bluuebunny/crossref_metadata_2025 huggingface dataset:
Homogeneous buoyancy-generated turbulence
Authors: Batchelor G. K. , Canuto V. M. , Chasnov J. R.
Published: July 2002

If a random and statistically homogeneous density distribution is created in a fluid, a random motion of the fluid is subsequently generated by buoyancy forces. This motion is resisted by viscous stresses, while the density variation is smoothed by molecular diffusion of the relevant scalar property of the fluid. Furthermore, advective mixing generates smaller-scale components of the scalar quantity, increasing the rate of smoothing. If the Reynolds number of the motion is sufficiently large, the motion becomes turbulent in the ordinary sense. Such turbulence may be said to be generated by an ‘‘active’’ conserved scalar quantity. To elucidate the nature of this buoyancy-driven turbulence, we consider an infinite fluid that is initially at rest everywhere, with a given distribution of the conserved scalar quantity. In order to have a well-defined initial state specified by a manageably small number of parameters, the initial distribution of the scalar is assumed to be statistically homogeneous and isotropic, and to be characterized by a single length L, and a measure of the magnitude of the scalar variations, say, the initial root-mean-square scalar fluctuation θ0. The other parameters on which the field of buoyancy-driven turbulence depends are the kinematic viscosity ν, the diffusivity of the conserved scalar quantity D, and the gravitational constant g.

Dimensionless variables are constructed by choosing L as the unit of length and [L/gθ0]1/2 as the unit of time. The choice of these units introduces two dimensionless parameters into the problem, a Reynolds number R=[gθ0L3]1/2/ν and a Schmidt number σ=ν/D. Since analytical treatment of the problem is limited to the special cases where the nonlinear interactions may be neglected (e.g., R and σR≪1), the primary means of inquiry is a numerical simulation of the flow field. We have performed numerical simulations (using a subgrid model) which sample the entire parameter space of the flow. Of particular interest is the asymptotic behavior of the flow as R and σR increase. One observes that a competition arises between the increase in magnitude of the fluid velocity according to the linear equations and the nonlinear generation of small-scale density and velocity fluctuations. The linear analysis predicts that, in dimensionless units, the time tm at which the mean-square velocity fluctuations reach a maximum, as well as the value of this maximum 〈uiui〉m, increases without limit as R and σR increase. In contrast, the results of the numerical simulation show that tm and 〈uiui〉m become independent of R and σR for sufficiently large values of these parameters. The linear amplification of the flow is thus checked by nonlinear effects. Buoyancy-generated turbulence at high Reynolds number has a period of growth from an initial state of rest, lives for a time in which there is a burst of activity, and then dies in consequence of this activity. This example of self-induced mixing can be given a quantitative description which may be useful in other contexts.
Three-dimensional structure of the curved mixing layer using image reconstruction and volume rendering
Authors: Karasso P. S. , Mungal M. G.
Published: July 2002

This study investigates the structure and mixing of the two-dimensional turbulent mixing layer when subjected to longitudinal streamwise curvature. The straight layer is now well known to be dominated by the primary Kelvin–Helmholtz (KH) instability as well as the secondary Taylor–Goertler (TG) instability. For equal density fluids, placing the high-speed fluid on the inside of a streamwise bend causes the TG instability to be enhanced (unstable case), while placing the low-speed fluid on the inside of the same bend leads to the suppression of the TG instability (stable case). The location of the mixing transition is correspondingly altered. Our goal is to study the changes to the mixing field and growth rate resulting from the competition between instabilities.

Our studies are performed in a newly constructed blow-down water facility capable of high Reynolds numbers and excellent optical access. Maximum flow speeds are 2 and 0.25 m/sec for the high- and low-speed sides, respectively, leading to maximum Reynolds numbers of 80 000 based on velocity difference and the width of the layer. We are able to dye one stream with a fluorescent dye, thus providing several planar views of the flow under laser sheet illumination. These views are superior to conventional approaches as they are free of wall effects and are not spatially integrating. However, our most useful diagnostic of the structure of the flow is the ability to record high-speed images of the end view of the flow that are then reconstructed by computer using the volume rendering technique of Jiménez et al.1 This approach is especially useful as it allows us to compare the structural changes to the flow resulting from the competition between the KH and TG instabilities. Another advantage is the fact that several hundred frames, covering many characteristic times, are incorporated into the rendered image and thus capture considerably more flow physics than do still images. We currently have our rendering techniques fully operational,2 and are presently acquiring high quality high-speed movies of the various flow cases.

