bluuebunny/crossref_metadata_2025 huggingface dataset:
A multiple-scale model for compressible turbulent flows
Authors: Liou William W. , Shih Tsan-Hsing , Duncan Beverly S.
Published: July 2002

A multiple-scale model for compressible turbulent flows is proposed in this paper. It is assumed that turbulent eddy shocklets are formed primarily by large energetic eddies. The extra straining of the large eddy, due to their interactions with shocklets, enhances the energy cascade to smaller eddies. Model transport equations are developed for the turbulent kinetic energies and the energy transfer rates of the different scale. The turbulent eddy viscosity is determined by the total turbulent kinetic energy and the rate of energy transfer from the large scale to the small scale, which is different from the energy dissipation rate. The model coefficients in the modeled turbulent transport equations depend on the ratio of the turbulent kinetic energy of the large scale to that of the small scale, which renders the model more adaptive to the characteristics of individual flow. The model is tested against compressible free shear layers, boundary layers, and a compression ramp flow. The results agree satisfactorily with measurements.
The higher moments in the Lundgren model conform with Kolmogorov scaling
Authors: Segel Daniel
Published: July 2002

We calculate the structure functions of the higher moments of the vorticity in the framework of Lundgren’s spiral model of turbulence. We show that they conform to the scaling expected from Kolmogorov’s scaling hypothesis of 1941, and explain the result.
Numerical simulations of thermal convection in a rotating spherical fluid shell at high Taylor and Rayleigh numbers
Authors: Sun Z.-P. , Schubert G.
Published: July 2002

In this study, we carry out numerical simulations of thermal convection in a rapidly rotating spherical fluid shell at high Taylor number Ta and Rayleigh number R with a nonlinear, three-dimensional, time-dependent, spectral-transform code. The parameters used in the simulations are chosen to be in a range which allows us to study two different types of convection, i.e., single column and multi-layered types, and the transition between them. Numerical solutions feature highly time-dependent north–south open columnar convective cells. The cells occur irregularly in longitude, are quasi-layered in cylindrical radius, and maintain alternating bands of mean zonal flow. The complex convective structure and the banded mean zonal flow are results of the high Taylor and Rayleigh numbers. The transition between the two types of convection appears to occur gradually with increasing Rayleigh and Taylor numbers. At a Taylor number of 107 the differential rotation pattern consists of an inner cylindrical region of subrotation and an outer cylindrical shell of superrotation manifest at the outer boundary as an equatorial superrotation and a high latitude subrotation. The differential rotation pattern is similar at Ta=108 and low Rayleigh number. Cylindrical shells of alternately directed mean zonal flow begin to develop at Ta=108 and R=50Rc and at Ta=109 and R=25Rc. This pattern is seen on the outer surface as a latitudinally-banded zonal flow consisting of an equatorial superrotation, a middle and high latitude subrotation, and a polar superrotation. At Ta=109 and R=50Rc the differential rotation appears at the surface as a broad eastward flow in the equatorial region with alternating bands of westward and eastward flow at high latitudes.
Stability analysis of source and sink flows
Authors: Shusser M. , Weihs D.
Published: July 2002

A linear stability analysis is performed for two- and three-dimensional steady source and sink flows. Cases studied include inviscid compressible and incompressible fluids. For two-dimensional flows viscous incompressible fluid is also examined. The one-dimensional nature of the unperturbed base flow suggested taking the vorticity as a perturbation in order to reduce the number of variables and to simplify the analysis. It is shown that source flows are always unstable. Sink flows are found unstable for inviscid compressible fluid and also for two-dimensional flow of viscous incompressible fluid for low Reynolds numbers. The different modes of instability existing in perturbed flow are obtained.
Simplified statistical approach to complex turbulent flows and ensemble-mean compressible turbulence modeling
Authors: Yoshizawa Akira
Published: July 2002

A simplified statistical method for complex turbulent flows is developed in close relation to ensemble-mean compressible turbulence modeling. Using this method, various correlations appearing in the ensemble-mean equations for a compressible fluid are evaluated, and their relationship with the mean quantities and the turbulence properties characterizing compressible flow is examined. These results are discussed in light of the previous finding of the direct numerical simulation of compressible turbulence. This method is also applicable to complex turbulence phenomena like magnetohydrodynamic flow.
Fluctuating kinetic energy budget during homogeneous flow of a fluid solid mixture
Authors: Liljegren L. M. , Foslein W.
Published: July 2002

Ensemble-averaging theorems are applied to derive transport equations for the fluctuating kinetic energy of a particulate mixture consisting of a continuous fluid and solid particles. The evolution of fluctuating kinetic energy in a homogeneous flow is examined and discussed.
Exploitation of Brownian motions for the optimal control of fiber orientation distributions
Authors: Szeri Andrew J.
Published: July 2002

