bluuebunny/crossref_metadata_2025 huggingface dataset:
Quality of life and its correlates among heroin users in Taiwan
Authors: Yen Chia‐Nan , Wang Carol Sheei‐Meei , Wang Tzu‐Yun , Chen Hsiao‐Fang , Chang Hsun‐Cheng
Published: April 2011

The aims of this study were to compare the quality of life (QOL) between subjects with and without heroin use and to examine the association of QOL with sociodemographic characteristics, characteristics of heroin use, family support, and depression among heroin users at entry to a methadone maintenance treatment program. A group of 123 heroin users who visited an outpatient addiction treatment clinic in southern Taiwan for methadone maintenance treatment were recruited into this study. We also recruited 106 subjects who had never used heroin as the control group. Their QOL status was assessed by the short form of the Taiwan Version of the World Health Organization Questionnaire on Quality of Life (the WHOQOL‐BREF Taiwan version). The level of QOL between subjects with and without heroin use was compared, and the correlates of QOL among heroin users were examined. Heroin users had poorer QOL than nonusers in the physical, psychological, and social relationship domains but not the environment domain of the WHOQOL‐BREF after controlling for the influences of other factors. In addition, heroin users with obvious depression had poorer QOL in all four domains than those without obvious depression. Also, heroin users who perceived higher family support had better QOL in the social relationship and environment domains. Heroin users had poorer QOL than nonusers in multiple domains. Relief of depressive symptoms and enhancement of family support should be important strategies to improve QOL in heroin users.
Eosinophilic meningitis risk associated with raw Ampullarium canaliculatus snails consumption
Authors: Wang Jiun‐Jye , Chung Li‐Yu , Lin Rong‐Jyh , Lee June‐Der , Lin Chaio‐Wen , Yen Chuan‐Min
Published: April 2011

In Taiwan, Angiostrongylus cantonensis infection has been reported in foreign laborers who had consumed raw Ampullarium canaliculatus snails. This study analyzed three foreign laborers who had contracted enzyme‐linked immunosorbent assay‐confirmed A cantonensis infection while working in Taiwan. All three workers had consumed either roasted snails or raw snails flavored with seasoning while drinking wine. This study investigated possible risk factors for A cantonensis, including naturally occurring A cantonensis in A canaliculatus snails, viability of third‐stage A cantonensis larvae in raw seasoned snails and in roasted snails, infectivity of larvae, and effects of alcohol while consuming snails. Positive infection rates in snails from five different irrigation canals in south Taiwan ranged from 12.3% to 29.4% and the average number of motile larvae per infected snail ranged from 36 to 65. The number of motile and coiled larvae in snail meat after 120 minutes seasoning was 93 (27.7%) and 233 (69.3%), respectively. After 20 minutes of roasting, most larvae in the snail meat were dead. The infectivities of motile and coiled larvae from snail meat after 60 minutes seasoning were 53.2% and 33.2%, respectively, and those from snail meat after 5 minutes roasting were 33.2% and 7.0%, respectively. Eating Taiwan A canaliculatus snails raw is extremely risky given their high infection rates and infection intensities. Even after 120 minutes seasoning or after 20 minutes roasting, snail meat should be considered unsafe for human consumption. Finally, experimental rodent studies indicated that consuming alcohol while ingesting larvae does not significantly reduced infectivity.
Analysis of vortex populations in turbulent wall-bounded flows
Authors: GAO Q. , ORTIZ-DUEÑAS C. , LONGMIRE E. K.
Published: April 2011

Vortical structures were identified and characterized using velocity fields of turbulent wall-bounded flows. Two direct numerical simulation data sets of fully developed channel flow at Reτ = 934 obtained by del Álamo et al. (J. Fluid Mech., vol. 500, 2004, p. 135) and Reτ = 590 obtained by Moser, Kim & Mansour (Phys. Fluids, vol. 11, 1999, p. 943) as well as dual-plane particle image velocimetry data at z+ = 110 in a zero-pressure-gradient turbulent boundary layer at Reτ = 1160 obtained by Ganapathisubramani, Longmire & Marusic (Phys. Fluids, vol. 18, 2006, 055105) were employed. The three-dimensional swirling strength based on the local velocity gradient tensor was employed to identify vortex core locations. The real eigenvector of the tensor was used both to refine the identification algorithm and to determine the orientation of each vortex core. The identification method allowed cores of nearly all orientations to be analysed. Circulation of each vortical structure was calculated using the vorticity vector projected onto the real eigenvector direction. Various population distributions were then computed at different wall-normal locations including core size, orientation, circulation and propagation velocity. The mean radius of the cores identified was found to increase with increasing wall-normal distance, and the mean circulation increases approximately quadratically with eddy radius. Orientations of cores with stronger circulation were distributed over a much narrower range than those for vortices with weaker circulation and were consistent with legs, necks and heads of forward-leaning hairpin structures.
Bifurcations and instabilities in sliding Couette flow
Authors: DEGUCHI K. , NAGATA M.
Published: April 2011

