phorinia are analysed using morphological characters. The analysis reveals that Phorinia is clearly monophyletic and closely related to Ctenophorinia Mesnil within the tribe Exoristini. The following three species-groups are defined in this genus: the aurifrons-group: Phorinia aurifrons Robineau-Desvoidy; P. breviata, sp. nov.; P. minuta, sp. nov.; P. aduncata, sp. nov. and P. convexa, sp. nov.; the bifurcata-group: P. bifurcata, sp. nov.; P. quadrata, sp. nov.; P. longiseta, sp. nov. and P. spinulosa, sp. nov.; and the flava-group: P. flava, sp. nov.; P. gracilis, sp. nov.; P. australiana, sp. nov.; P. denticulata, sp. nov.; P. insignita, sp. nov.; P. occidentalis, sp. nov. and P. orientalis, sp. nov.
The inverted chevron plot measured by NMR relaxation reveals a native-like unfolding intermediate in acyl-CoA binding protein
Authors: Teilum Kaare , Poulsen Flemming M. , Akke Mikael
Published: April 2006

The folding kinetics of bovine acyl-CoA binding protein was studied by
15
N relaxation dispersion measurements under equilibrium conditions. Relaxation dispersion profiles were measured at several concentrations of guanidine hydrochloride (GuHCl). The unfolding rate constant (
k
u
) was determined under conditions favoring folding, for which the folding rate constant (
k
f
) dominates the relaxation in stopped-flow kinetic measurements. Conversely,
k
f
was determined under conditions favoring unfolding, for which
k
u
dominates stopped-flow data. The rates determined by NMR therefore complement those from stopped-flow kinetics and define an “inverted chevron” plot. The combination of NMR relaxation and stopped-flow kinetic measurements allowed determination of
k
f
and
k
u
in the range from 0.48 M GuHCl to 1.28 M GuHCl. Individually, the stopped-flow and NMR data fit two-state models for folding. However, although the values of
k
f
determined by the two methods agree, the values of
k
u
do not. As a result, a combined analysis of all data does not comply with a two-state model but indicates that an unfolding intermediate exists on the native side of the dominant energy barrier. The denaturant and temperature dependencies of the chemical shifts and
k
u
indicate that the intermediate state is structurally similar to the native state. Equilibrium unfolding monitored by optical spectroscopy corroborate these conclusions. The temperature dependence of the chemical shifts identifies regions of the protein that are selectively destabilized in the intermediate. These results illustrate the power of combining stopped-flow kinetics and NMR spectroscopy to analyze protein folding.