bluuebunny/crossref_metadata_2025 huggingface dataset:
The electronic structure of polyaniline and doped phases studied by soft x-ray absorption and emission spectroscopies
Authors: Magnuson M. , Guo J.-H. , Butorin S. M. , Agui A. , Såthe C. , Nordgren J. , Monkman A. P.
Published: July 2002

The electronic structure of the conjugated polymer, polyaniline, has been studied by resonant and nonresonant soft x-ray emission spectroscopy using synchrotron radiation for the excitation. The measurements were made on polyaniline and a few doped (protonated) phases for both the carbon and nitrogen contents. The resonant x-ray emission spectra show depletion of the π electron bands due to the selective excitation which enhances the effect of symmetry selection rules. The valence band structures in the x-ray emission spectra attributed to the π bands show unambiguous changes of the electronic structure upon protonation. By comparing to x-ray absorption measurements, the chemical bonding and electronic configuration is characterized.
Ab initio calculation of elastic constants of SiO2 stishovite and α-quartz
Authors: Holm Bengt , Ahuja Rajeev
Published: July 2002

We present theoretical ab initio results for the elastic constants of two different phases of SiO2, stishovite and α-quartz using the plane-wave-pseudopotential method. Our calculated values are in very good agreement with the experimental data as well as some of the existing model calculations. SiO2 is one of the most abundant minerals in the earth’s mantle. Therefore, the knowledge of its elastic behavior will be very important from a seismology point of view. Our calculated sound velocities are presented and are accordingly also in good agreement with those derived from experiment.
A simulation study of the kinetics of passage of CO2 and N2 through the liquid/vapor interface of water
Authors: Somasundaram T. , in het Panhuis M. , Lynden-Bell R. M. , Patterson C. H.
Published: July 2002

The rate of passage of molecules of carbon dioxide and nitrogen through the vapor–liquid interface of water at 300 K is studied by simulation. Previous work has established the form of the free energy profile which has a minimum when the solute molecule is on the surface and a barrier between this state and solution in the bulk liquid. In one set of simulations, trajectories were initiated in the gas phase. From these, the average lifetime of molecules in the surface is determined to be considerably longer than the inverse of the energy relaxation rate, so that the sticking coefficient is one and exiting molecules have no memory of their original velocities. However, most molecules do return to the gas phase rather than entering the bulk solution. The rate of passage of molecules over the free energy barrier is studied using the reactive flux method with trajectories initiated near the top of the barrier. The results for nitrogen, in particular, give a good plateau in the time-dependent transmission coefficient and hence a reliable rate constant. The results from these two sets of simulations are combined to give an effective interface width which is used to determine the permeability of thin water films. These results are compared to experimental permeabilities of thin Newton black soap films. The rate-determining step for solution in bulk water is not passage through the few Ångstroms width of the interface we study, but rather the transport from the vicinity of the interface into the bulk over the larger distance scale of μm.
Global ab initio potential energy surfaces for the ClH2 reactive system
Authors: Bian Wensheng , Werner Hans-Joachim
Published: July 2002

Two new global ab initio potential energy surfaces (called BW1 and BW2) for the ClH2 reactive system are presented. These are based on internally contracted multireference configuration interaction calculations using a very large basis set, performed at 1200 geometries. Accurate analytical fits have been generated using the functional form proposed by Aguado and Paniagua. The BW1 surface is based on the original ab initio points. This surface slightly underestimates the dissociation energies of the diatomic fragments and overestimates the barrier height. Therefore, a second surface (BW2) has been computed by scaling the correlation energies at all geometries with a constant factor, which was chosen such that the dissociation energies of HCl and H2 are reproduced more accurately. The barrier heights for the collinear transition state of the Cl+H2→HCl+H reaction are computed to be 8.14 kcal/mol and 7.61 kcal/mol for the BW1 and BW2 surfaces, respectively. To these values the spin–orbit correction of 0.84 kcal/mol has to be added, yielding a best estimate for the true barrier height of 8.45 kcal/mol. In the entrance channel of the Cl+H2→HCl+H reaction a T-shaped van der Waals well with a depth of 0.51 kcal/mol is found, while in the exit channel a van der Waals well with a collinear geometry and a depth of 0.45 kcal/mol is predicted. For the H+ClH exchange reaction, which also has a collinear transition state, the barrier heights are computed to be 18.5 kcal/mol and 17.9 kcal/mol for BW1 and BW2, respectively. It is shown that the topology of the new surfaces differs qualitatively from previous semiempirical surfaces, and the implications on the dynamics of the H2+Cl reaction are discussed.
Pump/probe self heterodyned 2D spectroscopy of vibrational transitions of a small globular peptide
Authors: Hamm Peter , Lim Manho , DeGrado William F. , Hochstrasser Robin M.
Published: July 2002

