bluuebunny/crossref_metadata_2025 huggingface dataset:
Tilted phases of fatty acid monolayers
Authors: Kaganer V. M. , Peterson I. R. , Kenn R. M. , Shih M. C. , Durbin M. , Dutta P.
Published: July 2002

X-ray diffraction data from water-supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2′, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2′–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The distortions are related to symmetry of the phases and described by the order parameters responsible for tilt and herringbone ordering of the backbone planes of the molecules.
Calculations of the dynamic Debye–Scherrer diffraction patterns for small metal particles
Authors: Hall B. D. , Ugarte D. , Reinhard D. , Monot R.
Published: July 2002

Dynamic diffraction pattern profiles are calculated for randomly oriented aggregates of gold and silver in the size range from 147 to 5083 atoms and at incident electron energies of 40 kV and 100 kV. The Debye–Scherrer diffraction patterns were obtained by combining a series of multislice calculations performed on model particles over a range of orientations. Calculations are performed for both fcc and icosahedral structures. The results show that corrections to the kinematical theory (Debye equation) are more important than predicted by the two-beam theory of Blackman. One calculation, a fcc 923-atom silver aggregate at 100 kV, showed a distortion to the (111) Bragg peak causing it to to shift its center to a higher scattering angle. Scattering corrections to the icosahedral results are less important than for an equivalent sized fcc aggregate and preserve the general diffraction features seen in kinematic calculations.
Configuration interaction and the theory of electronic factors in energy transfer and molecular exciton interactions
Authors: Scholes Gregory D. , Harcourt Richard D.
Published: July 2002

The theory established in J. Chem. Phys. 101, 10521 (1994), for electronic factors which promote interchromophore electronic energy transfer, exciton interactions and which provide the stabilization of excimers, is extended; first so as to include the possible contribution of doubly excited configurations. It is ascertained that there is a resultant effect upon the (interchromophore orbital overlap-dependent) through-configuration interaction, and a significant correction to the simple expression obtained previously for the Coulombic interaction. These CI effects are admitted to the general theory of the previous work and the cases of nonidentical, identical, and orthogonal donor and acceptor are discussed. Second, a description of superexchange effects is admitted to the theory. Two possible formalisms are developed and discussed.
Solvent free energy curves for electron transfer reactions: A nonlinear solvent response model
Authors: Ichiye Toshiko
Published: July 2002

Marcus theory for electron transfer assumes a linear response of the solvent so that both the reactant and product free energy curves are parabolic functions of the solvent polarization, each with the same solvent force constant k characterizing the curvature. Simulation data by other workers indicate that the assumption of parabolic free energy curves is good for the Fe2+–Fe3+ self-exchange reaction but that the k of the reactant and product free energy curves are different for the reaction D0+A0→D1−+A1+. However, the fluctuations sampled in these simulations were not large enough to reach the activation barrier region, which was thus treated either by umbrella sampling or by parabolic extrapolation. Here, we present free energy curves calculated from a simple model of ionic solvation developed in an earlier paper by Hyun, Babu, and Ichiye, which we refer to here as the HBI model. The HBI model describes the nonlinearity of the solvent response due to the orientation of polar solvent molecules. Since it is a continuum model, it may be considered the first-order nonlinear correction to the linear response Born model. Moreover, in the limit of zero charge or infinite radius, the Born model and the Marcus relations are recovered. Here, the full free energy curves are calculated using analytic expressions from the HBI model. The HBI reactant and product curves have different k for D0+A0→D1−+A1+ as in the simulations, but examining the full curves shows they are nonparabolic due to the nonlinear response of the solvent. On the other hand, the HBI curves are close to parabolic for the Fe2+–Fe3+ reaction, also in agreement with simulations, while those for another self-exchange reaction D0−A1+ show greater deviations from parabolic behavior than the Fe2+–Fe3+ reaction. This indicates that transitions from neutral to charged species will have the largest deviations. Thus, the second moment of the polarization is shown to be a measure of the deviation from Marcus theory. Finally, since the HBI expressions for the free energy curves are not simple, the HBI curves are compared with various approximate parabolic descriptions of the curves, including Marcus parabolas.
A neutron scattering study of the structure of amorphous glucose
Authors: Tromp R. Hans , Parker Roger , Ring Steve G.
Published: July 2002

Wide angle neutron scattering combined with H/D substitution has been applied to determine the hydrogen bond structure in glassy and liquid glucose [C6H7O(OH)5]. H/D substitution involved only the H atoms belonging to OH groups. The resulting radial distribution function gHH(r) clearly shows the ordering of H atoms, in a way expected from hydrogen bonding. At −10 °C and 35 °C, at which temperatures glucose is in the glassy state, temperature dependence of the hydrogen bond structure is clearly observed. The number of hydrogen bonds in the glassy state is similar to that in the crystalline state. At 80 °C, in the undercooled liquid state, the number of hydrogen bonds is 20% lower and the hydrogen bond structure much less pronounced. The hydrogen bond H–H distance is found to be in the range of 2.4–2.6 Å, similar to the values found in the crystal (2.40 Å) and water at room temperature (2.44 Å). At the lower temperatures, preliminary GROMOS molecular dynamics simulation results of gHH(r) are in reasonable agreement with the experimental data. However, at 80 °C, the simulations seriously overestimate intermolecular structure.
Unimolecular reactions in the gas and liquid phases: A possible resolution to the puzzles of the trans-stilbene isomerization
Authors: Gershinsky Gidon , Pollak Eli
Published: July 2002

Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited S1 state indicated that the gas phase thermal rate at room temperature is much smaller than the thermal rate in the liquid phase. This was based on the observations that: (a) A combination of measured energy-dependent rates and RRKM calculations led to an isolated molecule thermal rate at T=300 K of 2×109 s−1; (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the S0 to S1 zero point to zero point electronic transition energy (000), gave a lifetime in the excited state of ∼780 ps. The liquid state lifetime in ethane is ∼30 ps. In this paper we present theoretical computations of the rate in the gas and liquid phases, based on a new potential model of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. We find that: (a) RRKM rates are in agreement with measured energy-dependent rates; (b) The thermal rate derived from the new RRKM rates is the same as the thermal rate in liquid ethane; (c) The laser excitation experiment of Balk and Fleming leads to laser cooling of the excited state suggesting that their measured lifetime is longer than the lifetime in the liquid. The surrounding liquid heats up the molecule on a time scale which is faster than the isomerization lifetime. Experiments are suggested to verify this interpretation.
Resonant dissociation of DNA bases by subionization electrons
Authors: Huels Michael A. , Hahndorf Ina , Illenberger Eugen , Sanche Léon
Published: July 2002

We have measured the electron energy dependence for production of a great variety of anion fragments, induced by resonant attachment of subionization electrons to thymine (T) and cytosine (C) within femto-second time scales. At the lowest electron energies we also observe stable molecular anions of these bases, viz., T− and C−. Our measurements suggest that this resonant mechanism may relate to critical damage of irradiated cellular DNA by subionization electrons prior to thermalization.
Computer simulation studies of liquid crystals: A new Corner potential for cylindrically symmetric particles
Authors: Zewdie Habtamu
Published: July 2002

We have developed a new Corner potential suitable for computer simulation studies of pure and mixture systems composed of rodlike, disklike, and spherical molecules. The pair potential is assumed to have the shifted Lennard–Jones 12-6 potential which has the general form of εf(σ0/(r−σ+σ0)). The strength parameter ε and the range parameter σ are then expanded in terms of a complete orthogonal basis set of functions, called S functions, to obtain expansion coefficients typical of mesogenic molecules. The coefficients for the range parameter are determined by mapping the expansion onto prolate and oblate spherocylinders which are considered to be more realistic models for rodlike and disklike molecules, respectively. The shape anisotropies considered are (L+D)/D=3 and (D+L)/L=3 for rodlike and disklike molecules, respectively. One of the important advantages of this expansion approach is that each coefficient in the expansion of the strength parameter can be adjusted to reflect the contribution from a specific type of interaction. To make a systematic evaluation of the success of this approach we have obtained the expansion coefficients for the strength parameter by mapping onto that of two well-studied models: The Gay–Berne (GB) model potential (GBI: μ=1, ν=2 and GBII: μ=2, ν=1), the potential model for site–site interaction between two molecules each represented by a linear array of four Lennard–Jones centers per molecule (RLJ4). To explore the value of the model potential for studies of liquid crystals, we have carried out a detailed Monte Carlo simulation. We have studied a system of rodlike molecules with shape anisotropy equal to 3 at three packing fractions (Nv0/V=0.4, 0.47, and 0.62). Five thermodynamically stable condensed phases have been identified and characterized as crystal, smectic B, smectic A, nematic, and isotropic phases. Such phase polymorphism contrasts with that for a system of hard prolate spherocylinders with the same shape anisotropy which is known to form only isotropic and crystalline phases. The range of stability and the nature of the transition between the phases have been determined. The influence of density on the range and stability of the phases is explored. Increasing the density is found to decrease the range of stability of the nematic phase in favor of the smectic A phase.
Extracting site-specific reaction rates from steady state surface morphologies: Kinetic Monte Carlo simulations of aqueous Si(111) etching
Authors: Flidr Jaroslav , Huang Yi-Chiau , Newton Theresa A. , Hines Melissa A.
Published: July 2002

The etching of Si(111) surfaces is studied using a chemically realistic, kinetic Monte Carlo simulation that incorporates the full surface geometry. These simulations show that reliable, site-specific reaction rates can be extracted from steady state etched surface morphologies. The kinetics of pit etching and step etching are examined separately and in concert. Comparisons are made to mean field approximations; however, mean field kinetics are shown to be quantitatively incorrect in their predictions of surface morphology. Statistical parametrizations of surface morphology, such as local width measurements, are also examined. Although these parametrizations are sensitive to the relative etch rates, they cannot be used to ascertain the actual etch mechanism. In general, steady state etch morphologies change gradually as the relative etch rates are varied over three-to-five decades of reactivity. Within this range, a change in surface morphology is observable when a single etch rate is changed by a factor of 2–3.
Influence functionals with semiclassical propagators in combined forward–backward time
Authors: Thompson Keiran , Makri Nancy
Published: July 2002

