bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonequilibrium fluctuation theory in electrochemical nucleation. II. Experimental determination of critical fluctuation in silver nucleation onto platinum electrode
Authors: Tadano Atsushi , Aogaki Ryoichi
Published: July 2002

Following part I, in this paper, the experiments for silver nucleation onto platinum electrode in AgNO3+NaNO3 solution were carried out to compare the theoretical equations with experimental data. The theoretical examination predicted that after double layer charging current a minimum current emerges, accompanied with rapid monolayer nucleus formation. Since the monolayer nuclei prepare the substrates for the following extensive nucleation, the stage when the nuclei are formed is of much importance. From the experiments for double layer charging current, it was decided that the nucleus formation is activated at the final stage of the charging. This result reinforces the theoretical prediction that the monolayer nuclei are formed by the minimum current. Extrapolation of the minimum current to the critical state gave the critical autocorrelation distance and the average critical concentration fluctuation on the completely active surface. The autocorrelation distance was determined as ãcr=7.82×10−4 m at 300 K, which is quite large in comparison with the scale of the fluctuation and in good agreement with the value obtained in nickel pitting dissolution. On the other hand, as discussed in the previous paper, since there is no intense specific adsorption of ions onto the electrode surface, the electrostatic interaction between the ions accelerates the simultaneous adsorption of Ag+ ion and NO3− ion. Therefore, it was concluded that the concentration of the supporting electrolyte NaNO3 also fluctuates with the Ag+ ionic concentration.
Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film
Authors: Asanuma Miki , Aogaki Ryoichi
Published: July 2002

Following part I, in this paper, the experiments for nickel pitting dissolution in NaCl solution were carried out to compare the theoretical equations with experimental data. From the theoretical examination on nonequilibrium fluctuations in part I, after double-layer charging current, a slow relaxation current, i.e., the diffusion current of the fluctuations was expected. The current must have been a function of passive film coverage. Therefore, if the passive film breaks so slowly, such diffusion current cannot be observed. However, it was actually found that the film is rapidly broken at the final stage of the double-layer charging, and the coverage is determined by the slope of the Cottrel plot of the diffusion current. Then, it was also shown that in the absence of Cl− ions, the film have already been broken at least about 15%, which may be attributed to the passivity breakdown effect of H+ ions. Finally, by the use of extrapolation of the data to the infinity of Cl− ionic concentration, the average critical concentration fluctuation without any passive film was obtained.
Isotropic and anisotropic intermolecular dynamics of liquids studied by femtosecond position-sensitive Kerr lens spectroscopy
Authors: Chang Yong Joon , Cong Peijun , Simon John D.
Published: July 2002

Femtosecond Fourier-transform position-sensitive Kerr lens spectroscopy is shown to be a powerful technique for obtaining the isotropic and anisotropic components of the low-frequency, intermolecular Raman spectra of liquids. The isotropic and anisotropic spectral features are compared for liquids benzene, toluene, and benzonitrile in order to characterize the intermolecular motions in terms of their relative contribution to the different elements of the Raman susceptibility. We observe that the spectral profiles for the isotropic and anisotropic components of the Raman susceptibility tensor of both benzene and toluene are identical within our experimental uncertainty. A frequency-independent depolarization ratio of 0.7±0.1 and 0.75±0.1 are obtained throughout the 0–150 cm−1 region, respectively, for benzene and toluene, respectively. This ratio indicates that the collective intermolecular vibrations can be described as being “depolarized.” On the other hand, we observe that the intermolecular depolarization ratio obtained for benzonitrile is frequency-dependent, gradually decreasing from 0.7 near 0 cm−1 to approximately 0.5 at 150 cm−1. This observation indicates that the intermolecular interactions in benzonitrile become increasingly “polarized” as the frequency is increased. These data are discussed in terms of the distinct molecular properties of each liquid and the differences in their overall intermolecular interaction energies.
Characterization of the I2− anion ground state using conventional and femtosecond photoelectron spectroscopy
Authors: Zanni Martin T. , Taylor Travis R. , Greenblatt B. Jefferys , Soep Benoit , Neumark Daniel M.
Published: July 2002

The X̃ 2Σu+ state of the I2− anion has been fit to a Morse potential using data from two techniques: conventional and femtosecond photoelectron spectroscopy (FPES). Conventional photoelectron spectroscopy is used to determine the adiabatic electron affinity of I2 as well as the well depth and equilibrium nuclear geometry of I2−. In the FPES experiment, the pump pulse induces coherent nuclear motion on the ground state of I2− by resonant impulsive stimulated Raman scattering (RISRS), and the vibrational frequency of the anion is determined from the resulting oscillatory structure in the time-dependent photoelectron spectra. We find the electron affinity (EA) of I2 to be 2.524±0.005 eV, the well depth (De) for I2− to be 1.014±0.005 eV, the equilibrium internuclear separation (Re) to be 3.205±0.005 Å, and the vibrational frequency to be 110±2 cm−1. These values for the I2− potential parameters differ significantly from previous results.
A point-charge model for the nuclear quadrupole moment: Coupled-cluster, Dirac–Fock, Douglas–Kroll, and nonrelativistic Hartree–Fock calculations for the Cu and F electric field gradients in CuF
Authors: Pernpointner Markus , Seth Michael , Schwerdtfeger Peter
Published: July 2002

