bluuebunny/crossref_metadata_2025 huggingface dataset:
Dependence of supercooling of a liquid on its overheating
Authors: Tong H. Y. , Shi F. G.
Published: July 2002

The degree of overheating of a melt often plays a significant role in determining its supercooling behavior and thus the nucleation and growth of crystals and their qualities. However, the relationship between the level of melt overheating and the resulting degree of supercooling is far from being understood. It is demonstrated that the dependence of the degree of supercooling on the level of melt overheating for bismuth is a function of the duration of melt overheating above its melting point. Depending on the duration, the degree of supercooling can increase either discontinuously or continuously with the level of melt overheating. These observations, which can be linked to the transient evolution of short-range structures during melting and solidification, suggest that the similar observations should also be observable for substances exhibiting substantially different short-range structures in the solid and liquid state.
Theory of interaction between helical molecules
Authors: Kornyshev A. A. , Leikin S.
Published: July 2002

This work builds a basis for understanding electrostatic and solvation forces between various types of helical molecules by explicitly incorporating the helical structure and symmetries into the theory. We derive exact expressions for interaction between molecules with cylindrical inner cores and arbitrary distribution of discrete surface charges and analyze forces between single-stranded, double-stranded, and multistranded helices. For example, we demonstrate that the traditional approximation by a homogeneously charged rod becomes inappropriate when even less than a third of the strand charge on a single-stranded helix is neutralized by countercharges (adsorbed or intrinsic to the helix by their nature). The traditionally expected force is then complemented by helix-specific interactions. These helix-specific forces allow commensurate helices (with the ratio of pitches equal to a rational number) to recognize each other at a distance and self-assemble into an aggregate. Under certain conditions, these forces may induce a spontaneous symmetry loss, e.g., two DNA-type double helices rotate around their long axes to a separation-dependent angle when the molecules come closer than a critical interaxial separation. In general, while a longer-range helix-specific attraction induces the self-assembly, a shorter-range helix-specific repulsion prevents the tight molecular contact creating a force balance responsible for a nonzero surface separation in equilibrium. The decay rates and the amplitudes of the attraction and of the repulsion depend on the helical pitch and on the number and relative disposition of the helical strands. The theory of these forces allows us to explain a number of puzzling features of interactions measured between biological helices, including DNA, collagen, and four-stranded guanosine macromolecules.
Computing vibrational energy relaxation for high-frequency modes in condensed environments
Authors: Rostkier-Edelstein Dorita , Graf Peter , Nitzan Abraham
Published: July 2002

In this paper we consider vibrational relaxation of high-frequency impurity modes in condensed environments as a computational problem. Linear response theory provides convenient routes for this computation: The vibrational relaxation rate is obtained as a Fourier transform of a force–force time correlation function. However, numerical difficulties arise for processes characterized by a direct relaxation of high-frequency modes into an environment characterized by a relatively low cutoff frequency. It is shown that modern signal processing procedures can significantly enhance the efficiency and accuracy of the needed computation. Since the relevant “signal” can be very small, the computation can be very sensitive to boundary conditions, and care must be taken to avoid artifacts. The computation may be facilitated by using the expected functional form, exponential dependence on the impurity frequency for high frequency, and fitting the parameters of this form from the simulation. It is emphasized that this exponential dependence seems to be the correct functional form, in spite of theoretical arguments in favor of a Gaussian dependence. The main difficulty in the numerical evaluation of the relaxation rate of high-frequency modes results from the fact that at low temperature the dynamical behavior of such modes is essentially quantum mechanical. We demonstrate this issue by considering vibrational relaxation of an impurity CO molecule in a low-temperature Ar matrix. The results obtained for this system by estimating the quantum correction to the classical force–force correlation function are consistent with experimental results, which indicate that under these conditions the relaxation of the vibrationally excited CO is dominated by radiative decay.
Relaxation of nitrobenzene from the excited singlet state
Authors: Takezaki M. , Hirota N. , Terazima M.
Published: July 2002

The lifetime of the lowest excited singlet (S1) state of nitrobenzene in ethanol was measured using a picosecond time-resolved transient grating method. The lifetime was found to be 6 ps and the rate constant of the intersystem crossing relaxation process from the S1 state was determined to be 1.3–1.6×1011 s−1. The relaxation mechanism from the S1 state is also discussed.
Electron and anion mobility in low density hydrogen cyanide gas. I. Dipole-bound electron ground states
Authors: Klahn Th. , Krebs P.
Published: July 2002

