bluuebunny/crossref_metadata_2025 huggingface dataset:
Ion–ion and ion–water interactions in an aqueous solution of erbium bromide (ErBr3). A differential anomalous x-ray scattering study
Authors: Beudert R. , Bertagnolli H. , Zeller M.
Published: July 2002

A differential anomalous x-ray scattering (DAS) study of a three molal aqueous solution of erbium bromide (ErBr3) is reported. X-ray scattering at various energies below the L3 edge of erbium and the K edge of bromine was used to elucidate the ion–ion and the ion–water interaction in this solution and to get more precise information on the local environment of both ions. After an intricate data evaluation procedure the differential atom pair correlation function (DAPCF) and its “negative image” the complement atom pair correlation function (CAPCF), were obtained. The analysis of the DACPF and the CAPCF provides no considerable inner-shell complexing of the cation and the anion, but a well defined and stable hydratation sphere around the triply charged Er3+–ion was found, which is in good agreement with results of conventional x-ray studies, extended x-ray absorption fine structure (EXAFS) measurements and Raman spectroscopy. In addition the analysis provides Er3+–Br1− interaction in the second coordination sphere and an overlap of the hydratation spheres of cat- and anions as well as of two anions. On the basis of these results a two-dimensional model of the ion–ion and the ion–water interaction is presented.
Absolute partial cross sections for electron-impact ionization of H2O and D2O from threshold to 1000 eV
Authors: Straub H. C. , Lindsay B. G. , Smith K. A. , Stebbings R. F.
Published: July 2002

Absolute partial cross sections for electron-impact ionization of H2O and D2O are reported for electron energies from threshold to 1000 eV. Data are presented for the production of H2O++OH++O+, O2+, H2+, and H+ from H2O and for the production of D2O+, OD+, O+, O2+, D2+, and D+ from D2O. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±4.5% for singly charged parent ions and is slightly greater for fragment ions. The cross sections for H2O and D2O are found to be the same to within experimental uncertainties except for the H2+ cross section which is approximately a factor of 2 greater than the D2+ cross section. Previous results are compared to the present measurements.
Transition path sampling and the calculation of rate constants
Authors: Dellago Christoph , Bolhuis Peter G. , Csajka Félix S. , Chandler David
Published: July 2002

We have developed a method to study transition pathways for rare events in complex systems. The method can be used to determine rate constants for transitions between stable states by turning the calculation of reactive flux correlation functions into the computation of an isomorphic reversible work. In contrast to previous dynamical approaches, the method relies neither on prior knowledge nor on explicit specification of transition states. Rather, it provides an importance sampling from which transition states can be characterized statistically. A simple model is analyzed to illustrate the methodology.
Identification and treatment of internal rotation in normal mode vibrational analysis
Authors: Ayala Philippe Y. , Schlegel H. Bernhard
Published: July 2002

A procedure that automatically identifies internal rotation modes and rotating groups during the normal mode vibrational analysis is outlined, and an improved approximation to the corrections for the thermodynamic functions is proposed. The identification and the characterization of the internal rotation modes require no user intervention and make extensive use of the information imbedded in the redundant internal coordinates. Rigid-rotor internal rotation modes are obtained by fixing stretching, bending, and out-of-plane bending motions and solving the vibrational problem for the constrained system. Normal vibrational modes corresponding to internal rotations are identified by comparing them with the constrained modes. The atomic composition of the rotating groups is determined automatically and the kinetic energy matrix for internal rotation is given by either the constrained Wilson-G matrix or the Kilpatrick and Pitzer protocol. The potential periodicity, the rotating tops’ symmetry numbers, and the well-multiplicity are obtained using simple rules. These parameters can be altered by user input. An improved analytical approximation to the partition function for a one-dimensional hindered internal rotation has been developed that reproduces the accurate values tabulated by Pitzer and Gwinn to ±0.4% with a maximum error of 2.1%. This approximation is shown to behave better than previously available approximations over a wider range of regimes. The one-dimensional rotor treatment is generalized to give useful approximations to the multidimensional rotor thermodynamic functions that can be a good start for more thorough studies.
A density functional study of clean and hydrogen-covered α-MoO3(010): Electronic structure and surface relaxation
Authors: Chen M. , Waghmare U. V. , Friend C. M. , Kaxiras Efthimios
Published: July 2002

