bluuebunny/crossref_metadata_2025 huggingface dataset:
Dewatering of flocculated suspensions by pressure filtration
Authors: Landman K. A. , Sirakoff C. , White L. R.
Published: July 2002

Pressure filtration is an important method for removing liquids from a suspension. Previous work used linear models or applied to stable suspensions. Nonlinear models for flocculated suspensions are studied here. The equations governing the consolidation of flocculated suspensions under the influence of an applied pressure are based on the assumption that when the volume fraction is high enough, the network formed from the aggregation of flocs possesses a compressive yield stress Py(φ) that is a function of local volume fraction φ only. There are two modes of operation of the pressure filter—the fluid flux or the applied pressure is specified—and both of these are studied. The resulting nonlinear partial differential equations involve the time-dependent piston position, and in the case of the suspension being initially unnetworked, another internal moving boundary below which the suspension is networked. The small time behavior of these systems is obtained with an asymptotic method. In general, at later times, the solution can only be found numerically and an algorithm for doing this is discussed. The important parameters and properties of the filter cake are described. The results suggest various ways of controlling the filtration process, which may be useful in the manufacture of ceramics.
Mixing by  side jets’’ in strongly forced and self-excited round jets
Authors: Monkewitz Peter A. , Pfizenmaier Eberhard
Published: July 2002

The topic of this paper is an experimental investigation of entrainment and mixing in transitional, i.e., initially laminar, axisymmetric jets that are either self-excited or strongly forced at Strouhal numbers of around 0.4, where self-excitation is achieved by heating the jet. In the potential core region such jets have been found to spread in a nonaxisymmetric fashion at unusually large rates, a phenomenon which is due to the formation of so-called ‘‘side jets’’ [Phys. Fluids A 1, 446 (1989)]. It has been speculated that the ejection of jet fluid into side jets, which are directed radially away from the jet axis, is related to an early appearance of lobes on the primary jet vortex rings. Quantitative LDA measurements in side jets suggest that their generation not only involves lobes in the primary vortex rings, but also pairs of longitudinal vortices in the braid regions between the vortex rings. Furthermore, it is shown that the velocity of side jets is comparable in forced cold jets and self-excited hot jets, if the primary vortex rings are of equal strength.
Chaotic transport in two- and three-dimensional flow past a cylinder
Authors: Batcho Paul , Karniadakis George Em
Published: July 2002

The response of transport measures (Nusselt number, drag and lift force) for two- and three-dimensional flow past a heated cylinder reaching a chaotic state is investigated numerically using a spectral element discretization at a Reynolds number Re=500. The undisturbed two-dimensional flow remains periodic at this Reynolds number, unless a suitable forcing is applied on the naturally produced system. Three-dimensional simulations establish that three-dimensionality sets in at Re≊200. Successive supercritical states are established through a series of period-doublings, before a chaotic state is reached at a Re≊500. For the two-dimensional forced flow, all transport measures oscillate aperiodically in time and undergo a ‘‘crisis,’’ i.e., a sudden and dramatic increase in their amplitude. The corresponding three-dimensional, naturally produced chaotic state corresponds to a less drastic change of the transport quantities with both rms and mean values lower than their two-dimensional counterparts.
Rigorous link between fluid permeability, electrical conductivity, and relaxation times for transport in porous media
Authors: Avellaneda M. , Torquato S.
Published: July 2002

A rigorous expression is derived that relates exactly the static fluid permeability k for flow through porous media to the electrical formation factor F (inverse of the dimensionless effective conductivity) and an effective length parameter L, i.e., k=L2/8F. This length parameter involves a certain average of the eigenvalues of the Stokes operator and reflects information about electrical and momentum transport. From the exact relation for k, a rigorous upper bound follows in terms of the principal viscous relation time Θ1 (proportional to the inverse of the smallest eigenvalue): k≤νΘ1/F, where ν is the kinematic viscosity. It is also demonstrated that νΘ1≤DT1, where T1 is the diffusion relaxation time for the analogous scalar diffusion problem and D is the diffusion coefficient. Therefore, one also has the alternative bound k≤DT1/F. The latter expression relates the fluid permeability on the one hand to purely diffusional parameters on the other. Finally, using the exact relation for the permeability, a derivation of the approximate relation k≂Λ2/8F postulated by Johnson et al. [Phys. Rev. Lett. 57, 2564 (1986)] is given.
Shock-jump relations in a turbulent flow
Authors: Lele Sanjiva K.
Published: July 2002

The exact jump relations applying across a shock in turbulent mean flow are formulated. These are analyzed to obtain answers to the following basic questions: Is the propagation speed of a shock wave which brings about a specified compression faster or slower than its classical value when moving through a turbulent medium? What is the magnitude of the turbulence associated correction? What is the magnitude of the correction to the pressure jump? Answers to these questions are developed by combining gas-dynamic analysis with homogeneous rapid distortion approximation.
Statistical analysis of the dynamic equations for higher-order moments in turbulent wall bounded flows
Authors: Jovanović J. , Durst F. , Johansson T. G.
Published: July 2002