We measured the mobility of excess electrons in the polar hydrogen cyanide gas (D=2.985 D) at low densities as a function of density and temperature by the so-called pulsed Townsend method. Experiments were performed at 294 and 333 K in the gas number density range 1.23×1017⩽n⩽3.61×1018 cm−3. We found a strong density dependence of the “zero-field” density-normalized mobility (μn). Only about 10% of the observed density variation can be qualitatively explained by coherent and incoherent multiple scattering effects. With increasing gas density an increasing number of linear HCN dimers is formed which due to the high dipole moment (D=6.552 D) represent much stronger electron scatterers than the HCN monomers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport process where short-lived dipole-bound electron ground states (lifetime ⩾12 ps) as quasilocalized states are involved. For comparison the electron mobility in saturated 2-aminoethanol vapor with a dipole moment of similar size (D=3.05 D) does not show any anomalous density behavior in the temperature range 298⩽T⩽435 K. In contrast to this the electron mobility in saturated but also in nonsaturated CH3CN gas (D=3.925 D) shows a density behavior similar to that in HCN.
A molecular dynamics study of macromolecules in good solvents: Comparison with dielectric spectroscopy experiments
Authors: Kaznessis Yiannis N. , Hill Davide A. , Maginn Edward J.
Published: July 2002

Molecular dynamics simulations are employed to study the static and dynamic properties of macromolecules in dilute and semidilute solutions of a good solvent. The results are compared with dielectric spectroscopy experiments. Crossover concentrations, ρ*, that demarcate the dilute and semidilute regimes are identified. The shift from self-avoiding-walk to random-walk behavior is also studied. An investigation is conducted of the normal-mode dielectric relaxation of type-A polar polymers. In dilute systems, a power law molecular weight dependence of the normal mode relaxation times τ∝N2.2 is observed, in accordance with DeGennes’s scaling analysis for self-avoiding-walk chains. This result does not agree with the experimental dependence of the normal mode relaxation time on the size of the polymers. The differences between the simulations and the experimental dynamic results in the dilute regime can be ascribed to the leading assumption of the model, the neglect of hydrodynamic interactions. For higher concentrations ρ>8ρ*, hydrodynamic and excluded volume effects are effectively screened, and the simulation results conform well with experiments. The dielectric spectrum broadens with increasing density, and for low densities the broadening can be explained in terms of chain overlapping. For densities higher than the entanglement density, entanglement effects broaden the spectrum considerably. A mapping of the model parameters to real polymer properties is also proposed.
Surface tension of amorphous polymer films
Authors: Hapke Thorsten , Pätzold Gerald , Heermann Dieter W.
Published: July 2002

We study the surface tension for thin, amorphous polymer films by means of computer simulation. Using molecular dynamics, we present surface tension measurements via the fluctuation spectrum of capillary waves in the long-wavelength limit for sufficiently large systems. We find good agreement with a theory based on continuum mechanics. In addition, we observe the spreading of the surface thickness with increasing lateral system size, an effect which allows another estimate of the surface tension. Furthermore, we studied the correlation between two surfaces and measured the transverse length scale by varying the film thickness. We also present data of the temperature dependence of the bulk density of the polymer film and the thickness of the surface region in the regime above the glass transition temperature.
Assessment of conventional density functional schemes for computing the polarizabilities and hyperpolarizabilities of conjugated oligomers: An ab initio investigation of polyacetylene chains
Authors: Champagne Benoı̂t , Perpète Eric A. , van Gisbergen Stan J. A. , Baerends Evert-Jan , Snijders Jaap G. , Soubra-Ghaoui Chirine , Robins Kathleen A. , Kirtman Bernard
Published: July 2002

DFT schemes based on conventional and less conventional exchange-correlation (XC) functionals have been employed to determine the polarizability and second hyperpolarizability of π-conjugated polyacetylene chains. These functionals fail in one or more of several ways: (i) the correlation correction to α is either much too small or in the wrong direction, leading to an overestimate; (ii) γ is significantly overestimated; (iii) the chain length dependence is excessively large, particularly for γ and for the more alternant system; and (iv) the bond length alternation effects on γ are either underestimated or qualitatively incorrect. The poor results with the asymptotically correct van Leeuwen–Baerends XC potential show that the overestimations are not related to the asymptotic behavior of the potential. These failures are described in terms of the separate effects of the exchange and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitive to the polarization charge induced by the external electric field at the chain ends.
Autoionization-detected infrared spectroscopy of intramolecular hydrogen bonds in aromatic cations. I. Principle and application to fluorophenol and methoxyphenol
Authors: Fujimaki Eiji , Fujii Asuka , Ebata Takayuki , Mikami Naohiko
Published: July 2002

A new infrared spectroscopic technique for jet-cooled molecular cations is applied to observe intramolecular hydrogen bonds in substituted phenol ions. Vibrational transitions of an ion core of high Rydberg states are measured by detecting molecular ions prepared through vibrational autoionization. The observed infrared spectra practically provide vibrational frequencies of the corresponding bare molecular ion. The OH stretching vibrations of ortho-, meta-, and para-isomers of fluorophenol and methoxyphenol cations are observed. The OH stretching vibrational frequency of the ortho-isomer shows a characteristic redshift due to the intramolecular hydrogen bond. The redshift increases with ionization, indicating a significant enhancement of the intramolecular hydrogen bond strength.
Path-integral diffusion Monte Carlo: Calculation of observables of many-body systems in the ground state
Authors: Hetényi Balázs , Rabani Eran , Berne B. J.
Published: July 2002

