bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of the radiation source strength in argon at temperatures between 5000 and 10 000 K
Authors: Owano T. G. , Gordon M. H. , Kruger C. H.
Published: July 2002

The volumetric radiative source strength has been measured for argon using a 50 kW rf induction plasma torch operated at atmospheric pressure with temperatures ranging between 5000 and 10 000 K. At temperatures below 7500 K the results differ by over an order of magnitude from the only previously published measurements [Phys. Fluids 10, 1125 (1967)]. Spatially resolved total radiation measurements were made with a calibrated pyroelectric detector system of flat response in the wavelength range 250 to 2500 nm. The corresponding temperature measurements were made through spectroscopic examination of relative and absolute line intensities, and absolute continuum intensities. The data were interpreted in terms of a partial equilibrium model in which the free and excited bound electrons are found to be mutually in equilibrium, irrespective of departures from equilibrium with the ground state. The differences from the previous radiation source strength measurements are interpreted in terms of unaccounted for nonequilibrium effects in that study. The present measurements are in satisfactory agreement with theoretical predictions. A simple yet complete set of calorimetric measurements taken independently supports these results.
Anomalous transport in tokamaks: Transport task force reviews
Authors: Callen J. D.
Published: July 2002

The status of studies of anomalous transport in toroidal magnetically confined plasmas and the Transport Task Force that was set up to increase emphasis in this area are briefly discussed. This note also serves as a preface to the six reviews of particular transport areas that are presented hereafter.
Parallel electric resistivity in the TFTR tokamak
Authors: Zarnstorff M. C. , McGuire K. , Bell M. G. , Grek B. , Johnson D. , McCune D. , Park H. , Ramsey A. , Taylor G.
Published: July 2002

The average parallel resistivity and the location of the q=1 surface are found to be consistent with the predictions of neoclassical transport theory and inconsistent with classical resistivity (uncorrected for toroidal effects) for Ohmic plasmas in the TFTR tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 51], both in near-equilibrium and during ramping of the plasma current. These observations are incompatible with theories predicting anomalous parallel resistivity in concert with anomalous perpendicular transport.
Laboratory generation and propagation of ripples
Authors: Henderson Diane M. , Lee Ron C.
Published: July 2002

The generation of gravity-capillary waves (ripples) by a flap-type wavemaker is examined theoretically and experimentally. Qualitative agreement is found in the trapped wave region; measured amplitudes exceed those predicted by the linear theory and the data extend further from the wavemaker. The air–water–aluminum contact line on the paddle face appears to cause a phase shift away from the paddle in the measured results. Excellent agreement is found with the linear dispersion relation for the progressive waves when the water is scrupulously cleaned of ions and organics, and filtered of minute particles. The presence of minute particles (only) necessitates introduction of the notion of ‘‘dynamic’’ surface tension which is about 30% lower than the measured static value. When particles are removed the dynamic and static values of surface tension are equivalent. The amplitudes of the progressive waves agree well with theory when the product of the wavenumber and water depth does not exceed seven, and the inviscid theory is modified to account for decay by viscosity. Agreement requires the assumption of a ‘‘fully contaminated’’ surface even when the water is particle filtered. Otherwise, the viscous decay is about twice that predicted by the fully contaminated surface model. Longitudinal instabilities of the progessive waves are observed when the product of wavenumber and water depth exceeds about seven. These instabilities appear to be modeled well by the nonlinear Schrödinger equation. Transverse instabilities are also observed in the data over the full range of wave frequencies investigated.
Generalized random forcing in random-walk turbulent dispersion models
Authors: Sawford B. L.
Published: July 2002

The Kramers–Moyal expansion is used to derive an infinite hierarchy of Eulerian moment conservation equations from a random-walk model with non-Gaussian random forcing, thus generalizing the Langevin-equation–Fokker–Planck analysis of van Dop et al. [H. van Dop, F. T. M. Nieuwstadt, and J. C. R. Hunt, Phys. Fluids 28, 1639 (1985)]. By imposing the condition that an initially well-mixed state should remain so, equations for random forcing moments of arbitrary order are derived in terms of the Eulerian velocity moments of the turbulence. This procedure makes explicit the equivalence of the different procedures used by van Dop et al. and Thomson [D. J. Thomson, Q. J. R. Meteorol. Soc. 110, 1107 (1984)] to derive the first few forcing moments and extends their results. It is then shown that the random forcing approximation implies an infinite hierarchy of Eulerian closure assumptions, the first few of which were derived by van Dop et al. The analysis is extended to a class of rescaled random-walk equations, and it is shown that the version developed by Wilson et al. [J. D. Wilson, B. J. Legg, and D. J. Thomson, Boundary-Layer Meteorol. 27, 163 (1983)] is unique in that it alone is realizable for inhomogeneous Gaussian turbulence and then has Gaussian random forcing.
Destabilization of global Alfvén eigenmodes and kinetic Alfvén waves by alpha particles in a tokamak plasma
Authors: Li Yan Ming , Mahajan Swadesh M. , Ross David W.
Published: July 2002