Our findings to date, based on planar time-averaged and instantaneous views, show the following: (1) a 50% increase in growth rate from the stable to the unstable case resulting from mild curvature; (2) an enhancement of the TG vortices in the unstable case, but without major disruption of the KH instability which remains relatively intact; and (3) the occurrence of the KH instability at angles tilted with respect to the splitter plate tip, in agreement with the predictions of linear stability theory. This final observation has not been reported to date, primarily because sheet techniques have not been used at Reynolds numbers as high as the present study. The presentation will provide detailed views of the changes between the stable, straight, and unstable cases using our volume rendering approach, and will provide statistical measures such as changes to vortex spacing and size, to quantify such changes.
A variational principle applied to the turbulent drag law
Authors: Stern Melvin E.
Published: July 2002

A general variational principle for turbulence is applied to the large Reynolds number flow behind a cylinder, leading to the conclusion that the drag force must be extremized subject to the dynamical constraints. By using only two approximate versions of these, a drag coefficient equal to 3/8 is obtained. Comparison is made with experimental values in the ‘‘supercritical’’ (Re=106) and ‘‘transcritical’’ (Re=107) regimes.
Particle dynamics in a viscously decaying cat’s eye: The effect of finite Schmidt numbers
Authors: Newton P. K. , Meiburg Eckart
Published: July 2002

The dynamics and mixing of passive marker particles for the model problem of a decaying cat’s eye flow is studied. The flow field corresponds to Stuart’s one-parameter family of solutions [J. Fluid Mech. 29, 417 (1967)]. It is time dependent as a result of viscosity, which is modeled by allowing the free parameter to depend on time according to the self-similar solution of the Navier–Stokes equations for an isolated point vortex. Particle diffusion is numerically simulated by a random walk model. While earlier work had shown that, for small values of time over Reynolds number t/Re≪1, the interval length characterizing the formation of lobes of fluid escaping from the cat’s eye scales as Re−1/2, the present study shows that, for the case of diffusive effects and t/Pe≪1, the scaling follows Pe−1/4. A simple argument, taking into account streamline convergence and divergence in different parts of the flow field, explains the Pe−1/4 scaling.
Axisymmetric waves on a vertical vortex in a stratified fluid
Authors: Miyazaki Takeshi , Fukumoto Yasuhide
Published: July 2002

Axisymmetric wave propagation along a vertical vortex core in a stably stratified fluid is considered theoretically. The fluid is assumed to be inviscid, incompressible, nondiffusive, and exponentially stratified. A linear analysis under the Boussinesq approximation shows that discrete inertial modes (bounded modes) are allowed in addition to continuous internal gravity waves (unbounded modes), when the stratification is not too strong. These inertial modes, whose eigenfunctions are confined to the vorticity region, disappear if the Brunt–Väisälä frequency N2 exceeds the maximum value of the Rayleigh function. Concrete results are given for the Burgers vortex. A weakly nonlinear analysis indicates that inertial modes (if permitted) are generated through the resonant interactions between two internal gravity waves. The time evolution of its amplitude is described by a cubic nonlinear Schrödinger equation, which admits envelope soliton solutions for shorter carrier waves only, viz., the soliton window has a low wave-number cutoff.
Stokes drag on a narrow annular disk sedimenting in the presence of fixed boundaries or other disks
Authors: Davis A. M. J.
Published: July 2002

The use of axisymmetric distributions of singularities to describe the translation of an annular disk in the presence of fixed boundaries or other disks is complicated by the appearance of a singularity at the inner rim. This makes the numerical solution of the integral equations more difficult and less accurate than for the corresponding motions of a unit disk discussed in an earlier paper [Phys. Fluids A 2, 301 (1990)]. However it is observed here that the asymptotic methods developed by a previous author [D. A. Spence, Proc. Cambridge Philos. Soc. 68, 529 (1970)], for the limit in which the ratio of radii approaches unity, can, to leading order, be used to obtain more simply estimates of the influence of fixed walls or other disks on the drag force and fluid flux through the hole. Interpolation then could provide estimates of these physical quantities for radii ratios that are close to neither unity nor zero.
Dissociation–recombination relaxation behind a normal shock
Authors: Kirkköprü Kadir , Kassoy D. R.
Published: July 2002

Mathematical methods developed originally for thermal explosion theory are used to study the evolution of a dissociation–recombination process occurring in a compressible gas behind a normal shock. The dissociation–recombination reaction, AB+M■A+B+M, is assumed to have a relatively large dissociation temperature, so that high activation energy perturbation techniques can be used to derive general parameter dependent analytical solutions. Spatial variation of the dependent variables is described in three zones, each of distinct length scale and physical character. Initially, small changes in temperature, concentrations, density, and pressure occur in the relatively high temperature dissociation initiation zone. In the subsequent broader major dissociation region, most of the specie AB is converted to A and B and there are significant variations in all physical variables. In the last and thickest zone, recombination becomes important as the reactive flow evolves to a final equilibrium state. The results provide an analytical counterpart to numerical solutions obtained earlier by Freeman [J. Fluid Mech. 4, 407 (1958)] and Matthews [Phys. Fluids 2, 170 (1959)] for the now classical problem of idealized diatomic gas dissociation.
Lagrangian turbulence and the Brownian motion paradox
Authors: Viecelli J. A.
Published: July 2002