We propose a new method to calculate ground state position dependent observables in quantum many-body systems. The method, which we call the path-integral diffusion Monte Carlo (PI-DMC) method, is essentially a combination of path-integral Monte Carlo (PIMC) and diffusion Monte Carlo (DMC) methods. The distribution resulting from a DMC simulation is further propagated in imaginary time by PIMC sampling. Tests of the new method for simple cases such as the harmonic oscillator, a double well, and a set of ten coupled harmonic oscillators show that the results for several observables converge rapidly to the exact result.
Molecular constants and Rydberg–Klein–Rees (RKR) potential curve for the Na2 1 3Σg− state
Authors: Liu Yaoming , Li Jian , Chen Dieyan , Li Li , Jones Kevin M. , Ji Bing , Le Roy Robert J.
Published: July 2002

Transitions into the doubly excited Na2 1 3Σg− state have been analyzed using near-dissociation expansions (NDE) to represent the vibrational energies and inertial rotational constants, while the centrifugal distortion constants were held fixed at “mechanically consistent” values calculated from the Rydberg–Klein–Rees (RKR) potential implied by those G(v) and Bv functions. The input data cover the range v=0 to 57 and N up to 47, and the fit yields vD=61.41(±0.10) and D0=3385.70(±0.2) cm−1.
Electronic absorption spectra of linear C6, C8 and cyclic C10, C12 in neon matrices
Authors: Grutter Michel , Wyss Muriel , Riaplov Evgueni , Maier John P. , Peyerimhoff Sigrid D. , Hanrath Michael
Published: July 2002

The electronic absorption spectra of the even-numbered carbon molecules C6–C14 have been measured in neon matrices. Bare carbon anions were produced in a cesium sputter source, mass selected, codeposited with neon at 6 K, and neutralized. The spectra show, apart from the known (1) Σu−3←X 3Σg− transition of linear C6, C8, and C10 in the visible, absorption bands in the UV region. The spectral data when considered in conjunction with ab initio calculations show that the linear forms of C6 and C8 have the next strong (2) Σu−3←X 3Σg− transition with band maximum near 238 and 277 nm, respectively, whereas the band systems of C10, C12, and C14 at 316, 332, and 347 nm are due to the monocyclic species.
Screen observables in relativistic and nonrelativistic quantum mechanics
Authors: Werner R.
Published: July 2002

Screen observables, which measure the arrival coordinates of free particles at a hyperplane containing a timelike direction, are defined by a covariance property with respect to an irreducible representation of the Poincaŕe or Galilei group. For each representation with m>0 the set of screen observables is constructed explicitly and a unique ideal screen observable of greatest intrinsic accuracy is singled out.
A class of exact plane wave solutions of the Maxwell–Dirac equations
Authors: Das A. , Kay D.
Published: July 2002

Exact solutions of Maxwell–Dirac equations are investigated for which the Dirac field is of the type ψ(x)=α(p)eipλxλ. In the subclass where the mass parameter m≠0, there exists no nontrivial solution of the problem. In the subclass where the mass parameter m=0, there exist infinitely many solutions inherent with arbitrary functions. Furthermore, every solution for m=0 must have a null four-current vector field associated with it.
Generating mass without the Higgs particle
Authors: LaChapelle J.
Published: July 2002

The conformal group is added to the gauge group of a model of elementary particles based on the fiber bundle formalism. The inertial mass of a particle is interpreted as a manifestation of its interaction with the gauge bosons associated with the generators of the translation and special conformal subgroups. The generation of mass by this mechanism (together with the fact that the Higgs field is not necessarily required to effect bundle reduction) makes the Higgs and the Yukawa terms in the Lagrangian unnecessary: the Lagrangian then consists only of a Yang–Mills term and a covariantly free matter field term. A particular choice of gauge reproduces the usual mass terms for both fermions and gauge bosons. The ‘‘no-go theorem’’ is not violated by the construction. The Poincaré generators commute with the internal symmetry generators after bundle reduction, and there is no mass splitting within symmetry multiplets. However, the left-handed/right-handed asymmetry allows the neutrino to remain massless; the definition of matter fields allows the up and down quarks to acquire different mass couplings; and the gauge bosons have different mass couplings determined by the inner product on the Lie algebra of the broken symmetry subgroup. The mass ratios of the gauge bosons—at tree level—are precisely those predicted by the Standard Model.
Nöther formalism for conserved quantities in classical gauge field theories
Authors: Fatibene L. , Ferraris M. , Francaviglia M.
Published: July 2002

Gauge theories coupled with the gravitational field and external matter fields are considered and Nötherian techniques are used to build the canonical conserved quantities. Their superpotentials are found and the relation between the canonical stress tensor and the Hilbert tensor is determined. The case of Yang–Mills theories coupled with gravity and scalar matter fields is considered in detail and it is shown that in this case the canonical stress tensor and the Hilbert tensor coincide.
Functional determinants on spheres and sectors
Authors: Dowker J. S.
Published: July 2002

The scalar functional determinants on sectors of the two-dimensional disc and spherical cap are determined for arbitrary angles (rational factors of π). The whole sphere and hemisphere expressions are also given, in low dimensions, for both the ordinary and the conformal Laplacian.
N=4 super KdV hierarchy in N=4 and N=2 superspaces
Authors: Delduc F. , Ivanov E. , Krivonos S.
Published: July 2002