In a recent letter [Chem. Phys. Lett. 291, 101 (1998)] we presented a semiclassical methodology for calculating influence functionals arising from many-body anharmonic environments in the path integral formulation of quantum dynamics. Taking advantage of the trace operation associated with the unobservable medium, we express the influence functional in terms of a single propagator along a combined forward–backward system path. This propagator is evaluated according to time-dependent semiclassical theory in a coherent state initial value representation. Because the action associated with propagation in combined forward and backward time is governed by the net force experienced by the environment due to its interaction with the system, the resulting propagator is generally a smooth function of coordinates and thus amenable to Monte Carlo sampling; yet, the interference between forward and reverse propagators is fully accounted for. In the present paper we present a more elaborate version of the semiclassical influence functional formalism, along with algorithms for evaluating the coherent state transform of the Boltzmann operator that enters the influence functional. This factor is evaluated by performing an imaginary time path integral, and various approximations of the resulting expression as well as sampling schemes are discussed. The feasibility of the approach is demonstrated via numerous test calculations involving a two-level system coupled to (a) a dissipative harmonic bath and (b) a ten-dimensional bath of coupled anharmonic oscillators.
Numerical prediction of the melting curve of n-octane
Authors: Polson James M. , Frenkel Daan
Published: July 2002

We compute the melting curve of n-octane using Molecular Dynamics simulations with a realistic all-atom molecular model. Thermodynamic integration methods are used to calculate the free energy of the system in both the crystalline solid and isotropic liquid phases. The Gibbs–Duhem integration procedure is used to calculate the melting curve, starting with an initial point obtained from the free energy calculations. The calculations yield quantitatively accurate results: in the pressure range of 0–100 MPa, the calculated melting curve deviates by only 3 K from the experimental curve. This deviation falls just within the range of uncertainty of the calculations.
Geometry of reorientational dynamics in supercooled poly(vinyl acetate) studied by 13C two-dimensional nuclear magnetic resonance echo experiments
Authors: Tracht U. , Heuer A. , Spiess H. W.
Published: July 2002

A study of the reorientational segmental dynamics in supercooled poly(vinyl acetate) is presented, yielding detailed information about geometry and time scale of the motion close to the glass transition. The geometry information is derived from systematic variation of the evolution time in 13C 2D echo NMR measurements. The dynamics can be described as a superposition of angular jumps of approximately 10° and rotational diffusional processes. Both processes are related to the macroscopic α-relaxation. On the time scale of one jump process the orientation of a segment changes by about 2° via small step diffusion (<0.6°). Furthermore the temperature dependence of this reorientatinal scenario is analyzed within the limits imposed by the experiment. All results are compared with previous 2H 2D NMR measurements on low-molecular glass formers. In both cases the loss of correlation, as described by conventional correlation times, results from a sequence of many distinct reorientational steps.
General relationship between complex impedance and linear stability in electrochemical systems
Authors: Naito M. , Tanaka N. , Okamoto H.
Published: July 2002

The impedance of the electrochemical system is derived in an explicit analytical form in relation to the stability of the system under various driving conditions. It is shown that the complex impedance is represented as the ratio of characteristic polynomials of the Jacobian matrices of linearized system under potentiostatic control and under galvanostatic control. Thus it is definitely shown that the zeros of the impedance are the eigenvalues of the Jacobian of the system under potentiostatic control, and that the poles are the eigenvalues of the Jacobian under galvanostatic control. The obtained impedance formulas are used to derive or prove several electrochemical characteristics. A direct analytical relationship between the hidden negative impedance and the galvanostatic Hopf bifurcation is also derived.
On partial differential equations related to Lorenz system
Authors: Hsieh D. Y.
Published: July 2002

Partial differential equations are constructed such that a truncation scheme as adopted by Lorenz [J. Atmos. Sci. 20, 130 (1963)] will lead to Lorenz equations. The partial differential equations are much simpler than those of the Rayleigh–Bernard problem and are essentially of the mixed type. Various aspects of the partial differential equations are explored. It is suggested that the switching back and forth between the elliptic and hyperbolic regimes represents the chaotic behavior of the system in the context of partial differential equations.
Transformation brackets between U(ν+1)⊃U(ν)⊃SO(ν) and U(ν+1)⊃SO(ν+1)⊃SO(ν)
Authors: Santopinto Elena , Bijker Roelof , Iachello Francesco
Published: July 2002

We derive a general expression for the transformation brackets between the chains U(ν+1)⊇U(ν)⊇SO(ν) and U(ν+1)⊇SO(ν+1)⊇SO(ν) for ν≥2.
Off-diagonal long range order and superconductive electrodynamics
Authors: Sewell Geoffrey L.
Published: July 2002