We carry out linear and nonlinear analyses on a flow between two infinitely long concentric cylinders with the radii a and b subject to a sliding motion of the inner cylinder in the axial direction. We confirm the linear stability result of Gittler (Acta Mechanica, vol. 101, 1993, p. 1) for the axisymmetric case, namely the flow is linearly stable against axisymmetric perturbations when the radius ratio η = a/b is greater than 0.1415. We extend his analysis to the non-axisymmetric case and find that the stability of the flow is still determined by axisymmetric perturbations. Our nonlinear analysis exhibits that (i) finite-amplitude axisymmetric solutions exist far below the linear critical Reynolds number for η < 0.1415 and (ii) non-axisymmetric travelling wave solutions appear abruptly at a finite Reynolds number even for η > 0.1415 where the linear critical state is absent.
On unsteady boundary-layer separation in supersonic flow. Part 1. Upstream moving separation point
Authors: RUBAN A. I. , ARAKI D. , YAPALPARVI R. , GAJJAR J. S. B.
Published: April 2011

This study is concerned with the boundary-layer separation from a rigid body surface in unsteady two-dimensional laminar supersonic flow. The separation is assumed to be provoked by a shock wave impinging upon the boundary layer at a point that moves with speed Vsh along the body surface. The strength of the shock and its speed Vsh are allowed to vary with time t, but not too fast, namely, we assume that the characteristic time scale t ≪ Re−1/2/Vw2. Here Re denotes the Reynolds number, and Vw = −Vsh is wall velocity referred to the gas velocity V∞ in the free stream. We show that under this assumption the flow in the region of interaction between the shock and boundary layer may be treated as quasi-steady if it is considered in the coordinate frame moving with the shock. We start with the flow regime when Vw = O(Re−1/8). In this case, the interaction between the shock and boundary layer is described by classical triple-deck theory. The main modification to the usual triple-deck formulation is that in the moving frame the body surface is no longer stationary; it moves with the speed Vw = −Vsh. The corresponding solutions of the triple-deck equations have been constructed numerically. For this purpose, we use a numerical technique based on finite differencing along the streamwise direction and Chebyshev collocation in the direction normal to the body surface. In the second part of the paper, we assume that 1 ≫ Vw ≫ O(Re−1/8), and concentrate our attention on the self-induced separation of the boundary layer. Assuming, as before, that the Reynolds number, Re, is large, the method of matched asymptotic expansions is used to construct the corresponding solutions of the Navier–Stokes equations in a vicinity of the separation point.
Approximate theory of acoustic waveguide of metamaterials
Authors: MEI CHIANG C. , LIU YING-HUNG
Published: April 2011

We theoretically examine the propagation of sound in a waveguide bounded by a metamaterial formed by an array of small Helmholtz resonators. The field equation is shown to be similar to that governing sound in a bubbly liquid. The effects of dissipation on the wave dispersion are examined. In particular, it is shown that the energy in a monochromatic wave train is not transported by the real part of the complex group velocity unless dissipation is absent. We further derived the envelope equation and show that in a one-dimensional waveguide, energy is transported forward despite the backward motion of the envelope peak.
Hydrodynamically coupled oscillators
Authors: TIRON R. , KANSO E. , NEWTON P. K.
Published: April 2011

A submerged spring–mass ring is analysed as a simple model for the way in which an underwater swimmer couples its body deformations to the surrounding fluid in order to accomplish locomotion. We adopt an inviscid, incompressible, irrotational assumption for the surrounding fluid and analyse the coupling response to various modes of excitation of the ring configuration. Due to the added mass effect, the surrounding fluid provides an environment which effectively couples the ‘normal modes’ of oscillation of the ring, leading to nonlinear trajectories if the ring is free to accelerate based on the effective forces the oscillations induce. Through a series of examples, we demonstrate various features that the model supports, including the locomotion on curved paths as a result of energy and angular momentum exchange with the surrounding fluid.
Stokes drift dynamos
Authors: HERREMAN W. , LESAFFRE P.
Published: April 2011

Fluid particles can have a mean motion in time, even when the Eulerian mean flow disappears everywhere in space. In the present article, we demonstrate that this phenomenon, known as the Stokes drift, plays an essential role in the problem of magnetic field generation by fluctuation flows (kinematic dynamo) at high Rm. At leading order, the dynamo is generated by the Stokes drift that acts as if it were a mean flow. This result is derived from a mean-field dynamo theory in terms of time averages, which reveals how classical expressions for alpha and beta tensors actually recombine into a single Stokes drift contribution. In a test case, we find fluctuation flows that have a G. O. Roberts flow as Stokes drift and this allows to confront our model to direct integration of the induction equation. We find an excellent quantitative agreement between the prediction of our model and the results of our simulations. We finally apply our Stokes drift model to prove that a broad class of inertial waves in rapidly rotating flows cannot drive a dynamo.
Two-phase gravity currents in porous media
Authors: GOLDING MADELEINE J. , NEUFELD JEROME A. , HESSE MARC A. , HUPPERT HERBERT E.
Published: April 2011

