bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical characterization of the dynamics of vortex filaments in round jets
Authors: Abid M. , Brachet M. E.
Published: July 2002

The dynamics of streamwise vorticity in axisymmetric jets is studied by direct numerical integration of the Navier–Stokes equations coupled with a passive scalar. Consistently with recent experiments and inviscid numerical simulations, the present viscous simulations show the appearance of pairs of axially counter-rotating vortex filaments. After their formation the filaments move away from the jet, dragging the tracer into finger-shaped structures. The three-dimensional topology of the rings, filaments, and fingers is described, well beyond the time of filament formation. Finally, visualizations of the pressure gradient field are presented, which suggest that the filaments can be directly observed experimentally by seeding the flow with microbubbles.
Shear flow instabilities and Rossby waves in barotropic flow in a rotating annulus
Authors: Solomon T. H. , Holloway W. J. , Swinney Harry L.
Published: July 2002

The primary instability of an azimuthal jet is studied experimentally in a rotating annulus with a rigid upper lid (infinite Rossby deformation radius) and a sloping bottom (topographical beta effect). An azimuthal jet is produced by the action of the Coriolis force on fluid pumped between concentric rings of sources and sinks in the bottom of the annulus. The flow is essentially two dimensional by the Taylor–Proudman theorem. Velocity measurements are made with hot-film probes and particle streak photography. For small forcing flux F, the jet is axisymmetric and has a 1/r velocity profile bounded by narrow free shear layers on each side. At a critical value of F, the inner shear layer becomes unstable to the formation of a propagating chain of vortices; at a larger value of F the outer shear layer also becomes unstable. The critical values of the mode numbers, wave speeds, and F at different annulus rotation rates are in good accord with a linear stability analysis by Lee and Marcus. At onset of instability the vortex chains are similar to those formed by a Kelvin–Helmholtz instability of a free shear layer, but this flow also has some properties typical of a Rossby wave flow—e.g., corotating jets have vortex chains with smaller wave speeds than counter-rotating jets. This asymmetry is enhanced as F is increased beyond instability onset, since growth in the size of the vortices results in a decrease of Rossby number. At sufficiently large F the vortex chains lock, resulting in a broad wavy jet with the same number of vortices on both sides. For corotating jets these states have significantly smaller wave speeds than the unlocked state, and this small speed is in accord with that predicted for a sinuous Rossby wave. However, the counter-rotating jets do not satisfy this relation but rather continue to have properties similar to Kelvin–Helmholtz waves.
Statistics of decaying Burgers turbulence
Authors: Gotoh Toshiyuki , Kraichnan Robert H.
Published: July 2002

Freely decaying Burgers turbulence at low and moderate Reynolds number ℛ is studied by mapping closure and by numerical simulations. Good agreement is found for the single-point probability distribution function (pdf) of velocity gradient ξ, for wave number spectra, and for various integral statistical quantities. The pdf of ξ exhibits a nearly exponential tail at large negative ξ, and a cusp with sharp cutoff at a positive value of ξ. The pdf of fluid density has a power-law tail, so that high-order moments of density do not exist. Asymptotic analysis of the mapping closure as ℛ→∞ yields an inertial-range wave number spectrum of kinetic energy with the form E(k)∝k−2. The pdf cusp becomes infinite as ℛ→∞ and gives rise to a flatness <ξ4≳/<ξ2≳2∝ℛ in the limit.
Bootstrap current control in stellarators
Authors: Shaing K. C. , Carreras B. A. , Dominguez N. , Lynch V. E. , Tolliver J. S.
Published: July 2002

The poloidal coil system of a stellarator can be used to modify the ‖B‖ spectrum and, as a consequence, change the magnitude of the bootstrap current. The addition of a small l=1 field component to a stellarator field can cancel or reverse the direction of the bootstrap current. It is shown that currentless operation in the collisionless regime is not impaired by bootstrap current, because the effects of bootstrap current can be eliminated by an appropriate external coil system. The advanced toroidal facility (ATF) [Fusion Technol. 10, 179 (1986)] can be used to test some of these results.
Efficiency optimization for free-electron laser oscillators
Authors: Serbeto A. , Levush B. , Antonsen T. M.
Published: July 2002

Using a normalized set of nonlinear equations, which describe a free-electron laser (FEL) oscillator, the efficiency of energy extraction from the electron beam to the radiation can be optimized. The optimum values of this efficiency are presented for (a) an untapered and (b) a tapered wiggler FEL oscillator.
Rayleigh–Taylor instability on the pusher–fuel contact surface of stagnating targets
Authors: Sakagami H. , Nishihara K.
Published: July 2002

