bluuebunny/crossref_metadata_2025 huggingface dataset:
Strontium contents of a Porites coral from Xisha Island, South China Sea: A proxy for sea‐surface temperature of the 20th century
Authors: Sun Yali , Sun Min , Wei Gangjian , Lee Typhoon , Nie Baofu , Yu Zhiwei
Published: April 2004

A Porites coral collected from Xisha Island, South China Sea, represents a skeleton secreted in the period from 1906 to 1994. The Sr contents of the coral vary linearly with the instrument‐measured sea‐surface temperature (SST), giving a Sr thermometer: SST = −1.9658 × Sr + 193.26. The reconstructed SST data show that the late 20th century was warmer (about 1°C) than the early 20th century and that two cooling (1915/1916 and 1947/1948) and three warming (1935/1936, 1960/1961, and 1976/1977) shifts occurred in the century. The temperature shifts are more pronounced for winters, implying a close effect of the west Pacific warm pool and Asian monsoon and suggesting that the former is a primary force controlling the climatic system of the region. Results of this study and previously published data indicate a close link of temperature shifts between the boreal summer and the austral winter or the boreal winter and the austral summer. The annual SST anomalies in the South China Sea and the South Pacific reveal the existence of harmonic but opposite SST variations between the two regions. On the decadal scale the comparative annual SST anomalies for the South China Sea and for the equatorial west Pacific show a similarity in temperature variations, implying that the South China Sea climate is coherent with climatic regime of the tropical west Pacific.
Statistical modeling of ionospheric foF2 over Wuhan
Authors: Liu Libo , Wan Weixing , Ning Baiqi
Published: April 2004

Half‐hourly values of the critical frequency of the ionospheric F region, foF2, obtained at Wuhan Ionospheric Observatory (geographic 114.4°E, 30.6°N; 45.2° dip), China, during the period from 1957 to 1991 have been used to investigate the solar activity dependence of the monthly median foF2, and to construct single‐station models using Fourier expansion and cubic‐B splines approaches. The climatological models incorporate local standard time, month, and solar cycle variations of foF2. Statistical analyses show that, over Wuhan, the monthly median foF2 has a significantly nonlinear dependence on the current and historical solar activities. Introducing the monthly median F107 in the prior month can significantly improve the accuracy of our models. Thus the complex influence of solar activities is approximately expressed by a general nonlinear function. Our models are in good agreement with observations, with standard deviations between about 0.26 to 0.58 MHz. In contrast, the IRI model tends to overestimate foF2 over Wuhan and has a lower accuracy with standard deviations between about 0.5 to 1 MHz.
Simulation of diurnal patterns of summer precipitation in the North American monsoon: An assessment using TRMM
Authors: Zou Cheng‐Zhi , Zheng Weizhong
Published: April 2004

The diurnal pattern of the July 2002 precipitation in the North American monsoon is simulated in a continuous one‐month integration using the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model Version 5 (MM5) coupled with the Simplified Simple Biosphere (SSiB) land surface model. The simulation is conducted in a two‐way, nested grid with a resolution of 45 km for the coarse domain and 15 km for the fine domain and only the fine domain that covers the core region of the North American monsoon is analyzed. Comparisons with the conventional gauge‐based observations show that the coupled model successfully simulates the strength, position, and orientation of the monthly mean heavy rainbelt along the western slopes of Sierra Madre Occidental. In addition, the spatial pattern and phasing of the diurnal cycle of precipitation in this region agrees fairly well with the satellite observations from the Tropical Rainfall Measuring Mission (TRMM).
Damping and pumping of a vortex Rossby wave in a monotonic cyclone: Critical layer stirring versus inertia–buoyancy wave emission
Authors: Schecter David A. , Montgomery Michael T.
Published: April 2004

This paper further examines the rate at which potential vorticity in the core of a monotonic cyclone becomes vertically aligned and horizontally axisymmetric. We consider the case in which symmetrization occurs by the damping of a discrete vortex Rossby (VR) wave. The damping of the VR wave is caused by its stirring of potential vorticity at a critical radius r*, outside the core of the cyclone. The decay rate generally increases with the radial gradient of potential vorticity at r*. Previous theories for the decay rate were based on “balance models” of the vortex dynamics. Such models filter out inertia–buoyancy (IB) oscillations, i.e., gravity waves. However, if the Rossby number is greater than unity, the core VR wave can excite a frequency-matched outward propagating IB wave, which has positive feedback. To accurately account for this radiation, we here develop a theory for the decay rate that is based on the hydrostatic primitive equations. Starting from conservation of wave activity (angular pseudomomentum), an expression for the decay rate is derived. This expression explicitly demonstrates a competition between the destabilizing influence of IB wave emission, and the stabilizing influence of potential vorticity stirring at r*. Moreover, it shows that if the radial gradient of potential vorticity at r* exceeds a small threshold, the VR wave will decay, and the vortex will symmetrize, even at large Rossby numbers.
On some common features of drop impact on liquid surfaces
Authors: Fedorchenko Alexander I. , Wang An-Bang
Published: April 2004

