bluuebunny/crossref_metadata_2025 huggingface dataset:
Small-angle neutron scattering from heavy water in the vicinity of the critical point
Authors: Bonetti M. , Romet-Lemonne G. , Calmettes P. , Bellissent-Funel M.-C.
Published: July 2002

Small-angle neutron scattering spectra from supercritical heavy water were recorded along a near-critical isochore at temperatures between (Tc,exp+1.63 K) and (Tc,exp+22.02 K), Tc,exp being the experimental critical temperature. At low values of the wave number transfer q, the scattered intensity shows a pronounced increase due to the divergence of density fluctuations at the critical point. In the studied q range, from 0.07 to 0.36 Å−1, all the spectra can be accurately described by the Fisher–Langer correlation function provided that a multiplicative additional term describing short-range correlations between water molecules is taken into account. The value found for the amplitude of the critical correlation length is ξ0=(1.36±0.06) Å, in fair agreement with the value predicted by the two-scale hypothesis.
Pulse radiolysis studies on charge carriers in conjugated polymers
Authors: Burrows H. D. , Miguel M. da G. , Monkman A. P. , Horsburgh L. E. , Hamblett I. , Navaratnam S.
Published: July 2002

The charged states of the conjugated polymers poly(2-methoxy,5-(2′-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) and poly(2,5-pyridinediyl) (PPY) have been studied by pulse radiolysis. Following pulse radiolysis of argon-saturated solutions of MEH-PPV in chloroform, a new absorption is seen to grow in over a few hundred microseconds. This has a principal absorption at 1.43 eV and a weaker, low energy band (⩽0.80 eV), and is assigned to the positive one-electron charge state (positive polaron) of MEH-PPV. The slow absorption decay is unaffected by oxygen. Negative charge states (negative polarons), with absorptions around 1.4 eV, are produced upon pulse radiolysis of MEH-PPV in argon-saturated solutions in tetrahydrofuran (THF) or benzonitrile. A small solvatochromic shift is observed. In contrast to the behavior of the positive polaron, the MEH-PPV negative charge carriers decay fairly rapidly, and are readily quenched by molecular oxygen. Previous results on chemically produced positive and negative charge states of conjugated polymers and oligomers are discussed on the basis of these assignments, and comparison is made with theoretical calculations. Using benzophenone as a charge scavenger, pulse radiolysis of formic acid is shown to generate one-electron reducing species. Various derivatives of PPY, including a regioregular polymer (rPPY) and a hexyl substituted compound (HPPY) have been studied by pulse radiolysis in formic acid solution. With rPPY, new absorptions are observed at 2.59 and 1.40 eV, and are assigned to the one-electron reduced species. These are strongly quenched by molecular oxygen. With HPPY, the lower energy transition is broadened to give a maximum below 1.21 eV and a shoulder at 1.65 eV. The differences between rPPY and HPPY are interpreted on the basis of differences in the rigidity of the polymer. The relevance of these assignments to the identification of charged species in photoinduced absorption measurements is indicated.
Theoretical and experimental investigation of pressure broadening and line shift of carbon monoxide in collision with hydrogen between 8 and 600 K
Authors: Mengel Markus , Flatin Daniel C. , De Lucia Frank C.
Published: July 2002

We have measured pressure broadening and line shift parameters for the J=1←0 and J=2←1 transitions of carbon monoxide (CO) in collision with hydrogen (H2) at various temperatures between 8 and 600 K. Because of the abundance of both species in the interstellar medium, the CO–H2 collision system is of astrophysical importance. Measurements below 35 K were made using the collisional cooling technique, while measurements at higher temperatures were made in a conventional equilibrium cell. These measurements were compared with theoretical results of quantum scattering calculations using the currently best available potential surface for the CO–H2 collision system. We find a good agreement between theory and experiment at higher temperatures (>30 K) while for lower temperatures considerable deviations occur.
Light scattering from nonequilibrium concentration fluctuations in a polymer solution
Authors: Li W. B. , Zhang K. J. , Sengers J. V. , Gammon R. W. , Ortiz de Zárate J. M.
Published: July 2002

