bluuebunny/crossref_metadata_2025 huggingface dataset:
The Rayleigh–Taylor and Kelvin–Helmholtz stability of a viscous liquid–vapor interface with heat and mass transfer
Authors: Adham-Khodaparast K. , Kawaji M. , Antar B. N.
Published: July 2002

Linear stability analysis of a liquid–vapor interface under adverse gravitational field and velocity streaming is considered. The liquid is assumed viscous, incompressible, and motionless over a vapor layer with a uniform horizontal velocity. It is shown that while the coupled viscosity-phase change mechanism of former studies adds considerably to the stability of the Rayleigh–Taylor problem, it has a deleterious effect on the Kelvin–Helmholtz mode of stability.
Decay of dipolar vortex structures in a stratified fluid
Authors: Flór J. B. , van Heijst G. J. F. , Delfos R.
Published: July 2002

In this paper the viscous decay of dipolar vortex structures in a linearly stratified fluid is investigated experimentally, and a comparison of the experimental results with simple theoretical models is made. The dipoles are generated by a pulsed horizontal injection of fluid. In a related experimental study by Flór and van Heijst [J. Fluid Mech. 279, 101 (1994)], it was shown that, after the emergence of the pancake-shaped vortex structure, the flow is quasi-two-dimensional and decays due to the vertical diffusion of vorticity and entrainment of ambient irrotational fluid. This results in an expansion of the vortex structure. Two decay models with the horizontal flow based on the viscously decaying Lamb–Chaplygin dipole, are presented. In a first model, the thickness and radius of the dipole are assumed constant, and in a second model also the increasing thickness of the vortex structure is taken into account. The models are compared with experimental data obtained from flow visualizations and from digital analysis of particle-streak photographs. Although both models neglect entrainment and the decay is modeled by diffusion only, a reasonable agreement with the experiments is obtained.
Vortex merging and spectral cascade in two-dimensional flows
Authors: Nielsen A. H. , He X. , Rasmussen J. Juul , Bohr T.
Published: July 2002

The merging of two identical vortices is studied numerically using a spectral code. It is noted that the enstrophy cascade is most active on the distorted vortex boundaries, with a Kolmogorov-like spectrum E(k)≊k−α, α≤4, developed at high wave numbers. The inverse energy cascade is completed when the vortices merge into one of larger size.
Intercluster interactions in rapid granular shear flows
Authors: Tan M-L. , Goldhirsch I.
Published: July 2002

One of the possible phases of a sheared system of inelastically colliding rigid smooth disks is one in which relatively dense strips aligned at 45° to the streamwise direction are interspersed among similarly aligned dilute strips. The dense strips may have secondary microstructures in the form of elongated clusters. The latter are formed by an instability, following which they are convected, stretched, and rotated by the shear field. This process causes cluster–cluster collisions, a result of which is the partial destruction of the colliding clusters, followed by the emergence of new clusters. In addition, it is demonstrated that clustering dynamics can be responsible for hysteresis and multistability in granular systems. The studies presented in this paper involve molecular dynamics simulations complemented by theoretical analysis.
Vorticity amplification in incompressible ideal swirling flow without a boundary
Authors: Carmack Lori A.
Published: July 2002

We numerically address the question of finite time singularity development in three-dimensional incompressible ideal fluids. We consider the axisymmetric with swirl case that allows for higher resolution, using a high-order accurate ENO (essentially nonoscillatory) scheme. We use numerical boundary conditions that correspond to a fluid without a physical boundary and investigate the role an imposed physical boundary plays on the results of the computation. In this study it is found that the maximum norm of the vorticity grows at such a rate as to support the conjecture that finite time singularities occur in such flows, and that the vorticity experiences significantly greater amplification when the simulation is performed within a container.
On recent intermittency models of turbulence
Authors: Boratav Oluş N.
Published: July 2002

Two recent intermittency models by Chen and Cao [Phys. Rev. E 52, R5757 (1995)] and Nelkin [Phys. Rev. E 52, R4610 (1995)] stemming from Novikov’s criticism [Phys. Rev. E 50, R3303 (1994)] on the She–Lévêque model [Phys. Rev. Lett. 72, 336 (1994)] are discussed. Both models are rewritten using μ6 (deviation from Kolmogorov 1941 of the sixth order structure function exponent) values computed numerically (=0.23 and 0.43 using longitudinal and lateral velocity differences). Scaling exponents are compared.
Motion of a permeable sphere at finite but small Reynolds numbers
Authors: Feng Zhi-Gang , Michaelides Efstathios E.
Published: July 2002

The problem of the motion of a porous sphere in a viscous fluid has three pertinent characteristic times: two for the external flow field of the viscous fluid and a third one for the internal flow field, inside the porous material. Because of this, a singular perturbation method must be used to obtain an analytical solution to the governing differential equations and for the determination of the flow field outside the porous sphere. Such a method is used here, and a solution is obtained, by using the so-called Saffman boundary condition at the interface between the porous sphere and the outside fluid. This solution is valid at finite but small Reynolds numbers. Thus, general expressions for the hydrodynamic force acting on the porous sphere and, hence, for the drag coefficient of the sphere are obtained. This general expression yields, as special cases, other known expressions for the drag coefficients, which were derived under more restrictive conditions, such as creeping flow, no-slip boundary conditions or zero permeability (solid) spheres.
Stationary and time dependent convection in the rotating cylindrical annulus with modulated height
Authors: Herrmann J. , Busse F. H.
Published: July 2002