We report extensive density functional theory calculations, using pseudopotentials with a plane-wave basis, for the properties of the (010) face of molybdenum trioxide (α-MoO3). The surface is modeled by a one-layer slab. Calculated bond lengths compare favorably with experimental measurements. The bonding of the different oxygen species to molybdenum is analyzed using the crystal orbital overlap population. This analysis indicates that the bonding is a combination of ionic and covalent character for all oxygen species. The terminal oxygen exhibits covalent bonding to Mo which is stronger than either of the two bridging oxygens. We also study the adsorption of hydrogen on this surface. Hydrogen is most strongly adsorbed over the terminal oxygen, followed by the asymmetric bridging oxygen, and then the symmetric bridging oxygen. This trend is explained in terms of simple chemical concepts. The inclusion of full surface relaxation is important for even a qualitative description of adsorbate bonding.
Bulk effects in Thermal Desorption Spectroscopy
Authors: Castro Facundo J. , Sánchez Alejandro D. , Meyer Gabriel
Published: July 2002

In this work we analyze Thermal Desorption Spectroscopy (TDS) experiments of gases considering simultaneously atomic diffusion in the bulk and recombinative reaction on the surface of a solid sample. We perform computer simulations in a three-dimensional cubic lattice, and compare the simulated spectra with numerical results from a simple reaction-diffusion model, and analytical results from models for the fast diffusion and fast reaction regimes. We analyze the effects on the spectra of each relevant parameter: diffusivity, reaction coefficient, heating rate, initial concentration of particles and sample size. We also develop a rate-limiting step identification procedure based on the analysis of peak position as a function of initial concentration. This last procedure applied to the simulations and to experimental TDS spectra produces very good results.
Average size and size distribution of large droplets produced in a free-jet expansion of a liquid
Authors: Knuth E. L. , Henne U.
Published: July 2002

The experimental parameters and fluid properties affecting the average size N̄ and the size distribution P(N) of droplets formed by fragmentation of a liquid after expansion into a vacuum are investigated. The mean droplet size is found to be a function of the surface tension of the liquid, the nozzle diameter, and a characteristic flow speed. The size distribution is found to be a linear exponential distribution; measurements deviate from this distribution at small sizes if a factor which is a function of the cluster size is included in the measuring process. Good agreement with measured distributions of both positive and negative droplet ions formed from neutral 4He droplets by electron impact is found. The strong dependence of mean droplet size on source–orifice diameter found in the present analysis indicates that earlier correlations of droplet size with specific entropy in the source were useful at best only for a fixed nozzle size.
Computer simulation studies of anisotropic systems. XXX. The phase behavior and structure of a Gay–Berne mesogen
Authors: Bates M. A. , Luckhurst G. R.
Published: July 2002

The Gay–Berne potential is proving to be a valuable model with which to investigate the behavior of liquid crystals using computer simulation techniques. The potential contains four independent parameters which control the anisotropy in the attractive and repulsive interactions. The choice of these parameters is not straightforward and it would seem that those employed in some simulations are not strictly appropriate for mesogenic rodlike molecules. Here we report a detailed computer simulation study of Gay–Berne particles interacting via a potential parametrized to reflect the anisotropic forces based on a fit to a realistic mesogenic molecule. The behavior of the phases and the transitions between them have been investigated for a system of 2000 particles using isothermal–isobaric Monte Carlo simulations. At low pressures, this Gay–Berne mesogen exhibits isotropic, smectic A and smectic B phases but, as the pressure is increased, so a nematic phase is added to the sequence. The nature of the phase transitions and the phase diagram are compared where possible with those of real mesogens. The structures of the four phases have been investigated in detail for a larger system of 16 000 particles using canonical molecular dynamics simulations at state points taken from the phase diagram determined from the Monte Carlo simulations. A wide range of singlet and pair distribution functions were evaluated together with orientational correlation coefficients and, for the smectic phases, a bond orientation correlation function. The results for these properties were used to identify the phases, to consider their structure at a quantitative level and, where possible, to make contact with experimental studies and the predictions of theories of liquid crystals. It would appear that with this parametrization, the Gay–Berne potential provides a powerful tool with which to understand the behavior of real liquid crystals and to test the predictions of theory.
Analysis of the hydrogen bonding and vibrational spectra of supercritical model water by molecular dynamics simulations
Authors: Martı́ J.
Published: July 2002

The properties of hydrogen bond networks in ambient, supercritical, and stretched model water at temperatures between 573 K and 873 K and densities between 0.1 g/cm3 and 1 g/cm3 have been investigated by molecular dynamics simulation. A flexible simple point charged pair potential model has been used and, after comparing two hydrogen bond definitions, a pure geometrical criterion has been employed. The structure found agrees well with recent experimental data. The presence of hydrogen bonds has been detected at every supercritical state, although the tetrahedral structure typical of liquid water at room temperature is substituted in supercritical model water by chains of hydrogen-bonded molecules allowing cavities. The calculated OH stretching peak shifts toward higher values when changing from ambient to supercritical conditions. Nevertheless, under such changes bending and libration bands are displaced to lower frequencies. The cage effect typical of liquid water at ambient conditions does not appear in the supercritical states.
Gaussian basis sets for use in correlated molecular calculations. IX. The atoms gallium through krypton
Authors: Wilson Angela K. , Woon David E. , Peterson Kirk A. , Dunning Thom H.
Published: July 2002