The unique properties of three-dimensional hydrodynamic turbulence depend on the nature of the long-range time correlations as well as the spatial correlations. Although Kolmogorov’s second similarity hypothesis predicts a power-law spatial scaling exponent for the Eulerian velocity fluctuations in agreement with experiments, it also leads, via the Lagrangian velocity time structure function relationship, to particle dispersion predictions that are inconsistent with enhanced diffusion. Recently, a new computational technique has been developed which can generate random power-law correlated fields in any number of dimensions with unlimited scale range. This new method is used to explore the consequences of a proposed set of assumptions about the nature of the time correlations and their relationship to the spatial correlations. In particular, the Brownian motion paradox is examined and it is shown that it can be resolved if the time domain constraint part of Kolmogorov’s second hypothesis is relaxed and replaced with an assumption of space-time isotropy. The proposed modification preserves the observed one-dimensional k−5/3 spatial energy spectrum, allows for enhanced diffusion consistent with Richardson’s law, is consistent with Taylor’s frozen turbulence assumption under the appropriate conditions, and yields an ω−5/3 frequency spectrum for the velocity fluctuations in a frame at rest with respect to the turbulence.
A minimum principle for Stokes flows containing rigid particles and an upper bound on the sedimentation speed
Authors: Luke Jonathan H. C.
Published: July 2002

A minimum principle for Stokes flows containing rigid particles is presented. This variational principle is the hydrodynamic analog of the minimum principle of electrostatics known as Thomson’s theorem. In comparison with other variational principles for Stokes flow (such as the classical minimum principle due to Helmholtz), the construction of admissible flows for the variational principle presented is quite simple owing to a representation as a single layer potential. An upper bound on the average sedimentation speed in a suspension follows immediately from the variational principle. Estimation of the sedimentation speed of a rectangular plate using admissible flows generated from a uniform stress distribution on the plate gives the sedimentation speed with a relative error of about 5%.
Detonation waves and the front tracking method
Authors: Bukiet Bruce G. , Menikoff Ralph
Published: July 2002

Detonation waves dominate the fluid flow in an explosive material. For numerical simulations, there is a need to account for the important properties of a propagating detonation wave without the fine grid necessary to resolve the very narrow reaction zone. The effects of the dynamics within the reaction zone on the fluid state behind the detonation wave are discussed. In contrast to a shock wave, the behavior of a detonation wave depends on the front curvature. Moreover, the flow behind a diverging underdriven detonation wave is supersonic relative to the front. This leads to composite waves and split waves which are analogous to those that occur for a fluid with a nonconvex equation of state. The wave structure and the wave curve for both converging and diverging detonation waves are described. The wave curve allows numerical calculations using the front tracking method to model multidimensional detonation problems. Implementing the analytic wave structure in numerical calculations is discussed.
Nonsteady detonation driven by a hypervelocity jet in a homogeneous explosive
Authors: Asay Blaine W. , McAfee John M. , Ferm Eric N.
Published: July 2002

Experiments are reported that show both steady and nonsteady detonation behavior of nitromethane (NM) that is being penetrated supersonically by a shaped-charge jet. The initial jet velocity and diameter were chosen so that prompt detonation (i.e., occurring immediately upon impact) did not occur and threshold initiation behavior could be observed. For the nonsteady case, after the jet entered the explosive, the penetration bow shock strengthened sufficiently to initiate a detonation whose velocity was greater than the jet penetration velocity. However, the diameter over which the reaction was established was insufficient, and the detonation subsequently failed. Upon failure, a penetration bow shock was again established and the initiation process was then repeated. Comparisons are made with results from tests in which jets of different velocities were used and also from tests in which the target material was water. The effects of jet particulation at late times were also observed.
Parametric excitation of buoyancy oscillation and formation of internal boundary layers in a stably stratified fluid
Authors: Miyazaki Takeshi
Published: July 2002

Resonant excitation of buoyancy oscillation by strained vertical vortices in a stably stratified fluid, which leads to formation of new internal boundary layers, is investigated theoretically. Any two-dimensional steady vorticity distributions, uniform in the vertical direction, are allowable under the Boussinesq approximation, i.e., at the zeroth order of Froude number (Fr) expansions. But, such an inviscid solution shows algebraic divergence at the first order. This anomaly is attributable to the resonance between the vertical oscillation due to a gravitational restoring force and the rotational motion of a fluid particle along a closed streamline. Weak viscous and diffusive effects are incorporated in order to remove the singularity and to obtain stationary solutions. Internal boundary layers of thickness proportional to Re−1/3(1+Pr−1)1/3 are formed, inside of which vertical velocity variations of magnitude N2 Fr Re1/3(1+Pr−1)−1/3 are observed. Here, the parameters Re, Pr, and N2 denote the Reynolds number, Prandtl number, and the normalized Brunt–Väisälä frequency, respectively.
Thermoelectric effects in decaying homogeneous magneto-gas turbulence
Authors: Shebalin John V.
Published: July 2002

In the formulation of compressible magnetohydrodynamics (MHD) (i.e., magneto-gas dynamics), a ‘‘generalized Ohm’s law’’ is required. In particular, an electron pressure term and a ‘‘Hall effect’’ term may appear as non-negligible additions to the Ohm’s law that is conventionally used for incompressible MHD. In ‘‘high-beta’’ (i.e., relatively low magnetic energy) situations, the Hall term may be neglected (at least initially) but, as it turns out, the electron pressure term cannot be neglected. Here, three-dimensional, high-beta, homogeneous, decaying, magneto-gas turbulence is examined with regard to this additional term. Through numerical simulation, it is found that ‘‘thermoelectric effects’’ are produced that significantly alter the evolution of the magnetic field and electric current strengths.
Coarse-grain entropy in two-dimensional turbulence
Authors: Dupree Thomas H.
Published: July 2002