The impact of a drop on liquid surfaces is studied experimentally and theoretically in the region of the fully developed splashing. In order to reveal the influence of viscosity and target liquid depth on the resulting flow patterns, the experiments were carried out with water and 70% glycerol–water solution, and for different target liquid depths. Based on the experimental observations, a dynamic model of the central jet formation at the cavity collapse is developed. This model predicts an emergence of a liquid flow up into the central jet and simultaneously a small flow velocity downward and allows us to evaluate the velocities of these two flows. A theoretical model for the cavity submergence is presented. This model gives the constant velocity of the cavity submergence which is half the initial drop impact velocity. Analytical solution for the gravity–capillary cavity collapse has been derived and provides a good fit to the experimental results. Theoretical analysis and experiments have shown that the maximum cavity radius and the cavity collapse time depend on both the Froude number and the dimensionless capillary length.
Scalar probability density function mixing models need not comply with the linearity and independence hypothesis
Authors: Hierro Juan , Dopazo César
Published: April 2004

In a mixture of scalar fields undergoing diffusive processes governed by Fick’s law, the concentration at each point evolves linearly in the concentrations at all points and independently from the other concentrations, when one considers a finite differences integration of their evolution equations. However, these properties must not necessarily be enforced in probability density function models, since they are relaxed when conditional expected values are taken.
How flexibility induces streamlining in a two-dimensional flow
Authors: Alben Silas , Shelley Michael , Zhang Jun
Published: April 2004

Recent work in bio-fluid dynamics has studied the relation of fluid drag to flow speed for flexible organic structures, such as tree leaves, seaweed, and coral beds, and found a reduction in drag growth due to body reconfiguration with increasing flow speed. Our theoretical and experimental work isolates the role of elastic bending in this process. Using a flexible glass fiber wetted into a vertical soap-film flow, we identify a transition in flow speed beyond which fluid forces dominate the elastic response, and yield large deformations of the fiber that greatly reduce drag. We construct free-streamline models that couple fluid and elastic forces and solve them in an efficient numerical scheme. Shape self-similarity emerges, with a scaling set by the balance of forces in a small “tip region” about the flow’s stagnation point. The result is a transition from the classical U2 drag scaling of rigid bodies to a new U4/3 drag law. We derive an asymptotic expansion for the fiber shape and flow, based on the length-scale of similarity. This analysis predicts that the fiber and wake are quasiparabolic at large velocities, and obtains the new drag law in terms of the drag on the tip region. Under variations of the model suggested by the experiment—the addition of flow tunnel walls, and a back pressure in the wake—the drag law persists, with a simple modification.
A generating mechanism for apparent fluid slip in hydrophobic microchannels
Authors: Tretheway Derek C. , Meinhart Carl D.
Published: April 2004

Fluid slip has been observed experimentally in micro- and nanoscale flow devices by several investigators [e.g., Tretheway and Meinhart, Phys. Fluids 14, L9 (2002); Zhu and Granik, Phys. Rev. Lett. 87, 096105 (2001); Pit et al., Phys. Rev. Lett. 85, 980 (2000); and Choi et al., Phys. Fluids 15, 2897 (2003)]. This paper examines a possible mechanism for the measured fluid slip, for water flowing over a hydrophobic surface. We extend the work of Lum et al. [J. Phys. Chem. B 103, 4570 (1999)], Zhu and Granick [Phys. Rev. Lett. 87, 096105 (2001)], Granick et al. [Nature Materials 2, 221 (2003)], and de Gennes [Langmuir 18, 3413 (2002)], who suggest slip develops from a depleted water region or vapor layer near a hydrophobic surface. By modeling the presence of either a depleted water layer or nanobubbles as an effective air gap at the wall, we calculate slip lengths for flow between two infinite parallel plates. The calculated slip lengths are consistent with experimental values when the gas layer is modeled as a continuum and significantly higher when rarefied gas conditions are assumed. The results suggest that the apparent fluid slip observed experimentally at hydrophobic surfaces may arise from either the presence of nanobubbles or a layer of low density fluid at the surface.
Probability density function computation of turbulent flows with a new near-wall model
Authors: Wacławczyk Marta , Pozorski Jacek , Minier Jean-Pierre
Published: April 2004

The modeling and computation of near-wall turbulent flows is addressed with the probability density function (PDF) method for velocity and the turbulent frequency. Near-wall extensions are considered in detail and a new model for viscous transport is proposed. A method of elliptic relaxation for a blending function is applied to model the pressure–strain term. A numerical integration scheme is developed to deal with the near-wall singularity of coefficients that appears in the discrete formulation. The PDF equation is solved by a Monte Carlo method and the whole approach appears as a self-contained Lagrangian simulation using stochastic particles. For the sake of a numerical example, the fully developed channel flow case is computed; results are compared with the available direct numerical simulation data.
An experimental investigation of symmetric and asymmetric turbulent wake development in pressure gradient
Authors: Thomas Flint O. , Liu Xiaofeng
Published: April 2004

