bluuebunny/crossref_metadata_2025 huggingface dataset:
Collisions of low-energy electrons with CO2
Authors: Lee Chuo-Han , Winstead Carl , McKoy Vincent
Published: July 2002

We report cross sections for collisions of low-energy electrons with carbon dioxide obtained by the Schwinger multichannel variational method. Elastic cross sections are obtained in the static-exchange-plus-polarization approximation. We pay particular attention to the position of the Πu2 resonance and to the strong enhancement in the integral cross section near zero energy, both prominent features whose accurate treatment requires an accounting for polarization effects. To include such effects in the resonant symmetry, we use an objective and physically motivated criterion to construct a set of configurations that accurately accounts for polarization while avoiding overcorrelation. In addition, we study the origin of the nonisotropic behavior of the elastic differential cross section at very low energies and conclude that it is caused by significant contributions from vibrationally excited CO2. Cross sections from threshold to 50 eV for excitation of the Σu+3, Δu1, Δu3, Σu−3, and Σu−1 states that arise from the (1πg→2πu) transition are presented for the first time.
The potential energy surface of the (N2O⋅H2O)+ cluster cation
Authors: Stevens Jonathan E. , Holthausen Max C. , Morokuma Keiji
Published: July 2002

The ground state potential energy surface of the (N2O⋅H2O)+ cluster cation is explored with ab initio and density functional theories. B3LYP optimizations are used to determine the structure of the products of the dissociation of the cluster ions as well as possible structures for the clusters themselves and transition states that connect various minima. Energetics for all optimized structures are determined with the G2M(RCC,MP2) method. The results are used to interpret collision-induced dissociation (CID) experiments which study the cluster ion, and which find that the cluster dissociates to form H2O++N2O, N2OH++OH, and N2O++H2O products. The calculations an (N2O–OH2)+ complex as well as a similar (H2O–N2O)+ complex, and show that these complexes access the experimentally observed H2O++N2O products and N2OH++OH products without any intervening reverse barrier. The stability of both these complexes, approximately −20 kcal/mol relative to the H2O++N2O products, agrees well with experimentally determined CID thresholds for all products. Additional calculations of the ground state potential energy surface of the cluster investigate the possibility of the formation of other products. Some preliminary studies of the excited states of the cluster cation are also performed; the results of these calculations lend insight into experimental photodissociation studies of the cluster ions. Mechanisms for the formation of H2O++N2O, N2OH++OH, and N2O++H2O products following photoexcitation of the cluster ions are discussed; the H2O++N2O and N2OH++OH products must be formed from a surface-hopping from an excited electronic state to states which correlate to ground state products. Similarly, N2O++H2O products may be formed from collision induced dissociation of clusters only by means of a surface-hopping mechanism.
Ab initio study of water. II. Liquid structure, electronic and thermodynamic properties over a wide range of temperature and density
Authors: Sato Hirofumi , Hirata Fumio
Published: July 2002

The electronic and liquid structures of water and its thermodynamic properties are studied over a wide range of temperature (0°–600 °C) and density (0.6–1.4 g/cm3) based on the ab initio molecular orbital theory combined with the integral equation method of liquid. Unlike standard treatments of water by means of the classical statistical mechanics including molecular simulations, the effective charges on oxygen and hydrogen atoms in water molecules are not “input parameters,” but naturally predicted from the theory in the course of self-consistent determination of the electronic structure and the density pair correlation functions in liquids. It is found that the molecular dipole moments and electronic polarization energies decrease with increasing temperature and/or density. The theoretical results for dipole moments are in quantitative accord with the experimental data, which has been determined based on the NMR chemical shift coupled with the molecular dynamics simulation [N. Matsubayashi, C. Wakai, and M. Nakahara, J. Chem. Phys. 110, 8000 (1999)]. The state dependence of the electronic structure is discussed in terms of the thermal activation of molecules and intermolecular interactions including the hydrogen bonds. The liquid structure of water is discussed in the wide range of thermodynamic states in terms of S(r), an average response of the pair correlation functions to temperature change which is introduced in the present study in order to make structural characteristics of water more distinctive. It is concluded from the behavior of the function that the short range structure of water retains the characteristics to ice, or the tetrahedral configuration, over relatively wide range of temperature in the normal density (1.0 g/cm3). The ice-like characteristics of water disappear to large extent both at high (1.4 g/cm3) and low (0.6 g/cm3) densities for different reasons: in the high density due to the packing effect, while in the low density due to essentially entropic cause, or increased configuration space available to a molecule. The distance between the nearest-neighbor molecules in water are insensitive to the density change compared with those corresponding to the Lennard-Jones fluid. The difference in the behaviors between the two fluids is explained in terms of the intermolecular interactions and liquid structures. The number of hydrogen bonds is calculated from the oxygen–hydrogen pair correlation function using a new definition based on S(r), which enables us to distinguish the hydrogen-bonded OH pairs from those just in contact due to packing effect. The temperature and density dependence of the quantity is discussed in terms of the liquid structure of water.
The electronic structure and surface chemistry of glycine adsorbed on Cu(110)
Authors: Nyberg M. , Hasselström J. , Karis O. , Wassdahl N. , Weinelt M. , Nilsson A. , Pettersson L. G. M.
Published: July 2002