We have performed light-scattering measurements in dilute and semidilute polymer solutions of polystyrene in toluene when subjected to stationary temperature gradients. Five solutions with concentrations below and one solution with a concentration above the overlap concentration were investigated. The experiments confirm the presence of long-range nonequilibrium concentration fluctuations which are proportional to (∇T)2/k4, where ∇T is the applied temperature gradient and k is the wave number of the fluctuations. In addition, we demonstrate that the strength of the nonequilibrium concentration fluctuations, observed in the dilute and semidilute solution regime, agrees with theoretical values calculated from fluctuating hydrodynamics. Further theoretical and experimental work will be needed to understand nonequilibrium fluctuations in polymer solutions at higher concentrations.
The vibrational spectra of molecular ions isolated in solid neon. XVI. SO2+, SO2−, and (SO2)2−
Authors: Forney Daniel , Kellogg C. Brian , Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a Ne:SO2 mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes absorptions assigned with the aid of isotopic substitution studies to SO, SO2+, SO2−, (SO2)2−, and, tentatively, SO−. The fundamental and first overtone absorptions of SO lie 0.9 and 1.8 cm−1, respectively, below the gas-phase band centers. Ab initio calculations at the Hartree–Fock level show an instability in the v3 vibration of SO2+ which is avoided by higher-level calculations. The ν3 and ν1 fundamentals of SO2− isolated in solid neon are identified at 1086.2 and 990.8 cm−1, respectively. In agreement with an earlier proposal, the 1042 cm−1 absorption originally assigned to ν3 of SO2− trapped in solid argon is reassigned to MSO2, with M an alkali metal. Near the photodetachment threshold for SO2− isolated in a neon matrix, electron capture by SO2 nearest-neighbor pairs results in growth of infrared absorptions of (SO2)2−, which has been shown by gas-phase studies to have a significantly higher photodetachment threshold than does SO2−. The isotopic substitution studies require that the two sulfur atoms in (SO2)2− be nonequivalent, favoring the linking of the two SO2 units by a S ⋯ O bond.
Solution of the Schrödinger equation for a particle in an equilateral triangle
Authors: Li Wai-Kee , Blinder S. M.
Published: July 2002

The complete solution for the quantum-mechanical problem of a particle in an equilateral triangle is derived. By use of projection operators, eigenfunctions belonging explicitly to each of the irreducible representations of the symmetry group C3V are constructed. The most natural definition of the quantum numbers p and q includes not only integers but also nonintegers of the class (1)/(3) and (2)/(3) modulo 1. Some relevant features relating to symmetry and degeneracy are also discussed.
Real anomalies
Authors: Lott J.
Published: July 2002

The relationship between global anomalies of quantum theory and the topology of spaces of real Fredholm operators is shown. The spectral properties of such operators and how they are seen in examples of global anomalies on both compact and noncompact space-times are discussed.
Holonomy groups in general relativity
Authors: Hall G. S. , Kay W.
Published: July 2002

The infinitesimal holonomy group structure of space-time is discussed and related to the Petrov type of the Weyl tensor and the algebraic (Segré) type of the energy-momentum tensor. The number of covariant derivatives of the curvature tensor required to determine the infinitesimal holonomy group is determined in each case and the complete classification scheme is tabulated. Some special cases of physical interest are investigated in more detail. A geometrical approach is followed throughout.
A harmonic oscillator system with the generalized commutation relations
Authors: Watanabe Shuji
Published: July 2002

The system of a one-dimensional harmonic oscillator is discussed with the generalized commutation relations obtained by Wigner. The Hamiltonian is shown to have self-adjoint extensions. The domain of self-adjointness is explicitly specified in some cases. The proof is carried out by the use of the Lax–Milgram lemma. A suitable rigged Hilbert space is found for this system to reformulate the earlier arguments. The main difficulty is that the momentum and Hamiltonian operators contain the reflection Rψ(x)=ψ(−x) as well as singular terms.
Multiple-soliton solutions of Einstein’s equations
Authors: Economou A. , Tsoubelis D.
Published: July 2002

Using the Belinsky–Zakharov generating technique and a flat metric as a seed, two- and four-soliton solutions of the Einstein vacuum equations for the cases of stationary axisymmetric, cylindrically symmetric, or plane symmetric gravitational fields are considered. Three- and five-parameter classes of exact solutions are obtained, some of which are new.
An algorithm to relate general solutions of different bidimensional problems
Authors: Hojman Sergio A. , Chayet Sergio , Núñez Darío , Roque Marco A.
Published: July 2002

A sufficient condition to relate the general solutions of two different bidimensional problems, both in classical and quantum mechanics, is found. The point transformation, as well as the explicit construction of the related potentials, is presented. The procedure is then used in geometrical optics, the vibrating membrane, and other physical systems in two dimensions. Several examples and applications are worked out.
An inverse boundary problem for the Schrödinger operator with magnetic field
Authors: Kurylev Yaroslav
Published: July 2002

The problem under consideration in this article is the inverse boundary spectral problem for the Schrödinger operator with magnetic field. The operator is considered on a compact Riemannian manifold (with nonzero boundary) so that the corresponding unperturbed operator is the Laplace–Beltrami operator on the manifold. It is shown that boundary spectral data determine the potential and the boundary impedance uniquely, while the magnetic field may be found to within the group of gauge transformations.
Time-dependent plane-wave spectrum representations for radiation from volume source distributions
Authors: Heyman Ehud
Published: July 2002

A new time-domain spectral theory for radiation from a time-dependent source distribution, is presented. The full spectral representation is based on a Radon transform of the source distribution in the four-dimensional space-time domain and consists of time-dependent plane waves that propagate in all space directions and with all (spectral) propagation speeds vκ. This operation, termed the slant stack transform, involves projection of the time-dependent source distribution along planes normal to the spectral propagation direction and stacking them with a progressive delay corresponding to the spectral propagation speed vκ along this direction. Outside the source domain, this three-fold representation may be contracted into a two-fold representation consisting of time-dependent plane waves that satisfy the spectral constraint vκ=c with c being the medium velocity. In the two-fold representation, however, the complete spectral representation involves both propagating time-dependent plane waves and evanescent time-dependent plane waves. We explore the separate role of these spectral constituents in establishing the causal field, and determine the space-time regions where the field is described only by the propagating spectrum. The spectral theory is presented here for scalar wave fields, but it may readily be extended to vector electromagnetic fields.
Killing vectors in asymptotically flat space–times. I. Asymptotically translational Killing vectors and the rigid positive energy theorem
Authors: Beig Robert , Chruściel Piotr T.
Published: July 2002