We present a general, model-independent, quantum statistical derivation of superconductive electrodynamics from the assumptions of off-diagonal long range order (ODLRO), local gauge covariance, and thermodynamic stability. On this basis, we obtain the Meissner and Josephson effects, the quantization of trapped magnetic flux, and the metastability of supercurrents. A key to these results is that the macroscopic wave function, specified by the ODLRO condition, enjoys the rigidity property that London [Superfluids, Vol. 1 (Wiley, London, 1950)], envisaged for the microstate of a superconductor.
Bispectrality for the quantum Ruijsenaars model and its integrable deformation
Authors: Chalykh Oleg A.
Published: July 2002

An elementary construction of the eigenfunctions for the quantum rational Ruijsenaars model with integer coupling parameter is presented. As a by-product, we establish the bispectral duality between this model and the trigonometric Calogero–Moser model. In particular, this gives a new way for calculating Jack polynomials. We propose also a certain one-parameter deformation of the Ruijsenaars model, proving its integrability and bispectrality. The generalizations related to other root systems and difference operators by Macdonald are considered.
Nonlinear localized excitations in magnets with a weak exchange interaction as a soliton problem
Authors: Gvozdikova M. V. , Kovalev A. S.
Published: July 2002

Spin dynamics of soliton-like localized excitations in a discrete ferromagnetic chain with an easy-axis anisotropy and weak exchange interaction is studied. The relation of these excitations and dynamic magnetic solitons in the long-wave approximation is determined, and the dependence of frequency of localized excitations on the exchange interaction parameter for a fixed value of the total number of spin deviations is constructed. It is shown that this dependence is modified significantly for values of exchange interaction of the order of the one-ion anisotropy value.
Low-frequency impedance and heat capacity of LuFe4Al8
Authors: Gurevich A. M. , Dmitriev V. M. , Eropkin V. N. , Ishchenko L. A. , Prentslau N. N. , Shlyk L. V.
Published: July 2002

Temperature dependences of low-frequency impedance (0–107 Hz) and heat capacity of LuFe4Al8 are studied at low temperature in the intervals 4.5–300 K and 1.5–120 K, respectively. The observed singularities suggest that antiferromagnetic phase ordering occurs at TN=100.5 K. A sharp increase in the resistance is observed at a certain temperature T1<TN below which a negative magnetoresistive effect is detected in a weak magnetic field (H<50 Oe). It is shown that the low- temperature anomalies at T<25 K are associated with the superconducting transition. This is supported by the presence of the Meissner effect, levitation, a decrease in ohmic losses, as well as the influence of a static magnetic field and direct current on the resistance of the compound. A magnetostructural phase transition, which is assumed to occur at T1<TN, may be responsible for the peculiarities observed in the physical properties of LuFe4Al8.
Simultaneous measurements of instantaneous concentrations of two species being mixed in a turbulent flow by using a combined laser-induced fluorescence and laser-scattering technique
Authors: Komori Satoru , Kanzaki Takao , Murakami Yasuhiro , Ueda Hiromasa
Published: July 2002

A technique for the simultaneous measurements of the instantaneous concentrations of two species being mixed in a turbulent flow is developed using a combined laser-induced fluorescence and laser-scattering technique, and the quantitative measurements are demonstrated in a simple grid-generated turbulent flow with two streams of nonpremixed species. It is shown that the present technique enables one to measure simultaneously the instantaneous concentrations of two species with spatial and time resolutions far less than the Kolmogorov length and time scales.
The interaction between a counter-rotating vortex pair in vertical ascent and a free surface
Authors: Marcus Daniel L. , Berger Stanley A.
Published: July 2002

The inviscid, two-dimensional interaction between a pair of counter-rotating line vortices and a free surface has been studied. A solution to the linearized, small-disturbance problem has been thoroughly explored. For the nonlinear problem numerical calculations were carried out for Froude numbers representing a range of very weak to very strong vortices. Strong vortices are little affected by the presence of the surface, rising to form a bubblelike disturbance; weaker vortices follow paths like vortices approaching a plane boundary. The experimentally observed scarring phenomena—surface depressions whose axes are perpendicular to the flow plane—are seen in the numerical results.
Goertler vortices in supersonic and hypersonic boundary layers
Authors: Spall R. E. , Malik M. R.
Published: July 2002

The problem of Goertler vortices in compressible boundary layers over concave walls is considered. At O(1) wavelengths, the instability is governed by parabolic partial differential equations that are solved numerically to determine the effect of various initial conditions on the development of Goertler vortex instability in compressible boundary layers. The results show that both the velocity and temperature fluctuations may lead to a Goertler vortex. The vortex growth rates determined from the present method are found to differ somewhat from those given by a normal mode solution. At both the supersonic and hypersonic Mach numbers, cooling has a small destabilizing effect. In addition, the most unstable disturbances shift toward lower wavelengths because of thinning of the boundary layer. Our results also show that compressibility has a stabilizing effect on the Goertler instability, while the effect of an adverse pressure gradient is found to be destabilizing. The behavior of the Goertler vortex structure with Mach number is also examined. At hypersonic Mach numbers, vortices are located near the edge of the boundary layer for adiabatic wall conditions. However, the entire boundary layer is affected when the wall is cooled.
Effect of wall suction on bursting in a turbulent boundary layer
Authors: Antonia R. A. , Bisset D. K. , Fulachier L. , Anselmet F.
Published: July 2002