We develop a model describing the buoyancy-driven propagation of two-phase gravity currents, motivated by problems in groundwater hydrology and geological storage of carbon dioxide (CO2). In these settings, fluid invades a porous medium saturated with an immiscible second fluid of different density and viscosity. The action of capillary forces in the porous medium results in spatial variations of the saturation of the two fluids. Here, we consider the propagation of fluid in a semi-infinite porous medium across a horizontal, impermeable boundary. In such systems, once the aspect ratio is large, fluid flow is mainly horizontal and the local saturation is determined by the vertical balance between capillary and gravitational forces. Gradients in the hydrostatic pressure along the current drive fluid flow in proportion to the saturation-dependent relative permeabilities, thus determining the shape and dynamics of two-phase currents. The resulting two-phase gravity current model is attractive because the formalism captures the essential macroscopic physics of multiphase flow in porous media. Residual trapping of CO2 by capillary forces is one of the key mechanisms that can permanently immobilize CO2 in the societally important example of geological CO2 sequestration. The magnitude of residual trapping is set by the areal extent and saturation distribution within the current, both of which are predicted by the two-phase gravity current model. Hence the magnitude of residual trapping during the post-injection buoyant rise of CO2 can be estimated quantitatively. We show that residual trapping increases in the presence of a capillary fringe, despite the decrease in average saturation.
Flow-destabilized seiches in a reservoir with a movable dam
Authors: HEWITT I. J. , SCOLAN H. , BALMFORTH N. J.
Published: April 2011

Using a combination of theoretical modelling and experiments, seiches in a reservoir are shown to become linearly unstable due to the coupling with flow under a dam that opens and closes in response to the upstream water pressure. The phenomenon is related to the mechanism commonly attributed to generate sound in musical instruments like the clarinet. Shallow water theory is used to model waves in the reservoir, and these are coupled, by an outflow condition, to a nonlinear oscillator equation for the dam opening. In general, several modes of oscillation are predicted to be unstable, and the frequency of the most unstable mode compares well with the dominant frequencies observed in the experiments. The experiments also show a systematic variation of the amplitude and spatial structure of the oscillations with the weight of the dam, reflecting the nonlinear coupling between the unstable modes of the system.
Hydrodynamic stability and breakdown of the viscous regime over riblets
Authors: GARCÍA-MAYORAL RICARDO , JIMÉNEZ JAVIER
Published: April 2011

The interaction of the overlying turbulent flow with a riblet surface and its impact on drag reduction are analysed. The ‘viscous regime’ of vanishing riblet spacing, in which the drag reduction produced by the riblets is proportional to their size, is reasonably well understood, but this paper focuses on the behaviour for spacingss+≃ 10–20, expressed in wall units, where the viscous regime breaks down and the reduction eventually becomes an increase. Experimental evidence suggests that the two regimes are largely independent, and, based on a re-evaluation of existing data, it is shown that the optimal rib size is collapsed best by the square root of the groove cross-section, ℓg+=Ag+1/2. The mechanism of the breakdown is investigated by systematic DNSs with increasing riblet sizes. It is found that the breakdown is caused by the appearance of long spanwise rollers belowy+≈ 20, with typical streamwise wavelengths λx+≈ 150, that develop from a two-dimensional Kelvin–Helmholtz-like instability of the mean streamwise flow, similar to those over plant canopies and porous surfaces. They account for the drag breakdown, both qualitatively and quantitatively. It is shown that a simplified linear instability model explains the scaling of the breakdown spacing with ℓg+.
Decay of an internal tide due to random topography in the ocean
Authors: BÜHLER OLIVER , HOLMES-CERFON MIRANDA
Published: April 2011

We present a theoretical and numerical study of the decay of an internal wave caused by scattering at undulating sea-floor topography, with an eye towards building a simple model in which the decay of internal tides in the ocean can be estimated. As is well known, the interactions of internal waves with irregular boundary shapes lead to a mathematically ill-posed problem, so care needs to be taken to extract meaningful information from this problem. Here, we restrict the problem to two spatial dimensions and build a numerical tool that combines a real-space computation based on the characteristics of the underlying partial differential equation with a spectral computation that satisfies the relevant radiation conditions. Our tool works for finite-amplitude topography but is restricted to subcritical topography slopes. Detailed results are presented for the decay of the gravest vertical internal wave mode as it encounters finite stretches of either sinusoidal topography or random topography defined as a Gaussian random process with a simple power spectrum. A number of scaling laws are identified and a simple expression for the decay rate in terms of the power spectrum is given. Finally, the resulting formulae are applied to an idealized model of sea-floor topography in the ocean, which seems to indicate that this scattering process can provide a rapid decay mechanism for internal tides. However, the present results are restricted to linear fluid dynamics in two spatial dimensions and to uniform stratification, which restricts their direct application to the real ocean.
On evaporation of sessile drops with moving contact lines
Authors: MURISIC N. , KONDIC L.
Published: April 2011