A generalized coarse-grain entropy for a simple two-dimensional system is derived and used to explain the emergence of coherent structures, which are the most probable states, and their equilibrium with random fluctuations. The relationship between the canonical distribution function and the most probable states, which appear at critical values of the inverse temperature β, is elucidated. Under some conditions, the coherent structures are localized and intermittent, i.e., occupy only a small fraction of the system. For small fluctuation amplitudes, the macrostate entropy is equal to minus the coarse-grain enstrophy, in agreement with the selective decay and minimum enstrophy theories. Maximizing the macrostate entropy leads to vortex coalescing, and in some cases, a process of concentration occurs in which vorticity and energy are concentrated into vortices of decreasing area.
Numerical modeling of a magnetically induced compression gas puff
Authors: Badaye M. , Gupta R. P.
Published: July 2002

A two-dimensional magnetohydrodynamic code is developed to study the dynamic performance of a magnetically induced compression (MIC) gas puff. It is shown that the plasma temperature in the gas puff plenum can rise up to 10–12 eV. The maximum calculated sheath velocity is in the 107 cm/sec range. Calculations also show that the pressure in the gas puff plenum can rise by a factor of 570–1360, depending on the initial pressure. The mass flow rate and the injected gas density fall in the proper range for the operation of a Z pinch only when a relatively high initial pressure is used. The simulated ion-induced and light-induced voltages in the ion and light detectors provide means of comparing the numerical results with measurements. The calculated results are found to match reasonably well with the measured values.
Comments on ‘‘Ion-temperature-gradient-driven transport in a density modification experiment on the Tokamak Fusion Test Reactor’’ [Phys. Fluids B 4, 953 (1992)]
Authors: Nordman H. , Weiland J.
Published: July 2002

Several points concerning the section ‘‘Comparison with the Swedish stability and transport analysis’’ in the paper by Horton et al. are discussed and clarified. In particular it is pointed out that the Swedish model gives about the right ratio of χe/χi for the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 63, 520 (1989)] density modification experiment.
Resistive ballooning in the toroidal reversed-field pinch
Authors: Dagazian R. Y. , Chance M. S.
Published: July 2002

Various toroidal reversed-field pinch (RFP) equilibria stable to ideal Mercier and ballooning modes are subjected to resistive ballooning analysis. The stability properties of these configurations are thus investigated as a function of quantities that are experimentally measurable. Resistive ballooning is seen as a likely candidate for the interpretation of a certain class of density fluctuation phenomena observed in these devices.
Adiabatic compression and interchange stability of rotating compact toroid plasmas
Authors: Hameiri Eliezer , Stevens Donald C. , Grossmann William
Published: July 2002

The adiabatic compression of plasmas with mass rotation is formulated as a variational principle based on some constants of the motion. A numerical code is constructed and Grad’s alternating dimensions method is used. The calculations simulate compression either by increasing the flux outside the plasma or by liner compression. Changes in Mach number and some stability criteria during compression are monitored. Interestingly, the Mach number remains almost constant despite a spin up of the plasma. An extension of the interchange stability criterion for rotating plasmas is developed. Interchange stable equilibria exhibit very high current density near the separatrix and tend to be more elongated. Interchange instability offers a mechanism to generate rotation throughout the plasma.
Generation of electromagnetic radiation by a field-aligned electron beam
Authors: Shoucri Magdi
Published: July 2002

A new mechanism for the direct generation of electromagnetic radiation by a field-aligned electron beam is presented. It is shown that, under the condition εc=ω2p/ω2c ≪1, a weakly relativistic beam can couple linearly the two branches of the extraordinary mode. It is suggested that such a mechanism can offer one more possible source for the auroral kilometric radiation.
Polarized electron-cyclotron emission in a poloidal-divertor tokamak
Authors: Sengstacke M. A. , Dexter R. N. , Prager S. C.
Published: July 2002

The polarization of electron-cyclotron emission at the second harmonic (ω=2ωce) has been measured both without and with a microwave absorber installed within view of the receiver to reduce wall reflection. Whereas without the absorber the radiation is dominantly unpolarized, with the absorber installed the radiation is significantly polarized, and the extraordinary mode intensity is typically three times the ordinary mode intensity. This indicates that wall reflections significantly depolarize the received radiation. Right-hand-cutoff effects are also observed and electron temperature is inferred from the emission from optically thin plasmas.
Taylor column sidewall drag in a rotating cylinder
Authors: Scott W. E.
Published: July 2002

A time-dependent analysis is made of the inviscid, incompressible Taylor column generated by the uniform motion of a thin disk of radius a rising slowly along the vertical axis of rotation of a tall, vertical, rotating, liquid-filled right circular cylinder of radius b. It is shown analytically that the effect of the sidewalls of the cylinder on the Taylor column drag on the disk may be neglected if a/b<0.1. Even if a/b∼0.4, the increase in drag above unbounded Taylor column value is less than 10%.
Geometric optics at lower hybrid frequencies
Authors: Riedel K. S.
Published: July 2002

