bluuebunny/crossref_metadata_2025 huggingface dataset:
Exile
Authors: Zabaleta Marta Raquel
Published: April 2003

Marta Raquel Zabaleta's autobiographical piece takes us through the trajectory of her exile as an Argentinian refugee, first in Glasgow and then in London. Forced to flee with her husband, a Chilean UN refugee, she describes the differences between the ways her husband and herself were treated by those in solidarity groups and other aid organizations and the particular difficulties faced by women refugees. She explores the isolating effects of having her professional identity and status erased as a refugee and of being relegated to the sole status of ‘wife’. Zabaleta also insists on the fundamental rights of refugees and asylum seekers to have both their histories and their desires for the future acknowledged by those in the host country.
The Maternity Care Needs of Refugee and Asylum Seeking Women in Ireland
Authors: Kennedy Patricia , Murphy-Lawless Jo
Published: April 2003

This article presents some of the findings from the original research carried out with asylum seeking and refugee women in Ireland who were pregnant or who had recently given birth. The explosion in numbers in Ireland from 1998 onwards has been such that this group now comprises more than one in five of every birth in the country's three major maternity hospitals, all based in Dublin. The article explores the background reasons for the major increase in recent years of this group of women. It discusses the difficult circumstances encountered by women who must engage with a system of maternity care unused to such complex needs, amidst a general policy climate of uncertainty and even hostility towards asylum seekers. The research findings contribute to the feminist literature on maternity and challenge us to examine the way in which globalization is impacting on women as mothers and the need for challenging Western states anew on the development of a more coherent model of maternity care in response to the needs of such women.
Everyday Worlds of Young Refugees in London
Authors: Candappa Mano , Igbinigie Itohan
Published: April 2003

This article examines the everyday lives of a sample of young refugees living in London, based on a study of the social roles and social networks of refugee children undertaken under the ESRC Children 5–16 Programme. It draws on findings from a survey of refugee and non-refugee children aged between 11 and 14 in two London schools, complemented by data from in-depth interviews with refugee children. The article focuses on the children's responsibilities towards home and family, friendships, and leisure activities. It highlights the experiences of the refugee children in the sample, and explores some gender differences between the social lives of refugee boys and girls, and between the lives of refugee children and those of their non-refugee peers.
Bosnian Refugee Women in (Re)settlement: Gender Relations and Social Mobility
Authors: Franz Barbara
Published: April 2003

Bosnian refugee women adapted more quickly than their male partners to their host environments in Vienna and New York City because of their self-understanding and their traditional roles and social positions in the former Yugoslavia. Refugee women's integration into host societies has to be understood through their specific historical experiences. Bosnian women in exile today continue to be influenced by traditional role models that were prevalent in the former Yugoslavia's 20th-century patriarchal society. Family, rather than self-fulfillment through wage labor and emancipation, is the center of life for Bosnian women. In their new environment, Bosnian refugee women are pushed into the labor market and work in low-skill and low-paying jobs. Their participation in the labor market, however, is not increasing their emancipation in part because they maintain their traditional understanding of zena (women) in the patriarchal culture. While Bosnian women's participation in low-skill labor appeared to be individual families’ decisions more in New York City than in Vienna, in the latter almost all Bosnian refugee women in my sample began to work in the black labor market because of restrictive employment policies. In contrast to men, women were relatively nonselective and willing to take any available job. Men, it seems, did not adapt as quickly as women to restrictions in the labor market and their loss of social status in both host societies. Despite their efforts, middle-class families in New York City and Vienna experienced substantial downward mobility in their new settings. Women's economic and social downward mobility in (re)settlement, however, did not significantly change the self-understanding of Bosnian women. Their families’ future and advancements socially and economically, rather than the women's own independence and emancipation remained the most important aspect of their being.
Refugee Women in Serbia – Invisible Victims of War in the Former Yugoslavia
Authors: Nikolic-Ristanovic Vesna
Published: April 2003

In this paper, I explore the experiences of women who found refuge in Serbia during the war in the former Yugoslavia. I look at the women's experiences of both leaving home and coping with everyday life in refuge. The exploration of refugee women's experiences is mainly based on analyses of their own stories, which I collected while researching women and war. In spite of all the hardship of their lives, refugee women who fled to Serbia have been treated by Western media, the public and aid organizations as ‘UNPEOPLE’ or as non-existent. Making their experiences visible as women, refugees and citizens is the main purpose of this article.
Drop breakup in the flow through fixed fiber beds: An experimental and computational investigation
Authors: Patel Prateek D. , Shaqfeh Eric S. G. , Butler Jason E. , Cristini Vittorio , Bławzdziewicz Jerzy , Loewenberg Michael
Published: April 2003

