bluuebunny/crossref_metadata_2025 huggingface dataset:
Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. II. Asynchronous concerted elimination of I2 from CH2I2
Authors: Zhang Qingguo , Marvet Una , Dantus Marcos
Published: July 2002

The photoinduced molecular detachment dynamics of CH2I2 have been investigated with femtosecond time resolution. Upon multiphoton excitation of CH2I2 with 312 nm femtosecond pulses, weak fluorescence in the 260–290 nm region was observed in addition to the I2 fluorescence in the 290–345 nm region studied in the previous paper. The weak fluorescence has also been interpreted as due to emission from I2, where I2 was produced from the photodissociation process CH2I2→CH2+I2*. In order to investigate the detailed dynamics of this reaction, femtosecond time-resolved data have been obtained by selective detection of the I2 fluorescence at 272 and 285 nm. From these transients, it has been found that the dissociation process takes place within the temporal width (50 fs) of the laser pulse and that the I2 photofragments exhibit coherent vibrational motion. The 272 nm transients also exhibit clear, fast decaying rotational anisotropy, quantitative analysis of which reveals a distribution of rather high rotational levels of I2. This permits us to conclude that the I2 detachment is an asynchronous concerted process; while breaking of the two C–I bonds and formation of the I–I bond happen in a single kinetic step, one of the C–I bonds breaks faster than the other. In addition, energy partitioning between the CH2 and I2 photofragments has also been explored based on the experimental observations. Since this study involves a multiphoton transition, a theoretical formulation for the time dependent rotational anisotropy is presented for the general case of multiphoton pump and multiphoton probe transitions.
Self-diffusion in CD4 and ND3: With notes on the dynamic isotope effect in liquids
Authors: Buchhauser J. , Groß T. , Karger N. , Lüdemann H.-D.
Published: July 2002

Self-diffusion coefficients D of CD4 and ND3 were measured with the NMR-PGSE-technique over a wide range of temperature and pressure (10–200 MPa, 150–450 K for CD4, 10–200 MPa, 200–450 K for ND3). When compared to the protonated species, both substances show a dynamic isotope effect Dr=DX–H/DX–D that was found to rise to 1.3 (CD4) and 1.4 (ND3) at the lowest temperatures studied. This behavior is similar to a number of other simple liquids (HF, CH3OH, H2O). Classical theories for single particle motion in liquids suggest a dependence of Dr on the square root of the inverse mass ratio, or the square root of the inverse ratio of the moments of inertia, if translation-rotation coupling is dominant. Dr should, however, be temperature-independent. The strong temperature-dependence of Dr and its high value at low temperatures found in many liquids can thus not be explained by single particle properties, but rather has to be viewed as a collective phenomenon. It was suggested earlier that the stronger hydrogen bonds expected in the deuterated liquids are responsible for this behavior. However, the fact that methane shows a similar dynamic isotope effect is an indication that more complex mechanisms are responsible for the deviations from classical models of liquid dynamics. Quantum mechanical calculations suggest that backscattering effects may describe this interesting phenomenon.
Longer time steps for molecular dynamics
Authors: Izaguirre Jesús A. , Reich Sebastian , Skeel Robert D.
Published: July 2002

Simulations of the dynamics of biomolecules have been greatly accelerated by the use of multiple time-stepping methods, such as the Verlet-I/r-RESPA (reversible reference system propagator algorithms) method, which is based on approximating “slow” forces as widely separated impulses. Indeed, numerical experiments have shown that time steps of 4 fs are possible for these slow forces but unfortunately have also shown that a long time step of 5 fs results in a dramatic energy drift. To overcome this instability, a symplectic modification of the impulsive Verlet-I/r-RESPA method has been proposed, called the mollified impulse method. The idea is that one modifies the slow part of the potential energy so that it is evaluated at “time averaged” values of the positions, and one uses the gradient of this modified potential for the slow part of the force. By filtering out excitations to the fastest motions, these averagings allow the use of longer time steps than does the impulse method. We introduce a new mollified method, Equilibrium, that avoids instability in a more effective manner than previous averaging mollified methods. Our experiments show that Equilibrium with a time step of 6 fs is as stable as the impulsive Verlet-I/r-RESPA method with a time step of 4 fs. We show that it may be necessary to include the effect of nonbonded forces in the averaging to make yet longer time steps possible. We also show that the slight modification of the potential has little effect on accuracy. For this purpose we compare self-diffusion coefficients and radial distribution functions against the Leapfrog method with a short time step (0.5 fs).
The interaction of gold clusters with methanol molecules: Ab initio molecular dynamics of Aun+CH3OH and AunCH3OH
Authors: Rousseau Roger , Marx Dominik
Published: July 2002