We present a combined density functional theory and x-ray emission spectroscopy study of the bonding and chemistry of glycine (NH2CH2COOH) chemisorbed on Cu(110). The amino acid deprotonates upon adsorption. The adsorbate exhibits a rich surface chemistry leading to several intermediate adsorption structures. The most stable geometry is found to involve both the carboxylic and amino functional end groups in the bond. This structure appears only after annealing to 400 K, which in the present work is attributed to a removal of surface or subsurface hydrogen from the metal. Comparison with experimental x-ray emission and near edge x-ray absorption fine structure (NEXAFS) spectra provide a detailed picture of the electronic structure for the most stable structure. This allows conclusions to be drawn regarding the covalent interaction of the adsorbate system. When combined with theoretical calculations addressing, e.g., the electrostatic adsorbate–substrate interaction, a complete picture of the surface chemical bond is obtained.
Nuclear spin–spin coupling constants from regular approximate relativistic density functional calculations. I. Formalism and scalar relativistic results for heavy metal compounds
Authors: Autschbach Jochen , Ziegler Tom
Published: July 2002

We present a relativistic formulation of the spin–spin coupling hyperfine terms based on the two component zeroth-order regular approximate Hamiltonian. The scalar relativistic parts of the resulting operators were used for an implementation into the Amsterdam density functional program. Application of the code for the calculation of one-bond metal-ligand couplings of systems containing W183, Pt195, Hg199, and Pb207 shows that scalar relativistic calculations are able to reproduce major parts of the relativistic effects on the coupling constants, which can be even larger in magnitude than the respective total nonrelativistic values. The spatial origin of the regular approximate relativistic analogue of the Fermi-contact contribution, which is usually responsible for the strong relativistic increase of the couplings, is analyzed. Its relativistic effects can be described by the relativistic increase of valence orbital density in the very vicinity of the heavy nucleus.
Density functional study of stoichiometric and O-rich titanium oxygen clusters
Authors: Albaret Tristan , Finocchi Fabio , Noguera Claudine
Published: July 2002

We propose a detailed description of the structural and electronic properties of neutral and charged TinO2n+m clusters (n=1–3 and m=0,1), through simulations based on the density functional theory in the local spin density approximation. In all the isomers studied, strongly bound titanyl groups are found. The order of stability of the low-energy stoichiometric clusters may change considerably from that found by the approaches based on classical electrostatics. The most stable isomers of the oxygen-rich neutral clusters show characteristic peroxide groups. All these facts stress the importance of the covalent contribution to the cohesion of the clusters. Large atomic relaxations, accompanying the change from a closed-shell to an open-shell electronic configuration when an electron is added or removed, can often induce reversals of stability among the isomers. A careful discussion of the computed electron affinities and excitation energies as a function of the size and the atomic conformation of the clusters is performed, in relation to recent experimental data.
The O(3,1) symmetry problem of the charge–monopole interaction
Authors: Fehér L. Gy.
Published: July 2002

The question of whether there exists a smooth, time-independent conserved vector observable in the classical mechanics of the charge–monopole interaction that spans an O(3,1) Lie algebra together with the angular momentum or not is examined. It turns out that any candidate for such dynamical symmetry algebra has hard singularities at that part of the phase space that corresponds to charge–monopole collisions. In the course of the investigation we use the transformation of Boulware et al. [D. G. Boulware, L. S. Brown, R. N. Cahn, S. D. Ellis, and C. Lee, Phys. Rev. D 14, 2708 (1976)] relating the charge–monopole system to a point mass moving in an inverse square potential. This transformation is shown to be a complete isomorphism between the scattering parts of the related Hamiltonian systems; its global behavior is described in terms of an U(1) principal fiber bundle of nontrivial topology. Several remarks on the symmetries of various monopole problems are made, e.g., the most general O(4) symmetry algebra is given for a special form of the charge–dyon interaction.
A modified renormalization procedure which may avoid use of bare parameters
Authors: Chimento Luis P. , Jakubi Alejandro S.
Published: July 2002

A new renormalization procedure is introduced via a method that assigns finite values to divergent expressions in quantum field theory. It is shown that this procedure works at the two-loop level in Feynman diagrams with overlapping divergences. Applications to λφ4 theory and quantum electrodynamics are made. Also, the Casimir effect is evaluated. Comparison of these results with those from standard renormalization methods proves them to be coincident.
Stochastic quantization of para-Fermi fields
Authors: Balakrishnan Janaki , Biswas S. N. , Goyal Ashok , Soni S. K.
Published: July 2002