A point charge model for the nuclear quadrupole moment tensor (PCNQM) is developed in order to determine accurate electric field gradients (EFG) at the relativistic and correlated levels. The symmetric s contributions arising from the Poisson equation are avoided by using an appropriate point charge distribution in three-dimensional space. It is shown that the PCNQM model yields virtually the same EFGs compared to the conventional method of expectation values, if the point charges are set at small displacements from the nucleus (d<10−13 m) and the SCF energy is converged out to 12 significant figures. We further demonstrate that the choice of the point charge ζ is not very critical to the PCNQM perturbation, and that the correlation energy at both the nonrelativistic and relativistic level of theory depends linearly on ζ. This suggests that accurate EFG tensors can be obtained by performing only two correlated calculations for each atom and tensor component. The PCNQM model is tested on one-electron atoms and on the Cu and F EFG in CuF. Relativistic and correlation effects on EFGs are discussed in detail. A Z-expansion on one-electron systems demonstrates that the relativistic correction scales ∼Z5. For the CuF molecule Douglas–Kroll and Dirac–Fock coupled cluster calculations are in good agreement with each other. At the best level of theory (coupled cluster Dirac–Fock plus correction from basis set incompleteness) we obtain a nuclear quadrupole coupling constant for 63Cu of 23 Mhz. This is in very good agreement with the experimental result of 22 MHz considering the large standard deviation of the 63Cu nuclear quadrupole moment applied, 220(10) mb.
Microscopic dynamics in water-swollen poly(vinyl alcohol)
Authors: Müller-Plathe Florian
Published: July 2002

Mixtures of water and poly(vinyl alcohol) (0–96%) have been investigated using molecular dynamics simulations with an atomistically detailed force field. The concentration and temperature (300 and 375 K) dependence of a number of dynamical processes has been studied: Water diffusion including its mechanism, water reorientation and its anisotropy, formation and destruction of hydrogen bonds, water clustering, dielectric properties, local mobility of the polymer. The time scales of these processes extend over several orders of magnitude. Their concentration dependence ranges from no dependence at all to higher than exponential dependence. The quantitative and qualitative results show good agreement with experimental values where available. Comparisons are made with previous simulations of an apolar polymer–solvent mixture (benzene–polystyrene). They indicate, that in spite of the substantial difference in interactions there are qualitative features common to both systems and probably generic to swollen polymers.
Capture and ionization of argon within liquid helium droplets
Authors: Callicoatt Berton E. , Förde Kirk , Ruchti Thomas , Jung Lilian , Janda Kenneth C. , Halberstadt Nadine
Published: July 2002

Liquid helium droplets of initial mean cluster size, 〈N〉, ranging from 600 to 8000 atoms are doped with argon using the pick-up technique. The doped clusters are ionized by electron impact, and the resulting fragment ions are monitored as a function of argon pressure in the pick-up volume. Analysis of the pressure dependent ion signals is used to determine (1) the probability for charge transfer from He+ to the Ar atoms within the droplet, and (2) the probability for fragmentation of the Ark subclusters upon ionization. The measured charge transfer probability from He+ to Ar ranges from 0.05±0.02 for clusters of mean original size 〈N〉=8000 to 0.26±0.05 for 〈N〉=600. Charge transfer to the Ark constituent results in the following qualitative trends; a single Ar atom yields HenAr+ ions; Ar2 mainly yields Ar2+, and Ar3 mainly fragments to yield Ar2+. Simulations of the results are performed to extract information on how the charge transfer and fragmentation processes within the ionized droplet dependent on the size of the helium droplet and the number of argon atoms captured. We use the positive-hole resonant-hopping mechanism to determine that the He+ hops 3–4 times prior to localization with either the Ar dopant or another He atom to form He2+. This corresponds to a time scale for He2+ formation of 60–80 fs.
Solvent effects on the vibrational frequency of I2− in size-selected I2−(Ar)n and I2−(CO2)n clusters
Authors: Zanni Martin T. , Greenblatt B. Jefferys , Neumark Daniel M.
Published: July 2002

The vibrational frequency of the I2− chromophore in size-selected I2−(Ar)n and I2−(CO2)n clusters has been measured to wave-number accuracy. The frequencies are determined by creating a coherent superposition of vibrational levels with a femtosecond laser pulse via a resonance impulsive stimulated Raman scattering process. The resulting wave-packet oscillations are detected with femtosecond photoelectron spectroscopy. Blueshifting of the frequency occurs upon solvation, with larger shifts observed for solvation with CO2. The nature of the shifting is discussed and related to specific cluster geometries.
A simplified density matrix minimization for linear scaling self-consistent field theory
Authors: Challacombe Matt
Published: July 2002