The equations for the higher-order moments of turbulent velocity fluctuations are considered. These are derived utilizing truncated, cumulant expansions as an approximation for the probability density distributions of the corresponding turbulence properties. By applying different degrees of truncations to these expansions, an alternative set of equations for the moments is formulated that contains only velocity correlations. From these equations, interrelations between the higher-order moments are deduced and are experimentally verified using data available in the literature and also data measured by the authors.
An update on the intermittency exponent in turbulence
Authors: Sreenivasan K. R. , Kailasnath P.
Published: July 2002

The issue of experimental determination of the intermittency exponent μ, is revisited and it is shown that the ‘‘best’’ estimate for it is 0.25±0.05. This ‘‘best’’ estimate is obtained from recent atmospheric data, and is based on several different techniques.
Comments on ‘‘Studies of Raman scattering from overdense targets irradiated by several kilojoules of 0.53 μm laser light’’ [Phys. Fluids 3
1, 3130 (1988)]
Authors: Simon Albert , Short Robert W.
Published: July 2002

Drake et al. [Phys. Fluids 31, 3130 (1988)] have considered several simple models of Raman scattering in laser-produced plasma and concluded that each of them is in conflict with at least one feature of the experimental data. It is shown here that the argument used against the enhanced plasma wave model [Phys. Fluids 29, 1704 (1986)] is incorrect and hence that this model remains as a possible explanation of the data.
Electron heat transport in a steep temperature gradient
Authors: Rogers J. H. , De Groot J. S. , Abou-Assaleh Z. , Matte J. P. , Johnston T. W. , Rosen M. D.
Published: July 2002

Temporal and spatial measurements of electron heat transport are made in the University of California Davis AURORA device (J. H. Rogers, Ph.D. dissertation, University of California, Davis, 1987). In AURORA, a microwave pulse heats a region of underdense, collisional, plasma (n/ncr ≲1, where ncr =1.8×1010 cm−3 is the critical density, Te0 ≊0.15 eV, and the electron scattering mean free path λ⊥≳2 cm). In this region, strong thermal heating (Tc ≲0.7 eV) as well as suprathermal heating (Th≊3 eV) is observed. The strong heating results in a steep temperature gradient that violates the approximations of classical heat diffusion theory (LT/λ⊥≳3 for thermal electrons, where LT=Tc(∂Tc/∂z)−1 is the cold electron temperature scale length. The time evolution of the electron temperature profile is measured using Langmuir probes. The measured relaxation of the temperature gradient after the microwave pulse is compared to calculations using the Fokker–Planck International code [Phys. Rev. Lett. 49, 1936 (1982)] and the multigroup, flux-limited, target design code lasnex [Comm. Plasma Phys. 2, 51 (1975)]. The electron distribution function at the end of the microwave pulse is used as initial conditions for both codes. The Fokker–Planck calculations are found to agree very well with the measurements. However, the flux-limited diffusion calculations do not agree with the measurements for any value of the flux limiter.
Axisymmetric toroidal equilibrium with flow and anisotropic pressure
Authors: Iacono R. , Bondeson A. , Troyon F. , Gruber R.
Published: July 2002

Axisymmetric toroidal plasma equilibria with mass flows and anisotropic pressure are investigated. The equilibrium system is derived for a general functional form of the pressures, which includes both fluid models, such as the magnetohydrodynamic (MHD) and the double-adiabatic models, and Grad’s guiding center model [Proceedings of the Symposium on Electromagnetics and Fluid Dynamics of Gaseous Plasmas, edited by J. Fox (Polytechnic Inst. of Brooklyn, New York, 1961), p. 37]. This allows for detailed comparisons between the models and clarifies how the ‘‘first hyperbolic region,’’ occurring in fluid theory when the poloidal flow is of the order of the poloidal sound speed, can be eliminated in guiding center theory. In the case of a pure toroidal rotation, macroscopic equations of state are derived from the guiding center model, characterized by a parallel temperature that is constant on each magnetic surface and a perpendicular temperature that varies with the magnetic field. The outward centrifugal shifts of the magnetic axis and of the mass density profile, caused by toroidal rotation, are increased by anisotropy if p∥
p⊥. In the guiding center model poloidal flow produces an inward shift of the density profile, in contrast with the MHD result.
Self-consistent turbulent dynamo modeling of reversed field pinches and planetary magnetic fields
Authors: Yoshizawa Akira
Published: July 2002

Turbulent dynamo modeling in a rotating frame is performed to enable studies of reversed field pinches of fusion plasma and planetary magnetic fields. The Reynolds stresses, turbulent electromotive force, and other important correlation functions, which appear in the equations for the mean velocity and magnetic field, are modeled using the mean fields and four bulk turbulence quantities (the magnetohydrodynamic turbulent energy, its dissipation rate, the cross helicity, and the residual helicity). Four equations for those bulk quantities are combined with the mean-field equations to lead to a self-consistent dynamo model. Specifically, the importance of the cross-helicity effect is pointed out. This model shows that the equilibrium state of reversed field pinches of plasma is a neutral state under the condition of vanishing cross helicity. The relevance to the study of the Earth’s and other planetary magnetic fields is discussed.
Reply to the comments of Drake
Authors: Simon Albert , Short Robert W.
Published: July 2002

