bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteady boundary layers with an intelligent numerical scheme
Authors: Cebeci Tuncer
Published: April 2006

A numerical method has been developed to represent unsteady boundary layers with large flow reversal. It makes use of the characteristic box scheme which examines the finite-difference grid in relation to the magnitude and direction of local velocity and reaches and implements a decision to ensure that the Courant, Friedricks & Lewey stability criterion is not violated. The method has been applied to the problem of an impulsively started circular cylinder and the results, though generally consistent with those of van Dommelen & Shen obtained with a Lagrangian method, show some differences. The time step is identified as very important and, with the present intelligent numerical scheme, the results were readily extended to times far beyond those previously achieved with Eulerian methods. Extrapolation of the results suggests that the much-discussed singularity for this unsteady flow is the same as that of the corresponding steady flow.
The spherical-droplet problem of evaporation and condensation in a vapour-gas mixture
Authors: Onishi Yoshimoto
Published: April 2006

The behaviour of a binary mixture of a vapour and an inert gas around the spherical condensed-phase droplet is studied analytically using kinetic theory. By the singular-perturbation method, the linearized Boltzmann equation of B–G–K type is first solved for problems with spherical symmetry under the diffusive boundary condition when the Knudsen number of the problem is small. The macroscopic equations and the appropriate boundary conditions in the form of the temperature and partial-pressure jumps on the interface between the droplet and the gas phase, which enable us to treat the problems at the level of ordinary fluid dynamics, are derived together with the Knudsen-layer structure formed near the interface. Then the velocity, temperature and pressure fields around the droplet are explicitly obtained, as well as the mass, heat and energy flows from it. The results obtained are capable of describing the transition from the diffusion-control to the kinetic-control mechanism in the mass-transfer process. The negative-temperature-gradient phenomenon, a common phenomenon for pure-vapour cases (absence of inert gas), is also possible, manifesting itself more easily as the kinetic-control mechanism becomes prevalent and the critical condition for its existence is given. The present analysis could be applied to other problems with spherical symmetry as well.
Airfoils with separation and the resulting wakes
Authors: Cebeci Tuncer , Clark R. W. , Chang K. C. , Halsey N. D. , Lee K.
Published: April 2006

A viscous/inviscid interaction method is described and has been used to calculate flows around four distinctly different airfoils as a function of angle of attack. It comprises an inviscid-flow method based on conformal mapping, a boundary-layer procedure based on the numerical solution of differential equations and an algebraic eddy viscosity. The results are in close agreement with experiment up to angles close to stall. In one case, where the airfoil thickness is large, small difficulties were experienced and are described. The method is shown to be capable of obtaining results with large flow separation and quantifies the role of transition on the lift coefficient.
Second-order modelling of particle dispersion in a turbulent flow
Authors: Shih Tsan-Hsing , Lumley J. L.
Published: April 2006

A set of second-order modelled equations for the motion of particles are presented. We consider the effects of the particle inertia and the crossing-trajectories effect on the particle dispersion. A simple case of a particle mixing layer in a decaying homogeneous turbulence for light and heavy particles is calculated. The results show that the crossing-trajectories effect on particle dispersion is very significant, while inertia only has a slight effect. This behaviour has been observed in experiments (Wells & Stock 1983) and is well predicted by an asymptotic analysis (Csanady, 1963). The calculation also shows that there is a significant difference between Favre-averaged particle velocity and conventional-averaged particle velocity in the low-particle-concentration region. All calculations are in good agreement with Wells & Stock's experimental data.
An asymptotic theory of wind-tunnel-wall interference on subsonic slender bodies
Authors: Malmuth N. D.
Published: April 2006

An asymptotic theory of solid cylindrical wind-tunnel-wall interference about subsonic slender bodies has been developed. The basic approximation used is one of large wall-radius to body-length ratio. Matched asymptotic expansions show that in contrast to the analogous two-dimensional problem of a confined airfoil, three regions exist. Besides the incompressible crossflow and nearly axisymmetric zones, a wall layer exists where reflection in the wall of the line source representing the body becomes of dominant importance. From the theory, the interference pressures are shown to be approximately constant for closed bodies. Also demonstrated is that D'Alembert's paradox holds for interference drag of such shapes. Numerical studies comparing the exact theory to the asymptotic model which provides drastic simplifications, show that the latter can be used with reasonable accuracy to describe flows, even where the characteristic tunnel-radius to body-length ratio is as low as 1.5.
Numerical study of ribbon-induced transition in Blasius flow
Authors: Spalart Philippe R. , Yang Kyung-Soo
Published: April 2006

The early three-dimensional stages of transition in the Blasius boundary layer are studied by numerical solution of the Navier-Stokes equations. A finite-amplitude two-dimensional wave and low-amplitude three-dimensional random disturbances are introduced. Rapid amplification of the three-dimensional components is observed and leads to transition. For intermediate amplitudes of the two-dimensional wave the breakdown is of subharmonic type, and the dominant spanwise wavenumber increases with the amplitude. For high amplitudes the energy of the fundamental mode is comparable to the energy of the subharmonic mode, but never dominates it; the breakdown is of mixed type. Visualizations, energy histories, and spectra are presented. The sensitivity of the results to various physical and numerical parameters is studied. The agreement with experimental and theoretical results is discussed.
On the stability of a columnar vortex to disturbances with large azimuthal wavenumber: the lower neutral points
Authors: Stewartson K. , Leibovich S.
Published: April 2006