A simplified version of the Li, Nunes and Vanderbilt [Phys. Rev. B 47, 10891 (1993)] and Daw [Phys. Rev. B 47, 10895 (1993)] density matrix minimization is introduced that requires four fewer matrix multiplies per minimization step relative to previous formulations. The simplified method also exhibits superior convergence properties, such that the bulk of the work may be shifted to the quadratically convergent McWeeny purification, which brings the density matrix to idempotency. Both orthogonal and nonorthogonal versions are derived. The AINV algorithm of Benzi, Meyer, and Tůma [SIAM J. Sci. Comp. 17, 1135 (1996)] is introduced to linear scaling electronic structure theory, and found to be essential in transformations between orthogonal and nonorthogonal representations. These methods have been developed with an atom-blocked sparse matrix algebra that achieves sustained megafloating point operations per second rates as high as 50% of theoretical, and implemented in the MondoSCF suite of linear scaling SCF programs. For the first time, linear scaling Hartree–Fock theory is demonstrated with three-dimensional systems, including water clusters and estane polymers. The nonorthogonal minimization is shown to be uncompetitive with minimization in an orthonormal representation. An early onset of linear scaling is found for both minimal and double zeta basis sets, and crossovers with a highly optimized eigensolver are achieved. Calculations with up to 6000 basis functions are reported. The scaling of errors with system size is investigated for various levels of approximation.
Coherent acoustic mode oscillation and damping in silver nanoparticles
Authors: Del Fatti N. , Voisin C. , Chevy F. , Vallée F. , Flytzanis C.
Published: July 2002

Using a femtosecond pump-probe technique, the fundamental mechanical radial mode of silver nanoparticles is coherently excited and probed via its interaction with the electron gas. The mechanical oscillations are launched by an indirect displacive process and are detected via the induced modulation of the surface plasmon resonance frequency. The measured fundamental radial mode period and damping time are found to be proportional to the nanoparticle radius in the range of 3–15 nm, in agreement with theoretical predictions.
An intramolecular theory of the mass-independent isotope effect for ozone. I
Authors: Hathorn B. C. , Marcus R. A.
Published: July 2002

An intramolecular theory of the unusual mass-independent isotope effect for ozone formation and dissociation is described. The experiments include the enrichment factor, its dependence on the ambient pressure, the ratio of the formation rates of symmetric and asymmetric ozone isotopomers, the enrichment of ozone formed from heavily enriched oxygen isotopes, the comparison of that enrichment to that when the heavy isotopes are present in trace amounts, the isotopic exchange rate constant, and the large mass-dependent effect when individual rate constants are measured, in contrast with the mass-independent effect observed for scrambled mixtures. To explain the results it is suggested that apart from the usual symmetry number ratio of a factor of 2, the asymmetric ozone isotopomers have a larger density of reactive (coupled) quantum states, compared with that for the symmetric isotopomers (about 10%), due to being more “RRKM-like” (Rice–Ramsperger–Kessel–Marcus): Symmetry restricts the number of intramolecular resonances and coupling terms in the Hamiltonian which are responsible for making the motion increasingly chaotic and, thereby, increasingly statistical. As a result the behavior occurs regardless of whether the nuclei are bosons (16O, 18O) or fermions (17O). Two alternative mechanisms are also considered, one invoking excited electronic states and the other invoking symmetry control in the entrance channel. Arguments against each are given. An expression is given relating the mass-independent rates of the scrambled systems to the mass-dependent rates of the unscrambled ones, and the role played by a partitioning term in the latter is described. Different definitions for the enrichment factor for heavily enriched isotopic systems are also considered. In the present paper attention is focused on setting up theoretical expressions and discussing relationships. They provide a basis for future detailed calculations.
The vibrational structure of benzene adsorbed on Si(001)
Authors: Staufer Markus , Birkenheuer Uwe , Belling Thomas , Nörtemann Folke , Rösch Notker , Widdra Wolf , Kostov Krassimir L. , Moritz Thomas , Menzel Dietrich
Published: July 2002

High resolution electron energy loss spectroscopy (HREELS) measurements and density functional model cluster calculations are presented to clarify the vibrational structure of the adsorption system C6H6/Si(001). All vibrational modes of the adsorption complex, which previously was identified to exhibit a cyclohexadiene-like structure, have been calculated and characterized according to the motion of the different atoms of the adsorption complex. Special emphasis is placed on the low-frequency modes. The coupling between the adsorbate and the substrate modes is analyzed with the help of a model that represents various limiting situations. Different coupling variants are found to apply to different collective modes of the adsorbate. The A1 and B1 modes can be described rather well by a model that only encompasses the adsorbate and the Si dimer underneath; for the A2 and B2 modes a frozen substrate description of the adsorption complex is more appropriate.
Multireference perturbation theory for large restricted and selected active space reference wave functions
Authors: Celani Paolo , Werner Hans-Joachim
Published: July 2002