The onset of convection driven by centrifugal buoyancy in the annular region between two coaxial corotating cylinders is investigated when the height of the annular region not only depends on the distance from the axis as in most previous works on the problem but on the azimuthal coordinate as well. The onset of convection in the form of stationary flows or in the form of thermal Rossby waves is investigated in dependence on the parameters of the problem. The evolution of the various modes with increasing amplitude of motion is followed through integrations in time of the fully nonlinear basic equations. Various forms of periodic, quasiperiodic and aperiodic time dependence are found.
The nonlinear evolution of centrifugal instabilities in curved, compressible mixing layers
Authors: Owen D. J. , Seddougui S. O. , Otto S. R.
Published: July 2002

The nonlinear development of Görtler vortices in a curved compressible mixing layer is studied. It has been shown both experimentally and theoretically that the curved mixing layer can support a centrifugal mode, which is believed to be similar to the Görtler vortex. This study follows the corresponding incompressible study of Seddougui & Otto [Proceedings of the IUTAM Symposium on Nonlinear Instability and Transition in Three-Dimensional Boundary Layers, edited by P. W. Duck and P. Hall (Kluwer Academic, Amsterdam, 1995)] and attempts to demonstrate the effect compressibility has on the growth of such modes. The formulation follows that of Hall & Lakin [Proc. R. Soc. London, Ser. A 415, 421 (1988)] for the corresponding incompressible boundary-layer problem, and utilises a stream function formulation to solve the governing equations. We will present the nonlinear structure of the so-called thermal modes, whose linear counterparts were first discovered in Owen, Seddougui & Otto [Phys. Fluids 9, 2506 (1997)]. A brief discussion will be made concerning the effect of the nonlinear modes on the inherent Kelvin–Helmholtz modes of instability, which may have significant ramifications in flows dominated by this form of instability.
Interactions between contacting fibers
Authors: Petrich Michael P. , Koch Donald L.
Published: July 2002

To investigate the nature of the forces involved in mechanical contact between fibers in a fluid, the interaction between a polymeric fiber settling under the influence of gravity and a fixed strand of the same material was observed. Initially after impact, the sedimenting fiber rotated about a fixed point. When the fiber came sufficiently close to vertical alignment so that the tangential component of gravity overcame the static friction, it slid along the surface of the fixed strand. The orientation of the fiber and its velocity were measured as a function of time, and compared to a simple model. The static coefficient of friction was found to be 0.38±0.06, in good agreement with published values. However, the velocity of the fiber after the onset of sliding was slower than that predicted by a theory incorporating friction and smooth cylinder lubrication, and an additional velocity-dependent resistance was required to model the fiber’s sliding motion. A possible source of this resistance is the coupling of normal and tangential lubrication forces as roughness elements are forced to lift past each other.
Numerical study of rotating turbulence with external forcing
Authors: Yeung P. K. , Zhou Ye
Published: July 2002

Direct numerical simulations at 2563 resolution have been carried out to study the response of isotropic turbulence to the concurrent effects of solid-body rotation and stochastic, isotropic forcing at the large scales. Because spectral transfer to the smaller scales is weakened by rotation, energy input from forcing gradually builds up at the large scales, causing the overall kinetic energy to increase. At intermediate wave numbers the energy spectrum undergoes a transition from a limited k−5/3 inertial range to k−2 scaling recently predicted in the literature. Although the Reynolds stress tensor remains approximately isotropic and three-component, evidence for anisotropy and quasi-two-dimensionality in length scales and spectra in different velocity components and directions is strong. The small scales are found to deviate from local isotropy, primarily as a result of anisotropic transfer from the large scales. To understand the spectral dynamics of this flow, we study the detailed behavior of nonlinear triadic interactions in wave number space. Spectral transfer in the velocity component parallel to the axis of rotation is qualitatively similar to that in nonrotating turbulence; however, the perpendicular component is characterized by a much weakened energy cascade at high wave numbers and a local reverse transfer at the largest scales. The broader implications of this work are briefly addressed.
Inverse equations
Authors: Keller Jakob J.
Published: July 2002