Alfvén wave instabilities in a reacting tokamak plasma are calculated both analytically and numerically. Two distinct classes of eigenmodes are considered, global Alfvén eigenmodes and kinetic Alfvén waves, each driven unstable by the free energy associated with the alpha particle density gradient. The growth rates of the global Alfvén eigenmodes are given for the first time. These are basically magnetohydrodynamic (MHD) modes, whose resonances with electrons and alpha particles are calculated using kinetic theory. The calculation of kinetic Alfvén wave growth rates is improved from earlier treatments. These modes depend on electron inertia and finite ion gyroradius for their existence and have no counterpart in MHD theory. In both sets of calculations toroidal coupling of the alpha particle response to sidebands in the poloidal mode number is fully taken into account. Global modes with small parallel phase velocity are identified as the most dangerous, both because of their substantial growth rates and the expected ineffectiveness of quasilinear stabilization. The kinetic waves, on the other hand, are likely to be stabilized easily by quasilinear flattening of the alpha particle profile. Calculations using both Maxwellian and slowing-down alpha distributions are performed, showing that, for the same alpha density and energy density, the slowing-down distribution yields somewhat larger growth rates.
Acoustic solitary vortices
Authors: Stenflo L.
Published: July 2002

Low-frequency, short-wavelength acoustic-gravity waves are considered. It is shown that solitary vortices can exist if the equilibrium temperature gradient is sufficiently large.
Concentration waves and flow modification in a particle-laden circular vortex
Authors: Druzhinin O. A.
Published: July 2002

Evolution of a two-dimensional axisymmetrical vortex laden with solid heavy particles is studied analytically and numerically. The particulate phase is assumed to be dilute enough to neglect the effects of particle–particle collisions. Only sufficiently small particle Stokes (St) and Reynolds numbers are considered, for which an approximate solution for the particle velocity can be derived. An analytical solution to a Cauchy problem is obtained for initially uniform concentration of particles in a circular flow describing the accumulation of particles in the form of a kinematic wave and the corresponding modification of the carrier flow. According to this solution, a steep peak of the concentration develops forming the wave crest which propagates out of the vortex. Due to the interaction between the two phases, a fluid velocity component directed towards the vortex center is generated, so that in the vicinity of the crest the vortex acquires a spiral-like shape. At later stages, the growth of the crest is inhibited and its propagation velocity decreases. Analysis of the problem for particles with larger Stokes numbers shows that the accumulation process is most intense when St is close to a critical value St* which generally depends on the vortex structure and, for the flow considered, is of the order unity.
Feedback control of von Kármán vortex shedding behind a circular cylinder at low Reynolds numbers
Authors: Park D. S. , Ladd D. M. , Hendricks E. W.
Published: July 2002

A computational study of the feedback control of von Kármán vortex shedding behind a circular cylinder at low Reynolds numbers is reported. The two-dimensional Navier–Stokes equations with feedback are solved numerically. The control actuators are a pair of blowing/suction slots located at ±110° from the leading stagnation point. A single feedback sensor is used and the actuators are 180° out of phase with each other. Complete suppression of vortex shedding is achieved for the simulation at Reynolds number Re=60. The suppression window in the feedback sensor location xs is narrow. With the feedback sensor location fixed at the optimum location, vortex shedding becomes suppressed with increasing feedback gain α. However, further increase of the feedback gain destabilizes the flow again. At Reynolds number Re=80, and above, the feedback control stabilizes the primary vortex shedding mode, but a secondary mode which may be lower or higher in frequency than the primary depending upon the phase of the feedback (feedback sensor location xs) and the feedback gain α arises.
Experimental investigation of a salt water turbulent boundary layer modified by an applied streamwise magnetohydrodynamic body force
Authors: Henoch C. , Stace J.
Published: July 2002

Single-component velocity field measurements, mean and fluctuating wall shear stress measurements, and photographic flow visualizations have been made of a magnetohydrodynamic (MHD) body-force modified turbulent boundary layer. The turbulent boundary layer flowed over a flat plate in salt water at zero pressure gradient; the MHD force was created by the interaction of a permanent magnetic field and an applied electric field from a magnet/electrode array integral to the surface of the plate. A MHD force, when applied to an electroconducting fluid and acting in a streamwise direction, can generate a near-wall jet, decreasing the boundary layer thickness and suppressing the intensity of the turbulent fluctuations across the boundary layer. At very high interactions, the force causes an increase in mean wall shear and in turbulence; in the zero free-stream velocity limit, the force acts as a pump. An increase in local skin friction, however, is offset by a grain in thrust due to the force. At moderate interactions, mean quantities are unaffected, but fluctuating wall shear stress and turbulence intensity are suppressed by up to 30% of their unperturbed values across the lower part of the boundary layer. At very low interactions, effects are seen only near the wall. An interaction parameter is derived that characterizes these regimes. The effects likely occur because the MHD force pumps high momentum fluid along the wall, disrupting the liftup of shear-generated wall vorticity. This jet effect is associated with increased convection of turbulent kinetic energy by the mean flow. With the force directed axial upstream, turbulence amplification is seen, along with a reduced mean velocity.
A mostly linear model of transition to turbulence
Authors: Baggett Jeffrey S. , Driscoll Tobin A. , Trefethen Lloyd N.
Published: July 2002