The effect of suction on the wall region of a turbulent boundary layer over a slightly heated wall has been quantified in terms of several aspects of the motion that are related to bursts and sweeps. These events are detected by the modified u-level method (with alignment on the ends of bursts), the uv-quadrant 2 method, and the uv-quadrant 4 method. The mean periods of bursts and sweeps are increased by suction. The regions upstream from u-level detected bursts are more coherent than the bursts themselves. Suction substantially increases the relative contributions from the organized motion to momentum and heat fluxes, ∼(uv), ∼(uθ), and ∼(vθ), and has a lesser effect on contributions to ∼(u2), ∼(v2), and ∼(θ2).
On the decay of compressible isotropic turbulence
Authors: Zeman Otto
Published: July 2002

This paper reports on the extension of second-order models for unforced isotropic turbulence to the compressible regime. The equations contain two principal compressibility terms: the dilatation dissipation and the pressure–dilatation correlation. The dilatation dissipation has been modeled in accordance with the shocklet dissipation theory of Zeman [Phys. Fluids A 2, 178 (1990)]; the pressure–dilatation term has been identified with the rate of change of compressible potential energy represented by the pressure fluctuation variance ∼(p2). The rate of change of ∼(p2) is assumed to be governed by a nonlinear relaxation mechanism that drives ∼(p2) to an equipartition value. By comparison with direct numerical simulations of 2-D and 3-D turbulence, it is shown that the extended turbulence energy equations are capable of simulating the compressibility effects associated with rms Mach number and the (initial) pressure fluctuations.
Diabatic supersonic flows of dense gases
Authors: Schnerr G. H. , Leidner P.
Published: July 2002

By adopting the van der Waals gas model the behavior of n-octane with addition or removal of heat in inviscid, steady, one-dimensional, supersonic flow is investigated. First, the given heat distribution is added in a constant area flow. Second, the effects in variable cross-sectional flows (nozzles) are shown for the additional condition of constant values of the Mach number or the static pressure, and for a given heat addition. As known from the isentropic flow of real gases, several gas-dynamic effects are inverted depending on the fundamental derivative Γ. Heat addition in a constant area duct at conditions near the thermodynamical critical point may cause an increase of the Mach number in transonic/supersonic flow, or a decrease in subsonic flow, which is the opposite behavior of a perfect gas. For heat addition at M=const a singularity is possible for Γ<0 in transonic/supersonic flow and for Γ≲1 in subsonic flow. Considering constant Mach number flows, the static pressure goes up for high values of Γ. Isobaric heat addition increases the Mach number for densities below the thermodynamical critical point. Perfect gas flow exhibits the opposite reaction in both cases without any singularity. Compared with the flow of a perfect gas (γ=1.4), in the dense gas the influence of the same rate of heat addition becomes much stronger.
A boundary integral equation method for the Stokes problem of multiparticle systems
Authors: Xijun Fan , Yeow Y. L.
Published: July 2002

A numerical procedure based on a Fredholm integral equation of the second kind was used to solve the resistance and mobility problems of Stokes flow past a general collection of particles. The procedure was found to outperform a number of other numerical methods in that it requires only a coarse discretization of the particle surfaces and produces highly accurate results.
Weakly nonlinear models for turbulent mixing in a plane mixing layer
Authors: Liou William W. , Morris Philip J.
Published: July 2002

New closure models for turbulent free shear flows are presented in this paper. They are based on a weakly nonlinear theory with a description of the dominant large-scale structures as instability waves. Two models are presented that describe the evolution of the free shear flows in terms of the time-averaged mean flow and the dominant large-scale turbulent structure. The local characteristics of the large-scale motions are described using linear theory. Their amplitude is determined from an energy integral analysis. The models have been applied to the study of an incompressible mixing layer. For both models, predictions of the mean flow development are made. In the second model, predictions of the time-dependent motion of the large-scale structures in the mixing layer are made. The predictions show good agreement with experimental observations.
Numerical characterization of localized solutions in plane Poiseuille flow
Authors: Price Tim , Brachet Marc , Pomeau Yves
Published: July 2002

Stable localized wave packets in two-dimensional incompressible Poiseuille flow are characterized by direct simulation of the Navier–Stokes equations. These asymmetric wave packets depend only on the Reynolds number when the channel periodicity length is sufficiently large. They are created through a saddle-node bifurcation at Re ∼2330, well below the critical Reynolds number for finite-amplitude Tollmien–Schlichting waves. Pairs of wave packets are found to interact repulsively, and no evidence for locking has been observed. Finally, an amplitude-equation model is suggested which incorporates the spatial asymmetry of the localized solutions.
Spatial correlations in turbulence: Predictions from the multifractal formalism and comparison with experiments
Authors: O’Neil John , Meneveau Charles
Published: July 2002