Geometric optics is analyzed at lower hybrid frequencies using a realistic scaling of the cold plasma conductivity tensor. An explicit example displays the inadequacy of standard geometric optics in treating spatially varying plasmas. A new term is added to the ray Hamiltonian to remove this inconsistency. An adiabatic invariant, corresponding to the poloidal rotation of the ray, is introduced. The cold plasma waves can develop a singularity only at those points where the flux surface is tangent to upper and lower hybrid resonance surfaces, thus supporting the conjecture that heating occurs only at these points. The only other spatial points where singularities may develop in the geometric optics ray equations are the cyclotron resonances.
Filamentation instability of a radially finite relativistic electron beam
Authors: Shanahan William R.
Published: July 2002

A study of the filamentation of a radially bounded relativistic electron beam propagating through a background plasma is presented. Through a careful consideration of all relevant boundary terms, both geometrically modified bulk as well as surface waves are exhibited. Approximate formulae for the growth rates of the two species of modes are derived. While no finite geometry effects arise to vitiate the stability criteria previously derived for the bulk modes in the homogeneous case, that found for the surface modes is slightly more stringent. Stability requires an external magnetic field twice as strong as that predicted by previous analyses. This added stringency is created by additional terms in the boundary conditions, which are displayed here for the first time. Selected numerical results demonstrate that the filamentation instability is unlikely to be the source of the strong beam disruption observed in recent experiments.
Kinetic equilibrium and stability properties of high-current betatrons
Authors: Petillo John J. , Davidson Ronald C.
Published: July 2002

Kinetic stability properties of an intense relativistic electron ring in modified and conventional betatron configurations are investigated using the linearized Vlasov–Maxwell equations. Included is the important influence of intense equilibrium self-fields. It is assumed that the ring is thin, and that ν/γb≪1, where ν is Budker’s parameter and γbmc2 is the characteristic electron energy. The stability analysis is carried out for eigenfrequency ω close to harmonics of the cyclotron frequency ωcz in the vertical betatron field. Also included in the analysis is the influence of transverse electromagnetic effects and surface-wave perturbations. Dispersion relations for longitudinal perturbations are obtained, where it is assumed that the ring is located inside a perfectly conducting toroidal shell. There are several noteworthy points. First, transverse electromagnetic effects can completely stabilize the negative-mass instability for sufficiently high-current rings when betatron

focusing forces exceed defocusing self-field forces (ω2cz>ω2pe/γ2b). Second, for ω2cz>ω2pe/γ2b with no charge neutralization or stabilizing spread in canonical angular momentum ( f=0 and Δ=0), surface-wave instabilities can be completely stabilized at a sufficiently low transverse beam temperature. Third, for ω2cz<ω2pe/γ2b, sufficiently low transverse temperature together with surface effects combine to drive a radial kink instability. Finally, the dispersion relation is analyzed numerically for parameters representative of the Naval Research Laboratory’s modified betatron (National Technical Information Service Document No. AD-A108359/1) and the Los Alamos National Laboratory’s Liner Driven Ring Accelerator (National Technical Information Service Document No. DE84007994) and Phermex Injected Conventional Betatron (National Technical Information Service Document No. DE86002441/XAB). Detailed stability results are presented for the projected operating regimes of these devices, including the effects of canonical angular momentum spread, inverse aspect ratio, slowly varying accelerating fields, location of the conducting wall, and radial elongation of the minor ring cross section.
Plasma column development in the CO2 laser-heated solenoid
Authors: Tighe W. , Offenberger A. A. , Capjack C. E.
Published: July 2002

Axial and radial plasma dynamics in the CO2 laser-heated solenoid have been studied experimentally and numerically. The axial behavior is found to be well described by a self-regulated bleaching wave model. The radial expansion is found to be strongly dependent on the focusing ratio of the input laser beam. With a fast focus ( f/5), the early radial expansion rate is twice that found with a slower focusing arrangement ( f/15). The faster focusing ratio also results in a significantly wider plasma column. On the other hand, no significant dependence of f/♯ on the axial propagation was found. A finite ionization time and the rapid formation of a density minimum on axis are observed and verify earlier experimental results. Detailed comparisons are made with a 2-D magnetohydrodynamic (MHD) and laser propagation code. The axial and radial plasma behavior and, in particular, the dependence of the radial behavior on the focal ratio of the laser are reasonably well supported by the simulation results. Computational results are also in good agreement with experimental measurements of temperature and density using stimulated scattering (Brillouin, Raman) and interferometry diagnostic techniques.
Particle confinement improvement during 2.45 GHz lower-hybrid current-drive experiments
Authors: Mayberry M. J. , Chen K-I. , Luckhardt S. C. , Porkolab M.
Published: July 2002

Particle confinement behavior during 2.45 GHz lower-hybrid current drive has been investigated on the Versator II tokamak [Phys. Fluids 29, 1985 (1986)]. It is found that during combined Ohmic and rf current drive the global particle confinement time τp increases by up to a factor of 2 compared to purely Ohmically driven discharges, as observed in earlier 800 MHz experiments at lower densities n̄e ≤6×1012 cm−3. In the present experiments, τp increases have been observed at densities up to n̄e =2×1013 cm−3.
Ringdown of inertial waves during spin-up from rest of a fluid contained in a rotating cylindrical cavity
Authors: Stergiopoulos S. , Aldridge K. D.
Published: July 2002