A simple model in three real dimensions is proposed, illustrating a possible mechanism of transition to turbulence. The linear part of the model is stable but highly non-normal, so that certain inputs experience a great deal of growth before they eventually decay. The nonlinear terms of the model contribute no energy growth, but recycle some of the linear outputs into inputs, closing a feedback loop and allowing initially small solutions to ‘‘bootstrap’’ to a much larger amplitude. Although different choices of parameters in the nonlinearity lead to a variety of long-term behaviors, the bootstrapping process is essentially independent of the details of the nonlinearity and varies predictably with the Reynolds number. The bootstrapping scenario demonstrated by this model is the basis of some recent explanations for the failure of classical hydrodynamic stability analysis to predict the onset of turbulence in certain flow configurations.
Drag and lift forces on microscopic bubbles entrained by a vortex
Authors: Sridhar G. , Katz J.
Published: July 2002

The forces that act on bubbles as they are entrained by a vortex are measured using particle image velocimetry. Triple exposure images are used to measure the velocity and acceleration of the fluid and the bubbles simultaneously. Distinction between phases is achieved by using fluorescent particles as liquid flow tracers. The buoyancy, pressure, and inertia forces are computed from the data, while the drag and the lift forces are determined from a force balance on each bubble. It is found that in the present range of bubble diameters, 500 μm<d<800 μm, and Reynolds numbers, 20<Re<80, the drag on a bubble is similar to that on a solid body. Vorticity does not have a significant effect on the drag coefficient. The lift coefficients are significantly higher than currently available analytical and numerical estimates. The coefficients are independent of the Reynolds number and are proportional to the fourth root of the local vorticity. Estimates of the Bassett force show that it can be neglected in the present experiment. Computed bubble trajectories, based on the measured lift and drag coefficients, compare well with experimental observations.
The breakdown of asymptotic hydrodynamic models of liquid spreading at increasing capillary number
Authors: Chen Q. , Ramé E. , Garoff S.
Published: July 2002

Complex hydrodynamics near the moving contact line control spreading of a fluid across a solid surface. In the confined region near the contact line, velocity gradients in the fluid are large and viscous forces control the shape of the fluid/fluid interface. The present model for liquid spreading describes the viscous effect on the dynamic interface shape to lowest order in capillary number, Ca. Using videomicroscopy and image analysis techniques, we have examined the shape of liquid/air interfaces very near moving contact lines for Ca≥0.10 where the interfaces are in capillary depression. We find that the theory correctly describes the data up to Ca=0.10 for distances from 20 to 400 μm from the contact line. As Ca increases, the model fails to describe the data in a region near the contact line, which grows as Ca increases. In this expanding region, the model predicts too large a curvature for the interface. We explore the origins of this breakdown by examining the fundamental assumptions of the model. The geometry-dependent part of the solution to O(1) in Ca is sufficient even at Ca=0.44. The breakdown of the model arises from the low order of the geometry-free part of the perturbation solution and/or contributions to the interface shape from the unique hydrodynamics very near the moving contact line.
A consistent hydrodynamic boundary condition for the lattice Boltzmann method
Authors: Noble David R. , Chen Shiyi , Georgiadis John G. , Buckius Richard O.
Published: July 2002

A hydrodynamic boundary condition is developed to replace the heuristic bounce-back boundary condition used in the majority of lattice Boltzmann simulations. This boundary condition is applied to the two-dimensional, steady flow of an incompressible fluid between two parallel plates. Poiseuille flow with stationary plates, and a constant pressure gradient is simulated to machine accuracy over the full range of relaxation times and pressure gradients. A second problem involves a moving upper plate and the injection of fluid normal to the plates. The bounce-back boundary condition is shown to be an inferior approach for simulating stationary walls, because it actually mimics boundaries that move with a speed that depends on the relaxation time. When using accurate hydrodynamic boundary conditions, the lattice Boltzmann method is shown to exhibit second-order accuracy.
Dynamics of coherent structures and transition to turbulence in free square jets
Authors: Grinstein F. F. , DeVore C. R.
Published: July 2002

We report results of time-dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex-sheet with slightly rounded corner-regions. The studies focus on the near field of jets with Mach number 0.3–0.6 and moderately high Reynolds numbers. A monotonically-integrated large-eddy-simulation approach is used, based on the solution of the unfiltered inviscid equations and appropriate inflow/outflow open boundary conditions. The simulations show that the initial development of the square jet is characterized by the dynamics of vortex rings and braid vortices. Farther downstream, strong vortex interactions lead to the breakdown of the vortices, and to a more disorganized flow regime characterized by smaller scale elongated vortices and spectral content consistent with that of the Kolmogorov (K41) inertial subrange. Entrainment rates significantly larger than those for round jets are directly related to the enhanced fluid and momentum transport between jet and surroundings determined by the vortex dynamics underlying the axis-rotation of the jet cross-section. The first axis-rotation of the jet cross-section can be directly correlated with self-induced vortex-ring deformation. However, subsequent jet axis-rotations are the result of strong interactions between ring and braid vortices, rather than being correlated with successive self-induced vortex-ring deformations, as previously conjectured based on laboratory observations. The interaction between braid and ring vortices has the effect of inhibiting the periodic self-induced axis-rotations observed in the case of isolated square vortex rings.
Marangoni effects on drop deformation in an extensional flow: The role of surfactant physical chemistry. I. Insoluble surfactants
Authors: Pawar Yashodhara , Stebe Kathleen J.
Published: July 2002