Dilute fixed fiber beds provide a model system for studying drop dynamics in disordered flows. Fluctuations about the mean uniform velocity are generated by fiber elements within the media, and the disturbance velocities far from any single fiber (at distances on the order of the pore size) have been predicted to be strong in terms of drop deformation and breakup by Mosler and Shaqfeh [Phys. Fluids 9, 5 (1997)]. In this work, we focus on the importance of near-field interactions, or the flow close to individual fibers. We present experimental observations of drop deformation and breakup during flow through a dilute bed of randomly placed fibers. We found breakup to result from only close interactions with fibers and describe two near-field breakup mechanisms which we term “graze” and “hairpin” processes. In addition, we present the breakup probability through the experimental fiber bed as a function of the appropriate Capillary number Ca. To better understand the near-field interactions, we used the boundary integral method to determine drop shape evolution in the flow around an infinite fiber within a porous medium, and our simulations capture the breakup mechanisms observed during experiments. To compare with experimental breakup probabilities, we have defined a critical offset for breakup during flow past a fiber and assuming straight center-of-mass trajectories, calculated breakup probabilities based on this simple model. These predictions compare well with the experimental measurements for Ca⩾2.
Disintegration of planar liquid film impacted by two-dimensional gas jets
Authors: Mehring C. , Sirignano W. A.
Published: April 2003

The distortion and break-up of a thin planar liquid film impacted by two gas jets while discharging from a twin-fluid atomizer is studied numerically. The gas momentum vector has components normal and parallel to the liquid stream. Viscosity and compressibility are neglected in both the liquid phase and the gas phase. The reduced-dimension (lubrication) approximation is employed to describe the nonlinear distortion and breakup of the thin film. The gas-phase dynamics are modelled by using a boundary-element-method formulation. For the considered parameter range and for a given energy expenditure, direct modulation of liquid-phase velocities at the nozzle exit is found to be more effective in causing film rupture than indirect modulation via adjacent impacting gas jets. In the former case, dilational film modulation results in shorter breakup lengths than sinuous modulation. On the other hand, for gas-jet modulated films, sinuous mode forcing is more effective than dilational forcing for the same energy input. Co-flowing gas streams significantly alter wavelengths and amplitudes of film disturbances generated by direct film modulation. Large ratios of gas-jet momentum to liquid-film momentum result in “immediate” film rupture in response to the dynamics of the impacting gas jets, whereas for lower ratios films disintegration occurs further downstream after continuous growth of the initial disturbances. Film distortion is characterized by the formation of fluid blobs or long band-like films depending on Weber number values and density ratio.
The effects of thermal diffusion on miscible, viscous displacement in a capillary tube
Authors: Kuang J. , Maxworthy T.
Published: April 2003

Experiments have been conducted to study the displacement of a high-viscosity fluid at low temperature by the same fluid of high temperature and low viscosity in cylindrical capillary tubes. A fraction, m, which is defined as the fraction of cold and more viscous fluid left on the wall of the tube behind the advancing finger, was measured as a function of Peclet number. The results show three regimes: a diffusion dominated regime I for Pe<1000 with m=0.5; a viscously dominated regime III for Pe>3000 with m averaging a value of 0.62 and a transition regime in which both diffusion and viscous effects are important, regime II, for 1000<Pe<3000. A significant difference between the present experiment and earlier species diffusion experiments suggests that it is very likely that new dynamical effects, e.g., Korteweg stresses, must be taken into account to explain the results from the latter experiments.
Wall migration and shear-induced diffusion of fluid droplets in emulsions
Authors: Hudson S. D.
Published: April 2003

The spatial distribution of drops in multiphase Stokes flow is derived theoretically as a function of two dimensionless parameters, accounting for wall migration, buoyancy, and shear-induced diffusion. The wall migration effect, which drives drops away from the walls and toward the center of the gap, is often significant even when droplets are 100 times smaller than the gap. By comparison with the experimental drop concentration profile, the shear-induced down-gradient diffusivity is measured and found to be approximately four to five times larger than the prediction for drop self-diffusivity. These are the first such measurements of the diffusivity of drops with clean interfaces and contrast markedly with previous measurements on surfactant-laden drops. Nonuniform concentration along the vorticity axis is also investigated briefly.
Flow over a surface with parallel grooves
Authors: Wang C. Y.
Published: April 2003

Stokes shear flow over a surface with evenly spaced, finite-depth rectangular grooves is solved analytically by eigenfunction expansions and matching. Macroscopically the rough surface is equivalent to a smooth surface with partial slip. The slip coefficients are determined for shear flow both along and transverse to the grooves. The partial slip condition is then applied to the flow through a channel with nonaligned grooves on the walls.
Molecular kinetic approach to the problem of compressible turbulence
Authors: Sakurai A. , Takayama F.
Published: April 2003