This paper examines and contrasts the response of planar turbulent wakes with initially symmetric and asymmetric mean velocity profiles to identical imposed streamwise pressure gradients. The focus on near wake behavior, profile asymmetry, and pressure gradient is motivated by their relevance to high-lift systems for commercial transport aircraft. In the experiments, the symmetric wake is generated by a flat plate with a tapered trailing edge. Active and passive flow control are applied on an identical second plate in order to generate the asymmetric wake. Both favorable and adverse constant pressure gradients are imposed on the wakes by means of a downstream diffuser section with fully adjustable top and bottom wall contour. For both symmetric and asymmetric wakes, the effect of pressure gradient on wake spreading, mean velocity defect decay, and the streamwise evolution of the turbulent stresses are documented in detail. In this manner, the role of asymmetry in strained wake development is isolated. We also gauge the ability of commonly used one- and two-equation turbulence models to predict the streamwise wake evolution observed in the experiments.
On ternary species mixing and combustion in isotropic turbulence at high pressure
Authors: Lou Hong , Miller Richard S.
Published: April 2004

Effects of Soret and Dufour cross-diffusion, whereby both concentration and thermal diffusion occur in the presence of mass fraction, temperature, and pressure gradients, are investigated in the context of both binary and ternary species mixing and combustion in isotropic turbulence at large pressure. The compressible flow formulation is based on a cubic real-gas state equation, and includes generalized forms for heat and mass diffusion derived from nonequilibrium thermodynamics and fluctuation theory. A previously derived formulation of the generalized binary species heat and mass fluxes is first extended to the case of ternary species, and appropriate treatment of the thermal and mass diffusion factors is described. Direct numerical simulations (DNS) are then conducted for both binary and ternary species mixing and combustion in stationary isotropic turbulence. Mean flow temperatures and pressures of 〈T〉=700 K and 〈P〉=45 atm are considered to ensure that all species mixtures remain in the supercritical state such that phase changes do not occur. DNS of ternary species systems undergoing both pure mixing and a simple chemical reaction of the form O2+N2→2NO are then conducted. It is shown that stationary scalar states previously observed for binary mixing persist for the ternary species problem as well; however, the production and magnitude of the scalar variance is found to be altered for the intermediate molecular weight species as compared to the binary species case. The intermediate molecular weight species produces a substantially smaller scalar variance than the remaining species for all flows considered. For combustion of nonstoichiometric mixtures, a binary species mixture, characterized by stationary scalar states, results at long times after the lean reactant is depleted. The form of this final scalar distribution is observed to be similar to that found in the binary flow situation. A series of lower resolution simulations for a variety of species is then used to show the dependence of the stationary scalar variance on the turbulence Reynolds number, turbulence Mach number, species molecular weight ratio, and relative proportion of two species present in the flow after completion of combustion.
A novel application of Curle’s acoustic analogy to aeolian tones in two dimensions
Authors: Hatakeyama Nozomu , Inoue Osamu
Published: April 2004

A new two-dimensional formula to describe aeolian tones radiated from rigid bodies in a uniform flow at low Mach numbers is proposed as an improved approximation of Curle’s dipole solution. This modified Curle’s dipole is composed mainly of two simple terms; one depends on time and the other does not, which represent the acoustic propagation and the hydrodynamic mean effect, respectively. The acoustic term includes the Doppler effect by regarding the sound speed to be directional in the source-fixed frame. The formula is verified in comparison with the results by direct numerical simulations (DNS) of the two-dimensional compressible Navier–Stokes equations for sounds from a circular cylinder in low Mach number flows. The results show that the modified Curle’s dipole approximates well the DNS results not only for the fluctuation pressure but also for the mean pressure in the far field. The mathematical basis of the formula is also presented in relation to the exact dipole term of the Ffowcs Williams–Hawkings equation.
Closed-loop-controlled vortex shedding and vibration of a flexibly supported square cylinder under different schemes
Authors: Zhang M. M. , Cheng L. , Zhou Y.
Published: April 2004

Different schemes were experimentally investigated of the closed-loop control of vortex shedding from a spring-supported square cylinder in cross flow. The control action was implemented through the perturbation of one cylinder surface, which was generated by three piezoelectric ceramic actuators, embedded underneath the surface and controlled by a proportional-integral-derivative controller. Three control schemes were investigated using different feedback signals, including the turbulent flow signal measured by a hot wire, flow-induced structural oscillation signal obtained by a laser vibrometer, and a combination of both signals. An investigation was conducted at the resonance condition, when the vortex-shedding frequency coincided with the natural frequency of the fluid-structure system. The flow and structural vibration were measured using particle image velocimetry, laser-induced fluorescence flow visualization, a laser Doppler anemometer, and a laser vibrometer. It was observed that the control scheme based on the feedback of both flow and structural oscillation led to the almost complete destruction of the Kármán vortex street and a reduction in the structural vibration, vortex shedding strength, and drag coefficient by 82%, 65%, and 35%, respectively, outperforming by far an open-loop control as well as the other two closed-loop schemes.
Time-dependent free surface Stokes flow with a moving contact line. II. Flow over wedges and trenches
Authors: Gramlich C. M. , Mazouchi Ali , Homsy G. M.
Published: April 2004

