bluuebunny/crossref_metadata_2025 huggingface dataset:
Ekman pumping in compact astrophysical bodies
Authors: Abney Mark , Epstein Richard I.
Published: April 2006

We examine the dynamics of a rotating viscous fluid following an abrupt change in the angular velocity of the solid bounding surface. We include the effects of a density stratification and compressibility which are important in astrophysical objects such as neutron stars. We confirm and extend the conclusions of previous studies that stratification restricts the Ekman pumping process to a relatively thin layer near the boundary, leaving much of the interior fluid unaffected. We find that finite compressibility further inhibits Ekman pumping by decreasing the extent of the pumped layer and by increasing the time for spin-up. The results of this paper are important for interpreting the spin period discontinuities (‘glitches’) observed in rotating neutron stars.
Weakly nonlinear internal waves in a two-fluid system
Authors: Choi Wooyoung , Camassa Roberto
Published: April 2006

We derive general evolution equations for two-dimensional weakly nonlinear waves at the free surface in a system of two fluids of different densities. The thickness of the upper fluid layer is assumed to be small compared with the characteristic wavelength, but no restrictions are imposed on the thickness of the lower layer. We consider the case of a free upper boundary for its relevance in applications to ocean dynamics problems and the case of a non-uniform rigid upper boundary for applications to atmospheric problems. For the special case of shallow water, the new set of equations reduces to the Boussinesq equations for two-dimensional internal waves, whilst, for great and infinite lower-layer depth, we can recover the well-known Intermediate Long Wave and Benjamin–Ono models, respectively, for one-dimensional uni-directional wave propagation. Some numerical solutions of the model for one-dimensional waves in deep water are presented and compared with the known solutions of the uni-directional model. Finally, the effects of finite-amplitude slowly varying bottom topography are included in a model appropriate to the situation when the dependence on one of the horizontal coordinates is weak.
Algebraic probability density tails in decaying isotropic two-dimensional turbulence
Authors: Jiménez Javier
Published: April 2006

The p.d.f. of the velocity gradients in two-dimensional decaying isotropic turbulence is shown to approach a Cauchy distribution, with algebraic s−2 tails, as the flow becomes dominated by a large number of compact coherent vortices. The statistical argument is independent of the vortex structure, and depends only on general scaling properties. The same argument predicts a Gaussian p.d.f. for the velocity components. The convergence to these limits as a function of the number of vortices is analysed. It is found to be fast in the former case, but slow (logarithmic) in the latter, resulting in residual u−3 tails in all practical cases. The influence of a spread Gaussian vorticity distribution in the cores is estimated, and the relevant dimensionless parameter is identified as the area fraction covered by the cores. A comparison is made with the result of numerical simulations of two-dimensional decaying turbulence. The agreement of the p.d.f.s is excellent in the case of the gradients, and adequate in the case of the velocities. In the latter case the ratio between energy and enstrophy is computed, and agrees with the simulations. All the one-point statistics considered in this paper are consistent with a random arrangement of the vortex cores, with no evidence of energy screening.
Non-continuum lubrication flows between particles colliding in a gas
Authors: Sundararajakumar R. R. , Koch Donald L.
Published: April 2006

Solid-body collisions between smooth particles in a gas would not occur if the lubrication force for a continuum incompressible fluid were to hold at all particle separations. When the gap between the particles is of the order of the mean free path λ0 of the gas, the discrete molecular nature of the gas becomes important. For particles of radii a smaller than about 50 μm colliding in air at a relative velocity comparable to their terminal velocity, the effects of compressibility of the gas in the gap are not important.

The nature of the flow in the gap depends on the relative magnitudes of the minimum gap thickness h0 ≡ aε, the mean-free path λ0, and the distance aε1/2 over which the effects of curvature become important. The slip-flow regime, a[Gt ]λ0, was analysed by Hocking (1973) using the Maxwell slip boundary condition at the particle surface. To find the lubrication force in the transition regime (aε ∼ O(λ0)), we use the results of Cercignani & Daneri (1963) for the flux as a function of the pressure gradient in a Poiseuille channel flow. When aε[Lt ]λ0[Lt ]aε1/2, one might expect the local flow in the gap to be governed by Knudsen diffusion. However, an attempt to calculate the Knudsen diffusivity between parallel plates leads to a logarithmic divergence, which is cut off by intermolecular collisions, and the flux is therefore proportional to h0c log(λ0/h0), where c is the mean molecular speed. The non-continuum lubrication force is shown to have a weak, log - log divergence as the particle separation goes to zero. As a result, the energy dissipated in the collision is finite. In the limit of large particle inertia, the energy dissipated is 6πμU0a2(log h0/λ0 – 1.28), where 2U0 is the relative velocity of the particles.

