bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of polydispersity on hard sphere crystals
Authors: Phan See-Eng , Russel William B. , Zhu Jixiang , Chaikin Paul M.
Published: July 2002

We use simple models and molecular dynamics simulations to determine the effects of polydispersity δ on the equation of state for hard sphere crystals. Experiments show that the osmotic pressure for poly-(methyl methacrylate) (PMMA) spheres with a poly-(12-hydroxy stearic acid) (PHSA) layer with a 5% polydispersity exceeds the value expected for hard spheres as the volume fraction φ increases, particularly for φ>0.60. Mean field theory predicts a higher osmotic pressure with increasing polydispersity, but the effects are only significant for δ>0.10. Molecular dynamics simulations with δ=0.05 bound the equation of state between a metastable disordered upper limit and a crystalline organized polydisperse (possibly) lower limit. The pressure for the PMMA-PHSA spheres lies close to the organized polydisperse limit, indicating a preference for a crystalline ordered arrangement where smaller particles surround larger ones. Thus, the higher osmotic pressure seen in the equation of state of PMMA-PHSA spheres is a direct effect of polydispersity, manifest as a pronounced reduction in the crystalline close packed volume fraction from φmax(FCC, δ=0)=0.7404 to φmax(FCC, δ=0.1)=0.665. The random close packing φmax(RCP) is almost independent of polydispersity. This leads to a crossing of values of φmax(FCC) and φmax(RCP) and hence a possible terminal polydispersity of 0.12±0.01, consistent with other simulations, theories, and experiments. Since our results do not include size fractionation of the liquid and solid, the exact meaning of this crossing is unclear and its agreement with previously reported terminal polydispersities may be coincidental.
Probing molecular site structure in low-temperature matrices: An EXAFS study of carbonyl sulfide in solid argon
Authors: Roubin P. , Varin S. , Crépin C. , Gauthier-Roy B. , Flank A.-M. , Delaunay R. , Pompa M. , Tremblay B.
Published: July 2002

We present here an EXAFS (extended x-ray absorption fine structure) experiment performed at the K edge of the sulfur atom of OCS isolated in an argon matrix. The aim was to elucidate the structure of the trapping site of the molecule. We show that the argon atoms are located at a mean distance of 3.78 Å from the sulfur atom, with a root mean square variation of the S–Ar distances of about 0.08 Å. Our results indicate that the host atoms occupy a statistical set of positions around the molecule, rather than well-defined sites.
The penalty method for random walks with uncertain energies
Authors: Ceperley D. M. , Dewing M.
Published: July 2002

We generalize the Metropolis et al. random walk algorithm to the situation where the energy is noisy and can only be estimated. Two possible applications are for long range potentials and for mixed quantum-classical simulations. If the noise is normally distributed, we are able to modify the acceptance probability by applying a penalty to the energy difference and thereby achieve exact sampling even with very strong noise. When one has to estimate the variance we have an approximate formula, good in the limit of a large number of independent estimates. We argue that the penalty method is nearly optimal. We also adapt an existing method by Kennedy and Kuti and compare to the penalty method on a one-dimensional double well.
Impact of nonadiabatic coupling between the conically intersecting X̃ 2A1 and Ã 2B2 states of NO2 on the negative ion photoelectron spectra of NO2−
Authors: Mahapatra S. , Köppel H. , Cederbaum L. S.
Published: July 2002

Negative ion photoelectron spectra of NO2− at 266 nm (4.660 eV) and 351 nm (3.532 eV) are theoretically calculated and compared with earlier experimental findings. A diabatic model Hamiltonian with linear vibronic coupling between the conically intersecting X̃ 2A1 and Ã 2B2 states of NO2 is employed for this purpose. While the linear vibronic coupling model successfully reproduces the low resolution experimental photoelectron spectrum at 266 nm, second order coupling terms are necessary for the high resolution spectrum at 351 nm. The vibronic structure of the X̃ 2A1 state of NO2 in the photoelectron spectrum is found to be almost unaffected by the nonadiabatic coupling, whereas, the latter has a strong impact on the vibronic structure of the Ã 2 B2 state. The magnitude of the vibronic coupling constant, which has otherwise been overestimated by earlier theoretical studies, is calculated ab initio and its limits of validity are confirmed by the results from different ab initio calculations.
Linear scaling computation of the Fock matrix. IV. Multipole accelerated formation of the exchange matrix
Authors: Schwegler Eric , Challacombe Matt
Published: July 2002

A new method for the multipole evaluation of contracted Cartesian Gaussian-based electron repulsion integrals is described, and implemented in linear scaling methods for computation of the Hartree–Fock exchange matrix. The new method, which relies on a nonempirical multipole acceptability criterion [J. Chem. Phys. 109, 8764 (1998)], renders the work associated with integral evaluation independent of the basis set contraction length. Benchmark calculations on a series of three-dimensional water molecule clusters and graphitic sheets with highly contracted basis sets indicate that the new method is up to 4.6 times faster than a well optimized direct integral evaluation routine. For calculations involving lower levels of contraction a factor of 2 speedup is typically observed. Importantly, the method achieves these large gains in computational efficiency while maintaining numerical equivalence with standard direct self consistent field theory.
The chemi-ionization of He*(2 1,3S)+Ar,Kr,Xe for collision energies from 0.003 to 6 eV
Authors: Feltgen R. , Ferkel H. , Helbing R. K. B. , Lindinger A. , Pikorz D. , Vehmeyer H.
Published: July 2002