Valence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions (although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms). After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: cc-pVDZ: (14s11p6d)/[5s4p2d]; cc-pVTZ: (20s13p9d1f )/[6s5p3d1f]; cc-pVQZ: (21s16p12d2 f1g)/[7s6p4d2 f1g]; cc-pV5Z: (26s17p13d3f2g1h)/[8s7p5d3f2g1h]. Augmented sets were obtained by adding diffuse functions to the above sets (one for each angular momentum present in the set), with the exponents of the additional functions optimized in calculations on the atomic anions. Test calculations on the atoms as well as selected molecules with the new basis sets show good convergence to an apparent complete basis set limit.
On non-negativity of Fukui function indices
Authors: Roy Ram Kinkar , Pal Sourav , Hirao Kimihiko
Published: July 2002

In this paper we have analyzed the factors which cause Fukui function (FF) indices to be negative, when evaluated in condensed form through crude finite difference approximation. Inability to take care of the relaxation effect and improper charge partitioning techniques have been cited to be the probable reasons. For the first time, we have shown that the “stockholders” charge partitioning technique (i.e., Hirshfeld’s analysis) produces non-negative FF values which, when evaluated through other kinds of charge partitioning techniques, become negative in some cases. Advantages of “stockholders” charge partitioning over other kinds of partitioning techniques are also discussed, particularly in case of evaluation of condensed FF.
Anomalous cryoprotective effectiveness of trehalose: Raman scattering evidences
Authors: Branca C. , Magazù S. , Maisano G. , Migliardo P.
Published: July 2002

Results of Raman scattering measurements performed on aqueous solutions of the homologous disaccharides (trehalose, maltose, and sucrose) are reported. To get some insight into the effects of disaccharides on the hydrogen bond network of water, and to clarify the reasons that make trehalose the most effective in protecting organisms from dehydration and freezing, we investigate the intramolecular OH stretching mode. To carry out this study, two different approaches are employed: namely, a decomposition of the isotropic spectra into an “open” and a “closed” contribution, and a spectral stripping procedure to extract the “collective” contribution from the polarized spectra. Both procedures agree in suggesting that disaccharides promote, with a different strength, a destructuring effect on the tetrahedral H-bond network of pure water. This result makes plausible the hypothesis that disaccharides obstruct the crystallization process reducing the amount of freezable water, namely destroying the network of water compatible with that of ice. What conclusively emerges from this Raman scattering study is that the greater bioprotective action of trehalose on biological structures is to be connected with its greater destructuring effect on the tetrahedral H-bond network of water.
Linear scaling coupled cluster and perturbation theories in the atomic orbital basis
Authors: Scuseria Gustavo E. , Ayala Philippe Y.
Published: July 2002

We present a reformulation of the coupled cluster equations in the atomic orbital (AO) basis that leads to a linear scaling algorithm for large molecules. Neglecting excitation amplitudes in a screening process designed to achieve a target energy accuracy, we obtain an AO coupled cluster method which is competitive in terms of number of amplitudes with the traditional molecular orbital (MO) solution, even for small molecules. For large molecules, the decay properties of integrals and excitation amplitudes becomes evident and our AO method yields a linear scaling algorithm with respect to molecular size. We present benchmark calculations to demonstrate that our AO reformulation of the many-body electron correlation problem defeats the “exponential scaling wall” that has characterized high-level MO quantum chemistry calculations for many years.
Styrene and phenylacetylene: Electronic effects of conjugating substituents “off” and “on” the axis of a benzene ring
Authors: Ribblett Jason W. , Borst David R. , Pratt David W.
Published: July 2002

Rotationally resolved fluorescence excitation spectra of several vibronic bands in the S1←S0 electronic transitions of styrene (STY) and phenylacetylene (PA) have been obtained. Confirming earlier low resolution results, we find that the origin band of PA is a b-type band but that the corresponding band of STY is an a-type band, showing that the S1 state of PA is Lb1 in character (like that of most other monosubstituted benzenes) but that the corresponding state of STY is La1. The observed changes in the rotational constants of PA and STY that occur when the photon is absorbed are consistent with these assignments. Reversal in the electronic character of the S1 state in STY is attributed to the presence of the “off-axis” conjugating –CH=CH2 group, a suggestion that is supported by the observed polarizations of higher vibronic bands in both molecules.
Molecular beam study of the adsorption and desorption of hydrogen sulfide on Au{100}
Authors: Bondzie V. , Dixon-Warren St. J. , Yu Y.
Published: July 2002