The primary aim of this paper is to study the feasibility of the molecular kinetic approach to the problem of turbulence oriented computation of compressible flows. The Boltzmann equation in its integral form is employed with the Bhatnagar–Gross–Krook or discrete Boltzmann equation model for the collision term. First, we consider the basic problem of the production of small eddies from large ones in turbulent flow field in terms of the Taylor–Green type initial value problem in two and three dimensions. Results computed with different collision models compare well with each other and with a corresponding direct numerical simulation computation based on the Navier–Stokes equations, indicating consistency of the present approach to the problem. Next, we compute a space periodic compressible flow inside a unit cube initiated from a random velocity field with uniform density and temperature. Results show the development of many small shock-wave-like structures along with vortices or eddy shocklets scattered throughout the entire flow field. While its energy spectrum pattern does not change much with time, as appropriate in this case, some quantities, like density distribution, change quickly from the initial uniform state to a turbulent state. Some geometric properties of the flow field are derived from the invariant of its velocity deformation tensor, which has features characteristic of compressible turbulence.
Hydrodynamic stability of Taylor–Dean flow between rotating porous cylinders with radial flow
Authors: Chang Min-Hsing
Published: April 2003

A linear stability analysis has been implemented for the Taylor–Dean flow between porous concentric rotating cylinders when radial flow is present. The stability equations with respect to both axisymmetric and nonaxisymmetric disturbances are derived and solved by a direct numerical procedure. Both types of radial flows, inward and outward flows, are considered. A parametric study covering wide ranges of α, the radial Reynolds number based on the radial velocity at the inner cylinder and inner radius, and β, a parameter characterizing the ratio of average pumping velocity due to azimuthal Poiseuille flow and rotation velocity due to inner cylinder rotation, is conducted for the situation of practical interest where the outer cylinder is stationary and the inner cylinder is rotating. The area where the onset mode is nonaxisymmetric is shown in the plane (β,α). A critical curve is discovered that the most stable state always occurs at the point on this curve for an assigned value of α. Moreover, a discontinuity of the critical axial wavenumber happens when the parameters α or β cross this curve. The critical mode transition of the onset of instability is demonstrated in detail and results for the variations of the critical Taylor number and axial wavenumber are presented. It is found that the superimposed radial flow may produce a stabilizing or destabilizing effect depending heavily on the value of β related to the basic state.
An experimental study on resonance of oscillating air/vapor bubbles in water using a two-frequency acoustic apparatus
Authors: Ohsaka K.
Published: April 2003

A two-frequency acoustic apparatus is employed to study the growth behavior of vapor-saturated bubbles driven in a volumetric mode. A unique feature of the apparatus is its capability of trapping a bubble by an ultrasonic standing wave while independently driving it into oscillations by a second lower-frequency acoustic wave. It is observed that the growing vapor bubbles exhibit a periodic shape transition between the volumetric and shape modes due to resonant coupling. In order to explain this observation, we performed an experimental investigation on resonant coupling of air bubbles and obtained the following results: First, the induced shape oscillations are actually a mixed mode that contains the volume component, thus, vapor bubbles can grow while they exhibit shape oscillations. Second, the acoustically levitated bubbles are deformed and therefore, degeneracy in resonant frequency is partially removed. As a result, the vapor bubbles exhibit the shape oscillations in both the axisymmetric mode and asymmetric (three-dimensional) modes. Nonlinear effects in addition to the frequency shift and split due to deformation creates overlapping of the coupling ranges for different modes, which leads to the continuous shape oscillations above a certain bubble radius as the bubble grows.
Reynolds number effects on the flow structure behind two side-by-side cylinders
Authors: Xu S. J. , Zhou Y. , So R. M. C.
Published: April 2003

The wake structure of two side-by-side cylinders was experimentally investigated using the laser-induced fluorescence flow visualization, particle image velocimetry and hot-wire techniques. The investigation was focused on the asymmetrical flow regime, i.e., T/d=1.2–1.6, where T is the center-to-center cylinder spacing and d is the cylinder diameter. Experiments were conducted in both the water tunnel and the wind tunnel at a Reynolds number (Re) range of 150–14 300. It has been found that, as Re increases, the flow structure behind the cylinders may change from one single vortex street to two streets with one narrow and one wide for the same T/d. The one-street flow structure is dominated by one frequency f0*=f0d/U∞≈0.09, where f0 is the dominant frequency and U∞ is the free-stream velocity. On the other hand, two frequencies, f0*≈0.3 and 0.09, characterized the two-street flow structure. These are associated with the narrow and wide street, respectively. It is further observed that the critical Re, at which the transition from single to two streets occurs, increases as T/d decreases. The present finding clarifies previous scattered reports for 1.2<T/d<1.5: the detection of one dominant frequency by some but two by others.
Orientation of carbon nanotubes in a sheared polymer melt
Authors: Hobbie E. K. , Wang H. , Kim H. , Lin-Gibson S. , Grulke E. A.
Published: April 2003

Optical measurements of the shear response of semidilute dispersions of polymer-dispersed multiwalled carbon nanotubes are presented. For a weakly elastic polymer melt, the data suggest that the semiflexible tubes orient along the direction of flow at low shear stress, with a transition to vorticity alignment above a critical shear stress, σc, corresponding to a critical Deborah number of approximately 0.15. In contrast, data for a highly elastic polymer solution suggest that the tubes orient with the flow field at high shear rates, in the limit of large Deborah number. The measurements are in qualitative agreement with previous experimental and theoretical studies of fiber orientation in elastic fluids under simple shear flow.
Multiple stationary states for deformable drops in linear Stokes flows
Authors: Bławzdziewicz J. , Cristini Vittorio , Loewenberg Michael
Published: April 2003