Structural, dynamical, and electronic properties of adducts obtained by adsorbing one methanol molecule onto charged and neutral gold clusters, Aun+–CH3OH and Aun–CH3OH, are investigated using Car–Parrinello ab initio molecular dynamics as a function of the cluster size n. The absorption process occurs by the formation of a Au⋆–O coordination bond to one particular gold atom Au⋆ without altering the structure of the underlying cluster. This chemical bond is much stronger for the charged metal clusters Aun+ than for the neutral analogs Aun. In the charged case, the C–O stretching vibration of the interacting methanol molecule is found to increase discontinuously as the underlying cluster structure changes from two-dimensional to three-dimensional. The weaker C–O bond in the neutral species however has “insufficient strength” to be sensitive to changes in coordination number and cluster structure. This leads to a constant C–O stretching frequency as the size of the cluster increases, including the regime where the Aun cluster changes from planar to three-dimensional.
Dissociation rate of hot benzene
Authors: Tsai Shang-Ting , Lin Chih-Kai , Lee Yuan T. , Ni Chi-Kung
Published: July 2002

The dissociation rate of benzene and d6-benzene were measured under collision-free condition by multimass ion imaging techniques. The value of 1±0.2×105 s−1 and 5±1×104 s−1 were obtained for benzene and d6-benzene, respectively, with internal energy of 618 kJ/mol. The dissociation rate of benzene with internal energy of 483 kJ/mol was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 3×103 s−1.
Nonlinear equations with superposition formulas and the exceptional group G2. I. Complex and real forms of g2 and their maximal subalgebras
Authors: Beckers J. , Hussin V. , Winternitz P.
Published: July 2002

In order to study nonlinear ordinary differential equations with superposition principles, related to the exceptional simple Lie group G2, the complex and real forms of its Lie algebra are examined and their maximal subalgebras are summarized. In particular the parabolic subalgebras of the noncompact real form gNC2(R) are determined. Explicit matrix realizations of the fundamental representation D(1,0) are used and studied in connection with invariant subspaces in a seven-dimensional (complex or real) vector space. The results are collected in three tables of specific interest for the study of nonlinear differential equations, which will be developed in Paper II of this series.
Exact self-gravitating disks and rings: A solitonic approach
Authors: Letelier P. S. , Oliveira S. R.
Published: July 2002

The Belinsky–Zakharov version of the inverse scattering method is used to generate a large class of solutions to the vacuum Einstein equations representing uniformly accelerating and rotating disks and rings. The solutions studied are generated from a simple class of static disks and rings that can be expressed in a simple form using suitable complex functions of the usual cylindrical coordinates.
Bound states in curved quantum waveguides
Authors: Exner P. , Seba P.
Published: July 2002

A free quantum particle living on a curved planar strip Ω of a fixed width d with Dirichlet boundary conditions is studied. It can serve as a model for electrons in thin films on a cylinder-type substrate, or in a curved quantum wire. Assuming that the boundary of Ω is infinitely smooth and its curvature decays fast enough at infinity, it is proved that a bound state with energy below the first transversal mode exists for all sufficiently small d. A lower bound on the critical width is obtained using the Birman–Schwinger technique.
Braided momentum in the q-Poincaré group
Authors: Majid Shahn
Published: July 2002

The q-Poincaré group of M. Schlieker et al. [Z. Phys. C 53, 79 (1992)] is shown to have the structure of a semidirect product and coproduct B× SOq(1,3) where B is a braided-quantum group structure on the q-Minkowski space of four-momentum with braided-coproduct Δ_p=p⊗1+1⊗p. Here the necessary B is not a usual kind of quantum group, but one with braid statistics. Similar braided vectors and covectors V(R′), V*(R′) exist for a general R-matrix. The abstract structure of the q-Lorentz group is also studied.
Invariant of dynamical systems: A generalized entropy
Authors: Mesón Alejandro M. , Vericat Fernando
Published: July 2002

In this work the concept of entropy of a dynamical system, as given by Kolmogorov, is generalized in the sense of Tsallis. It is shown that this entropy is an isomorphism invariant, being complete for Bernoulli schemes.
Conditions of Cayley’s type for ellipsoidal billiard
Authors: Dragović Vladimir , Radnović Milena
Published: July 2002

The billiard system within an ellipsoid in three-dimensional space is considered. We describe periodical trajectories of that mechanical system deriving analytical condition of Cayley’s type for generalized Poncelet’s theorem, using methods of algebrogeometric integration.
Numerical analysis of the shear and thermal creep flows of a rarefied gas over a plane wall on the basis of the linearized Boltzmann equation for hard-sphere molecules
Authors: Ohwada Taku , Sone Yoshio , Aoki Kazuo
Published: July 2002

Shear flow and thermal creep flow (flow induced by the temperature gradient along the boundary wall) of a rarefied gas over a plane wall are considered on the basis of the linearized Boltzmann equation for hard-sphere molecules and diffuse reflection boundary condition. These fundamental rarefied gas dynamic problems, typical half-space boundary-value problems of the linearized Boltzmann equation, are analyzed numerically by the finite-difference method developed recently by the authors, and the velocity distribution functions, as well as the macroscopic variables, are obtained with good accuracy. From the results, the shear and thermal creep slip coefficients and their associated Knudsen layers of a slightly rarefied gas flow past a body are derived. The results for the slip coefficients and Knudsen layers are compared with experimental data and various results by the Boltzmann–Krook–Welander (BKW) equation, the modified BKW equation, and a direct simulation method.
A binomial sampling model for scalar turbulent mixing
Authors: Valino L. , Dopazo C.
Published: July 2002

