bluuebunny/crossref_metadata_2025 huggingface dataset:
Global geometry optimization of atomic clusters using a modified genetic algorithm in space-fixed coordinates
Authors: Niesse J. A. , Mayne Howard R.
Published: July 2002

In a recent paper, Gregurick, Alexander, and Hartke [S. K. Gregurick, M. H. Alexander, and B. Hartke, J. Chem. Phys. 104, 2684 (1996)] proposed a global geometry optimization technique using a modified Genetic Algorithm approach for clusters. They refer to their technique as a deterministic/stochastic genetic algorithm (DS-GA). In this technique, the stochastic part is a traditional GA, with the manipulations being carried out on binary-coded internal coordinates (atom–atom distances). The deterministic aspect of their method is the inclusion of a coarse gradient descent calculation on each geometry. This step avoids spending a large amount of computer time searching parts of the configuration space which correspond to high-energy geometries. Their tests of the technique show it is vastly more efficient than searches without this local minimization. They report geometries for clusters of up to n=29 Ar atoms, and find that their computer time scales as O(n4.5). In this work, we have recast the genetic algorithm optimization in space-fixed Cartesian coordinates, which scale much more favorably than internal coordinates for large clusters. We introduce genetic operators suited for real (base-10) variables. We find convergence for clusters up to n=55. Furthermore, our algorithm scales as O(n3.3). It is concluded that genetic algorithm optimization in nonseparable real variables is not only viable, but numerically superior to that in internal candidates for atomic cluster calculations. Furthermore, no special choice of variable need be made for different cluster types; real Cartesian variables are readily portable, and can be used for atomic and molecular clusters with no extra effort.
Change of distribution of free volume holes during crystallization of poly(ethylene terephthalate) revealed by positron annihilation lifetime spectroscopy
Authors: Mohamed Hamdy F. M. , Ito Yasuo , Imai Masayuki
Published: July 2002

Change of free volume holes during crystallization of amorphous poly(ethylene terephthalate) at 130 °C was investigated by positron annihilation lifetime technique together with the measurements of specific density and wide angle x-ray scattering (WAXS). The o-Ps intensity, I3, decreased by 24.6% with the crystallization. This value agreed with the degree of crystallinity determined by the specific density and WAXS, which indicates that o-Ps annihilation takes place in the amorphous part. The average o-Ps lifetime increased slightly by the crystallization. Detailed analysis of the o-Ps lifetime by a continuous distribution model showed that the small size component of the distribution disappeared due to the crystallization. The result is favorably compared with the recently proposed picture of crystallization which deals with the conformational change of ethylene group.
Metal oxide and carbide thermochemistry of Y+, Zr+, Nb+, and Mo+
Authors: Sievers M. R. , Chen Yu-Min , Armentrout P. B.
Published: July 2002

Reactions of Y+, Zr+, Nb+, and Mo+ with molecular oxygen and carbon monoxide and the collision induced dissociations of their metal oxides with Xe are studied as a function of kinetic energy using guided ion beam mass spectrometry. A meter-long flow tube ion source is used to create Zr+, Nb+, and Mo+ ions in their electronic ground state terms and Y+ mostly in its ground state. The kinetic energy dependencies for the reactions of Y+, Zr+, and Nb+ with O2 show exothermic, barrierless behavior, while Mo+ reacts with O2 in a process with a small endothermicity. Reactions with CO lead to formation of MC+ and MO+ in endothermic processes. Analyses of the reaction cross sections obtained in this study yield 0-K bond dissociation energies (in eV) of D0(Y+–O)=7.24±0.18, D0(Y+–C)=2.91±0.12, D0(Zr+–O)=7.76±0.11, D0(Zr+–C)=4.72 ±0.11, D0(Nb+–O)=7.13±0.11, D0(Nb+–C)=5.16±0.15, D0(Mo+–O)=5.06±0.02, and D0(Mo+–C)=4.31±0.20. There is some question whether the YC+ and YO+ bond energies represent the correct adiabatic values. From this thermochemistry and literature values for D0(MO) and D0(MC), we also are able to calculate the ionization energies for the metal carbides and oxides (in eV): IE(YO)=6.39±0.22, IE(YC)=7.60±0.19, IE(ZrO)=6.87±0.18, IE(NbO)=7.65±0.22, IE(Nbc)=7.45±0.20, IE(MoO)=7.79±0.22, and IE(MoC)=7.73±0.26. These thermochemical values are compared with the literature and the periodic trends discussed.
Statistical rate theory description of beam-dosing adsorption kinetics
Authors: Elliott J. A. W. , Ward C. A.
Published: July 2002