Nonlinear evolution of the Rayleigh–Taylor instability on the pusher–fuel contact surface in the deceleration phase of implosion is investigated in cylindrical geometry using a two-dimensional fluid code, impact-2d [J. Comput. Phys. 49, 357 (1983)]. The linear growth rates obtained by simulations agree quite well with analytical values that include the cylindrical geometry effects. The saturation amplitudes of the exponential growth are found to be of the order of half the wavelength. The free-falling phase following the saturation is studied in detail. The perturbation amplitudes at the maximum compression are estimated as a function of an initial amplitude and its mode number. The reduction of the growth rate caused by electron thermal conduction is also investigated and a large reduction is observed for wavelengths shorter than ten times the electron mean-free path.
Reduction of 3ω/2 emission from laser-produced plasmas with broad bandwidth, induced spatial incoherence at 0.53 μm
Authors: Peyser T. A. , Manka C. K. , Obenschain S. P. , Kearney K. J.
Published: July 2002

Measurements of the 3ω0/2 emission from laser-irradiated targets at 0.53 μm were made at three angles over a wide range of laser bandwidths with and without induced spatial incoherence (ISI) echelons. The 3ω0/2 emission was found to be correlated with hard x rays but not Raman spectra, suggesting that the 3ω0/2 radiation was due to two-plasmon decay. Reduction of both 3ω0/2 emission and the accompanying hard x rays by ISI required five to ten times larger bandwidths than needed to suppress stimulated Raman scattering and stimulated Brillouin scattering.
Theory and simulation of a high-frequency magnetic drift wave
Authors: Huba J. D.
Published: July 2002

A theoretical and computational study of a high-frequency electromagnetic drift mode are presented. A fluid theory based upon the resistive magnetohydrodynamic (MHD) equations, which include the Hall term, as well as a kinetic theory based upon the Vlasov equation, are developed. The fluid theory is valid in the regime ω<ωlh (where ωlh is the lower hybrid frequency), and yields an approximate dispersion equation ω≂kyV2A/LnΩi −iηk2y, where ky is the wave number transverse to B0 and the density gradient, VA is the Alfvén velocity, Ln is the density gradient scale length, and η is the resistivity. This dispersion equation is valid in the limit Ln≪VA/Ωi. Using kinetic theory it is shown that the drift mode transitions to the lower hybrid mode in the limit ω≳ωlh. The simulation study is based upon the modified MHD equations and the nonlocal nature of the mode is investigated. Applications to sub-Alfvénic plasma expansions, electromagnetic waves in the Earth’s magnetosphere, and plasma switches are discussed.
Velocity ratio measurement using the frequency of the gyro backward wave
Authors: Muggli P. , Tran M. Q. , Tran T. M.
Published: July 2002

The operating diagram of a low quality factor, 8 GHz TE°01 gyrotron exhibits oscillations between 6.8 and 7.3 GHz. These oscillations are identified as the backward wave component of the TE°21 traveling mode. As the resonance condition of this mode depends on the average parallel velocity 〈v∥〉 of the beam electrons (ωBW≂Ωc/γ−k∥〈v∥〉), the measurement of ωBW for given Ωc and γ is used as a diagnostic for the beam electrons velocity ratio α=〈v⊥〉/〈v∥〉. The values of α, deduced from ωBW through the linear dispersion relation for the electron cyclotron instability in an infinite waveguide, are unrealistic. A nonlinear simulation code gives α values that are in very good agreement with the ones predicted by a particle trajectory code (+10% to +20%). It is found numerically that the particles’ velocity dispersion in v⊥ and v∥ increases ωBW. This effect explains part of the discrepancy between the values of α inferred from ωBW without velocity dispersion and the expected values.
Space and time behavior of parametric instabilities for a finite pump wave duration in a bounded plasma
Authors: Mounaix Philippe , Pesme Denis , Rozmus Wojciech , Casanova Michel
Published: July 2002

The space and time behavior of the decay waves is computed analytically in the regime of standard parametric decay. The plasma is assumed to be homogeneous and bounded. The pump wave has a finite pulse duration. The propagation of the pump wave is taken into account, its depletion is ignored. The parametric growth is solved in terms of fluctuating initial and boundary conditions corresponding to thermal noise at equilibrium. Fluctuating source terms, representing noise emission, are accordingly retained in the coupled mode equations. The initial stage of parametric growth is investigated in detail; the time from which the asymptotic concept of absolute or convective instability applies is computed. The connection between the Manley–Rowe and flux conservation relations is discussed.
Instability of a gas jet in liquid
Authors: Cheng Lap. Y.
Published: July 2002