When λ0[Gt ]aε1/2, we have a free molecular flow in the gap. In this case, owing to the curvature of the particles, the flux versus pressure gradient relation is non-local. We analyse the free molecular flow between two cylinders and obtain scalings for the lubrication force.
Sound radiation from a cylindrical duct. Part 2. Source modelling, nil-shielding directions, and the open-to-ducted transfer function
Authors: Chapman C. J.
Published: April 2006

This paper analyses the sound radiated from the front face of a hard-walled circular cylindrical duct in a subsonic mean flow when the duct contains acoustic sources typical of those in a ducted-fan aeroengine. Two main results are established for modes of any given frequency and circumferential order. The first result is that in certain easily calculated directions, called here the nil-shielding directions, the sound radiated by ducted sources is the same as the sound radiated by the corresponding open sources, i.e. by unducted sources of the same distribution and strength radiating into free space. Thus in these special directions the duct has no noise-shielding effect. The second result is that, in the Kirchhoff approximation, the sound radiated by the open sources in the nil-shielding directions determines the sound radiated by the ducted sources in all directions; i.e. the sound fields radiated by open and ducted sources are related by an open-to-ducted transfer function. This function is such that the sound radiated by the ducted sources is a linear combination of certain diffraction functions, in which the coefficients are given by the sound radiated by the open sources in the nil-shielding directions. The diffraction functions do not depend on the sources and are here calculated explicitly in terms of Bessel functions. The method used in the paper is Kirchhoff's approximation; within linear theory this gives the nil-shielding directions exactly, i.e. in agreement with the Wiener—Hopf solution, and gives the main beam of the radiated field, including the major side-lobes, to good accuracy. The results are relevant to the sound radiated into the forward arc by a ducted turbofan aeroengine.
Convective instability in rotating liquid 3He-4He mixtures
Authors: Thurlow M. S. , Brooks B. J. , Lucas P. G. J. , Ardron M. R. , Bhattacharjee J. K. , Woodcraft A. L.
Published: April 2006

Thermal convection is investigated experimentally in a dilute liquid mixture of 3He in 4He at four temperatures between 20 and 100 mK above the superfluid transition temperature, chosen for their proximity to the codimension-two and hydrodynamic tricritical points. Two experimental cells of aspect ratio 2.76 and 1.00 were used. For the cell with the higher aspect ratio, two convective transitions at each of the four temperatures were observed above a critical angular velocity, and only one observed below. At temperatures lower than that of the hydrodynamic tricritical point the transition with the lower critical Rayleigh number is hysteretic for all angular velocities; above this temperature hysteresis is absent. The critical Rayleigh numbers are compared with theoretical predictions that take into account the existence of convection modes with azimuthal angular dependence. In the case of the cell with the smaller aspect ratio thermal relaxation oscillations were observed when heating from below. Convective thresholds were again observed but their critical Rayleigh numbers are almost independent of angular velocity. Some suggestions are advanced for this unexpected behaviour.
An experimental study of the influence of riblets on transition
Authors: Grek G. R. , Kozlov V. V. , Titarenko S. V.
Published: April 2006

An experimental study of the effect of riblets on three-dimensional nonlinear structures, the so-called Λ-vortices on laminar-turbulent transition showed that riblets delay the transformation of the Λ-vortices into turbulent spots and shift the point of transition downstream. This result is opposite to the negative influence of such ribbed surfaces on two-dimensional linear Tollmien-Schlichting waves (the linear stage of transition). Thus, the ribbed surface influences laminar-turbulent transition structures differently: a negative influence on the linear-stage transition structures and a positive influence on the nonlinear-stage transition structures. It is demonstrated that transition control by means of riblets requires special attention to be paid to the choice of their location, taking into account the stage of transition.
Dynamic simulation of hydrodynamically interacting spheres in a quiescent second-order fluid
Authors: Phillips Ronald J.
Published: April 2006

A method is described for calculating the motion of N spherical particles suspended in a quiescent second-order fluid. The method requires calculation of only the low-Reynolds-number Newtonian velocity profile. This profile is used in conjunction with what has been called the ‘Reciprocal theorem method’ to evaluate particle velocities accurate to leading order in the Deborah number. If the Newtonian velocity field is found by a multipole moment expansion, then it is shown that the method can be integrated neatly into the Stokesian dynamics method of simulating Newtonian suspensions. Simulation results involving two, three, four and six particles are reported as illustrative examples, and are compared with corresponding results for particles in Newtonian fluids and with experimental results found in the literature. In addition, simulations of sedimenting suspensions are performed by using periodic boundary conditions to model an unbounded system, and the observed formation of clusters in the sedimenting system is shown to be in qualitative agreement with experimental observations.
On the observability of finite-depth short-crested water waves
Authors: Ioualalen M. , Roberts A. J. , Kharif C.
Published: April 2006

A numerical study of the superharmonic instabilities of short-crested waves on water of finite depth is performed in order to measure their time scales. It is shown that these superharmonic instabilities can be significant-unlike the deep-water case-in parts of the parameter regime. New resonances associated with the standing wave limit are studied closely. These instabilities ‘contaminate’ most of the parameter space, excluding that near two-dimensional progressive waves; however, they are significant only near the standing wave limit. The main result is that very narrow bands of both short-crested waves ‘close’ to two-dimensional standing waves, and of well developed short-crested waves, perturbed by superharmonic instabilities, are unstable for depths shallower than approximately a non-dimensional depth d= 1; the study is performed down to depth d= 0.5 beyond which the computations do not converge sufficiently. As a corollary, the present study predicts that these very narrow sub-domains of short-crested wave fields will not be observable, although most of the short-crested wave fields will be.
Penetrative turbulent convection into a rotating two-layer fluid
Authors: Narimousa Siavash
Published: April 2006