The inviscid instability of a columnar trailing-line vortex at large values of the azimuthal wavenumber n near neutral conditions is considered. This extends an earlier analysis (Leibovich & Stewartson 1983), which is not accurate near the limiting values of the axial wavenumber for which instabilities exist. Here an asymptotic expansion is derived for the solution in the neighbourhood of the lower neutral point and the results compared with existing computations a t moderate values of n. For these weak instabilities disturbances are centred near the axis of the vortex and the relevant equation is solved in the complex plane by a generalized saddle-point method. In addition, the marginal stability of the vortex is examined, and an estimate obtained of the value of the swirl parameter above which the vortex is stable at large values of n.
The resonant-triad nonlinear interaction in boundary-layer transition
Authors: Smith F. T. , Stewart P. A.
Published: April 2006

Recent controlled experiments by Kachanov & Levchenko (1984) and others indicate that, during some slower kinds of transition to turbulence in boundary layers, three-dimensionality can come into play initially as a resonant-triad phenomenon, depending on the disturbance sizes present. The triad interaction, suggested theoretically in the boundary-layer context by Craik (1971) and others, is studied in the present work by means of multi-structured analysis for high characteristic Reynolds numbers. A finite-amplitude/relatively high-frequency approach leads rationally to the nonlinear triad equations, solutions for which are then obtained analytically and computationally in certain central cases of interest (temporal and spatial). The solutions have a rather chaotic spiky appearance as continual energy exchange develops between the two- and three-dimensional nonlinear modes, whose large-scale response seems governed by inviscid dynamics but subject to important, continual ‘rejuvenation’ from small- (fast-) scale viscous action in-between. The three-dimensional growth rate is thereby increased, but not the two-dimensional. Subsequently the disturbed flow enters a higher-amplitude regime similar to that studied in some related papers by the authors and co-workers. Comparisons with the experiments are very supportive of the theory (in the small and in the large), yielding both qualitative and quantitative agreement.
Boundary mixing in a rotating, stratified fluid
Authors: Ivey G. N.
Published: April 2006

An earlier laboratory experiment of Ivey & Corcos (1982) on boundary mixing in a stratified fluid was extended by including the effects of rotation. An axisymmetric version of the original laboratory experiments was constructed and a series of experiments conducted on a rotating table with the tank filled with salt-stratified solution. The combination of vertical mixing at the boundary and horizontal mixing by mesoscale eddies lead to the weakening of the interior density gradient. While the mechanisms were thus complex, the experiments demonstrated that the boundary-mixing process may be parameterized by a relatively simple formulation dependent upon the turbulence properties at the boundary and the tank dimensions.
Roll-up of vorticity in adverse-pressure-gradient boundary layers
Authors: Goldstein M. E. , Durbin P. A. , Leib S. J.
Published: April 2006

It is shown how the unsteady, nonlinear critical-layer equation determines the evolution of instability waves in a weak adverse-pressure-gradient boundary layer. Numerical solutions show that the nonlinearity halts the growth of these inviscidly unstable waves. The stabilizing effect of nonlinearity, in the present case, can be described as a consequence of either the increase (toward zero) of the phase jump across the critical layer or the roll-up of the critical-layer disturbance vorticity.
Torsion effect on fully developed flow in a helical pipe
Authors: Kao Hsiao C.
Published: April 2006

Two approaches have been used to study the torsion effect on the fully developed laminar flow in a helical pipe of constant circular cross-section. The first approach is the series expansion method that perturbs the Poiseuille flow and is valid for low Dean numbers with both the dimensionless curvature and dimensionless torsion being much less than unity. The second is a numerical procedure that solves the complete Navier-Stokes equation and is applicable to intermediate values of the Dean number. The results obtained indicate that, as far as the secondary flow patterns are concerned, the presence of torsion can produce a large effect if the ratio of the curvature to the torsion is of order unity. In these cases the secondary flow, though still consisting of a pair of vortices, can be very much distorted. Under extreme conditions one vortex is so prevalent as to squeeze the second one into a narrow region. However, ordinarily the torsion effect is small and the secondary flow has the usual pattern of a pair of counter-rotating vortices of nearly equal strength. Concerning the flow resistance in the pipe the effect of torsion is always small in all the circumstances that have so far been considered.
The development of nonlinear waves on the surface of a horizontally rotating thin liquid film
Authors: Needham D. J. , Merkin J. H.
Published: April 2006

We consider the axisymmetric thin liquid film formed on a horizontally spinning disk. The asymptotic structure of the steady film is obtained, after which a theory is developed to describe the evolution of localized disturbances imposed upon the steady film. It is shown that this can lead to the propagation of large gradients in the film. Moreover, it is found that under certain conditions the steady film can become unstable.
Stability of rigid motions and coating films in bicomponent flows of immiscible liquids
Authors: Joseph Daniel D. , Preziosi Luigi
Published: April 2006