The linear stability of a jet submerged in a fluid of different density is analyzed, as a first step in studying the formation of bubbles caused by the injection of air into water. Numerical results for both the temporal and spatial growth rates for the axisymmetric mode have been obtained for different Weber numbers. For a jet submerged in the same fluid, correspondence between the temporal and spatial growth rates has been found for large Strouhal numbers. However, such a correspondence for an air jet in water, for which there is a drastic difference in fluid density, could not be found.
Fast ions and hot electrons in the laser–plasma interaction
Authors: Gitomer S. J. , Jones R. D. , Begay F. , Ehler A. W. , Kephart J. F. , Kristal R.
Published: July 2002

Data on the emission of energetic ions produced in laser–matter interactions have been analyzed for a wide variety of laser wavelengths, energies, and pulse lengths. Strong correlation has been found between the bulk energy per AMU for fast ions measured by charge cups and the x-ray-determined hot electron temperature. Five theoretical models have been used to explain this correlation. The models include (1) a steady-state spherically symmetric fluid model with classical electron heat conduction, (2) a steady-state spherically symmetric fluid model with flux limited electron heat conduction, (3) a simple analytic model of an isothermal rarefaction followed by a free expansion, (4) the lasnex hydrodynamics code [Comments Plasma Phys. Controlled Fusion 2, 85 (1975)], calculations employing a spherical expansion and simple initial conditions, and (5) the lasnex code with its full array of absorption, transport, and emission physics. The results obtained with these models are in good agreement with the experiments and indicate that the detailed shape of the correlation curve between mean fast ion energy and hot electron temperature is due to target surface impurities at the higher temperatures (higher laser intensities) and to the expansion of bulk target material at the lower temperatures (lower laser intensities).
Three-dimensional modulation of electron-cyclotron waves
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

A set of nonlinear mode coupling equations, describing the interaction of large amplitude electron-cyclotron waves with low-frequency electrostatic as well as electromagnetic fluctuations, is presented. Accounting for the contributions of the ponderomotive force and relativistic electron mass variation nonlinearities, a general dispersion relation has been obtained for the parametric processes. Modulation and filamentation instabilities are investigated.
Ablative stabilization in the incompressible Rayleigh–Taylor instability
Authors: Kull H. J. , Anisimov S. I.
Published: July 2002

Ablative Rayleigh–Taylor instabilities are analyzed in terms of an incompressible fluid model. A generalized surface wave dispersion relation can be derived for arbitrary step-profiles with ablative mass and heat flow. A systematic overview on different regimes of ablative stabilization and growth reduction is given. Convective stabilization by the incoming and outcoming flows are found included as upper and lower limits on the instability growth. Applications for self-consistent steady-state conditions are discussed and a comparison is made with previous numerical work.
Measurements of magnetic fields using the Zeeman effect in laser-produced plasmas
Authors: Briand J. , Kieffer J. C. , Gomes A. , Arnas C. , Dinguirard J. P. , Quemener Y. , Berge L. , El Tamer M. , Armengaud M.
Published: July 2002

In this article, experimental results on magnetic fields obtained from measurements of the Zeeman effect in laser-produced plasmas are presented. Observations of the coronal plasma produced by interaction between a carbon plane target and a 0.25 μm laser beam, show for the first time at such short wavelengths the existence of toroidal and axial fields with amplitudes reaching, respectively, 0.35 and 0.5 MG.
Collective focusing of an intense pulsed ion beam
Authors: Kraft Robert , Kusse Bruce , Moschella John
Published: July 2002

Under the assumptions of quasineutrality and small perturbation to the applied solenoidal magnetic field, collective focusing of a neutralized, intense pulsed ion beam injected into a solenoidal magnetic field region is demonstrated. The focal point was observed to occur within the solenoidal magnetic field and was at a location in between the single-particle electron and ion focal points. The focused beam current density was an order of magnitude above the 20 A/cm2 of the incident beam. The experimental results were found to be in good agreement with a theoretical model developed under the above assumptions. The presence of background plasma or a conducting liner was observed to remove the collective focusing, confirming the physical picture of the theoretical model.
Three-dimensional centrifugal-type instabilities in inviscid two-dimensional flows
Authors: Bayly B. J.
Published: July 2002