Pump/probe self-heterodyne experiments on the amide I band of a small de novo cyclic pentapeptide were utilized to demonstrate a novel form of two-dimensional (2D) vibrational spectroscopy. Spectrally resolved cross peaks are observed, which measure the coupling between different peptide units and which can be related to the structure of the peptide, in analogy to 2D-NMR spectroscopy. In contrast to our previous work, these experiments work in the time domain in the semiimpulsive limit, employing two intense ultrashort infrared laser pulses. A theoretical formalism is presented in order to model the interstate coherent wave packet generated by the excitation pulse and the resulting spectroscopic signal. The observed coherences provide an independent proof of excitonic coupling within the amide I manifold of the peptide backbone.
Systematic study of foreign-atom-doped fullerenes by using a nuclear recoil method and their MD simulation
Authors: Ohtsuki T. , Ohno K. , Shiga K. , Kawazoe Y. , Maruyama Y. , Masumoto K.
Published: July 2002

The formation of atom-doped fullerenes has been investigated by using several types of radionuclides produced by nuclear reactions. It was found that the endohedral fullerenes (Kr@C60,  Xe@C60) and their dimers, furthermore, heterofullerenes, such as AsC59, GeC59 and their dimers, are produced by a recoil process following nuclear reactions. Other nuclear reaction products (Na, Ca, Sc, etc.) may destroy most of the fullerene cage in the same process. Carrying out ab initio molecular-dynamics simulations based on an all-electron mixed-basis approach, we confirmed that the formation of Kr- (or Xe-) atom-doped endohedral fullerenes and of substitutional heterofullerenes doped with an As atom is really possible. The experimental and theoretical results indicate that the chemical nature of doping atoms is important in the formation of foreign-atom-doped fullerenes.
An interesting ligand for the preparation of luminescent plastics: The picrate ion
Authors: de Zea Bermudez V. , Carlos L. D. , Silva M. M. , Smith M. J.
Published: July 2002

Electrolytes formed with poly(oxyethylene), POE, and europium picrate, [Eu(pic)2(OH2)6]pic.6H2O (where pic denotes the picrate anion) or simply Eu(pic)3xH2O, have been investigated by infrared and photoluminescence spectroscopies. The complexes have been represented by POEnEu(pic)3xH2O (where n represents the molar ratio of (OCH2CH2) units per Eu3+ ion). Materials with n ranging from 133 to 11 have been examined. A tentative attribution for the absorption bands of the mid-infrared spectra is presented. The spectral changes detected in the mid-infrared region and the modifications observed in the XRD patterns at increasing salt content show that Eu(pic)3xH2O exerts an effective plasticizing role which leads to the complete supression of crystallinity at n=11. The emission spectra of the complexes and their signature in the νC–O spectral region provide conclusive evidence that the Eu3+ ions are coordinated to the oxygens atoms of the polyether chains over the whole range of compositions studied. The photoluminescence spectra of the POEnEu(pic)3xH2O electrolytes indicate that the Eu3+/ether oxygen complexation occurs with the concomitant partial substitution of water molecules from the cation coordination shell. The luminescence data obtained confirm that the latter aspect is quite advantageous from the standpoint of optical properties. The incorporation of the picrate salt into POE results in a substantial increase in the calculated total radiative contribution for the depopulation of the 5D0 level (from 0.58 to 1.94 ms−1) and in the quantum efficiency (from 5.6 to 41.9%). The analysis of the νaNO2, νsNO2 and νphenCO picrate anion characteristic bands of the spectra of the POE-based complexes suggests that the coordinated picrate ligands of the picrate salt are unaffected by the presence of POE and never leave the lanthanide coordination shell. The absence of diffraction peaks of Eu(pic)3xH2O in the diffractograms of the electrolytes and the recognition that the crystalline regions are essentially composed of pure POE allow us to conclude that the complexation effect occurs exclusively in the amorphous phase. It is probable that the charge carrier in these electrolytes is the non-hydrogen-bonded and uncoordinated picrate ligand of the salt, while the complex cation [Eu(pic)2(OH2)y(OR2)z]+ (where R2O stands for POE, y<6 and z⩾1) is expected to be immobile.
On apparent quantized transition-state thresholds in the photofragmentation of acetaldehyde
Authors: King Rollin A. , Allen Wesley D. , Schaefer Henry F.
Published: July 2002

Recent photofragmentation experiments have observed stepwise increases in the dissociation rate for CH3CHO (T1)→CH3 (X̃ 2A2″)+HCO (X̃ 2A′) as a function of excitation energy. In accord with the Rice–Ramsperger–Kassel–Marcus (RRKM) form of transition-state theory, these steps were interpreted as corresponding to vibrational levels of the fragmentation transition state on the triplet surface. We have investigated this acetaldehyde dissociation using coupled cluster (CC) and density functional (DFT) methods with [C,O/H] atomic-orbital basis sets ranging in quality from [4s2p1d/2s1p] to [6s5p4d3f2g1h/5s4p3d2 f1g]. A high-level focal point analysis, along with harmonic force field computations, results in predictions of the dissociation energy, D0=1583 cm−1, and the association barrier height, V0*=3149 cm−1. With a basis set of triple-ζ plus double-polarization plus f(TZ2Pf ) quality, the DFT method UB3LYP and the CC method RCCSD predict barrier frequencies of 355i cm−1 and 516i cm−1, respectively, while the empirical value inferred from RRKM models is only 60i cm−1. The RRKM-derived frequencies for the degrees of freedom orthogonal to the reaction path are more reasonable but still not in convincing agreement with electronic structure theory. Thus, while the experimental steps in the dissociation rate of acetaldehyde (as well as ketene) have yet to be satisfactorily explained, proven ab initio methods provide strong evidence that simple RRKM fits to the k(E) profile provide misleading vibrational frequencies of the transition state on the corresponding triplet potential energy surface.
State-to-state reaction probabilities using bond coordinates: Application to the Li+HF(v, j) collision
Authors: Lara Manuel , Aguado Alfredo , Paniagua Miguel , Roncero Octavio
Published: July 2002