Measurements of the collision energy dependence of the chemi-ionization cross sections of the model systems He*(2 1,3S)+R, R=Ar,Kr,Xe, are presented for the relevant energy range 0.003–6 eV. Except for He*(2 1S)+Xe, all systems show a pronounced minimum of ionization at thermal energies. In the hyperthermal range, however, the two spin systems are in sharp contrast to each other: the triplet systems exhibit a broad saturating maximum of ionization, whereas the singlet systems reveal a shallow shaped second minimum (pronounced for Xe) which can be rationalized within the one-electron model potential calculations of Siska [J. Chem. Phys. 71, 3942 (1979)]. Using an effective single-channel Schrödinger equation, one finds, for all systems, complex potentials depending on distance only which properly model the measured features. A more profound picture appears when differential elastic cross sections from experiments of other groups are included. In the triplet case, differential elastic and chemi-ionization data can be adequately described with one complex potential, i.e., the local approximation holds. This is not found for the singlet systems. The hyperthermal minima instead give evidence that the optical singlet potentials are nonlocal. Various aspects of this striking spin dependence of the characteristics of He*(2 1,3S)+R are discussed. In particular, the breakdown of the local approximation in the singlet case can be rigorously derived.
Proton transfer in 7-hydroxyquinoline⋅(NH3)n solvent clusters
Authors: Bach Andreas , Leutwyler Samuel
Published: July 2002

Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline⋅(NH3)n clusters with n=2–16. For n=2 and 3, the absorption and emission spectra exhibit discrete and narrow bands, characteristic of nonreactive hydrogen bonded ammonia-chain clusters. For n⩾4, the S1←S0 R2PI spectra are completely broadened, with an onset at ≈355 nm. For n=4–7, a weak fluorescence emission is observed, Stokes-shifted by ≈185 nm, with a maximum at ≈540 nm, which shows discrete structure on a broad background. From comparison to fluorescence emission observed in bulk solution, we conclude that S1 state enol→keto tautomerization occurs. For the n⩾7 or 8 clusters, the fluorescence emission spectra become completely unstructured and shift to the blue, peaking at 435–450 nm. This emission indicates the occurrence of either S1-state proton transfer to the ammonia solvent cluster and formation of the 7-HQ anion, or of the ground-state intracluster enol→keto tautomerization reaction.
“Aging” of the structure of crystals of hard colloidal spheres
Authors: Kegel Willem K. , Dhont Jan K. G.
Published: July 2002

We study the development of the structure of crystals of colloidal hard spheres in time when gravity effects are minimal and polydispersity is small (<3%). The initial stacking of the close-packed hexagonal layers that make up the crystals is varied by applying various types of shear stress during nucleation of the crystals. The experimental powder diffraction patterns are consistent with a fraction of a faulted-twinned face-centered cubic (fcc) structure that grows at the expense of randomly stacked crystallites. If a faulted-twinned fcc structure is generated initially, no change is found over a considerable time. The present observations rule out the possibility that a randomly stacked structure is the equilibrium structure of colloidal crystals of (nearly) hard spheres, and point to the thermodynamic or kinetic stability of faulted-twinned fcc crystals in these systems.
Vibrational relaxation of cyanide at the metal/electrolyte interface
Authors: Matranga Christopher , Guyot-Sionnest Philippe
Published: July 2002

The vibrational lifetime of the CN stretching mode has been measured at a Pt(111), polycrystalline Au and polycrystalline Ag electrochemical interface with sum-frequency generation (SFG). The lifetimes on Pt(111) and Au are potential dependent and range from 3 to 8 ps and 10 to 19 ps, respectively, with the longer lifetimes occurring at more positive potentials. Results for the Ag surface show solvent dependent lifetimes of ≈28 ps in a 0.1 M solution of NaClO4 in H2O and ≈60 ps when the solvent is changed to D2O. The results on all of these metals are surprising because of the comparatively short (1.5–3.0 ps) lifetimes predicted and measured for isoelectronic CO. The lifetimes on Ag are remarkable because of their similarity to those of freely-solvated CN− ions. We discuss our results in the context of a charge transfer to the substrate and also consider an image dipole mechanism.
Characterizing nanoparticle interactions: Linking models to experiments
Authors: Ramakrishnan S. , Zukoski C. F.
Published: July 2002

Self-assembly of nanoparticles involves manipulating particle interactions such that attractions are on the order of the average thermal energy in the system. If the self-assembly is to result in an ordered packing, an understanding of their phase behavior is necessary. Here we test the ability of simple pair potentials to characterize the interactions and phase behavior of silico tungstic acid (STA), a 1.2 nm particle. The strength of interaction is controlled by dispersing STA in different background salt concentrations. The experimental variables used in characterizing the interactions are the osmotic compressibility (dΠ/dρ), the second virial coefficient (B2), relative solution viscosity (η/ηc), and the solubility (ρσ3)sat. Various techniques are then developed to extract the parameters of square well, the adhesive hard sphere (AHS), and the Yukawa pair potentials that best describe the experimental data. The AHS model describes the solution thermodynamic behavior only where the system is weakly attractive but, as would be expected, fails when long range repulsions or nonmonotonic pair potentials become important. Model free representations are presented which offer the opportunity to extract pair potential parameters.
The rotational spectra of the HCCCNH+, NCCNH+, and CH3CNH+ ions
Authors: Gottlieb C. A. , Apponi A. J. , McCarthy M. C. , Thaddeus P. , Linnartz H.
Published: July 2002