In this paper the classical Rayleigh centrifugal instability theory is extended to general inviscid two-dimensional flows. Sufficient conditions for centrifugal instability are that the streamlines be convex closed curves in some region of the flow, with the magnitude of the circulation decreasing outward. If these conditions are satisfied, a class of three-dimensional short-wave instabilities can be constructed, which are localized near the streamline on which the exponent of a certain matrix Floquet problem is maximized.
Inertial organization of a two-dimensional turbulent vortex street
Authors: Thess A. , Sommeria J. , Jüttner B.
Published: July 2002

The formation of organized structures resulting from the evolution of two parallel opposite vortex sheets in a two-dimensional perfect fluid is studied, employing an equilibrium statistical theory. The system is confined in a channel in which periodic solutions of the statistical equilibrium equations are considered. It is found that the statistical equilibrium state of maximum entropy is a staggered arrangement of alternating vortex patches—the turbulent counterpart to the von-Kármán vortex street. However, these vortices spread over the whole width of the channel and the pattern with the maximum allowed wavelength is always preferred: the confinement by the boundary conditions is essential. The theoretical predictions are supported by direct numerical simulations of the two-dimensional Navier–Stokes equation at high Reynolds number.
The spacing of streaks in unsteady turbulent wall-bounded flow
Authors: Brereton G. J. , Hwang J.-L.
Published: July 2002

The spacing of streaks of low-speed fluid has been studied experimentally in a wall-bounded turbulent flow in which sinusoidal unsteadiness was superposed on an otherwise steady mainstream over a range of frequencies. The modulation of phase-conditioned streak spacing about its mean value does not follow the steady wall-flow relation of λuτ/ν equal to a constant. Instead, it is quite accurately described by a local length scale that models the momentary value of the total shear distortion of large eddies of the flow. This single shear-distortion length scale also correlates well with the streak spacing measured in steady wall-bounded flows and in unbounded homogeneous turbulent flow at high shear rates. The apparent generality of these results implies that the streak-spacing selection mechanism depends strongly on the strain history of large-scale coherent motions and so should be investigated in the context of the coupled straining processes of turbulence production.
Reduction in the dimensionality of turbulence due to a strong rotation
Authors: Hossain Murshed
Published: July 2002

Three-dimensional incompressible hydrodynamic turbulence is driven to a statistically steady state. A strong uniform rotation is then turned on. It is shown that the turbulence reduces to an approximate two-dimensional state. Furthermore, the energy inverse cascades to longer length scales.
Laminar boundary layer over riblets
Authors: Djenidi L. , Anselmet F. , Liandrat J. , Fulachier L.
Published: July 2002

Laser doppler anemometry (LDA) measurements and numerical calculations have been made for a laminar boundary layer on triangular riblets. Calculated mean velocity distributions along the riblet contour are in good agreement with the measured ones. The results show that no transversal motion exists above and within the riblet valleys (e.g., no secondary motion). It is found that despite the large wetted area increase, the frictional drag is not increased on riblets relative to a smooth wall. This result suggests that the viscous effects are at play in the drag reduction for a turbulent boundary layer, in the sense that they compensate for the increase in wetted area.
Lagrangian self-diffusion of Brownian particles in periodic flow fields
Authors: Mauri Roberto
Published: July 2002

The steady transport of Brownian particles convected by a periodic flow field is studied by following the motion of a randomly chosen tagged particle in an otherwise uniform solute concentration field. A nonlocal, Fickian constitutive relation is derived, in which the steady mass flux of Brownian particles equals a convolution integral of the concentration gradient times a (tensorial) diffusion function DL(R). In turn, the diffusion function is uniquely determined via the nth diffusivities, which are determined analytically in terms of the nth cumulants of the probability distribution by exploiting the translational symmetry of the velocity field. The Lagrangian, long-time self-diffusion function DL(R) is shown to be equal to the symmetric part of the Eulerian, gradient diffusion function DE(R). Since the latter characterizes the dissipative steady-state mass transport, while DL(R) describes the fluctuations of the concentration field about its uniform equilibrium value, the equality between DE(R) and DL(R) can be seen as an aspect of the fluctuation–dissipation theorem. Finally, the present results are applied to study the transport of solute particles immersed in a fluid flowing in rectilinear pipes and through periodic fixed beds of spheres at low Péclet number. In the first case, the first six nth diffusivities are determined; in the second, the first two diffusivities are calculated, showing that the enhancement to the second diffusivity due to convection is eight times larger in the direction parallel to the fluid flow than in the transversal direction.
The influence of riblets on a boundary layer with embedded streamwise vortices
Authors: Grek G. R. , Kozlov V. V. , Titarenko S. V. , Klingmann B. G. B.
Published: July 2002