A multireference second-order perturbation theory (MRPT2) has been developed which allows the use of reference wave functions with large active spaces and arbitrary configuration selection. Internally contracted configurations are used as a basis for all configuration subspaces of the first-order wave function for which the overlap matrix depends only on the second-order density matrix of the reference function. Some other subspaces which would require the third- or fourth-order density matrices are left uncontracted. This removes bottlenecks of the complete active space second order pertubation theory (CASPT2) caused by the need to construct and diagonalize large overlap matrices. Preliminary applications of the new method for 1,2-dihydronaphthalene (DHN) and free base porphin are presented in which the effect of applying occupancy restrictions in the reference wave function (restricted active space second-order perturbation theory, RASPT2) and reference configuration selection (general MRPT2) on electronic excitation energies is tested. In the case of the S0–S1 transition of DHN rapid convergence of the RASPT2 and MRPT2 excitation energies towards the CASPT2 value with increasing number of reference configurations is observed. In the calculations for the lowest five states of porphin all 24 valence π-orbitals were included in the active space of restricted active space self-consistent field (MCSCF) and MRPT2 calculations. From the RASSCF wave functions different subsets of reference configurations were selected on the basis of their coefficients. In this case convergence of the excitation energies with decreasing selection threshold is found to be rather slow, indicating the need for more elaborate selection schemes. The computed excitation energies are in good agreement with previous CASPT2 results obtained with much smaller active spaces.
Electron-phonon coupling dynamics in very small (between 2 and 8 nm diameter) Au nanoparticles
Authors: Hodak José H. , Henglein Arnim , Hartland Gregory V.
Published: July 2002

Ultrafast laser experiments were used to study electron-phonon coupling in Au nanoparticles in the 2.5 to 8 nm size range in aqueous solution. The electron-phonon coupling constants for these samples were found to be independent of the particle size. This is attributed to a weak interaction between the electron gas and the surface phonon modes in Au. Calculations were performed which show that the coupling between the hot electrons and the surface accounts for less than 10% of the total electron energy losses for these particles. Thus, bulk electron-phonon coupling dominates the relaxation of excited electrons in Au particles, for particles as small as several hundred atoms.
Einstein–Maxwell equations and the conformal Ricci collineations
Authors: Faridi Abbas M.
Published: July 2002

The Einstein–Maxwell field equations for non-null electromagnetic fields are studied under the assumption of admitting a conformal Ricci collineation. It is shown that a non-null electromagnetic field does not admit any conformal Ricci collineation, unless the generators of the symmetry groups are Killing vector fields. Furthermore, it is shown that the energy-momentum tensor of a non-null electromagnetic field can admit a conformal Ricci collineation, if and only if the collineation is homothetic. The restrictions on non-null Maxwell field, its sources, and its invariants implied by the symmetry condition are calculated. An example of a space-time satisfying the Einstein–Maxwell equations, and admitting a homothetic conformal vector field is also given.
The nonlinear Boltzmann equation with partially absorbing boundary conditions. Global existence and uniqueness results
Authors: Toscani G. , Protopopescu V.
Published: July 2002

The method applied by Bellomo and Toscani [J. Math. Phys. 26, 334 (1985)] for the Boltzmann equation in an infinite medium to establish global results for bounded media with partially absorbing boundary conditions is generalized. The method does not require that the equilibrium solution be the vacuum state and, accordingly, does not rely on positivity/monotonicity arguments. The growth produced by the nonlinearity is compensated by the combined effect of streaming and (partial) absorption (leakage) at the boundary.
The quantum relativistic two-body bound state. I. The spectrum
Authors: Arshansky R. , Horwitz L. P.
Published: July 2002

In the framework of a manifestly covariant quantum theory on space-time, it is shown that the ground state mass of a relativistic two-body system with O(3,1) symmetric potential is lower when represented by a wave function with support in an O(2,1) invariant subspace of the spacelike region. The wave functions for the relativistic bound states are obtained explicitly. Coulomb type binding, the harmonic oscillator, and the relativistic square well are treated as examples. The mass spectrum is determined by a differential equation in the invariant spacelike interval ρ, which can be put into correspondence with the radial part of a nonrelativistic Schrödinger equation with potential of the same form, where r is replaced by ρ. In the case that the binding is small compared to the particle masses, the mass spectrum (bounded below) is well-approximated by the results of the nonrelativistic theory. The eigenfunctions transform under the full Lorentz group as elements of an induced representation with O(2,1) little group. This representation is studied in a succeeding paper.
On the Landau–Zener formula for two-level systems
Authors: Jakšić Vojkan , Segert Jan
Published: July 2002

It is shown that the transition probability between eigenstates of a time-dependent two-level system is given by the Fourier transform in the adiabatic parameter of a function determined by the Hamiltonian. Sufficient and necessary conditions for the transition probability between levels to decrease exponentially in the adiabatic limit are established. Our method involves a Paley–Wiener type theorem to study the Fourier transform of functions belonging to a Hardy class on a strip.
Hierarchy of negative order equation and its Lax pair
Authors: Zhou Ruguang
Published: July 2002

A simple method is presented to construct the hierarchy of a negative order equations from a given eigenvalue problem. The hierarchy of the negative order equation we obtained is the natural continuation of a corresponding hierarchy of a higher order equation, although most of the negative order equations are complicated; we give their Lax representations. Three examples are discussed to illustrate the effectiveness of our method.
On the statistical independence of algebras of observables
Authors: Florig Martin , Summers Stephen J.
Published: July 2002

We reexamine various notions of statistical independence presently in use in algebraic quantum theory, establishing alternative characterizations for such independence, some of which are also valid without assuming that the observable algebras mutually commute. In addition, in the context which holds in concrete applications to quantum theory, the equivalence of three major notions of statistical independence is proven.
A Lanczos potential in Kerr geometry
Authors: Bergqvist Göran
Published: July 2002