Absolute rate theory and the sticking probability approach have been previously examined as possible means of predicting the rate of adsorption. However, when applied to examine adsorption kinetics, they have been found not to contain the coverage and pressure dependence required for several important systems including CO–Ni(111). Statistical rate theory (SRT) is being developed with the objective of predicting the rate of molecular (or atomic) transport across the interface between macroscopic phases in terms of experimentally controllable variables and material properties of the two phases. Previous applications of SRT to adsorption have been limited to systems for which both the gas phase pressure and the temperature could be assumed to be constant. Herein, the SRT approach is extended to systems in which the number of molecules in the system (and hence the gas phase pressure) is not constant. To examine this extension, SRT is used to formulate the equations governing the rate of adsorption in isothermal, beam-dosing experiments. These equations are then combined with the values of certain material properties that have previously been established and a hypothesis that the value of the equilibrium adsorption cross section is given by the area of an adsorption site. The kinetic data for CO adsorbing on Ni(111) data reported by three different laboratories are then examined. For each set of experimental data, constants had to be inferred that were related to the experimental apparatus used and as such they were not expected to have any coverage or pressure dependence. The good agreement found between the predicted and measured adsorption kinetics indicates that all of the necessary coverage and pressure dependence was explicitly predicted from the SRT approach.
Transitions to a new chiral phase in a Langmuir monolayer
Authors: Durbin M. K. , Malik A. , Richter A. G. , Ghaskadvi R. , Gog T. , Dutta P.
Published: July 2002

Isotherms and x-ray diffraction studies of eicosanoic acid Langmuir monolayers show a phase in which the molecular tilt is intermediate between nearest-neighbor (NN) and next-nearest-neighbor (NNN) directions. The transition from this “I” phase to an NN-tilted structure is first order, with a ∼60° change in the tilt direction, while the transition to an NNN-tilted structure is apparently continuous. These results can be explained using a Landau-type theory for uniaxially distorted lattices, which is a modification of an existing theory for hexagonal lattices [J. V. Selinger and D. R. Nelson, Phys. Rev. Lett. 61, 416 (1988)].
Dissipative particle dynamics: Bridging the gap between atomistic and mesoscopic simulation
Authors: Groot Robert D. , Warren Patrick B.
Published: July 2002

We critically review dissipative particle dynamics (DPD) as a mesoscopic simulation method. We have established useful parameter ranges for simulations, and have made a link between these parameters and χ-parameters in Flory-Huggins-type models. This is possible because the equation of state of the DPD fluid is essentially quadratic in density. This link opens the way to do large scale simulations, effectively describing millions of atoms, by firstly performing simulations of molecular fragments retaining all atomistic details to derive χ-parameters, then secondly using these results as input to a DPD simulation to study the formation of micelles, networks, mesophases and so forth. As an example application, we have calculated the interfacial tension σ between homopolymer melts as a function of χ and N and have found a universal scaling collapse when σ/ρkBTχ0.4 is plotted against χN for N>1. We also discuss the use of DPD to simulate the dynamics of mesoscopic systems, and indicate a possible problem with the timescale separation between particle diffusion and momentum diffusion (viscosity).
Biaxial phases in polydisperse mean-field model solution of uniaxial micelles
Authors: Henriques Eduardo F. , Henriques Vera B.
Published: July 2002

We consider a discretized Maier–Saupe model for a polydisperse solution of micelles of axial symmetry. An exact mean-field solution is presented. The phase diagram displays a biaxial phase at small average anisometries and finite dispersion in size, and two opposite uniaxial nematic phases for large average anisometries. Relations to the experimental phase diagram for ternary mixtures are discussed.
A theory for the liquid-crystalline phase behavior of the Gay–Berne model
Authors: Velasco E. , Mederos L.
Published: July 2002

A simple yet reasonably accurate perturbation theory for the Gay–Berne model, capable of describing the uniform isotropic and nematic phases, as well as the layered smectic-A phase, is presented. The theory, in line with a previously proposed theory, is based on a perturbative scheme, but the reference system, a hard Gaussian overlap model, is treated using the nonlocal approximation of Somoza and Tarazona. This approximate scheme, which reduces to the well-known decoupling approximation for nematics, is a simple generalization of the decoupling approximation designed to include smectic structures. The attractive free energy is calculated using a mean-field approximation. Underestimation of the attractive energy implied by this approximation is alleviated by introducing some scale factors, set to reproduce the critical point and two triple points involving the smectic phase. The choice of scale factors, which is valid for a particular set of molecular parameters, is shown to reproduce accurately the phase diagram corresponding to other parameter values. The theory is used to examine the global liquid-crystalline phase behavior of the Gay–Berne model, paying particular attention to the effect of the anisotropy attraction parameter κ′ on the location of the various phase boundaries. Comparison of the results with the available computer simulations for this system indicates that the theory leads to qualitatively correct predictions. The theory could be useful to predict the phase behavior of realistic systems with respect to molecular elongation and energy anisotropy.
Absorption and spontaneous emission in aggregates of conjugated polymers
Authors: Manas Eric S. , Spano Frank C.
Published: July 2002