The present study shows that surface manipulation makes it possible to suppress longitudinal vortex structures in a boundary layer, and thereby stabilize it with respect to high-frequency traveling waves and delay the transition to turbulence. The results should be of interest for the control of transition in cross-flow and Görtler flow.
Extensional flow of semidilute suspensions of rod-like particles through an orifice
Authors: Mongruel A. , Cloitre M.
Published: July 2002

This paper deals with the extensional flow of semidilute suspensions of non-Brownian rod-like particles through an orifice. The experimental setup allows simultaneously visualizations of the flow pattern and measurements of the flow rate-pressure drop relation. The flow pattern in the converging region is characterized by the appearance of a vortex ring which grows with the volume fraction of the particles and the aspect ratio. At the same time, the pressure drop experienced by the suspension as it flows through the orifice increases. The experimental results are interpreted in terms of a competition between the shear stress due to the solvent and the extensional stress generated by the particles. This interpretation is supported by the scaling of the vortex size and of the pressure drop with the volume fraction and the aspect ratio of the particles.
A dynamical model for turbulence. V. The effect of rotation
Authors: Canuto V. M. , Dubovikov M. S.
Published: July 2002

We study turbulence undergoing rapid rotation. We show that there exist two different regimes divided by the new number N=K/νΩ. N<1: rotation is so strong that it suppresses the energy cascade process and no inertial regime can develop. In the freely decaying case, only viscosity operates: an initially isotropic three-dimensional (3-D) turbulence remains so and never tends toward a 2-D state. N>1: the energy cascade, though inhibited, allows an inertial regime in which 3-D and a quasi-2-D state exist in equilibrium. The latter is restricted to a narrow band of values δkz∼kΩ−1k2, where kΩ=(Ω3ε−1)1/2. For k>kΩ, the Kolmogorov spectrum sets in, while for k<kΩ, the spectrum exhibits a new form, E(k)∼(εΩ)1/2k−2. A freely decaying turbulence within a time scale τΩ∼(Ωε−1)1/2L tends to a 2-D–3-D regime: K∼t−n while horizontal length scales grow as tm, where (m,n) are half the values of the Ω=0 case. As for the vertical length scales, one grows as the horizontal scales, while the other grows much faster, demonstrating the existence of the 2-D mode. The nonlinear interactions, though weakened, are the main cause of two-dimensionalization, which, however, is not of the Proudman–Taylor type since the latter requires negligible nonlinearities. We also derive the dynamic equation for the dissipation rate ε. All the results are shown to be in agreement with numerical simulation and experimental data.
Local similarity solutions for the stress field of an Oldroyd-B fluid in the partial-slip/slip flow
Authors: Salamon Todd R. , Bornside David E. , Armstrong Robert C. , Brown Robert A.
Published: July 2002

Local similarity solutions are presented for the stress field of a fluid described by the Oldroyd-B viscoelastic constitutive equation near the singularity caused by the intersection of a planar free surface and a solid surface along which Navier’s slip law holds, the partial-slip/slip problem. For the case where the velocity field is given by Newtonian kinematics, the elastic stress field is predicted to have a logarithmic singularity as the point of attachment of the free surface is approached. Asymptotic analysis for the fully-coupled flow, where the stress and flow fields are determined simultaneously, results in a local form for the flow and elastic stress fields that is similar in form to that for the decoupled case. For both the coupled and decoupled flow problems, the strength of the singularity depends on the dimensionless solvent viscosity and the slip coefficient, but not upon the Deborah number. The asymptotic results for the coupled flow differ from the predictions with Newtonian kinematics in that the strength of the singularity in the rate-of-strain and elastic stress fields scales with the inverse of the dimensionless solvent viscosity, and suggest that calculations with decreasing solvent viscosity become increasingly difficult. The fully-coupled analysis also suggests that the asymptotic behavior in the limit of vanishing solvent viscosity, the UCM limit, is qualitatively different from that for finite values of the solvent viscosity. The structure of the flow and stress fields for both the coupled and decoupled flow problems is reproduced by finite element calculations.
On the Rayleigh number dependence of convection with a strongly temperature-dependent viscosity
Authors: Trompert R. A. , Hansen U.
Published: July 2002