There are three issues in Drake’s comment. The first raises questions about the angular distribution predicted by our theory at lower laser intensities. It is shown that the qualitative behavior should be in accord with existing observations, but that a detailed comparison will require experimental data that is not yet available. The second is about the relative timing of the Raman scattering and disappearance of the quarter-critical surface. It is noted that existing measurements have considerable uncertainty and that there are additional complications for an exploding foil target. The last comment, about the probability of our power flow, is simply an opinion about a conjecture.
Intense laser plasma interactions: From Janus to Nova
Authors: Kruer William L.
Published: July 2002

An overview of intense laser plasma interactions is given. The relevant physical processes range from collisional absorption to the excitation of instabilities, such as stimulated Raman and Brillouin scattering. These processes and important consequences that include suprathermal particle generation and enhanced absorption or scattering have been examined in theory, computer simulation, and experiments. The interaction physics has been studied in experiments using electromagnetic waves with wavelengths ranging from a fraction of a micron to tens of meters. Virtually every major interaction process has now been identified in laser plasma experiments, which have grown remarkably in sophistication over the years. Various control techniques have been demonstrated, and some intensity-wavelength regimes for efficient laser plasma coupling have been successfully identified. Indeed, current implosion experiments show excellent coupling of laser energy into thermal plasma. Future challenges include a better understanding of nonlinear levels, the competition of instabilities in large plasmas, and the beneficial effects of laser beam smoothing.
Relaxation processes in a low-order three-dimensional magnetohydrodynamics model
Authors: Stribling Troy , Matthaeus William H.
Published: July 2002

The selective decay and dynamic alignment relaxation theories are used to interpret the time asymptotic behavior of a Galerkin model of three-dimensional (3-D) magnetohydrodynamics (MHD). A large number of simulations are performed that scan a parameter space defined by the rugged ideal invariants: energy, cross helicity, and magnetic helicity. Ranges of the initial parameters are found where one or both of the relaxation theories are needed to describe the time asymptotic properties of the system, as previously found in analogous studies of two-dimensional (2-D) MHD [Ting et al., Phys. Fluids 29, 3261 (1986)]. In many cases, the time asymptotic state can be interpreted as a relaxation to minimum energy. For certain parameter ranges spectral back transfer of cross helicity can lead to growth in velocity-magnetic field correlation [Stribling and Matthaeus, Phys. Fluids B 2, 1979 (1990)]. A simple decay model, based on absolute equilibrium theory, predicts a mapping of initial onto time asymptotic states, and accurately describes the long time behavior of the runs when magnetic helicity is present. We also discuss two processes, operating on time scales shorter than selective decay and dynamic alignment, in which the ratio of kinetic to magnetic energy relaxes to values O(1). The faster of the two takes states initially dominant in magnetic energy to a state of near-equipartition between kinetic and magnetic energy through power law growth of kinetic energy. The other process takes states initially dominant in kinetic energy to the near-equipartitioned state through exponential growth of magnetic energy.
Analytic expressions for mode conversion in a plasma at the peak of a parabolic density profile
Authors: Hinkel-Lipsker D. E. , Fried B. D. , Morales G. J.
Published: July 2002

For mode conversion in an unmagnetized plasma with a parabolic density profile of scale length L, analytic expressions, in terms of parabolic cylinder functions, for the energy flux coefficients (reflection, transmission, and mode conversion) and the fields for both the ‘‘direct’’ problem (incident electromagnetic wave converting to a Langmuir wave) and the ‘‘inverse’’ problem (incident Langmuir wave converting to an electromagnetic wave) are derived for the case where the incident wave frequency ω matches the electron plasma frequency ωp at the peak of the density profile. The mode conversion coefficient for the direct problem is equal in magnitude to that of the inverse problem, and the corresponding reflection and transmission coefficients satisfy energy conservation. In contrast to the linear profile problem, the conversion efficiency depends explicitly on the value of the collision frequency (in the cold, collisional limit) or electron temperature (in the warm, collisionless limit), but a transformation of parameters relates the results for these two limits.
Stability of resistive and ideal ballooning modes in the Texas Experimental Tokamak and DIII-D
Authors: McCarthy D. R. , Guzdar P. N. , Drake J. F. , Antonsen T. M. , Hassam A. B.
Published: July 2002

The stability of drift resistive ballooning modes is examined using the reduced Braginskii equations, which include electron temperature and magnetic fluctuations. The Texas Experimental Tokamak (TEXT) [Phys. Fluids B 2, 2879 (1990)] edge plasma is found to be unstable for a broad range of mode numbers. For low mode numbers (m<70), the plasma is unstable to the drift resistive ballooning mode, which has a growth rate that scales linearly with the resistivity (γ∼η). As the mode number increases, a transistion is found to the resistive ballooning mode with the usual scaling of γ∼η1/3. A similar analysis is made for parameters from DIII-D [Phys. Fluids B 2, 1405 (1990)] for both the L mode and the H mode. It is found that the L mode is unstable to the resistive mode, but β ≊ βI/6, where βI is the critical β for the ideal instability. The H mode is weakly unstable to the resistive mode and is only about a factor of 2 below the β threshold for the more robustly unstable ideal mode.
Structure formation and transport in dissipative drift-wave turbulence
Authors: Koniges A. E. , Crotinger J. A. , Diamond P. H.
Published: July 2002

