bluuebunny/crossref_metadata_2025 huggingface dataset:
Predissociation of the Na2 4 3Σg+ state
Authors: Li J. , Liu Y. , Chen H. , Gao H. , Xiang J. , Chen D. , Wu G. , Li Li , Field R. W.
Published: July 2002

The Na2 4 3Σg+ state dissociates adiabatically to the 3s+4p atomic limit. The vibrational levels of the 4 3Σg+ state below the 3s+3d atomic limit were observed previously by perturbation facilitated optical–optical double resonance (PFOODR) fluorescence excitation spectroscopy. Vibrational levels above the 3s+3d atomic limit are found to be predissociated and transitions into these levels have been observed by detecting atomic Na 3d→3p fluorescence. The predissociation is due to the interaction with the continuum of the 2 3Πg state (direct as well as accidental predissociation via bound vibrational levels of the 3 3Πg state). The potential energy curve of the 4 3Σg+ state has been constructed.
Conformational transition behavior around glass transition temperature
Authors: Liang Taining , Yang Yong , Guo Dawei , Yang Xiaozhen
Published: July 2002

The conformational transition behavior around the glass transition temperature has been studied by means of molecular dynamics through a model system, atactic polypropylene in bulk. Various criteria of the conformational transition were examined. A domain at barrier site is defined as the transition state of the conformers and the other conformational states are labeled in code. The transition is accepted when the code is changed. Variation of the width of the domain at barrier distinguishes the deep jumps of the torsion rotations from the shallow jumps that do not reach the well bottom within ±20° from the minimum. The behavior of the deep jumps obeys the rate equation of the transition-state theory, being independent of Tg. The shallow jumps were found significant in the characterization of the motion of polymer chains through Tg. This study thus proves that the changes of properties of polymers resulting from the glass transition are predominately mastered by the shallow jumps.
Lagrangian variational principle in stochastic mechanics: Gauge structure and stability
Authors: Loffredo Maria I. , Morato Laura M.
Published: July 2002

The Lagrangian variational principle with the classical action leads, in stochastic mechanics, to Madelung’s fluid equations, if only irrotational velocity fields are allowed, while new dynamical equations arise if rotational velocity fields are also taken into account. The new equations are shown to be equivalent to the (gauge invariant) system of a Schrödinger equation involving a four-vector potential (A,Φ) and the coupled evolution equation (of magnetohydrodynamical type) for the vector field A. A general energy theorem can be proved and the stability properties of irrotational and rotational solutions investigated.
Multidimensional solitons and their spectral transforms
Authors: Boiti M. , Leon J. P. , Pempinelli F.
Published: July 2002

The soliton solution to the hierarchy of two-dimensional nonlinear evolution equations related to the Zakharov–Shabat spectral problem (including the Davey–Stewartson equation) are derived and studied. The solitons are localized two-dimensional structures traveling on straight lines at constant velocities. Their spectral transform is not uniquely defined and this point is discussed by giving two explicit different spectral transforms of the one-soliton solution and also by giving the general dependence of the spectral transform on the definition of the basic Jost-like solutions.
The singular manifold method revisited
Authors: Pickering Andrew
Published: July 2002

For many completely integrable partial differential equations (PDEs) the singular manifold method of Weiss allows the recovery of the Lax pair and Darboux transformation (DT), and so also the Bäcklund transformation, from a truncated Painlevé expansion. Recently the so-called ‘‘two-singular manifold method’’ has been proposed in order to handle PDEs such as the modified Korteweg–de Vries (MKdV) equation. Here we present a more natural extension of the Weiss singular manifold method which makes use of only one singular manifold but is capable of dealing with such PDEs. In this approach we allow the possibility that the DT might in fact correspond to an infinite Painlevé expansion, for a certain choice of the arbitrary coefficients. This then leads us to a new and more consistent definition of ‘‘singular manifold equation’’ (SME); this can give SMEs different from those usually presented. The summation of infinite Painlevé expansions is effected by seeking a truncation in a new Riccati variable Z. The use of this variable greatly simplifies the recovery of Lax pairs from Painlevé analysis. Practical and theoretical aspects of our approach are illustrated using MKdV as an example. The results of this analysis are confirmed by the consideration of fifth-order MKdV. We then make a further extension of this method which allows it to be applied to a PDE in 2+1 dimensions, and so simultaneously to reductions of the latter to PDEs in 1+1 dimensions. A corollary of our analysis is a direct proof of the convergence of infinite WTC expansions for a certain choice of the arbitrary coefficients therein. In addition, the approach developed here allows us to place within the context of Painlevé analysis a larger class of exact solutions than was possible hitherto. Again, our analysis greatly simplifies the recovery of such solutions.
Nonlinear mixed-state longitudinal and transverse resistivities of superconductors with anisotropic pinning—a phenomenological approach
Authors: Shklovskij Valerij A.
Published: July 2002

In the presence of anisotropic pinning due to unidirected twins, the nonlinear vortex dynamics is discussed in terms of phenomenologically introduced anisotropic drag and pinning viscosities. A theoretical basis for experimental reconstruction of these viscosities is proposed. A nonlinear Ohm’s law is derived. Assuming the anisotropic pinning alone (a-pinning model): a) new scaling relations for the anisotropic Hall conductivity are predicted; b) nonlinear guiding effects are discussed; c) specific current and angular behavior of current-voltage characteristics are analyzed.
Semiclassical quantization condition for magnetic energy levels of electrons in metals with band-contact lines
Authors: Mikitik G. P. , Sharlai Yu. V.
Published: July 2002