The closure problem generated by the molecular mixing term in the turbulent convection of scalars is studied. The statistical average of this term both in moment formulations and in the probability density function (pdf) approach implicitly encloses the turbulence straining action on scalar gradients leading to a significant enhancement of the molecular dissipative effects. Previous pdf model equations are examined in terms of cumulants evolution and reasons for their failure are diagnosed. A new noninteractive model is proposed, combining a linear mean square estimation (LMSE) deterministic subprocess affecting all the Monte Carlo particles, used to represent the pdf, and a binomial sampling acting on a fraction of them. The scalar lower and/or upper bounds are naturally considered in the formulation. For unbounded scalars, or when the scalar standard deviation is much smaller than the absolute value of the difference between the bounds and the scalar mean, the binomial sampling tends to a Gaussian one. The extension of this model to investigate the joint statistics of velocity and scalars is also considered. Numerical results for the homogeneous turbulent mixing of one scalar with initial values of either 0 or 1 are presented. The evolution of the first four moments and the pdf is qualitatively reasonable. The correct asymptotic Gaussian state is predicted.
A binomial Langevin model for turbulent mixing
Authors: Valiño L. , Dopazo C.
Published: July 2002

A Langevin model with binomial random diffusion replacing the classical Wiener process is proposed to model the turbulent mixing of a scalar convected by a field of statistically homogeneous turbulence. A Monte Carlo simulation is performed. The results display an excellent agreement with existing data from the numerical experiment of Eswaran and Pope [Phys. Fluids 31, 506 (1988)].
Gaseous mixture slit flow at intermediate Knudsen numbers
Authors: Cercignani Carlo , Sharipov Felix
Published: July 2002

The problem of a two-dimensional binary gaseous mixture slit flow is investigated on the basis of numerical calculations of the linearized kinetic model proposed by McCormack [Phys. Fluids 16, 2095 (1973)]. The mass flow rate and the field flow of both components are calculated for several Knudsen numbers between 10 and 0.1. Kinetic coefficients satisfying Onsager’s reciprocity relations are found.
Head-on collision of a large vortex ring with a free surface
Authors: Song Museok , Bernal Luis P. , Tryggvason Grétar
Published: July 2002

The generation of free surface waves due to the head-on collision of a vortex ring with the free surface is studied experimentally and numerically. The interaction process is investigated by looking at the free surface evolution with the aid of a shadowgraph image, by measurement of the free surface elevation, and by visualizing the vortex ring with hydrogen bubbles. Three different free surface patterns are observed, depending on the strength of the vortex ring: (1) A single circular depression of the free surface for weak vortex rings and at very early times for strong vortex rings, (2) axisymmetric radially propagating waves in the early stage of the interaction of strong vortex rings with the surface, and (3) fully three-dimensional waves that depend on the local structure of the vortex core. Numerical simulations are used to address the observed axisymmetric phenomena.
Effects of surface roughness on a sphere sedimenting through a dilute suspension of neutrally buoyant spheres
Authors: Davis Robert H.
Published: July 2002

The influence of microscopic surface roughness on the statistical motion of a heavy sphere falling through a dilute suspension of neutrally buoyant spheres is analyzed for conditions of low Reynolds number and large Péclet number. The presence of roughness on their surfaces allows interacting spheres to come into contact. A stick/rotate model (based on rigid-body motion) and a roll/slip model (based on solid contact friction) are proposed for the motion normal to the line-of-centers of the heavy sphere and a neutrally buoyant sphere when in contact. A trajectory analysis is used to describe pairwise interactions and their contributions to the mean velocity, velocity variance, and hydrodynamic diffusivity of the heavy sphere as it undergoes a fluctuating motion due to interactions with the background spheres. The horizontal component of the hydrodynamic diffusivity becomes nonzero as a direct result of surface roughness and contact, since ideally smooth spheres undergo symmetric pairwise trajectories which result in zero net horizontal displacement. The effects of surface roughness on the vertical components of the diffusivity, mean velocity, and velocity variance are relatively small when the heavy and background spheres are of nearly equal size, but the effects of surface roughness are large for disparate size ratios. The limited experimental data available show better agreement with the roll/slip model than with the stick/rotate model.
General properties of a multilayer stratified fluids system
Authors: Yang Yumin , Zhang Qiang
Published: July 2002

Linearized Euler equations of a general stationary multiple layer stratified system for both compressible and incompressible inviscid fluids are analyzed. The main result is that many features of a multilayer system are universal, in the sense they do not depend on such details as the number of layers, their thicknesses, equations of state for the fluids, and equilibrium density distributions. Necessary and sufficient conditions of stability are determined. For compressible fluids, it is possible for the system to be unstable even if there is no density inversion anywhere. It is shown that a compressible system is always more unstable than the corresponding incompressible one. A universal upper bound for the growth rate for a given perturbation wave number is given. General Rayleigh–Taylor unstable modes are characterized, and the range of unstable wave numbers is determined. Properties of stable modes are discussed. Numerical algorithms for solving the eigenvalue problem of the set of linearized Euler equations are given.
Electron-temperature-gradient drift wave instabilities in magnetized electron–ion plasmas
Authors: Murtaza G. , Nadeem M. , Shukla P. K.
Published: July 2002