The shape of a drop centered in an axisymmetric extensional flow is determined by the viscous stresses that deform the drop and surface tension γ′ that resists the deformation. The ratio of these stresses is given by the capillary number, Ca. When Ca is small enough, the drop attains a steady shape. However, above a threshold value, Cacr, the drop elongates continuously, and no steady shape is attained. When surfactants are present on the drop interface, the surface tension is determined by the surface concentration profile, which varies throughout the deformation process. Initially, the surface tension is given by γeq′, in equilibrium with the uniform surface concentration Γeq′. When the flow is initiated, surfactant is swept toward the drop tips, reducing the surface tension there, and altering the interfacial stress balance tangentially through Marangoni stresses and normally through the Laplace pressure. In this paper, the effects of an insoluble surfactant on drop deformation are studied. In previous work, either a surface equation of state for the surface tension γ′ that is linear in the surface concentration Γ′ was used, an approximation that is valid only for dilute Γ′, or Γ′ sufficiently dilute for the linear approximation to be valid were studied. In this paper, a nonlinear surface equation of state that accounts for surface saturation and nonideal interactions among the surfactant molecules is adopted. The linear framework results are recovered for Γ′ that are sufficiently dilute. As Γ′ is increased, the effects of saturation and surfactant interactions are probed at constant initial Γeq′ and at constant initial γeq′. Finally, the case of strong intersurfactant cohesion is treated with a first-order surface phase transformation model. At moderate surface concentrations, these nonlinear phenomena strongly alter the steady drop deformations and Cacr relative to the uniform surface tension and linear equation of state results.
Destabilization of plane Poiseuille flow of insulating liquids by unipolar charge injection
Authors: Lara Juan L. , Castellanos Antonio , Pontiga Francisco
Published: July 2002

The electrohydrodynamic instabilities of plane Poiseuille flow when subjected to an orthogonal arbitrary unipolar injection of charge has been studied. Appropriate boundary conditions for the perturbed charge density in the space charge limited current regime in the presence of forced flows has been derived. The effect of the injection level on the stability of the flow is analyzed, as well as the role played by the ratio of the hydrodynamic to the true ionic mobility. It is shown that in the low Reynolds number region traverse rolls are destabilized more strongly as the ratio of mobilities decreases.
Simulation of flow through bead packs using the lattice Boltzmann method
Authors: Maier R. S. , Kroll D. M. , Kutsovsky Y. E. , Davis H. T. , Bernard R. S.
Published: July 2002

The lattice Boltzmann method (LBM) is used to simulate viscous fluid flow through a column of glass beads. The results suggest that the normalized velocity distribution is sensitive to the spatial resolution but not the details of the packing. With increasing spatial resolution, simulation results converge to a velocity distribution with a sharp peak near zero. A simple argument is presented to explain this result. Changes in the shape of the distribution as a function of flow rate are determined for low Reynolds numbers, and the large-velocity tail of the distribution is shown to depend on the packing geometry. The effect of a finite Reynolds number on the apparent permeability is demonstrated and discussed in relation to previous results in the literature. Comparison with velocity distributions from NMR (nuclear magnetic resonance) spectroscopy finds qualitative agreement after adjusting for diffusion effects in the NMR distributions.
The viscous drag force on a spherical bubble with a time-dependent radius
Authors: Magnaudet Jacques , Legendre Dominique
Published: July 2002

The expression of the hydrodynamic force experienced by a spherical bubble having a variable radius and moving in a viscous incompressible liquid is derived analytically in two different asymptotic situations. The solution is obtained by rewriting the initial problem in a frame of reference where the bubble has a fixed radius and where the relevant dimensionless parameters are conserved. It is shown that when the assumption of unsteady creeping motion is valid, the bubble radius variation combined with a constant rise velocity produces a nonzero history force. This contribution can have a significant effect on the bubble motion, especially for a collapsing bubble.
Asymmetric transport and non-Gaussian statistics of passive scalars in vortices in shear
Authors: del-Castillo-Negrete D.
Published: July 2002