We present the results of further analysis of the integrability properties of the N=4 supersymmetric Korteweg–de Vries (KdV) equation deduced earlier by two of us [F. Delduc and E. Ivanov, Phys. Lett. B 309, 312 (1993)] as a Hamiltonian flow on N=4 SU(2) superconformal algebra in the harmonic N=4 superspace. To make this equation and the relevant Hamiltonian structures more tractable, we reformulate it in the ordinary N=4 and further in N=2 superspaces. In N=2 superspace it is represented by a coupled system of evolution equations for a general N=2 superfield and two chiral and antichiral superfields, and involves two independent real parameters, a and b. We construct a few first bosonic conserved charges in involution, of dimensions from 1 to 6, and show that they exist only for the following choices of the parameters: (i) a=4, b=0; (ii) a=−2, b=−6; (iii) a=−2, b=6. The same values are needed for the relevant evolution equations, including N=4 KdV itself, to be bi-Hamiltonian. We demonstrate that the above three options are related via SU(2) transformations and actually amount to the SU(2) covariant integrability condition found in the harmonic superspace approach. Our results provide a strong evidence that the unique N=4 SU(2) super KdV hierarchy exists. Upon reduction to N=2 KdV, the above three possibilities cease to be equivalent. They give rise to the a=4 and a=−2 N=2 KdV hierarchies, which thus prove to be different truncations of the single N=4 SU(2) KdV one.
On quantization of Z2-graded algebras
Authors: Mudrov A. I.
Published: July 2002

Quantization of Z2-graded Lie bialgebras viewed as a generalization of sl(2) is investigated. A method of building multiparameter solutions is developed. We study the role of the quantum double construction in the selected class and prove that a quantized coboundary Z2-graded Lie bialgebra contains a quasitriangular Hopf subalgebra.
Low-temperature spectroscopy of nonequivalent Pr3+ optical centers in a Y2SiO5 crystal
Authors: Malyukin Yu. V. , Minkov B. I. , Borisov R. S. , Seminozhenko V. P. , Znamenskii N. V. , Manykin E. A. , Marchenko D. V. , Petrenko E. A.
Published: July 2002

An analysis of the absorption and luminescence spectra of a Y2SiO5:Pr3+ crystal has revealed the existence of two Pr3+ optical centers associated with the presence of impurity ions at nonequivalent cation sites. It is found that energy is transported nonradiatively between the nonequivalent Pr3+ optical centers. and a preliminary analysis of the transport mechanism is carried out.
Particle dispersion in a turbulent, plane, free shear layer
Authors: Lázaro B. J. , Lasheras J. C.
Published: July 2002

The particle dispersion mechanisms in an inhomogeneous, anisotropic, high Reynolds number, turbulent free shear flow are presented. Flow visualization, as well as laser attenuation and diffraction techniques, are used to characterize the evolution of the flow. It is shown that, at each downstream location, the spectral response of the particles to the coherent velocity field that develops in the layer as a result of a Kelvin–Helmholtz type of instability leads to a selective dispersion of the particles across the mixing layer. Furthermore, this particle dispersion layer is shown to be composed of a central core region, characterized by a small mean particle size and two external sublayers with considerably larger mean particle sizes. Finally, the experimental results suggest the dominant role played by the coherent, large scale vortical structure in the dispersion of the particles. Scaling arguments and simplified flow models supporting this hypothesis are also presented.
Time-resolved measurements of total temperature and pressure in the vortex street behind a cylinder
Authors: Ng W. F. , Chakroun W. M. , Kurosaka M.
Published: July 2002

Recent theoretical and numerical investigations revealed the prospect that the instantaneous total temperature becomes separated around vortices; once time averaged, however, the wake behind a bluff body takes the guise of a colder wake, displaying the so-called Eckert–Weise effect. For low Mach number flows, the instantaneous total pressure was also shown to become separated in such a similar manner that the near-wake patterns of instantaneous total pressure exhibit almost exact facsimiles of those of total temperature. In this paper the results from the time-accurate measurements of fluctuating total temperature and pressure are presented. The data were obtained by placing a time-resolved aspirating probe in the vortex street behind a cylinder. The free-stream Mach number was 0.4 and the Reynolds number based on the cylinder diameter was 2.3×105. The maximum magnitude of the rms fluctuations of total temperature are of the order of 15 ° K—about 5% of the upstream total temperature. Maximum fluctuations in total pressure are approximately 10% of the upstream total pressure. Not only are the time traces of the total temperature and pressure taken in the near-wake qualitatively similar, but also quantitatively they agree with the predictions.
The migration of a compound drop due to thermocapillarity
Authors: Morton David S. , Subramanian R. Shankar , Balasubramaniam R.
Published: July 2002