The strong dependence of the rheology of a fluid on temperature has a great impact on the style of thermally driven convection. When the viscosity contrast is sufficiently large, the viscosity of the coldest fluid at the top of a bottom-heated box is so high that this fluid layer becomes very stiff and a so-called cold “stagnant lid” develops on top of a hot convecting layer. Studying this style of convection is relevant for planetary mantles since the rheology of mantle material is likely to be very strongly temperature dependent. In this paper, the Rayleigh number dependence of stagnant-lid convection with a viscosity contrast of 106 is studied numerically in two and three dimensions in wide Cartesian domains. Like in constant-viscosity cases, the convection in the layer underneath the stagnant lid undergoes the typical transition from steady to time-dependent with the thinning of plumes and with the appearance of boundary layer instabilities as the Rayleigh number increases. A stagnant-lid style of convection was obtained in 2D and 3D for all supercritical Rayleigh numbers considered and the interior temperature appeared not to depend on the Rayleigh number. We have compared our results with other theoretical and numerical results and we found a close agreement.
Dipolar vortices in a strain flow
Authors: Trieling R. R. , van Wesenbeeck J. M. A. , van Heijst G. J. F.
Published: July 2002

The evolution characteristics of dipolar vortices in a strain flow were investigated both experimentally and theoretically. The laboratory experiments were performed in a stratified fluid, the strain flow being generated by four rotating horizontal discs, whereas the dipolar vortex was created by a pulsed injection of a small amount of fluid. Dye-visualization studies and particle-tracking techniques were used to obtain qualitative and quantitative information about the horizontal flow field. Depending on the initial orientation of the dipole, either a head–tail structure or a pair of elliptic-like monopolar vortices was formed. In the former case, the distance between the vortex centers was observed to remain nearly constant due to the opposing effects of strain and lateral diffusion, while in the latter case, the vortex centers were passively advected by the ambient flow. The head–tail formation could be explained kinematically by a simple point-vortex model. Full-numerical simulations based on the quasi-two-dimensional vorticity equation revealed a very good agreement with the laboratory observations.
Assessment of direct simulation Monte Carlo phenomenological rotational relaxation models
Authors: Wysong Ingrid J. , Wadsworth Dean C.
Published: July 2002

We address direct simulation Monte Carlo (DSMC) implementation of phenomenological models of the rotational relaxation process suitable for an arbitrary gas mixture composed of atomic and quantized diatomic species. The macroscopic relaxation process is parametrized by a constant or temperature-dependent collision number Zr such as that of Parker [Phys. Fluids 2, 449 (1959)]. The energy redistribution properties predicted by such a model at the collision level are compared with a recent quasiclassical state-to-state model. Modified forms of the constant collision number, and thus constant relaxation probability, serial quantized Borgnakke–Larsen algorithm [Phys. Fluids A 5, 2278 (1993)] and the null collision SICS-D algorithm [Phys. Fluids A 4, 1782 (1992)] are shown to be equivalent. The generalization to an energy-dependent relaxation probability [Phys. Fluids 6, 4042 (1994)] leads to a systematic bias toward delayed relaxation, due to approximations inherent in the analytical formulation. The error induced in the predicted relaxation behavior as a function of temperature is approximately equivalent in magnitude to a previously proposed, but unrelated, correction factor [Phys. Fluids 6, 2191 (1994)], and also to the variation in the temperature-dependent Parker collision number over a wide range of conditions. Comparisons between DSMC and state-to-state calculations of the rotational distribution function in a relaxing bath quantify the microscopic limitations of the phenomenological model. Finally, a direct comparison of DSMC results with experimental shock layer measurements demonstrates that the energy-dependent relaxation model has a negligible advantage over the constant probability model when the collision number is chosen judiciously.
Length scale for bubble problem in Rayleigh–Taylor instability
Authors: Abarzhi S. I.
Published: July 2002

In some researches on the bubble problem in the Rayleigh–Taylor instability, the bubble radius is identified with one-half spatial period. We show that these quantities are distinct. The bubble radius is significantly dependent on flow parameters, and its measurement would determine the accuracy of theories and simulations.
Symmetric and nonsymmetric Holmboe instabilities in an inviscid flow
Authors: Haigh S. P. , Lawrence G. A.
Published: July 2002