A Lanczos potential is a tensor of rank 3 for which a certain combination of derivatives always equals the Weyl tensor. General existence theorems have been proved before and some explicit expressions for Lanczos potentials in certain spacetimes have been found. In this paper we explicitly study a Lanczos potential in Kerr spacetime obtained from a previously studied flat connection. As an application it is shown that the mass can be expressed as an integral of the Lanczos potential.
On 2D Euler equations. I. On the energy–Casimir stabilities and the spectra for linearized 2D Euler equations
Authors: Li Yanguang (Charles)
Published: July 2002

In this paper, we study a linearized two-dimensional Euler equation. This equation decouples into infinitely many invariant subsystems. Each invariant subsystem is shown to be a linear Hamiltonian system of infinite dimensions. Another important invariant besides the Hamiltonian for each invariant subsystem is found and is utilized to prove an “unstable disk theorem” through a simple energy–Casimir argument [Holm et al., Phys. Rep. 123, 1–116 (1985)]. The eigenvalues of the linear Hamiltonian system are of four types: real pairs (c,−c), purely imaginary pairs (id,−id), quadruples (±c±id), and zero eigenvalues. The eigenvalues are computed through continued fractions. The spectral equation for each invariant subsystem is a Poincaré-type difference equation, i.e., it can be represented as the spectral equation of an infinite matrix operator, and the infinite matrix operator is a sum of a constant-coefficient infinite matrix operator and a compact infinite matrix operator. We have obtained a complete spectral theory.
Metastability and nucleation for conservative dynamics
Authors: Hollander F. den , Olivieri E. , Scoppola E.
Published: July 2002

In this paper we study metastability and nucleation for a local version of the two-dimensional lattice gas with Kawasaki dynamics at low temperature and low density. Let β>0 be the inverse temperature and let Λ̄⊂Λβ⊂Z2 be two finite boxes. Particles perform independent random walks on Λβ\Λ̄ and inside Λ̄ feel exclusion as well as a binding energy U>0 with particles at neighboring sites, i.e., inside Λ̄ the dynamics follows a Metropolis algorithm with an attractive lattice gas Hamiltonian. The initial configuration is chosen such that Λ̄ is empty, while a total of ρ|Λβ| particles is distributed randomly over Λβ\Λ̄ with no exclusion. That is to say, initially the system is in equilibrium with particle density ρ conditioned on Λ̄ being empty. For large β, the system in equilibrium has Λ̄ fully occupied because of the binding energy. We consider the case where ρ=e−Δβ for some Δ∈(U,2U) and investigate how the transition from empty to full takes place under the dynamics. In particular, we identify the size and shape of the critical droplet and the time of its creation in the limit as β→∞ for fixed Λ and limβ→∞(1/β) log|Λβ|=∞. In addition, we obtain some information on the typical trajectory of the system prior to the creation of the critical droplet. The choice Δ∈(U,2U) corresponds to the situation where the critical droplet has side length lc∈(1,∞), i.e., the system is metastable. The side length of Λ̄ must be much larger than lc and independent of β, but is otherwise arbitrary. Because particles are conserved under Kawasaki dynamics, the analysis of metastability and nucleation is more difficult than for Ising spins under Glauber dynamics. The key point is to show that at low density the gas in Λβ\Λ̄ can be treated as a reservoir that creates particles with rate ρ at sites on the interior boundary of Λ̄ and annihilates particles with rate 1 at sites on the exterior boundary of Λ̄. Once this approximation has been achieved, the problem reduces to understanding the local metastable behavior inside Λ̄ in the presence of a nonconservative boundary. The dynamics inside Λ̄ is still conservative and this difficulty has to be handled via local geometric arguments. Here it turns out that the Kawasaki dynamics has its own peculiarities. For instance, rectangular droplets tend to become square through a movement of particles along the border of the droplet. This is different from the behavior under the Glauber dynamics, where subcritical rectangular droplets are attracted by the maximal square contained in the interior, while supercritical rectangular droplets tend to grow uniformly in all directions (at least for not too long a time) without being attracted by a square.
Sweeping decorrelation in isotropic turbulence
Authors: Chen Shiyi , Kraichnan Robert H.
Published: July 2002

Tennekes [J. Fluid Mech. 67, 561 (1975)] estimated the time decorrelation of inertial-range excitation in isotropic turbulence by assuming effective statistical independence of the one-time distributions of inertial-range and energy-range excitation. This picture has been challenged by Yakhot, Orszag, and She [Phys. Fluids A 1, 184 (1989)], who studied forced turbulence by renormalization-group (RNG) methods. The analysis given in the present paper leads to the conclusion that (a) precise coherence between energy-range and inertial-range excitation is needed to inhibit sweeping effects; (b) in the case of randomly forced turbulence, this coherence is impossible and Tennekes’ picture is unavoidable; and (c) the RNG analysis does not demonstrate inhibition of sweeping; instead, it discards sweeping effects at the outset. To augment the present study, an advected passive scalar is examined by computer simulation. Sweeping effects on small scales survive even in the case of long-time advection by a frozen velocity field. The observed probability distributions resemble those for the alignment of vorticity and velocity observed in flow simulations.
Fourth-order nonlinear evolution equation for two Stokes wave trains in deep water
Authors: Dhar A. K. , Das K. P.
Published: July 2002