We consider theoretically, computationally and experimentally spontaneous evaporation of water and isopropanol drops on smooth silicon wafers. In contrast to a number of previous works, the solid surface we consider is smooth and therefore the droplets' evolution proceeds without contact line pinning. We develop a theoretical model for evaporation of pure liquid drops that includes Marangoni forces due to the thermal gradients produced by non-uniform evaporation, and heat conduction effects in both liquid and solid phases. The key ingredient in this model is the evaporative flux. We consider two commonly used models: one based on the assumption that the evaporation is limited by the processes originating in the gas (vapour diffusion-limited evaporation), and the other one which assumes that the processes in the liquid are limiting. Our theoretical model allows for implementing evaporative fluxes resulting from both approaches. The required parameters are obtained from physical experiments. We then carry out fully nonlinear time-dependent simulations and compare the results with the experimental ones. Finally, we discuss how the simulation results can be used to predict which of the two theoretical models is appropriate for a particular physical experiment.
Particle mesh Ewald Stokesian dynamics simulations for suspensions of non-spherical particles
Authors: KUMAR A. , HIGDON J. J. L.
Published: April 2011

A particle mesh Ewald (PME) Stokesian dynamics algorithm has been developed to model hydrodynamic interactions in suspensions of non-spherical dicolloidal particles. Dicolloids, which have recently been synthesized by a number of independent research groups (Johnson, van Kats & van Blaaderen (Langmuir, vol. 21, 2005, p. 11510), Mocket al. (Langmuir, vol. 22, 2006, p. 4037), Kim, Larsen & Weitz (J. Am. Chem. Soc., vol. 128, 2006, p. 14374)), consist of two intersecting spheres of varying radii and centre-to-centre separation. One-body resistance tensors and disturbance velocity fields are computed for general linear flows using a superposition of Stokes singularities along the symmetry axis of the dicolloid particles. The coefficients and the locations of the singularities are optimized to minimize the norm of the velocity error on the particle surface. The one-body solution provides all coefficients required for the far-field many-body interactions in the Stokesian dynamics algorithm. These generalize the analytical results for spheres employed in the classic algorithm. Modified lubrication interaction tensors are developed for dicolloids for the singular near-field lubrication interactions. Accuracy of the one-body solutions and two-body generalized Stokesian dynamics solutions are validated by comparison with high-precision numerical solutions computed with the spectral boundary element method of Muldowney & Higdon (J. Fluid Mech., vol. 298, 1995, p. 167). The newly developed PME Stokesian dynamics algorithm was used to study transport properties in dicolloidal suspensions over a range of volume fractions (φ ≤ 0.5). The effects of the degree of anisotropy on the properties of the suspension are discussed. For these mildly anisotropic particles, the transport properties remain close to those of spheres, however certain interesting trends emerge, with non-monotonic viscosity dependence as a function of increasing aspect ratio. The minimum viscosity in concentrated suspensions is lower than that for spheres with equal volume fraction over a range of volume fractions.
Spatial resolution correction for wall-bounded turbulence measurements
Authors: SMITS A. J. , MONTY J. , HULTMARK M. , BAILEY S. C. C. , HUTCHINS N. , MARUSIC I.
Published: April 2011

A correction for streamwise Reynolds stress data acquired with insufficient spatial resolution is proposed for wall-bounded flows. The method is based on the attached eddy hypothesis to account for spatial filtering effects at all wall-normal positions. This analysis reveals that outside the near-wall region the spatial filtering effect scales inversely with the distance from the wall, in contrast to the commonly assumed scaling with the viscous length scale. The new formulation is shown to work very well for data taken over a wide range of Reynolds numbers and wire lengths.
Granular jets and hydraulic jumps on an inclined plane
Authors: JOHNSON C. G. , GRAY J. M. N. T.
Published: April 2011

A jet of granular material impinging on an inclined plane produces a diverse range of flows, from steady hydraulic jumps to periodic avalanches, self-channelised flows and pile collapse behaviour. We describe the various flow regimes and study in detail a steady-state flow, in which the jet generates a closed teardrop-shaped hydraulic jump on the plane, enclosing a region of fast-moving radial flow. On shallower slopes, a second steady regime exists in which the shock is not teardrop-shaped, but exhibits a more complex ‘blunted’ shape with a steadily breaking wave. We explain these regimes by consideration of the supercritical or subcritical nature of the flow surrounding the shock. A model is developed in which the impact of the jet on the inclined plane is treated as an inviscid flow, which is then coupled to a depth-integrated model for the resulting thin granular avalanche on the inclined plane. Numerical simulations produce a flow regime diagram strikingly similar to that obtained in experiments, with the model correctly reproducing the regimes and their dependence on the jet velocity and slope angle. The size and shape of the steady experimental shocks and the location of sub- and supercritical flow regions are also both accurately predicted. We find that the physics underlying the rapid flow inside the shock is dominated by depth-averaged mass and momentum transport, with granular friction, pressure gradients and three-dimensional aspects of the flow having comparatively little effect. Further downstream, the flow is governed by a friction–gravity balance, and some flow features, such as a persistent indentation in the free surface, are not reproduced in the numerical solutions. On planes inclined at a shallow angle, the effect of stationary granular material becomes important in the flow evolution, and oscillatory and more general time-dependent flows are observed. The hysteretic transition between static and dynamic friction leads to two phenomena observed in the flows: unsteady avalanching behaviour, and the feedback from static grains on the flowing region, leading to levéed, self-channelised flows.
Granular column collapses down rough, inclined channels
Authors: LUBE GERT , HUPPERT HERBERT E. , SPARKS R. STEPHEN J. , FREUNDT ARMIN
Published: April 2011