In this work we present a time-dependent method to evaluate state-to-state reaction probabilities, based on bond coordinates and an adapted body-fixed frame. Such a method is expected to be rather efficient to describe A+BC→AB+C reactive collisions. In addition, the apparent complexity of the Hamiltonian expressed in these coordinates is reduced when applied to a wave packet represented in grids for the internal coordinates. The efficiency of this method as compared to the use of the most adapted Jacobi coordinates increases as the masses of the satellite atoms approach that of the heavier central atom and, what may be more important, avoids the problems associated with the singularities of the radial kinetic terms in the region of configuration space of interest. This method is used to study the Li+HF(v=0,1,j=0,J=0) reactive collision and the structure of the final state distribution of the LiF products is interpreted in terms of transition state resonances.
Maximal subalgebra associated with a first integral of a system possessing sl(3,R) algebra
Authors: Leach P. G. L. , Mahomed F. M.
Published: July 2002

Conventionally the symmetries of a dynamical system are used to determine the first integrals associated with the system. In this paper the reverse procedure is adopted for one-dimensional systems possessing SL(3,R) symmetry. The equivalence of such systems enables the free particle equation to be used as a vehicle. It is observed that for three particular first integrals, each gives rise to a triplet of generators having isomorphic algebras. It is then shown how the knowledge of a first integral and its associated triplet enables one to obtain the remaining integrals and triplets.
Quadratic alternative algebras
Authors: Elduque Alberto
Published: July 2002

Quadratic alternative algebras are completely classified, up to isomorphism, by means of the associated set of ‘‘vectors.’’ They include the quaternion and octonion algebras, but also many other nonassociative algebras used in physics.
Nonuniversal correlations for random matrix ensembles
Authors: Nagao Taro , Slevin Keith
Published: July 2002

The eigenvalue correlations of the generalized Gaussian and Laguerre random matrix ensembles are calculated exactly. The fluctuations are shown to be nonuniversal in certain intervals of the spectrum. A physical example from quantum transport where such nonuniversal effects occur is discussed.
A simple difference realization of the Heisenberg q-algebra
Authors: Atakishiyev Natig M. , Frank Alejandro , Wolf Kurt Bernardo
Published: July 2002

A realization of the Heisenberg q-algebra whose generators are first-order difference operators on the full real line is discussed herein. The eigenfunctions of the corresponding q-oscillator Hamiltonian are given explicitly in terms of the q−1-Hermite polynomials. The nonuniqueness of the measure for these q-oscillator states is also studied.
Functional integration over geometries
Authors: Mottola Emil
Published: July 2002

The geometric construction of the functional integral over coset spaces ℳ/𝒢 is reviewed. The inner product on the cotangent space of infinitesimal deformations of ℳ defines an invariant distance and volume form, or functional integration measure on the full configuration space. Then, by a simple change of coordinates parameterizing the gauge fiber 𝒢, the functional measure on the coset space ℳ/𝒢 is deduced. This change of integration variables leads to a Jacobian which is entirely equivalent to the Faddeev–Popov determinant of the more traditional gauge fixed approach in non-abelian gauge theory. If the general construction is applied to the case where 𝒢 is the group of coordinate reparameterizations of spacetime, the continuum functional integral over geometries, i.e. metrics modulo coordinate reparameterizations may be defined. The invariant functional integration measure is used to derive the trace anomaly and effective action for the conformal part of the metric in two and four dimensional spacetime. In two dimensions this approach generates the Polyakov–Liouville action of closed bosonic non-critical string theory. In four dimensions the corresponding effective action leads to novel conclusions on the importance of quantum effects in gravity in the far infrared, and in particular, a dramatic modification of the classical Einstein theory at cosmological distance scales, signaled first by the quantum instability of classical de Sitter spacetime. Finite volume scaling relations for the functional integral of quantum gravity in two and four dimensions are derived, and comparison with the discretized dynamical triangulation approach to the integration over geometries are discussed. Outstanding unsolved problems in both the continuum definition and the simplicial approach to the functional integral over geometries are highlighted.
The q-Coulomb problem
Authors: Finkelstein Robert J.
Published: July 2002

There exist operator solutions of the q-Coulomb problem in both configuration and momentum space. Since the arguments of the corresponding amplitudes are noncommuting, however, there are problems of physical interpretation. Here we answer the question of physical interpretation by associating with the operator amplitude in momentum space a numerically valued amplitude lying in a Hilbert space defined by the SUq(2) algebra. This new amplitude, now depending on commuting arguments, may be interpreted by the usual rules of quantum mechanics and may also be Fourier transformed to yield the amplitude in configuration space.
New integrable generalizations of Calogero–Moser quantum problem
Authors: Chalykh O. , Feigin M. , Veselov A.
Published: July 2002