Absorption and spontaneous emission from exciton states in ordered aggregates of π-conjugated polymers are studied theoretically. Each polymer contains N carbon atoms with average bond length a and is treated using a Pariser–Parr–Pople Hamiltonian. Coulombic interactions between molecules are treated to first order using an effective Hamiltonian approach. The aggregate electronic excited states are Wannier–Frenkel (WF) excitons with intramolecular Wannier exciton character and intermolecular Frenkel exciton character. For aggregates with in-phase double bond alignment, interpolymer interactions cause a blue shift of the optically allowed (νB−) WF excitons. The blue shift increases with N, peaks, and ultimately tends to a small value when the polymer length greatly exceeds d, the nearest neighbor separation. The radiative decay rate γνB− of the optically allowed νB− WF exciton is proportional to the number of polymers, M. Intermolecular interactions have little effect on the exciton sizes and energies when the polymer length is much greater than the exciton sizes where γνB− scales linearly with N due to delocalization of the electron-hole (e-h) center of mass along each polymer. In this limit, the radiative decay rate is enhanced two dimensionally, γνB−∝MN. However, when N is smaller, the exciton sizes and energies can be strongly affected by these interactions, leading to a markedly different N dependence for γνB−. For parameters which best describe realistic conjugated polymers, this system behaves like an H-aggregate, with a superradiant state at the top of the exciton band. The resulting emission will not be superradiant if the radiative decay rate is much smaller than the nonradiative population relaxation rate to lower energy, optically dark Frenkel exciton states. Applications of this analysis to recent experimental findings are discussed.
Hydrophobic hydration of acetone probed by nuclear magnetic resonance and infrared: Evidence for the interaction C–H⋯OH2
Authors: Mizuno Kazuko , Ochi Torayuki , Shindo Yohji
Published: July 2002

The concentration and the temperature dependencies of H1 and C13 chemical shifts in NMR of aqueous acetone mixtures were studied, together with the concentration dependence of the frequency of the C–H stretching vibration of acetone in IR spectra. H1 and C13 chemical shifts were measured at 1 °C, 23 °C, and 48 °C by the external double reference method using a capillary with a blown-out sphere at the bottom for tetramethylsilane as the external reference substance. By this method, it is possible to determine the volume magnetic susceptibility of a sample solution at each temperature, for which the observed chemical shifts may be corrected exactly. Thus, we revealed the detailed electronic polarization in acetone as well as water as functions of concentration and temperature. On diluting acetone with water, the chemical shift of water protons, δH2O, is 3 ppm at the mole fraction of water Xw=0,05 and increases to the value for pure water, ca. 5 ppm, at Xw=0.96, with increasing Xw. In the region of Xw>0.96, δH2O is slightly larger than the value, indicating the presence of more polarized water species than pure water. The chemical shifts of C–H proton, δCH_3, and C–H carbon, δC_H3, also increase slightly with increasing Xw up to Xw=0.96. The frequency for the C–H vibration of acetone, νC–H, increases from the value for pure acetone, 3005 cm−1, to 3013 cm−1 at Xw=0.96, while it decreases sharply with further increase in Xw. These results of IR and NMR measurements show that the hydration of acetone accompanies electronic redistribution in the C–H bonds in cooperated with the change in the polarization of the surrounding water molecules, and that two different types of hydration of acetone are predominant in different concentration regions, Xw<0.96 and Xw>0.96. In the region of Xw<0.96, the results can be explained satisfactorily if we consider that a part of the electron about the C–H proton is pushed out into the C–H bond due to a repulsive interaction between the C–H hydrogen and water oxygen. In the region of Xw>0.96, we can interpret the results well by considering that the pushing by the water oxygen becomes strong enough to induce the polarization of the C–H bonds compared to the pushing at Xw⩽0.96. Since the polarization of the C–H bond was found to increase with decreasing temperature, the repulsive interaction seems to have the property of hydrogen bonding and to be denoted as C–H⋯OH2(⋯OH2)n, where OH2(⋯OH2)n expresses water molecules hydrogen-bonded cooperatively and responsible for the more polarized water than pure water. The ratio of water to acetone seems to be a predominant factor to cause the transition of the hydration state from the repulsive interaction to hydrophobic hydration of acetone.
Barrier properties of oriented disk composites
Authors: Fredrickson Glenn H. , Bicerano Jozef
Published: July 2002

We examine the effective diffusion coefficient of composites containing randomly placed, impermeable, oriented disks of high aspect ratio. We extend previous work by providing a quantitative framework in which to examine disorder and polydispersity effects, as well as treating explicitly the crossover from the dilute to the semidilute regime as the concentration of disks is increased. Our results have applicability to matrices filled with macroscopic plates or flakes (for example, mica-filled polymers) as well as to polymer–clay hybrid materials (“nanocomposites”).
Molecular dynamics simulation of the liquid–vapor interface: Binary mixtures of Lennard-Jones fluids
Authors: Mecke Matthias , Winkelmann Jochen , Fischer Johann
Published: July 2002