Time-dependent complex eigenfrequencies for traveling inertial waves in a rotating cylinder of fluid have been measured while the fluid was spun up from rest. The waves were excited by precessing the lid of the cylindrical container at a frequency near a predicted eigenfrequency for a given rotation speed. Time sequences of disturbance pressure differences measured during subsequent free ringdown of the excited wave were inverted to recover inertial eigenfrequency and decay rate using a generalized least-squares procedure.
Comments on ‘‘The onset of Alfvénic turbulence’’ [Phys. Fluids 3
, 1371 (1987)]
Authors: Mjo/lhus Einar
Published: July 2002

The interpretation of the derivative nonlinear Schrödinger model for nonlinear Alfvén waves is questioned. In particular, the damping term used by Ghosh and Papadopoulos [Phys. Fluids 30, 1371 (1987)] is shown to be inconsistent.
Reply to comments of Mjo/lhus
Authors: Ghosh S. , Papadopoulos K.
Published: July 2002

A reply to the comments of Mjo/lhus on a recent paper [Phys. Fluids 30, 1371 (1987)] is presented. It is shown that although some of the criticism is valid, other points cited by Mjo/lhus do not appear to be mistakes on our part.
Gallery of fluid motion
Authors: Reed Helen L.
Published: July 2002

This article displays winning photographs from the fifth Annual Fluid Mechanics Photo Contest held at the November 1987 meeting of the American Physical Society, Division of Fluid Dynamics, Eugene, Oregon.
Density of states of inviscid incompressible two-dimensional fluid unit disk vortex systems
Authors: Johnson Darell J.
Published: July 2002

The density of states of discrete line vortex fluids in cylindrical pipes is experimentally determined for low vortex number systems via a Monte Carlo numerical computer simulation calculation. The results indicate that the theoretical results of Pointin and Lundgren [Phys. Fluids 19, 1459 (1976)] are valid for very small vortex number systems.
The local Alfvén resonance in a toroidal plasma
Authors: Dolgopolov V. V. , Kryukov A. V. , Romanov S. S.
Published: July 2002

The local Alfvén resonance in the toroidal plasma is investigated. Equations of resonance surfaces and fields near the resonance surfaces with and without rotational transform of the magnetic field are found. It is shown that under conditions of fixed wave frequency in the torus with rotational transform there are regions where the Alfvén resonance is forbidden at any value of wavenumber.
Bubble characteristics and trajectories in a microbubble boundary layer
Authors: Pal S. , Merkle C. L. , Deutsch S.
Published: July 2002

Optical and photographic surveys of microbubble boundary layers are presented. The results show that the outer edge of the bubble cloud diffuses away from the wall as the bubbles are swept downstream. The plate-on-bottom orientation contains a bubble-free region near the wall that cannot be discerned for the plate-on-top configuration. Skin friction measurements made when the bubble-free region extends to y+=200 show there is no longer any Cf reduction present suggesting bubbles are not effective when they are outside the near-wall region of the boundary layer. Bubble sizes, which increase with airflow and distance from the injection section and decrease with free-stream velocity, were measured to be between 150 and 1100 μm.
Bromine dot target plasma spectroscopy: A detailed analysis
Authors: Cerjan Charles
Published: July 2002

A detailed simulation of the spectra produced from a bromine dot target by laser irradiation is presented. This study incorporates the results of a two-dimensional hydrodynamic calculation and an extensive atomic rate data base to generate synthetic spectra under conditions that match those of an earlier experiment. A dynamic picture of the plasma’s evolution is given that is consistent with experimental results. Overall plasma characteristics are calculated that produce spectra largely in agreement with those observed. Also, sensitive dielectronic recombination features are reproduced corroborating the adequacy of this approach.
Direct solution for the power spectrum of the Lorenz attractor
Authors: Andrews P. L. , Waltz R. E.
Published: July 2002

A novel method is applied to the Lorenz system that deals directly with the power spectrum of the turbulent solutions. Analytic predictions are produced as to the level of turbulence and the spread in frequency space of the solution that are in good agreement with numerical computations. The method involves manipulation of the Fourier transformed Lorenz equations and their tempered distribution solutions.
Turbulence modification by large-scale organized electrohydrodynamic flows
Authors: Soldati Alfredo , Banerjee Sanjoy
Published: July 2002