We study Killing vector fields in asymptotically flat space–times. We prove the following result, implicitly assumed in the uniqueness theory of stationary black holes. If the conditions of the rigidity part of the positive energy theorem are met, then in such space–times there are no asymptotically null Killing vector fields, except if the initial data set can be embedded in Minkowski space–time. We also give a proof of the nonexistence of nonsingular (in an appropriate sense) asymptotically flat space–times that satisfy an energy condition and that have a null ADM four-momentum, under conditions weaker than previously considered.
Feynman integrals for a class of exponentially growing potentials
Authors: Kuna Tobias , Streit Ludwig , Westerkamp Werner
Published: July 2002

We construct the Feynman integrands for a class of exponentially growing time-dependent potentials as white noise functionals. We show that they solve the Schrödinger equation. The Morse potential is considered as a special case.
One-loop stress-tensor renormalization in curved background: The relation between ζ-function and point-splitting approaches, and an improved point-splitting procedure
Authors: Moretti Valter
Published: July 2002

We conclude the rigorous analysis of a previous paper [V. Moretti, Commun. Math. Phys. 201, 327 (1999)] concerning the relation between the (Euclidean) point-splitting approach and the local ζ-function procedure to renormalize physical quantities at one-loop in (Euclidean) Quantum Field Theory in curved space–time. The case of the stress tensor is now considered in general D-dimensional closed manifolds for positive scalar operators −Δ+V(x). Results obtained formally in previous works [in the case D=4 and V(x)=ξR(x)+m2] are rigorously proven and generalized. It is also proven that, in static Euclidean manifolds, the method is compatible with Lorentzian-time analytic continuations. It is proven that the result of the ζ-function procedure is the same obtained from an improved version of the point-splitting method which uses a particular choice of the term w0(x,y) in the Hadamard expansion of the Green’s function, given in terms of heat-kernel coefficients. This version of the point-splitting procedure works for any value of the field mass m. If D is even, the result is affected by an arbitrary one-parameter class of (conserved in absence of external source) symmetric tensors, dependent on the geometry locally, and it gives rise to the general correct trace expression containing the renormalized field fluctuations as well as the conformal anomaly term. Furthermore, it is proven that, in the case D=4 and V(x)=ξR(x)+m2, the given procedure reduces to the Euclidean version of Wald’s improved point-splitting procedure provided the arbitrary mass scale present in the ζ-function is chosen opportunely. It is finally argued that the found point-splitting method should work generally, also dropping the hypothesis of a closed manifold, and not depending on the ζ-function procedure. This fact is indeed checked in the Euclidean section of Minkowski space–time for A=−Δ+m2 where the method gives rise to the correct Minkowski stress tensor for m2⩾0 automatically.
On the replica symmetric equations for the Hopfield model
Authors: Pastur L. , Shcherbina M. , Tirozzi B.
Published: July 2002

We prove the central limit theorem for the cavity field in the case of the Hopfield model.
The Levi-Civita space–time as a limiting case of the γ space–time
Authors: Herrera L. , Paiva Filipe M. , Santos N. O.
Published: July 2002

It is shown that the Levi-Civita metric can be obtained from a family of the Weyl metric, the γ metric, by taking the limit when the length of its Newtonian image source tends to infinity. In this process a relationship appears between two fundamental parameters of both metrics.
Schrödinger operators in the twentieth century
Authors: Simon Barry
Published: July 2002

This paper reviews the past fifty years of work on spectral theory and related issues in nonrelativistic quantum mechanics.
Large-eddy simulation of passive scalar diffusion in isotropic turbulence
Authors: Lesieur Marcel , Rogallo Robert
Published: July 2002

A spectral large-eddy simulation of the velocity and passive temperature fields in three-dimensional isotropic turbulence is developed at high Reynolds numbers and a Prandtl number of the order of 1. The temperature spectrum in the large scales is shown to develop a range of the form ET(k)≈0.1η[〈u2〉/ε]k−1, where ε and η are the kinetic energy and temperature variance dissipation rates, respectively. This range, which is a result of shearing by large scale velocity gradients, is followed by a k−5/3 inertial-convective range at higher wavenumbers. The temperature variance decays much faster in time than the kinetic energy. Finally, the spectral eddy conductivity rises logarithmically with k toward small wavenumbers, contrary to the eddy viscosity that displays a plateau.
Enhanced separation of diffusing particles by chaotic advection
Authors: Aref Hassan , Jones Scott W.
Published: July 2002