Using linear stability analysis we studied the effect of displacing a thin density interface with respect to the center of the shear layer on the stability of an inviscid, stably stratified, parallel flow. When no interface displacement is present and the flow is unbounded, pure Holmboe instabilities exist at all bulk Richardson numbers and are the most unstable instabilities for values of the bulk Richardson number greater than 0.046. When the interface displacement is nonzero the two modes of a Holmboe instability split into a stronger and a weaker mode. As the height of the vertical domain decreases the roles of the two modes switch with the originally weaker mode becoming the stronger mode and vice versa. The importance of including the height of the vertical domain in the stability analysis was illustrated by comparing theoretical results with the field data of Yoshida et al. [Yoshida, Ohtani, Nishida, and Linden, in Physical Processes in Lakes and Oceans, edited by J. Imberger (American Geophysical Union, Washington, DC, 1998), pp. 389–400]. The assumption that the instabilities are initially two-dimensional is examined. When the flow is unbounded, both symmetric and nonsymmetric Holmboe instabilities are initially two-dimensional. When boundaries are included, the two-dimensional assumption is valid except when the total vertical domain is small in which case three-dimensional primary instabilities are possible.
Instabilities of steep short-crested surface waves in deep water
Authors: Kimmoun Olivier , Ioualalen Mansour , Kharif Christian
Published: July 2002

The stability of steep short-crested gravity waves in deep water is investigated, thanks to an improvement of the numerical procedure originally developed by Ioualalen and Kharif [J. Fluid. Mech. 262, 265 (1994)]. This study predicts their potential evolution: For very steep waves, it is shown that: (i) Near the two-dimensional standing wave limit, they are unstable to class I modulational perturbations; (ii) near the two-dimensional progressive wave limit, they are unstable to class II perturbations, yielding to “horseshoe-patterned” fields; and (iii) fully three-dimensional waves match the two limits continuously, i.e., class II surpasses class I for the steepest waves.
Vortex-dynamics model for entrainment in jets and plumes
Authors: Sreenivas K. R. , Prasad Ajay K.
Published: July 2002

Recent experimental results of Bhat and Narasimha (1996) have revealed a dramatic difference in the entrainment between jets and plumes subjected to off-source volumetric heating and their unheated counterparts. Experimental observations show that plumes entrain more rapidly than jets; the greater entrainment by the plume is typically attributed to the presence of buoyancy in the plume. In contrast, the addition of buoyancy away from the source by volumetric heating produces the opposite effect of reduced entrainment. Apart from buoyancy, other factors such as acceleration due to pressure gradients or other body forces can also affect the rate of entrainment. In this paper, we develop a model for entrainment to explain the mechanism by which buoyancy produces contrasting effects on entrainment in volumetrically heated flows in comparison to their unheated counterparts. The model highlights the role of density stratification in the process of vortex sheet roll-up in free shear flows. With this model, we are also able to explain the higher entrainment of the plume relative to the unheated jet. The model is further extended to explain entrainment behavior during acceleration due to an applied pressure gradient or other body forces.
Landau-fluid simulation of laser filamentation
Authors: Kaiser T. B. , Cohen B. I. , Berger R. L. , Lasinski B. F. , Langdon A. B. , Williams E. A.
Published: July 2002

The Landau-fluid model is a recently introduced fluid-moment closure scheme [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (1990)] that was designed to include kinetic dissipative effects like Landau damping in fluid calculations. The fluid-moment hierarchy is terminated by assuming linear relationships among the retained moments in Fourier-transform space, with coefficients determined by matching the plasma response to that obtained from a kinetic analysis. This paper generalizes the technique to the full range of ion and electron collisionality and applies it to a new fluid simulation code constructed to study laser filamentation in underdense plasmas [Berger et al., Phys. Fluids B 5, 2243 (1993)]. By matching the ion-acoustic complex frequency derived from the fluid model with that predicted by collisional, Fokker–Planck, and kinetic analyses, the specific heat ratio, thermal conductivity coefficient, and viscosity coefficient for ions and the thermal conductivity coefficient for electrons are determined as functions of the wave number k. For frequencies much less than the pump frequency this leads to a fourth-order polynomial dispersion relation whose spectrum includes damped ion-acoustic waves as well as filamentation modes whose stability depends on the pump strength. An analytic instability threshold condition on the laser intensity is derived from which the relative importance of ponderomotive and thermal drives can be assessed. Expressions for the linear susceptibilities in the presence of a finite-amplitude pump are also given, which might prove useful for understanding spectral linewidths for Thomson scattering.
Dynamics of low to high (‘‘L’’ to ‘‘H’’) confinement bifurcation: Poloidal flow and ion pressure gradient evolution
Authors: Carreras B. A. , Newman D. , Diamond P. H. , Liang Y.-M.
Published: July 2002