The dynamical behavior of suspensions of rod-like particles is explored in unsteady elementary flows characterized by the following special property: while there may be arbitrarily large excursions from an initial state by the carrier fluid, the fluid returns to the initial state at the end of the time interval. In such a closed flow, it is shown that the orientation distribution of a dilute non-Brownian suspension will also return to its initial state, after arbitrarily large excursions driven by the flow of the carrier fluid. In contrast, the orientation distribution in a Brownian suspension will not return to its initial state at the end of the interval in a closed flow. These effects are exploited in a constrained nonlinear optimal control problem for the orientation distribution of a Brownian suspension in an example closed flow. The result of the control problem is a family of protocols that are most effective at achieving various degrees of anisotropy of the suspension, within the class of flows under consideration.
Nonlinear spatially developing Görtler vortices in curved wall jet flow
Authors: Le Cunff Cédric , Zebib Abdelfattah
Published: July 2002

Linear and nonlinear spatial developments of two-dimensional wall jets on curved surfaces are computed using pseudospectral-finite difference methods. Inviscid analysis shows that the instability originates from the inner/outer region on a concave/convex wall; the shear layer is thus always unstable regardless of the curvature. This primary instability is a steady spanwise vortex structure similar to Görtler instability in a Blasius flow. In the present study, a perturbation of prescribed wave number α is assumed. In the limit of high Reynolds number (Re) and small curvature (ε), a parabolic set of nonlinear equations describes the spatial evolution of the disturbance. Direct marching simulation of the perturbation and a parabolic stability approach are employed. Both give the same results with different computational efficiencies. For the concave case at low Görtler numbers (G2=ε√Re), perturbations are unstable for small α. Their energy reaches a maximum and then decays. At high G, the most unstable disturbance occurring at larger α will grow exponentially and reach saturation. The convex case is the most unstable situation. But as for the concave case, the most dangerous disturbance moves from small to larger α as G increases. The numerical results are able to capture the primary instability as observed in the experiment of Matsson [Phys. Fluids 7, 3048 (1995)].
Experimental investigation of a two-layer shearing instability in a cylindrical Couette cell
Authors: Gallagher Christopher T. , Leighton David T. , McCready Mark J.
Published: July 2002

A new experiment is presented to investigate interfacial instability between two immiscible liquids with different viscosity. Two density-matched fluids are confined between the walls of a concentric cylinder Couette cell that is aligned parallel to gravity. The resulting interface is parallel to the walls of the Couette cell and gravity. At sufficiently fast rotation rates, interfacial waves form with amplitudes that vary azimuthally and with crests parallel to gravity. Experiments and detailed results on the behavior of the system including a phase map and video stills are presented as a function of depth ratio and rotation rate. Both two-dimensional periodic wave trains and localized waves are observed. The experimental wavelength of the two-dimensional waves agree with the predictions of the Orr–Sommerfeld equation.
A theoretical model for centering of a thin viscous liquid shell in free and forced capillary oscillations
Authors: Lee C. P. , Anilkumar A. V. , Wang T. G.
Published: July 2002

In previous numerical studies [Lee and Wang, J. Fluid Mech. 188, 411 (1988); Pelekasis et al., J. Fluid Mech. 230, 541 (1991)], it has been shown that when an inviscid and nonconcentric liquid shell undergoes a finite-amplitude capillary oscillation, its enclosed bubble undertakes a slow translational oscillation relative to the shell. In the present work, we study the effects of viscosity on the slow motion, in both free and forced capillary oscillations. It is found that in a free oscillation, the shell cannot become concentric because the oscillations are damped by viscosity before centering occurs. In a forced oscillation which is sustained by an external source such as a modulated acoustic radiation pressure, centering does occur when the slow oscillations are damped. The predicted centering of the shell takes place more slowly than that observed in experiments [Wang et al., J. Colloid Interface Sci. 165, 19 (1994)]. However, it is noted that a comparison with experiments is not appropriate at this time, since the shell in the experiments had an uncontrolled rotation in the acoustic potential well.
Influence of thermal boundary conditions on the stability of thermocapillary-driven convection at low Prandtl numbers
Authors: Priede Jānis , Gerbeth Gunter
Published: July 2002

We analyze the effect of various thermal boundary conditions on the linear stability of surface-tension-driven flow in an unbounded liquid layer subject to a longitudinal temperature gradient. An original approach is devised to estimate the critical instability parameters. The order of magnitude estimates are used to solve the problem asymptotically for small Prandtl numbers. The instability is shown to be essentially determined by the thermal boundary conditions. For insulating boundaries the critical wavenumber scales as kc∼Pr1/2 meaning that the most unstable wave is considerably longer than the depth of the layer. When the bottom is well conducting, the critical wavelength is comparable to the depth of the layer. For the case of insulating bottom and non-adiabatic free surface the critical wavenumber depends on the Biot number as kc∼Bi1/2. Even a weak thermal coupling between the free surface and the ambient medium such that Bi∼Pr can significantly influence the instability threshold.
Vortex ring structure at late stages of formation
Authors: Fabris Drazen , Liepmann Dorian
Published: July 2002

High resolution DPIV measurements are made of a vortex ring formed by a piston/cylinder mechanism (Reynolds number 3700 and 7500). A complex ring structure is identified. The stopping condition of the piston leads to vortical fluid collecting near the forward stagnation point of the ring. In addition, the roll-up of the core produces a vorticity distribution that departs from a simple Gaussian profile.
Combined flow and evaporation of fluid on a spinning disk
Authors: Birnie Dunbar P. , Manley Manuel
Published: July 2002