We refine the well-known quantization condition for magnetic energy levels of a semiclassical electron. The refined condition results in the energy shift of the levels when in k space the closed electron orbit links to the band-contact line (i.e., surrounds it). This effect is closely analogous to that of Aharonov–Bohm provided the band-contact line plays the role of the infinitely thin “solenoid” with the fixed “magnetic flux.” The predicted shift must manifest itself in oscillation phenomena.
Implosion and shock compression of two-dimensional eddy structures
Authors: Viecelli James A.
Published: July 2002

Integration of the compressible Navier–Stokes equations shows that large amplifications of eddy kinetic energy are possible during the cylindrical implosion of two-dimensional randomized eddy structures. In the limit of vanishingly small initial eddy kinetic energy and viscosity, the amplification of the kinetic energy, and the density fluctuations associated with the eddies, is consistent with assumption that the angular momentum of individual eddies is conserved during the implosion. With increasing initial eddy kinetic energy, nonlinear effects reduce the amplification by a factor approximately exponential in the product of the compression and the ratio of the initial eddy kinetic energy to the implosion kinetic energy.
Inviscid secondary instability of the far wake
Authors: Chow Kwok W.
Published: July 2002

The three-dimensional (3-D) instability of a parallel mean flow and a two-dimensional (2-D) wave is examined. Such secondary instability is shown to exist for the Gaussian profile as a model for the wake. Numerical results are obtained based upon a recently proposed wave–mean-flow interaction mechanism, with the assumptions of temporal problem and inviscid flows. Classical 2-D instability is strong in the near wake but decays to zero in the far wake, where it is overwhelmed by 3-D instability. It is conjectured that this will qualitatively explain the emergence of three-dimensionality in the far wake.
A microscopic vortical structure in fully developed turbulence
Authors: Nakano Tohru
Published: July 2002

A microscopic vortical structure in fully developed three-dimensional turbulence is considered. The strain rates due to eddies of a certain scale stretch and shrink the vorticity field of smaller scales, whose strain rates excite the vorticity of even smaller scales. The excitation propagates toward small scales, the self-similar structure being anticipated in wavenumber space. It is indicated that a pair of oppositely oriented vortex sheets are a candidate for such a self-similar structure: the two sheets approach each other and the vorticity is enhanced during the process. The time evolution of the vorticity is expressed in the dynamical scaling form proposed previously by Nelkin and the present author [Phys. Rev. A 31, 1980(1985)] in which the higher-order structure functions and generalized dimensions in fully developed turbulence were interpreted in good agreement with experiment. Hence the derived vortical structure is expected to be responsible for the intermittent effects.
On the thermodynamic criterion for detonability of a phase-transforming substance
Authors: Fowles G. Richard
Published: July 2002

The thermodynamic criterion for detonability of a phase-transforming substance derived by Kuznetsov [Sov. Phys. JETP 22, 1047 (1966)] is shown to be an approximation to the more general criterion for detonability, namely, (∂P/∂λ)E,V>0, where E is internal energy, V is volume, and λ is the reaction coordinate measured positive as the reaction proceeds toward equilibrium.
A similarity model of axisymmetric convection
Authors: Shapiro A. M. , Benton G. S.
Published: July 2002

An inverse distance scaling has been imposed on the velocity field for steady axisymmetric convection of an incompressible fluid with spatially variable density over a flat plate. The model is closed by explicitly specifying the solenoidal term of the vorticity equation as a function of the polar angle. Results indicate that axial jets can form in association with updrafts but not with downdrafts. On the other hand, wall jets can form in downdrafts but not in updrafts. In addition, the theory predicts a finite upper limit to the magnitude of the solenoidal field in updrafts beyond which solutions do not exist.
The interaction region of a turbulent duct flow
Authors: Teitel M. , Antonia R. A.
Published: July 2002

The interaction between the opposite shear layers in a fully developed turbulent duct flow has been examined with the use of an array of X probes deployed in the plane of mean shear. This array provides simultaneous information on the turbulence characteristics of the shear layers on either side of the centerline. The application of a uv quadrant analysis to the data indicates that quadrant 2 events on one side of the centerline can almost reach the opposite wall. The extent of the intrusion across the centerline increases as the Reynolds number decreases. The contribution from events in quadrant 2 to the Reynolds shear stress is smaller in the duct than in a boundary layer, reflecting the mutually inhibiting effect of flow structures associated with the opposite shear layers. In the central region of the duct, spectra of the lateral velocity fluctuation v suggest that the average wavelength of the organized motion is about 3.2h, where h is the duct half-width. This wavelength is also evident in v-correlation contours and seems independent of the Reynolds number.
A representation of the motion of a vortex sheet in a three-dimensional flow
Authors: Kaneda Yukio
Published: July 2002

A Lagrangian representation of the three-dimensional motion of an infinitely thin vortex sheet in an ideal fluid is derived by using an invariance property of the jump of the velocity potential across the vortex sheet.
A note on exact relations for solitary waves
Authors: Pennell S.
Published: July 2002