We consider the problem of a viscous liquid film with a moving contact line flowing under the influence of a body force over a substrate that includes a convex wedge or a trench. The time evolution of the liquid film profile is obtained by the numerical technique developed in part I. The effect of the capillary number Ca and the contact angle φ on the interaction between the dynamic contact line and topography is studied for both rectangular wedges and trenches of various widths and depths. Our results show that for smaller Ca the liquid film is pushed further along the wedge wall, resulting in a greater “runout” length before the film separates into a drop. When the wedge is the initial step down of a trench of finite depth and width, this runout length provides an upper bound on the depth of trenches that can be coated. For flows that reach the trench floor, details of the further development of the surface profile determine whether the liquid successfully coats the surface. We explore for various Ca and φ the dimensions of trenches that can be coated.
An internal splash: Levitation of falling spheres in stratified fluids
Authors: Abaid Nicole , Adalsteinsson David , Agyapong Akua , McLaughlin Richard M.
Published: April 2004

We experimentally explore the motion of falling spheres in strongly stratified fluids in which the fluid transitions from low density at the top to high density at the bottom and document an internal splash in which the falling sphere may reverse its direction of motion (from falling, to rising, to falling again) as it penetrates a region of strong density transition. We present measurements of the sphere’s velocity and exhibit nonmonotonic sphere velocity profiles connecting the maximum and minimum terminal velocities, matching earlier measurements [J. Fluid Mech. 381, 175 (1999)], but further exhibit the new levitation phenomenon. We give a physical explanation of this motion which necessarily couples the sphere motion with the stratified fluid, and vice versa, and supplement this with a simplified, reduced mathematical model involving a nonlinear system of ordinary differential equations which captures the nonmonotonic transition and agrees with the measured velocity profiles at all depths except those in the vicinity of the sharp transition for which the model deviates from the measured speeds. We repeat the experiments adjusting the distance between the camera and falling sphere thereby reducing the optical blur associated with the change in optical refractive index associated with the strong density transition. By directly measuring the residual optical distortion with a center plane, vertical ruler, we exhibit that the measured velocity profile within the transition layer is strongly sensitive to the details of the measured optical distortion, and show subsequent improved agreement between the measurement and the model. Through direct measurement of the nonlinear mapping between physical and imaged coordinates we document measured velocity error trends which may occur from inaccurately accounting for this optical distortion. We suggest strategies for correcting this localized measurement detail generally.
Turbulence and fluid-front area production in binary-species, supercritical, transitional mixing layers
Authors: Okong’o N. , Bellan J.
Published: April 2004

Databases of transitional states obtained from direct numerical simulations of temporal, supercritical mixing layers for two species systems, O2–H2 and C7H16–N2, are analyzed to elucidate species-specific turbulence aspects and features of fluid disintegration. Although the evolution of all layers is characterized by the formation of high-density-gradient magnitude (HDGM) regions, due to the specified, smaller initial density stratification, the C7H16–N2 layers display higher growth and increased global molecular mixing as well as larger turbulence levels than comparable O2–H2 layers. However, smaller density gradients and lower mass-fraction gradients at the transitional state for the O2–H2 system indicate that on a local basis, the layer exhibits an enhanced mixing, this being attributed to the increased mixture solubility and to mixture near-ideality. These thermodynamic features are found responsible for a larger irreversible entropy production (dissipation) in the O2–H2 compared to the C7H16–N2 layers. The largest O2–H2 dissipation is primarily concentrated in HDGM regions that are distortions of the initial density stratification boundary, whereas the largest C7H16–N2 dissipation is located in HDGM regions resulting from the mixing of the two fluids. To understand fluid disintegration, the area production of a fluid front perpendicular to the mass fraction gradient is calculated in a coordinate system moving with the relative velocity between the front and the flow. On a cross-stream local basis, the C7H16–N2 layers produce more area, and area production increases with smaller perturbation wavelengths combined with larger initial Reynolds numbers. The most active area-producing layer also exhibits the largest probability of having perpendicular vorticity and mass-fraction-gradient vectors. Analysis of the terms in the area production equation shows a large pressure-gradient-term root mean square contribution for the C7H16–N2 layers, due to the coincidence of regions with large magnitudes of pressure gradient with HDGM regions. Such coincidence is attributed to real-gas behavior, which is species-system specific, as the alignment of the pressure gradient and density gradient is similar for both species systems. The alignment of the mass fraction gradient with the strain rate is also species-system dependent. Independent of species system and of the initial conditions, the vorticity is preferentially aligned with the intermediate strain-rate eigendirection, indicating that eddy-viscosity-type models are not adequate for turbulent supercritical mixing.
Time-dependent free surface Stokes flow with a moving contact line. I. Flow over plane surfaces
Authors: Mazouchi Ali , Gramlich C. M. , Homsy G. M.
Published: April 2004