Combining the reversibility of advection by a Stokes flow with the irreversibility of diffusion leads to a separation strategy for diffusing substances. This basic idea goes back to Taylor and Heller. It is shown here that the sensitivity of the method can be greatly enhanced by making the advection chaotic. The separation is particularly efficient when the thinnest structures resulting from advection are made comparable in size to a diffusion length. Simple heuristic estimates based on an understanding of chaotic motion and diffusion lead to a certain scaling that is seen in numerical experiments on this separation method.
Transition to unsteady natural convection in a tall water-filled cavity
Authors: Le Quéré P.
Published: July 2002

The transition to unsteady natural convection in a water-filled differentially heated cavity of vertical aspect ratio 10 is studied numerically by integrating two-dimensional Navier–Stokes equations in the Boussinesq approximation. The numerical algorithm combines a pseudospectral Chebyshev space discretization with a third-order time-stepping scheme. The top and bottom horizontal walls are considered to be either insulated or perfectly conducting. The critical Rayleigh number for adiabatic walls is found to be more than 30 times larger than that corresponding to perfectly conducting walls. In both cases, however, examination of the space-time structure of the temperature and velocity fluctuations shows that the onset of unsteadiness corresponds to boundary layer instability. In the case of perfectly conducting end walls, the transition to unsteady solutions possesses the characteristic features of a supercritical Hopf bifurcation. This is used to accurately determine the critical value. The dependence of the basic oscillation period in the vicinity of the critical Rayleigh number is given. Finally, the influence of unsteadiness on the local heat transfer coefficient is shown.
The structure of a passive scalar field with a uniform mean gradient in rapidly sheared homogeneous turbulent flow
Authors: Rogers Michael M.
Published: July 2002

The effect of an arbitrarily oriented mean passive scalar gradient on one-point passive scalar statistics is studied in homogeneous turbulent shear flow in the limit of rapid shearing. By neglecting the nonlinear inertial transfer to small scales an analytical solution for individual Fourier modes is obtained for the case of unity Prandtl number. This solution is used to compute the development of one-point statistics for the velocity and scalar fields in the inviscid limit. Comparisons to direct numerical simulations of the full nonlinear equations for the same flow show that in addition to describing the early time response to the imposed shear, the linear solution gives reasonable estimates of several correlation coefficients for the developed shear flow.
Viscoelastic Poiseuille flow through a curved channel: A new elastic instability
Authors: Joo Yong Lak , Shaqfeh Eric S. G.
Published: July 2002

The linear stability of the inertialess, pressure-driven Poiseuille flow of an Oldroyd-B fluid through a slightly curved channel is considered. The flow is shown to be unstable in certain flow parameter regimes. The critical conditions and the structure of the vortex flow at the onset of instability are presented. These results reveal that there is a purely elastic, instability in the flow, and the instability is a stationary mode in contrast to the elastic, oscillatory instability that occurs in Taylor–Couette flow [see Larson, Shaqfeh, and Muller, J. Fluid Mech. 218, 573 (1990)]. In addition, the mechanism of the instability is investigated through an examination of the disturbance-energy equation.
Transition to unsteady nonperiodic state in a through-flow lid-driven cavity
Authors: Benson J. D. , Aidun C. K.
Published: July 2002

A flush-mounted hot-film anemometer is used to investigate the transition to unsteady flow in a lid-driven cavity with a small amount of through flow. The quantitative measurements with the hot-film probe confirm the transition from steady to time-periodic state from previous flow visualization studies and show a second transition in the primary state from time-periodic to quasiperiodic flow and a transition to a state with a broad band of frequencies typical of a chaotic state. From these results, it is concluded that the transition to unsteady nonperiodic state in this system is through a torus bifurcation.
Dynamics of nonstationary dipole vortices
Authors: Hesthaven J. S. , Lynov J. P. , Nycander J.
Published: July 2002

The dynamics of tilted dipole vortices in the equivalent barotropic vorticity (or Hasegawa–Mima) equation is studied. A recent theory is compared with numerical simulations and found to describe the short time behavior of dipole vortices well. In the long time limit the dipoles are found to either disintegrate or relax toward a steady eastward propagating dipole vortex. This relaxation is a consequence of nonviscous enstrophy loss by the dipole vortex.
Vortex shedding from long slender cylinders in near-axial flow
Authors: Bull M. K. , Dekkers W. A.
Published: July 2002

It has been found that vortex shedding can occur from long cylinders at small angles of yaw, i.e. in near-axial flow. The range of Reynolds number over which this can take place has been determined for yaw angles between 0° and 10°. Results for vortex-shedding frequencies are also presented, and compared with existing data for larger yaw angles. It is suggested that the phenomenon may be relevant to turbulence-generation mechanisms in thick axisymmetric turbulent boundary layers.
Generation of tripolar vortical structures on the beta plane
Authors: Hesthaven J. S. , Lynov J. P. , Rasmussen J. Juul , Sutyrin G. G.
Published: July 2002