Fluid flow and fluid evaporation both contribute to the overall rate of thinning during spinning of a fluid on a disk. Laser interferometry of solvent thinning behavior on spinning silicon wafers was performed to yield plots of solvent thickness evolution. These plots of thickness versus time were then analyzed to understand the respective contributions of viscous flow and evaporation to the thinning. A technique is described for extracting both the viscosity and the evaporation rate from the interference data. Well understood solvent systems are examined as test cases for this deconvolution. It is also demonstrated that nonevaporating fluids can be analyzed, even though their thickness evolution has no easily referenced endpoint to the thinning, in contrast to the volatile solvents which are rapidly spun dry.
Dynamic light scattering in shear flow
Authors: Hoppenbrouwers Marc , van de Water Willem
Published: July 2002

We describe a new dynamic light scattering technique for measuring diffusion in sheared suspensions. It involves a scattering geometry with two crossing laser beams. A detailed analysis of the correlation function of scattered light is given. The viability of our method is demonstrated in an experiment where the effect of Taylor diffusion on the scattered light correlation function is measured.
Vortex rings descending in a stratified fluid
Authors: Orlandi P. , Egermann P. , Hopfinger E. J.
Published: July 2002

Experimental and numerical results are presented of a laminar vortex ring descending in a linearly stratified fluid. It is observed that the ring shrinks in size as it penetrates the stratified environment and has nearly disappeared when it comes to a stop. This behavior is similar to what was observed by Maxworthy [J. Fluid Mech. 81, 465 (1977)] for a turbulent vortex ring in a stratified environment. The ring position and size collapse on a single curve when appropriately scaled. It is shown that baroclinic vorticity production of opposite sign and cross diffusion is the cause for the disappearance of the ring. The instantaneous ring Froude number is always ⩾1 and, therefore, no internal waves are generated.
Applications of stochastic estimation in the axisymmetric sudden expansion
Authors: Cole Daniel R. , Glauser Mark N.
Published: July 2002

Stochastic estimation was utilized to reconstruct estimates of multipoint conditional averages in an axisymmetric sudden expansion. Application of this technique requires, in addition to the two-point correlation tensor, time series data measured at selected positions simultaneously. One application of this technique yielded estimates of the spatial and temporally evolving velocity field in a given azimuthal plane of the sudden expansion. An application of this technique in the reattachment region clearly shows its unsteady character. The spectra measurements as well as the stochastic estimates utilizing multipoint conditions applied over two directions suggest that coherent structures from the near field shear layer propagate downstream into the reattachment region and cause this unsteadiness.
Scaling of longitudinal velocity increments in a fully developed turbulent channel flow
Authors: Antonia R. A. , Orlandi P. , Romano G. P.
Published: July 2002

The departure from Kolmogorov (1941) scaling of high-order moments of longitudinal velocity increments in a fully developed turbulent channel flow is investigated using data obtained with hot wire anemometry (HWA), laser Doppler velocimetry (LDV) and direct numerical simulation (DNS). The magnitude of the departure increases towards the wall, reaching a local maximum at a distance of about 20 wall units.
Some remarks concerning recent work on rotating turbulence
Authors: Zhou Ye , Speziale Charles G. , Rubinstein Robert
Published: July 2002

Some aspects of a recent paper on rotating turbulence by Canuto and Dubovikov [Phys. Fluids 9, 2132 (1997)] are examined from historical and scientific perspectives. Their claim to have discovered a new energy spectrum scaling law for rotating turbulence is examined in light of previous publications on this subject. We answer an objection raised to the consistency of this spectral scaling law with the Bardina-type dissipation rate transport equation derived from this law by two of the authors [Phys. Fluids 8, 3172 (1996)]. Finally, some difficulties with the alternative model for the dissipation rate transport equation proposed by Canuto and Dubovikov are described in both the weak and strong rotation limits.
Sheared salt fingers: Instability in a truncated system
Authors: Paparella Francesco , Spiegel Edward A.
Published: July 2002

We derive a model for fingering doubly diffusive convection from a truncated expansion in horizontal planform functions with the inclusion of a large-scale shearing mode. This produces nonlinear partial differential equations in time and in vertical coordinate. At a high enough Rayleigh number, both convection and shear modes are sustained and their interaction produces rich cyclic dynamics with the fingering layer dividing into two distinct finger layers that engender steps in the mean salinity before being disrupted by the beginning of a new cycle.
Stability of self-similar solutions for van der Waals driven thin film rupture
Authors: Witelski Thomas P. , Bernoff Andrew J.
Published: July 2002

Recent studies of pinch-off of filaments and rupture in thin films have found infinite sets of first-type similarity solutions. Of these, the dynamically stable similarity solutions produce observable rupture behavior as localized, finite-time singularities in the models of the flow. In this letter we describe a systematic technique for calculating such solutions and determining their linear stability. For the problem of axisymmetric van der Waals driven rupture (recently studied by Zhang and Lister), we identify the unique stable similarity solution for point rupture of a thin film and an alternative mode of singularity formation corresponding to annular “ring rupture.”
Flow structure and modeling issues in the closure region of attached cavitation
Authors: Gopalan Shridhar , Katz Joseph
Published: July 2002