Transport of passive scalars in a chain of vortices in a shear layer is studied using a model motivated by the quasigeostrophic equation, and a discrete map model. Surrounding the vortices there is a stochastic layer where particles alternate chaotically between being trapped in the vortices, and moving following the shear flow. Transport in the stochastic layer is asymmetric: Mixing between the vortices and the up-stream flow is, in general, different from mixing between the vortices and the down-stream flow. We use the Melnikov method to study this asymmetry, and to construct a generalized separatrix map model for asymmetric transport. The statistics of the passive scalar is non-Gaussian. In particular, there is anomalous advection, and anomalous (non-Brownian) diffusion. Thus, transport in this system cannot be described by an advection-diffusion equation with an effective diffusivity. The probability density function (PDF) of particle displacements δx, P(δx,t), is asymmetric and broader than Gaussian. At large times, P relaxes to a self-similar limit distribution of the form t−γ/2f(X/tγ/2), where X≡δx−〈δx〉, f is a scaling function, and γ is the anomalous diffusion exponent. As a result, the moments scale as 〈Xn〉∼tnγ/2. We present a systematic study of the dependence of the mean, the variance, the skewness, and the flatness, on the parameters controlling the asymmetry of the flow. The PDFs of the duration of flight (motion following the shear flow) events, and vortex trapping events, exhibit algebraic decay. In some cases, the flights correspond to Lévy flights. The results of the model are compared with recent experiments on chaotic advection and Lévy flights in a rotating annulus.
A new Strouhal–Reynolds-number relationship for the circular cylinder in the range 47<Re<2×105
Authors: Fey Uwe , König Michael , Eckelmann Helmut
Published: July 2002

Based on experiments a new law is proposed for the vortex shedding from a circular cylinder which describes in a consistent way the Strouhal–Reynolds-number dependency as Sr=Sr*+m/Re from the beginning of the vortex shedding at Re=47 up to the laminar–turbulent transition of the cylinder boundary layer at Re>2×105. The various vortex shedding processes, occurring with increasing Reynolds number, are described by different coefficients Sr* and m.
Pattern formation in thin liquid films with insoluble surfactants
Authors: de Souza E. Ramos , Gallez D.
Published: July 2002

The problem of pattern formation in thin liquid films with insoluble surfactants under attractive and repulsive forces is addressed. A thin fluid film bounded by a wall is modeled by a set of two nonlinear evolution equations for the film thickness and surfactant concentration on the free interface. We perform a bifurcation analysis valid for the general case of apolar and polar forces and predict a supercritical bifurcation to new stationary and periodic structures. Numerical simulations for the particular case of a negative apolar spreading coefficient (attractive van der Waals forces) and a positive polar spreading coefficient (repulsive hydration pressure) are discussed in terms of the analytical predictions. Nonlinearities in the competition between attractive and repulsive forces can lead to formation of periodic patterns for the film thickness with homogeneously distributed surfactants. Due to diffusion and Marangoni effects, insoluble surfactants alter the time required for pattern formation but do not alter the final pattern profile itself. Bifurcation analysis allows us then to predict the ranges of film parameters in which pattern formation, rupture, or total film spreading is possible.
Direct numerical simulation of laboratory experiments in isotropic turbulence
Authors: de Bruyn Kops S. M. , Riley J. J.
Published: July 2002

Massively parallel computers are now large enough to support accurate direct numerical simulations (DNSs) of laboratory experiments on isotropic turbulence, providing researchers with a full description of the flow field as a function of space and time. The high accuracy of the simulations is demonstrated by their agreement with the underlying laboratory experiment and on checks of numerical accuracy. In order to simulate the experiments, requirements for the largest and smallest length scales computed must be met. Furthermore, an iterative technique is developed in order to initialize the larger length scales in the flow. Using these methods, DNS is shown to accurately simulate isotropic turbulence decay experiments such as those of Comte-Bellot and Corrsin [J. Fluid Mech. 48, 273 (1971)].
Transport of a passive scalar at a shear-free boundary in fully developed turbulent open channel flow
Authors: Handler R. A. , Saylor J. R. , Leighton R. I. , Rovelstad A. L.
Published: July 2002

Direct numerical simulations of fully developed turbulence in an open channel geometry were performed in which a passive scalar was introduced. The simulations were intended to explore transport at free surfaces in two cases for which (1) the free surface was maintained at constant temperature and (2) the interfacial flux was fixed. These cases can be considered models for mass and evaporative heat transport where buoyancy and surface deformation effects are negligible. Significant differences were found in the thermal fields in these two cases. The turbulent statistics reveal that the surface flux in the constant temperature case was significantly more intermittent compared to the surface temperature field in the constant flux case. The surface temperature field in the latter case formed large patches of warm fluid, reminiscent of the so-called fish scale patterns revealed in recent infrared imagery of the air–water interface. The wake-like structure of the patches was evident despite the absence of surface shear. A model of surface renewal based on the existence of two disparate time scales (a fast hydrodynamic scale, and a slow, diffusional scale) was introduced to explain these differences in a heuristic manner. The model appears successful in explaining, in a qualitative sense, the surface thermal structure in each case. Correlations between the surface thermal fields (flux or temperature) and the subsurface hydrodynamics were also computed. A model based on the hypothesis that hairpin eddies are the dominant kinematic structure responsible for surface renewal is consistent with the correlations. However, these results cannot rule out the importance of other turbulent structures in free surface heat and mass transport processes.
Turbulence modeling using body force potentials
Authors: Perot Blair
Published: July 2002