We have performed molecular dynamics simulations to study structure and equilibrium properties of liquid–vapor interfaces of binary mixtures containing argon and methane over the entire range of compositions. The phase equilibrium data obtained from our simulations are compared with those from NpT+test particle calculations of an analogous system. Values of the surface tension were computed and extrapolated to the full Lennard-Jones potential using a new ansatz to determine tail contributions to the surface tension directly within the simulation run. This new method uses the actual partial density profiles obtained within the simulation and, therefore, does not require any prescribed functional form. From both the surface tension results and the partial density profiles the relative enrichment of argon at the interface was estimated.
Block copolymer films between neutral walls: A Monte Carlo study
Authors: Sommer Jens-Uwe , Hoffmann Alexander , Blumen Alexander
Published: July 2002

We study thin, symmetric AB-copolymer films between neutral walls using the bond fluctuation model. Near the surface, the monomer distribution with respect to the chains’ center of mass is nonisotropic: The chains flatten on the surface. The resulting A- and B-lamellae have interfaces practically perpendicular to the walls, as also observed in recent experiments. Our simulations indicate the onset of long-range surface ordering before the bulk orders. The strongest effects take place between the spinodal and the order–disorder transition point of the diblock copolymer melt.
A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives
Authors: Henkelman Graeme , Jónsson Hannes
Published: July 2002

The problem of determining which activated (and slow) transitions can occur from a given initial state at a finite temperature is addressed. In the harmonic approximation to transition state theory this problem reduces to finding the set of low lying saddle points at the boundary of the potential energy basin associated with the initial state, as well as the relevant vibrational frequencies. Also, when full transition state theory calculations are carried out, it can be useful to know the location of the saddle points on the potential energy surface. A method for finding saddle points without knowledge of the final state of the transition is described. The method only makes use of first derivatives of the potential energy and is, therefore, applicable in situations where second derivatives are too costly or too tedious to evaluate, for example, in plane wave based density functional theory calculations. It is also designed to scale efficiently with the dimensionality of the system and can be applied to very large systems when empirical or semiempirical methods are used to obtain the atomic forces. The method can be started from the potential minimum representing the initial state, or from an initial guess closer to the saddle point. An application to Al adatom diffusion on an Al(100) surface described by an embedded atom method potential is presented. A large number of saddle points were found for adatom diffusion and dimer/vacancy formation. A surprisingly low energy four atom exchange process was found as well as processes indicative of local hex reconstruction of the surface layer.
Intrinsic cascading contributions to the fifth- and seventh-order electronically off-resonant Raman spectroscopies
Authors: Cho Minhaeng , Blank David A. , Sung Jaeyoung , Park Kisam , Hahn Sangjoon , Fleming Graham R.
Published: July 2002

The cascading polarization contributing to the femtosecond fifth- and seventh-order coherent Raman scattering processes is obtained in terms of the third- and fifth-order Raman response functions. It is found that four cascading processes contribute to the fifth-order Raman scattering signal. In the case of the seventh-order Raman scattering, there are 34 possible cascading processes contributing to the seventh-order Raman scattering signal, and they are classified as χ(5)×χ(3)- and χ(3)×χ(3)×χ(3)-types depending on the involved low-order nonlinear optical processes. Due to the complicated interference among these polarization fields, it may not be possible to experimentally measure the pure seventh-order Raman scattering signal without contamination from the cascaded contributions.
Polymer conformations of gas-hydrate kinetic inhibitors: A small-angle neutron scattering study
Authors: King H. E. , Hutter Jeffrey L. , Lin Min Y. , Sun Thomas
Published: July 2002

We have used small-angle neutron scattering to characterize the polymer conformations of four nonionic water soluble polymers: poly(ethylene oxide), poly(N-vinyl-2-pyrollidone), poly(N-vinyl-2-caprolactam), and an N-methyl, N-vinylacetamide/N-vinyl-2-caprolactam copolymer. The last three of these are able to kinetically suppress hydrate crystallization, and their inhibitor activity ranges from moderate to very effective. This attribute is of significant commercial importance to the oil and gas industry, but the mechanism of the activity is unknown. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows little difference among the four polymers: the majority of the scattering is that expected for a polymer in a good solvent. Each solution also exhibits some additional low-q scattering which we attribute to aggregates. In the presence of a hydrate-crystal/liquid slurry, the three inhibitor polymers significantly change their conformation. Utilizing results from our previous contrast variation study, we show that this arises from polymer adsorbed to the hydrate-crystal surface. Furthermore, we find a strong correlation between the scattering intensity at low q values and the effectiveness of the inhibitor polymer. We suggest this is an indication that as surface adsorption increases, the inhibitor’s blocking of growth sites increases. Also measured for one of the kinetic-inhibitor polymers was the dilute-solution polymer conformation in a hydrate-forming propane/water fluid (hydrate crystal free). This system shows additional low-q scattering, possibly indicating a polymer–propane interaction prior to crystal formation. This may affect hydrate nucleation behavior and offer a second mechanism for kinetic hydrate inhibition.
Pyridine adsorption on the polar ZnO(0001) surface: Zn termination versus O termination
Authors: Hövel St. , Kolczewski C. , Wühn M. , Albers J. , Weiss K. , Staemmler V. , Wöll Ch.
Published: July 2002