Following previous work by Keller [“Inverse Euler equations,” Z. Angew. Math. Phys. 49, 363 (1998)], that is extended to nonconservative flows, the general time-independent flow equations are first written in a perfectly antisymmetric form, using a pair of stream functions as the dependent variables. In a second step the equations are written in an inverse form, using the two stream functions and the natural coordinate as independent variables. The special cases of incompressible flow and inviscid axisymmetric flow are also considered. The main advantage of using these inverse equations is associated with the possibilities of using static pressure distributions, mach number distributions, geometric constraints, etc., or any combination of geometric constraints and specifications of physical quantities to define the boundary conditions. In contrast to conventional inverse methods, that are based on iterative approximations to a desired pressure distribution along the surface of a flow device, for example, the use of inverse equations offers the possibility to arrive at the solution for any kind of boundary conditions in a single step. Furthermore, there is no need for complicated grid generation procedures, because the domain of definition in inverse space is typically a cube with Cartesian coordinates.
Formation of cusp on the free surface at low Reynolds number flow
Authors: Jeong Jae-Tack
Published: July 2002

Free surface deformation and cusp formation are analyzed by considering a viscous flow due to a vortex dipole immersed beneath the free surface, with the direction and strength of the vortex dipole arbitrary. In the analysis, the Stokes’ approximation is used and surface tension effects are included, but gravity is neglected. The solution is obtained analytically by using conformal mapping and complex variable techniques. From the solution, deformations of the free surface are shown and the formation of a cusp on the free surface are discussed. As the capillary number grows, it is known that the radius of curvature at the converging point on the free surface becomes extremely small, which means that in a real fluid a cusp should form on the free surface above some critical capillary number. For a particular range of vortex dipole inclination, however, there exists no cusp, even for infinite capillary numbers. Typically, streamline patterns for some capillary numbers of horizontal vortex dipole are also shown.
Turbulent force as a diffusive field with vortical sources
Authors: Wu Jie-Zhi , Zhou Ye , Lu Xi-Yun , Fan Meng
Published: July 2002

In Reynolds-average Navier–Stokes equation it is the divergence of Reynolds stress tensor, i.e., the turbulent force, rather than the tensor itself, is to be simulated and partially modeled. Thus, directly working on turbulent force could bring significant simplification. In this paper a novel exact equation for incompressible turbulent force f is derived: (∂/∂t −ν∇2)f=∇⋅S, where ν is the molecular viscosity and all source terms in tensor S to be modeled are vortical. The dominant mechanism is the advection and stretching (with an opposite sign) of a “pseudo-Lamb vector” by fluctuating velocity field. No coupling with pressure is involved. The equation follows from a study of the mean fluctuating Lamb vector and kinetic energy, which constitute the turbulent force. Both constituents are governed by the same kind of equations as f. This innovative turbulent-force equation is similar to Lighthill’s acoustic analogy and naturally calls one’s attention to studying the vortical sources of turbulent force. The methodology described here may lead to turbulence models which provide more complete treatment than that of two-equation models, but relatively easier computation than that of second-order closures.
A dynamical model for turbulence. VIII. IR and UV Reynolds stress spectra for shear-driven flows
Authors: Canuto V. M. , Dubovikov M. S. , Yu G.
Published: July 2002

The basic equations for the two-point Reynolds stresses derived in paper II are solved analytically in two regimes: the UV (ultraviolet) region corresponding to the inertial range and the IR (infrared) region corresponding to k→0. The analytic treatment is possible due to the existence of two smallness parameters: Ui,j(k2νt)−1 in the UV region and kL in the IR region; Ui,j is the mean velocity gradient, νt(k) is the turbulent viscosity, and L is the integral length scale. For an arbitrary flow, the Reynolds stress spectrum in the UV region is given by Eqs. (53545556575859). In the IR region, and in the first-order approximation in kL, the spectra coincide with those of the rapid distortion theory. Since they are flow dependent, we shall discuss a few representative cases. The resulting Reynolds stress spectra, which are shown to reproduce existing data, are the basis for the calculation of the one-point Reynolds stresses to be presented in paper IX. The model has no free parameters.
Propagation and reflection of internal waves
Authors: Sutherland B. R.
Published: July 2002

Fully nonlinear numerical simulations are performed to examine the behavior of large-amplitude internal gravity waves incident upon a level where the Doppler-shifted frequency of the waves is comparable to the background buoyancy frequency. Although linear theory predicts that the waves should reflect if the Doppler-shifted frequency is greater than the buoyancy frequency, it is found that nonlinear effects may greatly enhance the transmission of a wave packet across a reflecting level. If the Doppler-shifted frequency is moderately less than the buoyancy frequency, then nonlinear effects may greatly enhance the reflection of waves. A range of simulations is performed to characterize the reflection coefficient as a function of the amplitude and spatial extent of the wave packet. In comparison with horizontally periodic wave packets, it is found that the nonlinearly enhanced transmission of wave packets is more significant if they are horizontally compact. This occurs because the wave-induced mean flow effectively increases and decreases the horizontal phase speed of the waves on the incident and trailing flank of the wave packets, respectively, and this significantly broadens the frequency spectrum of the waves.
Effects of helicity on Lagrangian and Eulerian time correlations in turbulence
Authors: Rubinstein Robert , Zhou Ye
Published: July 2002