The interactions of flows generated by ionic discharges with wall turbulence are not only of interest for turbulence control, but also for devices of industrial importance, such as wire-plate electrostatic precipitators (ESPs). Under conditions of uniform discharge, in wire-plate ESPs, arrays of regular, spanwise vortices are found in the absence of a through-flow. These arise from ionic discharges from the spanwise wires placed between the grounded plates on each side. The interactions of such electrohydrodynamic (EHD) flows with a turbulent through-flow are still poorly understood. Direct numerical simulation (DNS) is an attractive method for investigating such problems since the details of the interactions can be unraveled, and the results are directly applicable to industrial-scale systems because their Reynolds numbers are typically quite low. In this study, pseudospectral channel flow simulations were performed with the electrohydrodynamic effects being modeled by a spatially varying body-force term in the equations of fluid motion. The interactions between EHD flows and wall structures were elucidated by examining the instantaneous structure of the flow field. Results indicate that the mean flow, the EHD flows, and the turbulence field undergo significant modifications caused by mutual interaction. First, it is found that EHD flows reduce drag, allowing larger flow rates for a given pressure drop. Second, the EHD flows themselves appear weakened by the presence of the through-flow, particularly in the central region of the channel. The EHD flows affect the turbulence field by both increasing dissipation and turbulence production, the overall turbulence level being determined by the balance between the increased dissipation and production. Even though high EHD flow intensities may increase streamwise and wall-normal turbulence intensities, the Reynolds stress is reduced, consistent with the observed reduction in drag. From a mechanistic viewpoint, there are indications that EHD flows of the type investigated here reduce drag by decreasing the relative importance of the positive Reynolds stress contributions, i.e., second (ejections) and fourth (sweeps) quadrant events, compared to the negative Reynolds stress contributions, i.e., first and third quadrant events.
Dynamic control of the reversed-field pinch in numerical magnetohydrodynamic simulations
Authors: Sätherblom H.-E.
Published: July 2002

First results from a numerical test of dynamic chaos control for the reversed-field pinch are presented. A control scheme, using a control signal in the form of a small time continuous perturbation, is implemented into a magnetohydrodynamic simulation code. The control signal, constituting the difference between a periodic external signal and the actual toroidally applied electric field, multiplied by an adjustable weight factor, is added to an existing code equation. This equation determines the loop voltage for a desired plasma current, and would correspond to modulations of the applied loop voltage in an experiment. The main effect of the dynamic control on the dynamics of the reversed-field pinch is an increase in the magnetic helicity with an unchanged value of the magnetic energy. The helicity increases in a stepwise manner, and occurs when the loop voltage undergoes a π phase shift.
Thresholds and the role of the radial electric field in confinement bifurcations in the H-1 heliac
Authors: Shats M. G. , Michael C. A. , Rudakov D. L. , Blackwell B. D.
Published: July 2002

The threshold conditions for the confinement bifurcations in the H-1 heliac [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] are studied experimentally. The thresholds include the magnetic field, the rf power, and the neutral gas filling pressure. It is shown that in any combination of these parameters it is the radial electric field Er that is driven to a critical value before the bifurcation. A mechanism of the electric field formation is suggested, which is based on the balance of the electron and ion losses. The ion loss, at low magnetic field and high ion temperature observed in H-1, is dominated by the direct orbit loss mechanism. This is shown by modeling of the exact ion orbits and by comparing a qualitative picture of the Er formation with the experimental data. Relative efficiency of the electron and ion heating in the inner regions of plasma define the conditions for either low-to-high (L–H) or high-to low (H–L) confinement bifurcations in H-1.
Linewidth of distributed feedback semiconductor lasers with partially reflecting facets
Authors: Agrawal Govind P. , Dutta N. K. , Anthony P. J.
Published: July 2002

The linewidth of distributed feedback semiconductor lasers with partially reflecting facets is analyzed theoretically to study its dependence on the grating phases at the facets. For most phase combinations, the linewidth is found to be reduced compared to its value expected in the absence of grating, i.e., the value expected for a similar Fabry–Perot laser. The extent of reduction increases with the coupling coefficient κ. The calculated results are in qualitative agreement with the recent observations on phase tunable distributed feedback lasers.
InGaAsP/InP optical switches using carrier induced refractive index change
Authors: Ishida K. , Nakamura H. , Matsumura H. , Kadoi T. , Inoue H.
Published: July 2002

InGaAsP/InP optical switches have been fabricated which use a carrier induced refractive index change. Switching has been achieved with a power isolation of 20.5 dB in a 1-mm-long device in multimode operation. This is a promising new step toward making optical integrated circuits.
Enhanced tail diffusion of ion implanted boron in silicon
Authors: Fan D. , Huang J. , Jaccodine R. J. , Kahora P. , Stevie F.
Published: July 2002

The enhanced diffusion tail of implanted boron in Si was studied using conventional furnace annealing. Surface layer stripping was applied to remove part of the sample before annealing in order to distinguish the effect of the defect-rich surface region from the tail region on the enhanced diffusion of boron. The boron concentration profiles were obtained with secondary ion mass spectrometry (SIMS). Spreading resistance profiles were also measured to compare with the SIMS profiles. The results show that implantation-induced damage in the surface region is responsible for the enhanced boron diffusion. Channeled boron in the tail of the implantation profile becomes activated during annealing and has little effect on the tail redistribution during annealing.
Effects of the layer thickness on the electronic character in GaAs-AlAs superlattices
Authors: Ihm J.
Published: July 2002