The interaction of pyridine (C5H5N) with the two differently terminated ZnO(0001) surfaces has been investigated using thermal desorption spectroscopy (TDS), x-ray photoelectron spectroscopy (XPS), and x-ray absorption spectroscopy (NEXAFS). The binding energy of pyridine on the O-terminated ZnO(000-1) surface amounts to 57 kJ/mol and the spectroscopic data reveal only small modifications of the pyridine electronic structure, indicating the presence of a physisorbed species. On the Zn-terminated ZnO(0001) surface, the binding energy is substantially higher, 112 kJ/mol, and the N1s NEXAFS data for the pyridine π* resonance shows a shift of 0.6 eV toward higher binding energies. This observation indicates a substantial interaction between the nitrogen lone pair and Zn, as is confirmed by precise ab initio calculations of the core-level excitation.
A reactive potential for hydrocarbons with intermolecular interactions
Authors: Stuart Steven J. , Tutein Alan B. , Harrison Judith A.
Published: July 2002

A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems.
A comparison of the permanent electric dipole moments of ZrS and TiS
Authors: Bousquet Robert R. , Namiki Kei-ichi C. , Steimle Timothy C.
Published: July 2002

Optical Stark measurements were performed on the (1,0) C 3Δ–X 3Δ band system of titanium monosulfide, TiS48, and the (0,0) C 1Σ+–X 1Σ+ band system of isovalent zirconium monosulfide, ZrS90. The molecules were produced by ablating a metal rod in the presence of a CS2/argon supersonic expansion. The field-free spectrum for (1,0) C 3Δ1–X 3Δ1 and (1,0) C 3Δ2–X 3Δ2 sub-bands of TiS48 were analyzed and an improved set of fine structure parameters was determined. The permanent electric dipole moments, μ, for TiS were determined to be 5.75(10) D and 4.41(9) D for the X 3Δ1(v=0) and C 3Δ1(v=1) states, respectively, and for ZrS to be 3.86(6) D and 4.71(6) D for the X 1Σ+(v=0) and C 1Σ+(v=0) states, respectively. A comparison of the transition metal sulfides to their respective oxides is presented.
Anomalous domain spacing difference between AB diblock and homologous A2B2 starblock copolymers
Authors: Matsen M. W. , Gardiner J. M.
Published: July 2002

Based on weak- and strong-segregation theories, symmetric AB diblock copolymers and the homologous A2B2 starblock copolymers formed by joining pairs of diblocks together by their junctions should have identical order–disorder transitions (ODT’s) and lamellar periods. However, recent experiments report that the starblock melts have elevated ODT’s and domain spacings. The ODT shift has been attributed to the increase in chain stretching, but there is still no convincing explanation for the larger domain size. Here, we show that this latter effect is correctly predicted by self-consistent field theory (SCFT) and that it results due to a reduction in translation entropy of the junctions. Furthermore, we derive a strong-segregation based approximation for this anomalous domain spacing difference.
Integrable Hamiltonian systems with velocity-dependent potentials
Authors: Dorizzi B. , Grammaticos B. , Ramani A. , Winternitz P.
Published: July 2002

The integrability of a two-dimensional Hamiltonian in which the potential depends explicitly on the momenta is investigated. Hamiltonians of this kind are encountered in the description of the motion of a particle in a magnetic field. Two integrable classes of potentials are identified and the second integral of motion is constructed for each of them. The singularity analysis of the equations of motion is also performed, confirming once more the relation between the (weak) Painlevé property and integrability.
Anharmonic oscillator: Constructing the strong coupling expansions
Authors: Turbiner A. V. , Ushveridze A. G.
Published: July 2002

Two novel approaches to construction of the strong coupling expansion for the anharmonic oscillator with the potential V(x)= 1/2 x2+(g/4)x4 are proposed. The first one is simply a straightforward solution of the Schrödinger equation via the ‘‘nonlinearization’’ technique, resulting in the rapidly convergent perturbation series. The second one is a version of the path integral perturbation theory, but with an unconventional choice of the zeroth approximation action. Nine leading coefficients of the strong coupling expansion are computed. They decrease rapidly, the ninth one being of the order of 10−9. Three leading corrections of the nonlinearization approach provide the ground-state energy within a relative accuracy of 10−7–10−9, at an arbitrary coupling g. The explicit formulas for corrections enable one to study the analyticity properties of the energy as a function of the coupling g. In the second approach the strong coupling expansion coefficients are computed as an infinite linear series of the weak coupling expansion coefficients. This greatly simplifies calculations, though at the price of slower and slower convergence of the series for the higher-order coefficients. If 41 terms of the weak coupling expansion are included, a relative accuracy of 10−6 and 10−2 is achieved for the second and ninth coefficients of the strong coupling expansion, respectively. In the Appendices the weak coupling expansion coefficients up to the 41st order are tabulated. In addition, both a proof of the convergence property of the nonstandard perturbation theory and also a derivation of certain functional inequalities are given, which make clear already at a qualitative level why and how the nonstandard perturbation theory acquires the convergence property.
General theory of fractal path integrals with applications to many-body theories and statistical physics
Authors: Suzuki Masuo
Published: July 2002