The application of the method of stochastic quantization originally attributed to Parisi and Wu has been extended to spinor fields obeying para-Fermi statistics. The connection between Euclidean and stochastic field theories is established in the conventional manner by proving the equivalence between a Langevin equation satisfied by para-Grassmann fields and a Fokker–Planck equation, the Hamiltonian of which has been constructed using para-Grassmann variables analogous to its construction from Grassmann variables in the Fermi case. As an example, a two-point Green function is calculated for any arbitrary value of order p of para-Fermi statistics, barring the pathological case p=2 which has been mentioned briefly.
On generalization of the Sinai theorem in the random site problem
Authors: Ahmed E. , Hegazi A. S.
Published: July 2002

The Sinai theorem is generalized to determine the transformations that preserve the critical concentration Pc and the critical exponent β in the random site problem.
Minisuperspaces with conformally and minimally coupled scalar fields
Authors: Page Don N.
Published: July 2002

One may perform a local field redefinition to transform between gravity minimally coupled to a free scalar field and gravity conformally coupled. However, the allowed field values differ in the two cases. For a minisuperspace consisting of a Friedman–Robertson–Walker geometry and a homogeneous scalar field, the conformal coupling allows a more general class of solutions of the Wheeler–DeWitt equation than does the minimal coupling. Nevertheless, there is a one-to-one correspondence between the bounded solutions in the two cases for k=1. This correspondence exploits an isomorphism between harmonic oscillator wavefunctions and solutions of the massive Klein–Gordon equation in the 1+1 dimensional Rindler wedge.
Quantum and braided linear algebra
Authors: Majid S.
Published: July 2002

Quantum matrices A(R) are known for every R matrix obeying the quantum Yang–Baxter equations. It is also known that these act on ‘‘vectors’’ given by the corresponding Zamalodchikov algebra. This interpretation is developed in detail, distinguishing between two forms of this algebra, V(R) (vectors) and V*(R) (covectors). A(R)→V(R21)⊗V*(R) is an algebra homomorphism (i.e., quantum matrices are realized by the tensor product of a quantum vector with a quantum covector), while the inner product of a quantum covector with a quantum vector transforms as a scaler. It is shown that if V(R) and V*(R) are endowed with the necessary braid statistics Ψ then their braided tensor-product V(R)⊗_V*(R) is a realization of the braided matrices B(R) introduced previously, while their inner product leads to an invariant quantum trace. Introducing braid statistics in this way leads to a fully covariant quantum (braided) linear algebra. The braided groups obtained from B(R) act on themselves by conjugation in a way impossible for the quantum groups obtained from A(R).
Quantization of the nilpotent orbits in so(1,2)* and massless particles on (anti-)de Sitter space–time
Authors: De Bievre Stephan , Renaud Jacques
Published: July 2002

The nontrivial nilpotent orbits in so(1,2)*≂su(1,1)* are the phase spaces of the zero mass particles on the two-dimensional (anti-)de Sitter space–time. As is well known, the lack of global hyperbolicity (respectively, stationarity) for the anti-de Sitter (respectively, de Sitter) space–time implies that the canonical field quantization of its free massless field is not uniquely defined. One might nevertheless hope to get the one-particle quantum theory directly from an appropriate ‘‘first’’ quantization of the classical phase space. Unfortunately, geometric quantization (the orbit method) does not apply to the above orbits and a naive canonical quantization does not yield the correct result. To resolve these difficulties, we present a simple geometric construction that associates to them an indecomposable representation of SO0(1,2) on a positive semidefinite inner product space. It is shown that quotienting out its one-dimensional invariant subspace yields the first term of the holomorphic discrete series of representations of SO0(1,2)≂SU(1,1)/Z2. We interpret these results physically by showing that the above positive semidefinite inner product space is naturally isomorphic to a space of solutions of the conformally invariant zero-mass Klein–Gordon equation on the (anti)-de Sitter space–time, equipped with the usual Klein–Gordon inner product, obtained by integrating over a suitable spacelike hypersurface. As such, our version of geometric quantization selects in a natural way a quantization of the free massless particle. We show it is conformally [i.e., SO0(2,2)] invariant and behaves correctly in the classical limit.
Quantum flows associated to master equations in quantum optics
Authors: Fagnola Franco , Rebolledo Rolando , Saavedra Carlos
Published: July 2002

A stochastic dilation of master equation for a maser is constructed using the technique of quantum stochastic differential equations with unbounded operator coefficients. The associated quantum Markov flow is characterized and turns out to extend a classical birth and death process. Furthermore, the stationary distribution of the aforementioned process, which is connected to the stationary state of the micromaser, is obtained in a closed form.
Invariant connections with torsion on group manifolds and their application in Kaluza–Klein theories
Authors: Kubyshin Yu. A. , Malyshenko V. O. , Marín Ricoy D.
Published: July 2002

Invariant connections with torsion on simple group manifolds S are studied and an explicit formula describing them is presented. This result is used for the dimensional reduction in a theory of multidimensional gravity with curvature squared terms on M4×S. The potential of scalar fields emerging from extra components of the metric and torsion is calculated, and the role of the torsion for the stability of spontaneous compactification is analyzed.
Reduced SL(2,R) WZNW quantum mechanics
Authors: Fülöp T.
Published: July 2002