A one-parameter deformation of Calogero–Moser quantum problem is introduced. It is shown that corresponding Schrödinger operator is integrable for any value of the parameter and algebraically integrable in case of integer value.
Fermion realization of the nuclear Sp(6,R) model
Authors: Escher Jutta , Draayer J. P.
Published: July 2002

A fermion realization of the nuclear Sp(6,R) model, which complements the traditional bosonic representation, is developed. A recursive process is presented in which symplectic matrix elements of arbitrary one-body fermion operators between states of excitation Nℏω and N′ℏω in the same or in different symplectic bands are related back to valence shell matrix elements, which can be evaluated by standard shell model techniques. Matrix elements so determined may be used to calculate observables such as electron scattering form factors which carry detailed structural information on nuclear wave functions.
Non-Kolmogorov probability models and modified Bell’s inequality
Authors: Khrennikov Andrei
Published: July 2002

We analyze the proof of Bell’s inequality and demonstrate that this inequality is related to one particular model of probability theory, namely Kolmogorov measure-theoretical axiomatics from 1933. We found a (numerical) statistical correction to Bell’s inequality. Such an additional term εφ on the right-hand side of Bell’s inequality can be considered as a probability invariant of a quantum state φ. This is a measure of nonreproducibility of hidden variables in different runs of experiments. Experiments to verify Bell’s inequality can be considered as just experiments to estimate the constant εφ. It seems that Bell’s inequality could not be used as a crucial reason to deny local realism. We consider deterministic as well as stochastic hidden variables models.
Probability distribution of distance in a uniform ellipsoid: Theory and applications to physics
Authors: Parry Michelle , Fischbach Ephraim
Published: July 2002

A number of authors have previously found the probability Pn(r) that two points uniformly distributed in an n-dimensional sphere are separated by a distance r. This result greatly facilitates the calculation of self-energies of spherically symmetric matter distributions interacting by means of an arbitrary radially symmetric two-body potential. We present here the analogous results for P2(r;ε) and P3(r;ε) which respectively describe an ellipse and an ellipsoid whose major and minor axes are 2a and 2b. It is shown that for ε=(1−b2/a2)1/2⩽1, P2(r;ε) and P3(r;ε) can be obtained as an expansion in powers of ε, and our results are valid through order ε4. As an application of these results we calculate the Coulomb energy of an ellipsoidal nucleus, and compare our result to an earlier result quoted in the literature.
Integration of the soliton hierarchy with self-consistent sources
Authors: Zeng Yunbo , Ma Wen-Xiu , Lin Runliang
Published: July 2002

In contrast with the soliton equations, the evolution of the eigenfunctions in the Lax representation of soliton equation with self-consistent sources (SESCS) possesses singularity. We present a general method to treat the singularity to determine the evolution of scattering data. The AKNS hierarchy with self-consistent sources, the MKdV hierarchy with self-consistent sources, the nonlinear Schrödinger equation hierarchy with self-consistent sources, the Kaup–Newell hierarchy with self-consistent sources and the derivative nonlinear Schrödinger equation hierarchy with self-consistent sources are integrated directly by using the inverse scattering method. The N soliton solutions for some SESCS are presented. It is shown that the insertion of a source may cause the variation of the velocity of soliton. This approach can be applied to all other (1+1)-dimensional soliton hierarchies.
Superconducting properties and structure of vanadium after cryogenic deformation
Authors: Aksenov V. K. , Chernyak N. A. , Volchok O. I. , Mats A. V. , Starodubov Ya. D.
Published: July 2002

The effect of low temperature (77 K) deformation by drawing (80%) on the superconducting properties and structure of vanadium is studied. The structural elements (fragment boundaries) responsible for the observed changes of critical parameters are isolated. The electron-phonon coupling constant and the electron mean free path undergo most significant changes in these regions of rotational deformation localization, which have a high density of defects and are powerful sources of internal stresses. The dislocation density at the fragment boundaries is estimated.
Numerical simulation of 3-D Bénard convection with gravitational modulation
Authors: Biringen S. , Peltier L. J.
Published: July 2002

In this numerical study, randomly and sinusoidally modulated gravitational fields imposed on three-dimensional Rayleigh–Bénard convection are investigated in an effort to understand the effects of vibration (G-Jitter) on fluid systems. The time-dependent, Navier–Stokes equations and the energy equation with Boussinesq approximations are solved by a semi-implicit, pseudospectral procedure. An analysis of energy balances indicates that with increasing modulation amplitude, transition from synchronous to relaxation oscillation goes through the subharmonic response. Random modulations are found to be less stabilizing than sinusoidal and are shown to impose local three-dimensionality on the flow for some parameter ranges both at terrestrial and zero base gravity conditions.
Impinging jets atomization
Authors: Ibrahim E. A. , Przekwas A. J.
Published: July 2002