We consider the problem of global stability of the rigid rotation of two fluids. The realized interfacial configurations minimize a potential. We derive the most general form of the potential in which the working of the contact line may be expressed as a potential. The resulting variational problem for the interfacial potential is solved when the contact-line conditions are prescribed and for coating flows in which the interface makes a tangent contact with the wetted rod. In the former case, good agreement with experiments is obtained except near lines of contact. This shows that a spinning rod interfacial tensiometer is viable. In the latter case of coating flow, we get good agreement with experiments when the effects of gravity are not too large. The problem of bifurcation of coating flow is discussed qualitatively and some experimental results are given. We show how bifurcating sequences fit well into our qualitative description of the solution which must minimize the interfacial potential as the angular velocity is increased. The last bifurcations lead to pendant drops on a rotating ‘ceiling’ under the influence of centripetal forces which replace gravity. The dynamics of rollers of oil in water, or part in water and part in air, are explained in terms of the wavelength dependence of rotating drops.
Sediment transport during unsteady settling in an inclined channel
Authors: Amberg Gustav , Dahlkild Anders A.
Published: April 2006

Settling of a dilute, monodisperse suspension between two inclined, narrowly spaced parallel plates is considered. Effects of sediment motion are accounted for. Lubrication theory and a simplified model for the particle motion lead to a system of two coupled nonlinear hyperbolic equations for the evolution of the two interfaces between clear fluid, suspension and sediment. Two problems are solved: batch settling and the filling of a channel that initially contains clear fluid. In the batch-settling case, the sediment has no major qualitative effect on the motion. In the filling problem, however, effects of sediment are important.
The boundary layer due to a three-dimensional vortex loop
Authors: Ersoy S. , Walker J. D. A.
Published: April 2006

The nature of the boundary layer induced by the motion of a three-dimensional vortex loop towards a plane wall is considered. Initially the vortex is taken to be a ring approaching a plane wall at an angle of attack in an otherwise stagnant fluid; the ring rapidly distorts into a loop shape due to the influence of the wall and the trajectory is computed from a numerical solution of the Biot-Savart integral. As the vortex loop moves, an unsteady boundary-layer flow develops on the wall. A method is described which allows the computation of the flow velocities on and near the symmetry plane of the vortex loop within the boundary layer. The computed results show the development of a variety of complex three-dimensional separation phenomena. Some of the solutions ultimately show strong localized boundary-layer growth and are suggestive that a boundary-layer eruption and a strong viscous-inviscid interaction will be induced by the moving vortex.
Standard covariant formulation for perfect-fluid dynamics
Authors: Carter B. , Gaffet B.
Published: April 2006

After a brief description of the Milne generalization of the Galilean invariance group for the space–time of Newtonian kinematics, it is shown how the generalized Eulerian dynamical equations for the motion of a multiconstituent perfect (nonconducting) fluid can be expressed in terms of interior products of current 4-vectors with exterior derivatives of the appropriate 4-momentum 1-forms (whose role is central in this approach) in a fully covariant standard form whose structure is identical in the Newtonian case to that of the corresponding equation for the case of (special or general) relativistic perfect fluid mechanics. In addition to space–time covariance, this standard form exhibits chemical covariance in the sense that it is manifestly invariant under redefinition of the number densities of the independent conserved chemical constituents in terms of linear combinations of each other. It is shown how, in the strictly conservative case when no chemical reactions occur, this standard form, can be used (via the construction of suitably generalized Clebsch potentials) for setting up an Eulerian (fixed-point) variation principle in a form that is simultaneously valid for both Newtonian and relativistic cases.
Excitation of instability waves in free shear layers Part 2. Experiments
Authors: Bechert D. W. , Stahl B.
Published: April 2006

The acoustical excitation of shear layers is investigated experimentally. Acoustical excitation causes, for example, the so-called ‘orderly structures’ in shear layers and jets. Also, the deviations in the spreading rate between different turbulent-shear-layer experiments are due to the same excitation mechanism. The present investigations focus on measurements in the linear interaction region close to the edge from which the shear layer is shed. We report on two sets of experiments (Houston 1981 and Berlin 1983/84). The measurements have been carried out with laminar shear layers in air using hot-wire anemometers and microphones. The agreement between these measurements and the theory is good. Even details of the fluctuating flow field correspond to theoretical predictions, such as the local occurrence of negative phase speeds.
Waves in liquids with vapour bubbles
Authors: Nigmatulin R. I. , Khabeev N. S. , Hai Zuong Ngok
Published: April 2006