Turbulent convection into stratified two-layer fluid systems in the presence of rotation has been investigated in a cylindrical laboratory tank. For a wide range of conditions the vertical stability of the flow depends only on the Richardson numberRi=g′h0/(B0R)2/3(here,g′ =gδ ρi/ρ0 is the jump in the reduced gravity across the density interface, δ ρiis the jump in density itself,h0is the depth of the top layer,B0is the surface buoyancy flux andRis the radius of the source). We have found that for values ofRigreater than a critical value ofRic≈ 11, the convective flow did not penetrate through the density interface, regardless of the values of the convective Rossby numberRo*= (B0/f3h02)1/2of the flow (herefis the Coriolis parameter). In this case after the convective layer interacted with the density interface the mixed fluid, of intermediate density, propagated radially along the interface in the form of an intrusion. Later, ifRo*was less than approximately 5, mesoscale vortices with mean diameterD≈ 8(RoR)2/3and maximum swirl velocityv≈ (B0R)1/3were generated at the edge of this propagating front (hereRoR= (B0/f3R2)1/2is the Rossby number based onR). WhenRiwas less than 11, the convective flow eventually penetrated through the density interface and into the bottom layer. This occurred through the formation of discrete ‘convective holes’ beneath the source. At large values ofRo*, and at early times, one turbulent plume penetrated into the bottom layer from each hole. Later the initial holes coalesced to create one large lesion beneath the source.
Upper bounds on the energy dissipation in turbulent precession
Authors: Kerswell R. R.
Published: April 2006

Rigorous upper bounds on the viscous dissipation rate are identified for two commonly studied precessing fluid-filled configurations: an oblate spheroid and a long cylinder. The latter represents an interesting new application of the upper-bounding techniques developed by Howard and Busse. A novel ‘background’ method recently introduced by Doering & Constantin is also used to deduce in both instances an upper bound which is independent of the fluid's viscosity and the forcing precession rate. Experimental data provide some evidence that the observed viscous dissipation rate mirrors this behaviour at sufficiently high precessional forcing. Implications are then discussed for the Earth's precessional response.
Experiments on the rise of air bubbles in clean viscous liquids
Authors: Maxworthy T. , Gnann C. , Kürten M. , Durst F.
Published: April 2006

A number of new experiments have been performed on the rise of air bubbles in clean mixtures of distilled water and pure, reagent grade, glycerine covering a range of the relevant parameter, the Morton number, Mo = gv4ρ3/σ3, of 1013. Here g is the acceleration due to gravity, v the kinematic viscosity, ρ the density and σ the surface tension of the mixture. In these careful measurements several scaling regimes have been found that have not been discussed before in the extensive literature on the subject. The transitions between these regimes have been delineated and attempts made to discuss the dynamical processes that might be important in each of them.
Molecular diffusion and viscous effects on concentration statistics in grid turbulence
Authors: Borgas Michael S. , Sawford Brian L.
Published: April 2006

Two-particle Lagrangian models in turbulence are used to consider dispersion in decaying isotropic homogeneous turbulence (i.e. approximating grid-generated wind-tunnel flows). Thomson's formulation is used, and his model is extended to incorporate molecular diffusivity and viscosity, i.e. the range of scales modelled includes the dissipation subranges as well as the intertial subrange. New terms are proposed which consistently provide well-mixed models and the impact on Saffman's well-known small-time results is considered. The new model is ideal for comparison with recent concentration-fluctuation measurements in decaying wind-tunnel turbulence and the results are encouraging. In particular, the fluctuation intensities, both along and across the wind tunnel, are well described by the new model. In addition, small source-size effects are far better explained when we include the molecular effects. A surprising results is the persistence in time of both source-geometry and molecular effects upon concentration fluctuations. Both of the effects are negligible at large times for mean concentrations, but persist for significant times for the fluctuations, indicating an important role for small-scale dynamics.
Active sonic boom control
Authors: Crow Steven C. , Bergmeier Gene G.
Published: April 2006

A theory and simulation code are developed to study non-steady sources as means to control sonic booms of supersonic aircraft. A key result is that the source of sonic boom pressure is not confined to the length of the aircraft but occupies an extensive segment of the flight path. An aircraft in non-steady flight functions as a synthetic aperture antenna, generating complex acoustic waves with no simple relation to instantaneous volume or lift distributions.

The theory applies linear acoustics to slender non-steady sources but requires no far-field approximation. The solution for pressure contains a term not seen in Whitham's theory for sonic booms of distant supersonic aircraft. The term describes a pressure field that decays algebraically behind the Mach cone and, in the case of steady flight, integrates to a ground load equal to the weight of the aircraft. The algebraic term is separate from those that describe the sonic boom.