A general local dispersion relation is derived for the circulating electron-temperature-gradient-driven drift waves in an inhomogeneous magnetized electron–ion plasma. By taking specific limits, two new instabilities are shown to exist. The resulting quasilinear electron energy and particle fluxes are also obtained.
Frequency spectrum in drift wave turbulence
Authors: Mattor N. , Terry P. W.
Published: July 2002

The frequency spectrum of drift wave turbulence is examined, with emphasis on the underlying dynamics, the nonlinear frequency shift, and the reliability of analytical techniques. It is found that the frequency spectrum can depend on three different effects, each of which contribute both a broadening and shift to the spectrum. First is the usual ‘‘eddy-damping’’ mechanism, which is derived here from a direct interaction approximation (DIA) analysis. Spectral broadening from the eddy-damping mechanism tends to be important at longer wavelengths, and gives a frequency broadening and a downshift, coming, respectively, from the real and imaginary parts of the renormalized diffusivity. A second effect that contributes to the frequency spectrum is the incoherent nonlinear drive, revealed by a DIA solution but often neglected in Markovian approximations. The nonlinear drive also tends to be important at longer wavelengths, and contributes both to the spectral width (from broadband driving), and a frequency downshift (from off-frequency driving). A third effect comes from turbulent fluctuations of the density gradient, which add to the equilibrium density gradient and thereby cause fluctuations in the linear drift frequency. This mechanism tends to become important at short wavelengths, contributing both a frequency broadening as revealed by a DIA analysis, and a frequency upshift, as revealed by a weak turbulence analysis. The relation of these results to the fluctuation–dissipation theorem, and the impact on drift wave stability are discussed.
Neoclassical bootstrap current and transport in optimized stellarator configurations
Authors: Maassberg H. , Lotz W. , Nührenberg J.
Published: July 2002

The neoclassical bootstrap current properties of optimized stellarators are analyzed in the relevant mean-free-path regimes and compared with the neoclassical transport properties. Two methods—global Monte Carlo simulation [Phys. Fluids 31, 2984 (1988)], and local analysis with the drift kinetic equation solver code [Phys. Fluids B 1, 563 (1989)]—are employed and good agreement is obtained. Full consistency with the elimination of the bootstrap current and favorable neoclassical transport are found.
Isodynamical (omnigenous) equilibrium in symmetrically confined plasma configurations
Authors: Bernardin M. P. , Moses R. W. , Tataronis J. A.
Published: July 2002

Isodynamical or omnigenous equilibrium has the property that the magnitude of the magnetic field is constant on magnetic surfaces. It is shown that in plasma confinement configurations with one ignorable coordinate there are three possible classes of solutions, characterized by the properties of the curvature of the magnetic axis, the magnitude of the magnetic field on axis, and the closure of magnetic surfaces about the magnetic axis. Solutions belonging to class (i) have a straight magnetic axis, a finite field on axis, and closed magnetic surfaces. Solutions in class (ii) have a curved magnetic axis, closed magnetic surfaces, and a magnetic field that vanishes on axis. Finally, solutions in class (iii) have a curved magnetic axis, a finite magnetic field on axis, and open magnetic surfaces.
Equilibrium fluctuation energy of gyrokinetic plasma
Authors: Krommes John A. , Lee W. W. , Oberman C.
Published: July 2002

The thermal equilibrium electric field fluctuation energy of the gyrokinetic model of magnetized plasma is computed and found to be smaller than the well-known result 〈δE⋅δE〉(k)/8π= 1/2 T/[1+(kλD)2] valid for arbitrarily magnetized plasmas. It is shown that, in a certain sense, the equilibrium electric field energy is minimum in the gyrokinetic regime.
Comments on ‘‘Stability of the radial electric field in a nonaxisymmetric torus’’ [Phys. Fluids 2
7, 1567 (1984)]
Authors: Catto Peter J. , Myra J. R.
Published: July 2002

The validity of a stability argument in which only the ambipolar potential is perturbed to select a suitable solution when multiple ambipolar roots occur is questioned.
Plasma dynamics of the interaction of intense ion beams with ‘‘sub’’ and ‘‘super’’ range plane targets
Authors: Long K. A. , Tahir N. A.
Published: July 2002

Analytic and numerical solutions for the problem of the interaction of intense ion beams with matter in the form of plane targets are considered in this paper. The theory of the interaction of protons with matter at low energies is discussed and calculations are presented for the energy loss of protons in aluminum and gold. Zero- and one-dimensional models are developed and the results are compared to numerical simulations carried out with the one-dimensional Lagrangian hydrodynamic code medusa [Comp. Phys. Comm. 1, 271 (1974)], which has been extended to include the various physical effects needed to carry out realistic simulations of the interaction of ion beams with matter. The theory and simulation of the acceleration of foils by intense ion beams is also considered and representative results are given. The theoretical results are used to investigate the optimum conditions in which to carry out stopping power experiments for ions in hot, dense plasmas, so that the theory can be tested. These results are needed in order to perform more realistic pellet calculations for inertial fusion.
Formation of a compact torus by a rotating relativistic electron beam
Authors: Jain K. K. , John P. I.
Published: July 2002