Numerical flow simulations are used to study the effect of coherent structures on transport in the context of a two-field, two-dimensional model of dissipative drift-wave turbulence. The presence and nature of structures are found to depend on the adiabaticity parameter α=k∥2 VT2/2νeiωs which controls the degree to which the electrons respond to parallel electric fields. Transport estimates based on quasilinear and mixing-length models are compared with the simulations. In the regime with long-lived coherent structures, the turbulent particle transport predicted by a standard quasilinear or mean-field estimate is found to exceed that actually observed in the presence of coherent structures.
Analytic expressions for mode conversion in a plasma with a parabolic density profile: Generalized results
Authors: Hinkel-Lipsker D. E. , Fried B. D. , Morales G. J.
Published: July 2002

This study provides an analytic solution to the general problem of mode conversion in an unmagnetized plasma. Specifically, an electromagnetic wave of frequency ω propagating through a plasma with a parabolic density profile of scale length Lp is examined. The mode conversion points are located a distance Δ0 from the peak of the profile, where the electron plasma frequency ωp(z) matches the wave frequency ω. The corresponding reflection, transmission, and mode conversion coefficients are expressed analytically in terms of parabolic cylinder functions for all values of Δ0. The method of solution is based on a source approximation technique that is valid when the electromagnetic and electrostatic scale lengths are well separated. For large Δ0, i.e., (cLp/ω)1/2≪Δ0<Lp, the appropriately scaled result [D. E. Hinkel-Lipsker et al., Phys. Fluids B 4, 559 (1992)] for a linear density profile is recovered as the parabolic cylinder functions asymptotically become Airy functions. When Δ0→0, the special case of conversion at the peak of the profile [D. E. Hinkel-Lipsker et al., Phys. Fluids B 4, 1772 (1992)] is obtained.
Nonlinear analysis of the two-stream instability for relativistic annular electron beams
Authors: Uhm Han S. , Chen Chiping
Published: July 2002

A nonlinear theory is developed for the two-stream interaction of two relativistic annular electron beams propagating through a grounded cylindrical conducting tube. The theory is based on the assumption that the beams experience energy modulation at a cavity before they enter the drift tube. Two coupled integrodifferential equations are derived which describe beam current modulation in terms of time t and propagation distance z. The evolution of the fundamental mode in the current modulation is investigated analytically by making use of these coupled equations. The amplitude of the fundamental mode, which is a function of the propagation distance, is expressed explicitly in terms of the initial energy modulation, the growth rate of the instability, and the beam intensity. It is found that self-field effects dominate two-stream effects at the beginning of the propagation. As the beams propagate further, the two-stream effects start to dominate and then the perturbations grow exponentially. The saturation mechanism is identified as multimode coupling. A theoretical model describing the mode coupling in the current modulation is also developed. The theory is verified by detailed comparisons between analytical and particle-in-cell simulation results.
Three-dimensional numerical simulations of the relaxation process in spheromak plasmas
Authors: Katayama Kozo , Katsurai Makoto
Published: July 2002

Nonlinear evolution of three-dimensional magnetohydrodynamic (MHD) instabilities of a toroidal spheromak in a cylindrical flux conserver has been studied by numerical simulations for various initial equilibrium states with different q profiles. In spheromaks with qa>1, where qa is the safety factor on the magnetic axis, nonlinear evolution of the resonant internal kink mode dominates with a poloidal mode number m=1 and a toroidal mode number n=1 that causes the poloidal flux amplification. This process corresponds to that of the internal disruption model for tokamaks by Kadomtsev [Sov. J. Plasma Phys. 1, 389 (1975)]. In spheromaks with a very high qa, namely qa≳3, the gross n=1 kink mode grows extensively in the region including the major axis of the torus, which causes the flux conversion from the toroidal to poloidal directions. For spheromaks with a low qa, namely qa≲0.5, the internal kink mode with a toroidal mode number n∼1/qa is first destabilized, and the excitation of the modes with lower n numbers down to n=1 proceeds, while the n=2 mode saturates. Nonlinear coupling of various modes leads to the flux conversion from the poloidal to toroidal directions. When a center conductor is present in this case, a reversed-field pinch (RFP) configuration once formed is sustained. Relaxations through pressure-driven modes are also discussed. All final states obtained in our simulations are quite near the Taylor state with an excess magnetic energy less than 10% of that of the Taylor state.
Rayleigh–Taylor stability for a normal shock wave–density discontinuity interaction
Authors: Fraley Gary
Published: July 2002