The rotational spectra of the HCCCNH+, NCCNH+, and CH3CNH+ ions have been observed in a supersonic molecular beam by Fourier transform microwave (FTM) spectroscopy. The rotational and centrifugal distortion constants were determined for all three, and the nitrogen quadrupole hyperfine coupling constants for HCCCNH+ and NCCNH+. From the respective Doppler shifts, it is found that the velocities of the ions are 3% larger than those of the parent unprotonated molecules, and the linewidths are greater by about 50%. The concentration of the ions near the nozzle is approximately 1011 cm−3, which is sufficiently high to be detectable in the visible and the IR by present laser techniques.
Some properties of hyperspherical harmonics
Authors: Wen Zhen-Yi , Avery John
Published: July 2002

A general formula is given for the canonical decomposition of a homogeneous polynomial of order λ in m variables into a sum of harmonic polynomials. This formula, which involves successive applications of the generalized Laplace operator, is proved in the Appendix. It is shown that the group-theoretical method for constructing irreducible Cartesian tensors follows from the general formula for canonical decomposition. The relationship between harmonic polynomials and hyperspherical harmonics is discussed, and an addition theorem for hyperspherical harmonics is derived. An expansion of a many-dimensional plane wave in terms of Gegenbauer polynomials and Bessel functions is derived and used to construct bicenter expansions of arbitrary functions in many-dimensional spaces. Finally, a formula is derived for the 3λ coefficients of hyperspherical harmonics. These coefficients give the values of integrals involving the products of three harmonics.
Elementary representations and intertwining operators for SU(2, 2). I
Authors: Dobrev V. K.
Published: July 2002

The structure of the group SU(2, 2) and of its Lie algebra is studied in detail. The results will be applied in subsequent parts devoted to the explicit construction of elementary representations of SU(2, 2) induced from different parabolic subgroups and of the intertwining operators between these representations. A summary of some results of Parts II and III is given.
Displacement theorems for spherical solutions of the linear Navier–Stokes equations
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

Displacement theorems are presented for the solutions in spherical coordinates of the linear Navier–Stokes equations for time-independent flow in an incompressible viscous fluid. The theorems allow one to expand spherical solutions about a chosen center into spherical solutions centered elsewhere.
Killing vectors of static spherically symmetric metrics
Authors: Bokhari Ashfaque H. , Qadir Asghar
Published: July 2002

An error in a previous theorem [J. Math. Phys. 28, 1019 (1987); 29, 525 (1988)] is corrected and the theorem is extended.
The Korteweg–de Vries hierarchy and long water-waves
Authors: Kraenkel R. A. , Manna M. A. , Pereira J. G.
Published: July 2002

By using the multiple scale method with the simultaneous introduction of multiple times, we study the propagation of long surface-waves in a shallow inviscid fluid. As a consequence of the requirements of scale invariance and absence of secular terms in each order of the perturbative expansion, we show that the Korteweg–de Vries hierarchy equations do play a role in the description of such waves. Finally, we show that this procedure of eliminating secularities is closely related to the renormalization technique introduced by Kodama and Taniuti.
Quantization of differential calculi on universal enveloping algebras
Authors: van den Hijligenberg N. , Martini R. , Post G. F.
Published: July 2002

A method to construct differential calculi on quantized universal enveloping algebras is discussed. These differential calculi are obtained by quantizing calculi on ‘‘classical’’ enveloping algebras provided with appropriate co-Poisson structures. The procedure is demonstrated by applying it to the standard quantizations of the Heisenberg algebra and the algebra gl(2).
A Riemannian generalization of a result of DeWitt concerning Ricci-flat Lorentz metrics
Authors: Gil-Medrano Olga
Published: July 2002

For any given nonzero real number, α, and any curve from some open interval, I, into the space ℳ, of Riemannian metrics on a compact manifold M, a metric, g̃, on the manifold I×M can be constructed in a natural way; the metric is Riemannian for α positive and Lorentzian for α negative. The aim of this paper is to show that g̃ is an Einstein metric, with constant λ, if and only if the curve of metrics is a solution of a certain Lagrangian on ℳ defined in terms of the DeWitt metric on ℳ, the total scalar curvature functional and the values α and λ. This result was obtained by DeWitt [Phys. Rev. 160, 1113 (1967)], for the case where the dimension of M is three, α is negative and λ=0, in the context of a formulation of Einstein equations of evolution as a dynamical system. DeWitt metric is a member of a one parameter family of pseudometrics on ℳ; it is shown here that, no other such a metric can be used to describe the relationship of the (n+1) geometry of I×M with the dynamics on ℳ, so a characterization of the DeWitt metric is obtained.
Statistics of resonance poles, phase shifts and time delays in quantum chaotic scattering: Random matrix approach for systems with broken time-reversal invariance
Authors: Fyodorov Yan V. , Sommers Hans-Jürgen
Published: July 2002