A closed field-reversed configuration has been produced by a long pulse duration (≊ 500 nsec) intense electron beam injected into hydrogen gas. The field reversal is created by the beam current during the duration of beam propagation, but is sustained by the beam-induced plasma currents after the exit of the beam. Reconnection of the antiparallel field lines has been observed, which results in formation of a prolate spheroid. The axial extent of the spheroid is 80.0 cm. It contracts immediately after its formation with the supersonic velocity. Attempts were made to close axial currents along the closed poloidal field lines.
Anode-type double layers in a nonuniform magnetic field
Authors: Cartier Steven L. , Merlino Robert L.
Published: July 2002

The properties of strong, magnetized, three-dimensional double layers are studied. The double layers are produced by drawing a discharge to a large anode plate located in the diverging magnetic field region of a cylindrical argon discharge. If the anode voltage is sufficiently high, the electrons that are accelerated through the anode sheath may become sufficiently energetic to ionize the background neutral gas and transform the anode sheath into a strong double layer. The resulting conical-shaped structures, which extend outward from the plate, have parallel, oblique, and perpendicular electric field components with respect to the magnetic field. The axial extent of these structures depends on the plate bias voltage, neutral gas pressure, and the magnetic field. At neutral gas pressures of a few millitorr, the double-layer structures are visually apparent because of the enhanced light emission from neutrals excited by the energetic electrons. Color photographs of some of these structures are shown. The scaling of the width of these double layers with electric field components perpendicular to B is also investigated.
Saturation induction and radiation in wave-driven plasmas
Authors: Rax Jean M.
Published: July 2002

A study of some problems associated with noninductive current drive is presented. Both slow and fast waves, weak and strong power, and transient and steady-state responses are considered. The wave-induced deformation of the electron distribution function is expressed as a linear functional of the absorbed power. This expression is then used to study three important phenomena: saturation of the absorption, coupling with an inductive field, and wave-induced electron cyclotron emission.
On the spanwise correlation of vortex shedding from a circular cylinder at high subcritical Reynolds number
Authors: Szepessy Stefan
Published: July 2002

The three-dimensionality of the vortex shedding from a circular cylinder at high subcritical Reynolds number (4.3×104) has been studied, focusing on the characteristics of the vortex shedding phase drift and correlation along the cylinder span. Short time integration of the correlation coefficient, based on eight shedding cycles, showed that there were strong oscillations in the degree of correlation of the vortex shedding, and that these oscillations showed strong regularity having periods around 10–20 times the Strouhal period. These oscillations started when exceeding separations of Δz/d≊1 between the measurement points, and remained up to the largest case studied (Δz/d=6). The phase drift angle between two points at different spanwise positions was analyzed, showing that at the transition separation (Δz/d≊1) the probability distribution changed from being narrow banded to become more broad banded and closely Gaussian for an intermediate spacing of 2<Δz/d<4, and almost uniform for Δz/d=6. For all different separations studied (0.25<Δz/d<6), the probability distribution of the phase drift angle had a zero mean, indicating that the vortex shedding was, on the average, parallel to the cylinder.
On the transition of the cylinder wake
Authors: Zhang Hong-Quan , Fey Uwe , Noack Bernd R. , König Michael , Eckelmann Helmut
Published: July 2002

The transition of the cylinder wake is investigated experimentally in a water channel and is computed numerically using a finite-difference scheme. Four physically different instabilities are observed: a local ‘‘vortex-adhesion mode,’’ and three near-wake instabilities, which are associated with three different spanwise wavelengths of approximately 1, 2, and 4 diam. All four instability processes can originate in a narrow Reynolds-number interval between 160 and 230, and may give rise to different transition scenarios. Thus, Williamson’s [Phys. Fluids 31, 3165 (1988)] experimental observation of a hard transition is for the first time numerically reproduced, and is found to be induced by the vortex-adhesion mode. Without vortex adhesion, a soft onset of three-dimensionality is numerically and experimentally obtained. A control-wire technique is proposed, which suppresses transition up to a Reynolds number of 230.
On the curved wall jet influenced by system rotation and self-similar suction or blowing
Authors: Matsson O. John E.
Published: July 2002

In this paper we use local nonparallel linear stability theory to study the jet on a concave and convex wall with spanwise system rotation and self-similar suction or blowing. It is found for low negative rotation, i.e. the Coriolis force counteracts the centrifugal force, that the critical Goertler number Go is increased for both the concave and convex wall jet. For the convex wall jet the critical Go is increased up to eight times compared with the nonrotation case. In this region of negative rotation, the principle of exchange of instabilities does not hold for the convex wall jet. For high negative and positive rotation the flow is destabilized on both types of walls. Suction stabilizes the concave wall jet while the convex wall jet is destabilized. For blowing, the concave wall jet is destabilized to a certain limit and then stabilized for increased blowing. The convex wall jet is stabilized for blowing. The combined effects of curvature, system rotation, and self-similar suction or blowing show that the highest critical Go can be increased for the rotating concave wall jet for both suction and blowing. For the rotating convex wall jet the highest critical Go is increased for suction and decreased for blowing.
On fluctuations of vortex breakdown location
Authors: Gursul Ismet , Yang Houbin
Published: July 2002