Questions about applicability of multiplicative cascade models for turbulent small-scale intermittency (such as lognormal, random curdling, β, α, p models, etc.) are addressed by using the multifractal formalism to predict new properties of two-point moments. These predictions are compared with experimental data. Measurements are performed in the wake of a cylinder and grid turbulence. Data at high Reynolds number in the atmospheric surface layer are also considered. The autocorrelation function of the local singularity strength α(x), as well as mixed moments of the form <εr(x)qεr(x+s)−q≳ are computed from the kinetic energy dissipation obtained from single-component, single-probe measurements using Taylor’s hypothesis. For flows at high-enough Reynolds number, the α(x) autocorrelation function exhibits logarithmic decay with distance, as predicted from a random multiplicative cascade process. Some discrepancies exist in the quantitative details, implying enhanced randomization. The mixed moments are found to exhibit a scaling transition, also in agreement with the multiplicative models. The results illustrate the usefulness of the two-point multifractal formalism in characterizing intermittency and, as far as two-point statistics is concerned, lend further (qualified) support to the multiplicative cascade models.
Linear stability of low mode number tearing modes in the banana collisionality regime
Authors: Fitzpatrick R.
Published: July 2002

The semicollisional layer equations governing the linear stability of small mode number tearing modes in a low beta, large aspect ratio, tokamak equilibrium are derived from an expansion of the gyrokinetic equation. In this analysis only the cases where the ion Larmor radius is either much less than, or much greater than, the layer width are considered. Both the electrons and the ions are assumed to lie in the banana collisionality regime. One interesting feature of the derived layer equations, in the limit of small ion Larmor radius, is a substantial reduction in the effective collisionality of the system due to neoclassical ion dynamics. Next, using a shooting code, a dispersion relation is obtained from the layer equations in the limits of small ion Larmor radius and a vanishingly small fraction of trapped particles. As expected, strong semicollisional stabilization of the mode is found, but, in addition, a somewhat weaker destabilizing effect is obtained in the transition region between the collisional and semicollisional regimes.
Kinetic effects on Alfvén wave nonlinearity. II. The modified nonlinear wave equation
Authors: Spangler Steven R.
Published: July 2002

The study of kinetic effects on Alfvén wave nonlinearity is continued. Previously obtained expressions for the perturbed (by an Alfvén wave) ion and electron distribution functions are used to obtain a nonlinear wave equation for parallel-propagating, circularly polarized waves. The results are cast in the form of a modified version of the familiar derivative nonlinear Schrödinger equation. The approach in obtaining this equation is a hybrid one; fluid theory is used to the greatest extent possible, and kinetic theory is introduced where the correction is believed to be most important. Fluid theory at two levels of sophistication is employed. The first uses a simple scalar pressure term. This approach yields physical insight and illuminates the field-aligned fluid flow and the associated plasma density perturbation as a major contributor to Alfvén wave nonlinearity. The second approach employs a tensor pressure term that in general will be necessary. The results indicate that kinetic effects in general produce a nonlinear wave equation that is of a different functional form than the derivative nonlinear Schrödinger equation, as previously reported by Mjo/lhus and Wyller [Phys. Scr. 33, 442 (1986); J. Plasma Phys. 40, 229 (1988)]. The coefficient of the derivative cubic term depends on the plasma beta in a way which, in general, is quite different from the fluid expression. In addition, a functionally novel term appears in the modified equation. The magnitude of this term, named the ‘‘nonlocal term’’ by Mjo/lhus and Wyller, can be large when the plasma beta is comparable to unity. The susceptibility of the modified equation to modulational instability is studied. Kinetic effects cause modulational instability of wave packets, even when fluid theory would predict modulational stability. This modulational instability occurs for both right- and left-hand polarized waves. This latter result might explain a property of waves near the Earth’s bow shock that was enigmatic within the context of fluid theory.
Edge turbulence scaling with shear flow
Authors: Zhang Y. Z. , Mahajan S. M.
Published: July 2002

A formula relating turbulence levels with arbitrary shear flow is derived. When the diffusion coefficient is made a functional of the corresponding turbulence level, it is found that the scaling laws governing turbulence suppression are considerably modified. The results are compared with known formulas in various limiting cases, indicating that turbulence suppression mainly pertains in the moderate shear flow regime. The results also show that a flattened (steep) radial equilibrium gradient tends to enhance (eliminate) turbulence suppression due to the shear flow.
Particle transport studies for single-null divertor discharges in DIII-D
Authors: Rensink M. E. , Allen S. L. , Futch A. H. , Hill D. N. , Porter G. D. , Mahdavi M. A.
Published: July 2002