Assuming the validity of random matrices for describing the statistics of a closed chaotic quantum system, we study analytically some statistical properties of the S-matrix characterizing scattering in its open counterpart. In the first part of the paper we attempt to expose systematically ideas underlying the so-called stochastic (Heidelberg) approach to chaotic quantum scattering. Then we concentrate on systems with broken time-reversal invariance coupled to continua via Mopen channels; a=1,2,…,M. A physical realization of this case corresponds to the chaotic scattering in ballistic microstructures pierced by a strong enough magnetic flux. By using the supersymmetry method we derive an explicit expression for the density of S-matrix poles (resonances) in the complex energy plane. When all scattering channels are considered to be equivalent our expression describes a crossover from the χ2 distribution of resonance widths (regime of isolated resonances) to a broad power-like distribution typical for the regime of overlapping resonances. The first moment is found to reproduce exactly the Moldauer–Simonius relation between the mean resonance width and the transmission coefficient. Under the same assumptions we derive an explicit expression for the parametric correlation function of densities of eigenphases θa of the S-matrix (taken modulo 2π). We use it to find the distribution of derivatives τa=∂θa/∂E of these eigenphases with respect to the energy (“partial delay times”) as well as with respect to an arbitrary external parameter. We also find the parametric correlations of the Wigner–Smith time delay τw(E)=(1/M)∑a ∂θa/∂E at two different energies E−Ω/2 and E+Ω/2 as well as at two different values of the external parameter. The relation between our results and those following from the semiclassical approach as well as the relevance to experiments are briefly discussed.
Homothetic motions of spherically symmetric space–times
Authors: Ahmad Daud , Ziad M.
Published: July 2002

The homotheties of spherically symmetric space–times admitting maximal isometry groups larger than SO(3) are found along with their metrics, using the homothety equations and without imposing any restriction on the stress-energy tensor. It turns out that there are either 11 or 7 or 5 homotheties. For the space–times with SO(3) as a maximal group, solution is provided in the form of derivatives of metric coefficients, which then requires a further classification, for example, according to different types of stress-energy tensor, as has been done by Eardley [Commun. Math. Phys. 37, 287 (1974)], Cahill and Taub [Commun. Math. Phys. 21, 1 (1971)], and McIntosh [Phys. Lett. A 50, 429 (1975)].
On the derivation of boundary integral equations for scattering by an infinite two-dimensional rough surface
Authors: DeSanto J. A. , Martin P. A.
Published: July 2002

A plane acoustic wave is incident upon an infinite, rough, impenetrable surface S. The aim is to find the scattered field by deriving a boundary integral equation over S, using Green’s theorem and the free-space Green’s function. This requires careful consideration of certain integrals over a large hemisphere of radius r; it is known that these integrals vanish as r→∞ if the scattered field satisfies the Sommerfeld radiation condition, but that is not the case here—reflected plane waves must be present. It is shown that the well-known Helmholtz integral equation is not valid in all circumstances. For example, it is not valid when the scattered field includes plane waves propagating away from S along the axis of the hemisphere. In particular, it is not valid for the simplest possible problem of a plane wave at normal incidence to an infinite flat plane. Some suggestions for modified integral equations are discussed.
Wave growth and spreading of a turbulent spot in plane Poiseuille flow
Authors: Henningson Dan S.
Published: July 2002

Previous numerical simulations have shown that waves at the wing tips of plane Poiseuille spots grow rapidly as they are overtaken by the turbulent region. Kinematic wave theory is used to investigate the growth mechanism by calculating the amplitude of waves along rays defined by the group velocity. A dispersion relation is found by solving an extended form of the Orr–Sommerfeld equation using spatially averaged mean velocity profiles. The results indicate that the waves grow because of a combination of wave energy focusing and ‘‘cross-flow’’ instability. The exponential growth resulting from the ‘‘cross-flow’’ inflectional profiles is the dominating factor. Good agreement is obtained between the characteristics of the waves calculated by the theory and those found in the numerical simulation.
The statistics of stretching and stirring in chaotic flows
Authors: Muzzio F. J. , Swanson P. D. , Ottino J. M.
Published: July 2002

The statistics of stretching and stirring in time-periodic chaotic flows is studied numerically by following the evolution of stretching of O(105) points. The ratio between stretchings accumulated by each point at successive periods is referred to as a multiplier, and the total stretching is the product of multipliers. As expected, the mean stretching of the population increases exponentially whereas the probability density function of multipliers converges—in just two periods or so—to a time-invariant distribution. There is, however, a considerable degree of order in the spatial distribution of stretching in spite of conditions of global chaos. The self-correlation of multipliers shows as well considerable structure and often there are segregated populations of points: the largest population consists of points that experience extensive stretching, efficient stirring, and have a distribution of stretching values that evolves asymptotically—in about ten periods—into a limiting time-invariant scaling distribution. The remaining points experience slow stretching and, although they also exhibit scaling behavior, are effectively segregated from the rest of the system in the time scale of our simulations.
Variable viscosity effects on convective instability in superposed fluid and porous layers
Authors: Chen Falin , Lu Jay W.
Published: July 2002