A general scheme of fractal decomposition of exponential operators is presented in any order m. Namely, exp[x(A+B)]=Sm(x)+O(xm+1) for any positive integer m, where Sm(x)=et1A et2B et3A et4B⋅⋅⋅etMA with finite M depending on m. A general recursive scheme of construction of {tj} is given explicitly. It is proven that some of {tj} should be negative for m≥3 and for any finite M (nonexistence theorem of positive decomposition). General systematic decomposition criterions based on a new type of time-ordering are also formulated. The decomposition exp[x(A+B)]=[Sm(x/n)]n +O(xm+1/nm) yields a new efficient approach to quantum Monte Carlo simulations.
Group-invariant solutions and optimal systems for multidimensional hydrodynamics
Authors: Coggeshall S. V. , Meyer-ter-Vehn J.
Published: July 2002

The group properties of the three-dimensional (3-D), one-temperature hydrodynamic equations, including nonlinear conduction and a thermal source, are presented. A subgroup corresponding to axisymmetric geometry is chosen, and the details of the construction of the one- and two-dimensional optimal systems are shown. The two-dimensional optimal system is used to generate 23 intrinsically different reductions of the 2-D partial differential equations to ordinary differential equations. These ordinary differential equations can be solved to provide analytic solutions to the original partial differential equations. Two example analytic solutions are presented: a 2-D axisymmetric flow with a P2 asymmetry and a 3-D spiraling flow.
Kinetic theory for binary mixtures of smooth, nearly elastic spheres
Authors: Jenkins J. T. , Mancini F.
Published: July 2002

Existing Chapman–Enskog solution procedures for binary mixtures of hard, perfectly elastic spheres are extended to hard, slightly dissipative spheres, and the associated constitutive relations are calculated. Then a steady, homogeneous shear flow is analyzed and the behavior of the mixture viscosity is determined as the diameter ratio, volume ratio, and total volume fraction are varied.
A computational study of developing 2-D laminar flow in curved channels
Authors: Snyder B. , Lovely C.
Published: July 2002

Uniform flow entering a curved channel induces an ‘‘upstream influence’’ that skews the axial velocity profile toward the center of bend curvature. Skew grows so abruptly that its evolution is difficult to examine experimentally. Therefore the developing viscous flow was simulated numerically, using an alternating direction-implicit (ADI) algorithm, to resolve how streamline curvature influences the formation of skew. Entering flow was found to develop on an inviscid length scale L∼O(a), where L is the streamwise path length and a is the channel half-thickness, with skew becoming maximum approximately 2a downstream from the inlet irrespective of Re (100≤2aW/ν≤3200) or curvature ratio (4≤R/a≤100). Downstream flow approaches the fully developed configuration much more gradually, on a scale L∼O(a Re), but skew never shifts to the outer wall of a channel bend. This observation suggests that swirl-driven momentum transport, not the direct action of inertial force, must account for the outward skew of fully developed flow in curved pipes.
Global stability of a lid-driven cavity with throughflow: Flow visualization studies
Authors: Aidun C. K. , Triantafillopoulos N. G. , Benson J. D.
Published: July 2002

Flow visualization studies of a lid-driven cavity (LDC) with a small amount of throughflow reveal multiple steady states at low cavity Reynolds numbers. These results show that the well-known LDC flow, which consists of a primary eddy and secondary corner eddies, is only locally stable, becomes globally unstable, and competes with at least three other steady states before being replaced by a time-periodic flow. The small amount of throughflow present in this system seems to have no qualitative effect on the fluid flow characteristics. These observations suggest that multiple stable steady states may also exist in closed LDC’s. Since stability properties of the closed LDC flows are virtually unexplored, we interpret our flow visualization results by first proposing an expected behavior of an idealized (free-slip end walls) LDC and then treating the problem at hand as a perturbation of the ideal case. The results also suggest that there are nonunique and competing sequences of transitions that lead the flow in a LDC from laminar steady state toward turbulence.
Analytic solutions of hydrodynamics equations
Authors: Coggeshall S. V.
Published: July 2002

Many similarity solutions have been found for the equations of one-dimensional (1-D) hydrodynamics. These special combinations of variables allow the partial differential equations to be reduced to ordinary differential equations, which must then be solved to determine the physical solutions. Usually, these reduced ordinary differential equations are solved numerically. In some cases it is possible to solve these reduced equations analytically to obtain explicit solutions. In this work a collection of analytic solutions of the 1-D hydrodynamics equations is presented. These can be used for a variety of purposes, including (i) numerical benchmark problems, (ii) as a basis for analytic models, and (iii) to provide insight into more complicated solutions.
An experimental and numerical study of nonlinear internal waves
Authors: Renouard Dominique P. , Tomasson Gunnar G. , Melville W. Kendall
Published: July 2002