The adsorption and desorption of hydrogen sulfide on clean reconstructed Au{100}-(5×20) and sulfided gold, denoted by Au{100}-(1×1)-SH, has been studied with a combination of temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and molecular beam (MB) methods. Precursor-mediated adsorption kinetics were observed for the adsorption of H2S on the Au{100}-(5×20) between 80 and 100 K. The H2S adsorbs reversibly into a weakly bound physisorbed state, which desorbs at ∼107 K. At 110 K reversible Langmuir adsorption into the precursor state was observed for H2S on the Au{100}-(5×20). The Au{100}-(1×1)-SH was prepared by electron irradiation of H2S adsorbed on the Au{100}-(5×20) substrate. In sharp contrast with the precursor adsorption kinetics observed on the clean Au{100}-(5×20) surface, the adsorption kinetics for H2S on the Au{100}-(1×1)-SH exhibited Langmuir adsorption kinetics at 80 K. The TPD of H2S from Au{100}-(1×1)-SH and H2S adsorbed on Au{100}-(1×1)-SH showed additional features at higher temperatures which were associated with the disproportionation of chemisorbed HS(ad). Finally, we have identified a new sulfided gold surface, denoted Au{100}-(2×2)-S which was prepared by annealing the Au{100}-(1×1)-SH to remove adsorbed hydrogen.
Grand canonical ensemble Monte Carlo simulation of a lipid bilayer using extension biased rotations
Authors: Jedlovszky Pál , Mezei Mihaly
Published: July 2002

The cavity-biased grand-canonical ensemble method was applied to the simulation of a lipid bilayer using an enhanced Monte Carlo sampling technique. The enhancements include controlling the torsion and molecular rotation step size based on the lipid’s conformation and controlling the order of torsion change attempts. It was found that the proposed sampling technique significantly enhances the rate of sampling of the lipid conformations while the grand-canonical ensemble implementation ensures that the water can both penetrate and escape pockets in the bilayer. The latter will be particularly important for simulating bilayers with embedded molecules.
Characterization of oxygen phases created during oxidation of Ru(0001)
Authors: Böttcher A. , Conrad H. , Niehus H.
Published: July 2002

Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction (LEED), and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru(0001). Emission of RuOx (x<4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (Tp<850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular RuxOy phase takes place within the temperature region of 900–1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly reduced when conducting the oxidation at temperatures higher than 1250 K. In this case only a relatively low amount of oxygen atoms adsorbed on the bare Ru surface can be achieved, neither oxides nor subsurface oxygen have been found. The presence of a RuO2 coating layer manifests itself by LEED patterns characteristic for a particular RuO2 single crystal face as well as by additional features in the valence ultraviolet photoelectron spectra. The oxidation of CO molecules reactively scattered at these oxygen-rich surfaces proceeds as long as mobile oxygen atoms are present in the subsurface region. The reaction is entirely quenched when the subsurface oxygen is replaced by an uniform film of RuO2.
Cosmologies with a noninteracting mixture of dust and radiation
Authors: Weber Edmond
Published: July 2002

In this paper a particular class of anisotropic cosmologies, the Kantowski–Sachs models, is considered. It is assumed that the matter content of the models consists of a noninteracting mixture of ordinary matter (‘‘dust’’) and thermal radiation. A qualitative study by means of a three-dimensional autonomous system is carried out, giving us the global behavior of the ‘‘dust’’ density, the radiation density, and the shear anisotropy during the models’ evolution. All the models have past and future cosmological singularities where both the dust density and the radiation density diverge. A particular interesting result is a set of solutions of three-measure zero, which is radiation dominated at one (past) singularity (of the ‘‘point’’ type) and evolving to a (future) singularity, where ordinary matter and thermal radiation become negligible.
Existence and uniqueness of the pressure profile p(A) of a current-carrying quasineutral equilibrium plasma
Authors: Kiessling Michael
Published: July 2002

It is proved that the nonlinear integral equation (quasineutrality condition) for the electrostatic potential φ of an isothermal quasineutral current-carrying equilibrium plasma with translational invariance has a solution φ(A) determined uniquely up to an arbitrary additive constant, where A is the magnetic flux function. This implies the existence and uniqueness of the pressure profile p(A). The existence proof is constructive, which implies its potential usefulness for the practical computation of current-carrying plasma equilibria.
Mode stability of the Kerr black hole
Authors: Whiting Bernard F.
Published: July 2002

Separate differential and integral transformations are introduced for the individual radial and angular equations governing the (infinitesimally) gauge invariant Newman–Penrose quantities which represent massless perturbations of the Kerr black hole. Using these new transformations it is shown, without need for numerical investigation or reference to the analytic behavior of the separation constant, that no unstable mode perturbations exist for any physical value of the spin of massless fields on the rotating black hole background.
Local symmetries and constraints
Authors: Lee Joohan , Wald Robert M.
Published: July 2002