A new feature of the long-time evolution of a strong vortex with initially monotonic potential vorticity is found by direct numerical solution of the quasigeostrophic equivalent barotropic equation. Two satellites, which emerge after splitting of an annulus, appear at the vortex periphery. Rotation and oscillation of the tripolar structure may lead to increased mixing near the boundary of the vortex core. The translation of strong monopoles is found to be well described, even for times longer than the linear Rossby wave period, by a recent approximate theory for the evolution of an azimuthal perturbation with mode number l=1.
The existence of stationary vortex solutions of the equations for nonlinear drift waves in plasmas and nonlinear Rossby waves
Authors: Nycander J.
Published: July 2002

The effect of including the electron temperature gradient and higher-order terms in the expansion of the density profile in the equations for nonlinear drift waves in plasmas is studied, as well as the effect of expanding the equation for nonlinear Rossby waves to higher orders in the amplitude. The two cases are very similar, and earlier studies have concluded that these effects would make a new kind of stationary monopole vortex solution possible. This solution would have a unique functional form in the whole plane, and for some parameter values no closed streamlines. It is shown that this conclusion is correct, and that such solutions do not exist. It is also pointed out that these additional effects become particularly important in the region of very long wavelengths, where they give rise to wave breaking and a transfer of energy toward shorter wavelengths. This can prevent the condensation to zonal flows that has been predicted earlier.
Poloidal and parallel plasma viscosities in tokamak geometry
Authors: Shaing K. C.
Published: July 2002

The poloidal and parallel plasma viscosities in tokamak geometry in Hamada coordinates are calculated from the drift kinetic equation, including a large mass flow velocity without imposing the usual constraint that VpB/(vtiBp) be small. Here, Vp is the poloidal plasma flow velocity, vti is the ion thermal speed, B is the magnetic field strength, and Bp is the poloidal magnetic field strength. With this extended validity, the poloidal and parallel viscosities are useful in modeling the radial electric field in the edge region of a tokamak in the enhanced confinement regime.
Considerations of ion-temperature-gradient-driven turbulence
Authors: Cowley S. C. , Kulsrud R. M. , Sudan R.
Published: July 2002

The ion-temperature-gradient-driven instability is considered in this paper. Physical pictures are presented to clarify the nature of the instability. The saturation of a single eddy is modeled by a simple nonlinear equation. It is shown that eddies that are elongated in the direction of the temperature gradient are the most unstable and have the highest saturation amplitudes. In a sheared magnetic field, such elongated eddies twist with the field lines. This structure is shown to be an alternative to the usual Fourier mode picture in which the mode is localized around the surface where k∥ =0. These elongated twisting eddies, which are an integral part of the ‘‘ballooning mode’’ structure, could survive in a torus. The elongated eddies are shown to be unstable to secondary instabilities that are driven by the large gradients in the long eddy. It is argued that the ‘‘mixing length’’ is affected by this nonlinear process, and is unlikely to be a linear eigenmode width.
Refined definition of the beam brightness
Authors: Rhee M. J.
Published: July 2002

An unambiguously refined definition of the beam brightness is presented. Using the definition, brightnesses of a few commonly used beam distributions are determined and expressed in terms of the beam current and the rms emittances.
Equilibrium of a liquid in a spherical shell caused by gravity, surface tension, and van der Waals forces
Authors: Mok Lawrence S. , Kim Kyekyoon , Bernat Thomas P.
Published: July 2002

A rigorous proof is presented which shows that the classical Young–Laplace equation cannot predict a continuous liquid layer inside a spherical-shell container. An augmented Young–Laplace equation is in turn derived to describe the equilibrium profiles of a continuous liquid hydrogen layer inside a spherical-shell inertial fusion target. The augmentation is achieved by adding to the total free energy a term originating from the attractive intermolecular forces of the van der Waals type.
Parallel and toroidal viscosity for nonaxisymmetric toroidal plasmas in the plateau regime
Authors: Coronado M. , Wobig H.
Published: July 2002

Calculations of the viscosity expressions appearing in the fluid approach to neoclassical transport of nonaxisymmetric toroidal plasmas are presented. By assuming the Chew–Goldberger–Low form for the viscosity tensor and a large-aspect-ratio plasma, the drift kinetic equation for the plateau regime is solved and expressions for the parallel and toroidal viscosity are calculated as functions of plasma and heat fluxes. The viscosity expressions found for the plateau regime are similar to the expressions valid in the collision-dominated regime.
Useful formulas in the rapid distortion theory of homogeneous turbulence
Authors: Lee Moon J. , Piomelli Ugo , Reynolds William C.
Published: July 2002

Rapid distortion theory is applied to study the response of homogeneous turbulence to imposed arbitrary irrotational mean strains. Expressions useful in turbulence modeling, such as the Reynolds stresses and vorticity correlations, have been obtained.
A symmetric regularized long-wave equation for shallow water waves
Authors: Shivamoggi Bhimsen K.
Published: July 2002

Nonlinear bidirectional shallow water waves are shown to be described by a regularized long-wave equation. Six constants of motion for this equation are exhibited.
Numerical solution of axisymmetric, unsteady free-boundary problems at finite Reynolds number. I. Finite-difference scheme and its application to the deformation of a bubble in a uniaxial straining flow
Authors: Kang I. S. , Leal L. G.
Published: July 2002