The SL(2,R) WZNW→Liouville reduction leads to a nontrivial phase space on the classical level both in 0+1 and 1+1 dimensions. To study the consequences in the quantum theory, the quantum mechanics of the 0+1-dimensional, point particle version of the constrained WZNW model is investigated. The spectrum and the eigenfunctions of the obtained (rather nontrivial) theory are given, and the physical connection between the pieces of the reduced configuration space is discussed in all the possible cases of the constraint parameters.
Lie–Poisson deformation of the Poincaré algebra
Authors: Stern A. , Yakushin I.
Published: July 2002

We find a one-parameter family of quadratic Poisson structures on R4×SL(2,C) which satisfies the properties: (a) that it reduces to the standard Poincaré algebra for a particular limiting value of the parameter (which we associate with the ‘‘canonical limit’’), as well as, (b) that it is preserved under the Lie–Poisson action of the Lorentz group (and the Lie–Poisson transformations reduce to canonical ones in the canonical limit). As with the Poincaré algebra, our deformed Poincaré algebra has two Casimir functions which correspond to ‘‘mass’’ and ‘‘spin.’’ The constant mass and spin surfaces in R4×SL(2,C) define symplectic leaves which we parametrize with space–time coordinates, momenta, and spin. We thereby obtain realizations of the deformed Poincaré algebra for both spinning and spinless particles. The formalism can be applied for finding a one-parameter family of canonically inequivalent descriptions of the photon.
Soliton solutions and nontrivial scattering in an integrable chiral model in (2+1) dimensions
Authors: Ioannidou T.
Published: July 2002

The behavior of solitons in integrable theories is strongly constrained by the integrability of the theory; i.e., by the existence of an infinite number of conserved quantities that these theories are known to possess. One usually expects the scattering of solitons in such theories to be rather simple, i.e., trivial. By contrast, in this paper we generate new soliton solutions for the planar integrable chiral model whose scattering properties are highly nontrivial; more precisely, in head-on collisions of N indistinguishable solitons the scattering angle (of the emerging structures relative to the incoming ones) is π/N. We also generate soliton–antisoliton solutions with elastic scattering; in particular, a head-on collision of a soliton and an antisoliton resulting in 90° scattering.
Oscillation spaces: Properties and applications to fractal and multifractal functions
Authors: Jaffard Stéphane
Published: July 2002

We establish a wavelet characterization of the p oscillation, and give two applications: a wavelet formula for the fractal dimension of graphs and a multifractal formalism for “chirp-type” Hölder singularities.
Limitations on the smooth confinement of an unstretchable manifold
Authors: Venkataramani S. C. , Witten T. A. , Kramer E. M. , Geroch R. P.
Published: July 2002

We prove that an m-dimensional unit ball Dm in the Euclidean space Rm cannot be isometrically embedded into a higher-dimensional Euclidean ball Brd⊂Rd of radius r<1/2 unless one of two conditions is met: (1) the embedding manifold has dimension d⩾2m; (2) the embedding is not smooth. The proof uses differential geometry to show that if d<2m and the embedding is smooth and isometric, we can construct a line from the center of Dm to the boundary that is geodesic in both Dm and in the embedding manifold Rd. Since such a line has length 1, the diameter of the embedding ball must exceed 1.
Critical current in a granular superconductor
Authors: Ignatjev V. K.
Published: July 2002

The mechanism of hypervortex pinning in a granular superconductor at intergranular contacts and intragranular defects is proposed for describing the field dependence of the critical current density and pinning potential. The results are in good agreement with those obtained experimentally. The proposed model describes the peak effect and the percolation mechanism of conductivity in ceramic superconductors.
The gamma distribution as a model for temperature dissipation in intermittent turbulence
Authors: Andrews L. C. , Shivamoggi B. K.
Published: July 2002

Following a recent theory suggesting that the local average rate of energy dissipation εr follows statistics predicted by the gamma distribution rather than the lognormal distribution, the gamma distribution is further examined here as a model for the local temperature dissipation rate χr. Comparisons of measured moments 〈χmr〉 with those predicted by both the gamma and lognormal distributions clearly favor the gamma model for predicting the power-law exponents of r/L. Based on gamma models for both εr and χr, the bivariate gamma distribution is then examined as a possible model for the joint distribution of dissipation rates εr and χr. In particular, the bivariate gamma model and the bivariate lognormal model are both compared with the inertial-range behavior of measured temperature structure functions up to order 12. Some additional comparisons of these models are also made with data involving mixed moments of the velocity and temperature structure functions.
A fourth-order evolution equation for deep water surface gravity waves in the presence of wind blowing over water
Authors: Dhar A. K. , Das K. P.
Published: July 2002