The solution for the perturbation growth of a shock wave striking a density discontinuity at a material interface is developed. The Laplace transformation of the perturbation results in an equation which has a simple solution for weak shock waves. The solution for strong shock waves may be given by a power series. It is assumed that the equation of state is that of an ideal gas. The four independent parameters of the solution are the ratio of specific heat for each material, the density ratio at the interface, and the incoming shock strength. Properties of the solution which are investigated include the asymptotic behavior at large times of the perturbation velocity at the interface, the vorticity near the interface, and the rate of decay of the solution at large distances from the interface. The last is much weaker than the exponential decay in an incompressible fluid. The asymptotic solution near the interface, in addition to a constant term, consists of a number of slowly decaying discrete frequencies. The number is roughly proportional to the logarithm of the density ratio at the surface for strong shocks, and decreases with shock strength. For weak shocks the solution is compared with results for an incompressible fluid. Only interface perturbation velocities which tend to zero at large times lead to a limited deformation of the interface. It is found that these are possible only for density ratios less than about 1.5.
Collisional effects on the Weibel instability
Authors: Wallace J. M. , Brackbill J. U. , Cranfill C. W. , Forslund D. W. , Mason R. J.
Published: July 2002

The effects of electron Rutherford scattering on the Weibel instability are investigated using the implicit plasma simulation code venus [J. Comput. Phys. 46, 271 (1982); 63, 434 (1986)]. Collisions decrease the Weibel growth rate below the collisionless value, in agreement initially with simple linear theory. Likewise, collisions decrease the saturation level, in agreement with a magnetic trapping mechanism. When the collision rate exceeds the collisionless Weibel growth rate, the instability is suppressed altogether. The results provide benchmarks for analysis of proposed short-wavelength laser-fusion schemes.
Transport processes in random arrays of cylinders. II. Viscous flow
Authors: Sangani A. S. , Yao C.
Published: July 2002

The method described in Part I [Phys. Fluids 31, XXXX (1988)] is extended to treat the problem of determining the permeability of random arrays of infinitely long cylinders. The results for the transverse and longitudinal permeabilities averaged over several configurations of random arrays of cylinders are presented as a function of the area fraction of the cylinders. A detailed comparison is made with the estimates of the permeability obtained by various approximate and asymptotic theories to determine their range of validity.
Turbulence structure in free-surface channel flows
Authors: Rashidi M. , Banerjee S.
Published: July 2002

A turbulence structure in horizontal liquid streams bounded by a free surface and a wall has been investigated using 10–25 μm oxygen bubbles as tracers. High speed video movies indicate that the dominant flow structure is caused by the periodic ejection of intensely turbulent fluid with low streamwise momentum from the wall region into the relatively quiescent bulk fluid which it displaces and mixes with slowly. The motion of these bursts is constrained by the free interface. Between bursts and the interface a high speed region with a steep velocity gradient develops as a consequence. This in turn causes progress of the burst fluid toward the interface to slow down and eventually to turn back toward the wall, giving rise to characteristic rolling structures, which rotate clockwise if the flow is viewed as going from left to right. To complement the video studies, quantitative data were obtained by analyzing bubble streak lines generated by photography of optically chopped flashes. These data show that in the vicinity of the interface the velocity fluctuations normal to it are damped whereas those parallel to it are enhanced. Analysis of conditional samples of the data indicate that fluid with relatively low streamwise momentum tends to move toward the interface while fluid with high momentum moves away giving rise to rotating structures that roll along with the flow in agreement with the video studies. A high degree of correlation between ejection events near the wall and the fluid motion near the interface also confirm that the bursts extend across the flow stream. This has important implications for surface renewal theories of turbulent transport at fluid–fluid interfaces.
Two-dimensional numerical analysis of the Poiseuille–Bénard flow in a rectangular channel heated from below
Authors: Nicolas X. , Mojtabi A. , Platten J. K.
Published: July 2002

The Poiseuille–Bénard flow (PBF) is studied by a two-dimensional numerical simulation for a Prandtl number equal to 6.4 (that of water at 23 °C) and for a wide range of Rayleigh (Ra) and Reynolds (Re) numbers: Ra⩽6000 and Re⩽3. The two observed flow configurations are (1) thermally stratified Poiseuille flow and (2) thermoconvective transversal rolls superimposed to the basic Poiseuille flow. The time evolution of the velocity components, the spatial development of the transversal rolls, their frequency, wavelength and velocity, the Nusselt number, together with the stability map in the Ra–Re plane, are studied in detail. Whenever possible, quantitative comparisons are made with published results: most of the experimental data, based on laser-Doppler anemometry (LDA), are recovered with amazing accuracy; a good agreement with results of convective stability deduced from a weakly nonlinear Ginzburg–Landau theory is also obtained.
Discontinuous behavior of liquids between parallel and tilted plates
Authors: Concus Paul , Finn Robert
Published: July 2002

Discontinuous behavior of liquids between parallel and tilted plates in the absence of gravity is discussed. A principal finding, derived mathematically from the classical Young–Laplace–Gauss formulation for capillary free surfaces, is that in a large range of configurations liquid bridges between parallel plates are unstable with respect to small, even infinitesimal, tilting of one of the plates. Under a computationally based hypothesis of uniqueness of spherical bridges in a wedge, it is shown that the same discontinuous behavior prevails for all but very particular circumstances. The various liquid configurations, which form the basis for an experiment on board the Space Station Mir, are characterized and illustrated.
Computation of flow through a fluid-sediment interface in a benthic chamber
Authors: Basu A. J. , Khalili A.
Published: July 2002