Flow visualization and velocity measurements are performed in order to investigate the unsteady nature of vortex breakdown location over a delta wing. The results indicate that the fluctuations of vortex breakdown location occur at much lower frequencies than the frequency of the hydrodynamic instability of the flow in the wake of the vortex breakdown. It is suggested that the helical mode instability does not influence the unsteady nature of breakdown location.
The role of surface tension in the dominant balance in the die swell singularity
Authors: Salamon Todd R. , Bornside David E. , Armstrong Robert C. , Brown Robert A.
Published: July 2002

The two-dimensional, free-surface flow of a Newtonian fluid exiting from a planar die is computed by finite element analysis using quasiorthogonal mesh generation and local mesh refinement with irregular, embedded elements to obtain extreme resolution of the velocity and pressure fields near the die edge, where the fluid sheet attaches to the solid boundary. Calculations for the limit of large surface tension, the stick-slip problem, reproduce the singular behavior near the die edge expected from asymptotic analysis using a self-similar form for the velocity field. Results for finite capillary number (Ca) predict that the meniscus separates from the die at a finite contact angle and suggest that the capillary force enters the dominant normal stress balance at the die edge through an infinite curvature, as previously suggested by Schultz and Gervasio. The size of this region with large positive curvature increases with increasing Ca, and the strength of the singularity is in good agreement with theoretical predictions for a straight meniscus attached to the die at the appropriate contact angle predicted by the simulations. The contact angle appears to be determined from matching of the inner solution structure valid near the singularity with the bulk flow, in agreement with arguments made by Ramalingam; increasing the Reynolds number decreases the contact angle, corroborating this effect. Introducing fluid slip along the surface of the die changes the structure of the singularity in the pressure and stresses, but does not alleviate the singular behavior. In fact, the calculations with slip coefficients small enough not to change the bulk solution are more difficult than calculations with the no-slip boundary condition.
A two-phase flow model of the Rayleigh–Taylor mixing zone
Authors: Chen Yupin , Glimm James , Sharp David H. , Zhang Qiang
Published: July 2002

The Rayleigh–Taylor instability of an interface separating fluids of distinct density is driven by an acceleration across the interface. Low order statistical moments of fluctuating fluid quantities characterize the hydrodynamics of the mixing zone. A new model is proposed for the momentum coupling between the two phases. This model is validated against computational data for compressible flows, including flows near the incompressible limit. Our main result is a zero parameter first order closure for ensemble averaged two phase flow equations. We do not, however, fully solve the closure problem, as the equations we derive are missing an (internal) boundary condition along any surface for which either phase goes to zero volume fraction. In this sense, the closure problem is reduced from a volume to a surface condition, rather than being solved completely. We compare two formulations of the statistical moments, one based on two phase flow and the other on turbulence models. These formulations describe different aspects of the mixing process. For the problem considered, the two phase flow moments appear to be preferable, in that they subsume the turbulence moments but not conversely.
Hydrodynamics of monolayer domains at the air–water interface
Authors: Lubensky David K. , Goldstein Raymond E.
Published: July 2002

Molecules at the air–water interface often form inhomogeneous layers in which domains of different densities are separated by sharp interfaces. Complex interfacial pattern formation may occur through the competition of short- and long-range forces acting within the monolayer. The overdamped hydrodynamics of such interfacial motion is treated here in a general manner that accounts for dissipation both within the monolayer and in the subfluid. Previous results on the linear stability of interfaces are recovered and extended, and a formulation applicable to the nonlinear regime is developed. A simplified dynamical law valid when dissipation in the monolayer itself is negligible is also proposed. Throughout the analysis, special attention is paid to the dependence of the dynamical behavior on a characteristic length scale set by the ratio of the viscosities in the monolayer and in the subphase.
Inviscid and inviscid-limit behavior of a surface quasigeostrophic flow
Authors: Ohkitani Koji , Yamada Michio
Published: July 2002

The growth of the gradient of a scalar temperature in a quasigeostrophic flow is studied numerically in detail. We use a flow evolving from a simple initial condition which was regarded by Constantin et al. as a candidate for a singularity formation in a finite time. For the inviscid problem, we propose a completely different interpretation of the growth, that is, the temperature gradient can be fitted equally well by a double-exponential function of time rather than an algebraic blowup. It seems impossible to distinguish whether the flow blows up or not on the basis of the inviscid computations at hand. In the viscous case, a comparison is made between a series of computations with different Reynolds numbers. The critical time at which the temperature gradient attains the first local maximum is found to depend double logarithmically on the Reynolds number, which suggests the global regularity of the inviscid flow.
Nonlinear Lagrangian equations for turbulent motion and buoyancy in inhomogeneous flows
Authors: Heinz Stefan
Published: July 2002

Linear and nonlinear Lagrangian equations are derived for stochastic processes that appear as solutions of the averaged hydrodynamic equations, since their moments satisfy the budgets given by these equations. These equations include the potential temperature, so that non-neutral flows can be described. They will be compared with nonlinear and non-Markovian equations that are obtained using concepts of nonequilibrium statistical mechanics. This approach permits the description of turbulent motion and buoyancy, where memory effects and driving forces with arbitrary colored noise may occur. The equations depend on assumptions that concern the dissipation and pressure redistribution. In the approximations of Kolmogorov and Rotta for these terms, the dissipation time scale remains open, which can be determined by the calculation of the production–dissipation ratio of turbulent kinetic energy. The features of these equations are illustrated by the calculation of turbulent states in the space of invariants.
On binary impacts of small liquid-filled shells
Authors: Louge Michel Y. , Tuozzolo Christopher , Lorenz Adam
Published: July 2002