The stability of a train of nonlinear surface gravity waves in deep water in the presence of wind blowing over water is considered. An evolution equation is derived for the wave envelope that is correct to fourth order in the wave steepness. The importance of the fourth-order term in the evolution equation was pointed out by Dysthe [Proc. R. Soc. London Ser. A 369, 105 (1979)]. From this evolution equation the expressions for the maximum growth rate of the instability and the frequency at marginal stability are derived, and graphs are plotted for those two expressions against the wave steepness.
Numerical analysis of steady flows of a gas evaporating from its cylindrical condensed phase on the basis of kinetic theory
Authors: Sugimoto Hiroshi , Sone Yoshio
Published: July 2002

Steady evaporating flows from a cylindrical condensed phase in an infinite expanse of its vapor gas are investigated numerically on the basis of the Boltzmann–Krook–Welander equation. Not only the mass flow rate and the energy flow rate from the cylinder, but also the local variables of the gas over the whole flow field are obtained for a wide range of the Knudsen number and the pressure ratio, which is defined by the pressure at infinity divided by the saturation gas pressure at the temperature of the condensed phase. The acceleration of gas flows, especially to a supersonic flow, near the cylinder and the deceleration to the stationary state at infinity are clarified. The discontinuity of the velocity distribution function in the gas, a typical behavior of a gas around a convex body, is analyzed accurately with the difference scheme devised for this purpose, and its relation to the S layer [Phys. Fluids 16, 1422 (1973)] is discussed.
The spreading of heat or soluble surfactant along a thin liquid film
Authors: Jensen O. E. , Grotberg J. B.
Published: July 2002

The spreading of a localized distribution of surfactant on a thin viscous film is considered, in the situation in which the surfactant is soluble in the bulk layer and the boundary beneath the fluid is impermeable to surfactant. The surfactant distribution is controlled by advection and diffusion both at the surface of the film, where the surfactant forms a monolayer, and in the bulk. The bulk and surface surfactant concentrations are related by linearized sorption kinetics. The surfactant diffuses rapidly across the thin fluid layer, and lubrication theory is used to derive evolution equations for the film height and the surface and cross-sectionally averaged bulk surfactant concentrations. A special case of the governing equations describes the Marangoni flow induced by a locally hot region of the layer. It is shown that in comparison to the spreading of insoluble surfactant, transient desorption of surfactant from the monolayer to the bulk causes the spreading rate to diminish, although once the bulk and surface concentrations are locally in equilibrium, film deformations are more severe, with a sharp pulse in the film height created just upstream of the leading edge of the surfactant distribution.
Current sheets and nonlinear growth of the m=1 kink-tearing mode
Authors: Waelbroeck F. L.
Published: July 2002

A calculation is presented that accounts for rapid nonlinear growth of the m=1 kink-tearing instability. The equilibrium analysis contained in the Rutherford theory [Phys. Fluids 16, 1903 (1973)] of nonlinear tearing-mode growth is generalized to islands for which the constant-ψ approximation is not valid. Applying the helicity-conservation assumption introduced by Kadomtsev [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1977), Vol. I, p. 555], the presence of a current-sheet singularity is shown that gives rise to a narrow tearing layer and rapid reconnection. This rapid reconnection, in turn, justifies the use of the helicity conservation assumption. The existence of a family of self-similar m=1 equilibrium islands is demonstrated. The formalism introduced here is shown to apply both to the case of the m=1 kink-tearing mode and to the case of forced reconnection. These two cases are compared and contrasted.
Status of local transport measurements and analysis in toroidal devices
Authors: Burrell K. H. , Gentle K. W. , Luhmann N. C. , Marmar E. S. , Murakami M. , Schoenberg K. F. , Tang W. M. , Zarnstorff M. C.
Published: July 2002

This paper reviews the status of methods used to analyze local transport of particles, energy, and angular momentum in plasmas contained in toroidal fusion devices. The standard technique at present is based on determination of particle, energy, and momentum fluxes through analysis based on measured profiles of density, temperature and angular rotation speed and on calculation of all sources of particles, energy, and momentum. Recent experiments have also been done using perturbations of local density, temperature, or angular rotation speed caused by modulating the input sources. The local transport information is then extracted from the time history of the perturbations. These two techniques are contrasted and the strengths of each are discussed in this paper. Recommendations for further work needed to make progress in understanding transport are also given.
One-dimensional modeling of the wavelength sensitivity, localization, and correlation in reflectometry measurements of plasma fluctuations
Authors: Bretz N.
Published: July 2002