In this paper we present a single-domain approach for the solution of flow in a composite region made up of a pure fluid layer and an underlying saturated porous layer. As an example, we compute the unsteady, axisymmetric flow and scalar transport in a stationary cylindrical container with a rotating lid, filled to the midheight with a porous material and to the top with water. A generalized equation known as the Brinkman-extended Darcy equation is solved inside the porous medium, along with the incompressible Navier–Stokes equations in the upper fluid layer. Comparisons with experimental data previously obtained by the authors for flow in the same geometry show good agreement, thus verifying the accuracy of the present computations. The results indicate that a single-domain approach can provide good predictions of interfacial flow, thereby obviating the need for ad hoc interface conditions. The existence of a thin Brinkman layer below the interface is observed. Radial profiles of computed velocity components adjacent to the interface show remarkable similarity, despite vast differences in magnitudes, showing that good matching between the two different flows has been achieved by the present single-domain approach.
On the role of buoyancy in determining the stability of curved mixing layers
Authors: Otto S. R. , Stott Jillian A. K. , Denier James P.
Published: July 2002

It is well known that buoyancy forces and centrifugal effects can render a flow unstable to longitudinal vortex structures. Such competing instability mechanisms can be found in flows such as the curved mixing layer formed by the passage of two streams of fluid at different temperatures in the wake of a curved body. Via an asymptotic consideration of the problem we are able to characterize the interplay between these mechanisms. We are also able to determine the level of convex curvature required to stabilize unstably stratified mixing layers and the level of concave curvature required to destabilize stably stratified mixing layers.
Soap film flows: Statistics of two-dimensional turbulence
Authors: Vorobieff P. , Rivera M. , Ecke R. E.
Published: July 2002

Soap film flows provide a very convenient laboratory model for studies of two-dimensional (2-D) hydrodynamics including turbulence. For a gravity-driven soap film channel with a grid of equally spaced cylinders inserted in the flow, we have measured the simultaneous velocity and thickness fields in the irregular flow downstream from the cylinders. The velocity field is determined by a modified digital particle image velocimetry method and the thickness from the light scattered by the particles in the film. From these measurements, we compute the decay of mean energy, enstrophy, and thickness fluctuations with downstream distance, and the structure functions of velocity, vorticity, thickness fluctuation, and vorticity flux. From these quantities we determine the microscale Reynolds number of the flow Rλ≈100 and the integral and dissipation scales of 2D turbulence. We also obtain quantitative measures of the degree to which our flow can be considered incompressible and isotropic as a function of downstream distance. We find coarsening of characteristic spatial scales, qualitative correspondence of the decay of energy and enstrophy with the Batchelor model, scaling of energy in k space consistent with the k−3 spectrum of the Kraichnan–Batchelor enstrophy-scaling picture, and power-law scalings of the structure functions of velocity, vorticity, vorticity flux, and thickness. These results are compared with models of 2-D turbulence and with numerical simulations.
Electrostatic shock wave in dusty plasmas
Authors: Dwivedi C. B. , Pandey B. P.
Published: July 2002

The present investigation reports the existence of a one-dimensional (1-D) shock wave profile for a weakly nonlinear modified acoustic-like/dust-acoustic waves. Here, unlike in the conventional plasma systems, the dissipative effect to balance the nonlinear steepening arises self-consistently due to dust charge fluctuation dynamics in response to the collective plasma oscillations. The possibility of other localized coherent structures like dissipative/dispersive solitons in higher-order solutions has been highlighted. Its implications in laboratory and space plasmas are discussed.
Radiative instabilities in the tokamak scrape-off layer during edge-localized mode activity
Authors: Helander P. , Krasheninnikov S. I. , Morozov D. Kh. , Soboleva T. K.
Published: July 2002

In order to reduce the heat flux entering the divertor, it is desirable to have strong impurity radiation in the scrape-off layer (SOL) of reactor-size tokamaks like the International Thermonuclear Experimental Reactor [International Thermonuclear Experimental Reactor (ITER) Conceptual Design Activity Final Report, ITER Documentation Series No. 16 (International Atomic Energy Agency, Vienna, 1991)]. Under such circumstances, however, the SOL plasma is likely to be unstable to the radiative condensation instability. In the present paper, an investigation is undertaken to study the effects of edge-localized mode (ELM) activity on this instability. In the linear regime, it is demonstrated that high-frequency (‘‘grassy’’) ELM’s may parametrically excite acoustic waves. The possibility of nonlinear radiative collapse with concomitant stratification of the plasma is discussed, and solutions describing nonlinear traveling waves are derived in which the plasma goes over from equilibrium state to another.
Bursting processes in plasmas and relevant nonlinear model equations
Authors: Basu B. , Coppi B.
Published: July 2002