We report the peculiar impact properties of small spherical shells filled with a viscous liquid. Upon collisions of two identical liquid-filled shells, the fluid is progressively set in rotation by the shell spin induced by tangential impact forces. An analysis of the corresponding fluid motion predicts a collision outcome unlike that of solid spheres where angular velocity is uniform. Observations of colliding vitamin-E pills reveal that the point of contact is rarely involved in gross slip. In the direction of the line of centers, collisions are adequately described by a kinematic restitution coefficient. In the perpendicular direction, they generally exhibit rolling contact.
Instability of a liquid sheet of parabolic velocity profile
Authors: Ibrahim E. A.
Published: July 2002

A power series mathematical solution is advanced for the problem of instability of an inviscid liquid sheet of parabolic velocity profile emanated from a nozzle into an inviscid gas. A comparison of the instability of a sheet of parabolic velocity profile with one of a uniform velocity profile is performed in order to deduce the effects of velocity profile relaxation on instability. The results show that for both antisymmetrical and symmetrical disturbances departure from uniformity of the velocity profile causes the instability to be reduced. The physical interpretation and practical implications of the present findings are discussed.
Velocity measurements in turbulent gaseous mixtures induced by Richtmyer–Meshkov instability
Authors: Poggi Françoise , Thorembey Marie-Hélène , Rodriguez Gérard
Published: July 2002

Instantaneous velocity measurements in a gaseous mixture arising from the shock wave-induced Richtmyer–Meshkov instability are conducted for the first time in a shock tube. Laser Doppler anemometry gives us the evolution of the axial velocity fluctuations of the mixing zone, before and after its interactions with reflected shock waves from the shock tube end wall. Experimental results demonstrate that a turbulent mixing zone is generated by the incident shock wave. Afterwards, the axial variance decreases before being amplified by the first reshock interaction through a baroclinic effect. Before the second reshock arrival, we measure once again a decrease of the turbulence level which is explained by both diffusion and dissipation.
The growth of leading-edge distortions on a viscous sheet
Authors: Hocking L. M. , Debler W. R. , Cook K. E.
Published: July 2002

The results of a set of experiments to determine some features of the undulations that develop on the leading edge of a sheet of fluid on an inclined plane are presented. A range of fluid viscosities, fluid volumes, and plate angles was used. In nearly all the cases, the observed disturbances had a triangular or sawtooth shape, with only a single example of a finger or parallel-sided shape appearing. The power-law exponents for the position down the plate of both the tips of the disturbances and their roots, that is, the points where they join the uniform sheet above them, were calculated from a series of photographs, and the corresponding wavelengths measured. The exponents are broadly in line with those that can be deduced from a simple model including viscosity, gravity, and volume flux, and ignoring all capillary effects. This conclusion suggests that the criterion for distinguishing the two types of disturbance does not depend on the global dynamics of the developing structures, and that a detailed analysis of the tip and root regions, where capillarity will be significant, is needed for further progress to be made.
The rear meniscus of a long bubble steadily displacing a Newtonian liquid in a capillary tube
Authors: Giavedoni María D. , Saita Fernando A.
Published: July 2002

In this work the interfacial shapes and the flow occurring at the trailing meniscus of a long bubble is numerically analyzed. The technique employed solves the complete set of governing equations simultaneously. The numerical results reported complete previous descriptions of the creeping flow regime; the influence of the inertia forces on the free surface shapes, interfacial undulations, and flow patterns is also analyzed.
Spreading of a wetting film under the action of van der Waals forces
Authors: Pismen Len M. , Rubinstein Boris Y. , Bazhlekov Ivan
Published: July 2002

The profiles of a spreading wetting film are computed using a variable grid implicit scheme. The form of Tanner’s law is deduced from the scaling, and the dependence of its coefficient on ratio of the van der Waals to capillary length and on the inclination angle is determined.
Shock interactions with heavy gaseous elliptic cylinders: Two leeward-side shock competition modes and a heuristic model for interfacial circulation deposition at early times
Authors: Ray Jaideep , Samtaney Ravi , Zabusky Norman J.
Published: July 2002

We identify two different modes, types I and II, of the interaction for planar shocks accelerating heavy prolate gaseous ellipses. These modes arise from different interactions of the incident shock (IS) and transmitted shock (TS) on the leeward side of the ellipse. A time ratio tT/tI(M,η,λ,γ0,γb), which characterizes the mode of interaction, is derived heuristically. Here, the principal parameters governing the interaction are the Mach number of the shock (M), the ratio of the density of the ellipse to the ambient gas density, (η>1), γ0, γb (the ratios of specific heats of the two gases), λ (the aspect ratio). Salient events in shock–ellipse interactions are identified and correlated with their signatures in circulation budgets and on-axis space–time pressure diagrams. The two modes yield different mechanisms of the baroclinic vorticity generation. We present a heuristic model for the net baroclinic circulation generated on the interface at the end of the early-time phase by both the IS and TS and validate the model via numerical simulations of the Euler equations. In the range 1.2⩽M⩽3.5, 1.54⩽η⩽5.04, and λ=1.5 and 3.0, our model predicts the baroclinic circulation on the interface within a band of ±10% in comparison to converged numerical simulations.
On transport-driven currents
Authors: Ma Sifeng , Dawson John M.
Published: July 2002