Particle image velocimetry (PIV) and high-speed photography are used to measure the flow structure at the closure region and downstream of sheet cavitation. The experiments are performed in a water tunnel of cross section 6.35×5.08 cm2 whose test area contains transparent nozzles with a prescribed pressure distribution. This study presents data on instantaneous and averaged velocity, vorticity and turbulence when the ambient pressure is reduced slightly below the cavitation inception level. The results demonstrate that the collapse of the vapor cavities in the closure region is the primary mechanism of vorticity production. When the cavity is thin there is no reverse flow downstream and below the cavitation, i.e., a reentrant flow does not occur. Instead, the cavities collapse as the vapor condenses, creating in the process hairpin-like vortices with microscopic bubbles in their cores. These hairpin vortices, some of which have sizes as much as three times the height of the stable cavity, dominate the flow downstream of the cavitating region. The averaged velocity distributions show that the unsteady collapse of the cavities in the closure region involves substantial increase in turbulence, momentum, and displacement thickness. Two series of tests performed at the same velocity and pressure, i.e., at the same hydrodynamic conditions, but at different water temperatures, 35 °C and 45 °C, show the effect of small changes in the cavitation index (σ=4.69 vs. σ=4.41). This small decrease causes only a slight increase in the size of the cavity, but has a significant impact on the turbulence level and momentum deficit in the boundary layer downstream. Ensemble averaging of the measured instantaneous velocity distributions is used for estimating the liquid void fraction, average velocities, Reynolds stresses, turbulent kinetic energy and pressure distributions. The results are used to examine the mass and momentum balance downstream of the cavitating region. It is shown that in dealing with the ensemble-averaged flow in the closure region of attached cavitation, one should account for the sharp (but still finite) gradients in the liquid void fraction. The 2-D continuity equation can only be satisfied when the gradients in void fraction are included in the analysis. Using the momentum equation it is possible to estimate the magnitude of the “interaction term,” i.e., the impact of the vapor phase on the liquid momentum. It is demonstrated that, at least for the present test conditions, the interaction term can be estimated as the local pressure multiplied by the gradient in void fraction.
Temporal and spatial unmixedness downstream of a plate array
Authors: Guillaume D. W. , LaRue J. C.
Published: July 2002

The effect of a plate array on a turbulent velocity and turbulent concentration field is determined. Profiles of mean and root-mean-square velocity and concentration, profiles of temporal and spatial unmixedness, and profiles of the variance of the gradient of velocity and the variance of the gradient of concentration are presented. Velocity and concentration integral length scales are compared. A biplane injection grid is used to produce the turbulent concentration and turbulent velocity field. Helium is injected through the jets at the grid nodes as air passes through the grid. The time-resolved velocity and concentration data are obtained using a two-sensor probe that consists of a hot wire and a TSI 1440-20 aspirating concentration probe. The addition of a plate array is shown to decrease the spatial unmixedness to a nearly zero value in about half the downstream distance observed without plates. Further, an increase in dissipation is shown with the array in place that reduces the temporal unmixedness to a value less than the value observed without the plates in about one-third the downstream distance.
On the rapid estimation of permeability for porous media using Brownian motion paths
Authors: Hwang Chi-Ok , Given James A. , Mascagni Michael
Published: July 2002

We describe two efficient methods of estimating the fluid permeability of standard models of porous media by using the statistics of continuous Brownian motion paths that initiate outside a sample and terminate on contacting the porous sample. The first method associates the “penetration depth” with a specific property of the Brownian paths, then uses the standard relation between penetration depth and permeability to calculate the latter. The second method uses Brownian paths to calculate an effective capacitance for the sample, then relates the capacitance, via angle-averaging theorems, to the translational hydrodynamic friction of the sample. Finally, a result of Felderhof is used to relate the latter quantity to the permeability of the sample. We find that the penetration depth method is highly accurate in predicting permeability of porous material.
A linear process in wall-bounded turbulent shear flows
Authors: Kim John , Lim Junwoo
Published: July 2002

A linear process in wall-bounded turbulent shear flows has been investigated through numerical experiments. It is shown that the linear coupling term, which enhances non-normality of the linearized Navier–Stokes system, plays an important role in fully turbulent—and hence, nonlinear —flows. Near-wall turbulence is shown to decay without the linear coupling term. It is also shown that near-wall turbulence structures are not formed in their proper scales without the nonlinear terms in the Navier–Stokes equations, thus indicating that the formation of the commonly observed near-wall turbulence structures are essentially nonlinear, but the maintenance relies on the linear process. Other implications of the linear process are also discussed.
Production of C60 plasma
Authors: Sato N. , Mieno T. , Hirata T. , Yagi Y. , Hatakeyama R. , Iizuka S.
Published: July 2002