An investigation of wave processes in liquids with vapour bubbles with interphase heat and mass transfer is presented. A single-velocity two-pressure model is used which takes into account both the liquid radial inertia due to medium volume changes, and the temperature distribution around the bubbles. An analysis of the microscopic fields of physical parameters is aimed at closing the system of equations for averaged characteristics. The original system of differential equations of the model is modified to a form suitable for numerical integration. An elliptic equation is obtained to determine the field of the mixture average pressure at an arbitrary time through the known fields of the remaining quantities. The existence of the steady structure of shock waves, either monotonic or oscillatory, is proved. The effect of the initial conditions, shock strength, volume fraction, and dispersity of the vapour phase and of the thermophysical properties of the phases on shock-wave structure and relaxation time is studied. The influence of nonlinear, dispersion and dissipative effects on the wave evolution is also investigated. The shock adiabat for reflected waves is analysed. The results obtained have proved that the interphase heat and mass transfer determined by the thermal diffusivity of the liquid greatly influences the wave structure. The possible enhancement of disturbances in the region of their initiation is shown. The model has been tested for suitability and the results of calculations have been compared with experimental data.
Hydrodynamic arrest of a flat body moving towards a parallel surface at arbitrary Reynolds number
Authors: Lawrence C. J. , Weinbaum S.
Published: April 2006

The motion of a flat body towards a parallel plane surface in incompressible fluid is considered both in the presence and absence of an applied force for a non-vanishing initial velocity. In the inviscid limit, a first integral of the equations is obtained and analytic solutions are presented for the cases of finite body inertia with zero applied force and finite applied force with negligible body inertia. In the former case when the ratio of body inertia to fluid inertia is large, a singular behaviour is observed in the arrest of the body before impact wherein the time-dependent pressure and radial velocity of the fluid exhibit a sharp peak and there is a large transfer of kinetic energy from the body to the thin fluid layer. For a real fluid, a general procedure is described to obtain solutions at arbitrary Reynolds number for naturally occurring initial velocity conditions. Solutions to the full Navier-Stokes equations are obtained for an arbitrary Reynolds number based on gap height which are valid provided the flow remains laminar and the gap height is small. In general, the equations of motion of the body and fluid are both dynamically and kinematically coupled. The dynamic coupling, however, is removed when the body inertia is neglected. In particular, the cases of hydrodynamic arrest with zero applied force, and draining of the fluid under a constant applied force are considered. The natural initial conditions lead to a new exact similarity solution of the Navier-Stokes equations which is valid for an instantaneous time-dependent Re based on gap height of greater than approximately 100, wherein the top and bottom boundary layers remain distinct. The longer time portions of the motion and the final arrest are described by a numerical calculation for intermediate Reynolds number and a low-Reynolds-number analysis.
The effect of dead zones on longitudinal dispersion in streams
Authors: Purnama Anton
Published: April 2006

Dead zones tend to hold back the downstream travel, to increase the longitudinal spreading and to provide a long tail of low concentration for passive contaminant releases in natural streams. Here it is shown how the presence of a random distribution of dead zones can be accommodated into the method of moments by choosing an appropriate composite averaging. The individual roles of the cross-stream velocity shear, the dead-zones mean volume fraction and the dead-zones probability distribution are clearly revealed in the longitudinal shear-dispersion coefficient. The inevitable deviations from Gaussianity are examined by means of skewness and kurtosis. Simple examples are used to quantify the effects of the dead zones upon contaminant dispersion in Couette flow, pipe and plane Poiseuille flows.
Cumulative effects of weak pressure waves during the induction period of a thermal explosion in a closed cylinder
Authors: Klein R. , Peters N.
Published: April 2006

It is shown that the accumulation of small-amplitude gasdynamic perturbations is able to accelerate the process of self-ignition of a homogeneous explosive mixture. For analytical simplicity, the chemical kinetics are represented by a one-step irreversible reaction with a large activation energy. A one-dimensional piston-cylinder geometry is considered, which allows a slow compression of the gas during the induction period. It is assumed that the rates of temperature increase due to the reaction and due to the compression are comparable. A two-timescale asymptotic expansion for small Mach numbers is performed considering the distinguished limit ME = O(1), where E is the non-dimensional activation energy, and M is the Mach number based on a characteristic speed of the gas motion. Gasdynamic-chemical interaction is observed at the second order, where the secular equations describing the evolution on the slow timescale explicitly show a cumulative influence.
Minimizing shoreline pollution in rivers with tributaries
Authors: Smith Ronald
Published: April 2006

If an unpolluted tributary joins a river less than about thirty channel breadths downstream of an effluent outlet, then the additional dilution can reduce the peak pollution level experienced at the shoreline. This paper identifies the optimal sites for steady discharges to take full advantage of the extra dilution.
On the entrainment rate across a density interface
Authors: Nokes R. I.
Published: April 2006

Mixed-layer deepening due to grid-generated turbulence is studied experimentally with the aim of explaining the contradictory results of previous studies. Entrainment rates are calculated at fixed distances from the grid in order to avoid the necessity of using an empirical expression for the decay of the turbulent velocity scale. It is shown that an incorrect form of this decay law can cause large errors in the predicted Richardson number dependence of the entrainment rate. For this study this dependence can be expressed as a power law of the form E = KRi−1,2. The spread of the results imply that an error of at least ± 10% is realistic in the determination of the exponent.