Using a small-deformation expansion and numerical simulations we study stationary shapes of viscous drops in two-dimensional linear Stokes flows with nonzero vorticity. We show that high-viscosity drops in flows with vorticity magnitude β≪1 have two branches of stable stationary states. One branch corresponds to nearly spherical drops stabilized primarily by rotation, and the other to elongated drops stabilized primarily by capillary forces. For drop-to-continuous-phase viscosity ratios beyond a critical value λc, the rotationally stabilized solution exists in the absence of capillary stresses, because the rate of drop deformation (but not rotation) decreases with drop viscosity. We show that λc=10β−2, and the capillary stresses required for drop stability vanish at λc with exponent 1/2, as required by flow-reversal symmetry.
Thermocapillary actuation of liquid flow on chemically patterned surfaces
Authors: Darhuber Anton A. , Davis Jeffrey M. , Troian Sandra M. , Reisner Walter W.
Published: April 2003

We have investigated the thermocapillary flow of a Newtonian liquid on hydrophilic microstripes which are lithographically defined on a hydrophobic surface. The speed of the microstreams is studied as a function of the stripe width w, the applied thermal gradient |dT/dx| and the liquid volume V deposited on a connecting reservoir pad. Numerical solutions of the flow speed as a function of downstream position show excellent agreement with experiment. The only adjustable parameter is the inlet film height, which is controlled by the ratio of the reservoir pressure to the shear stress applied to the liquid stream. In the limiting cases where this ratio is either much smaller or much larger than unity, the rivulet speed shows a power law dependency on w, |dT/dx| and V. In this study we demonstrate that thermocapillary driven flow on chemically patterned surfaces can provide an elegant and tunable method for the transport of ultrasmall liquid volumes in emerging microfluidic technologies.
Marangoni instability at a contaminated liquid–vapor interface of a burning thin film
Authors: Armendáriz Javier , Matalon Moshe
Published: April 2003

We consider the evaporation and subsequent burning of thin films of liquid fuels on which a nonsoluble surface active agent (surfactant) is present. This work complements a previous study where we have considered the same problem but in the absence of surfactant. Surfactant may result from impurities of the liquid fuel or from backward diffusion of unoxidized combustion intermediaries and heavy soot precursors. When burning occurs in a quiescent ambient, the mathematical problem can be systematically reduced to a pair of nonlinear evolution equations for the film’s thickness and surfactant’s concentration. These equations contain, in particular, the temperature and mass flux at the liquid–vapor interface as additional parameters, determined from full consideration of the gas-phase processes. We show that in the absence of combustion or, when the heat released by the chemical reactions is relatively small, thermo-capillary effects tend to destabilize a nominally planar interface. The presence of surfactant brings about a slower growth and can possibly stabilize the film. Combustion generally acts to reverse these trends: When the heat release is large, thermo-capillary effects stabilize the liquid–vapor interface while the presence of surfactant leads to destabilization.
On the new vortex shedding mode past a rotating circular cylinder
Authors: Stojković D. , Schön P. , Breuer M. , Durst F.
Published: April 2003

To examine in detail the behavior of a new vortex shedding mode found in a previous investigation [Phys. Fluids 14, 3160 (2002)], a two-dimensional numerical study on the laminar incompressible flow past a rotating circular cylinder in the Reynolds number range 60⩽Re⩽200 and at rotational rates 0⩽α⩽6 was carried out. The results obtained clearly confirm the existence of the second shedding mode for the entire Reynolds number range investigated. A complete bifurcation diagram α(Re) was compiled defining both kind of shedding modes. The unsteady periodic flow in the second mode is characterized by a frequency much lower than that known for classical von Kármán vortex shedding of the first mode. The corresponding Strouhal number shows a strong dependence on the rotational velocity of the cylinder, while only a weak dependence is observed for the Reynolds number. Furthermore, the amplitudes of the fluctuating lift and drag coefficients are much larger than those characterizing classical vortex shedding behind nonrotating or slowly rotating cylinders. Additionally, negative values for the mean drag denoting thrust are found within the second shedding mode.
Polymer dynamics in a model of the turbulent buffer layer
Authors: Stone Philip A. , Graham Michael D.
Published: April 2003

A Brownian dynamics study of bead–spring–chain polymer dynamics is undertaken in a model flow that captures key features of the buffer region of near-wall turbulence—wavy streamwise vortices superimposed on a mean shear. In this flow and in any Lagrangian chaotic flow, a Hookean dumbbell polymer will stretch indefinitely if and only if the Weissenberg number based on the largest Lyapunov exponent for the velocity field is ⩾12. In the flow investigated here, this criterion is found to be good predictor of when the stretch of finitely extensible chains approaches its maximum value. The chains become highly stretched in the streamwise streaks and relax as they move into and around the vortex cores, leading to large differences in stress in different regions of the flow. Hydrodynamic and excluded volume interactions between polymer segments have no qualitative effects once results are normalized for the change in relaxation time due to their inclusion. The results from the bead–spring–chain models are used to assess the utility of the simpler FENE-P model. Although the FENE-P model does not capture the hysteresis in stress that is seen with the bead–spring–chain models, it otherwise qualitatively captures the behavior of the bead–spring chains. Most importantly, large polymer stress in the flow is seen at the same spatial positions for both the FENE-P and the more detailed models.
Room-temperature fabrication of high-resolution carbon nanotube field-emission cathodes by self-assembly
Authors: Oh S. J. , Cheng Y. , Zhang J. , Shimoda H. , Zhou O.
Published: April 2003