Important intrinsic plasma instabilities manifest themselves in the form of periodic bursts of fluctuations rather than as a state of stationary fluctuations, which a conventional application of quasilinear theory would lead to expect. A set of coupled nonlinear equations for the time evolution of the fluctuation amplitude and of the driving factor of the relevant instability is shown to have the features necessary to reproduce the variety of bursts that are observed experimentally. These are the periodicity, the duration, and the shape of the bursts, special consideration being given to the excitation of modes by high-energy particle populations in thermalized plasmas and to a model for the transition from a bursting state to one of stationary fluctuations. A model is introduced that is relevant to the case where the spatial dependence of the mode amplitude is important. The application of the given analysis to the bursty wave emissions observed in space is discussed.
Transport matrix and electric field formation in stochastic magnetic field
Authors: Kubota Tetsuyuki , Itoh Sanae-I.
Published: July 2002

The anomalous transport of high-temperature plasma in the region of a stochastic magnetic field line is studied. Plasmas in toroidal systems such as tokamaks are considered. The plasma transport matrix in the stochastic region is derived, with the assumption of a quasilinear approximation for the diffusivity of the magnetic field line, DM. By use of this transport matrix, the radial electric field formation and the associated temperature distribution in the stochastic region are obtained. The magnitude of radial electric field is found to depend on the value of DM and on the heat fluxes and temperatures of electrons and ions in the nonstochastic region. The buildup of the radial electric field, in turn, affects the transport coefficients as well as the temperature gradients in that region.
Three-dimensional fluid simulations of tokamak edge turbulence
Authors: Zeiler A. , Biskamp D. , Drake J. F. , Guzdar P. N.
Published: July 2002

Three-dimensional (3-D) simulations of drift-resistive ballooning turbulence are presented. The turbulence is basically controlled by a parameter α, the ratio of the drift wave frequency to the ideal ballooning growth rate. If this parameter is small [α≤1, corresponding to Ohmic (OH) or low confinement phase (L-mode) plasmas], the system is dominated by ballooning turbulence, which is strongly peaked at the outside of the torus. If it is large [α≥1, corresponding to high confinement phase (H-mode) plasmas], field line curvature plays a minor role. The turbulence is nonlinearly sustained even if curvature is removed and all modes are linearly stable due to magnetic shear. In the nonlinear regime without curvature the system obeys a different scaling law compared to the low-α regime. The transport scaling is discussed in both regimes and the implications for OH, L-mode, and H-mode transport are discussed.
The generalized Ohm’s law in collisionless magnetic reconnection
Authors: Cai H. J. , Lee L. C.
Published: July 2002

The generalized Ohm’s law and the force balance near neutral lines in collisionless magnetic reconnection is studied based on two-dimensional full particle simulations in which the ion–electron mass ratio is set to be 1836. The off-diagonal elements of a plasma pressure tensor are found to be responsible for the breakdown of the frozen-in condition in collisionless reconnection. While the off-diagonal elements of the electron pressure tensor are dominant terms in the generalized Ohm’s law near neutral lines, the ion off-diagonal pressure terms are of significant importance when ions are main current carriers. The spatial scale of electron off-diagonal pressure term Pxy(e) is also found to be proportional to the Dungey length scale, (meEy/eβ2)1/3, where β=∂Bz/∂x.
Omnigenity and quasihelicity in helical plasma confinement systems
Authors: Cary John R. , Shasharina Svetlana G.
Published: July 2002

Omnigenous stellarators, those with bounce averaged drifts lying within the magnetic surfaces, form a much larger class than quasihelical stellarators (i.e., with magnetic-field strength depending on only a single linear combination of the toroidal flux angles) while nevertheless having very good transport properties. It is shown by construction that exactly omnigenous magnetic strengths can be very far from quasihelical, but that all such magnetic strengths are nonanalytic. However, by truncation of the Fourier representation one can obtain analytic magnetic strengths that are very close to being omnigenous while still very far from quasihelical. This indicates that the condition of quasihelicity can be significantly relaxed and yet retain good neoclassical transport. Finally, it is conjectured that reasonably good transport can be obtained by requiring omnigenity for only the deeply trapped and marginally trapped particles. Such fields will have no transition particles and, hence, no separatrix crossing chaos. These conditions reduce to the requirements that the field line maxima and minima of the magnetic field and the separatrix action be constant on a magnetic surface.
Spectroscopic determination of the magnetic-field distribution in an imploding plasma
Authors: Davara G. , Gregorian L. , Kroupp E. , Maron Y.
Published: July 2002

The time-dependent radial distribution of the magnetic field in a high density z-pinch plasma has been determined by observation of the contribution of the Zeeman effect to the spectral profiles of ionic emission lines. The dominance of the line profiles by the Stark broadening required high-accuracy profile measurements and the use of polarization spectroscopy. The plasma implodes in ≃600 ns, and the field distribution was measured up to 90 ns before stagnation on axis. During the implosion the plasma was found to conduct the entire circuit current. By comparing the data to the solution of the magnetic diffusion equation the electrical conductivity of the plasma was determined, found to be in agreement with the Spitzer value. These measurements, together with our previously measured ion velocity distributions, allowed for the determination of the time-dependent relative contributions of the magnetic and thermal pressure to the ion radial acceleration across the plasma shell.
Finite bootstrap current density and finite neoclassical reduction of electrical conductivity at the magnetic axis of a tokamak
Authors: Wang Shaojie
Published: July 2002