The reflection of electromagnetic waves from a plasma cutoff layer has been used to examine properties of density fluctuations in fusion plasmas. In this paper an exact one-dimensional model is used to show the relation between changes in the phase of the reflected wave and the location, magnitude, and correlation properties of density fluctuations. For long-wavelength density perturbations the reflected phase can be simply related to the amplitude of fluctuating density and the density scale length, Ln, near the cutoff layer. However, the phase response falls substantially as the fluctuation wavelength approaches the free space wavelength of the reflected wave, λ0, and the location of the maximum response moves out in front of the cutoff layer following the wave matching condition kΛ= 2k ≊ 2η(x)k0. Thus, a measurement of the reflected phase is strongly weighted to and localized for phenomena whose wavelength is longer than the characteristic scale (λ20Ln)1/3. Because of this weighting and because the region of maximum response moves away from the cutoff layer for short-wavelength fluctuations, there is also a limitation in any estimate of the density correlation length from the reflected phase. The correlation of phases between several different probe frequencies can be used to estimate a density correlation length no less than about four times the free space probe wavelength.
A numerical study of the high-n shear Alfvén spectrum gap and the high-n gap mode
Authors: Chu M. S. , Greene J. M. , Lao L. L. , Turnbull A. D. , Chance M. S.
Published: July 2002

The toroidicity-induced gaps of the shear Alfvén wave spectrum in tokamaks are shown to satisfy an envelope equation. The structure of these gaps, and the location of the high-n gap modes, which are localized modes with frequency in the gap, are studied for general numerically generated equilibria. The dependence of the frequencies of the gaps and the gap modes on the equilibrium properties, such as elongation, triangularity, and β of the plasma are explored.
On nonlinear effects in electron-cyclotron resonance plasma heating by microwave radiation
Authors: Litvak A. G. , Sergeev A. M. , Suvorov E. V. , Tokman M. D. , Khazanov I. V.
Published: July 2002

Possible nonlinear effects in electron-cyclotron plasma production and heating by microwave radiation are analyzed. They include nonlinear regimes of electron acceleration in strong microwave fields under cyclotron resonance conditions, nonstationary regimes of microwave beam self-focusing, and some parametric processes. Presented results may be of importance for predictions of plasma response to electron-cyclotron resonance (ECR) heating or preionization by powerful radiation.
Instability resulting from stratification in thermal conductivity
Authors: Yih Chia-Shun
Published: July 2002

There are many instances of hydrodynamic instability induced by a variation, or stratification, in either a fluid property or a flow property. In this article a new instability is presented. It is shown that when there is a variation in thermal conductivity in the fluid, instability can occur in the presence of a longitudinal gravitational field.
Condition for pressure isotropy
Authors: Newcomb William A.
Published: July 2002

For a collisionless, relativistic electron fluid, conditions on the velocity field are formulated which are necessary for the maintenance of pressure isotropy in the local fluid rest frame. A complete characterization of all velocity fields which satisfy these conditions is worked out in the nonrelativistic limit, and a few simple examples are given in the relativistic case.
Asymptotically correct collisional presheaths
Authors: Main Geoffrey L.
Published: July 2002

Few exact solutions for collisional presheaths exist because of the difficulty of simultaneously satisfying both the collisional Boltzmann equation and the Poisson equation. The exact solutions that do exist are for very specialized collision terms such as constant cross-section charge exchange with cold neutrals. The present paper presents an asymptotic method which is applicable to a variety of collision terms and is applied in particular to constant collision frequency charge exchange with noncold neutrals. Constant collision frequency and constant cross-section collision with cold neutral results are also presented. The first-order terms for the presheath potential rise and ion distribution functions are calculated and it is shown that second- and higher-order terms can be calculated using a multiexponential expansion for presheath potential rise. The first-order cold neutral constant cross-section results correspond well to the exact solution. The calculated presheath potential rises are of the order expected from the Bohm criterion, and in some of the specialized cold neutral cases, exactly kTe/2. The presheath potential rise is reduced by a neutral plasma potential gradient which accelerates ions toward the presheath. In all cases the collisional presheath is asymptotically matched to both the neutral plasma and the collisionless sheath.
Shock wave derivatives
Authors: Emanuel G. , Liu Min-Shan
Published: July 2002

A general theory is developed for obtaining the tangential and normal derivatives of thermodynamic and kinematic properties just downstream of a curved, unsteady shock wave. The flow upstream of the shock need not be steady or uniform and the gas need not be thermally or calorically perfect. Because of the complexity of the results, explicit formulas are provided for the above derivatives when the flow is steady and two-dimensional or axisymmetric, and the gas is perfect.
Rayleigh–Bénard and interfacial instabilities in two immiscible liquid layers
Authors: Wahal Sanjay , Bose Arijit
Published: July 2002

The linear stability of two immiscible liquid layers heated from below through a rigid perfectly conducting boundary with a free deformable upper surface and a deformable liquid–liquid interface is examined. Three modes of instability are allowed simultaneously in the analysis: surface tension driven at each of the two interfaces, buoyancy driven because of the presence of adverse density gradients in each liquid, and an interfacial mode (the Rayleigh–Taylor instability), related to the density difference and interfacial tension at the interfaces. For purely surface tension driven convection, the presence of a middle interface that can suppress normal deformations makes the two liquid layers more stable than a single layer of the same total depth. The interaction of the buoyancy and interfacial modes leads to overstability when the physical properties of the two liquids are only slightly different from each other. For certain Rayleigh numbers, both stationary and oscillatory modes display positive growth constants over a certain range of wavenumbers. As the Marangoni number is increased, the coupling between the surface tension and buoyancy mechanisms makes the system more unstable but removes the oscillatory eigenmodes. The addition of trace amounts of insoluble surface active agents at the two interfaces has a very strong stabilizing influence by introducing the expected hydrodynamic rigidity to the surfaces. However, their more interesting effect is their ability to change the nature of the most unstable eigenmode from stationary to oscillatory.
A note on the spectra and decay of rotating homogeneous turbulence
Authors: Zeman O.
Published: July 2002