Taylor series expansions of turbulent time correlation functions are applied to show that although to second order, helicity does not affect Lagrangian time correlations, the corresponding helicity effect on Eulerian time correlations is nonzero. This result is applied to two problems in which Eulerian time correlations are relevant: sound radiation and scattering by turbulence. Helicity shifts radiated sound to lower frequencies and increases the angle between scattered and incident sound waves.
Universal properties of chaotic transport in the presence of diffusion
Authors: Rom-Kedar V. , Poje A. C.
Published: July 2002

The combined, finite time effects of molecular diffusion and chaotic advection on a finite distribution of scalar are studied in the context of time periodic, recirculating flows with variable stirring frequency. Comparison of two disparate frequencies with identical advective fluxes indicates that diffusive effects are enhanced for slower oscillations. By examining the geometry of the chaotic advection in both high and low frequency limits, the flux function and the width of the stochastic zone are found to have a universal frequency dependence for a broad class of flows. Furthermore, such systems possess an adiabatic transport mechanism which results in the establishment of a “Lagrangian steady state,” where only the asymptotically invariant core remains after a single advective cycle. At higher frequencies, transport due to chaotic advection is confined to exchange along the perimeter of the recirculating region. The effects of molecular diffusion on the total transport are different in these two cases and it is argued and demonstrated numerically that increasing the diffusion coefficient (in some prescribed range) leads to a dramatic increase in the transport only for low frequency stirring. The frequency dependence of the total, long time transport of a limited amount of scalar is more involved since faster stirring leads to smaller invariant core sizes.
Comparison of Burnett and DSMC predictions of pressure distributions and normal stress in one-dimensional, strongly nonisothermal gases
Authors: Mackowski Daniel W. , Papadopoulos Dimitrios H. , Rosner Daniel E.
Published: July 2002

The Burnett equations have been shown to provide improved descriptions, relative to the Navier–Stokes equations, of flow structure in high-velocity (i.e., hypersonic) gases. We examine here the accuracy of the Burnett constitutive equation for fluid stress as applied to stationary gases. Specifically, we investigate the effects of “thermal stress” (fluid stress induced by a temperature gradient), as predicted by the Burnett equation, on the pressure distributions and normal stress in a stationary, buoyancy-free, hard-sphere gas for the case of one-dimensional heat transfer. We show, using first-law principles and the Burnett equation, that thermal stress results in a reduction in normal stress in the nonisothermal gas relative to that in the equilibrium state. The normal stress, in turn, can be obtained as an eigenvalue to a second-order ordinary differential equation, representing the Burnett equation, for the pressure distribution in the gas. Simple asymptotic solutions to the Burnett equation are developed, and are used in combination with order-Kn pressure slip relations to formulate pressure boundary conditions at the heated and cooled surfaces. The approximate solutions, as well as exact numerical calculations, are compared with pressure distributions generated from the direct-simulation Monte Carlo (DSMC) method. The Burnett and DSMC predictions of pressure are in good agreement for effective Knudsen numbers (based on the temperature gradient in the gas) less than 0.1. In particular, the Burnett equations can provide a reasonable description of the Knudsen (or rarefaction) layers adjacent to the heated and cooled surfaces that bound the gas, and can also describe the variation in pressure in the bulk gas. In addition, theoretical predictions of the reduction in normal stress correspond well to DSMC-derived values.
Evolution and convection of large-scale structures in supersonic reattaching shear flows
Authors: Smith K. M. , Dutton J. C.
Published: July 2002

Double-pulsed Mie scattering studies were performed to characterize the evolution of large-scale structures embedded within a planar supersonic base flow. Images were obtained at several streamwise stations along the shear layers, at reattachment, and in the near-wake regions. From these time-correlated images, the evolution characteristics of the large-scale structures were examined over a range of nondimensional time delays, as defined by local integral length and velocity scales. The double-pulsed images indicated that for short time delays (i.e., less than the representative eddy rollover time), the structures exhibited a simple translation in the streamwise direction. As the time delay was increased, rotation and elongation of the structures were observed in addition to the translation feature. Time delays that appreciably exceeded the local eddy rollover time generally resulted in a dramatic loss of structure identity. No eddy interactions, such as pairing, were observed at any of the imaging locations. Images obtained near reattachment provided evidence of shocklets moving in concert with the local eddies. In the initial portions of the shear layers, the mean convection velocity was measured to be significantly higher than the isentropic estimate, which is consistent with the results of previous convection velocity studies using mixing layers composed of supersonic/subsonic freestream combinations. The eddies decelerate through the recompression and reattachment regions, presumably due to the influence of the adverse pressure gradient. Downstream of reattachment, the large-scale structures accelerate as the wake develops.
Application of mapping closure to non-Gaussian velocity fields
Authors: Takaoka M.
Published: July 2002

An attempt is made to assess the mapping closure for turbulent velocity fields with non-Gaussian probability density functions (PDFs). The a priori assumption of the Gaussian velocity statistics is not made in the present mapping closure, which generalizes the results for the Burgers turbulence by Kraichnan [Phys. Rev. Lett. 65, 575 (1990)] and Gotoh and Kraichnan [Phys. Fluids A 5, 445 (1993)]. The PDFs calculated from the mapping closure are compared to the results obtained from the direct numerical simulation for non-Gaussian initial fields. The statistics of second- and higher-order derivative terms are discussed also.
Large-scale properties of passive scalar advection
Authors: Balkovsky E. , Falkovich G. , Lebedev V. , Lysiansky M.
Published: July 2002