Reynolds averaged Navier–Stokes (RANS) turbulence models are usually concerned with modeling the Reynolds stress tensor. An alternative approach to RANS turbulence modeling is described where the primary modeled quantities are the scalar and vector potentials of the turbulent body force—the divergence of the Reynolds stress tensor. This approach is shown to have a number of attractive properties, most important of which is the ability to model nonequilibrium turbulence situations accurately at a cost and complexity comparable to the widely used two-equation models such as k-ε. Like Reynolds stress transport equation models, the proposed model does not require a hypothesized constitutive relation between the turbulence and the mean flow variables. This allows nonequilibrium turbulence to be modeled effectively. However, unlike Reynolds stress transport equation models, the proposed system of partial differential equations is much simpler to model and compute. It involves fewer variables, no realizability conditions, and removes the strong coupling between the equations. A detailed analysis of the turbulent body force potentials and their physical significance reveals that they represent the relevant information contained in the Reynolds stress tensor and are fundamental turbulence quantities in their own right. Model predictions for a number of basic turbulent flows are presented including: Channel flow at various Reynolds numbers, mixing layer, rotating channel flow, adverse pressure gradient boundary layers, low Reynolds number backward facing step, and transition to turbulence in channel flow.
Particle segregation in monodisperse sheared suspensions in a partially filled rotating horizontal cylinder
Authors: Tirumkudulu Mahesh , Mileo Antonio , Acrivos Andreas
Published: July 2002

Recent experiments in a partially filled horizontal Couette device [Tirumkudulu et al., Phys. Fluids 11, 507 (1999)] have revealed a new instability in the flow of suspensions containing narrow-sized neutrally buoyant particles in a very viscous Newtonian liquid where the suspension under shear segregates into regions of high and low particle concentration along the length of the cylinder. We conducted similar experiments but in a different geometry, viz., a horizontal cylinder, in which the cylinder was partially filled with that same suspension and rotated about its horizontal axis. We found that, within a certain range of parameter space, the initially uniform suspension divided into cylindrical bands of high particle concentration separated by regions of pure liquid. Although it seems likely that this instability is initiated by particle concentration fluctuations, which, in turn, lead to fluctuations in the effective viscosity of the suspension, we have been unable to provide a quantitative explanation for this phenomenon.
Plasma production from helicon waves
Authors: Degeling A. W. , Jung C. O. , Boswell R. W. , Ellingboe A. R.
Published: July 2002

Experimental measurements taken in a large magnetoplasma show that a simple double half-turn antenna will excite m=1 helicon waves with wavelengths from 10–60 cm. Increased ionization in the center of the downstream plasma is measured when the axial wavelength of the helicon wave becomes less than the characteristic length of the system, typically 50–100 cm. A sharp maximum in the plasma density downstream from the source is measured for a magnetic field of 50 G, where the helicon wave phase velocity is about 3×108 cm s−1. Transport of energy away from the source to the downstream region must occur to create the hot electrons needed for the increased ionization. A simple model shows that electrons in a Maxwellian distribution most likely to ionize for these experimental conditions also have a velocity of around 3×108 cm s−1. This strong correlation suggests that the helicon wave is trapping electrons in the Maxwellian distribution with velocities somewhat slower than the wave and accelerating them into a quasibeam with velocity somewhat faster than the wave. The nonlinear increase in central density downstream as the power is increased for helicon waves with phase velocities close to the optimum electron velocity for ionization lends support to this idea.
Force-free thin flux tubes: Basic equations and stability
Authors: Zhugzhda Y. D.
Published: July 2002

The thin flux tube approximation is considered for a straight, symmetrical, force-free, rigidly rotating flux tube. The derived set of equations describes tube, body sausage, and Alfvén wave modes and is valid for any values of β. The linear waves and instabilities of force-free flux tubes are considered. The comparison of approximate and exact solutions for an untwisted, nonrotating flux tube is performed. It is shown that the approximate and exact dispersion equations coincides, except the 20% discrepancy of sausage frequencies. An effective cross section is proposed to introduce the removal of this discrepancy. It makes the derived approximation correct for the force-free thin flux tube dynamics, except the detailed structure of radial eigenfunction. The dispersion of Alfvén torsional waves in a force-free tubes appears. The valve effect of one directional propagation of waves in rotating twisted tube is revealed. The current and rotational sausage instabilities of a force-free, thin flux tube are considered.
Bootstrap current and neoclassical transport in tokamaks of arbitrary collisionality and aspect ratio
Authors: Houlberg W. A. , Shaing K. C. , Hirshman S. P. , Zarnstorff M. C.
Published: July 2002