Fourth-order nonlinear evolution equations, which are a good starting point for the study of nonlinear water waves as first pointed out by Dysthe [Proc. R. Soc. London Ser. A 369, 105 (1979)] and later elaborated by Janssen [J. Fluid Mech. 126, 1 (1983)], are derived for a deep-water surface gravity wave packet in the presence of a second wave packet. Here it is assumed that the space variation of the amplitude takes place only in a direction along which the group velocity projection of the two waves overlap. Stability analysis is made for a uniform Stokes wave train in the presence of a second wave train. Graphs are plotted for maximum growth rate of instability and for wave number at marginal stability against wave steepness. Significant deviations are noticed from the results obtained from the third-order evolution equations which consist of two coupled nonlinear Schrödinger equations.
A nonlinear stability problem of convection in a porous vertical slab
Authors: Qin Y. , Kaloni P. N.
Published: July 2002

A nonlinear analysis of convection in a porous vertical slab, using the Brinkman model, is given. A sufficient condition for the nonlinear disturbances to be asymptotically stable is derived. This imposes certain restrictions on the Rayleigh number and on the initial temperature gradient.
Effective diffusion in time-periodic linear planar flow
Authors: Indeikina Alexandra , Chang Hsueh-Chia
Published: July 2002

It is shown that when a point source of solute is inserted into a time-periodic, unbounded linear planar flow, the large-time, time-average transport of the solute can be described by classical anisotropic diffusion with constant effective diffusion tensors. For a given vorticity and forcing period, elongational flow is shown to be the most dispersive followed by simple shear and rotational flow. Large-time diffusivity along the major axis of the time-average concentration ellipse, whose alignment is predicted from the theory, is shown to increase with vorticity for all flows and decrease with increasing forcing frequency for elongational flow and simple shear. For the interesting case of rotational flow, there exist discrete resonant frequencies where the time-average major diffusivity reaches local maxima equal to the time-average steady flow case with zero forcing frequency.
Power balance analysis of ion Bernstein wave heating experiments in the Alcator C tokamak
Authors: Moody John D. , Porkolab Miklos
Published: July 2002

Energy and particle transport processes in Alcator C [Phys. Rev. Lett. 60, 298, (1988)] discharges heated by directly launched ion Bernstein waves (IBW) are investigated using the onetwo transport code [Nucl. Fusion 26, 329 (1986)]. The dependence of the observed ion heating rate (ΔTH/Prf) on plasma density is shown to result mainly from the inherent ion energy confinement that is characteristic of these discharges in the Ohmic phase and not on IBW propagation and absorption characteristics. The inferred value of the Ohmic ion thermal conduction exhibits an increasing anomaly with increasing plasma density. At a plasma density of n̄e ≲1×1020 m−3, the inferred ion thermal conduction is nearly equal to the Chang–Hinton neoclassical prediction. However, at a plasma density of n̄e ≳2×1020 m−3, the inferred ion thermal conduction increases to about 5–8 times the Chang–Hinton neoclassical value. This increasing anomaly, which may result from the ion temperature gradient driven instability, can essentially account for the observed ion heating rate behavior during IBW heating. The effect of edge turbulence on IBW propagation is modeled using a Monte Carlo direct sampling simulation. The result is mainly a radial broadening of the calculated power deposition profile with increasing plasma density.
Modified polynomial function model for reversed-field pinches
Authors: Shen W. , Sprott J. C.
Published: July 2002

A generalized analytic representation for the magnetic field and current density profiles in a reversed-field pinch (RFP) is proposed. These profiles have zero current density at the wall and finite plasma pressure. The profiles are characterized by two free parameters here taken to be the field-reversal parameter (F) and pinch parameter (Θ). From the profiles, many useful quantities such as magnetic energy, beta, inductance, resistance, and Ohmic input power are calculated. These quantities provide a basis for analyzing experimental data and performing electrical circuit modeling of RFP discharges.
Dynamics of ponderomotive self-focusing in plasmas
Authors: Cohen Bruce I. , Lasinski Barbara F. , Langdon A. Bruce , Cummings Julian C.
Published: July 2002

The space-time evolution of nonlinear self-focusing of a coherent electromagnetic beam in a plasma is calculated. Parameters are considered for which the dominant nonlinearity is the ponderomotive force and the plasma response is hydrostatic. A simple nonlinear wave equation with a saturable, exponential nonlinearity is presented and solved in the paraxial limit. Nondimensional scaled variables are introduced, and both steady-state and time-dependent numerical solutions of the scaled equation are calculated in the limit of cylindrical symmetry. Self-focusing can be important both for high-power lasers in inertial-confinement-fusion applications and for heating of magnetically confined plasmas with intense, pulsed free-electron lasers.
A second-order theory for transverse ion heating and momentum coupling due to electrostatic ion cyclotron waves
Authors: Miller Ronald H. , Winske Dan , Gary S. Peter
Published: July 2002