A brief description of a numerical technique suitable for solving axisymmetric, unsteady free-boundary problems in fluid mechanics is presented. The technique is based on a finite-difference solution of the equations of motion on a moving orthogonal curvilinear coordinate system, which is constructed numerically and adjusted to fit the boundary shape at any time. The initial value problem is solved using a fully implicit first-order backward time differencing scheme in order to insure numerical stability. As an example of application, the unsteady deformation of a bubble in a uniaxial extensional flow for Reynolds numbers is considered in the range of 0.1≤R≤100. The computation shows that the bubble extends indefinitely if the Weber number is larger than a critical value (W>Wc). Furthermore, it is shown that a bubble may not achieve a stable steady state even at subcritical values of Weber number if the initial shape is sufficiently different from the steady shape. Finally, potential-flow solutions as an approximation for R→∞ have also been obtained. These show that an initially deformed bubble undergoes oscillatory motion if W<Wc, with a frequency of oscillation (based on the surface tension time scale) that decreases as Weber number increases and equals zero at the critical Weber number beyond which steady solutions could not be obtained in the earlier numerical work of Ryskin and Leal [J. Fluid Mech. 148, 37 (1984)]. This clearly indicates that the point of nonconvergence of the steady-state numerical problem is, in fact, a limit point in the branch of steady-state solutions.
Drift vortices in inhomogeneous plasmas: Stationary states and stability criteria
Authors: Laedke E. W. , Spatschek K. H.
Published: July 2002

This paper contains a theoretical treatment of the dynamical behavior of two-dimensional nonlinear drift waves in plasmas. A simple model is investigated that allows gradients in temperature as well as in particle number density. The general class of stationary states can be specified and from the general description simplifed states can be (re)derived. A quite general method, resulting in general criteria, is proposed to study the dynamics of drift vortices under perturbations. The results are applied to several cases.
Evolution of bioconvection patterns in a culture of motile flagellates
Authors: Harashima Akira , Watanabe Masataka , Fujishiro Issei
Published: July 2002

Numerical experiments were carried out on the pattern formation of bioconvection that is observed in cultures of motile aquatic microorganisms. New features were revealed on how the bioconvective system evolves and how the number of falling fingers is selected at each stage of evolution. At the onset of convection, the relevant dynamical regime is that of the Rayleigh–Taylor instability. Then, readjustment of the wavenumber occurs as the adjacent convection cells combine with each other. The trajectory in (total kinetic energy, total potential energy) space shows that evolution of the system proceeds in the direction of intensifying the downward advection of microorganisms and reducing the total potential energy of the system. Finally the system reaches a stationary state, where the aspect ratio of the convection cells resembles that of Bénard–Rayleigh convection and is optimum for the efficient downward advection of microorganisms. Furthermore, it is demonstrated that trajectories of the two cases deviate from this major evolution. In the case where the diffusion time of the system is large, the system shows remarkable oscillation and repeats the Rayleigh–Taylor instability intermittently. In the case where the viscous effect is large, the system ceases to evolve before reaching the optimum mode.
Chaotic advection in pulsed source–sink systems
Authors: Jones Scott W. , Aref Hassan
Published: July 2002

The onset of chaos in passive advection of particles by flow caused by a pulsed source–sink system is documented. This type of model is of interest in various applications. It is of fundamental interest as the first example of a flow without circulation about any contour at any instant displaying chaotic particle paths. Standard chaos diagnostics such as Poincaré sections and Lyapunov exponents are studied as are more conventional flow visualization measures such as streaklines. Numerical stirring experiments for various collections of particles are performed and the properties of a certain one-dimensional map induced by the two-dimensional flow are examined.
An adaptive turbulence filter for decomposition of organized turbulent flows
Authors: Brereton G. J. , Kodal A.
Published: July 2002

A new decomposition has been developed in which turbulent processes in shear flows may be represented as a combination of organized and more random turbulent motions. Each component is modeled as a summation of its characteristic eddies, of strength that varies in time and space as a function of the entire process. The contribution of all turbulent eddies of the more random component are estimated with an adaptive turbulence filter, which recognizes this component as the orthogonal partner to organized motion, with a power density spectrum of appropriate shape. The decomposition recovers organized motion from time and space series of data in a physically meaningful way, and can be used to characterize interaction between coherent and more random motions. It also provides an estimate for the turbulence in shear flows that are too complex for a meaningful average motion to be identified.
Direct simulation of transition in Stokes boundary layers
Authors: Verzicco R. , Vittori G.
Published: July 2002

Numerical simulations of the Stokes boundary layer over a three-dimensional wavy wall are performed in order to investigate the role played by infinitesimal wall imperfections in triggering transition to turbulence. Our results show flow patterns qualitatively similar to those experimentally detected. In particular the laminar, disturbed-laminar and intermittent turbulent regimes are recovered. The characteristics of the above flow regimes are analyzed.
Application of PDF methods to compressible turbulent flows
Authors: Delarue B. J. , Pope S. B.
Published: July 2002