The variable viscosity effects on the stability characteristics of the motionless fluid in superposed fluid and porous layers bounded by conducting rigid boundaries are investigated. It is found that the stability characteristics in terms of the critical Rayleigh number Rmc, the critical wave number amc, and the critical flow patterns, are profoundly influenced by the viscosity variation, which is represented by γ=ln(νmax/νmin), where ν is the kinematic viscosity of the fluid. The intrinsic features of the critical flow are mainly determined by both the values of γ and ζ, the depth ratio of the fluid layer depth to the porous layer depth. Three critical flow patterns are identified on the basis of varying γ and ζ. The transition between any two of these three flow patterns is a bimodal instability. The present results may provide valuable information for understanding the convective motion of the melt during the solidification process of alloys.
Saturation of stimulated Raman scattering by Langmuir and ion-acoustic wave coupling
Authors: Kolber T. , Rozmus W. , Tikhonchuk V. T.
Published: July 2002

Studies of stimulated Raman scattering (SRS) based on the one-dimensional Zakharov–Maxwell equations for homogeneous, finite plasmas are presented for a wide range of parameters relevant to current laser–plasma interaction experiments. It was found that the primary mechanism responsible for saturation of SRS is the parametric decay instability (PDI) of the resonantly driven Langmuir wave. The different spatiotemporal evolution of SRS (convective) and PDI (absolute) leads to new physics in SRS nonlinear evolution, including disruption of PDI cascade, localization of Langmuir fields, and burstlike behavior of SRS reflectivity. The asymptotic saturation levels are well approximated by a simple scaling law, which is proportional to the PDI threshold and depends on the SRS convective gain factor.
Observation of the backward electrostatic ion-cyclotron wave
Authors: Goree J. , Ono M. , Wong K. L.
Published: July 2002

The backward branch of the electrostatic ion-cyclotron wave has been observed for the first time. The wave, which was driven by a phased antenna structure inserted in a neon plasma, exists in the parameter ranges 2Ti/mi≪(ω/k∥)2≪2Te/me, nΩi<ω<(n+1)Ωi, Te≳Ti, and ωpi>Ωi. Double-tip probe interferometry data agree with the theoretical dispersion relation. The antenna couples into the wave more readily on the side of the antenna where it has its smallest wavenumber.
Nozzle flows with nonequilibrium condensation
Authors: Clarke Joseph H. , Delale Can F.
Published: July 2002

The compressible nozzle flow of a mixture of a condensible vapor and an inert carrier gas with nonequilibrium condensation is considered. An asymptotic method is used with respect to two disparate parameters appearing in the nonequilibrium condensation rate equation. A high activation limit and another limit consideration that signify a large nucleation time followed by a small droplet growth time are employed. Under the limits, the coupled integral rate equation is so controlled by the activation function of homogeneous nucleation over its six zones that there appears to be a one-to-one correspondence between these zones and the six characteristic condensation zones of the nozzle flow itself. The six zones are the initial growth of condensate, further growth, rapid growth, onset, nucleation with growth, and droplet growth zone. The last zone can be greatly simplified if terms that imply a reasonably sizable order of error are neglected. This results in the rectilinear, one-dimensional flow equations of a normal shock wave structured or resisted by droplet growth.
Natural convection of water near its density maximum in a rectangular enclosure: Low Rayleigh number calculations
Authors: Nansteel M. W. , Medjani K. , Lin D. S.
Published: July 2002

The convection of cold water in the vicinity of its density maximum in a rectangular enclosure is studied in the limit of small Rayleigh number. Results for the stream function (to lowest order) and the first corrections (for convective effects) to the temperature field and heat transfer are obtained. Results are discussed in the context of a single parameter, which fixes the orientation of the wall temperatures with respect to the extremum temperature. The effects of changing the enclosure aspect ratio are also discussed.
Drag reduction caused by the injection of polymer thread into a turbulent pipe flow
Authors: Usui Hiromoto , Maeguchi Katsuhiro , Sano Yuji
Published: July 2002

Drag reduction caused by the injection of concentrated polymer solutions into a turbulent pipe flow was studied. Measurements were made of the radial distribution of fluctuating velocities by means of video image analysis. The results showed that a higher velocity was observed for injected polymer threads and both the radial fluctuation and the Reynolds stress were significantly suppressed. It was suggested that the wall turbulence structure might be controlled by suppressing the large scale turbulent motion in the turbulent core region.
On the structure and control of near wall turbulence
Authors: Jiménez Javier
Published: July 2002

A simplified self-sustaining cycle is proposed for the events in the near wall region of a turbulent boundary layer. An approximate quantitative analysis of the resulting model predicts the right dimensions for the sublayer streaks and for other flow structures. The model is checked further by applying it to a set of numerical simulations in which the longitudinal and transverse no-slip conditions are applied at different positions with respect to the wall, and to the analysis of flow over riblets. Substantial changes in the flow statistics are obtained, including drag reductions, and the resulting trends are also predicted correctly by the model.
Hydrodynamics of a one-dimensional granular medium
Authors: Sela N. , Goldhirsch I.
Published: July 2002