A multi-species fluid model is described for the steady state parallel and radial force balance equations in axisymmetric tokamak plasmas. The bootstrap current, electrical resistivity, and particle and heat fluxes are evaluated in terms of the rotation velocities and friction and viscosity coefficients. A recent formulation of the neoclassical plasma viscosity for arbitrary shape and aspect ratio (including the unity aspect ratio limit), arbitrary collisionality, and orbit squeezing from strong radial electric fields is used to illustrate features of the model. The bootstrap current for the very low aspect ratio National Spherical Torus Experiment [J. Spitzer et al., Fusion Technol. 30, 1337 (1996)] is compared with other models; the largest differences occur near the plasma edge from treatment of the collisional contributions. The effects of orbit squeezing on bootstrap current, thermal and particle transport, and poloidal rotation are illustrated for an enhanced reverse shear plasma in the Tokamak Fusion Test Reactor [D. Meade and the TFTR Group, Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 9]. Multiple charge states of impurities are incorporated using the reduced ion charge state formalism for computational efficiency. Because the force balance equations allow for inclusion of external momentum and heat sources and sinks they can be used for general plasma rotation studies while retaining the multi-species neoclassical effects.
Generalized theory of helicon waves. I. Normal modes
Authors: Chen Francis F. , Arnush Donald
Published: July 2002

The theory of helicon waves is extended to include finite electron mass. This introduces an additional branch to the dispersion relation that is essentially an electron cyclotron or Trivelpiece–Gould (TG) wave with a short radial wavelength. The effect of the TG wave is expected to be important only for low dc magnetic fields and long parallel wavelengths. The normal modes at low fields are mixtures of the TG wave and the usual helicon wave and depend on the nature of the boundaries. Computations show, however, that since the TG waves are damped near the surface of the plasma, the helicon wave at high fields is almost exactly the same as is found when the electron mass is neglected.
Instability of a magnetized plasma jet
Authors: Biskamp D. , Schwarz E. , Zeiler A.
Published: July 2002

The stability characteristics of a plasma jet V(x)=sech2 x embedded in a parallel magnetic field B(x)=B0 tanh x are investigated. The Kelvin–Helmholtz kink mode, which dominates at small B0, is stabilized at B0=0.46, while the pinch mode remains unstable up to B0=0.96. Contrary to a nonmagnetized jet, for finite B0 the kink mode saturates quasilinearly by broadening the jet, such that B0/Vmax becomes larger than the marginal value 0.46. Including finite resistivity the nonlinear behavior is eventually governed by the tearing mode for all values of B0≠0, consisting of a single plasmoid moving along the jet. Hence the shear flow cannot stabilize the tearing mode, so that the apparent tearing stability of resistive current sheets observed in numerical simulations of magnetic reconnection is only due to finite-length effects.
Shock propagation in a low-density foam filled with fluid
Authors: Hazak Giora , Velikovich Alexander L. , Gardner John H. , Dahlburg Jill P.
Published: July 2002

It is found that when a planar shock is imparted to a system of a foam of low mean density filled with fluid (fiber density>ambient fluid density), an undercompressed quasisteady post-shock state is established. In this state, pressure and density are lower than the values predicted by Hugoniot relations for a uniform fluid with the same average fluid-foam density. It is shown that this undercompression phenomenon is due to residual correlations of fluctuations left after the saturation of the initially rapid mixing of fiber material with the background fluid. Generalized Hugoniot relations are derived for quasiplanar shocks in nonuniform systems, with foam as an example. The behavior of fluctuations in such systems is studied. Formulas for the level of fluctuations induced by a shock as it propagates through foam and for the relaxation time of the fluctuations, are derived.
Stability of a compact three-period stellarator with quasiaxial symmetry features
Authors: Cooper W. A.
Published: July 2002

The ideal magnetohydrodynamic stability properties of a three-field period stellarator with quasiaxial symmetry features proposed by Garabedian and Ku [Phys. Plasmas 6, 645 (1999)] are investigated. The stability limit imposed by local ballooning and Mercier modes allows β∼5.3% to be achieved by tailoring the pressure profile. For this system, the global external kink modes are shown to be stable. To demonstrate this, a sequence of configurations has been developed that varies from the reference system proposed to the equivalent axisymmetric tokamak. A scan of kink stability reveals that the configuration with a plasma boundary shape that corresponds to 80% of the basic three-period stellarator and 20% of the tokamak becomes marginal to global external modes. The normal magnetic field line curvature and the local magnetic shear play crucial roles in the stability properties. The local magnetic shear develops a helical stripe with increasing three-dimensional plasma deformation that inhibits the formation of global structures. The normal magnetic field line curvature, however, becomes locally amplified by the three-dimensional plasma deformation in regions where the local magnetic shear vanishes. As a result, the susceptibility to localized ballooning mode destabilization is enhanced.
Homogeneous gain saturation in GaAs/AlGaAs quantum well lasers
Authors: Göbel E. O. , Höger R. , Kuhl J. , Polland H. J. , Ploog K.
Published: July 2002

We have studied the dynamical behavior of laser emission in optically pumped GaAs/AlGaAs lasers with picosecond time resolution. We find a saturation of the spontaneous emission above threshold and a homogeneous reduction of carrier lifetime down to about 70 ps. These data suggest a homogeneous gain saturation in GaAs/AlGaAs quantum well lasers, similar to conventional double heterostructure lasers.
Self-induced polarization changes in birefringent optical fibers
Authors: Winful Herbert G.
Published: July 2002