In this letter, we report a process for room-temperature assembly of patterned and periodic structures of carbon nanotubes (CNTs). Well-defined patterns with less than a 10-μm linewidth and variable thickness were readily deposited. The CNTs show long-range in-plane orientation ordering and adhere strongly to the substrates. The self-assembled macroscopic structures emit electrons under applied electrical field with the emission characteristics comparable to those from the high-temperature or/and low-resolution deposition processes. This room-temperature self-assembly method can be utilized for assembly and integration of nanostructured materials for a variety of devices, including the field-emission displays.
Streak breakdown instability in pipe Poiseuille flow
Authors: Meseguer Álvaro
Published: April 2003

This work is devoted to the study of the stability of Hagen–Poiseuille flow or pipe flow. The analysis is focused on the streak breakdown process by which two-dimensional streamwise-independent finite amplitude perturbations transiently modulate the basic flow leading to a profile that contains saddle points and is linearly unstable with respect to very small streamwise-dependent perturbations. This mechanism is one possible route of transition to turbulence in subcritical shear flows. The exploration is carried out for initial disturbances of different finite amplitudes and axial and azimuthal periodicity. This study covers a wide range of Reynolds numbers and the double threshold curve obtained for transition is consistent with experimental observations.
On a generalized approach to the linear stability of spatially nonuniform thin film flows
Authors: Davis Jeffrey M. , Troian Sandra M.
Published: April 2003

The presence of a deformable free surface in thin films driven to spread by body or shear forces gives rise to base states that are spatially nonuniform. This nonuniformity produces linearized disturbance operators that are non-normal and an eigenvalue spectrum that does not necessarily predict stability behavior. The falling film provides a simple example for demonstrating a more generalized, rigorous nonmodal approach to linear stability for free surface flows. Calculations of the pseudospectra and maximum disturbance amplification in this system, however, reveal weak effects of non-normality and transient growth such that the modal growth rate is rapidly recovered. Subdominant modes contribute little energy to the leading eigenvector because the oscillatory behavior is rapidly damped by surface tension. Generalization of these results to numerous other lubrication flows involving surface tension suggests similarly weak non-normality and transient growth.
Control of turbulent boundary layers
Authors: Kim John
Published: April 2003

The objective of this paper is to give an overview of recent progress on boundary layer control made by the author’s research group at University of California, Los Angeles. A primary theme is to highlight the importance of a certain linear mechanism and its contribution to skin-friction drag in turbulent boundary layers—and the implication that significant drag reduction can be achieved by altering this linear mechanism. Examples that first led to this realization are presented, followed by applications of linear optimal control theory to boundary-layer control. Results from these applications, in which the linear mechanism in turbulent channel flow was targeted, indirectly confirm the importance of linear mechanisms in turbulent—and hence, nonlinear—flows. Although this new approach has thus far been based solely on numerical experiments which are yet to be verified in the laboratory, they show great promise and represent a fundamentally new approach for flow control. The success and limitations of various controllers and their implications are also discussed.
On interfacial gravity-capillary solitary waves of the Benjamin type and their stability
Authors: Calvo David C. , Akylas T. R.
Published: April 2003

Symmetric solitary waves on the interface between two fluid layers are discussed for the case when the upper fluid is bounded above and the lower fluid is infinitely deep. In this flow geometry, under the conditions that the fluid densities are nearly equal and interfacial tension is relatively large, Benjamin [J. Fluid Mech. 245, 401 (1992)] derived a weakly nonlinear long-wave evolution equation which admits a new kind of elevation solitary wave that features decaying oscillatory tails. We present computations of solitary waves of this type and examine their stability to small perturbations on the basis of the full Euler equations using numerical methods. Computed solitary-wave profiles are found to be in good agreement with solutions of the Benjamin equation even for Weber and Froude numbers well outside the formal range of validity of weakly nonlinear long-wave analysis. Furthermore, our computations reveal that, unless the upper fluid is very light relative to the lower fluid, moderately steep interfacial solitary waves behave qualitatively as predicted by the Benjamin equation: elevation waves with a tall center crest, akin to those found by Benjamin, are stable and coexist with depression solitary waves (not reported by Benjamin) which are unstable. As expected, for low enough density ratio, these two solution branches exchange stabilities and computed profiles resemble those of free-surface gravity-capillary solitary waves on deep water.
Evaporation and condensation Knudsen layers for nonunity condensation coefficient
Authors: Meland Roar , Ytrehus Tor
Published: April 2003