The general relationship between local symmetries occurring in a Lagrangian formulation of a field theory and the corresponding constraints present in a phase space formulation are studied. First, a prescription—applicable to an arbitrary Lagrangian field theory—for the construction of phase space from the manifold of field configurations on space-time is given. Next, a general definition of the notion of local symmetries on the manifold of field configurations is given that encompasses, as special cases, the usual gauge transformations of Yang–Mills theory and general relativity. Local symmetries on phase space are then defined via projection from field configuration space. It is proved that associated to each local symmetry which suitably projects to phase space is a corresponding equivalence class of constraint functions on phase space. Moreover, the constraints thereby obtained are always first class, and the Poisson bracket algebra of the constraint functions is isomorphic to the Lie bracket algebra of the local symmetries on the constraint submanifold of phase space. The differences that occur in the structure of constraints in Yang–Mills theory and general relativity are fully accounted for by the manner in which the local symmetries project to phase space: In Yang–Mills theory all the ‘‘field-independent’’ local symmetries project to all of phase space, whereas in general relativity the nonspatial diffeomorphisms do not project to all of phase space and the ones that suitably project to the constraint submanifold are ‘‘field dependent.’’ As by-products of the present work, definitions are given of the symplectic potential current density and the symplectic current density in the context of an arbitrary Lagrangian field theory, and the Noether current density associated with an arbitrary local symmetry. A number of properties of these currents are established and some relationships between them are obtained.
On the integrability aspects of the one-dimensional classical continuum isotropic biquadratic Heisenberg spin chain
Authors: Porsezian K. , Daniel M. , Lakshmanan M.
Published: July 2002

The integrability aspects of a classical one-dimensional continuum isotropic biquadratic Heisenberg spin chain in its continuum limit up to order [O(a4)] in the lattice parameter ‘‘a’’ are studied. Through a differential geometric approach, the dynamical equation for the spin chain is expressed in the form of a higher-order generalized nonlinear Schrödinger equation (GNLSE). An integrable biquadratic chain that is a deformation of the lower-order continuum isotropic spin chain, is identified by carrying out a Painlevé singularity structure analysis on the GNLSE (also through gauge analysis) and its properties are discussed briefly. For the nonintegrable chain, the perturbed soliton solution is obtained through a multiple scale analysis.
Local behavior in supersymmetric quantum mechanics
Authors: Wehrhahn R. F. , Cooper Ian L.
Published: July 2002

In supersymmetric quantum mechanics, the differential equations corresponding to exactly solvable potentials may be treated by algebraic methods. By use of a system of geodesic polar coordinates on a Riemannian manifold, and subsequent transformation to a Schrödinger equation with a potential in two ways, it is demonstrated that the local behavior of the exactly solvable potentials considered in supersymmetric quantum mechanics corresponds to isotropic harmonic oscillator and Pöschl–Teller potential problems.
On the integrability of pure Skyrme models in two dimensions
Authors: Leznov A. N. , Piette B. , Zakrzewski W. J.
Published: July 2002

We point out that some recently studied pure skyrme models in (2+0) dimensions are completely integrable. We discuss some implications of this fact.
Discrete spectral triples and their symmetries
Authors: Paschke Mario , Sitarz Andrzej
Published: July 2002

We classify zero-dimensional spectral triples over complex and real algebras and provide some general statements about their differential structure. We investigate also whether such spectral triples admit a symmetry arising from the Hopf algebra structure of the finite algebra. We discuss examples of commutative algebras and group algebras.
Quantifying excitations of quasinormal mode systems
Authors: Nollert Hans-Peter , Price Richard H.
Published: July 2002

Computations of the strong field generation of gravitational waves by black hole processes produces waveforms that are dominated by quasinormal (QN) ringing, a damped oscillation characteristic of the black hole. We describe here the mathematical problem of quantifying the QN content of the waveforms generated. This is done in several steps: (i) We develop the mathematics of QN systems that are complete (in a sense to be defined) and show that there is a quantity, the “excitation coefficient,” that appears to have the properties needed to quantify QN content. (ii) We show that incomplete systems can (at least sometimes) be converted to physically equivalent complete systems. Most notably, we give a rigorous proof of completeness for a specific modified model problem. (iii) We evaluate the excitation coefficient for the model problem, and demonstrate that the excitation coefficient is of limited utility. We finish by discussing the general question of quantification of QN excitations, and offer a few speculations about unavoidable differences between normal mode and QNM systems.
Resolution of differences between collision number definitions in particle and continuum simulations
Authors: Lumpkin Forrest E. , Haas Brian L. , Boyd Iain D.
Published: July 2002

The use of the same symbol, Z, representing a ‘‘collision number’’ for thermal relaxation, has lead to confusion regarding its definition in the context of both continuum and particle simulations. Examination of the relaxation mechanics employed in particle simulations demonstrates that these definitions differ by a numerical factor that depends upon the intermolecular potential. Particle and continuum simulations employing appropriate definitions of Z lead to identical results during isothermal and adiabatic stationary relaxation.
Variable soft sphere molecular model for inverse-power-law or Lennard-Jones potential
Authors: Koura Katsuhisa , Matsumoto Hiroaki
Published: July 2002