We consider statistics of the passive scalar on distances much larger than the pumping scale. Such statistics is determined by statistics of Lagrangian contraction, that is by probabilities of initially distant fluid particles coming close. At the Batchelor limit of spatially smooth velocity, the breakdown of scale invariance is established for scalar statistics.
Macroscopic structures of inhomogeneous, Navier–Stokes turbulence
Authors: Turner Leaf
Published: July 2002

Macroscopic properties of inhomogeneous, Navier–Stokes turbulence are calculated using a fundamental theoretical approach. Starting from a set of solenoidal basis vectors suitable for describing a turbulence bounded by infinite, parallel free-slip planes, invoking a random-phase approximation, and using the eddy-damped quasinormal Markovian closure, various quantities of dynamical interest are calculated. Quantities that affect the evolution of both the momentum density and the kinetic energy density of the turbulently evolving fluid are studied. These quantities include the ensemble-averaged values of gradients of the triple-velocity correlation and the pressure–velocity correlation. When the energy spectrum in wave-number space lacks certain reflection symmetries, a nontrivial mean flow velocity will be seen to emerge out of the turbulent eddies. In turn, these mean flow velocities will affect the evolution of the turbulence. Of course, such physically interesting mean-field structures cannot arise in dynamics suffused by the assumption of statistical homogeneity.
The decay of grid turbulence in polymer and surfactant solutions
Authors: van Doorn Eric , White Christopher M. , Sreenivasan K. R.
Published: July 2002

The decay of turbulence behind a towed grid is studied in polymer and surfactant solutions with the use of particle image velocimetry. Unlike in water, the turbulent energy components show marked anisotropy, and decay more slowly. These differences are stronger for initial periods of time, but persist through the entire period of decay. The major difference between the polymer and surfactant solutions is that the small scales are more strongly damped in the former.
Up to the unsteadiness of axisymmetric thermocapillary flows in a laterally heated liquid bridge
Authors: Kasperski G. , Batoul A. , Labrosse G.
Published: July 2002

Axisymmetric flows of fluids of various Prandtl numbers, Pr, in a cylindrical and undeformable liquid bridge submitted to a lateral heating are studied in a zero gravity environment, up to their transitions to unsteadiness. The governing equations are solved numerically by a pseudospectral scheme. The results depend on Pr, in a nonmonotonic fashion, as, for instance, the threshold of the onset to unsteadiness. It has always been reached, except for our Pr=1 experiment, run up to a Marangoni number value Ma=106. The steady flows at very high Reynolds numbers (up to 107) offer a rich structure. A first incursion into unsteadiness shows different flow structure evolutions, depending on whether Pr is smaller or larger than one.
Inflow conditions for large-eddy simulations of mixing layers
Authors: Li N. , Balaras E. , Piomelli U.
Published: July 2002

A method to generate inflow boundary conditions for large-eddy simulations (LES) of turbulent free shear layers is presented. A time series of instantaneous velocity planes, with duration approximately equal to the integral time scale of the flow, is extracted from a periodic simulation and saved on disk. This signal is transformed into a periodic one, by using a conventional windowing technique, and is re-used in the actual simulation as many times as required to obtain converged statistics. The method is applied in a LES of a spatially developing turbulent mixing layer. It is shown that the periodicity induced by the inflow signal decays rapidly in about 25% of the domain, and that the length of the period has small effect on the statistics, which agree well with the reference experimental data. The method appears to be a cost-effective strategy in the generation of inflow data in a large variety of flows.
Oscillatory shear layers in source driven flows in an unbounded rotating fluid
Authors: Tilgner A.
Published: July 2002

The internal structure of oscillatory shear layers occurring in rapidly rotating fluids is investigated. An analytical treatment is possible for flows driven by a distribution of sources in an unbounded fluid. “Shear layers” are shown to be envelopes of wave packets of inertial waves. The modification of these layers by a magnetic field and a stable stratification are also studied.
Circulating flows inside a drop under time-periodic nonuniform electric fields
Authors: Lee S. M. , Im D. J. , Kang I. S.
Published: July 2002

The circulating flows formed inside a spherical drop under time-periodic nonuniform electric fields are considered. For simplicity, it is assumed that there are axisymmetric electric fields and that the flow fields are in the Stokes flow regime. An analytical solution of the streamfunction distribution inside and outside the drop is obtained. The flow field is found to be dependent on the frequency of the time-periodic electric field and the ratios of the material properties such as the viscosity, the electrical conductivity, and the electrical permittivity. As part of the solution, an analytical expression of the dielectrophoretic migration velocity of a drop under a time-periodic electric field is also obtained. The result shows an interesting physics—that dielectrophoretic migration is possible in a time-periodic electric field even in the situation where dielectrophoresis would be impossible in a static electric field. By using the analytical solution of the streamfunction, fluid mixing inside a drop is analyzed based on the Poincaré maps. The mass transfer enhancement factor due to fluid mixing has also been computed by solving the unsteady mass transfer equation numerically. The existence of an optimal frequency has been confirmed as in other mass transfer enhancement processes by time-periodic forcing.
Effects of orbit squeezing on poloidal mass flow and bootstrap current in tokamak plasmas
Authors: Shaing K. C. , Hsu C. T. , Hazeltine R. D.
Published: July 2002