Two non-steady source phenomena are evaluated: periodic velocity changes (surge), and periodic longitudinal lift redistribution (slosh). Surge can attenuate a sonic boom and covert it into prolonged weak reverberation, but accelerations needed to produce the phenomenon seem too large for practical use. Slosh may be practical and can alter sonic booms but does not, on average, result in boom attenuation. The conclusion is that active sonic boom abatement is possible in theory but maybe not practical.
Approximations to wave trapping by topography
Authors: Chamberlain P. G. , Porter D.
Published: April 2006

The trapping of linear water waves over two-dimensional topography is investigated by using the mild-slope approximation. Two types of bed profile are considered: a local irregularity in a horizontal bed and a shelf joining two horizontal bed sections at different depths. A number of results are derived concerning the existence of trapped modes and their multiplicity. It is found, for example, that the maximum number of modes which can exist depends only on the gross properties of the topography and not on its precise shape. A range of problems is solved numerically, to inform and illustrate the analysis, using both the mild-slope equation and the recently derived modified mild-slope equation.
Instabilities in quasi-two-dimensional magnetohydrodynamic flows
Authors: BüHler L.
Published: April 2006

The improvement of heat transfer conditions in liquid-metal magnetohydrodynamic (MHD) flows is of prime importance for self-cooled fusion blanket design concepts. For many years the research was based on stationary inertialess assumptions since it was expected that time-dependent inertial flows would be suppressed by strong electromagnetic damping, especially in the extreme range of fusion relevant parameters. In the present analysis the stationary inertialess assumptions are abandoned. Nevertheless, the classical ideas usually used to obtain inertialess asymptotic solutions are drawn on. The basic inertial equations are reduced to a coupled two-dimensional problem by analytical integration along magnetic field lines. The magnetic field is responsible for a quasi-two-dimensional flow; the non-uniform distribution of the wall conductivity creates a wake-type profile, the MHD effect reducing to a particular forcing and friction. The solution for the two-dimensional variables, the field aligned component of vorticity, the stream function, and the electric potential are obtained by numerical methods. In a flat channel with non-uniform electrical wall conductivity, time-dependent solutions similar to the Kármán vortex street behind bluff bodies are possible. The onset of the vortex motion, i.e. the critical Reynolds number depends strongly on the strength of the magnetic field expressed by the Hartmann number. Stability analyses in viscous hydrodynamic wakes often use the approximation of a unidirectional flow which does not take into account the spatial evolution of the wake. The present problem exhibits a wake-type basic flow, which does not change along the flow path. It represents, therefore, an excellent example to which the simple linear analysis on the basis of Orr-Sommerfeld-type equations applies exactly. Once unstable, the flow first exhibits a regular time periodic vortex pattern which is rearranged further downstream. One can observe an elongation, pairing, or sometimes more complex merging of vortices. All these effects lead to larger flow structures with lower frequencies. The possibility for a creation and maintenance of time-dependent vortex-type flow pattern in MHD flows is demonstrated.
Stability of stationary endwall boundary layers during spin-down
Authors: Lopez J. M. , Weidman P. D.
Published: April 2006

Since Bödewadt's (1940) seminal work on the boundary layer flow produced by a fluid in solid-body rotation over a stationary disk of infinite radius there has been much interest in determining the stability of such flows. To date, it appears that there is no theoretical study of the stability of Bödewadt's self-similar solution to perturbations that are not self-similar. Experimental studies have been compromised due to the difficulty in establishing these steady flows in the laboratory. Savaç (1983, 1987) has studied the endwall boundary layers of flow in a circular cylinder following impulsive spin-down. During the first few radians of rotation, the endwall boundary layers have a structure very similar to Bödewadt layers. For certain conditions, SavaÇ has observed a series of axisymmetric waves travelling radially inwards in the endwall boundary layers. The conjecture is that these waves represent a mode of instability of the Bödewadt layer. Within a few radians of rotation however, the centrifugal instability of the sidewall layer dominates the spin-down process and the endwall waves are difficult to examine further.

Here, the impulsive spin-down problem is examined numerically for Savaç’ (1983, 1987) conditions and good agreement with his experiments is achieved. New experimental results are also presented, which include quantitative space-time information regarding the axisymmetric waves. These agree well with both the numerics and the earlier experimental work. Further, a related problem is considered numerically. This flow is also initially in solid-body rotation, but only the endwalls are impulsively stopped, keeping the sidewall rotating. This results in a flow virtually identical to the usual spin-down flow for the first few radians of rotation, except in the immediate vicinity of the sidewall. The sidewall layer is no longer centrifugally unstable and the circular waves on the endwalls are observed without the influence of the sidewall instability.
Self-similar multi-layer exchange flow through a contraction
Authors: Engqvist Anders
Published: April 2006

The multi-layer exchange equations for gravitationally driven flows between two basins with stable Boussinesq type of stratification in discrete layers, specified far upstream on either side of a connecting strait, result in a hydraulic control condition that must be satisfied at the narrowest part of the contraction, the control point. If one stagnant layer is present at the control point, the control condition that applies to all layers collectively may be separated into two such conditions that apply independently to two groups of layers going in opposite directions separated by the stagnant layer. Such bidirectional flow regimes exist if the structure of the prespecified density profiles permits each of the opposing groups to vertically reduce their thickness by the ratio 2/3 relative to their upstream thicknesses, leaving space for the stagnant layer to protrude through the contraction. Under these restrictions, the bidirectional flow is controlled by the fastest propagating wave mode and the stationary solution then relies on the superposition of two previously known unidirectional self-similar flow regimes that are completely decoupled. Techniques for their numerical computation are presented. The transition into loosely coupled and fully coupled flow is discussed. The decoupling principle also applies when several non-adjacent stagnant layers are simultaneously present at control in which case multiple groups of decoupled layers flow in alternating directions.
Wave pattern formation in a fluid annulus with a radially vibrating inner cylinder
Authors: Krasnopolskaya T. S. , Heijst G. J. F. Van
Published: April 2006

The phenomenon of pattern formation of free-surface waves of a fluid confined in an annulus the inner wall of which vibrates radially, is investigated both theoretically and experimentally. Although the waves are excited by harmonic axisymmetric deformations of the inner shell, depending on the vibration frequency both axisymmetric and non-symmetric wave patterns may arise.