The variable soft sphere (VSS) molecular model is introduced for both the viscosity and diffusion cross sections (coefficients) to be consistent with those of the inverse-power-law (IPL) or Lennard-Jones (LJ) potential. The VSS model has almost the same analytical and computational simplicity (computation time) as the variable hard sphere (VHS) model in the Monte Carlo simulation of rarefied gas flows. The null-collision Monte Carlo method is used to make comparative calculations for the molecular diffusion in a heat-bath gas and the normal shock wave structure in a simple gas. For the most severe test of the VSS model for the IPL potential, the softest practical model corresponding to the Maxwell molecule is chosen. The agreement in the molecular diffusion and shock wave structure between the VSS model and the IPL or LJ potential is remarkably good.
Inertial transfers in the helical decomposition
Authors: Waleffe Fabian
Published: July 2002

The directions of the triadic energy transfers assumed in a previous paper [Phys. Fluids A 4, 350 (1992)] agree with the spectral closures, in a similarity range, if the exponent of the power-law energy spectrum is less than unity. The helical interactions showing a strong local energy transfer when the triad is nonlocal, sum up to a reverse cascade unless the spectrum falls off faster than a −7/3 power of the wave number. The energy cascades from each type of helical interaction are calculated for a −5/3 inertial range using the eddy damped quasinormal Markovian (EDQNM) model. One type of interaction is responsible for 86% of the cascade. The contributions of the two classes of helical interactions to the subgrid-scale eddy viscosity are presented, together with the contributions from the forward and reverse cascading interactions. An application of the assumption on the triadic energy transfers to turbulence under rapid rotation gives a simple argument for the tendency toward nonlinear two-dimensionalization of the flow.
A lattice Boltzmann model for multiphase fluid flows
Authors: Grunau Daryl , Chen Shiyi , Eggert Kenneth
Published: July 2002

A lattice Boltzmann equation method for simulating multiphase immiscible fluid flows with variation of density and viscosity, based on the model proposed by Gunstensen et al. for two-component immiscible fluids [Phys. Rev. A 43, 4320 (1991)] is developed. The numerical measurements of surface tension and viscosity agree well with theoretical predictions. Several basic numerical tests, including spinodal decomposition, two-phase fluid flows in two-dimensional channels, and two-phase viscous fingering, are shown in agreement of experiments and analytical solutions.
Radio-frequency wave interchange stability experiments below the ion cyclotron frequency
Authors: Browning J. J. , Hershkowitz N. , Intrator T. , Majeski R. , Meassick S.
Published: July 2002

It has been demonstrated in the Phaedrus-B tandem mirror [Phys. Fluids 31, 714 (1988)] that radio-frequency (rf) waves applied below the ion cyclotron frequency (ω/Ωi ≊0.75) can stabilize a mirror plasma against the interchange instability. Results presented here demonstrate stabilization over a range of frequencies (0.65≤ω/Ωi≤0.8) with the mMHD =−1 instability stabilized and higher-order (‖mMHD‖≥2) low-amplitude modes present. Radial profiles of the rf magnetic fields at a number of axial locations as well as parallel wavenumber measurements indicate mrf =−1 slow shear Alfvén wave excitation over this frequency range. Measurements at frequencies below 0.75Ωi indicate attenuation of rf waves by gas baffles and a gas box.
The merging of quasiperpendicular collisionless shocks
Authors: Cargill P. J.
Published: July 2002

The overtaking of one collisionless shock by another is studied by means of hybrid numerical simulations. The two shocks merge into a stronger shock and trailing nonshock discontinuities. The strong shock continues to propagate in the same direction as the two weaker shocks. The merging is shown to occur by a self-consistent process involving the interaction of ions reflected at the overtaking shock with the plasma upstream of the leading shock. The characteristic time scale for the merging is typically Ω−1i, where Ωi is the ion gyrofrequency. For exactly perpendicular shocks, the trailing discontinuity is a tangential discontinuity. It has a width of 2–3 ion Larmor radii. For oblique shocks, a contact discontinuity is present in the downstream plasma state. These results are of relevance to shock interactions in the very distant solar wind as well as in other energetic astrophysical situations such as solar flares.
Stability of annular equilibrium of energetic large orbit ion beam
Authors: Wong H. Vernon , Berk H. L. , Lovelace R. V. , Rostoker N.
Published: July 2002