We study a gravity driven two-dimensional viscous flow with a moving contact line. Using a boundary integral method, the Stokes equations are written as a system of algebraic equations for the flow variables on the domain boundary. The equations are solved for an initial free surface profile, providing the local velocity of points on the free surface, which are then kinematically advected to obtain the new profile at the next time step; repeated time-stepping represents the flow profile evolution. We have studied the time evolution and fully developed steady solutions of the viscous flow for relatively high capillary numbers, Ca∼O(0.1), and a wide range of the contact angle, φ. Our results show a profile with a ridge, followed by a capillary wave decaying to the incoming parallel flow, whose size is a function of both Ca and φ. For a range of high contact angles and high Ca we find steady solutions with an overhang or nose and have characterized these in the (Ca,φ) plane. Finally, we find that the profiles exhibit an apparent or dynamic contact angle that varies with Ca and φ, and which agrees with Cox–Voinov-type relationships.
Non-Boussinesq simulations of Rayleigh–Bénard convection in a perfect gas
Authors: Robinson Frank , Chan Kwing
Published: April 2004

We present direct numerical simulations of Boussinesq and non-Boussinesq Rayleigh–Bénard convection in a rigid box containing a perfect gas. For small stratifications, which includes Boussinesq fluids, the first instability after steady rolls was an oscillatory instability (a Hopf bifurcation). The resulting convection was characterized by two hot and two cold blobs circulating each convective roll. The same sign thermal perturbations (blobs) are at diametrically opposite points on the circular rolls, i.e., they are symmetric about the roll center. The time for a hot (or cold) blob to circulate a roll was between two and three roll turnover times. When the stratification was of sufficient strength, there was a dramatic change in the nature of the bifurcation. The sign of the thermal perturbations became antisymmetric with respect to the roll center, i.e., a hot blob was diametrically opposite a cold blob. In this case, a hot or cold blob circulated around each roll in about one turnover time. In a stratified layer, the Rayleigh number varies with height. We found that at the Hopf bifurcation, the Rayleigh number at the base was closest to the Boussinesq value. The change in instability appeared to be related to an increase in the speed (or Mach number) of the circulating rolls. It did not seem to be affected by the transport property variation with temperature. If the along roll aspect ratio was less than 2 or the walls perpendicular to the roll axis periodic, then only the symmetric instability could be found. We describe how our results might be reproduced in a laboratory experiment of convection in cryogenic helium gas.
On the effect of the Boussinesq–Basset force on the radial migration of a Stokes particle in a vortex
Authors: Candelier F. , Angilella J. R. , Souhar M.
Published: April 2004

The trajectory of an isolated solid particle dropped in the core of a vertical vortex is investigated theoretically and experimentally, in order to analyze the effect of the history force on the radial migration of the inclusion. Both the Stokes number (based on the particle radius and the fluid angular velocity) and the particle Reynolds number are small. The particle is heavier than the fluid, and is therefore expelled from the center of the vortex. An experimental device using spherical particles injected in a rotating cylindrical tank filled with silicone oil has been built. Experimental trajectories are compared to analytical solutions of the motion equations, which are obtained by making use of classical Laplace transforms. The analytical expression of the history force and the ejection rate are carried out. This force does not vanish, but increases exponentially and has to be taken into account for efficient predictions. In particular, calculations without history force overestimate particle ejection. The relative difference between the ejection rate with and without history force scales like the square root of the Stokes number, so that differences of the order of 10% are visible as soon as the Stokes number is of the order of 0.01. Also, agreement between experimental and theoretical trajectories is observed only if the acceleration term in the history integral involves the time derivative of the fluid velocity following the particle, rather than the acceleration of fluid points at the particle location, even for small particle Reynolds numbers. Finally, analytical calculations show that the particle ejection rate is more sensitive to the Boussinesq–Basset force than to Saffman’s lift.
Effect of substrate movement on shock formation in pressure-driven coating flow
Authors: Muhammad Tauqeer , Khayat Roger E.
Published: April 2004

The transient two-dimensional pressure-driven flow of a thin Newtonian fluid film over a flat moving straight substrate is examined in this theoretical study. The interplay among inertia, initial conditions, and substrate movement is examined for a fluid emerging from a channel. The movement of substrate is found to have a significant effect on the flow behavior. Some initial conditions give rise to the formation of a wave that propagates with time and results in a shocklike structure in the flow. The substrate movement is found to delay shock formation. There exists a critical substrate velocity at which the shock is permanently obliterated.
Steady streaming due to vibrating walls
Authors: Carlsson Fredrik , Sen Mihir , Löfdahl Lennart
Published: April 2004

An analytical study of the flow induced by vibrating solid walls is presented. Transverse motions in the form of standing waves vibrated at small amplitudes are considered and the governing equations are perturbed using the slope as a small parameter. Steady streaming occurs at second order in the perturbation solution involving the formation of cellular flow patterns in the channel. Two dimensionless parameters determine the steady flow: frequency and channel half-width. Investigating the dependence of the flow on these parameters reveals four distinct types of cellular flows.
An experimental study of particle effects on drop formation
Authors: Furbank Roy J. , Morris Jeffrey F.
Published: April 2004