A particle method applying the probability density function (PDF) approach to turbulent compressible flows is presented. The method is applied to several turbulent flows, including the compressible mixing layer, and good agreement is obtained with experimental data. The PDF equation is solved using a Lagrangian/Monte Carlo method. To accurately account for the effects of compressibility on the flow, the velocity PDF formulation is extended to include thermodynamic variables such as the pressure and the internal energy. The mean pressure, the determination of which has been the object of active research over the last few years, is obtained directly from the particle properties. It is therefore not necessary to link the PDF solver with a finite-volume type solver. The stochastic differential equations (SDE) which model the evolution of particle properties are based on existing second-order closures for compressible turbulence, limited in application to low turbulent Mach number flows. Tests are conducted in decaying isotropic turbulence to compare the performances of the PDF method with the Reynolds-stress closures from which it is derived, and in homogeneous shear flows, at which stage comparison with direct numerical simulation (DNS) data is conducted. The model is then applied to the plane compressible mixing layer, reproducing the well-known decrease in the spreading rate with increasing compressibility. It must be emphasized that the goal of this paper is not as much to assess the performance of models of compressibility effects, as it is to present an innovative and consistent PDF formulation designed for turbulent inhomogeneous compressible flows, with the aim of extending it further to deal with supersonic reacting flows.
Simulations of a randomly advected passive scalar field
Authors: Chen Shiyi , Kraichnan Robert H.
Published: July 2002

The advection of a passive scalar field by a rapidly decorrelating random velocity field with power-law scaling is computed by simulations in a cyclic square at resolutions of 40962 and 81922 grid points. Structure functions of the scalar field are measured and inertial-range scaling exponents are determined. The conditional mean of the scalar-field dissipation term and its moments are found. The results are compared with theoretical predictions and with other recent simulations.
Three-dimensional numerical experiments on thermal convection in a very viscous fluid: Implications for the dynamics of a thermal boundary layer at high Rayleigh number
Authors: Parmentier E. M. , Sotin C.
Published: July 2002

In this study we present results from three-dimensional numerical experiments on thermal convection in a volumetrically heated, infinite Prandtl number fluid cooled from above. At high Rayleigh number, a thin thermal boundary layer forms adjacent to the cold top boundary. On the basis of our numerical results we study the thermal structure and dynamics of this boundary layer and the population of plumes that it creates. Cold thermal plumes that develop by boundary layer instability form continuous nearly vertical columns that migrate horizontally sweeping off the unstable boundary layer. A plume usually persists until it coalesces with another plume. The average spacing of plumes, inferred from the variation of the observed number of plumes with Rayleigh number, is proportional to (δd)1/2, where δ and d are the thermal boundary layer and fluid layer thicknesses, respectively. Based on a “kinetic theory” of plume populations, we show that this is consistent with an equilibrium plume population in which the creation of plumes by boundary layer instability and their disappearance by coalescing with other plumes are balanced. This scaling of average plume spacing is a consequence of the width of velocity plumes in a very viscous (infinite Prandtl number) fluid comparable to the fluid layer depth. For finite Prandtl number, the same analysis but with temperature and velocity plumes of comparable width predicts a plume spacing proportional to the boundary layer thickness.
Alternate method for treating Alfvén waves driven unstable by α particles
Authors: Hedrick C. L. , Leboeuf J.-N. , Prochaska M. D. , Spong D. A. , Carreras B. A.
Published: July 2002

For treating Alfvén waves driven unstable by α particles, an alternative to the Oak Ridge National Laboratory gyro-Landau fluid approach is suggested. Its primary advantage is that this method can simultaneously treat fluid nonlinearities and kinetic quasilinear effects. It is argued that more perturbative approaches could overestimate the kinetic quasilinear effects.
Controlling the resistive hose instability in relativistic electron beams
Authors: Fernsler R. F. , Slinker S. P. , Lampe M. , Hubbard R. F.
Published: July 2002

The resistive hose instability has long been recognized as the major impediment to the propagation of intense, relativistic electrons beams in dense gas. However, hose is a convective instability, and therefore its growth is limited by the length of the beam pulse, the local growth rate, and the speed at which the instability convects through the pulse. The convective speed and the growth rate depend on the beam and plasma parameters, and these vary strongly from beam head to tail. In this paper, hose theory is reformulated to incorporate these variations, and the reformulated model is then used to compute the maximum hose growth possible in a given beam pulse. In air, the model predicts that hose grows by many orders of magnitude when the beam current is less than 10 kA or has a rise time more than a few nanoseconds long. But the growth is predicted to be less than a factor of 20 if the current is 50 kA or more, the rise time is subnanosecond, and the beam radius is properly tapered from head to tail. The model is supported by extensive numerical simulations and is in general agreement with available experimental data. Many of the issues discussed here may have application to other instabilities as well.
Experimental observation of very low-frequency macroscopic modes in a dusty plasma
Authors: Praburam G. , Goree J.
Published: July 2002