Free-surface solitary waves and interfacial solitary waves in a two-fluid system with a rigid lid are known to satisfy certain exact relations involving integral quantities (mass, kinetic and potential energy, etc.). It is shown here that similar relations are satisfied by interfacial solitary waves in a two-fluid system with a free upper surface and by surface solitary waves in a three-dimensional one-fluid system.
Nonlinear convection in a rotating layer with finite conducting boundaries
Authors: Riahi D. N.
Published: July 2002

Small-amplitude steady thermal convection is a horizontal layer of fluid with finite conducting boundaries and rotating about a vertical axis is investigated. Both analytical and numerical techniques are used to solve the nonlinear and stability problems. Conditions on the parameters γb, γt, and τ are determined under which either square cells or two-dimensional rolls can be the preferred form of convection (γb and γt are the ratios of the thermal conductivities of the lower and upper boundaries to that of the fluid and τ is the rotation parameter). Transition from steady rolls to time-dependent flow occurs for τ beyond some value, which decreases with decreasing γb or γt. Square cell convection is not subject to such transition, but there is a transition from steady squares to steady rolls for τ beyond a critical value. The dependence on γb, γt, and τ of the time-dependent transition and of the heat transported by convection are also discussed.
Streamwise vortices in an underexpanded axisymmetric jet
Authors: Krothapalli A. , Buzyna G. , Lourenco L.
Published: July 2002

This Letter describes an experimental investigation demonstrating the existence of streamwise vortices in an underexpanded jet issuing from a convergent axisymmetric nozzle at a moderate pressure ratio of 5.1, and at a Reynolds number of 1.8×106. These vortices are quite strong and are located in the supersonic region of the jet between the intercepting shock and the jet boundary. They seem to play a significant role by enhancing mixing between the ambient and jet fluids in the supersonic region of the jet. The scale of these vortices increases with downstream distance by a process of vortex merging.
Chaotic advection of fluid particles in a field of point vortices and vortex rings
Authors: Meleshko V. V. , Konstantinov M. Yu. , Gurzhi A. A. , Konovaljyk T. P.
Published: July 2002

Processes of formation of vortex rings and mushroom structures in the ocean, in geological structures in Earth’s mantle, and in various applications in chemical technology,1 provide conditions for advanced interest in the phenomena of internal mixing. In order to model such processes, an approach based on a direct study of the trajectories of motion of Lagrangian particles in a Eulerian velocity field currently attracts special attention. Although the ideas of such an approach may be found in the works of Maxwell2 and Reynolds,3 comparatively little has been done toward obtaining a clear view of what goes on, except in the simplest cases. To the more complicated cases belong the processes of chaotic advection of particles in bounded domains.4 This paper presents the results of a study of properties of chaotic particle advection of an ideal fluid in unbounded and bounded domains under free motion of point vortices or vortex rings. Data on the process of particle motion for particles that initially form the ‘‘atmosphere’’ of a vortex pair when it interacts with a single vortex, or with other noncollinear vortex pairs, are presented for a two-dimensional unbounded domain. Depending on the type of interaction—either ‘‘direct’’ or ‘‘exchange’’ collision or mutual trapping5–7—an estimate of the effectiveness of stirring of atmosphere particles and chaotization of their trajectories is given.

The problem of advection of particles in a two-dimensional rectangular domain with a free vortex is solved. Although the locked trajectory of the vortex and the period of its revolution are explicitly calculated by means of Jacobi and Weierstrass elliptic functions, a complicated dependence of the streamfunction on time leads to the formation of ‘‘whorls’’ and ‘‘tendrils,’’ and to effective stirring of some domains. Poincaré sections of particles for characteristic values of the geometrical parameters of the domain and initial positions of the vortex are given. Various types of interaction such as ‘‘leap-frogging,’’ passing and mutual trapping,8 are studied for the axisymmetric motion of two coaxial vortex rings with the intensities of vorticity of the same or different signs. An analysis of chaotization of the motion for particles initially forming the atmosphere of the rings has been carried out for some cases of interaction.
Spectral simulation for the development of vortex arrays in stratified fluids
Authors: He Xiong , Lewalle Jacques , Herring Jackson
Published: July 2002

The simulation of three-dimensional vortex flows in a density-stratified fluid is based on a generalization of the Boussinesq approximation. The generalization yields a nonhomogeneous term in the continuity equation. The spectral method is applied to the corresponding Navier–Stokes relations and an algebraic system equation is solved numerically with iteration on time. The numerical evolution of vortex arrays shows the effects of the stratification, including a trend toward 2-D motion from 3-D motion and an inhibition in turbulence development.
Obstructed flow between shrouded corotating disks
Authors: Tzeng Huey-Ming , Chang Chu-Ji
Published: July 2002

Time-resolved laser Doppler anemometry (LDA) measurements were conducted on an obstructed flow between shrouded corotating disks. With small obstruction the flow is characterized by a turbulent wake in the outer radii and a globally oscillatory flow in the inner radii. This oscillatory motion is shown to enhance the radial transport of flow variables. A steady, three-dimensional turbulence model, which predicts well the time-averaged mean and fluctuating fields of a flow between corotating disks with large obstruction, is incapable of reproducing the flow field with small obstruction where oscillatory flow is dominant.
The boundary layer on a finite flat plate
Authors: McLachlan Robert I.
Published: July 2002