For diffuse reflection at the interphase, the gas-kinetic solution for nonunity evaporation and condensation coefficients is completely described by an equivalent unity evaporation and condensation coefficient solution. Although the general transformation was derived 30 years ago, there is still some misunderstanding about how to transform the Knudsen layer. In this work the transformation is extended to the case of unequal evaporation and condensation coefficients and it is shown how to relate the Knudsen layer solutions to the ideal unity condensation coefficient solution both for evaporation and condensation.
The significance of vortex ring formation to the impulse and thrust of a starting jet
Authors: Krueger Paul S. , Gharib M.
Published: April 2003

The recent work of Gharib, Rambod, and Shariff [J. Fluid Mech. 360, 121 (1998)] studied vortex rings formed by starting jets generated using a piston-cylinder mechanism. Their results showed that vortex rings generated from starting jets stop forming and pinch off from the generating jet for sufficiently large values of the piston stroke to diameter ratio (L/D), suggesting a maximization principle may exist for propulsion utilizing starting jets. The importance of vortex ring formation and pinch off to propulsion, however, rests on the relative contribution of the leading vortex ring and the trailing jet (which appears after pinch off) to the impulse supplied to the flow. To resolve the relative importance of the vortex ring and trailing jet for propulsion, a piston-cylinder mechanism attached to a force balance is used to investigate the impulse and thrust generated by starting jets for L/D ratios in the range 2–8. Two different velocity programs are used, providing two different L/D values beyond which pinch off is observed, in order to determine the effect of vortex ring pinch off. Measurements of the impulse associated with vortex ring formation show it to be much larger than that expected from the jet velocity alone and proportionally larger than that associated with a trailing jet for L/D large enough to observe pinch off. The latter result leads to a local maximum in the average thrust during a pulse near L/D values associated with vortex rings whose circulation has been maximized. These results are shown to be related to the nozzle exit over-pressure generated during vortex ring formation. The over-pressure is in turn shown to be associated with the acceleration of ambient fluid by vortex ring formation in the form of added and entrained mass.
A methodology for optimal laminar flow control: Application to the damping of Tollmien–Schlichting waves in a boundary layer
Authors: Airiau Christophe , Bottaro Alessandro , Walther Steeve , Legendre Dominique
Published: April 2003

A methodology for determining the optimal steady suction distribution for the delay of transition in a boundary layer is presented. The flow state is obtained from the coupled system of boundary layer equations and parabolized stability equations (PSE), to account for the spatially developing nature of the flow. The wall suction is defined by an optimal control procedure based on the iterative solution of the equations for the state and the dual state; the latter is available from the adjoint boundary layer equations and the adjoint PSE. The technique is applied to the control of two-dimensional Tollmien–Schlichting (TS) waves. Results show that the onset of the instability can be significantly postponed and/or the growth rate considerably reduced by applying an appropriate suction through the whole wall length, in a wide frequency band. Control over panels of finite length completes the study and brings useful, preliminary information on the practicality of the approach in view of implementation. Finally, a simplified methodology which does not rely on the PSE is discussed, based on the minimization of the shape factor. Satisfactory results are achieved with this simpler approach which might, thus, constitute a method of choice when results are needed rapidly, i.e., during on-line control of TS waves.
Diffusivities and front propagation in sedimentation
Authors: Mucha P. J. , Brenner M. P.
Published: April 2003

Continuum models for particles sedimenting in a fluid often assume that the diffusivity is a local function of the particulate volume fraction. Since the hydrodynamically induced diffusivity is a result of the velocity fluctuations of particles, the recent identification [e.g., Tee et al., Phys. Rev. Lett. 89, 054501 (2002)] of particle density stratification as a controlling parameter for the velocity fluctuations also extends to the diffusivities. In particular, the stratification control strongly affects the diffusivity in the vicinity of the falling sediment front between particle-laden fluid below and clarified fluid above. The resulting scaling for stratification-controlled diffusivities in creeping flow sedimentation is presented and compares favorably with measurements from dilute-limit particle simulations. Steadily falling concentration profiles for dilute sedimentation with these diffusivities are then presented, and an extension of the model to higher volume fractions is discussed.
Inertia-induced breakup of highly viscous drops subjected to simple shear
Authors: Khismatullin Damir B. , Renardy Yuriko , Cristini Vittorio
Published: April 2003

We investigate the inertia-driven breakup of viscous drops suspended in a less viscous liquid under simple shear. For Stokes flow, it is known that there is a critical value of the viscosity ratio, beyond which breakup does not occur. We find that for viscosity ratios larger than this, inertia can be used as a mechanism of breakup. Inertia increases the angle of tilt of the drops and effectively leads to emulsification for a wider range of viscosity ratios than in Stokes flow.
Langevin models of turbulence: Renormalization group, distant interaction algorithms or rapid distortion theory?
Authors: Laval J.-P. , Dubrulle B. , McWilliams J. C.
Published: April 2003