The question whether one-dimensional granular systems can be described by hydrodynamic equations is the main theme of the present work. Numerical simulations are used to create a database with which theory is compared. The system investigated in the numerical work is that of a one-dimensional collection of point particles colliding inelastically. The dependence of the dynamical properties on both the degree of inelasticity and the number of particles is investigated. A hydrodynamic theory which describes the large-scale motion of such systems has been developed. It is shown that the standard set of hydrodynamic fields (density, velocity, and granular temperature) is insufficient for this purpose and that an additional hydrodynamic field corresponding to the third moment of the fluctuating velocity field must be added to that set. The results of a linear stability analysis of the derived hydrodynamic equations are in a close agreement with those of the numerical simulations. The question of the effects of velocity correlations on the hydrodynamics is addressed as well. It is shown that these correlations, though not negligible, do not affect the hydrodynamic equations. The form of the single particle initial distribution function is shown to slightly affect the form of the hydrodynamic equations for transient times. Except for this minor effect the hydrodynamic equations possess a universal form. Possible implications for higher dimensional systems are mentioned.
Hopf bifurcation in wakes behind a rotating and translating circular cylinder
Authors: Hu Guo-Hui , Sun De-Jun , Yin Xie-Yuan , Tong Bing-Gang
Published: July 2002

A low-dimensional Galerkin method, initiated by Noack and Eckelmann [Physica D 56, 151 (1992)], for the prediction of the flow field around a stationary two-dimensional circular cylinder in a uniform stream at low Reynolds number is generalized to the case of a rotating and translating cylinder. The Hopf bifurcation describing the transition from steady to time-periodic solution is investigated. A curve indicating the transitional boundary is given in the two-dimensional parameter plane of Reynolds number Re and rotating parameter α. Our results show that rotation may delay the onset of vortex street and decrease the vortex-shedding frequency.
Two-dimensional velocity profiles and laminar boundary layers in flowing soap films
Authors: Rutgers M. A. , Wu X-l. , Bhagavatula R. , Petersen A. A. , Goldburg W. I.
Published: July 2002

In this study we examine laminar velocity profiles of freely suspended flowing soap films. We introduce a new device which supports large uniform films for indefinite periods of time. The geometry of the flow is two-dimensional (2D), yet the measured velocity profiles depart from ideal 2D behavior. The main reason for this departure is that the soap film experiences an air drag force across its entire surface. Describing the air with Prandtl boundary layer theory, we predict the observed flow patterns with good accuracy. The downstream development of the profiles is self similar. Our models set an apparent upper limit on the film 2D viscosity of 5⋅10−6 surface poise for dilute soap concentrations. This measurement implies that the surfactant layers on the film may not contribute measurably to the 2D viscosity. For higher soap and glycerol concentrations the opposite appears to be true.
Hydrodynamic stability of flow between rotating porous cylinders with radial and axial flow
Authors: Johnson Eric C. , Lueptow Richard M.
Published: July 2002

A linear stability analysis was carried out for axial flow between a rotating porous inner cylinder and a concentric, stationary, porous outer cylinder when radial flow is present for several radius ratios. The radial Reynolds number, based on the radial velocity at the inner cylinder and the inner radius, was varied from −15 to 15, and the axial Reynolds number based on the mean axial velocity and the annular gap was varied from 0 to 10. Linear stability analysis for axisymmetric perturbations results in an eigenvalue problem that was solved using a numerical technique based on the Runge–Kutta method combined with a shooting procedure. At a given radius ratio, the critical Taylor number at which Taylor vortices first appear for radial outflow decreases slightly for small positive radial Reynolds numbers and then increases as the radial Reynolds number becomes more positive. For radial inflow, the critical Taylor number increases as the radial Reynolds number becomes more negative. For a given radial Reynolds number, increasing the axial Reynolds number increases the critical Taylor number for transition very slightly. The critical wave velocity decreases slightly for small positive radial Reynolds numbers, but increases for larger positive and all negative radial Reynolds numbers. The perturbed velocities are very similar to those for no axial flow.
Effects of pressure fluctuations on turbulence growth in compressible homogeneous shear flow
Authors: Hamba Fujihiro
Published: July 2002

Direct numerical simulation results are used to study compressibility effects on turbulent energy growth in homogeneous shear flow. Normalized amplitude of the pressure fluctuations is shown to decrease as the turbulent Mach number increases. This decrease causes the reduction in the pressure–strain term, changes the anisotropy of the Reynolds stress, and reduces the growth rate of turbulent kinetic energy as is the case of mixing layers. The transport equation for the pressure variance is examined in detail. It is shown that when the turbulent Mach number is high the pressure-variance dissipation term is not negligible and contributes to the reduction in the pressure fluctuations. The pressure-variance dissipation is closely related to the high-wavenumber part of the pressure-variance spectrum. Profile of the spectrum is observed to depend on the turbulent Mach number. A statistical theory called the two-scale direct-interaction approximation is applied to obtain model expressions for pressure-related terms. The pressure–dilatation correlation is a dominant term in the pressure-variance equation; its model expression as derived contains the material derivative of turbulent kinetic energy. This quantity represents nonequilibrium properties of turbulent field and can explain the very small value of the pressure dilatation in boundary layers and channel flows. The model expression for the pressure–strain correlation is also derived; the normalized pressure variance can be a parameter that describes compressibility effect on the pressure strain.
Electrically driven convection in a thin annular film undergoing circular Couette flow
Authors: Daya Zahir A. , Deyirmenjian V. B. , Morris Stephen W.
Published: July 2002