We present experimental results for the collapse of rectangular columns of sand down rough, inclined, parallel-walled channels. Results for basal inclination θ varying between 4.2° and 25° are compared with previous results for horizontal channels. Shallow-water theory can be usefully combined with scaling relationships obtained by dimensional analysis to yield analytical functions of the maximum runout distance, the maximum deposit height and the time to reach the maximum runout. While the theory excellently predicts the maximum lengths of the deposit it generally overestimates the runout time. The inertial flows are characterized by a moving internal interface separating upper flowing and lower static regions of material. In an initial free-fall phase of collapse the deposited area (= volume per unit width) below the internal interface varies with the square-root of time, independent of the initial height of the column and channel inclination. In the subsequent, lateral spreading phase the deposition rate decreases with increasing basal inclination or with decreasing initial height. The local deposition rate at any fixed distance is a constant, dependent on the column aspect ratio, the channel inclination and the longitudinal position, but invariant with flow velocity and depth. In the lateral spreading phase, vertical velocity profile in the flowing layer take a universal form and are independent of flow depth and velocity. They can be characterized by a shear rate as a function of channel inclination and a length scale describing the fraction of the column involved in flow.
Global linear and nonlinear stability of viscous confined plane wakes with co-flow
Authors: TAMMISOLA OUTI , LUNDELL FREDRIK , SCHLATTER PHILIPP , WEHRFRITZ ARMIN , SÖDERBERG L. DANIEL
Published: April 2011

The global stability of confined uniform density wakes is studied numerically, using two-dimensional linear global modes and nonlinear direct numerical simulations. The wake inflow velocity is varied between different amounts of co-flow (base bleed). In accordance with previous studies, we find that the frequencies of both the most unstable linear and the saturated nonlinear global mode increase with confinement. For wake Reynolds numberRe= 100 we find the confinement to be stabilising, decreasing the growth rate of the linear and the saturation amplitude of the nonlinear modes. The dampening effect is connected to the streamwise development of the base flow, and decreases for more parallel flows at higherRe. The linear analysis reveals that the critical wake velocities are almost identical for unconfined and confined wakes atRe≈ 400. Further, the results are compared with literature data for an inviscid parallel wake. The confined wake is found to be more stable than its inviscid counterpart, whereas the unconfined wake is more unstable than the inviscid wake. The main reason for both is the base flow development. A detailed comparison of the linear and nonlinear results reveals that the most unstable linear global mode gives in all cases an excellent prediction of the initial nonlinear behaviour and therefore the stability boundary. However, the nonlinear saturated state is different, mainly for higherRe. ForRe= 100, the saturated frequency differs less than 5% from the linear frequency, and trends regarding confinement observed in the linear analysis are confirmed.
Stability and dynamics of the laminar wake past a slender blunt-based axisymmetric body
Authors: BOHORQUEZ P. , SANMIGUEL-ROJAS E. , SEVILLA A. , JIMÉNEZ-GONZÁLEZ J. I. , MARTÍNEZ-BAZÁN C.
Published: April 2011

We investigate the stability properties and flow regimes of laminar wakes behind slender cylindrical bodies, of diameter D and length L, with a blunt trailing edge at zero angle of attack, combining experiments, direct numerical simulations and local/global linear stability analyses. It has been found that the flow field is steady and axisymmetric for Reynolds numbers below a critical value, Recs (L/D), which depends on the length-to-diameter ratio of the body, L/D. However, in the range of Reynolds numbers Recs(L/D) < Re < Reco(L/D), although the flow is still steady, it is no longer axisymmetric but exhibits planar symmetry. Finally, for Re > Reco, the flow becomes unsteady due to a second oscillatory bifurcation which preserves the reflectional symmetry. In addition, as the Reynolds number increases, we report a new flow regime, characterized by the presence of a secondary, low frequency oscillation while keeping the reflectional symmetry. The results reported indicate that a global linear stability analysis is adequate to predict the first bifurcation, thereby providing values of Recs nearly identical to those given by the corresponding numerical simulations. On the other hand, experiments and direct numerical simulations give similar values of Reco for the second, oscillatory bifurcation, which are however overestimated by the linear stability analysis due to the use of an axisymmetric base flow. It is also shown that both bifurcations can be stabilized by injecting a certain amount of fluid through the base of the body, quantified here as the bleed-to-free-stream velocity ratio, Cb = Wb/W∞.
On the velocity distribution in homogeneous isotropic turbulence: correlations and deviations from Gaussianity
Authors: WILCZEK MICHAEL , DAITCHE ANTON , FRIEDRICH RUDOLF
Published: April 2011

We investigate the single-point probability density function of the velocity in three-dimensional stationary and decaying homogeneous isotropic turbulence. To this end, we apply the statistical framework of the Lundgren–Monin–Novikov hierarchy combined with conditional averaging, identifying the quantities that determine the shape of the probability density function. In this framework, the conditional averages of the rate of energy dissipation, the velocity diffusion and the pressure gradient with respect to velocity play a key role. Direct numerical simulations of the Navier–Stokes equation are used to complement the theoretical results and assess deviations from Gaussianity.
Modelling of block-scale macrodispersion as a random function
Authors: DE BARROS FELIPE P. J. , RUBIN YORAM
Published: April 2011