Experimental observations have revealed that waves are excited in two different resonance regimes. The first type corresponds to forced resonance, in which axisymmetric patterns are realized with eigenfrequencies equal to the frequency of excitation. The second kind is parametric resonance, in which case the waves are ‘transverse’, with their crests and troughs aligned perpendicular to the vibrating wall. These so-called cross-waves have frequencies equal to half of that of the wavemaker.

Both kinds of resonance were investigated theoretically using Lamé's method of superposition. It was shown experimentally that the pure forced resonant standing waves are not realized when the amplitude of excitation is beyond the threshold of parametric resonance for non-symmetric waves. The experimental observations agree very well with the theoretical results.
Development of structure within the turbulent wake of a porous body. Part 2. Evolution of the three-dimensional features
Authors: Huang Z. , Kawall J. G. , Keffer J. F.
Published: April 2006

The present investigation deals with the development stage (x/d = 20–100) of the porous-body wake, generated by a mesh strip. Pattern-recognition analysis was applied to the velocity signals from hot-wire measurements in the vertical centreplane and various horizontal planes in the flow field. Spanwise coherent structures (with ωz) and lateral coherent structures (ωy) have been identified. An examination of the fine-scale turbulence is used to show that the two kinds of structures are indeed connected. In the near region of the wake, the lateral structures are ‘ribs’ attached to the spanwise vortices. In the intermediate region, the interaction of the two structures changes the original spanwise vortices into three-dimensional structures. Beyond this zone, in the far region, the’ legs’ of the original spanwise structure merge with the’ ribs’, and it can be inferred that the coherent structures within the flow are essentially randomly distributed hairpin-like vortices.
Numerical investigation of the dynamic influence of the contact line region on the macroscopic meniscus shape
Authors: Bazhlekov Ivan B. , Chesters Allan K.
Published: April 2006

The influence of different boundary conditions applied in the contact line region on the outer meniscus shape is analysed by means of a finite-element numerical simulation of the steady movement of a liquid-gas meniscus in a capillary tube. The free-surface steady shape is obtained by solving the unsteady creeping-flow approximation of the Navier–Stokes equations starting from some initial shape. Comparisons of the outer solutions obtained using two different inner models, together with that published by Lowndes (1980), indicate the relative insensitivity of the outer solution to the type of model utilized in the contact line region.
Nonlinear deformation and breakup of stretching liquid bridges
Authors: Zhang X. , Padgett R. S. , Basaran O. A.
Published: April 2006

In this paper, the nonlinear dynamics of an axisymmetric liquid bridge held captive between two coaxial, circular, solid disks that are separated at a constant velocity are considered. As the disks are continuously pulled apart, the bridge deforms and ultimately breaks when its length attains a limiting value, producing two drops that are supported on the two disks. The evolution in time of the bridge shape and the rupture of the interface are investigated theoretically and experimentally to quantitatively probe the influence of physical and geometrical parameters on the dynamics. In the computations, a one-dimensional model that is based on the slender jet approximation is used to simulate the dynamic response of the bridge to the continuous uniaxial stretching. The governing system of nonlinear, time-dependent equations is solved numerically by a method of lines that uses the Galerkin/finite element method for discretization in space and an adaptive, implicit finite difference technique for discretization in time. In order to verify the model and computational results, extensive experiments are performed by using an ultra-high-speed video system to monitor the dynamics of liquid bridges with a time resolution of 1/12 th of a millisecond. The computational and experimental results show that as the importance of the inertial force – most easily changed in experiments by changing the stretching velocity – relative to the surface tension force increases but does not become too large and the importance of the viscous force – most easily changed by changing liquid viscosity – relative to the surface tension force increases, the limiting length that a liquid bridge is able to attain before breaking increases. By contrast, increasing the gravitational force – most readily controlled by varying disk radius or liquid density – relative to the surface tension force reduces the limiting bridge length at breakup. Moreover, the manner in which the bridge volume is partitioned between the pendant and sessile drops that result upon breakup is strongly influenced by the magnitudes of viscous, inertial, and gravitational forces relative to surface tension ones. Attention is also paid here to the dynamics of the liquid thread that connects the two portions of the bridge liquid that are pendant from the top moving rod and sessile on the lower stationary rod because the manner in which the thread evolves in time and breaks has important implications for the closely related problem of drop formation from a capillary. Reassuringly, the computations and the experimental measurements are shown to agree well with one another.
A uniformly asymptotic approximation for the development of shear dispersion
Authors: Phillips C. G. , Kaye S. R.
Published: April 2006