The low-frequency stability of a long thin annular layer of energetic ions in a background plasma with finite axial and zero azimuthal magnetic field is studied analytically. It is found that although the equilibrium is susceptible to the kink instability, low mode number perturbations can be stabilized in the limit of Ni/Nb→0 when the current layer is close to the maximum field-reversal parameter. A brief discussion of the tearing mode stability criteria of such strong current layers with respect to the placement of conducting walls is also presented.
Fluid models of phase mixing, Landau damping, and nonlinear gyrokinetic dynamics
Authors: Hammett G. W. , Dorland W. , Perkins F. W.
Published: July 2002

Fluidlike models have long been used to develop qualitative understanding of the drift-wave class of instabilities (such as the ion temperature gradient mode and various trapped-particle modes) which are prime candidates for explaining anomalous transport in plasmas. Here, the fluid approach is improved by developing fairly realistic models of kinetic effects, such as Landau damping and gyroradius orbit averaging, which strongly affect both the linear mode properties and the resulting nonlinear turbulence. Central to this work is a simple but effective fluid model [Phys. Rev. Lett. 64, 3019 (1990)] of the collisionless phase mixing responsible for Landau damping (and inverse Landau damping). This model is based on a nonlocal damping term with a damping rate ∼ vt‖k∥‖ in the closure approximation for the nth velocity space moment of the distribution function f, resulting in an n-pole approximation of the plasma dispersion function Z. Alternatively, this closure approximation is linearly exact (and therefore physically realizable) for a particular f0 which is close to Maxwellian. ‘‘Gyrofluid’’ equations (conservation laws for the guiding-center density n, momentum mnu∥, and parallel and perpendicular pressures p∥ and p⊥) are derived by taking moments of the gyrokinetic equation in guiding-center coordinates rather than particle coordinates. This naturally yields nonlinear gyroradius terms and an important gyroaveraging of the shear. The gyroradius effects in the Bessel functions are modeled with robust Padé-like approximations. These new fluid models of phase mixing and Landau damping are being applied by others to a broad range of applications outside of drift-wave turbulence, including strong Langmuir turbulence, laser–plasma interactions, and the α-driven toroidicity-induced Alfvén eigenmode (TAE) instability.
Beat-wave heating with density profile steepening
Authors: Kruer W. L. , Estabrook Kent
Published: July 2002

Electron heating by the beat between 0.53 and 1.06 μm light in a self-consistently steepened density profile is examined.
Gas flow in open vertical slots with large horizontal temperature differences and arbitrary external temperature
Authors: Chenoweth D. R. , Paolucci S.
Published: July 2002

Exact solutions to the steady Navier–Stokes equations are given for laminar convective motion in open vertical slots immersed in a perfect gas. The solutions are for large temperature differences using Sutherland law transport properties and an arbitrary external temperature. The solutions are valid in the fully developed region where the opposite hot and cold vertical wall boundary layers are fully merged. It is found that the temperature, velocity, mass flow rate, and vertical heat flux are very sensitive to property variations, even though the horizontal heat flux is not. Furthermore, it is found that if only one end of the slot is open, the velocity and vertical heat flux are independent of the external temperature. Comparisons with constant transport property solutions and the well-known Boussinesq limiting solution for small temperature differences are given for examples using air or hydrogen; these show that important but significantly different effects exist as a result of density and transport property variations.
The effect of nonzero viscosity ratio on the stability of fingers and bubbles in a Hele–Shaw cell
Authors: Tanveer S. , Saffman P. G.
Published: July 2002

The linear stability of a steadily moving bubble or a finger in a Hele–Shaw cell is considered in the case when gravity and the ratio between the viscosities of the less and more viscous fluids are nonzero. The effect of gravity is easily incorporated by a transformation of parameters introduced previously by Saffman and Taylor [Proc. R. Soc. London Ser. A 245, 312 (1958)] for the steady flow, which makes the time-dependent flows with and without gravity equivalent. For the nonzero viscosity ratio, the transformation of parameters introduced by Saffman and Taylor also makes steady finger and bubble flows with nonzero and zero viscosity ratios equivalent. However, for the unsteady case, there is no such equivalence and so a complete calculation is carried out to investigate the effect of the nonzero viscosity ratio on the stability of fingers and bubbles. The incorporation of the finite viscosity ratio is found not to qualitatively alter the linear stability features obtained in earlier work for the zero viscosity ratio, although there are quantitative differences in the growth or decay rate of various modes. For any surface tension, numerical calculation suggests that the McLean–Saffman branch of bubbles [Phys. Fluids 30, 651 (1987)] of arbitrary size is stable, whereas all the other branches are unstable. For a small bubble that is circular, the eigenvalues of the stability operator are found explicitly. The previous analytic theory for the stability of the finger in the limit of zero surface tension is extended to include the case of the finite viscosity ratio. It is found that, as in the case of bubbles, the finite viscosity ratio does not alter qualitatively any of the features obtained previously for the zero viscosity ratio.
A note on the history force on a spherical bubble at finite Reynolds number
Authors: Mei Renwei , Klausner James F. , Lawrence Christopher J.
Published: July 2002