A second-order theory for electrostatic instabilities driven by counterstreaming ion beams is developed which describes momentum coupling and heating of the plasma via wave–particle interactions. Exchange rates between the waves and particles are derived, which are suitable for the fluid equations simulating microscopic effects on macroscopic scales. Using a fully kinetic simulation, the electrostatic ion cyclotron instability due to counterstreaming H+ beams has been simulated. A power spectrum from the kinetic simulation is used to evaluate second-order exchange rates. The calculated heating and momentum loss from second-order theory is compared to the numerical simulation.
Thermal nonequilibrium hypersonic shock layer near the stagnation point
Authors: Yen S. M.
Published: July 2002

At high altitudes, the condition in the shock layer near the stagnation point deviates significantly from thermal equilibrium. This condition is comparable to that in a relatively strong shock wave. The shock layer is, therefore, a Knudsen layer. Together with the strong shock wave preceding the shock layer, the hypersonic flow problem near a stagnation point is that of a triple Knudsen layer.
Self-focusing and ion wave generation in laser-produced plasmas
Authors: Rankin R. , Marchand R. , Capjack C. E.
Published: July 2002

Two-dimensional hydrodynamic simulations of laser light self-focusing in a hydrogen plasma are presented. The simulation code includes a model for laser beam propagation which accounts for inverse bremsstrahlung absorption, refraction, diffraction, and ponderomotive forces. A Gaussian hot spot, superimposed upon a collimated, spatially uniform laser beam, is used to initiate self-focusing. Intense filaments provide a driving source for ion waves near the axis of the laser beam. The radially propagating ion waves cause spatiotemporal modulations of the flux where it is initially uniform, as well as the more usual focusing that occurs along the axis. Some of the factors affecting the generation of the ion waves are considered. In particular, the effect of changing the amplitude and width of the imposed nonuniformity is investigated. The intensity thresholds for thermal and ponderomotively driven self-focusing have also been determined by artificially turning the ponderomotive force on and off.
A new quotidian equation of state (QEOS) for hot dense matter
Authors: More R. M. , Warren K. H. , Young D. A. , Zimmerman G. B.
Published: July 2002

The quotidian equation of state (QEOS) is a general-purpose equation of state model for use in hydrodynamic simulation of high-pressure phenomena. Electronic properties are obtained from a modified Thomas–Fermi statistical model, while ion thermal motion is described by a multiphase equation of state combining Debye, Grüneisen, Lindemann, and fluid-scaling laws. The theory gives smooth and usable predictions for ionization state, pressure, energy, entropy, and Helmholtz free energy. When necessary, the results may be modified by a temperature-dependent pressure multiplier which greatly extends the class of materials that can be treated with reasonable accuracy. In this paper a comprehensive evaluation of the resulting thermodynamic data is given including comparison with other theories and shock-wave data.
Sweeping and straining effects in sound generation by high Reynolds number isotropic turbulence
Authors: Zhou Ye , Rubinstein Robert
Published: July 2002

The sound radiated by isotropic turbulence is computed using inertial range scaling expressions for the relevant two-time and two-point correlations. The result depends on whether the decay of Eulerian time correlations is dominated by large scale sweeping or by local straining: the straining hypothesis leads to an expression for total acoustic power given originally by Proudman, whereas the sweeping hypothesis leads to a more recent result due to Lilley.
Development of a two-equation heat transfer model based on direct simulations of turbulent flows with different Prandtl numbers
Authors: Nagano Y. , Shimada M.
Published: July 2002

Low-Reynolds-number type k−ε and kt−εt models have been constructed with the aid of direct numerical simulation (DNS) databases. The proposed models incorporate new velocity and time scales to represent various sizes of eddies in velocity and thermal fields with different Prandtl numbers. The validity of the present k−ε model was tested by application to basic and complex flows such as flows with injection and suction, flows with strong adverse and favorable pressure gradients, and flows with separation and reattachment, while comparing the relevant DNS and reliable experimental data. Fundamental properties of the proposed kt−εt model were first verified in basic flows under arbitrary wall thermal boundary conditions and next in backward-facing step flows at various Prandtl numbers through a comparison of the predictions with the DNS and measurements. These comparisons have proven that the proposed models for both velocity and thermal fields have wide applicability to science and engineering and have sufficient capability to perform highly stable computations at any Prandtl numbers, irrespective of flow configurations.
Modelling of differential diffusion effects in nonpremixed nonreacting turbulent flow
Authors: Kronenburg A. , Bilger R. W.
Published: July 2002

The modelling of differential diffusion with a conditional moment closure (CMC) method is considered. Direct numerical simulations for scalar fields in homogeneous isotropic decaying turbulence are used to quantify the non-unity Schmidt number effects. Unlike the equal molecular diffusion case, where terms involving deviations from the conditional average usually make a negligible contribution, these terms contribute significantly to the equations governing the evolution of the conditional average scalar in the presence of different molecular diffusion coefficients. Together with the conditional scalar dissipation they come to balance conditional scalar diffusion, the driving force of the differential diffusion effects. The shape of the conditional averages of these terms in mixture fraction space coincides with the approximate shape of the conditional average differential diffusion, Qz. Therefore, dividing by Qz they are only functions of time. Conditional moment closure leads to good predictions of Qz provided the time function is properly modelled. A model for the time scale is suggested.
Sonoluminescence light emission
Authors: Hilgenfeldt Sascha , Grossmann Siegfried , Lohse Detlef
Published: July 2002