We investigate the linear stability of a thin, suspended, annular film of conducting fluid with a voltage difference applied between its inner and outer edges. For a sufficiently large voltage, such a film is unstable to radially driven electroconvection due to charges which develop on its free surfaces. The film can also be subjected to a Couette shear by rotating its inner edge. This combination is experimentally realized using films of smectic A liquid crystals. In the absence of shear, the convective flow consists of a stationary, azimuthally one-dimensional (1D) pattern of symmetric, counter-rotating vortex pairs. When Couette flow is applied, an azimuthally traveling pattern results. When viewed in a co-rotating frame, the traveling pattern consists of pairs of asymmetric vortices. We calculate the neutral stability boundary for arbitrary radius ratio α and Reynolds number Re of the shear flow, and obtain the critical control parameter Rc(α,Re) and the critical azimuthal mode number mc(α,Re). The Couette flow suppresses the onset of electroconvection, so that Rc(α,Re)>Rc(α,0). The calculated suppression is compared with experiments performed at α=0.56 and 0⩽ℛe⩽0.22.
Generalized weighting scheme for δf particle-simulation method
Authors: Hu Genze , Krommes John A.
Published: July 2002

An improved nonlinear weighting scheme for the δf method of kinetic particle simulation is derived. The method employs two weight functions to evolve δf in phase space. It is valid for quite general, non-Hamiltonian dynamics with arbitrary sources. In the absence of sources, only one weight function is required and the scheme reduces to the nonlinear algorithm developed by Parker and Lee [Phys. Fluids B 5, 77 (1993)] for sourceless simulations. (It is shown that their original restriction to Hamiltonian dynamics is unnecessary.) One-dimensional gyrokinetic simulations are performed to show the utility of this two-weight scheme. A systematic kinetic theory is developed for the sampling noise due to a finite number of marker trajectories. The noise intensity is proportional to the square of an effective charge qeff=̇q(w̄/𝒟), where w̄ ∼δf/f is a typical weight and 𝒟 is the dielectric response function.
Experimental studies of the radial diffusion of energetic electrons in lower-hybrid wave heated plasmas in the Alcator-C tokamak
Authors: Knowlton S. , Porkolab M. , Takase Y.
Published: July 2002

An upper limit for the cross-field spatial diffusion coefficient of energetic electrons in moderate density tokamak discharges has been measured. The energetic electrons were generated by square wave-modulated lower-hybrid (LH) wave injection. The decay of the fast electron population for electron energies εd≥100 keV following LH radio-frequency (RF) power shutoff was inferred from the time dependence of the radial profiles of the hard x-ray bremsstrahlung emission. The experimental results were fitted to a diffusive model that includes collisional losses and electric field acceleration. The fast electron diffusion coefficient consistent with the data is Df≤0.2 m2/s, at least in the inner half of the plasma. The corresponding global confinement time of the energetic electrons is estimated to be at least a factor of 2 greater than the energy confinement time of the bulk plasma in discharges of density n̄e=8×1019 m −3.
Analytical model of the ‘‘tilting’’ instability
Authors: Rosenbluth M. N. , Shapiro V. D.
Published: July 2002

Tilting instability is an instability of convective motion in two-dimensional (2-D) ideal fluid transforming convection into sheared flow. An analytical model of the tilting instability is proposed that clearly exhibits inverse cascade phenomenon, conserving both energy and enstrophy. Obtained solution describes the evolution of the nonlinear stage in which initial fluid convection is transformed completely into the large-scale flow.
Nonlinear coupling of disparate helical tearing modes
Authors: Li Ding , Huo Yuping
Published: July 2002

An analytical model has been developed for the nonlinear interaction of linear tearing modes with different helicities in cylindrical geometry. The linear tearing modes are nonlinearly coupled together by the v×B induced electrical field as soon as they exist. According to the standard scaling of linear tearing mode, the nonlinear coupling is mainly through the convective term in evolution equation of poloidal magnetic flux perturbation at resistive layer. The set of nonlinear equations, therefore, can be derived for the time evolution of the flux perturbations of nonlinear coupling modes by asymptotic matching to eliminate the space variable. The nonlinear coupling effect depends on the relative amplitudes of the tearing modes and the nonlinear coupling parameters {αmn}, which are determined by the relative slopes of equilibrium current density in singular layers. The marginally stable m/n mode could be destabilized by the nonlinear coupling with the other modes only if αmn<0. The flux perturbations include both the exponential growth and algebraical evolution. The latter is caused by the nonlinear coupling and becomes more important even dominant when the flux perturbations increase.
Singular surfaces in the open field line region of a diverted tokamak
Authors: Reiman A.
Published: July 2002

The structure of the open field lines of a slightly nonaxisymmetric, poloidally diverted tokamak is explored by numerical integration of the field line equations for a simple model field. In practice, the nonaxisymmetry could be produced self-consistently by the nonlinear evolution of a free-boundary magnetohydrodynamic (MHD) mode, or it could be produced by field errors, or it could be imposed externally by design. In the presence of a nonaxisymmetric perturbation, the tokamak is shown to develop open field line regions of differing topology separated by singular surfaces. It is argued that the singular surfaces can be expected to play a role analogous to that of rational toroidal flux surfaces, in terms of constraining ideal MHD perturbations and thus constraining the free-energy that can be tapped by ideal MHD instabilities. The possibility of active control of free-boundary instabilities by means of currents driven on the open singular surfaces, which are directly accessible from the divertor plates, is discussed.
Trapped electron correction to beam driven current in general tokamak equilibria
Authors: Lin-Liu Y. R. , Hinton F. L.
Published: July 2002