The turbulent velocity decay law is also deduced from the data, and it is found that the decay cannot be represented by a simple power law. Indeed two distinct flow regions, with differing decay rates, are present.
The centring dynamics of a thin liquid shell in capillary oscillations
Authors: Lee Chun P. , Wang Taylor G.
Published: April 2006

The physical mechanism governing the centring of a hollow liquid shell in capillary oscillations, which has been observed in experiments, is investigated theoretically. First, the shell is assumed to be inviscid and to have a thickness that is much less than its spherical radius. A system of one-dimensional nonlinear equations of motion is derived using a thin-sheet model. From a numerical study the nonlinear effects of the wave are found to cause the core to oscillate slowly relative to the shell while the centre of mass of the whole system remains stationary. The effects of small viscosity are then considered in an approximation. Finally the strength of the centring mechanism is compared with that of the decentring effect due to buoyancy. The findings are consistent with the limited experimental information available.
The long-wave instability of a defect in a uniform parallel shear
Authors: Lerner J. , Knobloch E.
Published: April 2006

The stability properties of an inviscid, parallel, incompressible, free shear flow are studied. The shear profile is that of an unbounded, plane Couette flow containing a defect, or transition zone, whose magnitude ε is assumed to be small. The linearized eigenvalue problem is solved first for discretized models. When the defect has a finite thickness, the instability is confined to longitudinal wavenumbers, k [les ] 0(ε), in contrast to the more common 0(1) bandwidth, in units of inverse shear length. This observation motivates the application of a long-wave expansion to a smooth defect profile. A double expansion in both k and ε captures the whole waveband of the instability, and yields convergent expansions for the unstable eigenfunctions and for the dispersion relation describing their growth rate. The fastest growing modes are determined, and their back-reaction on the basic shear is calculated.
Viscous oscillations of a supported drop in an immiscible fluid
Authors: Strani M. , Sabetta F.
Published: April 2006

The small-amplitude free vibrations of a spherical drop immersed in an outer
immiscible fluid and in partial contact with a solid support are considered when both fluids are assumed to be viscous and incompressible, while gravity effects are neglected. Using the normal-mode decomposition and the Green-function method, the solution of the linearized Navier-Stokes equations is reduced to the solution of an eigenvalue problem. The model includes as particular cases the viscous model for a free drop proposed by Prosperetti (1980) and the inviscid model for a supported drop previously proposed by the authors.

The influence of the viscosity and of the support size are analysed both for the bubble and for the drop. At large values of the viscosity, the free drop shows significant differences with respect to the unsupported drop and a singular behaviour of the eigenvalue problem as the support size tends to zero.

The comparison with the available experimental data shows a quite satisfactory agreement for both the vibration frequency and the damping constant, provided that the support angle is not too large.
The energy and action of small waves riding on large waves
Authors: Henyey Frank S. , Creamer Dennis B. , Dysthe Kristian B. , Schult Roy L. , Wright Jon A.
Published: April 2006

We derive the dynamics of small waves riding on larger waves using a canonical, Hamiltonian formulation. The small waves are treated linearly and their energy is derived to all orders in the scale separation between the waves. Our results are similar to those of Longuet-Higgins (1987), but we have extended his calculations to include gravity-capillary waves and to allow for a more general, two-dimensional, large-wave field. Our result for the small-wave Hamiltonian is expressed in both Eulerian (horizontal) coordinate system and a non-inertial system determined by the large wave's surface. On further assuming scale separation between the small and large waves the averaged Lagrangian equations and the action density are derived. Action conservation is explicitly demonstrated.
On the thermodynamic stability of steady-state adiabatic systems
Authors: Henderson L. F.
Published: April 2006

This paper begins by reviewing Bethe's (1942) work on the subject. He considered the propagation of a normal shock wave in a medium with an arbitrary equation of state. Difficulties arise if one attempts to extend his theory to systems containing plane oblique shocks or the reflection or refraction of such shocks. The object of the present paper is to resolve these difficulties. General conditions for the local thermodynamic equilibrium and thermodynamie stability, of a non-equilibrium system in steady-state, adiabatic, flow are summarized by the principle of maximum entropy production, which gives
\[
\Delta s\geqslant 0;\quad {\rm d}(\Delta s)= 0;\quad {\rm d}^2(\Delta s) < 0,
\]
for ht, constant, where s is the specific entropy and ht is the specific total enthalpy; it is deduced from the second law. Conversely the consequences of Δs < 0, d(Δs) ≠ 0, d2(Δs) = 0, are discussed and may lead to either an impossibility or to some form of instability such as unsteadiness, or a change in the structure of the system (a catastrophe).
The superharmonic normal mode instabilities of nonlinear deep-water capillary waves
Authors: Hogan S. J.
Published: April 2006

We present results of the calculation of superharmonic normal mode perturbations to the exact nonlinear deep-water capilary wave solution of Crapper (1957). By using the method of Longuet-Higgins (1978a), we are able for the first time to consider all waveheights up to and including the maximum for two-dimensional perturbations. We find agreement with the recent asymptotic analysis of Hogan, Gruman & Stiassnie (1988). Superharmonic instabilities are found at various waveheights less than the maximum.
Cyclic recurrence in nonlinear unidirectional ocean waves
Authors: Bryant Peter J.
Published: April 2006