Conventional neoclassical theory predicts that the bootstrap current density and the neoclassical reduction of electrical conductivity approach to zero at the magnetic axis, since the fraction of trapped particles goes to zero at the magnetic axis. In this paper, it is shown that there exist finite bootstrap current density, and finite neoclassical reduction of electrical conductivity at the magnetic axis due to the effect of the finite trapping ratio at the magnetic axis.
Generalized action invariants for drift waves-zonal flow systems
Authors: Smolyakov A. I. , Diamond P. H.
Published: July 2002

Generalized action invariants are identified for various models of drift wave turbulence in the presence of the mean shear flow. It is shown that the wave kinetic equation describing the interaction of the small scale turbulence and large scale shear flow can be naturally writen in terms of these invariants. Unlike the wave energy, which is conserved as a sum of small- and large-scale components, the generalized action invariant is shown to correspond to a quantity which is conserved for the small scale component alone. This invariant can be used to construct canonical variables leading to a different definition of the wave action (as compared to the case without shear flow). It is suggested that these new canonical action variables form a natural basis for the description of the drift wave turbulence with a mean shear flow.
Melting of Ni40Pd40P20 glass
Authors: Kui H. W. , Turnbull D.
Published: July 2002

Upon reheating, most glassy metal alloys crystallize at a temperature not far removed from the glass temperature Tg and far below the liquidus temperature Tl. We have reported that the alloy Ni40Pd40P20, which exhibits a scaled glass temperature ∼0.68, has been melt quenched, under a flux of dehydrated B2O3, to glass at rates as low as 1°/s. Here we report experiments in which some specimens of this alloy, when similarly fluxed during reheating, have been reheated to temperatures within 50° of Tl and 280° above Tg at rates ∼2.5°/s, and then cooled again to Tg, without crystallization. From this behavior we infer that the steady frequency of homogeneous nucleation of crystals in the alloy is <10−1 cm−3 s−1.
Low loss InGaAs/InP multiple quantum well waveguides
Authors: Koren U. , Miller B. I. , Koch T. L. , Boyd G. D. , Capik R. J. , Soccolich C. E.
Published: July 2002

Double heterostructure planar waveguides with an InGaAs/InP multiple quantum well (MQW) core and InP cladding layers were grown by atmospheric pressure metalorganic chemical vapor deposition. Ridge waveguides had a low propagation loss of 0.8 dB/cm for 1.52 μm input light. The indices of refraction for the guided TE and TM modes have been measured and the bulk dispersion curves of the MQW material for the 1.46–1.55 μm wavelength region were derived.
Transmission electron microscopy of aluminum implanted and annealed (100) Si: Direct evidence of aluminum precipitate formation
Authors: Sadana D. K. , Norcott M. H. , Wilson R. G. , Dahmen U.
Published: July 2002

Aluminum implantation into (100) Si has been studied here by transmission electron microscopy (TEM), secondary ion mass spectrometry, and spreading resistance methods. For the implantation conditions (2×1015 cm−2, 200 keV, room temperature) used in our experiments a buried amorphous layer is created from 50 to 300 nm below the surface. During N2 annealing at 600–1000 °C, two discrete layers of dislocations at (110 and 330 nm) bounding a band of precipitates (>1011 cm−2) were created, accompanied by a pronounced redistribution of Al in the implanted region. Three Al peaks at depths corresponding to the defect layers observed by TEM appeared. The positions and number of the peaks remained almost unaffected as a function of annealing temperature. High resolution TEM showed that the band of precipitate contained oriented Al particles and that these particles also nucleated on dislocations. The stability of the Al peak and associated low electrical activity (<10%) has been explained by taking into account the formation of Al precipitates and their interaction with the oxygen in the surrounding regions.
Annealing treatment effects on structure and superconductivity in Y1Ba2Cu3O9−x
Authors: Beyers R. , Lim G. , Engler E. M. , Lee V. Y. , Ramirez M. L. , Savoy R. J. , Jacowitz R. D. , Shaw T. M. , La Placa S. , Boehme R. , Tsuei C. C. , Park Sung I. , Shafer M. W. , Gallagher W. J.
Published: July 2002

We report the effects of heat treatment and ambient on the structure and superconducting properties of Y1Ba2Cu3O9−x. The structure undergoes an orthorhombic-to-tetragonal transition on heating at about 700 °C, caused by oxygen loss and disordering of oxygen vacancies on the copper plane between the barium layers. Heat treatments that promote maximum ordering of the oxygen vacancies result in superior superconducting properties.
Oxygen precipitation in heavily boron-doped silicon crystals
Authors: Tsai H. L. , Stephens A. E. , Meyer F. O.
Published: July 2002

Transmission electron microscopy was applied to study oxygen precipitation in P+ wafers by comparison with that in P− wafers grown in the same puller under the same growth schedule. Besides the higher precipitate density in P+ wafers for all heat treatments, it was shown that nucleation centers can be easily re-established in P+ wafers at a low temperature after a prior denuding anneal. The precipitate morphology was found to depend on the precipitate growth rate and/or the presence of impurities. The morphology evolution from a small polyhedron to a large plate during the precipitate growth is proposed to explain the observed shapes.