An ultrafine-particle plasma consisting of electrons, positive K+ ions, and large negative C−60 ions is produced by introducing ‘‘Buckminsterfullerene, C60’’ particles into a low-temperature (≂0.2 eV) potassium plasma column confined by a strong axial magnetic field. With an increase in the C−60 fraction, the electron shielding decreases, yielding clear effects on plasma collective phenomena, which are demonstrated for low-frequency electrostatic plasma-wave propagations and instabilities. This plasma might be useful for producing new C60-based materials.
Beam divergence due to convective drift instabilities in an applied-B ion diode
Authors: Krall Nicholas A.
Published: July 2002

A low-frequency instability is identified, which is convected through an applied-B ion diode by the ion beam, is driven by the cross-field flow of electrons relative to drifting ions, and produces a spread in beam velocity. The consequences of this convective instability are explored in detail, and many of the aspects needed for application to ion beam physics are calculated and combined into a single unified theory. Many aspects of this instability agree with the frequency range, growth properties, spatial structure, saturation level, and beam spread seen in experiments and computer simulations.
Momentum and heat transfer of a translating liquid droplet under a thermal plasma environment
Authors: Paik Seungho , Nguyen Hoa D.
Published: July 2002

Momentum and heat transfer associated with a translating liquid droplet with/without mass efflux from the droplet surface in a quiescent argon thermal plasma environment is studied. A Chebyshev–Legendre spectral method with a fractional time stepping method is expanded to treat the compressible nature of the thermal plasma flow as well as the internal droplet incompressible flow. Steady-state solutions for the case of plasma flow over a constant temperature solid sphere and incompressible flow over a droplet without mass efflux from the droplet are calculated and compared with previously published results. The interfacial phenomena are studied with interfacial physical properties. Transient solutions with droplet internal motion and convective heat transfer with/without mass efflux from the sphere surface are presented. Results indicate that uniform droplet internal temperature is established more quickly than that of a solid sphere in the plasma environment for no mass efflux case. Also, a symmetric interfacial velocity distribution develops along the droplet interface as Reynolds number increases. In case of mass efflux from the sphere, results show that the drag force and heat transfer rate are reduced in the presence of radial mass efflux at the droplet surface.
Spectral characteristics of low-frequency turbulence in toroidal collisional magnetoplasma
Authors: Prasad G. , Bora D. , Saxena Y. C.
Published: July 2002

The dispersion and spectral characteristics of low-frequency fluctuations excited in collisional toroidal magnetoplasma is reported. The plasma produced under the operating conditions have magnetized electrons, unmagnetized ions, E∥∇n⊥B and ∇n antiparallel to effective gravity g. The low frequency (<ion-gyrofrequency) electrostatic fluctuations observed in density and potential are turbulent, travel mainly in azimuthal direction and are excited by the cross-field instability. The spectral indices of the k spectra for the density and potential fluctuations, as computed using pair probe technique, are −3.8±0.5 and −6.0±0.6, respectively. These results are in good agreement with the in situ measurements of fluctuations in equatorial electrojet and the earlier laboratory experiments on the cross-field instability. Numerical simulations and nonlinear theories, however, predict flatter spectra.
Relaxation of a nonideal magnetohydrodynamic plasma in a cylindrical column
Authors: Cheremnykh O. K. , Andrushchenko Zh. M. , Edenstrasser J. W. , Taranov V. B.
Published: July 2002

The time evolution of a nonideal magnetohydrodynamic (MHD) plasma is investigated. An approach for the reduction of the nonlinear vector MHD equations for a current carrying plasma with a strong longitudinal external magnetic field to a set of scalar partial differential equations is supposed. Analytical time-dependent solutions of the nonlinear resistive and dissipative MHD equations for a cylindrical plasma column are presented. It is shown that the internal magnetic field and the velocity have different damping times and that in the asymptotic limit t→∞ the plasma slowly relaxes toward the static equilibrium state. Analytical reduced MHD solutions describing slowly evolving states in tokamaks are found.
Dissociative recombination in cascaded arc generated Ar–N2 and N2 expanding plasma
Authors: Dahiya R. P. , de Graaf M. J. , Severens R. J. , Swelsen H. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

The expanding plasma obtained from a cascaded arc thermal source is analyzed with double probe, mass spectrometric, and Faraday cup measurements. In the argon–nitrogen mixtures a decrease in ion fluence is observed, contrary to pure argon plasmas in which recombination is insignificant. The recombination in argon–nitrogen plasmas is caused by charge exchange between atomic ions and N2 molecules followed by dissociative recombination. Hence, these processes account for the enhanced axial decay of the plasma density and also for the change in the ion mass spectra of the ion beam extracted from the expanding plasma. The total ion beam current density is also governed by charge exchange followed by dissociative recombination and is thus dependent on the recirculating neutral molecules.
On the dynamics of a plasma vortex street and its topological signatures
Authors: Siregar E. , Stribling W. T. , Goldstein M. L.
Published: July 2002