A fully nonlinear model is developed for the unidirectional propagation of periodic gravity wave groups in deep water, in which the shape of the group envelopes changes cyclically. It is intended to describe the slow-time evolution of wave groups on the open ocean surface, and to generalize the cyclic recurrence that can occur during the sideband modulation of Stokes waves and Schrödinger wave groups. The weak nonlinear interactions are shown to concentrate the wave energy at the centre of each group at regular intervals, causing the waves there to be of greater height locally in space and time. This is suggested as one mechanism for the local wave breaking that is observed on the open ocean surface. The cyclically recurring wave groups may be interpreted as the limit-cycle stage in a progression from uniform wave groups to chaos on the forced, damped, ocean surface.
One-dimensional closure models for three-dimensional incompressible viscoelastic free jets: von Kármán flow geometry and elliptical cross-section
Authors: Bechtel S. E. , Forest M. G. , Holm D. D. , Lin K. J.
Published: April 2006

In this paper we derive one-space-dimensional, reduced systems of equations (one-dimensional closure models) for viscoelastic free jets. We begin with the three-dimensional system of conservation laws and a Maxwell-Jeffreys constitutive law for an incompressible viscoelastic fluid. First, we exhibit exact truncations to a finite, closed system of one-dimensional equations based on classical velocity assumptions of von Kármán (1921). Next, we demonstrate that the three-dimensional free-surface boundary conditions overconstrain these truncated systems, so that only a very limited class of solutions exist. We then proceed to derive approximate one-dimensional closure theories through a slender-jet asymptotic scaling, combined with appropriate definitions of velocity, pressure and stress unknowns. Our non-axisymmetric one-dimensional slender-jet models incorporate the physical effects of inertia, viscoelasticity (viscosity, relaxation and retardation), gravity, surface tension, and properties of the ambient fluid, and include shear stresses and time dependence. Previous special one-dimensional slender-jet models correspond to the lowest-order equations in the present asymptotic theory by an a posteriori suppression to leading order of some of these effects, and a reduction to axisymmetry. We thereby: (i) derive existing one-dimensional models from the three-dimensional free surface boundary-value problem; (ii) clarify the sense of the one-dimensional approximation; (iii) deduce new one-dimensional closure models for non-axisymmetric viscoelastic free jets.
The nonlinear breakup of a magnetic layer: instability to interchange modes
Authors: Cattaneo F. , Hughes D. W.
Published: April 2006

Motivated by considerations of the solar toroidal magnetic field we have studied the behaviour of a layer of uniform magnetic field embedded in a convectively stable atmosphere. Since the field can support extra mass, such a configuration is top-heavy and thus instabilities of the Rayleigh-Taylor type can occur. For both static and rotating basic states we have followed the evolution of the interchange modes (no bending of the field lines) by integrating numerically the nonlinear compressible MHD equations. The initial Rayleigh-Taylor instability of the magnetic field gives instabilities to strong shearing motions, thereby exciting secondary Kelvin-Helmholtz instabilities which wrap the gas into regions of intense vorticity. The subsequent motions are determined primarily by the strong interactions between vortices which are responsible for the rapid disruption of the magnetic layer.
Finite-amplitude evolution of two-layer geostrophic vortices
Authors: Helfrich Karl R. , Send Uwe
Published: April 2006

The finite-amplitude evolution of circular two-layer quasi-geostrophic vortices with piecewise uniform potential vorticity in each layer (also termed ‘heton’ clouds by Hogg & Stommel 1985a and Pedlosky 1985) is studied using the contour dynamics method. The numerical investigations are preceded by a linear stability analysis which shows the stabilizing influence of deepening the lower layer. Net barotropic flow may be either stabilizing or destabilizing. The contour dynamics calculations for baroclinic vortices show that supercritical (i.e. linearly unstable) conditions may lead to explosive break up of the vortex via the generation of continuous hetons at the cloud boundary. The number of vortex pairs is equal to the azimuthal mode number of the initial disturbance. An additional weakly supercritical regime in which amplitude vacillation occurs, but not explosive growth, is identified. Vortices with net barotropic circulation behave similarly except that the layer with vorticity opposite to the barotropic circulation will break up first. Strong barotropic circulation can inhibit the development of hetons. The stronger layer may eject thin filaments, but remain mostly intact. Calculations for initial conditions composed of several unstable modes show that the linearly most unstable mode dominates at finite amplitude.
London and Droitwich, c. 650–750: trade, industry and the rise of Mercia
Authors: Maddicott J.R.
Published: April 2006

This article argues that the power of the early Mercian kings from
Wulfhere to Aethelbald, c.650-750, rested partly on their ability to
exploit the growing economy which characterised this period; and that
such exploitation provides a hitherto unacknowledged reason for the
rise of Mercia to supremacy. The argument rests on the Mercian rulers'
control of two particular places, London and Droitwich, the first the
country's foremost port, the second a major industrial site concerned
with the production of salt. After the growth of Mercian authority
over both places has been traced, it is suggested that taxation of
their activities may have resulted in substantial profits, via tolls
in both centres and, in Droitwich, via a further levy on the
manufacture of salt. The history of taxation on salt is traced back
from the early modern period in order to see what light later
practices shed upon those of the early middle ages. The article
concludes by suggesting that the Mercian rulers were fortunate in
exercising power at a time when economic growth, partly church-led,
was open to royal exploitation and that those rulers had a conscious
appreciation of the advantages so to be gained.
Microfabricated rubber microscope using soft solid immersion lenses
Authors: Gambin Yann , Legrand Olivier , Quake Stephen R.
Published: April 2006