In this paper an investigation of the particle confinement for beam-heated single-null discharges in the open divertor configuration of Doublet III-D (DIII-D) [E. J. Doyle et al., Phys. Fluids B 3, 2300 (1991)] is described. Results are based on a Monte Carlo neutral transport model with a relatively simple plasma model that utilizes experimental data on density, temperature, and heat flux profiles in the edge plasma. For a typical discharge, it is found that the particle confinement time in the quiescent H-mode phase is only about a factor of 2 larger than during the L-mode phase, an increase comparable to the energy confinement time increase. For both H-mode and L-mode phases the particle confinement time is about a factor of 4 larger than the energy confinement time. It is also found that the core plasma fueling rate is higher in the H mode due to the increased transparency of a thinner scrape-off layer. The longer particle confinement time and the increased fueling rate both contribute to the observed density rise during the quiescent period following the L–H transition. Flux surface-averaged transport modeling of the time evolution for the core plasma density profile during H mode suggests that a strong inward particle pinch is necessary near the separatrix.
The evolution of strongly modulated, low-frequency, moderate-amplitude wave packets in a dispersive plasma
Authors: Vasquez Bernard J. , Cargill Peter J.
Published: July 2002

The evolution of strongly modulated wave packets in a dispersive plasma that propagate parallel to the magnetic field is studied. Modulation effects are shown to reduce significantly (≊30%) the rate of spreading from that due to dispersion alone. For fluidlike behavior, nonlinearity has its greatest impact on evolution when the linear sound speed and initial wave packet speeds are well matched, resulting in a strong coupling between the wave magnetic and sonic components. Ion kinetic processes reduce the impact of nonlinearity and cause the rate of spreading to approach that expected from dispersion alone as the ratio of ion and electron temperatures, Ti/Te→4. For β≳1 and Ti/Te∼1, the coupled waveforms correspond qualitatively to kinetic treatments of the derivative nonlinear Schrödinger (DNLS) equation.
Differential filters for the large eddy numerical simulation of turbulent flows
Authors: Germano M.
Published: July 2002

Differential filters for the large eddy numerical simulation of turbulent flows are defined and their properties are discussed. Their main advantages are that the correlations can be expressed exactly in the so-called resolvable scale and the attenuation of the filtered function in the Fourier space can be carefully controlled
Some aspects of the thermal mixing layer in grid turbulence
Authors: Ma Bai-Kun , Warhaft Z.
Published: July 2002

By generating a temperature step in grid turbulence using two new methods (a mandoline and a toaster) the evolution of some first-, second-, third-, and fourth-order moment quantities of the temperature field in a thermal mixing layer are studied. The results confirm the earlier experimental findings of LaRue and Libby [Phys. Fluids 24, 597 (1981)] and LaRue et al. [Phys. Fluids 24, 1927 (1981)] who used a half-heated grid to generate the thermal field. It is also shown that although second-order quantities such as the heat flux coefficient and the centerline temperature variance appear to have reached their equilibrium values and that all quantities collapse well using the half-width of the mean temperature profile, the data is not consistent with self-similarity.
Kolmogorov similarity in freely decaying turbulence
Authors: Kida S. , Murakami Y.
Published: July 2002

A direct numerical simulation of the three-dimensional Navier–Stokes equation at high Reynolds numbers is performed by the spectral method with 3×3403 effective modes (853 independent degrees of freedom) starting from a high-symmetric flow. Kolmogorov’s [C. R. (Dokl.) Acad. Sci. URSS 30, 301 (1941)] similarity forms of the energy spectra (the one-dimensional longitudinal and lateral energy spectra as well as the three-dimensional one) in the universal range are observed in a decaying period after the enstrophy takes the maximum value. During this period the energy decays exponentially in time and the microscale Reynolds number changes from 100 to 60. At the lower part of the universal range Kolmogorov’s inertial range spectrum E(k,t)=Aε(t)2/3k−5/3 is observed over nearly one decade of the wavenumber, where the Kolmogorov constant A≊1.8. At the higher part of the universal range, on the other hand, it has an exponential tail with an algebraic correction, E(k,t)/[ε(t)ν5]1/4 =B(k/kd(t))m exp[−β(k/kd(t))], R)], where B≊8.4, m≊−1.6, β≊4.9, and kd(t)=[ε(t)/ν3]1/4 is the energy dissipation wavenumber.
Stability of roll and hexagonal patterns in Bénard–Marangoni convection
Authors: Cerisier P. , Jamond C. , Pantaloni J. , Perez-Garcia C.
Published: July 2002

New experimental results concerning the stability of convective patterns in Bénard–Marangoni convection are presented. The hexagonal pattern usually seems to be the most stable. By means of a new thermal technique a predetermined pattern can be imposed and the evolution of a pattern of rolls thus created is observed. For small depths of the fluid layer, hexagons are very stable but they become less and less so as the depth of the layer increases, whereas the imposed rolls become more and more stable. This is linked to the relative importance of the driving mechanisms—surface tension or buoyancy forces—and may correspond to a change in the order of this nonequilibrium phase transition.
Distortion and evolution of a localized vortex in an irrotational flow
Authors: Lingevitch Joseph F. , Bernoff Andrew J.
Published: July 2002