It is shown, by solving the drift kinetic equation, that the asymptotic values of the poloidal mass flow and the bootstrap current in the banana regime of large-aspect-ratio tokamak plasmas are not affected by orbit squeezing. However, because the definition of ion collisionality υ*i is reduced by a factor S−3/2, with S=1−(ρpid ln Er/dr) (cEr/Bpνti), the values of both the poloidal mass flow and the bootstrap current at the banana–plateau transition are different from those of the conventional neoclassical theory. Here, ρpi is the ion poloidal gyroradius, Er is the radial electric field, c is the speed of light, Bp is the poloidal magnetic strength, νti is the ion thermal speed, and r is the local radius. The implications for the experimental observations of poloidal rotation speeds in high mode (H mode) are discussed.
Longitudinal oscillations in hot isotropic Maxwellian plasmas
Authors: Schlickeiser Reinhard
Published: July 2002

Using the relativistic Vlasov equation and the appropriate Maxwell–Boltzmann–Jüttner distribution for the unperturbed plasma thermal equilibrium state, it is demonstrated that the longitudinal dispersion relation in warm isotropic Maxwellian plasmas has two real roots with superluminal phase speed below a characteristic wave number kc that is determined by the plasma frequency and the plasma temperature. The superluminal oscillations undergo no Landau damping. The relativistically correct analysis leads to markedly different results than previous nonrelativistic work which incorrectly yielded a nonzero imaginary part of the roots for all phase speeds.
Analysis of the tokamak sheath region with in-sheath ionization and transport of surface emitted particles
Authors: Hua T. Q. , Brooks J. N.
Published: July 2002

A self-consistent numerical analysis has been performed of the oblique incidence magnetic field tokamak sheath, with in-sheath ionization and transport of surface desorbed and reflected hydrogen molecules and atoms, and sputtered and evaporated surface atoms. The analysis uses the newly developed bphi Monte Carlo/cloud-in-cell kinetic code together with particle–surface interaction models. For typical plasma boundary conditions (Te=100 eV, ne=1×1020 m−3), nearly all desorbed hydrogen molecules are ionized in the sheath; this lowers sheath potential by up to 30% and increases heat transmission by up to 150%. Backscattered hydrogen, and elastic collisions between primary ions and hydrogen molecules, in contrast, have a negligible effect on sheath parameters. The magnetic sheath region has a significant effect on sputtered tungsten atoms, resulting in high local redeposition, but is not itself changed by the sputtering. Ionization of thermally evaporated surface atoms (from overheating) can increase sheath heat transmission significantly, a process which may contribute to surface hot-spot formation.
Interactions between magnetohydrodynamical discontinuities
Authors: Dai Wenlong , Woodward Paul R.
Published: July 2002

Interactions between magnetohydrodynamical (MHD) discontinuities are studied through numerical simulations for the set of one-dimensional MHD equations. The interactions include the impact of a shock on a contact discontinuity, the collision of two shocks, and the catchup of a shock over another shock. The shocks involved in the interactions may be very strong. Each shock in an interaction may be either a fast or a slow shock.
Response to ‘‘Comments on ‘Electron acoustic surface waves in a two-electron component plasma’ ’’ [Phys. Fluids B 5, 4502 (1993)]
Authors: Bharuthram R. , Misthry S. S. , Yu M. Y.
Published: July 2002

In M. Shoucri’s Comments, we must replace (9) by the Boltzmann density law and set δvh=0 in (14), so that the term −ω2ph/ω2 in (20) vanishes, revovering the results of Ref. 2. (AIP)
Stimulated Brillouin scattering and ponderomotive self-focusing from a single laser hot spot
Authors: Eliseev V. V. , Rozmus W. , Tikhonchuk V. T. , Capjack C. E.
Published: July 2002

The parametric interaction of a focused Gaussian laser beam with ion plasma modes has been studied by using two dimensional nonparaxial spectral code [Phys. Fluids B 5, 3748 (1993)]. It is shown that the reflectivity of stimulated Brillouin scattering (SBS) from the single laser hot spot is dominated by backscattering. The reflectivity scales with the stationary gain coefficient and very closely follows predictions of a simple three wave coupling model. This is in spite of essentially nonlinear SBS behavior, which develops in the inhomogeneous density channels formed by the ponderomotive force of the beam, where it competes with the self-focusing instability (SFI). For the relatively short focal lengths (f/number<10) the SFI is diffraction limited and evolves until hot spot is focused to a size of few laser wavelengths. The SFI is responsible for the broad and anisotropic angular distribution of the transmitted light.
Electron cyclotron resonance heating near the turning point in inhomogeneous magnetic field
Authors: Tatematsu Y. , Kiwamoto Y. , Saito T. , Katanuma I. , Yoshimura Y. , Tamano T.
Published: July 2002