Numerical modelling of solute dispersion in natural heterogeneous porous media is facing several challenges. Amongst these we highlight the challenge of accounting for high-frequency variability that is filtered out by homogenization at the subgrid scale and the uncertainty in the dispersive flux for transport under non-ergodic conditions. These two effects when combined lead to inaccurate representation of the dispersive fluxes. We propose to compensate for this deficiency by defining a block-scale dispersion tensor and modelling it as a random space function ℳij. The derived dispersion tensor is a function of several length scales and time. Grid blocks will be assigned dispersion coefficients generated from the ℳijdistribution. We will show the dependence of ℳijon the spatial variability of the conductivity field, on the contaminant source size, on the travel time and on the grid-block scale. For an ergodic source, a statistically uniform conductivity field and very large grid blocks, ℳijis equal to the macrodispersion coefficients proposed by Dagan (J. Fluid Mech., vol. 145, 1984, p. 151) with zero variance. For an ergodic source and non-uniform conductivity field with a finite-size grid block, ℳijapproaches the model proposed by Rubinet al. (J. Fluid Mech., vol. 395, 1999, p. 161). In both cases, ℳijis defined by its mean value with zero variance. ℳijis subject to uncertainty when the source is non-ergodic and when the grid block is defined by a finite scale. When the grid-block scale approaches zero, which means that the spatial variability is captured completely on the numerical grid, ℳijapproaches zero with zero variance. In addition, we provide a complete statistical characterization of ℳijby invoking the concept of minimum relative entropy, thus providing upper bounds on the uncertainty associated with ℳij.
The impact of a translating plunging jet on a pool of the same liquid
Authors: GÓMEZ-LEDESMA R. , KIGER K. T. , DUNCAN J. H.
Published: April 2011

An experimental study on the impact of a translating two-dimensional transient jet on an initially quiescent liquid pool is studied experimentally using high-speed cinematic visualization and particle image velocimetry methods. Six jet conditions (covering a range of jet thicknesses, velocities and inclination angles relative to vertical) are considered, with measurements performed over a range of horizontal translation speeds for each jet condition. For all conditions studied herein, the jet penetrates into the pool and forms two craters – one upstream and one downstream of the jet. Gravity acts to close these craters, which after a short time pinch off at intermediate depths, thereby entrapping cavities of air. The translation speed of the jet is found to have a dramatic effect on the cavity shapes, pinch-off depths and pinch-off times. A simple theory based on a potential flow and a hydrostatically driven collapse is used to model this flow, and the resulting jet tip trajectories and cavity shapes compare favourably with the experimental data.
Axisymmetric, constantly supplied gravity currents at high Reynolds number
Authors: SLIM ANJA C. , HUPPERT HERBERT E.
Published: April 2011

We consider theoretically the long-time evolution of axisymmetric, high Reynolds number, Boussinesq gravity currents supplied by a constant, small-area source of mass and radial momentum in a deep, quiescent ambient. We describe the gravity currents using a shallow-water model with a Froude number closure condition to incorporate ambient form drag at the front and present numerical and asymptotic solutions. The predicted profile consists of an expanding, radially decaying, steady interior that connects via a shock to a deeper, self-similar frontal boundary layer. Controlled by the balance of interior momentum flux and frontal buoyancy across the shock, the front advances as (g′sQ/r1/4s)4/154/5, where g′s is the reduced gravity of the source fluid, Q is the total volume flux, rs is the source radius and is time. A radial momentum source has no effect on this solution below a non-zero threshold value. Above this value, the (virtual) radius over which the flow becomes critical can be used to collapse the solution onto the subthreshold one. We also use a simple parameterization to incorporate the effect of interfacial entrainment, and show that the profile can be substantially modified, although the buoyancy profile and radial extent are less significantly impacted. Our predicted profiles and extents are in reasonable agreement with existing experiments.
Fluxes across double-diffusive interfaces: a one-dimensional-turbulence study
Authors: GONZALEZ-JUEZ ESTEBAN , KERSTEIN ALAN R. , LIGNELL DAVID O.
Published: April 2011

This work is a parametric study of the fluxes of heat and salt across unsheared and sheared double-diffusive interfaces using one-dimensional-turbulence (ODT) simulations. It is motivated by the need to understand how these fluxes scale with parameters related to the fluid molecular properties and background shear. Comparisons are made throughout with previous models and available measurements. In unsheared interfaces, ODT simulations show that the dimensionless heat fluxNuscales with the stability parameterRρ, Rayleigh numberRaand Prandtl numberPrasNu~ (Ra/Rρ)0.37±0.03whenPrvaries from 3 to 100 and asNu~ (Ra/Rρ)0.31Pr0.22±0.04whenPrvaries from 0.01 to 1. HereRa/Rρcan be seen as the ratio of destabilizing and stabilizing effects. The simulation results also indicate that the ratio of salt and heat fluxesRfis independent ofPr, scales with the Lewis numberLeasRf~Le0.41±0.04whenRρis large enough and deviates from this expression for low values ofRρ, when the interface becomes heavily eroded. In sheared interfaces, the simulations show three flow regimes. When the Richardson numberRi≪ 1, shear-induced mixing dominates, the heat flux scales with the horizontal velocity difference across the interface andRf=Rρ. NearRi~ 1 the heat and salt fluxes are seen to increase abruptly as the shear increases. The flow structure and scaling of the fluxes are similar to those of unsheared interfaces whenRi≫ 1.
Dynamics of high-Deborah-number entry flows: a numerical study
Authors: AFONSO A. M. , OLIVEIRA P. J. , PINHO F. T. , ALVES M. A.
Published: April 2011