Nonlinear internal waves were measured on the large rotating platform at the Institut de Mécanique de Grenoble (I.M.G.). The experimental data complement the results presented in Renouard et al. [J. Fluid Mech. 177, 381 (1987)] and support the assumption that the solitary Kelvin wave is accompanied by Poincaré waves. Based on the assumption of weak nonlinear, dispersive, and rotational effects, governing equations of the Boussinesq type are derived to model the evolution of an initial disturbance in a two-layer rotating fluid. The numerical study is based on these equations which are analogous to the Boussinesq equations of shallow-water theory and are not constrained to almost unidirectional propagation. Comparison of numerical solutions of the equations and experimental results are very good for moderately nonlinear conditions. These results provide supporting evidence for the resonant interaction of nonlinear Kelvin waves and linear Poincaré waves, as described by Melville et al. [J. Fluid Mech. 206, 1 (1989)].
Magnetohydrodynamic equilibrium of plasma ball lightning
Authors: Wu Hanming , Chen Yunming
Published: July 2002

A study is done of magnetohydrodynamic (MHD) equilibrium of a self-confined plasma ball imbedded in an atmosphere with constant pressure p0. It is found that there is more than one equilibrium state for the plasma ball and the average pressure inside the ball is less than p0. It is also shown that the force-free magnetic configuration cannot exist in the equilibrium state. In comparison with a spheromak, the magnetic axis in ball lightning drifts toward the center about 10%.
Observation of plasma production in a multipole device
Authors: Okuno Yoshihiro , Yagura Shinya , Fujita Hiroharu
Published: July 2002

Plasma production is observed in a simple discharge model of one filament cathode and a multipole magnet anode with variable electrode separation. Two-dimensional velocity distribution functions of electrons and ions and two-dimensional spatial profiles of the potential and electron density are measured with a directional energy analyzer, an emissive probe, and a Langmuir probe. The electrode distance is found to affect the plasma production for isotropic velocity distribution, quiescent plasma, and homogeneous spatial distributions of the potential and electron density. The results may help to explain the fundamental plasma physics of a multipole double plasma machine.
Long-pulse millimeter-wave free-electron laser and cyclotron autoresonance maser experiments
Authors: Danly B. G. , Hartemann F. V. , Chu T. S. , Legorburu P. , Menninger W. L. , Temkin R. J. , Faillon G. , Mourier G.
Published: July 2002

Experimental results on high-power long-pulse free-electron laser (FEL) and cyclotron autoresonance maser (CARM) experiments are summarized. Single-mode operation of a free-electron laser oscillator at 27.4 GHz with a Bragg resonator has been obtained, with an output power of 990 kW for a beam energy of 320 keV and transmitted current of 30 A, corresponding to an efficiency of 10.3%. Free-electron maser (FEM) amplifier operation at 35 GHz has yielded a gain of 26 dB with an output power of 800 kW, corresponding to an efficiency of 8.6%. CARM oscillator experiments at 32 GHz with a different electron gun have yielded lower powers because of poor beam quality; planned CARM experiments are discussed.
Differential filters of elliptic type
Authors: Germano M.
Published: July 2002

Linear differential filters, i.e., filters in which the filtered function f̄ and the original function f are connected by a linear differential equation, are studied on a general basis concerning the elliptic operators of second order. In addition, a particular example of a parabolic filter depending on space and extended to past times is given, and its interest in the context of the large eddy simulation of turbulence is discussed.
Turbulent wakes of stratified flow past a cylinder
Authors: Xu Yunxiu , Fernando Harindra J. S. , Boyer Don L.
Published: July 2002

Laboratory measurements were carried out to investigate the evolution of a turbulent wake behind a right circular cylinder moving in a linearly stratified fluid. The flow field is mainly determined by the internal Froude number Fi and the Reynolds number Re, but at high Re, Fi becomes the only governing parameter. Measurements show that stratified turbulent wakes can be classified into three flow regimes, based on Fi. When Fi<2, the wakes do not grow downstream, and remain at an approximately constant height. For 2<Fi<3, the wakes grow to a maximum height at Nt≊5 and then collapse physically; for Fi≳3, the maximum height is achieved at Nt≊2.5, before the collapse begins. The evolution of such other length scales as the Ozmidov, Kolmogorov, overturning, and Thorpe scales and the maximum Thorpe displacements was measured, and their behavior in the above Fi ranges was delineated. Length scale diagrams for the evolution of stratified turbulence in cylinder wakes were constructed and compared with previous theoretical predictions. The present results provide new insights into the evolution, collapse, and long-term evolution of stratified turbulent flows.
Pulsed gradient NMR measurements and numerical simulation of flow velocity distribution in sphere packings
Authors: Lebon L. , Oger L. , Leblond J. , Hulin J. P. , Martys N. S. , Schwartz L. M.
Published: July 2002