Images of a cloud of grains in a dusty plasma reveal a pair of very low-frequency modes, termed here the filamentary and great void modes. The plasma was a radio-frequency discharge formed between parallel-plate graphite electrodes. A cloud of 100 nm carbon particles was produced by accretion of carbon atoms produced by sputtering the graphite. The cloud was illuminated with a laser sheet and imaged with a video camera. The great void mode was a spoke-shaped region of the cloud that was free of dust and rotated azimuthally in the discharge. The filamentary mode had the appearance of turbulent striations, with a smaller amplitude than the great void. The filamentary mode sometimes appeared as a distinctive vortex, curling in the poloidal direction. Both modes had a very low frequency, on the order of 10 Hz. Two possible causes of the modes are discussed. The low phase velocity of the modes may be consistent with a dust-acoustic wave. Alternatively, the great void may be an ionization wave that moved the dust about, since a modulation in the glow was seen moving at the same speed as the void. It is argued that existing theories of waves in dusty plasmas assume weakly collisional plasmas, which may be unsuitable for explaining experimental results in laboratory dusty plasmas, since they are often strongly coupled.
Passive particle dynamics in a flow exhibiting transition to turbulence
Authors: Benkadda S. , Gabbai P. , Zaslavsky G. M.
Published: July 2002

The behavior of a passive particle in a flow that exhibits bifurcations in the transition to a turbulent regime is investigated. The flow considered is a variant of the Charney–Hasegawa–Mima equation. The scalar particle dynamics is considered for different regimes of the main flow. A regime of anomalous diffusion (hypodiffusion) is observed when the field has few harmonics whereas normal diffusion occurs in the strange attractor regime. The analysis of the singular orbit reveals the presence of traps and flights that control the transport.
A new Lagrangian formulation for laser-plasma interactions
Authors: Brizard Alain J.
Published: July 2002

A new Lagrangian structure for cold relativistic plasma electrodynamics is presented. This new formulation uses the fluid velocity v instead of the canonical-momentum Clebsch potential ψ [X. L. Chen and R. N. Sudan, Phys. Fluids B 5, 1336 (1993)]. As a simple application, it is used to derive (through the Noether method) new exact conservation laws associated with nonlinear laser wake-field equations in the multi-dimensional quasi-static approximation.
Charge compensated ion beam propagation in a reactor sized chamber
Authors: Vay J. L. , Deutsch C.
Published: July 2002

A fully electromagnetic particle in cell-Monte Carlo (PIC-MCC) code is considered for the ballistic transport of intense ion beams in a reaction chamber field with Flibe gas surrounding a pellet with a thermonuclear fuel in it. A specific emphasis is given to a self-consistent treatment of beam boundary conditions. Spurious electromagnetic waves are evacuated out of the grid, and a modified Maxwell system corrects for Gauss theorem error. A dynamical grid with self-adaptating field follows beam convergence. Final ion propagation in the Hylife II [R. Moir, Fusion Technol. 29, 306 (1991)] scheme and also in the space charge compensated one is investigated at length. For the first, a partial beam neutralization is identified only through electron background. The second displays an acceptable focalization at pellet, the background electron temperature has a significant influence on beam minimum radius. Transverse emittance is given specific attention.
Simulation of ultrarelativistic beam dynamics in plasma wake-field accelerator
Authors: Lotov K. V.
Published: July 2002

To study the long-term dynamics of ultrarelativistic particle beams (drivers) in a plasma wake-field accelerator, a two-dimensional hybrid code, LCODE, has been developed, in which the plasma is treated as an electron fluid, while the beam is treated as an ensemble of particles. With LCODE the evolution of the driver composed of several short particle bunches is studied. When properly positioned, the bunches are shown to achieve a radial equilibrium in which all driver particles are focused by the plasma, and the focusing force is balanced by driver transverse pressure. After establishment of the radial equilibrium, the driver loses much of its energy with little distortion in shape. Losing a major part of the initial energy, driver particles get defocused and lost. Due to this “self-cleaning,” the driver generates a good wake-field even after the particle loss of more than the half.
Sheared rotation effects on kinetic stability in enhanced confinement tokamak plasmas, and nonlinear dynamics of fluctuations and flows in axisymmetric plasmas
Authors: Rewoldt G. , Beer M. A. , Chance M. S. , Hahm T. S. , Lin Z. , Tang W. M.
Published: July 2002

Sheared rotation dynamics are widely believed to have significant influence on experimentally-observed confinement transitions in advanced operating modes in major tokamak experiments, such as the Tokamak Fusion Test Reactor (TFTR) [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)], with reversed magnetic shear regions in the plasma interior. The high-n toroidal drift modes destabilized by the combined effects of ion temperature gradients and trapped particles in toroidal geometry can be strongly affected by radially-sheared toroidal and poloidal plasma rotation. In previous work with the FULL linear microinstability code, a simplified rotation model including only toroidal rotation was employed, and results were obtained. Here, a more complete rotation model, which includes contributions from toroidal and poloidal rotation and the ion pressure gradient to the total radial electric field, is used for a proper self-consistent treatment of this key problem. Relevant advanced operating mode cases for TFTR are presented. In addition, the complementary problem of the dynamics of fluctuation-driven E×B flow is investigated by an integrated program of gyrokinetic simulation in annulus geometry and gyrofluid simulation in flux tube geometry.