We show here a technique of soft lithography to microfabricate efficient solid immersion lenses (SIL) out of rubber elastomers. The light collection efficiency of a lens system is described by its numerical aperture (NA), and is critical for applications as epifluorescence microscopy [B. Herman, Fluorescence Microscopy (BIOS Scientific, Oxford/Springer, United Kingdom, 1998). While most simple lens systems have numerical apertures less than 1, the lenses described here have NA=1.25. Better performance can be engineered though the use of compound designs; we used this principle to make compound solid immersion lenses (NA=1.32). An important application of these lenses will be as integrated optics for microfluidic devices. We incorporated them into a handheld rubber microscope for microfluidic flow cytometry and imaged single E. Coli cells by fluorescence.
Is Promoting an African American Unfair? The Triple Interaction of Participant Ethnicity, Target Ethnicity, and Ethnic Identity1
Authors: Konrad Alison M. , Ross Gerald , Linnehan Frank
Published: April 2006

This research examined the effect of manipulating a hypothetical candidate's ethnicity on the perceived fairness of promotions. In an experimental study, 142 undergraduates were assigned randomly to rate the fairness of promotions going to either a White or an African American candidate. Findings indicated that a significant three‐way interaction between participant's ethnicity, candidate's ethnicity, and scores on Phinney's (1992) multigroup ethnic identification index associated with perceptions of promotion decisions. Both White and African American participants with a strong ethnic identity gave higher fairness ratings when a member of their own ethnic group was promoted. These results demonstrate the importance of assessing the extent to which people identify with their ethnic group in addition to assessing their demographic categories.
Effects of Directed Thinking on Intentions to Engage in Beneficial Activities: Idea Generation or Mental Simulation?
Authors: Ten Eyck Laura L. , Labansat Heather A. , Gresky Dana M. , Dansereau Donald F. , Lord Charles G.
Published: April 2006

How might people best persuade themselves to engage in beneficial activities, such as dieting, exercise, and studying? One strategy is to think about actions. Another strategy is to think about reasons. In previous research, students who were directed to think about actions increased their study intentions more than did students who were directed to think about reasons. The present experiment tested whether thinking about actions was effective because of idea generation (coming up with the thoughts) or because of mental simulation (imagining the scenarios). In immediate and delayed measures, directed thinking about actions (but not reasons) proved generally more effective when students focused on mental simulation than when they focused on idea generation. Theoretical and applied implications are discussed.
The effect of fluvastatin on plasma adiponectin levels in dyslipidaemia
Authors: Sonmez Alper , Dogru Teoman , Tasci Ilker , Yilmaz M. Ilker , Pinar Murat , Naharci Ilkin , Bingol Necati , Kilic Selim , Demirtas Ayla , Bingol Sezin , Ozgurtas Taner , Erikci Selahattin
Published: April 2006

Objective  There is controversy about the effects of statins on insulin resistance and plasma adiponectin. The aim of this study was to investigate the effects of fluvastatin treatment on these parameters in a group of dyslipidaemic patients who had no confounding factors for insulin resistance or alterations in plasma adiponectin.

Design and patients  Forty‐nine patients [27 males, 22 females; mean age 47·2 ± 10·3 years; body mass index (BMI) 29·64 ± 3·2 kg/m2] with dyslipidaemia and 20 controls (six males, 14 females; mean age 45·3 ± 9·31 years; BMI 30·07 ± 4·04 kg/m2) were enrolled. All patients were treated initially with therapeutic lifestyle changes (TLC) for 6 weeks. Six out of 49 subjects were excluded from the study. Then, 24 out of 43 patients with high blood cholesterol despite TLC were allocated to fluvastatin 80 mg daily plus TLC, and the remaining 19 patients with normal cholesterol were subjected to TLC alone for additional 12 weeks.

Measurements  Plasma adiponectin, immunoreactive insulin levels, BMI, waist circumference, blood pressure, lipids, and glucose were determined. The insulin sensitivity index was quantified using the homeostasis model assessment (HOMA).

Results  TLC caused significant improvement in plasma insulin (P = 0·02) and elevation in plasma adiponectin (P = 0·02). Fluvastatin treatment decreased total cholesterol and low density lipoprotein (LDL)‐cholesterol significantly (P = 0·01 and P = 0·02, respectively). No significant effect of fluvastatin was observed on plasma insulin or adiponectin or on the HOMA index.