The displacement of water molecules associated with the flow of water inside a nonconsolidated packing of 800 μm OD glass spheres has been measured by a pulsed gradient NMR technique. Using a stimulated spin-echo sequence, mean displacements of up to 300 μm corresponding to measurement times of up to 200 ms can be analyzed. The measurement can be quantitatively calibrated using the pure molecular self-diffusion of water at zero flow conditions. For molecular displacements much smaller than the pore size, the distribution of the flow velocity component along the mean flow direction is determined at Reynolds numbers high enough so that longitudinal molecular diffusion is negligible. An exponential decay of the probability distribution of the displacements is observed at large distances. The results are very similar to those obtained by numerical solution of the Stokes equation in random sphere packings. At longer displacement distances, a secondary peak of the displacement distribution is observed: It is interpreted as the first step toward the transition toward classical dispersion at displacements much larger than the pore size. The influence of molecular diffusion and of the heterogeneities of the magnetic permeability also are discussed.
Capillary effects during droplet impact on a solid surface
Authors: Pasandideh-Fard M. , Qiao Y. M. , Chandra S. , Mostaghimi J.
Published: July 2002

Impact of water droplets on a flat, solid surface was studied using both experiments and numerical simulation. Liquid–solid contact angle was varied in experiments by adding traces of a surfactant to water. Impacting droplets were photographed and liquid–solid contact diameters and contact angles were measured from photographs. A numerical solution of the Navier–Stokes equation using a modified SOLA-VOF method was used to model droplet deformation. Measured values of dynamic contact angles were used as a boundary condition for the numerical model. Impacting droplets spread on the surface until liquid surface tension and viscosity overcame inertial forces, after which they recoiled off the surface. Adding a surfactant did not affect droplet shape during the initial stages of impact, but did increase maximum spread diameter and reduce recoil height. Comparison of computer generated images of impacting droplets with photographs showed that the numerical model modeled droplet shape evolution correctly. Accurate predictions were obtained for droplet contact diameter during spreading and at equilibrium. The model overpredicted droplet contact diameters during recoil. Assuming that dynamic surface tension of surfactant solutions is constant, equaling that of pure water, gave predicted droplet shapes that best agreed with experimental observations. When the contact angle was assumed constant in the model, equal to the measured equilibrium value, predictions were less accurate. A simple analytical model was developed to predict maximum droplet diameter after impact. Model predictions agreed well with experimental measurements reported in the literature. Capillary effects were shown to be negligible during droplet impact when We≫Re1/2.
Hindered diffusion of spherical macromolecules through dilute fibrous media
Authors: Clague David S. , Phillips Ronald J.
Published: July 2002

Results are presented for the effect of solute–fiber hydrodynamic interactions on the hindered diffusion of a spherical macromolecule in random media comprised of cylindrical fibers. Hydrodynamic interactions are calculated by representing the sphere as a collection of point singularities and accounting for the fibers by using a numerical version of slender-body theory. Electrostatic and other nonhydrodynamic interactions are neglected. The calculations show that the hydrodynamic mobility of the solute decreases in an exponential-like fashion as the fiber volume fraction is increased. Also, at a given volume fraction, a medium of thinner fibers hinders solute transport more than a medium of thicker fibers. The results compare well with experimental data, both for protein diffusion in solutions of the polysaccharide Dextran and for protein diffusion in cross-linked agarose gels.
Cascades of energy and helicity in the GOY shell model of turbulence
Authors: Ditlevsen P. D.
Published: July 2002

The effect of extreme hyperviscous damping, νknp,p=∞ is studied numerically in the GOY shell model of turbulence. It has resently been demonstrated [Leveque and She, Phys. Rev. Lett. 75, 2690 (1995)] that the inertial range scaling in the GOY model is non-universal and dependent on the viscous damping. In the present study it is shown that the deviation from Kolmogorov scaling is due to the cascade of the second inviscid invariant. This invariant is non-positive definite and in this sense analogous to the helicity of 3D turbulent flow.
Differential subgrid stress models in large eddy simulations
Authors: Fureby C. , Tabor G. , Weller H. G. , Gosman A. D.
Published: July 2002

Here we study differential stress equation models for the subgrid scale SGS stress tensor in large eddy simulations of urbulent incompressible flow. A study of the SGS stress equation is performed using the principle of frame indifference and the concept of realizability. Closure models are proposed that satisfy these constraints together with the additional requirement that the modeled balance equation must degenerate into the ordinary balance equation for SGS kinetic energy when contracted. The SGS stress equation model is applied to forced homogeneous isotropic turbulence and fully developed turbulent channel flow. For low Re numbers the differential stress equation model behaves in a similar manner to the linear combination model. At higher Re numbers it behaves increasingly like an eddy-viscosity model, but is better able to handle flow and grid anisotropy than traditional SGS models.
An experimental investigation of the intrinsic convection in a sedimenting suspension
Authors: Peysson Yannick , Guazzelli Élisabeth
Published: July 2002

The sedimentation of spheres in a Newtonian fluid is experimentally studied under creeping flow conditions. The mean particle settling velocity and the particle velocity fluctuations are measured across the width of the sedimentation cell. We show that there can be a global intrinsic convection of the suspension superimposed on the settling motion of the particles. Unlike the predictions of the dilute theories of intrinsic convection, this effect is found to be small and even to disappear with increasing concentrations. We also find that there is an ordering of the suspension near the wall, which may be responsible for the observed small magnitude of the convection.