High-elasticity simulations of flows through a two-dimensional (2D) 4 : 1 abrupt contraction and a 4 : 1 three-dimensional square–square abrupt contraction were performed with a finite-volume method implementing the log-conformation formulation, proposed by Fattal & Kupferman (J. Non-Newtonian Fluid Mech., vol. 123, 2004, p. 281) to alleviate the high-Weissenberg-number problem. For the 2D simulations of Boger fluids, modelled by the Oldroyd-B constitutive equation, local flow unsteadiness appears at a relatively low Deborah number (De) of 2.5. Predictions at higherDewere possible only with the log-conformation technique and showed that the periodic unsteadiness grows withDeleading to an asymmetric flow with alternate back-shedding of vorticity from pulsating upstream recirculating eddies. This is accompanied by a frequency doubling mechanism deteriorating to a chaotic regime at highDe. The log-conformation technique provides solutions of accuracy similar to the thoroughly tested standard finite-volume method under steady flow conditions and the onset of a time-dependent solution occurred approximately at the same Deborah number for both formulations. Nevertheless, for Deborah numbers higher than the critical Deborah number, and for which the standard iterative technique diverges, the log-conformation technique continues to provide stable solutions up to quite (impressively) high Deborah numbers, demonstrating its advantages relative to the standard methodology. For the 3D contraction, calculations were restricted to steady flows of Oldroyd-B and Phan-Thien–Tanner (PTT) fluids and very highDewere attained (De ≈ 20 for PTT with ϵ = 0.02 andDe≈ 10000 for PTT with ϵ = 0.25), with prediction of strong vortex enhancement. For the Boger fluid calculations, there was inversion of the secondary flow at highDe, as observed experimentally by Sousaet al. (J. Non-Newtonian Fluid Mech., vol. 160, 2009, p. 122).
Effects of radiative heat transfer on the structure of turbulent supersonic channel flow
Authors: GHOSH S. , FRIEDRICH R. , PFITZNER M. , STEMMER CHR. , CUENOT B. , EL HAFI M.
Published: April 2011

The interaction between turbulence in a minimal supersonic channel and radiative heat transfer is studied using large-eddy simulation. The working fluid is pure water vapour with temperature-dependent specific heats and molecular transport coefficients. Its line spectra properties are represented with a statistical narrow-band correlated-k model. A grey gas model is also tested. The parallel no-slip channel walls are treated as black surfaces concerning thermal radiation and are kept at a constant temperature of 1000 K. Simulations have been performed for different optical thicknesses (based on the Planck mean absorption coefficient) and different Mach numbers. Results for the mean flow variables, Reynolds stresses and certain terms of their transport equations indicate that thermal radiation effects counteract compressibility (Mach number) effects. An analysis of the total energy balance reveals the importance of radiative heat transfer, compared to the turbulent and mean molecular heat transport.
Polar confinement of the Sun's interior magnetic field by laminar magnetostrophic flow
Authors: WOOD TOBY S. , McINTYRE MICHAEL E.
Published: April 2011

The global-scale interior magnetic field Bi needed to account for the Sun's observed differential rotation can be effective only if confined below the convection zone in all latitudes including, most critically, the polar caps. Axisymmetric solutions are obtained to the nonlinear magnetohydrodynamic equations showing that such polar confinement can be brought about by a very weak downwelling flow U ~ 10−5cms−1 over each pole. Such downwelling is consistent with the helioseismic evidence. All three components of the magnetic field B decay exponentially with altitude across a thin, laminar ‘magnetic confinement layer’ located at the bottom of the tachocline and permeated by spiralling field lines. With realistic parameter values, the thickness of the confinement layer ~10−3 of the Sun's radius, the thickness scale being the magnetic advection–diffusion scale δ = η/U where the magnetic (ohmic) diffusivity η ≈ 4.1 × 102cm2s−1. Alongside baroclinic effects and stable thermal stratification, the solutions take into account the stable compositional stratification of the helium settling layer, if present as in today's Sun, and the small diffusivity of helium through hydrogen, χ ≈ 0.9 × 101cm2s−1. The small value of χ relative to η produces a double boundary-layer structure in which a ‘helium sublayer‘ of smaller vertical scale (χ/η)1/2δ is sandwiched between the top of the helium settling layer and the rest of the confinement layer. Solutions are obtained using both semi-analytical and purely numerical, finite-difference techniques. The confinement-layer flows are magnetostrophic to excellent approximation. More precisely, the principal force balances are between Lorentz, Coriolis, pressure-gradient and buoyancy forces, with relative accelerations negligible to excellent approximation. Viscous forces are also negligible, even in the helium sublayer where shears are greatest. This is despite the kinematic viscosity being somewhat greater than χ. We discuss how the confinement layers s at each pole might fit into a global dynamical picture of the solar tachocline. That picture, in turn, suggests a new insight into the early Sun and into the longstanding enigma of solar lithium depletion.
Numerical study of flow fields in an airway closure model
Authors: TAI C.-F. , BIAN S. , HALPERN D. , ZHENG Y. , FILOCHE M. , GROTBERG J. B.
Published: April 2011