A new dynamical turbulence model is validated by comparisons of its numerical simulations with fully resolved, direct numerical simulations (DNS) of the Navier–Stokes equations in three-dimensional, isotropic, homogeneous conditions. In this model the small-scale velocities are computed using a Langevin, linear, inhomogeneous, stochastic equation that is derived from a quasi-linear approximation of the Navier–Stokes equations, in the spirit of rapid distortion theory (RDT). The values of the turbulent viscosity involved in our Langevin model are compared with a theoretical prescription based on the renormalization group and the distant interaction algorithms (DSTA) model. We show that the empirical turbulent viscosities derived from simulations of the Langevin model are in good quantitative agreement with the DSTA predictions. Finally, Langevin simulations are compared with DNS and large eddy simulations based on the eddy-damped quasi-normal Markovian closure. The Langevin RDT model is able to reproduce the correct spectrum shape, intermittency statistics, and coherent flow structures for both the resolved and the largest sub-grid scales. It also predicts the evolution of the resolved scales better than the alternative models.
Full velocity-scalar probability density function computation of heated channel flow with wall function approach
Authors: Pozorski Jacek , Wacławczyk Marta , Minier Jean-Pierre
Published: April 2003

A joint velocity-scalar probability density function (PDF) method is presented to model and simulate turbulent flows with passive inert scalars (here temperature). The full PDF approach is applied for wall-bounded flows. In the present work, the boundary conditions are imposed in the logarithmic region and the modeling is therefore performed in the wall-function spirit. The PDF equation is solved by a Monte Carlo method and the whole approach appears as a Lagrangian simulation using stochastic particles. The purpose of the work is to analyze the behavior of classical PDF models in the near-wall region and to develop new particle boundary conditions for the velocity and scalars attached to each particle. First of all, the logarithmic region is described as an equilibrium zone and resulting analytical formulas for second-order temperature–velocity statistics 〈θ2〉, 〈uθ〉, 〈vθ〉 are derived. Boundary conditions for scalars are then developed and formulated in terms of instantaneous particle variables. These results are useful to discuss consistency issues between the formulation of scalar mixing models and the statement of boundary conditions. Finally, heated channel flow is simulated with a stand-alone PDF code for two different heat-flux conditions and results are compared with available direct numerical simulation and experimental data.
The atomic detail of a wetting/de-wetting flow
Authors: Freund Jonathan B.
Published: April 2003

Atomistic simulation is used to study flow in the vicinity of the advancing (wetting) and receding (de-wetting) solid–liquid–vapor contact lines of a two-dimensional (in the mean) liquid drop in thermodynamic equilibrium with its own vapor and moving steadily on an atomically smooth solid surface under the influence of an applied body force. Wetting and de-wetting are found to be asymmetric, counter to some predictions based on linear theory. There is a rolling flow of the liquid in the drop and a dividing streamline extends from the wetting common line into the displaced vapor as observed macroscopically. At the de-wetting common line there is substantial evaporation, which alters the flow field behind the drop. Evaporation is also observed above the advancing contact line, but there is a net mass flux into the drop right at the contact line. There is evidence of slip flow just in advance of the wetting contact line, but not at the de-wetting line. Results are discussed in the context of phase-field models.
From stationary to complex time-dependent flows at moderate Rayleigh numbers in two-dimensional annular thermal convection
Authors: Net Marta , Alonso Arantxa , Sánchez Juan
Published: April 2003

Two-dimensional nonlinear thermal convection in a cylindrical annulus is numerically analyzed for a Boussinesq fluid of low Prandtl number σ=0.025. For a fixed value of the radius ratio, η=0.3, different types of steady columnar patterns are found. The stability of these convection patterns and the spatial interaction between them, which result in the formation of mixed modes, are investigated by considering the full nonlinear set of Navier–Stokes equations. Special attention is paid to the strong spatial interaction of the initially unstable modes with wavenumbers n=2 and n=4, which leads, through global bifurcations, to multiple stable quasi-periodic states of the system. A detailed picture of the nonlinear dynamics until temporal chaotic patterns set in is presented and understood in terms of local and global symmetry-breaking bifurcations of the O(2)-symmetric system.
Tilted media in a perpendicular recording system for high areal density recording
Authors: Zou Y. Y. , Wang J. P. , Hee C. H. , Chong T. C.
Published: April 2003

To overcome the high sensitivity of the signal-to-noise ratio (SNR) to the switching field distribution in the perpendicular media, we proposed to use tilted media in the perpendicular recording system in this letter. It was found that a much better tolerance of the easy-axis distribution could be achieved when tilted media were used instead of perpendicular media in a perpendicular recording system. We then analyzed the range of the switching field at the freezing points, and found that high Ku magnetic material is feasible in tilted media. The results indicated that the areal density could be more than 62% higher than that of perpendicular media when 45° easy-axis tilted media with Ku=7.0×106 erg/cm3 were used. Finally, simulation of the switching dynamics revealed that a much faster magnetization switching could be achieved in 45°-tilted media than in perpendicular media.
Analysis of self-focusing beams of concentric symmetry in As2S3 glass using a near-field scanning optical microscope
Authors: Lee Jaekwang , Park Soon-Bin , Cho Kyuman , Song Ki-Bong
Published: April 2003