A plasma vortex street configuration can evolve when two velocity and one magnetic shear layer interact strongly. A study of the interaction between two- and three-dimensional plasma modes and a mean sheared magnetic field is undertaken using a three-dimensional magnetohydrodynamic spectral Galerkin computation. The initial state is a simple magnetic shear in a plane perpendicular to the plasma velocity shear plane. In a very weak magnetic field, secondary instabilities (three-dimensional modes), expressed by the kinking of vortex tubes, lead to plasma flow along and around the axes of the vortex cores, creating characteristic patterns of kinetic helicity and linkages between vortex filaments. Three-dimensionality leads to the vortex breakdown process. A strong sheared magnetic field inhibits the kinking of vortex tubes, maintaining two-dimensionality. This inhibits vortex breakdown over long dynamical times. There is an anticorrelation in time between linkage indices of the vortex filament (related to kinetic helicity), suggesting that the ellipticity axes of the vortex cores along the street undergo a global inphase evolution. This anticorrelation has a dynamical interpretation. It extends to a relaxing plasma in the Navier–Stokes flow notion that helical regions of opposite helicities interact and screen each other off so that the global helicity remains bounded.
Simulation of ultrahigh recycling divertors for tokamaks
Authors: Petravic M.
Published: July 2002

The handling of power carried by the charged particles into the scrape-off layer of a tokamak reactor remains a major obstacle for its continuous and reliable operation. Ways of reducing this power through radiation have been studied numerically using fluid models for both the plasma and neutral gas. A new model for the combined plasma and neutral gas transport in two dimensions capable of simultaneously representing regions of fully ionized plasma, partially ionized plasma, and pure neutral gas has been assembled and implemented in the planet code [J. Nucl Mat. 196/198, 883 (1992)]. Divertor plasma temperatures of just below 1 eV have been achieved in a pure hydrogen plasma, resulting in an ionization-free region together with ionization and recombination fronts detached from the material walls. In this regime energy reaches the walls almost exclusively in the form of radiation which, in principle, solves the divertor heat load problems.
Particle confinement in time-dependent fields
Authors: Soběhart J. R. , Farengo R.
Published: July 2002

The confinement of charged particles by linear, time-dependent fields with arbitrary pulse shape is studied. It is assumed that the trapped particles are perturbed by collisions with a neutral background gas. The spatial dispersion and energy of the trapped particles are determined using an asymptotic expansion based on a singular perturbation approach in terms of the momentum exchanged during collisions. Parametric instabilities, which result in the destruction of the confinement, are identified for different pulse shapes.
A two-dimensional kinetic model of the scrape-off layer
Authors: Catto Peter J. , Hazeltine R. D.
Published: July 2002

A two-dimensional (radius and poloidal angle), analytically tractable kinetic model of the ion (or energetic electron) behavior in the scrape-off layer of a limiter or divertor plasma in a tokamak is presented. The model determines the boundary conditions on the core ion density and ion temperature gradients, the power load on the limiter or divertor plates, the energy carried per particle to the walls, and the effective flux limit. The self-consistent electrostatic potential in the quasineutral scrape-off layer is determined by using the ion kinetic model of the layer along with a Maxwell–Boltzmann electron response that occurs because most electrons are reflected by the Debye sheaths (assumed to be infinitely thin) at the limiter or divertor plates.
Shear flow generation from the interaction of neoclassical and drift wave transport processes
Authors: Su X. N. , Yushmanov P. N. , Dong J. Q. , Horton W.
Published: July 2002

Self-consistent shear flow generation from the interaction of neoclassical and drift wave turbulence effects is investigated. The neoclassical poloidal flow damping is shown to compete with the plasma flow generation driven through the divergence of the Reynolds stress. When there is no external driving force except for the free energy released from toroidal shear flow, the turbulent fluctuations occur as a transient pulse which takes the system along an equilibrium path to a relaxed state. External torques, such as parallel neutral beam injection, are needed to maintain significant fluctuation levels. For a system driven by a fixed ion temperature gradient, although linearly the poloidal shear flow generated substantially reduces the growth rate, the simulation results show that a sequence of nonlinear pulses occurs that eventually build the fluctuations up to a level that is not significantly affected by the poloidal flow. In this new, highly nonlinear state the transport is intermittent, with high fluxes occurring through a sequence of pulses of duration 100 Ln/cs for typical system parameters.
Electrostatic and kinetic energies in the wake wave of a short laser pulse
Authors: Teychenné Denis , Bonnaud Guy , Bobin Jean-Louis
Published: July 2002

The electrostatic and kinetic energies available in the longitudinal electron plasma wave created in the wake of an ultraintense laser pulse are obtained analytically. The analysis is one dimensional and assumes a square-shaped pulse that propagates in a highly underdense plasma with the velocity of light in vacuum. The length scale for laser depletion is given as a function of the laser irradiance and the electron density.
Comment on ‘‘Dielectric energy versus plasma energy, and Hamiltonian action-angle variables for the Vlasov equation’’ [Phys. Fluids B 4, 3038 (1992)]
Authors: Best Robert W. B.
Published: July 2002

Morrison and Pfirsch derived expressions for plasma wave energy using the formalism of Van Kampen modes, apparently unaware of earlier work of Best on the same subject by the same method. It is stressed in this Comment that energy density, rather than total energy, is the relevant physical quantity.
An explanation for experimental observations of harmonic cyclotron emission induced by fast ions
Authors: Chen K. R. , Horton W. , Van Dam J. W.
Published: July 2002