The problem of finding the flow over a finite flat plate aligned with a uniform free stream is revisited. Multigrid is used to obtain accurate numerical solutions up to a Reynolds number of 4000. Fourier boundary conditions keep the computational domain small, with no loss of accuracy. Near the trailing edge, excellent agreement with first-order triple-deck theory is found. However, previous comparisons between computations, experiments, and triple-deck theory are shown to be misleading: In fact, triple-deck theory only accounts for half the drag excess (that part not due to the first-order Blasius boundary layer) even at R=4000. The remainder is shown to be due to, among other things, a large displacementlike effect in the boundary layer, i.e., an O(R−1) increase in skin friction extending over the whole plate.
An experimental and numerical investigation of buoyancy-driven two-phase displacement
Authors: Weidner D. E. , Schwartz L. W.
Published: July 2002

The motion of the interface between two fluids in a Hele–Shaw cell for the case of a cell oriented with the plates vertical is considered. The bottom edge of the cell may be at any angle to the horizontal and only density differences between the two fluids drive the flow. A boundary integral technique is employed to numerically predict the motion of the interface and numerical simulations are compared with experimental results. Unlike pressure-driven Hele–Shaw flow, where the simplified equations predict only qualitative features of the displacement profiles, here the agreement is quite good, in general. Theory predicts and experiment confirms that the displacement profiles are not a function of fluid viscosity.
On the Fourier space decomposition of free shear flow measurements and mode degeneration in the pairing process
Authors: Rajaee Mojtaba , Karlsson Sture K. F.
Published: July 2002

A two-dimensional shear layer perturbed by low level perturbation is studied experimentally. The driving signal is composed of two sine waves of fundamental and subharmonic modes of the most unstable natural frequency, with an adjustable phase angle. The flow is examined in a Fourier space and a phase measuring technique is employed to capture the disturbance field, including the subharmonic and its three higher harmonics. The flow field composed of the mean motion and the above-mentioned modes is reassembled. By this quantitative method the crucial connection between the evolution of superposed instability waves, based on hydrodynamic stability theory, and the experimentally observed large-scale orderly structures is made feasible. By considering the fundamental frequency field superimposed on the mean flow, two different Fourier mode structures, having two different phase speeds, are observed in the pairing region. This mode degeneration process is also observed in the fields of other higher harmonics of the subharmonic. Binary-mode as well as modal energy exchanges with the mean flow are discussed in this paper.
Reply to comments by P. Ripa
Authors: Sakuma H. , Ghil M.
Published: July 2002

A physical explanation is provided for the difference between the authors’ stability criterion, valid in the shallow-water equations, and earlier criteria for the quasigeostrophic potential vorticity equation. Given this difference, a counterexample of P. Ripa [Phys. Fluids A 4, ▪▪▪ (1992)] for the authors’ criterion applied to the latter equation is shown to be irrelevant for the former system. This raises interesting questions as to the reliability of the quasigeostrophic approximation in studying stability of planetary flows.
Boundary-layer formation for viscosity approximations in transonic flow
Authors: Gamba Irene Martínez
Published: July 2002

The boundary-layer formation for viscosity approximations in one-dimensional transonic flow models in a bounded interval with a source or a force-collision term is considered. The energy equation is replaced by a pressure–density relationship. The values of the density are imposed at the ends of the interval. The models are used in steady-state hydrodynamic modeling of semiconductor devices where the force term is a force-collision integral operator of the density. Also, the model represents the steady-state nozzle flow, where the source term accounts by geometrical effects. It is shown that the viscosity approximation converges to a solution of the equation arising from these models. As expected, a limiting solution can develop shocks that only occur from supersonic to subsonic regions satisfying the jump condition (i.e., equal flux) and the classical entropy condition (i.e., pressure and density increase across a shock in the direction of the particle path). More interesting is the boundary behavior of the limiting solution. The boundary layer has a condition that determines the possible range of discontinuities for the density. Values of the density at the upstream boundary in the limit ε→0, must remain subsonic if the prescribed value is subsonic. Values of the density at the upstream boundary in the limit ε→0, if different from the prescribed subsonic or sonic one, must become supersonic with bigger flux than the flux of the prescribed one. This can be thought of as trying to impose boundary values inside the range of a forming shock.
Vortical structures from a two-dimensional nozzle with a bluff-body slot
Authors: Chin Long P. , Tankin Richard S.
Published: July 2002

Vortical structures in a two-dimensional (2-D) nozzle with a bluff-body slot under isothermal flow conditions have been examined experimentally and numerically. For these experiments, a 2-D sheet lighting technique coupled with a chemically reacting system was used to observe the vortical shedding processes. By varying the slot flow (flow from the central bluff-body slot), four different types of vortical structures are observed−separated by regions of stability (transition regions). From the sets of data, a flow map was constructed showing the zones occupied by the four different types of vortical structures and the regions of stability. The Reynolds number of the slot flow varied from 7.2 to 291.8; the Reynolds number of the air flow (flow surrounding the bluff-body slot) varied from 50 to 392. For the numerical calculations, the same range of Reynolds numbers for the slot flow was chosen; the Reynolds number of the air flow was fixed at 392. The numerical scheme used is a modified MAC method. Simulated vortical structures were visualized by calculating the flow streaklines. These streaklines agreed with experimental observations. Flow streamlines also have been constructed numerically to characterize the flow fields, particularly the recirculating flow zones, for each type of vortical structures.
Critical Mach number estimates for dense gases
Authors: Cramer M. S. , Tarkenton L. M. , Tarkenton G. M.
Published: July 2002