In the limit that the electron thermal velocity greatly exceeds the fast ion velocity for electrical currents driven by neutral beams, the trapped electron correction to the Ohkawa current and the electron density gradient contribution to bootstrap current are shown to share the same transport coefficient in the banana regime. Therefore, existing analytic expressions for the bootstrap coefficient valid for arbitrary aspect ratio tokamaks can also be used to calculate the trapped electron effect.
Covariant descriptions of the relativistic guiding-center dynamics
Authors: Beklemishev Alexei , Tessarotto Massimo
Published: July 2002

The relativistic guiding center dynamics of charged particles is described in terms of noncanonical variables. The gyrokinetic transformation is obtained using the perturbative Lagrangian approach with a fully relativistic, four-dimensional covariant formulation. It is shown that the definition of the ignorable gyrophase (as well as those of the magnetic moment and the gyrocenter energy) is not unique, and allows for several free functions in the gyrokinetic transformation. This freedom can be interpreted as a choice of the reference frame. One of these frames, namely that moving with the relativistic version of the E_×B_-drift velocity, generates the simplest and intuitive description.
Atomic deuterium passivation of boron acceptor levels in silicon crystals
Authors: Mikkelsen J. C.
Published: July 2002

B-doped Si wafers were subjected to atomic-deuterium (D) plasmas to simulate the reactions of atomic hydrogen with substitutional B acceptor levels. Secondary ion mass spectrometry was used to profile the in- and out-diffusion of D, and spreading resistance was used to measure the distribution of net acceptor levels after deuteration and subsequent thermal annealing. A direct correlation was found between passivation of B acceptors and the presence of atomic deuterium. The passivated acceptor levels can be restored by thermal annealing above 200 °C. In addition, B enhances the production of another D-containing species, possibly molecular D2, which is not readily removed from crystalline Si below 400 °C. These results are contrasted to other reports of interaction of atomic H with p-type dopants, as well as with structural and chemical defects.
Electric field dependence of hydrogen neutralization of shallow-acceptor impurities in single-crystal silicon
Authors: Johnson N. M.
Published: July 2002

An electric field applied to p-type single-crystal silicon during hydrogenation dramatically affects the neutralization of shallow-acceptor impurities (e.g., boron). This is demonstrated with capacitance-voltage and secondary-ion mass spectrometry measurements on n+-p junction diodes that were reverse biased during exposure to monatomic deuterium at elevated temperatures. The results reveal that there is no neutralization within the field-induced space-charge layer, which extends from the metallurgical p-n junction into the p-type material. Rather, deuterium-boron pairing occurs in the electrically neutral material beginning at the edge of the depletion layer. Consequently, the distribution of deuterium (hydrogen) is also altered by the applied field. The effect of an electric field on hydrogen-boron pairing in p-type silicon is discussed in terms of the role of free holes in the chemical reaction and possible charge states of the diffusing hydrogen atoms.
Passivation of recombination centers in n-WSe2 yields high efficiency (>14%) photoelectrochemical cell
Authors: Tenne R. , Wold A.
Published: July 2002

It is shown that careful photoetching of n-WSe2 leads to a large improvement in its photoresponse. Conversion efficiencies in excess of 14% were obtained in polyiodide solution under simulated solar light. A significant red shift in the photocurrent spectrum is observed after photoetching as well as hexagonal etch pits which reduce the reflectivity of the surface.
Field drift of the hydrogen-related, acceptor-neutralizing defect in diodes from hydrogenated silicon
Authors: Tavendale A. J. , Alexiev D. , Williams A. A.
Published: July 2002

Field drift of the hydrogen-related, acceptor-neutralizing defect has been detected in reverse-biased Schottky barrier and junction diodes made from plasma-hydrogenated, p-type, boron-doped silicon. Significant differences in diffusion depths and drift rates between hydrogenated and deuterated silicon indicate that the mobile neutralizing species is possibly uncomplexed monoatomic hydrogen with a donor level above the mid band gap. Hydrogen-boron pairing explains qualitatively the boron acceptor concentration profiles in hydrogen-neutralized, p-type silicon.
Strong polarization-sensitive electroabsorption in GaAs/AlGaAs quantum well waveguides
Authors: Weiner J. S. , Miller D. A. B. , Chemla D. S. , Damen T. C. , Burrus C. A. , Wood T. H. , Gossard A. C. , Wiegmann W.
Published: July 2002

We report the first measurements of perpendicular field electroabsorption (quantum confined Stark effect) in GaAs/AlGaAs quantum wells for light propagating parallel to the plane of the layers. This geometry is well suited for integrated optics. The absorption edge shifts to longer wavelengths with increasing field by as much as 40 meV, giving a modulation depth>10 dB. The strong dichroism present in this geometry is retained even at high fields, making polarization-sensitive electro-optical devices possible. We also demonstrate in the waveguide geometry optical bistability due to the self-electro-optic effect with 20:1 on/off ratio.