The formation of drops of particulate suspensions composed of spherical, neutrally buoyant, noncolloidal particles in a viscous liquid is examined experimentally. The suspensions are investigated over a range of particle volume fractions, φ=nπdp3/6 where n is the particle number density and dp is the particle diameter (dp=212–250 μm in most of the experiments), and for flow through three tubes of outer diameters d=0.16, 0.32 and 0.64 cm; the corresponding inner diameters are 0.10, 0.22, and 0.53 cm. Drop formation in the dripping mode and the transition from dripping to jetting are investigated. In the dripping mode, the behavior of low-φ suspensions (φ⩽0.10) is markedly different from that of higher-φ suspensions (0.15⩽φ⩽0.40), with a transition in the qualitative behavior over a relatively narrow range of concentrations. Pinch-off structures for low-φ suspensions are similar to that of the pure liquid, consisting of a long slender liquid thread connecting a hemispherical cap of liquid at the orifice to the nearly spherical forming drop; the structures differ from those of pure liquids in that individual particles, or groups of particles, can be captured in the thread at rupture, leading to new modes of satellite drop formation. At higher φ, the presence of large numbers of particles in the thinning thread during necking results in thick conelike structures termed “spindles” owing to this labeling of structures of similar geometry observed in electrohydrodynamic sprays. Particles are found to substantially suppress the number of satellite drops at higher φ, but the few satellite drops produced are much larger than observed in pure liquid drop formation. The transition from dripping to jetting occurs at a smaller flow rate at finite φ, with much shorter coherent lengths after jetting than in the case of a pure liquid. The transition becomes less abrupt and more difficult to identify at the highest concentrations examined.
A study of laminar flow of polar liquids through circular microtubes
Authors: Phares Denis J. , Smedley Gregory T.
Published: April 2004

Recently, the validity of using classical flow theory to describe the laminar flow of polar liquids and electrolytic solutions through microtubes has been questioned for tube diameters as large as 500 μm [Brutin and Tadrist, Phys. Fluids 15, 653 (2003)]. This potential increase in flow resistance, which has been attributed to electrokinetic effects and enhanced surface roughness effects, is critical to the understanding of certain biofluid systems. We seek to characterize this phenomenon for a variety of capillary/liquid systems. Using a numerical solution to the Poisson–Boltzmann equation, we have calculated the electroviscous effect for the systems under consideration. We have also measured the pressure drop as a function of flow rate across well-characterized stainless steel and polyimide microtubes ranging in diameter from 120 μm to 440 μm. Deionized water, tap water, a saline solution, and a variety of glycerol/water mixtures were used. The calculations and measurements suggest that any deviation from Poiseuille flow for tubes larger than 50 microns in diameter is more likely caused by the enhanced importance of surface roughness in microtubes than by electrokinetic effects.
Hydrodynamic instabilities of near-critical CO2 flow in microchannels: Lattice Boltzmann simulation
Authors: Holdych D. J. , Georgiadis J. G. , Buckius R. O.
Published: April 2004

Motivated by systematic CO2 evaporation experiments which recently became available (J. Pettersen, “Flow vaporization of CO2 in microchannel tubes,” Doctor technicae thesis, Norwegian University of Science and Technology, 2002), the present work constitutes an exploratory investigation of isothermal flow of CO2 near its liquid–vapor critical point through a long 5 μm diameter microchannel. A modified van der Waals constitutive model—with properties closely approximating those of “real” near-critical CO2—is incorporated in a two-dimensional lattice Boltzmann hydrodynamics model by embedding a dimensionless parameter X, with X→1 denoting the “real” fluid. The hydrodynamic phenomena resulting by imposing a constant pressure gradient along a periodic channel are investigated by considering two regimes in tandem: (1) transition from bubbly to annular flow with a liquid film formed at the channel walls and (2) destabilization of the liquid film by the Kelvin–Helmholtz instability. Due to numerical constraints, intrinsic modeling errors are introduced and are shown to be associated with discrepancies in the relative vapor–liquid interfacial thickness, which is expressed by X. The effects of these errors are investigated both theoretically and numerically in the physical limit X→1. Numerically determined flow patterns compare qualitatively well with direct visualization results obtained by Pettersen. Overall, the characteristics of isothermal near-critical two-phase flow in microchannels can be reproduced by the appropriate modification of the thermophysical properties of CO2.
Flow induced ultrasound scattering: Experimental studies
Authors: Seifer Sh. , Steinberg V.
Published: April 2004

Sound scattering by a finite width beam on a single rigid body rotation vortex flow is detected by a linear array of transducers (both smaller than a flow cell), and analyzed using a revised scattering theory. Both the phase and amplitude of the scattered signal are obtained on 64 elements of the detector array and used for the analysis of velocity and vorticity fields. Due to averaging on many pulses the signal-to-noise ratio of the phases difference in the scattered sound signal can be amplified drastically, and the resolution of the method in the detection of circulation, vortex radius, vorticity, and vortex location becomes comparable with that obtained earlier by time-reversal mirror method [P. Roux, J. de Rosny, M. Tanter, and M. Fink, Phys. Rev. Lett. 79, 3170 (1997)]. The revised scattering theory includes two crucial steps, which allow overcoming limitations of the existing theories. First, the Huygens construction of a far-field scattering signal is carried out from a signal obtained at any intermediate plane. Second, a beam function that describes a finite width beam is introduced, which allows using a theory developed for an infinite width beam for the relation between a scattering amplitude and the vorticity structure function. Structure functions of the velocity and vorticity fields deduced from the sound scattering signal are compared with those obtained from simultaneous particle image velocimetry measurements. Good quantitative agreement is found.
Nonlocal expression for scalar flux in turbulent shear flow
Authors: Hamba Fujihiro
Published: April 2004