An explanation, supported by numerical simulations and analytical theory, is given for the harmonic cyclotron emission induced by fast ions in tokamak plasmas—in particular, for the emission observed at low harmonics in deuterium–deuterium and deuterium–tritium experiments in the Joint European Torus [e.g., Phys. Rev. Lett. 60, 33 (1988)]. It is shown that the first proton harmonic, whose field energy amplitude scales as the 0.84 power of the proton density, is one of the highest spectral peaks, whereas the first alpha harmonic is weak. The relative spectral amplitudes of different harmonics are compared. The results are consistent with the experimental observations. The simulations verify that the instabilities are caused by a weak relativistic mass effect. Simulation also shows that a nonuniform magnetic field leads to no appreciable change in the growth rate and saturation amplitude of the waves.
Parametric excitation of drift waves in a sheared slab geometry
Authors: Davydova T. A. , Jovanović D. , Vranješ J. , Weiland J.
Published: July 2002

The threshold for parametric excitation of drift waves in a sheared slab geometry is calculated for a drift-wave pump which has a standing wave structure along the magnetic field, and the coupling is achieved by the Hasegawa–Mima vector product-type nonlinearity. The usual shear damping is counteracted by the parametric interaction which causes a reversal of the direction of the drift wave group velocity, and the nonlinear eigenvalue problem is solved analytically using Taylor’s strong coupling approximation.
A graphical method for estimating the tunneling factor for mode conversion processes
Authors: Swanson D. G.
Published: July 2002

The fundamental parameter characterizing the strength of any mode conversion process is the tunneling parameter, which is typically determined from a model dispersion relation which is transformed into a differential equation. Here a graphical method is described which gives the tunneling parameter from quantities directly measured from a simple graph of the dispersion relation. The accuracy of the estimate depends only on the accuracy of the measurements.
Damping and absorption of high-frequency waves in dusty plasmas
Authors: De Angelis U. , Forlani A. , Bingham R. , Shukla P. K. , Ponomarev A. , Tsytovich V. N.
Published: July 2002

By employing standard statistical techniques of linear fluctuation theory, it is shown how the ensemble averages over statistical distributions of charged massive grains in a dusty plasma can result in ‘‘average’’ dispersion relations for high-frequency electromagnetic and electrostatic waves. The dispersion relations admit solutions for complex frequencies and wave numbers, suggesting that the waves can be damped (Im ω≠0) or absorbed (Im k≠0) in a dusty plasma.
A unified description of drift eigenmodes in a torus
Authors: Mattor Nathan
Published: July 2002

There are two known types of radial eigenmodes for drift waves in toroidal geometry. These are shown to be limits of a single unified description, analogous to the librational and rotational modes of a pendulum.
Free-electron lasers: Scaling laws for microundulator operation in magnetoresonance with a strong guide field
Authors: Spindler G. , Renz G.
Published: July 2002

Numerical simulations give evidence that a weak microundulator (field on axis: Bu≊50 G, period: λu≊5 mm), operating in magnetoresonance with a strong guide field (Bg≊3×104 G), is capable of producing millimeter and submillimeter radiation with an output power around 1 MW. The electronic efficiency appears to range between 1% and 5%, depending on the radial profile of the electron beam density at the undulator entrance. As an example, the energy of the electron beam is 300 kV to yield a radiation frequency slightly below 200 GHz. Analytical scaling laws for the electron dynamics at magnetoresonance predict that the relative spectral width of the spontaneous emission is proportional to (Bu/Bg)2/3, thus giving support to the surprising results in the case of a weak microundulator.
Kinetic modeling of fast electron dynamics and self-consistent magnetic fields in a reversed field pinch
Authors: Giruzzi G. , Martines E.
Published: July 2002

The dynamics of fast electrons in a reversed field pinch configuration is investigated by numerically solving the appropriate kinetic equation in three dimensions (two dimensions in velocity space and one dimension in real space). To this end, a Fokker–Planck code has been developed, including Coulomb collisions, direct current (dc) electric field, radial diffusion due to magnetic turbulence, ambipolar electric fields, and the self-consistent evaluation of the magnetic fields generated by the plasma itself. This has allowed the theoretical validation of the kinetic dynamo model in a realistic geometry. In contrast to fluid-turbulent theories, such a model predicts that the radial diffusion of fast electrons associated with stochastic magnetic fields might be able to sustain the reversed field configuration. Quantitatively, it is found that the level of magnetic turbulence necessary to obtain the toroidal field reversal at the plasma edge is compatible with levels typically measured in reversed field pinch devices. In particular, the main parameters of standard discharges in the largest existing facility of this type, RFX (reversed field experiment) [Proceedings of the 14th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Würzburg, 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 2, p. 583], have been successfully simulated.
Nonlinear hydromagnetic waves in a thermally stratified spherical shell: Exact toroidal field solutions
Authors: Hamabata Hiromitsu
Published: July 2002