An analysis of the characteristics of the spray produced by an impinging-jet injector is presented. Predictions of the spray droplet size and distribution are obtained through studying the formation and disintegration of the liquid sheet formed by the impact of two cylindrical jets of the same diameter and momentum. Two breakup regimes of the sheet are considered depending on Weber number, with transition occurring at Weber numbers between 500 and 2000. In the lower Weber number regime, the breakup is due to Taylor cardioidal waves, while at Weber number higher than 2000, the sheet disintegration is by the growth of Kelvin–Helmholtz instability waves. Theoretical expressions to predict the sheet thickness and shape are derived for the low Weber number breakup regime. An existing mathematical analysis of Kelvin–Helmholtz instability of radially moving liquid sheets is adopted in the predictions of resultant drop sizes by sheet breakup at Weber numbers greater than 2000. Comparisons of present theoretical results with experimental measurements and empirical correlations reported in the literature reveal favorable agreement.
A new mechanism for rapid transition involving a pair of oblique waves
Authors: Schmid P. J. , Henningson D. S.
Published: July 2002

A pair of small but finite-amplitude oblique waves were used as initial condition in numerical simulations of transition to turbulent flow. A rapid energy growth to a transition peak occurred. Although this transition scenario is triggered by a nonlinear interaction, the dominant mechanism yielding substantial disturbance amplitudes was found to be linear. A number of simulations were made that showed that the overall features of this transition scenario remains the same for a range of parameter values.
Nonlinear transient free-surface flow and dip formation due to a point sink
Authors: Miloh Touvia , Tyvand Peder A.
Published: July 2002

The early free-surface flow due to an impulsively started point sink in a semi-infinite fluid is analytically studied by employing a power series expansion in time. The full surface elevation to second order is calculated, as well as the third-order elevation at the surface center just above the sink. Gravity enters into the problem only in the third-order surface elevation. A Froude number is defined here with respect to the sink strength and the initial submergence depth. A critical value for the Froude number is analytically determined which is related to dip formation at the free surface. Its value is 15−1/2=0.2582. This critical value is shown to be in good agreement with experimental data and earlier works on dip formation in a container that is drained by a hole in the bottom.
Tracer dispersion in 2-D fractures with flat and rough walls in a radial flow geometry
Authors: Ippolito I. , Hinch E. J. , Daccord G. , Hulin J. P.
Published: July 2002

Tracer dispersion is studied in a plane fracture geometry with a radial flow between closely spaced parallel walls with either two smooth surfaces or one smooth and one rough surface. An echo dispersion technique in which the fluid is first injected into the fracture during a time Tinv and then pumped back through a detector is used in the experiments and is complemented by Monte-Carlo-type numerical simulations. For the smooth wall case, the Taylor dispersion mechanism is dominant when longitudinal molecular diffusion is negligible: it is verified numerically and experimentally that its transition to irreversibility only depends on the ratio of Tinv to the transverse diffusion time τm across the fracture thickness. In addition, the variation of Δ̄T̄2/(4Tinvτm) with respect to Tinv/τm is the same as for a flow of parallel geometry (Δ̄T̄2 being the mean square deviation of the transit time). Longitudinal molecular diffusion increases the global dispersion like Tinv/(PeQτm) at low Péclet numbers PeQ and long times due to the longitudinal velocity gradient. When one of the walls is rough, one expects to have a geometrical dispersion locally proportional to the velocity. A corresponding linear variation of Δ̄T̄2/(4Tinvτm) with respect to √Tinv/PeQ is observed experimentally.
Convective filamentation instability of circularly polarized Alfvén waves
Authors: Kuo S. P. , Whang M. H. , Schmidt G.
Published: July 2002

A convective instability that leads to the filamentation of large-amplitude Alfvén waves and generates nonoscillatory spatially growing density fluctuations is investigated by using a two-fluid plasma model. Based on linear perturbation analysis for a four-wave parametric coupling process, the dispersion relations of the instability for both a right- and left-hand circularly polarized Alfvén pump are derived. They are then solved numerically for the dependences of the threshold fields and growth rates on the wavenumber k of the nonoscillatory mode, the beta of the plasma, and the frequency of the pumps, for both right- and left-hand circularly polarized Alfvén pumps. The relevance of the proposed instability with some observations in the space plasmas is discussed.
Linear and nonlinear properties of infernal modes
Authors: Charlton L. A. , Carreras B. A. , Lynch V. E.
Published: July 2002

Infernal modes are pressure-driven instabilities that can occur in low-shear regions of tokamak plasmas for beta values well below the ballooning stability limit. They may be either resistive or ideal. In this paper, stability regions are determined in the plane of elongation versus triangularity to demonstrate the effects of shaping. Both ellipticity and triangularity are needed for stability. The effects of toroidicity, compressibility, and resistivity are also analyzed. A study of the nonlinear evolution of the mode with resistivity shows that complete flux rearrangement can occur suggesting a mechanism for observed internal disruptions.
The resistance and mobility functions of two equal spheres in low-Reynolds-number flow
Authors: Kim Sangtae , Mifflin Richard T.
Published: July 2002