An exact expression for the scalar flux was derived using the Green’s function for a scalar. The nonlocal eddy diffusivity involved in the expression represents a contribution to the scalar flux from the mean scalar gradient at remote points in space and time. The direct numerical simulation of channel flow was carried out to validate the nonlocal expression. The velocity and scalar fields as well as the Green’s function were calculated in the cases of one- and two-dimensional mean scalar fields and of oscillating mean scalar field. It was shown that the nonlocal expression is accurate in all the cases. A local expression for the scalar flux was also examined to show that the local approximation is not accurate enough and that the nonlocal effect is important. Some attempts were made to model the nonlocal effect. The nonlocal diffusivity was expressed algebraically using an exponential function, the local expression was modified by adding higher-order terms, and a differential equation for the nonlocal diffusivity was proposed. It was demonstrated that the analysis with the nonlocal expression gains insight into modeling the scalar transport in turbulent shear flows.
Numerical simulation of violent bubble motion
Authors: Xi Wang Qian
Published: April 2004

A robust mixed-Eulerian-Lagrangian modeling is presented for the simulation of violent bubble motion, which is characterized by implementing a hybrid surface interpolation for calculating the material velocity. A linear averaging approximation is deployed for the parts of the surface where the change of the surface normal within the element size is not small, and a polynomial scheme coupled with the weighted moving least squares method for the rest of the surface. Solid angles on the free surface (an open surface) are computed analytically in a closed form using the spherical triangle theory. The algorithm is validated against an experiment and an axisymmetric bubble code. Numerical analyses are carried out for the evolution of a gas bubble near an inclined rigid wall and the interaction of two gas bubbles and a free surface. The robustness of the algorithm is demonstrated through simulating sharp bubble jets, a bubble collapsing nearby a rigid wall with a large part of the bubble surface flattened against the wall, and bubbles collapsing in very close to a free surface producing sharp free surface spikes.
Pollinators in seed orchards of Eucalyptus nitens (Myrtaceae)
Authors: Hingston A. B. , McQuillan P. B. , Potts B. M.
Published: April 2004

Flowers of the commercially important tree Eucalyptus nitens (Deane & Maiden) Maiden were visited by a diverse array of insects, but not by birds, in Tasmanian seed orchards. Most species of insects that visited the flowers of E. nitens are likely to be effective pollinators because all common species of visitors carried many grains of Eucalyptus pollen, and the open floral structure facilitates frequent insect contact with stigmas. Seed production also suggested that a wide variety of insects were effective pollinators because flowers were consistently well pollinated, despite differences in flower-visitor communities among orchards and particular branches of flowers. The generalised entomophilous pollination system of E. nitens suggests that effective pollinators should occur in seed orchards of this tree throughout the world, provided that flowering occurs at a time of year conducive to insect activity. Although a wide variety of insects appear to be effective pollinators of E. nitens, introduced honeybees, Apis mellifera L., that are often deployed as pollinators in seed orchards were consistently not attracted to the flowers. The reliance on wild insects as pollinators suggests that seed production in E. nitens may benefit from reduced use of broad-spectrum insecticides in, and near, seed orchards.
Fruit characteristics of invasive bitou bush, Chrysanthemoides monilifera (Asteraceae), and a comparison with co-occurring native plant species
Authors: Gosper C. R.
Published: April 2004

Bitou bush, Chrysanthemoides monilifera (L.) Norlindh ssp. rotundata (DC.) Norlindh, is an invasive weed of coastal vegetation types in eastern Australia. It produces fleshy fruits, which are dispersed by birds and mammals. Compared with a range of vertebrate-dispersed fruits of native plant species that co-occur with naturalised C. monilifera, or occur elsewhere in south-eastern Australia, C. monilifera fruits and endocarps are intermediate in all dimensions and in the ratio of endocarp to pulp. C. monilifera fruits contain moderate quantities of sugars, but low quantities of protein and fat relative to many native fruits. Phenology, however, substantially distinguishes C. monilifera from co-occurring native plant species. C. monilifera produces fruits throughout the year, with the peak of production occurring in autumn to winter, a time when few native species in coastal vegetation produce fruits. Additionally, 9 of the 12 native species that do fruit, at least partly, over this period, differ substantially from C. monilifera in fruit morphology. C. monilifera, along with other invasive species, has substantially altered the temporal pattern of fruit availability in coastal vegetation. These fruiting characteristics have rendered C. monilifera fruits attractive to frugivores and have contributed to its successful spread.
The potential of five Western Australian native Acacia species for biological control of Phytophthora cinnamomi
Authors: D'Souza Nola K. , Colquhoun Ian J. , Shearer Bryan L. , Hardy Giles E. St. J.
Published: April 2004