Single bubble sonoluminescence is not an exotic phenomenon but can quantitatively be accounted for by applying a few well-known, simple concepts: the Rayleigh–Plesset dynamics of the bubble’s radius, polytropic uniform heating of the gas inside the bubble during collapse, the dissociation of molecular gases, and thermal radiation of the remaining hot noble gas, where its finite opacity (transparency for its own radiation) is essential. A system of equations based on these ingredients correctly describes the widths, shapes, intensities, and spectra of the emitted light pulses, all as a function of the experimentally adjustable parameters, namely, driving pressure, driving frequency, water temperature, and the concentration and type of the dissolved gas. The theory predicts that the pulse width of strongly forced xenon bubbles should show a wavelength dependence, in contrast to argon bubbles.
Absolute and convective instabilities of a swirling jet/wake shear layer
Authors: Loiseleux Thomas , Delbende Ivan , Huerre Patrick
Published: July 2002

The absolute (AI)/convective (CI) nature of the instability is determined in the family of swirling jet/wake shear layers considered by Martin and Meiburg [Phys. Fluids 6, 424 (1994)] and Lim and Redekopp [Eur. J. Mech. B/Fluids 17, 165 (1998)]. This idealized model includes as essential ingredients both the centrifugal instability associated with the swirl difference and the Kelvin–Helmholtz instability associated with the swirl and axial velocity differences between the core and the outer flow. Centrifugally stabilizing or destabilizing swirl differences are found to promote AI, but a centrifugally destabilizing configuration is more effective in triggering such a transition. For sufficiently large swirl differences, both co-flowing jets and wakes may become AI. In the case of jets, a centrifugally destabilizing swirl difference first brings about AI via the axisymmetric mode m=0 in a large range of mean swirl values. By contrast, a centrifugally stabilizing swirl difference triggers AI via the helical mode m=−∞. In the case of wakes, a centrifugally destabilizing swirl difference leads to AI via the bending mode m=1 whereas a centrifugally stabilizing swirl difference triggers AI via various negative helical modes m=−1,−2, etc.
Absolute and convective instability character of slender viscous vortices
Authors: Yin Xie-Yuan , Sun De-Jun , Wei Ming-Jun , Wu Jie-Zhi
Published: July 2002

Motivated by the need for effective vortex control, the character of absolute and convective instabilities (AI/CI) of incompressible and high-Mach number slender vortices with axial-velocity deficit is studied. Attention is focused on the disturbance modes which lead to the maximum absolute growth rate, and their dependence on flow conditions such as axial-flow profile, Reynolds number, and Mach number. A significant difference between the AI/CI and temporal-instability characters of the vortices occurs as the axial velocity deficit reduces. These theoretical results are applied to the flow region where vortex breakdown happens. It is found that the breakdown region is absolutely unstable, where waves are dominated by the spiral disturbance with lowest azimuthal wave number, in reasonable agreement with measurement.
Extensional viscosity of dilute polystyrene solutions: Effect of concentration and molecular weight
Authors: Gupta Rahul K. , Nguyen D. A. , Sridhar T.
Published: July 2002

This paper reports a detailed investigation of the steady shear and extensional properties of mono-disperse polystyrene solutions for a range of molecular weights from 1.95 to 20 million and a range of concentrations from 69 ppm to 777 ppm. The steady shear and dynamic properties are reasonably well described by the Zimm model. The relaxation time and polymer viscosity evaluated using the Zimm model exhibit the expected scaling with concentration and molecular weight for theta solutions. In an extensional flow, these solutions show strain hardening and need about 5.5 to 6 strain units to reach steady state. The stress growth depends both on strain rate and strain. However, at moderate values of strain and when the Weissenberg number exceeds 6, the extensional stress growth depends only on total strain. The steady state extensional viscosity for each fluid was observed to depend on the magnitude of Weissenberg number (Wi). For Wi<6, the steady state extensional viscosity was observed to be an increasing function of the strain rate. At a Wi∼10, the steady state extensional viscosity exhibits a maximum and surprisingly, for Wi>10, the steady state extensional viscosity is a decreasing function of the strain rate. The results indicate that even dilute solutions, with concentrations as low as 0.21 times the critical concentration, do show extension thinning under certain conditions. The transient and steady state extensional viscosities are found to be proportional to molecular weight (Mw) and concentration (c). This result is rather unexpected as the Zimm model would predict a scaling with c and Mw0.5 for the transient viscosity and scaling with c and Mw1.5 for the steady state viscosity. On the other hand, the Rouse model predicts a scaling with c and Mw (for transient extensional viscosity) and with c and Mw2 (for steady state extensional viscosity). Hence the data appear to follow a Rouse-like behavior for the transient viscosity. This implies that the advent of the stretching reduces the hydrodynamic interaction and a free draining behavior is obtained. As a result, predictions using the Zimm model parameters, estimated from shear data, are unable to predict the transient extensional viscosity. The data are analyzed using a constitutive equation that incorporates an anisotropic drag coefficient along with a Rouse spectrum of relaxation time. Such a model captures the extensional behavior of the solutions.