An empirical electron viscosity is incorporated into Ohm’s law to examine the effect of current diffusion on bootstrap currents. It is shown that for sufficiently large viscosity, no seed current is needed for steady state operation of tokamaks. This is expressed by the scaling law ΓR2m=4/27 where Rm is the magnetic Reynolds number and Γ is viscosity normalized to νeia2 (νei is the electron–ion collision frequency and a is the system size).
Laser diagnostic experiments on KrF laser ablation plasma-plume dynamics relevant to manufacturing applications*
Authors: Gilgenbach R. M. , Ching C. H. , Lash J. S. , Lindley R. A.
Published: July 2002

A brief review is given of the potential applications of laser ablation in the automotive and electronics manufacturing industries. Experiments are presented on KrF laser ablation of three materials relevant to manufacturing applications: aluminum metal vs aluminum–nitride (AlN) and alumina (Al2O3) ceramics. Plasma and neutral-atom diagnostic data are presented from resonant-holographic-interferometry, dye-laser-resonance-absorption photography, and HeNe laser deflection. Data show that plasma electron densities in excess of 1018 cm−3 exist in the ablation of AlN, with lower densities in Al and Al2O3. Aluminum neutral and ion expansion velocities are in the range of cm/μs. Ambipolar electric fields are estimated to be 5–50 V/cm.
Power transfer and current generation of fast ions with large-kθ waves in tokamak plasmas
Authors: Heikkinen J. A. , Sipilä S. K.
Published: July 2002

The direction and magnitude of power and momentum exchange between fast ions and electrostatic waves in slab and toroidal systems are obtained from global Monte Carlo simulations that include the quasilinear wave-induced ion diffusion both in velocity space and through a radially localized (lower hybrid) wave structure with propagation in one preferential poloidal direction in tokamaks. The model considers a full linearized collision model, finite fast ion orbits, and losses in toroidal geometry, and can properly treat the boundary effects on the particle–wave interaction in the configuration space. For an isotropic steady ion source, reduction of wave Landau damping but no wave amplification by wave localization is found for a Gaussian wave intensity distribution in radius, irrespective of the steepness of the radial gradient of the fast ion source rate. Enhanced wave-driven fast ion current, with magnitude, direction, and profile determined by the boundary conditions, net power transfer, and fast ion radial transport, is found to follow from the asymmetry in the parallel wave number spectrum created by the finite poloidal magnetic field. In the presence of intense well-penetrated waves the current carried by fusion α particles can be controlled by the choice of the poloidal wave number spectrum and the total current can greatly exceed the neoclassical bootstrap current of the α particles in a reactor.
Plateau regime dynamics of the relaxation of poloidal rotation in tokamak plasmas
Authors: Lebedev V. B. , Yushmanov P. N. , Diamond P. H. , Novakovskii S. V. , Smolyakov A. I.
Published: July 2002

A theory of the relaxation dynamics of the radial electric field toward its neoclassical value in the regime of subsonic poloidal rotation is presented. It is shown that the relaxation occurs via damped oscillations on time scales proportional to the ion transit time.
A hybrid Eulerian Vlasov code. I. Study of high-frequency beatwave experiment and Manley–Rowe action evolution in a finite causal system
Authors: Ghizzo A. , Bertrand P. , Lebas J. , Johnston T. W. , Shoucri M.
Published: July 2002

For the first time, beatwave simulations relevant to the UCLA experiment (University of California at Los Angeles) [see Clayton et al., Phys. Rev. Lett. 70, 37 (1993), and also Phys. Plasmas 1, 1753 (1994)] have been made with a relativistic Eulerian Hilbert–Vlasov code for a realistically high ratio of driver frequency to plasma wave frequency (ω0/ωp≊30). Some of the more striking features that have emerged from the Hilbert–Vlasov simulations are discussed in this paper, with particular emphasis on particle dynamics in phase space with beam injection, and action transfer results obtained from the derivation of the integrated Manley–Rowe relations derived for a finite causal system.
Momentum transport and flow damping in the reversed-field pinch plasma
Authors: Almagri A. F. , Chapman J. T. , Chiang C. S. , Craig D. , Den Hartog D. J. , Hegna C. C. , Prager S. C.
Published: July 2002

A biased electrode is used in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] to manipulate plasma flow in order to study flow damping and momentum transport. Finite radial conductivity allows a radial current, which provides the toroidal torque that spins up the plasma. The applied torque is balanced by a viscous force that opposes toroidal flow acceleration. From the plasma flow damping the viscosity is inferred to be anomalous. The radial transport of toroidal momentum is comparable to that of particles and energy, and is consistent with transport by stochastic magnetic field lines.