Five Acacia species native to Western Australia were assessed for their potential to protect the highly susceptible species Banksia grandis Wield from infection by the plant pathogen Phytophthora cinnamomi Rands. In a rehabilitated bauxite pit at Jarrahdale 55 km south-east of Perth and in a glasshouse trial, B. grandis planted either alone or with A. pulchella R.Br., A. urophylla Benth., A. extensa Lindl., A. lateriticola Maslin or A. drummondii Lindl., was soil inoculated with P. cinnamomi. It could only be shown that A. pulchella significantly protected B. grandis from P. cinnamomi infection in the rehabilitated bauxite pit trial up to 1 year after inoculation. This confirms the potential of this species for biological control of the pathogen in infested plant communities. The observed protection was not the result of a decrease in soil temperature or moisture. Protection was not emulated in a glasshouse trial where optimum environmental conditions favoured P. cinnamomi. Despite a delay in infection of B. grandis planted with Acacia spp., none of the five species definitively protected B. grandis from P. cinnamomi. However, in the glasshouse trial, A. pulchella, A. extensa, A. lateriticola and A. drummondii did significantly reduce the soil inoculum of P. cinnamomi, indicating a possible biological control effect on the pathogen. The mechanisms of biological control are discussed and the implications for management of rehabilitated bauxite mined areas and forests severely affected by P. cinnamomi are considered.
Reproductive phenology of the cerrado plant community in Emas National Park (central Brazil)
Authors: Batalha Marco Antônio , Martins Fernando Roberto
Published: April 2004

The cerrado, a savanna-like formation, is the second-most widespread vegetation type in Brazil. Its core region occupies the Brazilian Central Plateau, with a seasonal climate, including wet summer and dry winter. We studied the reproductive phenological patterns of the cerrado plant community in Emas National Park (central Brazil). Monthly field trips revealed 601 species, classified, according to their dispersal syndromes, into anemo-, auto-, or zoochorous species, when they were dispersed by wind, explosion or gravity, or animals, respectively. We analysed the flowering and fruiting patterns of the community in relation to dispersal syndromes, comparing the herbaceous and the woody components. The herbaceous component was characterised by a majority of autochorous species, and the woody component, by a majority of zoochorous species. There was a striking seasonality in the community-wide pattern of flowering and fruiting, although this was different between the herbaceous and the woody components. Woody species flowered mainly during late dry and early wet seasons, whereas herbaceous species flowered especially during late wet season. In the dry season, when their diaspores can be dispersed more efficiently, the proportions of anemo- and autochorous fruiting species were higher. During the rainy season, when their fruits can be kept attractive for longer time, the number of fruiting zoochorous species reached its peak.
Morphology and spatial distribution of alien sea-rockets (Cakile spp.) on South Australian and Western Canadian beaches
Authors: Cody Timothy W. D. , Cody Martin L.
Published: April 2004

Sea-rockets (Cakile spp., Brassicaceae) are annual plants of sandy beaches. Cakile edentula (Bigel.) Hook. is native to the eastern coast of North America, C. maritima Scop. to western Europe and the Mediterranean basin. The two species differ in several morphological features, including leaf form, fruits and petal size. Both are long-established aliens on beaches in western Canada and southern Australia, at sites where we examined their morphological and distributional attributes.
The two Cakile species co-occur at Pachina Beach, British Columbia, Canada, with C. edentula more common and widely distributed over broader range of beach elevations and C. maritima restricted to the upper beach. Although a few putative hybrids occur, the species are morphologically quite distinct. In contrast, on Westlake Shores beach, South Australia, Cakile is at least in part perennial, with widely variable morphologies, and the taxon is not separable into two morphologically distinct entities. Species boundaries have blurred apparently because of introgression. Factors that may have lead to this contrasting situation in South Australia are discussed.
One-pot synthesis of enantiopure
syn
-1,3-diacetates from racemic
syn
/
anti
mixtures of 1,3-diols by dynamic kinetic asymmetric transformation
Authors: Edin Michaela , Steinreiber Johannes , Bäckvall Jan-E.
Published: April 2004

A one-pot synthesis of enantiomerically pure
syn
-1,3-diacetates starting from readily accessible racemic diastereomeric mixtures of 1,3-diols has been realized by combining (
i
) enzymatic transesterification, (
ii
) ruthenium-catalyzed epimerization of a secondary alcohol in a diol or diol monoacetate, and (
iii
) intramolecular acyl migration in a
syn
-1,3-diol monoacetate. The
in situ
coupling of these three processes results in an efficient enantioselective synthesis of acyclic
syn
-1,3-diacetates via combined deracemization–deepimerization and constitutes a dynamic kinetic asymmetric transformation concept. Several differently substituted unsymmetrical, acyclic
syn
-1,3-diacetates were obtained in yields up to 73% with excellent enantioselectivities (>99%) and good diastereomeric ratios (>90%
syn
).