The liquid lining in small human airways can become unstable and form liquid plugs that close off the airways. Direct numerical simulations are carried out on an airway model to study this airway instability and the flow-induced stresses on the airway walls. The equations governing the fluid motion and the interfacial boundary conditions are solved using the finite-volume method coupled with the sharp interface method for the free surface. The dynamics of the closure process is simulated for a viscous Newtonian film with constant surface tension and a passive core gas phase. In addition, a special case is examined that considers the core dynamics so that comparisons can be made with the experiments of Bian et al. (J. Fluid Mech., vol. 647, 2010, p. 391). The computed flow fields and stress distributions are consistent with the experimental findings. Within the short time span of the closure process, there are large fluctuations in the wall shear stress. Furthermore, dramatic velocity changes in the film during closure indicate a steep normal stress gradient on the airway wall. The computational results show that the wall shear stress, normal stress and their gradients during closure can be high enough to injure airway epithelial cells.
THINGS OF THE GROUND: CHILDREN'S MEDICINE, MOTHERHOOD AND MEMORY IN THE CAMEROON GRASSFIELDS
Authors: Argenti Nicolas
Published: April 2011

Soon after birth, infants in the Cameroon Grassfields chiefdom of Oku are submitted by their parents to rites known generically as ‘children's medicine’ (k∂fu ∂bwan). Ostensibly performed to protect infants from harm and illness, the rites are in fact fraught with tension: they embrace contradictory perspectives regarding the social role of the mother and belie the normative ideal extolling her as a figure of nurture and protection. The article argues that, beyond their overt purpose and symbolism as rites of passage, the rites evoke collective memories of child abductions and contemporary anxieties regarding the anticipated departure of older children and adolescents into foster care or migrant labour. Going beyond a classic tripartite model, the article takes a long-term view that sees life-crisis rituals as a form of collective memory that bears witness to social tensions that cannot be resolved – in this case the contradictions inherent within the hallowed image of the mother and the compromised nature of parental love.
WRITING ABOUT CONFLICT IN AFRICA: STAKES AND STRATEGIES
Authors: McGovern Mike
Published: April 2011

Writing about conflict in Africa is a tricky thing. Publications from non-governmental organizations and human rights campaigners often read as if they were calibrated to maximize public distress, and thus the political or financial support that would keep human rights institutions in business. Many journalistic accounts are stitched together from the rhetorical and analytical remnants of a colonial and sometimes racist common sense. Against this backdrop, fine-grained empirical studies like those typically produced by anthropologists, historians and geographers take on a particular salience. They stake out a privileged space for explaining other logics, other incentives, and different causal relations that could make sense out of wars, insurgencies and other forms of violence that appear irrational to Europeans and North Americans.
BETWEEN THE UMBRELLA AND THE ELEPHANT: ELECTIONS, ETHNIC NEGOTIATIONS AND THE POLITICS OF SPIRIT POSSESSION IN TESHI, ACCRA
Authors: Venkatachalam Meera
Published: April 2011

This article focuses on a number of Ga spirit mediums located in Teshi, a neighbourhood of the Ghanaian capital, Accra. These individuals host foreign spirits from areas north of Ga territory, such as the modern Ashanti, Gonja and Dagomba regions. Such encounters of cross-cultural spirit possession have often been analysed in the scholarly literature as an embedded history of contact between peoples. These histories of ethnic or cultural contact – which inform cross-cultural spirit possession – are constantly re-imagined by spirit mediums and the broader community they service. How this re-imagination occurs, in conjunction with developments in the contemporary political and public spheres, is a theme that remains understudied. The perceived shifts in the contours of ethnic alliances and rivalries on a national scale, against the backdrop of modern Ghanaian party politics and the ever-changing relationships between the Ga and their northern neighbours, led to a thematic reconfiguration of possession practices in 2004. This ethnographic vignette details how spirit mediums were able to apply the ethnic and conceptual cultural divisions intrinsic to this corpus of ritual practice to a critique of national political events, producing a commentary, through possession, on the changing discourses on ethnicity and ethnic relations in the Ghanaian state.
LANREWAJU ADEPỌJU AND THE MAKING OF MODERN YORUBA POETRY
Authors: Okunoye Oyeniyi
Published: April 2011

This essay presents Lanrewaju Adepọju, whose work and ideas have been very influential on contemporary Yoruba poetry, as a local intellectual. In estimating his contribution to modernising ewì, an open poetic form that inhabits the interface between the oral and the written, the essay draws on biographical information, an extensive personal interview and relevant textual illustration. It correlates Adepọju's vision of poetry with the development of his creative consciousness and draws attention to aspects of his poetics and politically implicated poetry that deserve closer engagement. The article also offers a translation of a sample poem by Adepọju, while the online version of the essay offers more of his poems as well as an interview.