The quasistatic thermocapillary motion of a compound drop in an unbounded fluid possessing a uniform temperature gradient is analyzed. For completeness, gravitational effects are included in the treatment. The general model is formulated, and the equations for the concentric case are solved using spherical polar coordinates, while the eccentric case is handled using bispherical coordinates. Results are given for the velocity of the drop as well as that of the droplet with respect to the drop, along with useful approximations. Illustrative results are presented graphically for the thermocapillary migration of a compound drop in the special case when the droplet is a gas bubble. In addition to the velocities of the drop and the bubble, representative isotherms and streamlines also are presented which display interesting qualitative features.
Application of the velocity-dissipation probability density function model to inhomogeneous turbulent flows
Authors: Pope S. B.
Published: July 2002

Recently Pope and Chen [Phys. Fluids A 2, 1437 (1990)] developed a turbulence model based on the one-point Eulerian joint probability density function (pdf) of velocity and dissipation. The modeling is performed by constructing stochastic processes for the velocity and dissipation following fluid particles. In the original work, these models were constructed by reference to the known statistics of homogenous turbulence, and the applicability of the model was restricted to this narrow class of flows. In this paper the model is extended to inhomogeneous flows, and calculations are presented to demonstrate aspects of the model’s performance. The model equation admits a similarity solution corresponding to the log-law region of the turbulent boundary layer, and the principal statistics obtained from this solution are in good agreement with experimental data. Application of the model to the momentumless wake and to the plane mixing layer demonstrate its ability to represent turbulent/nonturbulent intermittency in these free flows: in the intermittent regions, the pdf of dissipation is bimodal, with a spike at zero corresponding to nonturbulent fluid. Fluid-particle paths in the turbulent mixing layer obtained from the model correspond to large-scale coherent motions, rather than to the small-scale incoherent motion characteristic of diffusive transport.
A unified nonequilibrium model for hypersonic flows
Authors: Thivet F. , Perrin M. Y. , Candel S.
Published: July 2002

Kinetic theory is applied to a mixture of monoatomic and diatomic molecules in order to derive the macroscopic equation corresponding to the relaxation of vibrational energy. Diatomic molecules are described using the harmonic oscillator model. The SSH theory is extended and applied to evaluate the vibrational energy relaxation times which govern the process. The theoretical expressions are used to calculate the relaxation times of the main diatomic species in air: oxygen and nitrogen. These relaxation times are in good agreement with available experimental data. A comparison with previous models is then carried out. The coupled thermodynamic and chemical relaxation occurring after a strong normal shock wave in a one-dimensional Eulerian flow is then studied. The computed results illustrate the interactions between three different phenomena: nonequilibrium thermodynamics, nonequilibrium chemistry, and dynamics of the flow. A complete analysis of these interactions is provided, and the influence of thermodynamic nonequilibrium is pointed out.
Shock formation in a poloidally rotating tokamak plasma
Authors: Shaing K. C. , Hazeltine R. D. , Sanuki H.
Published: July 2002

When the Mach number Mp of the poloidal rotation in a tokamak approaches unity, the poloidal variations of plasma density and potential appear to have the characteristics of a shock whose front lies on a plane (ribbon) of a fixed poloidal angle η0. The shock first appears, when 1−Mp≲(ε)1/2 (ε is the inverse aspect ratio), on the inside of the torus at a shock angle η0≥π if the plasma rotates counterclockwise poloidally. As Mp increases, η0 moves in the direction of the poloidal rotation. At Mp=1, η0=2π. When Mp −1≲(ε)1/2, the shock angle is at η0≲π. The parallel viscosity associated with the shock is collisionality independent, in contrast to the conventional neoclassical viscosity. The viscosity reaches its maximum at Mp=1, which is the barrier that must be overcome to have a poloidal supersonic flow. Strong up–down asymmetric components of poloidal variations of plasma density and potential develop at Mp ≂1. In the edge region, the convective poloidal momentum transport weakens the parallel viscosity and facilitates the L–H transition.
Theory of shear flow effects on long-wavelength drift wave turbulence
Authors: Carreras B. A. , Sidikman K. , Diamond P. H. , Terry P. W. , Garcia L.
Published: July 2002

A simple, paradigmatic model of long-wavelength drift wave turbulence in the presence of a sheared poloidal flow is analyzed in detail. Linear theory predicts that velocity shear induces a strong stabilizing effect by shifting the eigenmode away from the k⋅B=0 resonant surface, thereby enhancing ion damping. However, multiple-helicity numerical calculations indicate that velocity shear has little or no effect on saturated fluctuation levels. Analysis suggests that this result is related to the incidence of a spiky, radially intermittent profile of the turbulent fluctuation levels, induced by low-q mode rational surfaces, and occurs when the turbulent diffusivity exceeds the product of diamagnetic frequency and gyroradius squared. An analytical theory that explains the observed suppression of velocity and magnetic shear damping is presented.
Kelvin–Helmholtz instabilities of high-velocity magnetized, anisotropic shear layers
Authors: Roy Choudhury S. , Patel V. L.
Published: July 2002