The self-focusing (SF) effect for concentric laser beams passing through As2S3 glass was studied using a near-field scanning optical microscope. In the case of radially symmetric laser beams at the wavelength 633 nm, there was no special bright spot caused by the SF effect at local areas, except for the central part of the light beam. The minimum beam size that was arrested by nonparaxiality was less than 0.3 μm, which is approximately equal to the wavelength of light in the As2S3 glass, 0.265 μm. Qualitatively, the experimental results agreed with the theoretical prediction of the nonparaxial approximation.
Electro-optic sampling system with a single-crystal 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate sensor
Authors: Zheng X. , Wu S. , Sobolewski Roman , Adam R. , Mikulics M. , Kordoš P. , Siegel M.
Published: April 2003

We present an electro-optic (EO) sampling system based on an organic ionic salt crystal, 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate (DAST) transducer. Compared to LiTaO3, the DAST lower dielectric permittivity and much higher electro-optic coefficient dramatically improveme electric-field coupling into the EO crystal, which results in a much better signal-to-noise ratio of the sampling system. Submillivolt signals can be easily measured with the DAST sensor. Time resolution of the DAST-based EO system is the same as that of the LiTaO3-based sampler.
Stretchable gold conductors on elastomeric substrates
Authors: Lacour Stéphanie Périchon , Wagner Sigurd , Huang Zhenyu , Suo Z.
Published: April 2003

Stripes of thin gold films are made on an elastomeric substrate with built-in compressive stress to form surface waves. Because these waves can be stretched flat they function as elastic electrical conductors. Surprisingly, we observe electrical continuity not only up to an external strain of ∼2% reached by stretching the films first flat (∼0.4%) and then to the fracture strain of free-standing gold films (∼1%), but up to ∼22%. Such large strains will permit making stretchable electric conductors that will be essential to three-dimensional electronic circuits.
Polarization dynamics in a ferroelectric relaxor excited by short pulses
Authors: Boscolo I. , Cialdi S.
Published: April 2003

Polarization dynamics in a relaxor ferroelectric is investigated by exciting samples, set at different temperatures, with short high voltage (≈1 kV) rectangular pulses (≈200 ns) and then by measuring the charge carried to the sample, i.e., the polarization. Further important information is extracted from the trailing edge of the charge signals, which detect the relaxation of the polarization. Polarization dynamics of zero-field-cooled samples show a logarithmic increase during the voltage pulse plateau and a logarithmic decay after the pulse. The slopes depend on the field amplitude and temperature. The model of reorientable noninteracting polar clusters adequately describes the experimental behaviors.
Waveguide fabrication in phosphate glasses using femtosecond laser pulses
Authors: Chan James W. , Huser Thomas R. , Risbud Subhash H. , Hayden Joseph S. , Krol Denise M.
Published: April 2003

We report on the response of glass to focused femtosecond (fs) laser pulses during waveguide fabrication in a commercial sodium aluminum phosphate glass (Schott IOG-1). Single-pass longitudinal translation of IOG-1 glass with respect to the focused laser beam at a rate of 20 μm/s and pulse energies of 3.5 μJ results in the formation of two waveguides located on opposite sides of the laser-exposed region, which itself does not guide light. This behavior is different from that of the more widely studied silica glass system. The precise location of the waveguides in IOG-1 glass depends on the relative tilt of the fs laser beam with respect to the sample translation direction. Fluorescence imaging of the modified glass using a confocal microscope setup reveals the formation of color center defects in the exposed region but not within the waveguides.
Polarization enhancement and coercive field reduction in W- and Mo-doped Bi3.35La0.75Ti3O12 thin films
Authors: Wang Xusheng , Ishiwara Hiroshi
Published: April 2003

Bi 3.35 La 0.75 ( Ti 1−x W x ) 3 O 12 (BLTW) and Bi3.35La0.75(Ti1−xMox)3O12 (BLTM) (x=0.3%, 0.5%, 1%, and 2%) films were fabricated on Pt/Ti/SiO2/Si〈100〉 structures by a sol-gel method at 750 °C. The B-site substitution with high-valent cations, W6+ or Mo6+, in Bi3.35La0.75Ti3O12 (BLT) enhanced the remanent polarization and reduced the coercive field of the films. The remanent polarization (2Pr) values of the BLTW05 (x=0.5%) and BLTM05 (x=0.5%) films were 26 and 27 μC/cm2, respectively, which were higher than that of BLT (20 μC/cm2). The coercive field (2Ec) values of the BLTW05 and BLTM05 films were 125 and 126 kV/cm, respectively, which were much lower than that of BLT (190 kV/cm) and close to that of SrBi2Ta2O9 (∼110 kV/cm). These films also showed fatigue-free response up to 2×109 switching cycles and lower leakage current densities than 4×10−7 A/cm2 up to 200 kV/cm.