In this Letter, the dynamics of spectral energy transfer in rotating homogeneous turbulence is investigated. It is shown that the wave number kΩ=(Ω3/ε)1/2 (defined by the rotation speed Ω and dissipation ε) determines the turbulence length scale, above which rotation effects on spectral transfer and on energy spectrum form are important. From the rotation-modified spectrum, turbulence decay laws are also inferred.
Fractal properties of flocs formed by fluid shear and differential settling
Authors: Huang Hening
Published: July 2002

The fractal properties of fine-grained sediment flocs formed by fluid shear and by differential settling were studied through floc porosity–size relationships. Flocs were produced from flocculation tests in a Couette-type flocculator in which fluid shear was the dominant collision mechanism for flocculation, and in a disk-type flocculator in which differential settling was the dominant collision mechanism. Floc samples were then introduced into a settling tube to measure their sizes and settling speeds by a double-exposure photographic method. The porosities of flocs were determined from the settling speed data using a porous sphere settling model. It was found that the flocs produced in the Couette-type flocculator could be regarded as fractals, with a fractal dimension ranging from 1.83 to 1.97. However, the flocs produced in the disk-type flocculator did not exhibit simple scaling behavior and could not be regarded as fractals. The differences in fractal property are due to the differences in floc formation schemes associated with the collision mechanisms. The similarities between experimentally obtained flocs and computer simulated clusters are discussed. The multistage growth model, a formation scheme for flocs formed by fluid shear, is consistent with the cluster–cluster model used in computer simulations. A formation scheme for flocs formed by differential settling is postulated; it is consistent with the particle–cluster model used in computer simulations.
A branching liquid jet
Authors: Lin S. P. , Webb R. D.
Published: July 2002

A new phenomenon of successive branching of a liquid jet emanating from a nozzle into the atmosphere has been reported [Phys. Fluids A 3, 241 (1991)]. It was observed that an intact jet can be made to bifurcate successively into a two-, three-, and multiple-pronged jet by oscillating the nozzle along its axis at a successively higher frequency. It is shown here that the observed multiple-pronged jet was actually multiple streams of droplets after the jet breakup generated by the lateral component of the nozzle oscillation, which was induced at the free end when the piezoelectric nozzle was axially vibrated. A plausible theoretical explanation of the formation of the multiple streams is given.
Deterministic forcing of homogeneous, isotropic turbulence
Authors: Sullivan Neal P. , Mahalingam Shankar , Kerr Robert M.
Published: July 2002

A deterministic low-wave-number forcing scheme designed to obtain statistically stationary homogeneous, isotropic turbulence in incompressible flows, and address criticisms of earlier schemes is proposed. Three-dimensional turbulent kinetic energy spectra collapse well and are more consistent with the experimentally determined Kolmogorov coefficient. Spectra for unforced scalar fields at different Prandtl numbers advected by the forced velocity fields collapse under Batchelor scaling, and do not show as strong a low-wave-number anomaly as earlier simulations that use forcing on both the velocity and scalar fields.
Rayleigh–Taylor and Richtmyer–Meshkov instabilities in finite-thickness fluid layers
Authors: Mikaelian Karnig O.
Published: July 2002

Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities are considered in a fluid layer of thickness t having perturbations of arbitrary wave number k at either one or both interfaces. The evolution of the perturbation amplitudes η1,2(τ), τ =time, is given analytically in terms of a coupling angle θ which measures the strength of the coupling between interfaces 1 and 2. A new type of freeze-out in shocked layers is reported according to which, the proximity of the two interfaces can, under proper conditions, lead to the complete freeze-out of one, but not both, perturbations. For example, to freeze the first interface one needs η1(0)/η2(0)= sin θ. Freeze-out cannot be achieved in the RT case; instead, one can kill one of the modes. For example, setting η1(0)/η2(0)= tan(θ/2) will kill the exponentially growing mode, leaving only the oscillatory mode at both interfaces.
Horseshoe vortex systems resulting from the interaction between a laminar boundary layer and a transverse jet
Authors: Kelso R. M. , Smits A. J.
Published: July 2002