A study is made of the linear stability of finite-thickness, anisotropic compressible shear layers v=ẑvz(x), with a parallel uniform magnetic field, B=ẑB0. The stability of such sheared flows, described by the double adiabatic equations of Chew, Goldberger, and Low [Proc. R. Soc. London Ser.  A  236, 112 (1956)], and involving the nonlocal coupling of the firehose and mirror modes caused by the velocity shear, is relevant in a number of astrophysical, geophysical, and space plasma configurations. The scalar perturbation quantities have the form  f (x) exp[i(kzz −ωt)]. The dimensionless variables characterizing a shear layer with a given velocity profile, assumed to be a linear profile in the present work, are the sonic Mach number, M ≡ (2vzm/S⊥), the ratio of the magnetic field energy density to the perpendicular thermal energy density, q2 ≡ (vA/S⊥)2, and the anisotropy parameter, r2 ≡ (S∥/S⊥)2. Here, vz(x=±∞)=±vzm, S∥, and S⊥ are the sound speeds parallel and perpendicular to the magnetic field, and vA is the Alfvén speed. The dimensionless variable characterizing the perturbation is the wavenumber B ≡ kzL, where L is the shear layer thickness. It is shown that the resonance of the sound and firehose modes drives unstable standing and traveling waves for a shear layer having a vortex sheet profile [where vz(x) is a step function]. For the vortex sheet, the unstable standing wave modes first appear at M=2(3)1/2r for r>(1+q2)1/2/2, and the unstable traveling wave modes first appear at M=0 for r<((1+q2)1/2/2). Numerical methods are used to generate values of ωr and ωi>0 (corresponding to unstable wave motion) for the ‘‘linear’’ shear layer in the (B, M) plane for various values of q and r.

The coverage of the (B, M) plane is for B≤5, M≤10, and for discrete values of q≤0.5 and r≤2. Two regimes of instability are found to occur in the (B, M) plane with the structure of the unstable modes of the ‘‘linear’’ layer being very different from that for the anisotropic vortex sheet. The unstable modes are standing waves with ωr=0, and traveling waves with ωr≠0.
Displacement effect of a flame in a stagnation-point flow
Authors: Eteng E. , Ludford G. S. S. , Matalon M.
Published: July 2002

The interaction of a premixed flame with the flow near the front stagnation point of a body has rightly received considerable attention in combustion theory. Virtually all the investigations so far have been concerned with high strain rates, when the flame lies inside the boundary layer and can be extinguished. By contrast, we consider moderate strain rates, when the flame stands clear of the boundary layer and pushes the incident flow away from the body. The object is to determine this displacement effect, which is much more pronounced than that of the boundary layer.
An electromagnetic trapped-particle sideband instability
Authors: Cohen Bruce I. , Cohen Ronald H.
Published: July 2002

The sideband instability of a large-amplitude electromagnetic wave that traps electrons near the cyclotron resonance in a magnetized plasma is analyzed. The wave-trapped particles produce a weak instability. The growth rate and the criterion for the instability to exceed the threshold determined by collisional damping of the sidebands are given. This instability will have little consequence for the parameters expected in the microwave tokamak experiment at Lawrence Livermore National Laboratory [Proceedings of the 14th Annual Conference on Plasma Physics, St. Andrews, Scotland, 1987 (Pergamon, London, 1988), Paper No. 25].
Apparent dynamic contact angle of an advancing gas–liquid meniscus
Authors: Kalliadasis Serafim , Chang Hsueh-Chia
Published: July 2002

The steady motion of an advancing meniscus in a gas-filled capillary tube involves a delicate balance of capillary, viscous, and intermolecular forces. The limit of small capillary numbers Ca (dimensionless speeds) is analyzed here with a matched asymptotic analysis that links the outer capillary region to the precursor film in front of the meniscus through a lubricating film. The meniscus shape in the outer region is constructed and the apparent dynamic contact angle Θ that the meniscus forms with the solid surface is derived as a function of the capillary number, the capillary radius, and the Hamaker’s constant for intermolecular forces, under conditions of weak gas–solid interaction, which lead to fast spreading of the precursor film and weak intermolecular forces relative to viscous forces within the lubricating film. The dependence on intermolecular forces is very weak and the contact angle expression has a tight upper bound tan Θ=7.48 Ca1/3 for thick films, which is independent of the Hamaker constant. This upper bound is in very good agreement with existing experimental data for wetting fluids in any capillary and for partially wetting fluids in a prewetted capillary. Significant correction to the Ca1/3 dependence occurs only at very low Ca, where the intermolecular forces become more important and tan Θ diverges slightly from the above asymptotic behavior toward lower values.
Computer simulations of rapid granular flows of spheres interacting with a flat, frictional boundary
Authors: Louge Michel Y.
Published: July 2002

This paper employs computer simulations to test the theory of Jenkins [J. Applied Mech. 59, 120 (1992)] for the interaction between a rapid granular flow of spheres and a flat, frictional wall. This paper examines the boundary conditions that relate the shear stress and energy flux at the wall to the normal stress, slip velocity, and fluctuation energy, and to the parameters that characterize a collision. It is found that while the theory captures the trends of the boundary conditions at low friction, it does not anticipate their behavior at large friction. A critical evaluation of Jenkins’ assumptions suggests where his theory may be improved.
The spectral relaxation model of the scalar dissipation rate in homogeneous turbulence
Authors: Fox R. O.
Published: July 2002