An approximate expression for the history force on a spherical bubble is proposed for finite Reynolds number, Re. At small time, the history-force kernel is a constant, which decreases with increasing Re, and the kernel decays as t−2 for large time. For an impulsively started flow over a bubble, accurate finite difference results show that the history force on the bubble decays as t−2 at large time. Satisfactory agreement is observed between the presently proposed history force and the numerical solution.
An improved measure of strain state probability in turbulent flows
Authors: Lund Thomas S. , Rogers Michael M.
Published: July 2002

Probability density functions (PDFs) of the strain-rate tensor eigenvalues are examined. It is found that the accepted normalization used to bound the intermediate eigenvalue between ±1 leads to a PDF that must vanish at the end points for a non-singular distribution of strain states. This purely kinematic constraint has led previous investigators to conclude incorrectly that locally axisymmetric deformations do not exist in turbulent flows. An alternative normalization is presented that does not bias the probability distribution near the axisymmetric limits. This alternative normalization is shown to lead to the expected flat PDF in a Gaussian velocity field and to a PDF that indicates the presence of axisymmetric strain states in a turbulent field. Extension of the new measure to compressible flow is discussed. Several earlier results concerning the likelihood of various strain states and the correlation of these with elevated kinetic energy dissipation rate are reinterpreted in terms of the new normalization. Most importantly, it is found that regions of axisymmetric expansion play a much more dominant role in the turbulent dissipation process than was previously believed.
Near field dynamics of subsonic free square jets. A computational and experimental study
Authors: Grinstein F. F. , Gutmark E. , Parr T.
Published: July 2002

Results of a combined numerical and experimental investigation of the near field of low-subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically-integrated large-eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis-switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time-averaged properties of the jet cross sections. The different experimental jets were chosen such that important parameters affecting the initial conditions could be tested. Depending on the particular initial conditions of the subsonic jets studied, several or no axis switchings were observed in the first few diameters of jet development. Observed trends reported, include the effects of initial conditions such as, ratio of equivalent diameter to characteristic-momentum-thickness, turbulence level, nonuniform azimuthal momentum-thickness distributions, and Reynolds number.
Scaling of temperature- and stress-dependent viscosity convection
Authors: Solomatov V. S.
Published: July 2002

Simple scaling analysis of temperature- and stress-dependent viscosity convection with free-slip boundaries suggests three convective regimes: the small viscosity contrast regime which is similar to convection in a fluid whose viscosity does not depend on temperature, the transitional regime characterized by self-controlled dynamics of the cold boundary layer and the asymptotic regime in which the cold boundary becomes stagnant and convection involves only the hottest part of the lid determined by a rheological temperature scale. The first two regimes are usually observed in numerical experiments. The last regime is similar to strongly temperature-dependent viscosity convection with rigid boundaries studied in laboratory experiments.
A theory of three-dimensional interfacial vorticity dynamics
Authors: Wu Jie-Zhi
Published: July 2002

A three-dimensional theory of vorticity dynamics on an incompressible viscous and immiscible fluid–fluid interface, or interfacial vorticity dynamics for short, is presented as a counterpart of the vorticity dynamics on an arbitrarily curved rigid wall [J. Fluid Mech. 254, 183 (1993)]. General formulas with arbitrary Reynolds numbers Re are derived for determining (1) how much vorticity exists on an interface S, (2) how much vorticity is created from S and sent into the fluid per unit area in per unit time, and (3) the force and moment acted on a closed interface by the created vorticity thereon. The common feature and fundamental difference between interfacial vorticity dynamics and its rigid-wall counterpart are analyzed. In particular, on a free surface, the primary driving mechanism of vorticity creation is the balance between the shear stress (measured by tangent vorticity) and the tangent components of the surface-deformation stress alone, which results in a weak creation rate of O (Re−1/2) at large Re. Therefore, the exact form of the theory with its full complexity is of importance mainly at low Reynolds numbers, especially in understanding the small-scale coherent structures of interfacial turbulence. The vorticity creation rate at high-Re approximations, including an interfacial boundary layer of finite thickness and the limit of Re→∞ (the so-called Euler limit), is also studied, both allowing for a rotational inviscid outer flow. While for the former this leads to a generalization of Lundgren’s theory [in Mathematic Aspects of Vortex Dynamics, edited by R. E. Caflish (SIAM, Philadelphia, PA, 1989), pp. 68–79] and amounts to solving a linear boundary-layer problem, for the latter the creation rate can be directly obtained from an inviscid solution, leading to a dynamic evolution equation of interfacial vortex sheet. In three dimensions, a vortex sheet may bifurcate into a normal vorticity field, upon which the dependence of the sheet velocity is determined. A few examples are examined to illustrate different aspects and approximation levels of the general theory.