The horseshoe vortex system resulting from the interaction between a laminar boundary layer and a round transverse jet was studied over a range of Reynolds numbers and velocity ratios using hydrogen bubble wire visualization in a water channel. The study shows that the horseshoe vortex system can be steady, oscillating, or coalescing, depending on the flow conditions. Topological concepts are used to interpret the observed flow patterns and compare these patterns with those observed and computed upstream of wall-mounted circular cylinders. The Strouhal numbers of the observed oscillating and coalescing systems agree reasonably well with those appearing in the literature for wall-mounted circular cylinders. The relationship between the unsteady horseshoe vortex motions and the unsteady vortex motions in the wake is studied for a velocity ratio of 4. Here it is shown that the oscillating regime occurs at the same frequency as the wake and the coalescing regime occurs at approximately double the frequency of the wake. The results indicate that the wake intermittently becomes coupled to the horseshoe vortex motions and that this occurs either at the horseshoe vortex frequency in the case of the oscillating system or a subharmonic in the case of the coalescing system.
Small-scale properties of nonlinear interactions and subgrid-scale energy transfer in isotropic turbulence
Authors: Kerr Robert M. , Domaradzki J. Andrzej , Barbier Gilles
Published: July 2002

Using results of direct numerical simulations of isotropic turbulence the subgrid-scale energy transfer in the physical space is calculated exactly employing a spectral decomposition of the velocity field into large (resolved) and small (unresolved) scales. Comparisons with large-scale quantities reveal large qualitative correlations between regions of subgrid transfer and the boundaries of regions of large vorticity production. This suggests a novel analysis of the nonlinear term, where it is decomposed into four components determined by four combinations of the resolved and unresolved velocity and vorticity fields. It is found that there is a 90% vector-correlation between the subgrid transfer and the component of the full transfer associated with the resolved velocity and unresolved vorticity, but that 90% of the total subgrid energy production is determined by the component associated with the unresolved velocity and resolved vorticity. These results suggest subgrid-scale models that have higher correlation values with the exact subgrid scale terms than a number of other physical quantities that are traditionally considered to govern the dynamics of the large scales of turbulence. The distinguishing feature of the new models is that they emphasize the nonlinear dynamics of the smallest resolved scales in the modeling procedures.
Power law wave number spectra of fractal particle distributions advected by flowing fluids
Authors: Namenson Arthur , Antonsen Thomas M. , Ott Edward
Published: July 2002

Recently, it has been shown that an initial cloud of particles advected by a fluid may, under common circumstances (e.g., when the particles float on the fluid surface), eventually becomes distributed on a fractal set in space. This paper considers the characterization of such fractal spatial patterns by wave number spectra. If a scaling range exists in which the spectrum has an observable power law dependence, k−ρ, then the exponent ρ is given by ρ=D2+1−M, where D2 is the correlation dimension of the fractal attractor and M is the dimension of the relevant space. We find that observability of the power law may be obscured by fluctuations in the k-spectrum, but that averaging can be employed to compensate for this. Theoretical results and supporting numerical computations utilizing a random map are presented. In the companion paper by Sommerer [Phys. Fluids 8, 2441 (1996)], an experimental example utilizing particles floating on the surface of a flowing fluid is given. (More generally we note that our result for the k-spectrum power law exponent ρ should apply to fractal patterns in other physical contexts.)
On the three-dimensional instability of a swirling, annular, inviscid liquid sheet subject to unequal gas velocities
Authors: Panchagnula Mahesh V. , Sojka Paul E. , Santangelo Philip J.
Published: July 2002

A linear model describing the instability behavior of annular, swirling, inviscid sheets subject to inner and outer gas flows of differing velocities is presented. The model considers three-dimensional disturbances and contains previous flat sheet, cylindrical jet, and annular jet analyses as limiting cases. Model predictions show that, in the absence of swirl, (i) an increase in axial Weber number causes the range of unstable axial disturbance modes to increase, (ii) when the axial Weber numbers are small (<8), inner gas flows lead to slightly faster growing axial instability modes than outer gas flows at equivalent inner and outer Weber numbers, but inner and outer gas flows have the same effect when Weber numbers are high (≳10), (iii) the wavenumber for the axial mode having the highest growth rate decreases with a decrease in axial Weber number, (iv) an increase in the density of the atomizing gas results in a slight increase in the wavenumber of the axial disturbance mode having the highest growth rate. When swirl is present, model predictions demonstrate that (v) swirl reduces the wavenumber for the axial disturbance mode having the highest growth rate and reduces growth rates as well, (vi) an increase in the swirl Weber number beyond the stabilizing region increases the range of unstable axial and circumferential modes and increases growth rates as well for nonzero axial Weber numbers, (vii) increasing the swirl Weber number increases the axial wavenumber for the disturbance mode having the highest growth rate, but a circumferential mode number of zero is retained until the swirl Weber number exceeds about 8, at which point the axial wavenumber for the disturbance having the highest growth rate falls to zero and the circumferential wavenumber jumps to a finite value of n at which time further increases in swirl Weber number serve to increase n, (viii) up to two local nondimensional growth rate maxima can exist, and the instability domain can be simply connected or can consist of two separate regions separated by an area where disturbances are stable. The topology of the growth rate surface depends on the ratio of the annulus inner to outer radii. These findings are used to explain some observations of practical atomizer performance.