A model for the effect of scalar spectral relaxation on the scalar dissipation rate of an inert, passive scalar (Sc≥1) in fully developed homogeneous turbulence is presented. In the model, wave-number space is divided into a finite number [the total number depending on the turbulence Reynolds number Reλ and the Schmidt number (Sc)] of intermediate stages whose time constants are determined from the velocity spectrum. The model accounts for the evolution of the scalar spectrum from an arbitrary initial shape to its fully developed form and its effect on the scalar dissipation rate for finite Reλ and Sc≥1. Corrsin’s result [AIChE J. 10, 870 (1964)] for the scalar mixing time is attained for large Reλ in the presence of a constant mean scalar gradient and a stationary, isotropic turbulence field. Comparisons with DNS results for stationary, isotropic turbulence and experimental data for decaying, homogeneous grid turbulence demonstrate the satisfactory performance of the model.
Enclosure gas flows driven by non-isothermal walls
Authors: Papadopoulos Dimitrios H. , Rosner Daniel E.
Published: July 2002

The phenomenon of thermal creep, according to which a rarefied gas will ‘‘slip’’ at a fluid/solid interface in the presence of appreciable temperature gradients along the interface, has been previously treated by analytical techniques in the one-dimensional, semi-infinite, linearized case. The more general problem of multidimensional gas flows at high ΔT/T values typically defies analytical or semi-analytical solution. We employ a direct simulation Monte Carlo method to a hard-sphere gas in order to study the characteristics of thermal creep convective motion in a Cartesian, two-dimensional, confined flow geometry which captures important features of ampoules used in microgravity experiments on crystal growth. Vortex roll formation is indeed observed in these numerical experiments even for zero-gravity conditions, driven by thermal creep at the non-isothermal boundaries and in close resemblance to the classical fluid dynamic problem of the wall-driven cavity.
A blinking rotlet model for chaotic advection
Authors: Meleshko V. V. , Aref H.
Published: July 2002

The Stokes flow due to a rotlet in a circle is determined. The solution shows that for a certain position of the rotlet, the flow has a second stagnation point symmetrically placed inside the circle. Thus, a ‘‘blinking rotlet’’ model can be constructed in which the rotlet that is ‘‘off’’ does not disturb the flow. This model seems preferable to the ‘‘blinking vortex’’ flow when discussing chaotic advection by a Stokes flow, and is useful for comparisons with recent experimental and computational investigations of this phenomenon in a cylindrical tank with two rotating cylinders.
Study of compressibility modifications to the k−ε turbulence model
Authors: Krishnamurty Venkata S. , Shyy Wei
Published: July 2002

In modeling the effect of compressibility on the turbulence structure, additional physical mechanisms arise in production and dissipation of turbulent kinetic energy. Several proposed treatments dealing with the dilatation dissipation and the pressure dilatation correlation are discussed in the context of the k−ε two-equation model. Modifications accounting for the turbulent mass flux, enthalpic production, and baroclinic torque are also assessed along with the nonequilibrium treatment. These compressibility models are evaluated with the aid of experimental data for supersonic flow over an axisymmetric afterbody. With the free-stream Mach number of 2.46, the compressibility and nonequilibrium modifications give marginally better results for the turbulence structure and offer added insight into the mean and fluctuating flow fields.
Nonlinear convective/absolute instabilities in parallel two-dimensional wakes
Authors: Delbende Ivan , Chomaz Jean-Marc
Published: July 2002

The linear versus nonlinear convective/absolute instability of a family of plane wake profiles at low Reynolds number is investigated by numerically comparing the linearized and the fully nonlinear impulse responses. Through an analysis of the linear flow response obtained by direct numerical simulation (DNS), the linear temporal and spatiotemporal instability properties are retrieved, in excellent agreement with the properties obtained by Monkewitz [Phys. Fluids 31, 3000 (1994)] from the study of the associated viscous dispersion relation. Nonlinear terms are then shown to limit the amplitude to a saturation level within the response wave packet, while leaving the trailing and leading edges unaffected. For this family of open shear flows, the velocities of the fronts, formed between the trailing or leading edge and the central saturated region, are thus selected according to the linear Dee and Langer criterion [Phys. Rev. Lett. 50, 383 (1983)], whereas the front solutions are fully nonlinear. This property may be of importance in justifying the use of linear instability properties to predict the onset and the frequency of the von Kármán vortex street, as determined by Hammond and Redekopp [J. Fluid Mech. 331, 231 (1997)].
Circulation-induced shape deformations of drops and bubbles: Exact two-dimensional models
Authors: Crowdy Darren
Published: July 2002

In this paper simple two-dimensional mathematical models for understanding the fluid dynamical problem of how circulation affects the free surface shapes of inviscid drops and bubbles with surface tension are presented. This theoretical paradigm is of interest in many areas of science including large-scale transport processes in chemical engineering. Exact solutions for the finite-amplitude steady-state equilibria of the mathematical models are found. Equilibrium states are shown to exist right up to steady capillary pinch-off in the case of a bubble, the bubbles just before pinch-off having large perimeter-to-area ratios.
Investigation of the Weissenberg effect in suspensions of magnetic nanoparticles
Authors: Odenbach S. , Rylewicz T. , Rath H.
Published: July 2002