This paper examines the interaction of an axisymmetric vortex monopole, such as a Lamb vortex, with a background irrotational flow. At leading order, the monopole is advected with the background flow velocity at the center of vorticity. However, inhomogeneities of the flow will cause the monopole to distort. It is shown that a shear-diffusion mechanism, familiar from the study of mixing of passive scalars, plays an important role in the evolution of the vorticity distribution. Through this mechanism, nonaxisymmetric vorticity perturbations which do not shift the center of vorticity are homogenized along streamlines on a Re1/3 time scale, much faster than the Re decay time scale of an axisymmetric monopole. This separation of time scales leads to the quasisteady evolution of a monopole in a slowly varying flow. The asymptotic theory is verified by numerically computing the linear response of a Lamb monopole to a time-periodic straining flow and it is shown that a large amplitude, 𝒪(Re1/3), distortion results when the monopole is forced at its resonant frequency.
Drop formation in liquid–liquid systems before and after jetting
Authors: Richards John R. , Beris Antony N. , Lenhoff Abraham M.
Published: July 2002

The formation of drops resulting from the breakup of an axisymmetric Newtonian liquid jet injected vertically into another immiscible Newtonian liquid at various Reynolds numbers is investigated here. The full transient from startup to breakup into drops was simulated numerically by solving the time-dependent axisymmetric equations of motion and continuity using a combination of the volume-of-fluid (VOF) and continuous-surface-force (CSF) methods. The numerical simulation results compare well with previous experimental data and are significantly more accurate than previous simplified analyses based on drop formation before and after jetting over a wide range of conditions.
Three-dimensional Stokes flow in a cylindrical container
Authors: Shankar P. N.
Published: July 2002

Consider a cylindrical container of circular section filled with a viscous fluid. We consider flow in such a cylinder driven by the motion of flat belts across the partially open end walls of the container. The flow field is determined by the use of a vector eigenfunction expansion. The critical points that are exhibited in the plane of symmetry include elliptic points, foci, and saddles. As the parameters are varied one can have bifurcations in which one type of critical point bifurcates to a collection of others. An example of a limiting surface is also demonstrated. Since the flow fields considered have little symmetry, the three-dimensional streamlines are for the most part not closed. As a consequence the flow fields tend to globalize structures that would otherwise have been isolated. This feature can have important consequences for mixing.
A Lagrangian analysis of advection-diffusion equation for a three dimensional chaotic flow
Authors: Tang X. Z. , Boozer A. H.
Published: July 2002

The advection-diffusion equation is studied via a global Lagrangian coordinate transformation. The metric tensor of the Lagrangian coordinates couples the dynamical system theory rigorously into the solution of this class of partial differential equations. If the flow has chaotic streamlines, the diffusion will dominate the solution at a critical time, which scales logarithmically with the diffusivity. The subsequent rapid diffusive relaxation is completed on the order of a few Lyapunov times, and it becomes more anisotropic the smaller the diffusivity. The local Lyapunov time of the flow is the inverse of the finite time Lyapunov exponent. A finite time Lyapunov exponent can be expressed in terms of two convergence functions which are responsible for the spatio-temporal complexity of both the advective and diffusive transports. This complexity gives a new class of diffusion barrier in the chaotic region and a fractal-like behavior in both space and time. In an integrable flow with shear, there also exist fast and slow diffusion. But unlike that in a chaotic flow, a large gradient of the scalar field across the KAM surfaces can be maintained since the fast diffusion in an integrable flow is strictly confined within the KAM surfaces.
On Landau damping in hot equilibrium plasmas. I. Longitudinal oscillations along an external magnetic field
Authors: Fichtner Horst , Schlickeiser Reinhard
Published: July 2002

A relativistic theory of longitudinal oscillations in hot, magnetized, isotropic equilibrium plasmas is presented. The special theory of relativity enters the description, with the requirements that the particle masses are depending on the particle velocities that themselves are limited by the speed of light. Using the relativistic Maxwell–Boltzmann–Jüttner distribution correctly describing the equilibrium state of a plasma, the dispersion relations for subluminal, as well as superluminal waves, are formulated using the Trubnikov representation. Besides a discussion of the dispersion curves, it is shown that superluminal oscillations are always undamped, whereas for subluminal waves the relativistic generalization of the Landau damping is derived. It is shown that the resulting damping rates are significantly lower than those obtained with the nonrelativistic theory.
Relativistic incoherent Thomson scattering spectrum for generalized Lorentzian distributions
Authors: Naito O. , Yoshida H. , Hatae T. , Nagashima A. , Matoba T.
Published: July 2002

The applicability of incoherent Thomson scattering measurement is expanded to a class of non-Maxwellian plasmas which can be described by generalized Lorentzian (kappa) distributions. A fully relativistic scattering spectrum is presented in an analytic form which allows the possibility of a more robust estimation of the bulk electron temperature in the existence of high energy tail component.