The present study provides estimates for the critical Mach number at free-stream pressures and temperatures corresponding to the dense gas regime. A range of stable fluorocarbons is identified for which the critical Mach number exceeds 0.95 for practical airfoil or turbine blade configurations. Conditions under which the classical transonic similarity laws break down are also delineated.
An experimental study of nonlinear waves produced by an accelerating plate
Authors: Yang S. A. , Chwang Allen T.
Published: July 2002

The free-surface flow generated by an impulsively accelerating, surface-piercing, vertical plate has been studied experimentally in an open channel of constant depth. The flat vertical plate is fixed on a towing carriage that is set off by suddenly dropping a weight bucket through a connecting steel cable in a pulley system. The free-surface profile in front of the plate and the pressure distribution on the plate surface are measured for three different accelerations of the plate. A capacitance-type wave gauge is used to measure the variations of the water surface, while a variable reluctance pressure transducer is used to measure the pressure on the plate surface. The acceleration of the plate is obtained by means of an accelerometer. All response voltage outputs are recorded on an IBM PC-XT personal computer with a data-acquisition electrical board. Experimental measurements are compared with the numerical, viscous-flow results of Yang and Chwang (IIHR Report No. 332; Iowa Institute of Hydraulic Research, The University of Iowa, 1989) and the analytical, inviscid-flow solution of Chwang [Phys. Fluids 26, 383 (1983)]. The agreement of the free-surface profile and the pressure distribution between the numerical results and the present experimental measurements is fairly good. However, the inviscid-flow solution overpredicts the wave amplitude and the pressure distribution on the plate. In the physical experiments, the water surface is observed to rise in front of the vertical plate where the potential-flow theory becomes singular.
Experimental observations of an unsteady detached shear layer in enclosed corotating disk flow
Authors: Humphrey J. A. C. , Gor D.
Published: July 2002

The flow in the unobstructed space between a pair of disks corotating at high speed in a fixed cylindrical enclosure can be divided into five regions amenable to theoretical analysis [C. A. Schuler, Ph.D. Thesis, University of California at Berkeley (1990); C. A. Schuler et al., Phys. Fluids A 2, 1760 (1990)]. One of these, region III in Fig. 2, is an axially-aligned detached shear layer predicted by the analysis to be located at rIII/R2≊Γ1/2 and of thickness δIII/R2≊(2 Re)−1/4, where R2 is the radius of the disks, Re is the Reynolds number based on R2 and the tip speed of rotation of the disks (ωR2), and Γ is an experimentally determined constant. Through viscous diffusion, the detached shear layer allows the transition that must take place between the bulk of the three-dimensional flow in the interdisk space (region II) and the two-dimensional flow in solid body rotation surrounding the hub that spins the disks (region IV). Present findings, based on flow visualization, confirm these hitherto untested theoretical expressions and reveal that beyond a critical value of the Reynolds number the detached shear layer oscillates in the cross-stream (r-z) plane of the flow. The unsteadiness appears to originate at the enclosure side wall where the disk Ekman layers collide as a result of being redirected from the radial into the axial direction. These observations agree with the direct numerical simulations of Schuler [Ph.D. Thesis, University of California at Berkeley (1990)] which also show that the onset of flow unsteadiness in the cross-stream plane coincides with the appearance of an integer number of circumferentially-periodic large-scale flow structures with large component of axial vorticity, of the type found by Hide and Titman [J. Fluid Mech. 29, 39 (1967)] in a similar flow configuration as of a critical value of the Reynolds number.
Pressure gradient effect in natural convection boundary layers
Authors: Higuera F. J. , Liñán A.
Published: July 2002

The high Grashof number laminar natural convection flow around the lower stagnation point of a symmetric bowl-shaped heated body is analyzed. A region is identified where the direct effect on the flow of the component of the buoyancy force tangential to the body surface is comparable to the indirect effect of the component normal to the surface, which acts through the gradient of the nonuniform pressure that it induces in the boundary layer. Analysis of this region provides a description of the evolution of the flow from a pressure-gradient dominated regime to a buoyancy dominated regime. Numerical results are presented for the flows above and below heated power-law body shapes, and the upstream propagation of small perturbations to the stationary flow is discussed. An asymptotic analysis is carried out for the flow below nearly flat horizontal bodies, for which the change from pressure-gradient-driven to buoyancy-driven flow occurs very rapidly in a short region. The influence of body edges located in the region of interest is also discussed.
Electron cyclotron emission from nonthermal tokamak plasmas
Authors: Harvey R. W. , O’Brien M. R. , Rozhdestvensky V. V. , Luce T. C. , McCoy M. G. , Kerbel G. D.
Published: July 2002