Cyclotron resonance heating of electrons in a magnetic beach is studied by rigorous numerical analysis of the electron trajectories. It is confirmed that a heating response function [Y. Kiwamoto et al., Phys. Plasmas 1 834 (1994)] connecting the velocity distributions in both sides of the resonance layer can be applied over a wider range of electron velocities than originally expected. For electrons mirror-reflected close to the resonance layer, however, the electron velocity distribution after heating substantially deviates from the prediction of the response function. The deviation is quantitatively evaluated to obtain a criterion of applicability of the response function.
Ion divergence in magnetically insulated diodes
Authors: Slutz S. A. , Lemke R. W. , Pointon T. D. , Desjarlais M. P. , Johnson D. J. , Mehlhorn T. A. , Filuk A. , Bailey J.
Published: July 2002

Magnetically insulated ion diodes are being developed to drive inertial confinement fusion. Ion beam microdivergence must be reduced to achieve the very high beam intensities required to achieve this goal. Three-dimensional particle-in-cell simulations [Phys. Rev. Lett. 67, 3094 (1991)] indicate that instability-induced fluctuations can produce significant ion divergence during acceleration. These simulations exhibit a fast growing mode early in time, which has been identified as the diocotron instability. The divergence generated by this mode is modest, due to the relatively high-frequency (≳1 GHz). Later, a low-frequency low-phase-velocity instability develops with a frequency that is approximately the reciprocal of the ion transit time. This instability couples effectively to the ions, and can generate unacceptably large ion divergences (≳30 mrad). Linear stability theory reveals that this mode has structure parallel to the applied magnetic field and is related to the modified two-stream instability. Measurements of ion density fluctuations and energy-momentum correlations have confirmed that instabilities develop in ion diodes and contribute to the ion divergence. In addition, spectroscopic measurements indicate that lithium ions have a significant transverse temperature very close to the emission surface. Passive thin-film lithium fluoride (LiF) anodes have larger transverse beam temperatures than laser-irradiated active sources. Calculations of the ion beam source divergence for the LiF film due to surface roughness and the possible loss of adhesion and fragmentation of this film are presented.
Model for relaxation in reversed-field pinch plasmas
Authors: Wang A. K. , Qiu X. M.
Published: July 2002

In this article, a model for anomalous ion heating, a dynamo current-sustained edge toroidal field, and a sawtooth oscillation during the relaxation in the reversed-field pinch (RFP) plasma is presented. The dynamo (α), the turbulent resistivity (β) and viscosity (χ), dependent on the magnetohydrodynamics (MHD) fluctuations, are incorporated into the model. Turbulent viscous dissipation of the fluctuation energy is proposed as the mechanism of the anomalous ion heating. This is a straightforward corollary of the turbulent viscosity heating of ions in that the temperature of the heavier ions is higher than that of the lighter ions and that the ion temperature increases with the MHD fluctuation level. Correspondingly, the turbulent resistivity heats electrons anomalously. It is shown that the dynamo current, generated by the back-transfer of fluctuating magnetic field helicity to mean magnetic field, sustains the RFP magnetic configuration. In the edge the total current density is approximately equal to the dynamo current density, while at the core the dynamo current opposes the applied electric-field-driven current, flattening the current profile. Provided the α dynamo has a periodic behavior in time, the physical quantities of the RFP plasma have a sawtooth time dependence. The local poloidal current density in the edge increases during the sawtooth crash and peaks at the end of the crash, as do the ion and electron temperatures. In contrast, the toroidal current density at the core decreases during the crash and arrives at its minimum at the end of the crash. Qualitatively, the conclusions drawn from the present model are in good agreement with many of the experimental results [Scime et al., Phys. Rev. Lett. 68, 2165 (1992); Ji et al., ibid. 73, 668 (1994)] and the numerical simulations.
Plasma beta limits for magnetic annihilation models
Authors: Inverarity G. W. , Priest E. R.
Published: July 2002

Magnetic annihilation occurs when two oppositely directed magnetic fields are brought together by a plasma flow. Several exact nonlinear solutions exist which typically depend on the ratios of plasma pressure to magnetic pressure (the plasma beta), inflow speed to global Alfvén speed (the Alfvén Mach number) and of the advective to diffusive terms of the induction equation (the Lundquist number). Ensuring that the plasma pressure is everywhere positive restricts the freedom of choice of these parameters, however. Restrictions on the plasma beta are derived for the cases of two- and three-dimensional annihilation and two-dimensional reconnective annihilation. At the inflow speeds typically required for fast reconnection in diffuse astrophysical plasmas the minimum plasma beta is several orders of magnitude larger than the observed values of unity or less. In other words, at the observed plasma beta the models are only valid for extremely small annihilation rates.
Stationary large-scale magnetic fields generated by turbulent motion in a spherical region
Authors: Yoshizawa Akira , Yokoi Nobumitsu
Published: July 2002