The resistance and mobility functions which completely characterize the linear relations between the force, torque, and stresslet and the translational and rotational velocities of two spheres in low-Reynolds-number flow have been calculated using a boundary collocation technique. The ambient velocity field is assumed to be a superposition of a uniform stream and a linear (vorticity and rate-of-strain) field. This is the first compilation of accurate expressions for the entire set of functions. The calculations are in agreement with earlier results for all functions for which such results are available. The technique is successful at all sphere–sphere separations except at the almost-touching (gaps of less than 0.005 of the sphere diameter) configuration. New results for the stresslet functions have been used to determine Batchelor and Green’s [J. Fluid Mech. 56, 401 (1972)] order c2 coefficient in the bulk stress (7.1 instead of their 7.6). The two-sphere functions have also been used to determine the motion of a rigid dumbbell in a linear field. We also show that certain functions have extrema.
Experiments on annular liquid jet instability and on the formation of liquid shells
Authors: Kendall James M.
Published: July 2002

An annular jet flow of liquid surrounding a flow of gas at its core is extremely unstable. Experiments are described in which such a flow is generated by an annular nozzle operated at fairly specific conditions. It is shown that periodic, axisymmetric oscillations arise spontaneously within the cylindrical sheet emerging from the nozzle and grow with such rapidity along the axial dimension that a sealing-off and encapsulation of the core gas occurs within a few jet diameters. This is closely followed by a pinchoff of the liquid between adjacent bubbles. The liquid shells set free thereby assume spherically symmetric form under capillary forces, and each contains a precisely uniform measure of gas and of liquid on account of the extremely high frequency-stability of the process. Description is given of the fluid dynamic processes by which the shells are formed, and mention is made of exploiting the instability for the production of rigid shells for technological applications.
Calculation of swirling jets with a Reynolds stress closure
Authors: Gibson M. M. , Younis B. A.
Published: July 2002

The transport equations for the Reynolds stresses are closed by modeling the turbulence and mean-strain parts of the pressure-strain-rate correlation. The model constants are determined from simple relationships deduced from measurements in rectilinear and longitudinally curved shear flows. It is found that the effects of complex strain fields are more correctly predicted when the influence of the mean-strain part is reduced from levels indicated by rapid distortion theory, and the turbulence part is adjusted to conform approximately with the measured rates of return to isotropy. The case of the swirling jet is used to illustrate the improved performance of the model.
Consistency conditions for random-walk models of turbulent dispersion
Authors: Pope S. B.
Published: July 2002

Random-walk models have long been used to calculate the dispersion of passive contaminants in turbulence. When applied to nonstationary and inhomogeneous turbulence, the model coefficients are functions of the Eulerian turbulence statistics. More recently the same random-walk models have been used as turbulence closures in the evolution equation for the Eulerian joint probability density function (pdf) of velocity. There are, therefore, consistency conditions relating the coefficients specified in a random-walk model of dispersion and the Eulerian pdf calculated using the same random-walk model. It is shown that even if these conditions are not satisfied, the dispersion model does not violate the second law of thermodynamics: all that is required to avoid a second-law violation is that the mean pressure gradient be properly incorporated. It is also shown that for homogeneous turbulence the consistency conditions are satisfied by a linear Gaussian model; and that for inhomogeneous turbulence they are satisfied by a nonlinear Gaussian model.
Spatial simulations of oblique transition in a boundary layer
Authors: Berlin Stellan , Lundbladh Anders , Henningson Dan
Published: July 2002

Simulations of oblique transition in the spatial domain are presented, covering the complete transition process into the turbulent regime. It is conjectured that the three stages identified here and elsewhere are universal for oblique transition in all shear flows: first a nonlinear generation of a streamwise vortex by the oblique waves, second a transient growth of streaks from the vortex by the lift-up effect, and third a breakdown of the streaks due to secondary instability.
Stochastic equations with multifractal random increments for modeling turbulent dispersion
Authors: Borgas M. S. , Sawford B. L.
Published: July 2002

Previously studied stochastic models of one-particle dispersion in stationary, isotropic, and homogeneous turbulence are reconsidered and intermittency corrections sought. Known Lagrangian intermittency effects, in the form of multifractal scaling, independently derived from Eulerian measurements [M. S. Borgas, Philos. Trans. R. Soc. London Ser. A 342, 379 (1993)], are used to develop a new model. The previous models and approaches are shown to be inadequate. The new model incorporating Lagrangian intermittency satisfies Thomson’s well-mixed criterion [J. Fluid Mech. 180, 529 (1987)] and gives almost-Gaussian mean-concentration distributions for Gaussian sources. The trajectories generated by the model are not fractal, in agreement with the results of Borgas. The practical impact of intermittency upon dispersion is found to be small.
Creeping flow through sinusoidally constricted capillaries
Authors: Hemmat M. , Borhan A.
Published: July 2002

The steady pressure-driven flow of an incompressible Newtonian fluid through an axisymmetric capillary whose diameter varies sinusoidally in the axial direction is studied numerically under conditions of negligible inertial effects. The boundary integral formulation is used to obtain detailed velocity and pressure distributions within the capillary, and to determine the effects of amplitude and wavelength of corrugation on the kinematic structure of the flow. In particular, the critical values of the geometric parameters leading to flow reversal within the expansion region of the capillary are established. The dependence of the pressure drop through the capillary on the geometric parameters is also discussed with reference to the predictions of various effective pore models.
The GHS interaction model for strong attractive potentials
Authors: Kunc J. A. , Hash D. B. , Hassan H. A.
Published: July 2002