Electron cyclotron emission can be a sensitive indicator of nonthermal electron distributions. A new, comprehensive ray-tracing and cyclotron emission code that is aimed at predicting and interpreting the cyclotron emission from tokamak plasmas is described. The radiation transfer equation is solved along Wentzel–Kramers–Brillouin (WKB) rays using a fully relativistic calculation of the emission and absorption from electron distributions that are gyrotropic and toroidally symmetric, but may be otherwise arbitrary functions of the constants of motion. Using a radial array of electron distributions obtained from a bounce-averaged Fokker–Planck code modeling dc electron field and electron cyclotron heating effects, the cyclotron emission spectra are obtained. A pronounced strong nonthermal cyclotron emission feature that occurs at frequencies relativistically downshifted to second harmonic cyclotron frequencies outside the tokamak is calculated, in agreement with experimental results from the DIII-D [J. L. Luxon and L. G. Davies, Fusion Technol. 8, 441 (1985)] and FT-1 [D. G. Bulyginsky et al., in Proceedings of the 15th European Conference on Controlled Fusion and Plasma Heating, Dubrovnik, 1988 (European Physical Society, Petit-Lancy, 1988), Vol. 12B, Part II, p. 823] tokamaks. The calculations indicate the presence of a strong loss mechanism that operates on electrons in the 100–150 keV energy range.
Orbit effects on the particles and energy flow in the poloidal field null region of a tokamak axisymmetric divertor
Authors: Core W. G. F. , Lazzaro E.
Published: July 2002

Ion transport through the separatrix of a tokamak with a poloidal divertor is reconsidered, to reconcile the interpretation of results from different diagnostics and understand the unexpected multiplet structure of the heat deposition on the targets.
On a ballistic method for double layer regeneration in a Vlasov–Poisson plasma
Authors: Bazdenkov Sergey V. , Sato Tetsuya
Published: July 2002

Double layers generated by a current-driven instability are usually alive for only a short time and destroyed by emitting solitons and other nonlinear waves. It is analytically shown that they can be regenerated in a one-dimensional Vlasov–Poisson plasma system where the upstream velocity distribution is swiftly heated to restore an unstable condition in the downstream region due to ballistic deformation.
On the Smoluchowski paradox in a sedimenting suspension
Authors: Beenakker C. W. J. , Mazur P.
Published: July 2002

It is shown, by explicit calculation, that the influence of a plane wall supporting the suspension on the sedimentation velocity is such that the convergence problems of this quantity encountered in an unbounded suspension do not occur—even in the limit of an infinitely distant wall.
The helical nature of unforced turbulent flows
Authors: Pelz Richard B. , Shtilman Leonid , Tsinober Arkady
Published: July 2002

Results of direct numerical simulations of the decay of nearly isotropic turbulence exhibit clearly that much of the flow evolves in orientation to a state in which the vorticity vector is nearly aligned with the velocity vector.
Soliton decay of nonlinear Alfvén waves: Numerical studies
Authors: Ponce Dawson Silvina , Ferro Fontán Constantino
Published: July 2002

The derivative nonlinear Schrödinger equation is numerically solved for arbitrary initial conditions by an extension of the Ablowitz–Ladik scheme [Stud. Appl. Math 57, 1 (1977)]. The numerical nonlinear difference code, which takes advantage of the inverse scattering method, simulates the original differential equation reproducing common features, like solitons and an infinite set of constants of motion. The long-time behavior is analyzed in terms of the sign of one of the constants of motion. The formation of a soliton train is seen whenever the constant has a negative value. This fact is the global expression of the Mj≂lhus local criterion to distinguish between modulationally stable and unstable cases.
Compression of ultrahigh magnetic fields in a gas-puff Z pinch
Authors: Felber F. S. , Malley M. M. , Wessel F. J. , Matzen M. K. , Palmer M. A. , Spielman R. B. , Liberman M. A. , Velikovich A. L.
Published: July 2002

Evidence is presented that an axial magnetic field of 100 kG has been compressed to 42 MG by an imploding neon gas-puff Z pinch on a 800 kJ, 7.5 MA, 60 nsec pulsed power generator. Machine electrical diagnostics and x-ray pinhole photographs indicate that this 420-fold field compression was produced by imploding plasma at high magnetic Reynolds number.
Neutral beam injection and plasma convection in a magnetic field
Authors: Okuda H. , Hiroe S.
Published: July 2002

Injection of a neutral beam into a plasma in a magnetic field has been studied by means of numerical plasma simulations. It is found that, in the absence of a rotational transform, the convection electric field arising from the polarization charges at the edges of the beam is dissipated by turbulent plasma convection, leading to anomalous plasma diffusion across the magnetic field. The convection electric field increases with the beam density and beam energy. In the presence of a rotational transform, polarization charges can be neutralized by the electron motion along the magnetic field. Even in the presence of a rotational transform, a steady-state convection electric field and hence anomalous plasma diffusion can develop when a neutral beam is constantly injected into a plasma. Theoretical investigations of the convection electric field are described for a plasma in the presence of a rotational transform.
Abrupt transitions in high-concentration, particle-driven gravity currents
Authors: Hallworth Mark A. , Huppert Herbert E.
Published: July 2002