In this paper we consider the development of shear dispersion following the introduction of a diffusing tracer substance into a tube or duct containing flowing fluid, with emphasis on the characterization of the temporal variation of concentration at a fixed axial position. Asymptotic results are derived by assuming that the distance downstream of the point of tracer introduction, appropriately non-dimensionalized, is large. First, we consider the central moments of the temporal concentration variation, including their dependence on transverse position and on the initial transverse distribution of tracer. The moments for finite Péclet number are expressed in terms of their infinite-Péclet-number counterparts, and the latter are given explicitly for Poiseuille flow. Then, assuming the Péclet number is infinite, we derive an approximate solution for the Green's function expressing tracer concentration following its introduction at an arbitrary point within the tube. The solution is expressed in terms of three numerically evaluated functions of a dimensionless time variable, with parametric dependence on the distance downstream of the point of tracer release. The method is illustrated by calculation of the approximate solution for dispersion in Poiseuille flow. Unlike previous approximations, the present solution is uniformly asymptotic and represents the tails of the concentration distribution as well as the approximately Gaussian central part; in these three regions, simpler analytic forms of the approximation are given. Comparison with previous computational solutions suggests the present approximation remains reasonably accurate even at quite short distances from the point where tracer is released.
Pushing Back the Boundaries: Social Policy, Domestic Violence and Women's Organisations in Peru
Authors: BOESTEN JELKE
Published: April 2006

This article looks at the extent to which Peruvian women have been able to use a legal and institutional framework designed to combat domestic violence, despite its significant defects, in order to transform the authoritarian forms of communication that prevail in their families, organisations, and communities. Using in-depth interviews with women in Lima, Huancavelica and Ayacucho, the article suggests that while democratisation from above may often result in limited transformations in many policy areas, we must be attentive to the way in which democratisation can be appropriated, transformed and reinvigorated from below.
Pathophysiology and therapy for haemoglobinopathies; Part I: sickle cell disease
Authors: Madigan Catherine , Malik Punam
Published: April 2006

In sickle cell disease, a single base pair substitution in the gene encoding the β-globin chain of the haemoglobin molecule gives rise to a surprisingly broad spectrum of pathophysiological and clinical manifestations. Inflammation, endothelial activation, red blood cell membrane abnormalities and altered availability of vasoactive factors characterise this disorder. Clinically, patients suffer from a host of seemingly unrelated maladies, from pain episodes to strokes, life-threatening infections and pulmonary hypertension. Deepened understanding of this complex disease now allows us to begin to turn away from simple supportive treatments, and move towards therapies aimed at specific pathophysiological targets. This article, the first of two reviews on the pathophysiology of haemoglobinopathies, discusses the molecular basis of sickle cell disease, and elaborates on the many factors that exacerbate or ameliorate the disease process. It then focuses on the promising targeted therapies currently in use or under investigation. An accompanying article on haemoglobinopathies (Part II) focuses on thalassaemias.
Die Entwicklung der laparoskopischen Dickdarmchirurgie mit spezieller Berücksichtigung der Karzinombehandlung
Authors: Buchmann  , Dinçler
Published: April 2006

Die Entwicklung der laparoskopischen Chirurgie begann 1901 mit der Coelioskopie zu diagnostischen Zwecken und 1983 mit der ersten Appendektomie. Eine weltweite Verbreitung erlebte sie seit 1987 mit der Cholezystektomie. 1991 wurde erstmals sowohl die Hemikolektomie rechts als auch die Sigmaresektion beschrieben. Die anfängliche Euphorie dämpften die ersten Mitteilungen von Port-site-Metastasen. Die Kontroverse, ob beim Karzinom laparoskopisch operiert werden dürfe oder nicht, löste einen Forschungsboom aus. Im Jahr 2000 stand eindeutig fest, dass Port-site-Metastasen etwa gleich häufig wie «Drain-site-Metastasen» bei der offenen Chirurgie sind (um 0.9%). Die randomisierten Studien zum Vergleich laparoskopisch versus offen belegen keine Unterschiede bezüglich der Langzeitergebnisse beim Kolonkarzinom. In erfahrenen Händen gilt dies auch für das Rektumkarzinom. Die Lernkurve ist dabei von grosser Bedeutung. Sie wird mit 30 bis 70 Operationen angegeben, je nachdem ob nur die Operationsdauer oder zusätzlich andere Kriterien berücksichtigt werden (z.B. Umsteigerate, Komplikationen). Mit zunehmender Übung sinkt auch der intraoperative Materialaufwand, sodass die minimal-invasive Technik gesamthaft gesehen billiger wird als die offene Methode. Bei solchen Kalkulationen muss berücksichtigt werden, dass die rasche Wiedereingliederung eines Menschen in den Alltag volkswirtschaftlich gesehen profitabel ist und dass nach laparoskopischen Eingriffen gewisse Faktoren des Lebensqualitätsscores (SF-36) in der Langzeitbeobachtung signifikant besser sind. 2006 hat sich die laparoskopische kolorektale Chirurgie etabliert. Weil sie das Immunsystem weniger beeinträchtigt, wird spekuliert, dass sie sogar bessere Langzeitergebnisse zeigen wird. Dies müssen aber erst künftige Studien beweisen.
Ein rotes Bein
Authors: Geeser
Published: April 2006