A new version of the generalized hard sphere (GHS) model is presented that is valid for interactions where both repulsion and attraction of the colliding particles are important. The parameters of the cross section for scattering of heavy particles interacting through a 12-6 Lennard–Jones potential are given for collisions involving like and unlike particles.
NMR imaging of velocity profiles and velocity distributions in bead packs
Authors: Kutsovsky Y. E. , Scriven L. E. , Davis H. T. , Hammer B. E.
Published: July 2002

Spatially resolved velocity profiles and spatially nonresolved velocity distributions of steady flow in a tube and bead packs were measured. Two different NMR experiments were used to measure velocity distributions. In one, the velocity histogram was calculated from spatially resolved velocity phase encoded images acquired in a 6 mm bead pack. In the other, a Fourier flow method was used to measure the velocity distribution directly in a 0.25 mm bead pack. Axial velocity profiles in the pore space of the 6 mm bead pack at Reynolds numbers of 14.9, 29.9, and 44.8 proved to be roughly parabolic, with maxima near the pore centers. Both NMR methods yielded the same dimensionless velocity distributions that contain negative as well as positive velocity components. The velocity distribution function derived from a bundle-of-tubes-model accounts for the positive part of the velocity distribution.
Elliptical vortices in shear: Hamiltonian moment formulation and Melnikov analysis
Authors: Ngan Keith , Meacham Steve , Morrison P. J.
Published: July 2002

The equations of motion for interacting elliptical vortices in a background shear flow are derived from a Hamiltonian moment formulation. The equations reduce to the sixth order system of Melander et al. [J. Fluid Mech. 167, 95 (1986)] when a pair of vortices is considered and shear is neglected. The equations for a pair of identical vortices are analyzed using a number of methods, with particular emphasis on the implications for vortex merger. The splitting distance between the stable and unstable manifolds connecting the hyperbolic fixed points of the intercentroidal motion—the separatrix splitting—is estimated with a Melnikov analysis. This analysis differs from the standard time-periodic Melnikov analysis on two counts: (a) the ‘‘periodic’’ perturbation arises from a second degree of freedom in the system which is not wholly independent of the first degree of freedom, the intercentroidal motion; (b) this perturbation has a faster time scale than the intercentroidal motion. The resulting Melnikov integral appears to be exponentially small in the perturbation as the latter goes to zero. Numerical simulations, notably Poincaré sections, provide a global view of the dynamics and indicate that, as observed in previous studies, there are essentially two modes of merger. The effect of the shear on chaotic motion is also discussed.
Rotation of a sphere in Brinkman fluids
Authors: Solomentsev Yuri E. , Anderson John L.
Published: July 2002

The velocity field about a sphere of radius a rotating in a Brinkman fluid is derived and the hydrodynamic torque is calculated. The effect of the Darcy permeability (k) on the torque is O(a2/k), compared to O(a/k1/2) for the translational drag. The Brinkman effect on rotation is substantially weaker than on translational motion for any value of a/k1/2. Measurements of hydrodynamic torque and drag on spheres of different size could be used to determine the length scale at which the Brinkman model fails, thus providing insight into the microstructure of the fluid.
Linear stability analysis of an insoluble surfactant monolayer spreading on a thin liquid film
Authors: Matar Omar K. , Troian Sandra M.
Published: July 2002

Recent experiments by several groups have uncovered a novel fingering instability in the spreading of surface active material on a thin liquid film. The mechanism responsible for this instability is yet to be determined. In an effort to understand this phenomenon and isolate a possible mechanism, we have investigated the linear stability of a coupled set of equations describing the Marangoni spreading of a surfactant monolayer on a thin liquid support. The unperturbed flows, which exhibit simple linear behavior in the film thickness and surfactant concentration, are self-similar solutions of the first kind for spreading in a rectilinear geometry. The solution of the disturbance equations determines that the rectilinear base flows are linearly stable. An energy analysis reveals why these base flows can successfully heal perturbations of all wavenumbers. The details of this analysis suggest, however, a mechanism by which the spreading can be destabilized. We propose how the inclusion of additional forces acting on the surfactant coated spreading film might give rise to regions of adverse mobility gradients known to produce fingering instabilities in other fluid flows.
Two-dimensional vortex motions of fluid in harbor-like basins at large Reynolds numbers
Authors: Goncharov V. P. , Pavlov V. I.
Published: July 2002

This article considers the topology of the vortex regimes generated in harbor-like basins by the external potential long shore current at large Reynolds numbers. The proposed theory discusses the issues of what solution compatible with the Prandtl-Batchelor theorem for inviscid fluids, and under what conditions, may be realized as an asymptotic state of the open hydrodynamical system. The analysis developed is based on the variational principle. We formulated a validity criterion according to which stationary regimes of dissipative systems may be considered as extremals of a variational inviscid problem. In particular, such a situation is possible when the dissipative functionals represent some functions of motion invariants of the same problem. It is shown that the steady state corresponds to the circulational regime in which the system has minimal energy with the fixed enstrophy. This state is fixed by the Reynolds number. The approach that is formulated is applied to the model of a rectangular harbor-like basin in order to obtain the relation between the Reynolds number, the geometry factor and the topological number characterizing the number of vortex cells.
Periodic arrays of gyrotactic plumes in bioconvection
Authors: Ghorai S. , Hill N. A.
Published: July 2002