The propagation of nonlinear hydromagnetic waves in a highly conducting, self-gravitating fluid in a spherical geometry, subject to the convective forces produced by a radial temperature gradient, is treated in a Boussinesq approximation. Exact wave solutions of the nonlinear magnetohydrodynamic equations (in the Boussinesq approximation) in the presence of convective forces are obtained for the toroidal velocity and magnetic fields. The solution represents waves propagating along the mean magnetic field with the velocity depending on the mean magnetic (or velocity) field strength and the strength of stratification, under the influence of the azimuthal magnetic and velocity fields and convective forces. The solutions may be applicable to the hydromagnetic waves in the Earth’s core and the solar convection zone.
Evolution of high βp plasmas with improved stability and confinement*
Authors: Politzer P. A. , Casper T. , Forest C. B. , Gohil P. , Heidbrink W. W. , Hyatt A. W. , James R. A. , Jong R. , Lao L. L. , Makowski M. , Meyer W. , Porter G. D. , Sager G. T. , Stallard B. W. , St. John H. , Thompson S. J. , Turnbull A. D. , Wròblewski D.
Published: July 2002

Experiments to explore the long-time evolution of noninductive, high βp plasmas in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], have identified a new, quiescent, high performance regime. The experiments were carried out at low current (400–800 kA) with medium power neutral beam injection (3–10 MW). This regime is characterized by high q0 (≳2) and moderate li(∼1.3). It is reached by slow relaxation of the current profile, on the resistive time scale. As the profiles relax, q0 rises and li falls. When q0 goes above 2 (approximately), magnetohydrodynamic (MHD) activity disappears, and the stored energy rises. Most dramatic is the strong peaking of the central density, which increases by as much as a factor of 2. The improved central confinement appears similar to the PEP/reversed central shear/second stable core modes seen in tokamak experiments, but in this case without external intervention or transient excitation. At high current, a similar, but slower relaxation is seen. Also notable in connection with these discharges is the behavior of the edge and scrape-off layer (SOL). The edge localized modes (ELM’s) as seen previously, are small and very rapid (to 1 kHz). The SOL exhibits high density (≥1×1019 m−3), which shows little or no falloff with radius. Also the power deposition at the divertor surface is very broad, up to four times the width usually seen. This regime is of particular interest for the development of steady-state tokamak operating scenarios, for the Tokamak Physics Experiment (TPX), and following reactors.
Amplitude equations for electrostatic waves: Universal singular behavior in the limit of weak instability
Authors: Crawford John David
Published: July 2002

An amplitude equation for an unstable mode in a collisionless plasma is derived from the dynamics on the unstable manifold of the equilibrium F0(v). The mode eigenvalue arises from a simple zero of the dielectric εk(z); as the linear growth rate γ vanishes, the eigenvalue merges with the continuous spectrum on the imaginary axis and disappears. The evolution of the mode amplitude ρ(t) is studied using an expansion in ρ. As γ→0+, the expansion coefficients diverge, but these singularities are absorbed by rescaling the amplitude: ρ(t)≡γ2r(γt). This renders the theory finite and also indicates that the electric field exhibits trapping scaling E∼γ2. These singularities and scalings are independent of the specific F0(v) considered. The asymptotic dynamics of r(τ) can depend on F0 only through exp iξ where dεk/dz=‖εk′‖exp−iξ/2. Similar results also hold for the electric field and distribution function.
Transport suppression by diamagnetic phase shift as a possible mechanism of the low to high mode transition
Authors: Isichenko M. B. , Diamond P. H.
Published: July 2002

In the presence of a pressure gradient, the phase velocity of drift wave or ion temperature gradient (ITG) mode turbulence is different from the average poloidal E×B rotation velocity of the guiding centers. This results in an Ẽ×B turbulent particle diffusion being suppressed by the phase shift due to the diamagnetic rotation velocity uθ=(c/enB)dP/dr. This shift cannot be eliminated by a change of frame and affects the plasma transport. For uθ well above the turbulent Ẽ×B velocity, the radial diffusivity is suppressed as D∝u−3θ. This results in a nonmonotonous particle flux Γ(∇n)≂−D0∇n[1+α(∇n)2]−3/2 such that the transport can develop a bifurcation at a realistic density gradient.
Analysis of weakly nonlinear three-dimensional Rayleigh–Taylor instability growth
Authors: Dunning M. J. , Haan Steven W.
Published: July 2002

Understanding the Rayleigh–Taylor instability, which develops at an interface where a low density fluid pushes and accelerates a higher density fluid, is important to the design, analysis, and ultimate performance of inertial confinement fusion targets. Existing experimental results measuring the growth of two-dimensional (2-D) perturbations (perturbations translationally invariant in one transverse direction) are adequately modeled using the 2-D hydrodynamic code LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 11, 51 (1975)]. However, of ultimate interest is the growth of three-dimensional (3-D) perturbations such as those initiated by surface imperfections or illumination nonuniformities. Direct simulation of such 3-D experiments with all the significant physical processes included and with sufficient resolution is very difficult. This paper addresses how such experiments might be modeled. A model is considered that couples 2-D linear regime hydrodynamic code results with an analytic model to allow modeling of 3-D Rayleigh–Taylor growth through the linear regime and into the weakly nonlinear regime. The model is evaluated in 2-D by comparison with LASNEX results. Finally the model is applied to estimate the dynamics of a hypothetical 3-D foil.