Realistic tight-binding calculations for the GaAs-AlAs superlattice clearly demonstrate that if the GaAs layer thickness is less than ∼30 Å, the lowest conduction-band state is confined to the AlAs barrier region instead of the GaAs quantum well region resulting in a spatial separation of electrons and holes. This observation exemplifies a failure of the widely used Kronig–Penney model and emphasizes the importance of the k vector selection rule. This property can be used in photoluminescence spectroscopy to determine the band offset of thin layer superlattices for an essentially arbitrary composition of AlxGa1−xAs. A complete two-dimensional map of the electronic character as a function of the GaAs and the AlAs layer thickness is obtained. The possibility that Xxy states may lie below Xz states for narrow wells is proposed, and implications of the present calculation on interpreting spectroscopic data are discussed.
Observation of resonant tunneling through a compositionally graded parabolic quantum well
Authors: Sen Susanta , Capasso Federico , Gossard Arthur C. , Spah Richard A. , Hutchinson Albert L. , Chu S. N. G.
Published: July 2002

We report the first observation of electron resonant tunneling through parabolic quantum wells, compositionally graded by means of short-period (15 Å) AlxGa1−xAs/GaAs superlattices grown by molecular beam epitaxy. In one structure, comprising a 300-Å-wide well compositionally graded from AlAs to GaAs, five equally spaced resonances are observed in the current-voltage (I-V) characteristic in good agreement with the theory. In another structure with 432-Å-wide wells graded from Al0.30Ga0.70As to GaAs, up to 16 resonances are observed in the I-V. The first ten correspond to resonant tunneling through the quasi-bound states of the double barrier while the others are ascribed to electron interference effects associated with virtual levels in the quasi-continuum energy range above the collector barrier.
Symmetric self-electro-optic effect device: Optical set-reset latch
Authors: Lentine A. L. , Hinton H. S. , Miller D. A. B. , Henry J. E. , Cunningham J. E. , Chirovsky L. M. F.
Published: July 2002

We demonstrate an integrated symmetric self-electro-optic effect device consisting of two quantum well p-i-n diodes electrically connected in series. The device acts as a bistable optical memory element with individual set (S) and reset (R) inputs and complementary outputs (optical S-R latch). The switching point is determined by the ratio of the two inputs, making the device insensitive to optical power supply fluctuations when both power beams are derived from the same source. The device also shows time-sequential gain, in that the state can be set using low-power beams and read out with subsequent high-power beams. The device showed bistability for voltages greater than 3 V, incident optical switching energy densities of ∼16 fJ/μm2, and was tested to a switching time of 40 ns.
Scanning tunneling and atomic force microscopy combined
Authors: Bryant P. J. , Miller R. G. , Yang R.
Published: July 2002

The technique described here provides scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in one instrument. Both STM and AFM operations are accomplished by the same device applied to the same sample area. The same metallic probe interacts with the sample in both modes of operation. Switching from STM to AFM can occur automatically or on command. Images can be recorded separately or in several combined modes and directly compared. Electronic and geometric contributions are identifiable in the separate and combined images. Conductors, semiconductors, and insulators may be investigated. Representative images of each are shown.
Comparative organization of cattle chromosome 5 revealed by comparative mapping by annotation and sequence similarity and radiation hybrid mapping
Authors: Ozawa Akihito , Band Mark R. , Larson Joshua H. , Donovan Jena , Green Cheryl A. , Womack James E. , Lewin Harris A.
Published: July 2002

A whole genome cattle-hamster radiation hybrid cell panel was used to construct a map of 54 markers located on bovine chromosome 5 (BTA5). Of the 54 markers, 34 are microsatellites selected from the cattle linkage map and 20 are genes. Among the 20 mapped genes, 10 are new assignments that were made by using the comparative mapping by annotation and sequence similarity strategy. A LOD-3 radiation hybrid framework map consisting of 21 markers was constructed. The relatively low retention frequency of markers on this chromosome (19%) prevented unambiguous ordering of the other 33 markers. The length of the map is 398.7 cR, corresponding to a ratio of ≈2.8 cR
5,000
/cM. Type I genes were binned for comparison of gene order among cattle, humans, and mice. Multiple internal rearrangements within conserved syntenic groups were apparent upon comparison of gene order on BTA5 and HSA12 and HSA22. A similarly high number of rearrangements were observed between BTA5 and MMU6, MMU10, and MMU15. The detailed comparative map of BTA5 should facilitate identification of genes affecting economically important traits that have been mapped to this chromosome and should contribute to our understanding of mammalian chromosome evolution.
Unbalanced expression of the different subunits of elongation factor 1 in diabetic skeletal muscle
Authors: Reynet Christine , Kahn C. Ronald
Published: July 2002

In studies using subtraction cloning to screen for alterations in
mRNA expression in skeletal muscle from humans with Type 2 diabetes
mellitus and control subjects, one of the most prominent differences
was in the mRNA for elongation factor (EF)-1α. With Northern blot
analysis, EF-1α expression was enhanced by 2- to 6-fold in both Types
1 and 2 human diabetics. In contrast, no changes in expression of
EF-1β or -γ were noted. We observed similar results in animal
models of Type 1 diabetes. EF-1α expression, but not EF-1β or -γ
expression, was also enhanced in streptozotocin-induced diabetic rats,
and this effect was reversed by insulin treatment. An increased level
of EF-1α mRNA was also observed in nonobese diabetic mice. This
unbalanced regulation of the expression of the different subunits of
EF-1 may contribute to alterations not only in protein synthesis but
also in other cellular events observed in the diabetic state.