Using the continuum model of Pedley et al. [J. Fluid Mech. 195, 223 (1988)] for bioconvection in a suspension of swimming, gyrotactic micro-organisms, the existence and stability of periodic arrays of two-dimensional plumes in deep chambers are investigated. The system is governed by the Navier–Stokes equations for an incompressible fluid coupled with a micro-organism conservation equation. These equations are solved numerically using a conservative finite-difference scheme. In sufficiently deep chambers, the plumes are sometimes unstable to varicose or meandering modes. A linear stability analysis for an infinitely deep plume predicts the growth rates of these instabilities and agrees well with the numerical results.
Ion current collection in spacecraft wakes
Authors: Biasca Rodger , Wang Joseph
Published: July 2002

This work investigates the current–voltage characteristics of a highly negatively biased, isolated probe in the plasma wake of a much larger, unbiased object. The system is investigated for both flowing and stationary plasmas. For the stationary plasma case, spherical probe theory and a simple correction factor to account for the shadow of the plate adequately explain the results. The introduction of a flowing plasma leads to a much more complicated situation. In this case, since the potential field is asymmetric and the ions no longer conserve angular momentum, spherical probe theory is inadequate. The current–voltage characteristics take a much more complicated form than in the stationary case. Indeed, the simulations show that the current–voltage characteristics of varying axial probe positions cross each other. The high complexity of the ion orbits makes theoretical predictions of where the crossover occurs difficult. However, the potential structure around the probe and the ion trajectories are analyzed to explain the current–voltage characteristics. These results show regions can exist in energy–angular momentum space which cause incoming ions to orbit the probe and then escape the simulation. The fraction of particles which do orbit and then escape as a function of probe potential and position determines the shape of the current–voltage characteristic.
The evolution of ultra-intense, short-pulse lasers in underdense plasmas
Authors: Decker C. D. , Mori W. B. , Tzeng K.-C. , Katsouleas T.
Published: July 2002

The propagation of short-pulse lasers through underdense plasmas at ultra-high intensities (I≥1019 W/cm) is examined. The pulse evolution is found to be significantly different than it is for moderate intensities. The pulse breakup is dominated by leading edge erosion and plasma wave wake formation rather than from Raman forward scattering type instabilities. A differential equation which describes local pump depletion is derived and used to analyze the formation and evolution of the erosion. Pulse erosion is demonstrated with one dimensional particle in cell (PIC) simulations. In addition, two dimensional simulations are presented which show pulse erosion along with other effects such as channeling and diffraction. Possible applications for plasma based accelerators and light sources are discussed.
Dusty plasma ionization instability with ion drag
Authors: D’Angelo N.
Published: July 2002

The effect of ion drag on negatively charged dust grains is considered as a possible mechanism of excitation of the dust-acoustic (DA) ionization instability. It is found that DA waves are more and more damped as the coefficient for ion drag, μ, increases from zero to a critical value, μcrit. For μ>μcrit a zero-frequency (nonpropagating) perturbation grows when the drag of the ions on the dust grains overcomes the effect of the perturbation electric field.
Electromagnetic ion-temperature-gradient modes and anomalous transport in a nonuniform magnetized plasma with equilibrium flows
Authors: Rafiq Tariq , Qamar Anisa , Mirza Arshad M. , Murtaza G. , Shukla P. K.
Published: July 2002

The linear and nonlinear properties of electrostatic and electromagnetic waves in the presence of ion-temperature gradient, magnetic-field gradient, density gradient, and velocity gradients are examined. In the linear limit, a dispersion relation is obtained that admits new instabilities of drift waves. It is found that parallel velocity shear couples the electrostatic and magnetostatic modes and can cause an instability. An estimate of the anomalous ion energy transport and particle flux on the basis of mixing length hypothesis is made and the results are discussed for some interesting limiting cases. Furthermore, stationary solutions of the nonlinear equations without dissipation are also presented. The findings of the present investigation should be useful in understanding fluctuations and transport phenomena in space and laboratory plasmas.
Ion kinetic effects on the wake potential behind a dust grain in a flowing plasma
Authors: Winske D. , Daughton W. , Lemons D. S. , Murillo M. S.
Published: July 2002

The structure of the wake potential downstream of a stationary dust grain in a flowing plasma is studied on ion time scales using particle-in-cell simulation methods. The scaling of the wake is investigated as a function of Mach number and other parameters as well as the dimensionality of the system. The results are compared and discussed in relation to various theoretical expressions for the wake. Consistent with theory, in one dimension the wake wavelength scales as MλDe(1−M2)−1/2 for M<1, where M is the Mach number and λDe is the electron Debye length, while no wake forms for M>1. In two dimensions, a wake is formed for both M<1 and M>1, while the wake wavelength scales as MλDe in both regimes. The amplitude of the wake peaks at M≈1 in both the one- and two-dimensional simulations.