We have investigated the appearance of normal stress differences in suspensions of magnetic nanoparticles by observation of the rise of the fluid at a rotating axis. The combination ν10+4ν20 of the low shear rate limits of the normal stress coefficients ν10 and ν20 has been varied by means of the action of a magnetic field applied to the fluid. Its value has been determined for different shear rate and magnetic field strength. To enhance observability of the rise of the fluids, the experiments have been carried out under conditions of reduced gravity during parabolic flights.
Particle clustering due to hydrodynamic interactions
Authors: Wylie Jonathan J. , Koch Donald L.
Published: July 2002

Dynamic simulations of an isotropic suspension of particles in a viscous gas are performed. The energy in the suspension decays with time as a result of viscous dissipation in the gas. The rate of viscous dissipation in sufficiently energetic suspensions, those with a high Stokes number, is consistent with the theory of Sangani et al. [J. Fluid Mech. 313, 309–341 (1996)] for homogeneous hard-sphere suspensions. As the suspension loses energy, the dissipation rate decreases dramatically and the particles cluster as indicated by the presence of many more near neighbors than would be found in a hard sphere distribution.
Role of the radial electric field in the transition from L (low) mode to H (high) mode to VH (very high) mode in the DIII-D tokamak*
Authors: Burrell K. H. , Doyle E. J. , Gohil P. , Groebner R. J. , Kim J. , La Haye R. J. , Lao L. L. , Moyer R. A. , Osborne T. H. , Peebles W. A. , Rettig C. L. , Rhodes T. H. , Thomas D. M.
Published: July 2002

The hypothesis of stabilization of turbulence by shear in the E×B drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L (low)–H (high) transition; in addition, the observed levels of E×B shear significantly exceed the value theoretically required to stabilize turbulence. Furthermore, this same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from the H mode to the VH (very high) mode. Consequently, the most fundamental question for H-mode studies now is: How is the electric field Er formed? The radial force balance equation relates Er to the main ion pressure gradient ∇Pi, poloidal rotation vθi, and toroidal rotation vφi. In the plasma edge, observations show ∇Pi and vθi are the important terms at the L–H transition, with ∇Pi being the dominant, negative term throughout most of the H mode. In the plasma core, Er is primarily related to vφi. There is a clear temporal and spatial correlation between the change in E×B shear and the region of local confinement improvement when the plasma goes from the H mode to the VH mode. Direct manipulation of the vφi and E×B shear using the drag produced by a nonaxisymmetric magnetic perturbation has produced clear changes in local transport, consistent with the E×B shear stabilization hypothesis. The implications of these results for theories of the L–H and H–VH transitions will be discussed.
Thermal instability of a compressible finite Larmor radius, Hall plasma in porous medium
Authors: Sharma R. C. , Sunil
Published: July 2002

The thermal instability of a compressible plasma in porous medium is considered in the presence of a uniform horizontal magnetic field to include the Hall current and finite Larmor radius effects. The Hall current and finite Larmor radius (FLR), individually, have destabilizing and stabilizing effects, respectively, on the system. In the simultaneous presence of both, there is a competition between the destabilizing role of Hall current and stabilizing role of FLR and each—Hall current and FLR—succeeds in stabilizing a certain wave-number range. In the absence of magnetic field (and hence absence of Hall current and FLR), the destabilizing effect of medium permeability is depicted but in the presence of magnetic field (and hence presence of Hall current and FLR), the medium permeability may have a stabilizing or destabilizing effect on thermal instability of the plasma. In contrast to the case of vertical magnetic field (and corresponding Hall current and FLR), the oscillatory modes are not allowed for, (cpβ/g)≳1, in the case of horizontal magnetic field (and corresponding Hall current and FLR). The effect of compressibility is found to postpone the onset of thermal instability in plasma.
Closed form analytic solutions describing glow discharge plasma
Authors: Pai S. T. , Guo X. M. , Zhou T. D.
Published: July 2002

On the basis of an analytic model developed previously [S. T. Pai, J. Appl. Phys. 71, 5820 (1992)], an improved version of the model for the description of dc glow discharge plasma was successfully developed. A set of closed form solutions was obtained from the governing equations. The two-dimensional, analytic solutions are functional and completely satisfy the governing equations, the actual boundary conditions, and Maxwell equations. They can be readily used to carry out numerical calculations without the necessity of employing any assumed boundary conditions. Results obtained from the model reveal that as the discharge gap spacing or pressure increases the maximum value in the electron density distribution moves toward the cathode. At a sufficiently large value of gap spacing, the positive column phenomenon begins to appear in the discharge region. The model has the capability of treating the positive column and negative glow as a continuous system without the necessity of studying them separately. The model also predicts a sharp rise of the positive ion density near the cathode and field reversal in the anode region. Variation of the electrode radius produces little effect on the axial spatial distribution of physical quantities studied.