Wir berichten von einem 62-jährigen Patienten mit bekannten postthrombotischen Veränderungen im Bereich der V. poplitea links und passageren atemabhängigen Thoraxschmerzen in der Anamnese, welcher sich aktuell mit einer Thrombophlebitis der gesamten V. saphena magna rechts präsentiert. Differentialdiagnosen zur Thrombophlebitis umfassen im Wesentlichen eine TVT, eine Zellulitis/Pannikulitis, eine Lymphangitis, Insektenstiche, das Erythema nodosum, die kutane Polyarteriitis nodosa (PAN), das sarkoide Granulom und das Kaposi-Sarkom. Aufgrund des spontanen Auftretens der Thrombophlebitis und der indirekten Hinweise für thromboembolische Ereignisse in der Anamnese haben wir eine gerinnungsphysiologische Abklärung durchgeführt, welche eine homozygote Faktor V Leiden-Mutation als wahrscheinliche Ursache ergab. Da wir von einer symptomatischen Gerinnungsstörung ausgehen, haben wir dem Patienten bei einem 50- bis 100-fach erhöhten Thromboserisiko die lebenslange Antikoagulation empfohlen. Falls eine bezüglich thromboembolischen Ereignissen blande Anamnese vorgelegen hätte, hätten wir dem Patienten lediglich zu einer prophylaktischen Verwendung von niedermolekularen Heparinen in Risikosituationen geraten, da in diesem Fall bedeutende protektive Faktoren postuliert werden müssen.
Modeling of stress transfer in the Coquimbo region of central Chile
Authors: Gardi A. , Lemoine A. , Madariaga R. , Campos J.
Published: April 2006

We study the seismicity and stress transfer in the Coquimbo region of central Chile, where an exceptional series of more than 12 earthquakes of magnitudes from 6 to 7.6 has occurred since July 1997. In this area, the oceanic Nazca plate is subducted under the continental lithosphere of South America. Below 50 km, the downgoing slab slips aseismically with respect to the South American plate at a rate close to 6.5 cm/yr. The Coquimbo region was the site of major earthquakes of M > 8 in 1880 and 1943. After many years of quiescence, the seismic activity of the plate interface suddenly increased in mid‐1997 and continued at least until 2004. The first group of events occurred in July 1997 in the middle of the locked plate interface. In October 1997, the activity moved inland to the Punitaqui‐Ovalle area, just above the transition from the seismogenic zone to that of aseismic slip. The main event of the series was the M = 7.6 15 October 1997 Punitaqui earthquake. This is an intraslab compressional earthquake that occurred at ∼60 km depth, on a subvertical plane located very close to the downdip edge of the seismogenic coupled interface. We performed simulations of Coulomb stress transfer for earthquakes near the bottom of the seismogenic zone. We found that a simple model of stress transfer from the aseismic slip at depths greater than 50 km can explain the triggering not only of the Punitaqui earthquake but also of the July 1997 sequence. Additional simulations show that the seismicity that followed the 1997 event for almost 7 years can also be simply explained as a result of increased Coulomb stresses on the seismogenic plate interface as a result of the 1997 event and aseismic slip.
ALTAIR incoherent scatter observations of the equatorial daytime ionosphere
Authors: Kudeki Erhan , Milla Marco , Friedrich Martin , Lehmacher Gerald , Sponseller Dale
Published: April 2006

The first high‐resolution electron density maps of the low‐latitude D‐ and E‐region ionosphere in the Pacific sector acquired with ALTAIR UHF measurements are presented. The electron density data were collected in connection with a rocket based common volume experiment described by Friedrich et al. (2006). The observed phenomena in the density maps include E‐region density layers of tidal origin, shorter lived layers with small vertical scales, and lower E‐region electron density enhancement in response to enhanced X‐ray flux.
Regional high‐resolution spatiotemporal gravity modeling from GRACE data using spherical wavelets
Authors: Schmidt M. , Han S.‐C. , Kusche J. , Sanchez L. , Shum C. K.
Published: April 2006

We determine a regional spatiotemporal gravity field over northern South America including the Amazon region using GRACE inter‐satellite range‐rate measurements by application of a wavelet‐based multiresolution technique. A major advantage of this method is that we are able to represent the Amazon hydrological signals in form of time series of detail signals with level‐dependent temporal resolution: the coarser structures generally require only ten days, whereas the medium and finer details are computable from one month of data. To this end, we employ the basic property of multiresolution representations, which is to split a signal into detail signals, each related to a specific resolution level and computable from data covering a specific part of the spectrum. Our results, which for the first time fully exploit the spatial and temporal resolutions of GRACE data in modeling Amazon hydrological fluxes, are in good agreement with hydrological models and GPS‐derived height variations.
Formation of nitric acid/water ice particles in cirrus clouds
Authors: Kärcher B. , Voigt C.
Published: April 2006