A systematic series of experiments on the instantaneous release of two-dimensional, heavy, particle-driven gravity currents has been conducted. High-concentration currents propagated in a qualitatively different way than low concentration currents. In particular, beyond a critical initial volume fraction of particles, the resulting dense current came to an abrupt halt at some point down the channel, depositing the bulk of its initial sediment load as a relativly thick layer of fairly constant thickness, characterized by a pronounced, steep snout. A very much thinner layer of sediment extended for some distance beyond the arrest point. This layer was deposited from the subsequent propagation of a slower moving, low concentration residual cloud.
A weakly nonlinear theory for the dynamical Rayleigh–Taylor instability
Authors: Berning M. , Rubenchik A. M.
Published: July 2002

The dynamics of an interface between two incompressible, inviscid, irrotational, and immiscible liquids with densities ρ1 and ρ2 under the influence of a time-dependent gravitational field g(t) is investigated. A Hamiltonian formulation of the system is adopted leading to a perturbative expansion of the equations of motion for the canonical variables. Equations, accurate up to third order in the perturbation amplitude are derived. They are able to describe the initial stage of instability “saturation.” The latter equations are integrated iteratively for two standard limiting cases: constant gravity (classical Rayleigh–Taylor instability), g(t)≡g0, and impulsive Richtmyer–Meshkov loading, g(t)=v0δ(t−t0). The comparative growth of various two-dimensional structures and rectangular and hexagonal cells is evaluated. Surface tension effects are considered.
Low-temperature magnetic spectroscopy with a dc SQUID
Authors: Rozen J. R. , Awschalom D. D.
Published: July 2002

New magneto-optical methods combining integrated dc superconducting quantum interference device and fiber optic technology have allowed energy- and polarization-dependent magnetization measurements to be made on a 10-μm-diam sample of Cd1−xMnxTe. Direct optically induced magnetization data probe the energetics of magnetic polaron formation and reveal the ultimate spin orientation with respect to the incident photon angular momentum.
Molecular beam epitaxial growth of the II-V semiconductor compound Zn3As2
Authors: Chelluri B. , Chang T. Y. , Ourmazd A. , Dayem A. H. , Zyskind J. L. , Srivastava A.
Published: July 2002

We report the first molecular beam epitaxial growth of the II-V semiconducting compound Zn3As2. Commensurate growth of the tetragonal crystal was obtained in the temperature range 300–360 °C on InP. Growth on GaAs was incommensurate due to larger lattice mismatch. The unintentionally doped material is p type. At room temperature, p∼5×1018 cm−3, μ=43 cm2/V s, and the absorption edge is at 0.99 eV. Low-temperature photoluminescence and Hall data suggest that the lowest acceptor level is very close in energy to the valence-band maxima. Optical absorption data suggest that the band gap associated with these valence-band maxima is indirect in nature. The direct band gap is slightly larger.
Base transport dynamics in a heterojunction bipolar transistor
Authors: Hayes J. R. , Levi A. F. J. , Gossard A. C. , English J. H.
Published: July 2002

We have used hot-electron spectroscopy to study nonequilibrium electron transport in the base of a heterojunction bipolar transistor. Electrons injected from an n-type AlGaAs emitter into a p-type GaAs base were found to be strongly scattered such that they could be characterized by an effective electron temperature after traversing several hundred angstroms. The effective electron temperature, measured at 4.2 K, was found to be 150 K for a sample having a 900-Å base region and 500 K for a sample having a 450-Å base region.
Direct observation by reflection high-energy electron diffraction of amorphous-to-crystalline transition in the growth of Sb on GaAs(110)
Authors: Savage D. E. , Lagally M. G.
Published: July 2002

Direct evidence, using reflection high-energy electron diffraction, is found for a transition, at a specific thickness, from an amorphous to a crystalline state in Sb deposited on GaAs(110). After crystallization, Sb exists as small, 30–40 Å crystallites with a (0001) contact plane but randomly azimuthally oriented. A new surface phase, GaAs(110) p(3×2)-Sb, is found and appears to be a consequence of the crystallization transition.
Temperature-dependent factors contributing to T0 in graded-index separate-confinement-heterostructure single quantum well lasers
Authors: Leopold M. M. , Specht A. P. , Zmudzinski C. A. , Givens M. E. , Coleman J. J.
Published: July 2002

The temperature dependence of threshold current in graded-index, separate-confinement-heterostructure, single quantum well lasers has been investigated and analyzed. The conventional parameter used to describe this temperature dependence, T0, is measured and shown to increase with cavity length. The temperature dependences of the loss coefficient α and the differential gain β have also been measured. Both parameters decrease linearly with temperature in the range 20–70 °C. Competition between α(T) and β(T) is shown to account for the dependence of T0 on cavity length and to suggest guidelines for designing high T0 lasers.
Bulk-quality bipolar transistors fabricated in low-temperature (Tdep =800 °C) epitaxial silicon
Authors: Burger W. R. , Reif R.
Published: July 2002