Exact solutions are presented for the intensity-dependent polarization state of a light wave in a birefringent optical fiber. The theory takes into account both the linear polarization evolution and the nonlinear ellipse rotation. It is shown that, contrary to current belief, self-induced polarization changes are possible even with equal excitation of the fiber’s principal axes. The theory is applicable to the operation of recently demonstrated fiber-optic logic gates, pulse shapers, and intensity discriminators.
InGaAsP laser with semi-insulating current confining layers
Authors: Dutta N. K. , Zilko J. L. , Cella T. , Ackerman D. A. , Shen T. M. , Napholtz S. G.
Published: July 2002

The fabrication and performance characteristics of a InGaAsP laser structure with semi-insulating current confining layers are reported. The semi-insulating layers are Fe-doped InP and are grown using the metalorganic chemical vapor deposition growth technique. The lasers have threshold currents in the range 20–30 mA and external differential quantum efficiency ∼0.2 mW/mA/facet at 30 °C. The bandwidth for small-signal response is ∼2 GHz which suggests that the laser structure is suitable for high bit rate lightwave transmission systems. Initial aging results yield an estimated operating lifetime of 10 years at 20 °C.
Tunneling microscopy in an electrochemical cell: Images of Ag plating
Authors: Sonnenfeld Richard , Schardt Bruce C.
Published: July 2002

A scanning tunneling microscope (STM) operating in an electrochemical cell was used to observe Ag films that had been electrodeposited on a graphite substrate. Before Ag deposition and after stripping the deposited Ag, the atomic features of the graphite substrate were observed. The observed STM images are consistent with an island film growth mechanism. This initial study demonstrates that STM studies of electrochemical processes occurring at electrodes in solution are now possible.
Identification of a bistable defect in silicon: The carbon interstitial-carbon substitutional pair
Authors: Song L. W. , Benson B. W. , Watkins G. D.
Published: July 2002

By using a combination of deep level transient spectroscopy (DLTS) and electron paramagnetic resonance techniques applied to samples of varying compositions, we identify the bistable defect at Ec −0.17 eV in irradiated n-type silicon as a carbon interstitial-carbon substitutional pair. It arises upon annealing of interstitial carbon, which is also the precursor to a remarkable recently discovered four-level multistable defect which we now tentatively identify as a carbon-phosphorus pair. We demonstrate a new simple method for distinguishing the bistable carbon-carbon pair defect from the oxygen-vacancy pair under the same DLTS peak.
Epitaxial ordering of oxide superconductor thin films on (100) SrTiO3 prepared by pulsed laser evaporation
Authors: Wu X. D. , Dijkkamp D. , Ogale S. B. , Inam A. , Chase E. W. , Miceli P. F. , Chang C. C. , Tarascon J. M. , Venkatesan T.
Published: July 2002

Thin films of Y-Ba-Cu-O superconductors on (100) SrTiO3 were prepared using the pulsed excimer laser evaporation technique. After an annealing treatment in oxygen the films exhibited superconductivity with zero resistance at 85 K with transition widths (90%–10%) as narrow as a 2 K. A considerable amount of orientation of the films with respect to the substrates was observed by x-ray diffraction and ion channeling techniques. Interface reaction revealed by Rutherford backscattering and Auger electron spectrometry resulted in a thickness dependence of the superconducting properties of the films.
Ablation and etching of polymethylmethacrylate by very short (160 fs) ultraviolet (308 nm) laser pulses
Authors: Srinivasan R. , Sutcliffe Emile , Braren Bodil
Published: July 2002

The effect of pulse width on the ablation of polymers has been extended to ultrashort pulses (160 fs) of 308 nm wavelength. Polymethylmethacrylate has negligible absorption at this wavelength for one-photon excitation. With the ultrashort pulse clean etching without any sign of thermal damage can be achieved at fluences as little as 0.2–0.3 J/cm2. This is the first demonstration that the high power of ultrashort pulses of ultraviolet light can produce photochemical etching by taking advantage of multiphoton excitation to dissociative states.
Y1Ba2Cu3O7−δ thin films grown by a simple spray deposition technique
Authors: Gupta A. , Koren G. , Giess E. A. , Moore N. R. , O’Sullivan E. J. M. , Cooper E. I.
Published: July 2002

The preparation of high Tc superconducting thin films of Y1Ba2Cu3O7−δ on (100) single crystals of MgO, ZrO2 with 9% Y2O3 (yttria stabilized zirconia, or YSZ), and SrTiO3 using a simple spray deposition technique is described. Typical film growth procedure involves (a) the spraying of a stoichiometric solution of the nitrate precursors on the heated substrate (180 °C), (b) prebaking in air of the sprayed film (20 min at 500 °C), and (c) oven annealing of the film under flowing O2 (900–950 °C followed by slow cooling to 200 °C in about 3 h). X-ray diffraction analysis of the films after each of the growing steps mentioned above shows primarily the presence of crystalline phases of the nitrates, the oxides, and the orthorhombic superconducting phase, respectively. Resistivity versus temperature measurements show that the onset and completion of the superconductive transition occur at 92 and 87 K, respectively, in films on YSZ substrate; at 95 and 80 K, respectively, in films on SrTiO3 substrate; and at 82 and 77 K, respectively, in films on MgO substrate.