Stationary large-scale magnetic fields generated by an electrically conducting fluid in a spherical region are examined analytically, using the concept of the turbulent dynamo based on helicity and cross-helicity effects. Under this concept, the toroidal magnetic field is induced through the combination of a rotational motion and the turbulent cross-helicity effect. This field generates the poloidal one through the turbulent residual-helicity (alpha) effect. A new magnetic-field generation mechanism in the vicinity of the poles is also described. These findings are discussed in the context of the dimension of the convection part of a stellar object.
Plasma–maser theory for magnetized electron–positron plasma
Authors: Kakati H. , Goswami K. S.
Published: July 2002

The plasma maser theory for interaction between the nonresonant Bernstein mode and the resonant Langmuir mode in a magnetized electron–positron plasma is presented. The growth rate is calculated as if found to be larger than the corresponding interaction in an electron–ion plasma. The higher growth rate in an electron–positron plasma is due to the contribution from the resonant positrons in addition to the resonant electrons.
The clump lifetime revisited: Exact calculation of the second-order structure function for a model of forced, dissipative turbulence
Authors: Krommes John A.
Published: July 2002

The role of the clump lifetime τcl, logarithmically dependent on small scales, in determining the fluctuation level for forced, dissipative turbulence is reconsidered with the aid of an exactly solvable rapid-change model of passive advection. It is shown in mathematical detail that the common procedure of approximating the steady-state fluctuation level I by τclP, where P is the production rate or source of fluctuations, is invalid when a clean distinction can be made between energy-containing and inertial ranges. The correct result is I≈τDP, where τD is a macroscopic diffusion time that does not depend on the small scales.
Deuterium atomic cross section uncertainties and molecular recycling effects in the divertor region of tokamaks
Authors: Thomas E. W. , Stacey W. M.
Published: July 2002

How predictions of deuterium atomic reaction rates in tokamak divertors are altered by changes in cross section data which are comparable with the uncertainty in published data is investigated. An extended neutral diffusion theory calculation, which includes D atoms and ground and excited state D2 molecules, is performed on a fixed background plasma. For the atoms, the principal relevant mechanisms are elastic scattering, charge exchange, ionization, and recombination. It is evaluated that the charge exchange and electron processes are known to an accuracy better than +/−25%, and it is concluded that this level of accuracy is sufficient for the present state of divertor calculations. Of significant concern is the fact that classical elastic cross sections which are widely used in divertor calculations differ substantially from a more accurate set of cross sections based on quantum mechanics, and that the latter produce predictions of neutral density and ionization rate which differ by orders of magnitude from predictions based on the classical cross sections. Opacity of the plasma to Lyman alpha radiation influences transport for high density plasma situations. The explicit representation of incident particles recycling as molecules is shown to significantly affect the neutral atom density and reaction rates near the divertor plate. In particular, the dissociation of even a small fraction of excited molecules significantly reduces the plasma ion population and increases the neutral atom population just in front of the plate in nearly detached plasmas.
Brownian motion of a dust particle in a plasma
Authors: Mendonça J. T. , Shukla P. K. , Martins A. M. , Guerra R.
Published: July 2002

A new version of the Brownian motion describing the motion of a dust particle in a turbulent plasma is considered. Here, the stochastic force acting on the dust particle is due to the fluctuations of the plasma potential and not due to the usual molecular collisions. Another significant difference is due to the fact that the dust electric charge is not constant but fluctuates with the potential. A four-dimensional formulation of the problem is also given.
Simulation of the nonlinear dynamics of grid sheath oscillations in double plasma devices
Authors: Rohde A. , Piel A. , Klostermann H.
Published: July 2002

The oscillation of ion bunches around a strongly negative grid in a double plasma device is studied with one–dimensional particle–in–cell simulation. The system exhibits feedback amplified virtual anode oscillations in the target chamber. The undriven system is shown to perform relaxation processes which can be described by the van der Pol equation for high nonlinearity. The periodically driven system exhibits nonlinear phenomena like mode locking and periodic pulling, which are in quantitative agreement with theoretical predictions for the driven van der Pol oscillator. For large driver amplitudes period doubling and weak chaos are found.
A coupled plasma-neutrals model for divertor simulations
Authors: Stacey W. M.
Published: July 2002

The integral balance, or “two-point,” class of divertor plasma models is extended to explicitly represent effects associated with recycling neutrals and is coupled to a two-dimensional neutral transport model. New analytical solutions for the plasma density, velocity, and temperature distributions along the field lines are derived. The resulting divertor model is intermediate in complexity between the simple “two-point” models and the coupled two-dimensional plasma fluid–Monte Carlo or Navier Stokes neutrals models; it is suitable for coupling with core transport codes and for performing a wide range of divertor physics studies. Qualitative comparison with experiment and model problem calculations indicates that the new model produces many of the features found in divertor experiments.