In this letter we report for the first time the fabrication of bulk-quality bipolar transistors in low-temperature (Tdep =800 °C) epitaxial silicon. The epitaxial layers were deposited by an ultra-low pressure chemical vapor deposition technique utilizing an in situ predeposition argon sputter clean. The critical parameter affecting the quality of the epitaxial films is the deposition temperature. The junction leakage current decreases by two orders of magnitude, the ideality factor improves from 1.36 to 1.04, and the minority-carrier lifetime increases from 1.3 to 46 μs as the deposition temperature is increased from 750 to 800 °C. Analysis of the base-emitter and base-collector junction characteristics indicates that bipolar transistors fabricated in epitaxial films deposited at 800 °C have characteristics at least as good as control fabricated in bulk silicon.
Reduction of tritium permeation through 304L stainless steel using aluminum ion implantation
Authors: Maienschein J. L. , Musket R. G. , McMurphy F. E. , Brown D. W.
Published: July 2002

We have demonstrated the reduction of tritium gas permeation through 304L stainless steel using aluminum ion implantation. The implanted aluminum was selectively oxidized to form a thin, coherent, and stable Al2O3-rich layer on the stainless-steel surface. Tritium permeability at 100 and 170 °C was reduced 600–700-fold compared to clean stainless steel and 20-fold compared to stainless steel with native oxide. Since greater reductions are expected with improved Al2O3 layers, this approach is a promising alternative for reduction of permeation of all hydrogen isotopes through stainless steel and other materials.
Al/Si(100) Schottky barrier formation using nozzle jet beam deposition
Authors: Wong J. , Mei S-N. , Lu T-M.
Published: July 2002

High Schottky barrier height of φb=0.77 eV for Al/n-Si was obtained by the nozzle jet beam deposition method in a conventional vacuum condition without post-annealing. Previously, this high barrier height was only observable in the vacuum cleaved or sputter-etched samples in an ultrahigh vacuum condition. The Schottky barrier height was reduced if a partially ionized jet beam with 0.1% ions was used in the deposition process and if a bias of ≥0.5 kV was applied to the substrate. The reduction of the Schottky barrier height is attributed to the surface damage caused by the energetic ion bombardment. It is shown that a high barrier height (≂0.8 eV) and a low leakage current were retained after a relatively low-temperature (≂450 °C) annealing process if the applied bias was ≤1 kV.
GaAs/GaAlAs quantum well laser with a lateral spatial variation in thickness grown by metalorganic chemical vapor deposition
Authors: Fekete D. , Bour D. , Ballantyne J. M. , Eastman L. F.
Published: July 2002

A self-aligned GaAs/GaAlAs tapered laser with a tapered quantum well active region is described. The laser is grown by low pressure metalorganic chemical vapor deposition in a horizontal reactor. The effect of the tapered quantum well is in concentrating the injected charges in the modal volume and thus 6.2 mA threshold current is obtained. At a high power level the laser operates in single longitudinal and transversal modes.
1/f noise in quartz crystal resonators in relation to acoustic losses and frequency dispersion
Authors: Planat M. , Gagnepain J. J.
Published: July 2002

The 1/f frequency fluctuation of quartz crystal resonators between liquid helium and room temperature is investigated in relation with acoustic attenuation. Large attenuation is observed at 50 K, due to ionic Na+ impurity relaxation, at 20 K, corresponding to the interaction of the sound wave with thermal phonons, and below 7 K, which is a behavior already reported in amorphous glasses. A strong correlation is found between 1/f noise level and acoustic attenuation around the phonon-phonon relaxation peak; this correlation is not observed on the Na+ impurity peak. Below 7 K, 1/f noise also increases in relation with losses. A possible physical mechanism for 1/f noise in quartz resonators is discussed.
Ga0.47In0.53As/InP superlattices grown by chemical beam epitaxy: Absorption, photoluminescence excitation, and photocurrent spectroscopies
Authors: Tsang W. T. , Schubert E. F. , Chu S. N. G. , Tai K. , Sauer R.
Published: July 2002

Superlattices of Ga0.47In0.53As/InP grown by chemical beam epitaxy (CBE) were studied in detail by transmission electron microscopy, absorption, photoluminescence (PL), photoluminescence excitation (PLE), and photocurrent (PC) spectroscopies. Results from all these characterization techniques independently confirmed the superior qualities of the CBE-grown Ga0.47In0.53As/InP superlattices over previously reported results. Photoluminescence linewidths were close to those obtained from CBE-grown single quantum wells. All of the observed excitonic absorption peaks in absorption, PLE, and PC spectra were well resolved and clearly assigned including the forbidden E13h transition for the first time. In fact, these excitonic structures were still observed in the PC spectra at temperatures as high as 102 °C.
Upper critical fields and high superconducting transition temperatures of La1.85Sr0.15CuO4 and La1.85Ba0.15CuO4
Authors: Capone D. W. , Hinks D. G. , Jorgensen J. D. , Zhang K.
Published: July 2002

The superconducting transition temperature Tc of La1.85Sr0.15CuO4 is measured to be 34.7 K (midpoint) with a transition width (10–90%) of 2.6 K. The critical field slope at Tc is 2.13 T/K. Using the Werthamer, Helfand, and Hohenberg expression in the dirty limit we estimate the upper critical field Hc2(0) to be at least 50 T for this system. The measured transition width broadens initially with increasing field to 4 K, then saturates at this value for fields above 2 T. The critical field slope for the lower Tc material La1.85Ba0.15CuO4 is measured to be 1.78 T/K, which yields an Hc2(0) of at least 36 T.