Conclusions  Fluvastatin does not improve plasma adiponectin levels and insulin sensitivity, despite its beneficial effects on lipid levels. Our data, however, were limited by the fact that a more accurate method of assessing insulin sensitivity, the euglycaemic–hyperinsulinaemic glucose clamp technique, was not used.
The N‐terminal ATPase AT‐hook‐containing region of the Arabidopsis chromatin‐remodeling protein SPLAYED is sufficient for biological activity
Authors: Su Yanhui , Kwon Chang Seob , Bezhani Staver , Huvermann Bärbel , Chen Changbin , Peragine Angela , Kennedy John F. , Wagner Doris
Published: April 2006

The SNF2‐like chromatin‐remodeling ATPase SPLAYED (SYD) was identified as a co‐activator of floral homeotic gene expression in Arabidopsis. SYD is also required for meristem maintenance and regulates flowering under a non‐inductive photoperiod. SNF2 ATPases are structurally and functionally conserved from yeast to humans. In addition to the conserved protein features, SYD has a large unique C‐terminal domain. We show here that SYD is present as two forms in the nucleus, full‐length and truncated, with the latter apparently lacking the C‐terminal domain. The ratio of the two forms of endogenous SYD differs in juvenile and in adult tissues. Furthermore, an SYD variant lacking the C‐terminal domain (SYDΔC) rescues the syd null mutant, indicating that the N‐terminal ATPase AT‐hook‐containing region of SYD is sufficient for biological activity. Plants expressing SYDΔC show molecular and morphological phenotypes opposite to those of the null mutant, suggesting that the construct results in increased activity. This increased activity is at least in part due to elevated SYD protein levels in these lines. We propose that the C‐terminal domain may control SYD accumulation and/or specific activity in the context of the full‐length protein. The presence of the C‐terminal domain in rice SYD suggests that its role is probably conserved in the two classes of flowering plants.
Higher cut‐off index value of immunoglobulin M antibody to hepatitis B core antigen in Taiwanese patients with hepatitis B
Authors: Huang Yi‐Wen , Lin Chih‐Lin , Chen Pei‐Jer , Lai Ming‐Yang , Kao Jia‐Horng , Chen Ding‐Shinn
Published: April 2006

Background:  The cut‐off index value of immunoglobulin M (IgM) antibody to hepatitis B core antigen (anti‐HBc; AxSYM CORE‐M, Abbott) for diagnosing acute hepatitis B is 1.2. A high false‐positive rate of IgM anti‐HBc was observed in acute flare‐ups of chronic hepatitis B in Taiwanese patients. Thus the purpose of the present paper was to study the optimal index value of IgM anti‐HBc in Taiwanese subjects.

Methods:  The peak index values of 42 IgM anti‐HBc‐positive patients were collected. There were 20 acute hepatitis B patients and 22 patients with chronic hepatitis B with acute flare. The biochemical, virological, and serological data were obtained.

Results:  There were significant differences in mean age (36 vs 47 years, P = 0.01), serum alanine aminotransferase level (2042 U/L vs 1193 U/L, P = 0.02) and peak index value of IgM anti‐HBc (2.9 vs 1.5, P < 0.01) between patients with acute hepatitis B and those with acute flare of chronic hepatitis B. Eleven (50%) of 22 patients with chronic hepatitis B with acute flare had index value of >1.2. The optimal cut‐off index value to differentiate acute hepatitis B from chronic hepatitis B with acute flare was 2.4–2.5, with a sensitivity of 90% and specificity of 90%.

Conclusions:  The cut‐off index value of IgM anti‐HBc to differentiate acute hepatitis B from chronic hepatitis B with acute flare among Taiwanese patients should be set at 2.4–2.5 instead of 0.8–1.2.
Involvement of retinoic acid‐induced transglutaminase activity in zonal differences of hepatocyte proliferation after partial hepatectomy
Authors: Ohtake Yosuke , Maruko Akiko , Abe Shinya , Nagashima Tomomi , Fukumoto Manabu , Ohkubo Yasuhito
Published: April 2006

Background:  The authors have recently demonstrated that there is inverse correlation between transglutaminase (TGase) activity and DNA synthesis in periportal hepatocytes (PPH) and perivenous hepatocytes (PVH) at 1 day after partial hepatectomy in rats. In order to ensure the involvement of TGase in the differential growth capacities between periportal and perivenous regions of regenerating liver, the aim of this study was to investigate the effect of retinoic acid, an inducer of TGase expression, on zonal differences of hepatocyte proliferation between PPH and PVH isolated from regenerating rat liver.

Methods:  Regenerating liver was prepared by 70% partial hepatectomy. PPH and PVH subpopulations were isolated by the digitonin/collagenase perfusion technique. Cell cycle was evaluated for incorporation of BrdU into hepatocytes and detected by flow cytometric analysis. TGase activity was determined by incorporation of 14C‐putrescine into dimethylcasein.

Results:  When retinoic acid was injected immediately after partial hepatectomy, TGase activity greatly increased in both PPH and PVH at 1 day after partial hepatectomy, and activity was higher in PPH than in PVH. DNA synthesis in both subpopulations did not increase 1 day after partial hepatectomy, with peaks of DNA synthesis shifting to 2 days, and synthesis was higher in PVH than in PPH.

Conclusion:  These results suggest that TGase might be involved in differential growth capacities between periportal and perivenous regions of regenerating rat liver after partial hepatectomy.