Nitric acid (HNO3) in cirrus ice crystals has been measured in the last decade during airborne field campaigns at latitudes 53°S–68°N. The HNO3 content in ice crystals, expressed in terms of HNO3/H2O molar ratio, and the fraction of HNO3 in ice derived from those measurements exhibit a clear upward trend with decreasing temperature. The observations are explained by a novel model describing dissolution of HNO3 in liquid aerosol particles serving as freezing nuclei and subsequent trapping of HNO3 during ice crystal growth. The efficiency of trapping increases with decreasing temperature. Efficient trapping occurs via diffusional burial of the ambient HNO3 below about 203 K, because of long residence times of HNO3 molecules at the ice surface. This opens the possibility for HNO3‐induced modifications of processes affecting ice crystal growth. At warmer temperatures, molecular processes in the ice surface layer cause an increasingly rapid escape of adsorbed HNO3 into the gas phase and render trapping less efficient despite faster ice growth rates.
Anthropogenic impacts on continental surface water fluxes
Authors: Haddeland Ingjerd , Skaugen Thomas , Lettenmaier Dennis P.
Published: April 2006

Impacts of reservoirs and irrigation water withdrawals on continental surface water fluxes are studied within the framework of the Variable Infiltration Capacity (VIC) model for a part of North America, and for Asia. A reservoir model, designed for continental‐scale simulations, is developed and implemented in the VIC model. The model successfully simulates irrigation water requirements, and captures the main effects of reservoir operations and irrigation water withdrawals on surface water fluxes, although consumptive irrigation water use is somewhat underestimated. For the North American region, simulated irrigation water requirements and consumptive irrigation water uses are 191 and 98 km3year−1, while the corresponding numbers for the Asian region are 810 and 509 km3year−1, respectively. The consumptive uses represent a decrease in river discharge of 4.2 percent for the North American region, and 2.8 percent for the Asian region. The largest monthly decrease is about 30 percent, for the area draining the Western USA in June. The maximum monthly increase in streamflow (28 percent) is in March for the Asian Arctic region.
Gi-coupled receptors mediate phosphorylation of CPI-17 and MLC20 via preferential activation of the PI3K/ILK pathway
Authors: Huang Jiean , Mahavadi Sunila , Sriwai Wimolpak , Hu Wenhui , Murthy Karnam S.
Published: April 2006

Sustained smooth-muscle contraction or its experimental counterpart, Ca2+ sensitization, by Gq/13-coupled receptor agonists is mediated via RhoA-dependent inhibition of MLC (myosin light chain) phosphatase and MLC20 (20 kDa regulatory light chain of myosin II) phosphorylation by a Ca2+-independent MLCK (MLC kinase). The present study identified the corresponding pathways initiated by Gi-coupled receptors. Somatostatin acting via Gi1-coupled sstr3 receptor, DPDPE ([D-Pen2,D-Pen5]enkephalin; where Pen is penicillamine) acting via Gi2-coupled δ-opioid receptors, and cyclopentyl adenosine acting via Gi3-coupled adenosine A1 receptors preferentially activated PI3K (phosphoinositide 3-kinase) and ILK (integrin-linked kinase), whereas ACh (acetylcholine) acting via Gi3-coupled M2 receptors preferentially activated PI3K, Cdc42 (cell division cycle 42)/Rac1, PAK1 (p21-activated kinase 1) and p38 MAPK (mitogen-activated protein kinase). Only agonists that activated ILK induced sustained CPI-17 (protein kinase C potentiated inhibitor 17 kDa protein) phosphorylation at Thr38, MLC20 phosphorylation at Ser19, and contraction, consistent with recent evidence that ILK can act as a Ca2+-independent MLCK capable of phosphorylating the MLC phosphatase inhibitor, CPI-17, at Thr38. ILK activity, and CPI-17 and MLC20 phosphorylation were inhibited by LY294002 and in muscle cells expressing ILK(R211A) or treated with siRNA (small interfering RNA) for ILK. ACh acting via M2 receptors activated ILK, and induced CPI-17 and MLC20 phosphorylation and muscle contraction, but only after inhibition of p38 MAPK; all these responses were inhibited in cells expressing ILK(R211A). Conversely, ACh activated PAK1, a step upstream of p38 MAPK, whereas the three other agonists did so only in cells transfected with ILK(R211A) or siRNA for ILK. The results demonstrate reciprocal inhibition between two pathways downstream of PI3K, with ILK inhibiting PAK1, and p38 MAPK inhibiting ILK. Sustained contraction via Gi-coupled receptors is dependent on CPI-17 and MLC20 phosphorylation by ILK.
Enhanced hole transport in C60-doped hole transport layer
Authors: Lee Jun Yeob , Kwon Jang Hyuk
Published: April 2006

The carrier mobility of C60-doped hole transport layer was investigated according to the C60 content. The addition of C60 in the 1, 3, 5-tris (N, N-bis-(4, 5-methoxyphenyl)-aminophenyl) benzol (TDAPB) hole transport material resulted in the increase of hole mobility of TDAPB. The C60 molecule acted as a p-type dopant in the hole transport layer, resulting in the improved hole transport in TDAPB. The hole mobility of C60-doped TDAPB was 9.0×10−4cm2∕Vs compared with 1.0×10−4cm2∕Vs for nondoped TDAPB.