A self-consistent model of the low to high (‘‘L’’ to ‘‘H’’) transition is derived from coupled nonlinear envelope equations for the fluctuation level and radial electric field shear, Er′, as determined by ion pressure gradient, ∇Pi, and poloidal flow. These equations extend the phase transition model of the L to H bifurcation by including ∇Pi effects. In this model, the transition occurs when the turbulence drive is large enough to overcome the damping of the total E×B flow. Near the critical power for transition, poloidal flow shear dominates Er′, but at high power, ∇Pi gives the main contribution. The inclusion of ∇Pi also introduces a quenched fluctuation state that is accessible at high power and may be the experimentally observed H-mode state for P≫Pcrit. In this state, the radial electric field is determined only by ∇Pi because no fluctuation energy is available to produce a turbulent Reynolds stress.
Wave interaction in gyrotrons with off-axis electron beams
Authors: Nusinovich G. S. , Dumbrajs O. , Levush B.
Published: July 2002

The effect of displacement of an annular electron beam with respect to the resonator axis on gyrotron operation is studied. The coupling between co- and counter-rotating TEm,p waves caused by eccentricity is analyzed both in the linear and nonlinear regimes. Nonlinear regimes are studied first in the quasilinear approximation, when the dependence of the electron gain functions on the field amplitude is described by a polynomial approximation. This makes it possible to study analytically some nonlinear effects caused by eccentricity. Then a self-consistent set of equations is studied numerically in the case of the MIT gyrotron [Blank et al., IEEE Trans. Plasma Sci. PS-22, 883 (1994)] operating in the TE16,2 mode at the fundamental cyclotron harmonic, where such effects have been studied experimentally, and of a gyrotron operating in the TE1,2 mode at the second harmonic.
Subpicosecond KrF*-laser plasma interaction at intensities between 1014 and 1017 W/cm2
Authors: Teubner U. , Gibbon P. , Förster E. , Falliès F. , Audebert P. , Geindre J. P. , Gauthier J. C.
Published: July 2002

The interaction of high-intensity subpicosecond KrF*-laser pulses with aluminium plasmas is investigated at intensities between 1014 and 1017 W/cm2. Using a one-dimensional hydrocode, the laser energy absorption and time evolution of plasma parameters have been studied as a function of laser intensity, incidence angle, and polarization. Complementary particle-in-cell simulations have also been performed to check the collisionless absorption component carried by hot electrons and ions. These simulations are compared to previous experiments on laser pulse absorption and x-ray generation.
Structure of homogeneous nonhelical magnetohydrodynamic turbulence
Authors: Miller R. S. , Mashayek F. , Adumitroaie V. , Givi P.
Published: July 2002

Results are presented for three-dimensional direct numerical simulations of nonhelical magnetohydrodynamic (MHD) turbulence for both stationary isotropic and homogeneous shear flow configurations with zero mean induction and unity magnetic Prandtl number. Small scale dynamo action is observed in both flows, and stationary values for the ratio of magnetic to kinetic energy are shown to scale nearly linearly with the Taylor microscale Reynolds numbers above a critical value of Reλ≊30. The presence of the magnetic field has the effect of decreasing the kinetic energy of the flow, while simultaneously increasing the Taylor microscale Reynolds number due to enlargement of the hydrodynamic length scales. For shear flows, both the velocity and the magnetic fields become increasingly anisotropic with increasing initial magnetic field strength. The kinetic energy spectra show a relative increase in high wave-number energy in the presence of a magnetic field. The magnetic field is found to portray an intermittent behavior, with peak values of the flatness near the critical Reynolds number. The magnetic field of both flows is organized in the form of ‘‘flux tubes’’ and magnetic ‘‘sheets.’’ These regions of large magnetic field strength show a small correlation with moderate vorticity regions, while the electric current structures are correlated with large amplitude strain regions of the turbulence. Some of the characteristics of small scale MHD turbulence are explained via the ‘‘structural’’ description of turbulence.
The effects of the rotation in plasma
Authors: Arteha S. N.
Published: July 2002

Electric and magnetic self-fields can exist in the rotating plasma. A self-sustained rotation can be established in the plasma. The disturbed distribution function of rotating plasma is derived from the Vlasov equation. The propagation of waves in rotating plasma differs from that in the usual plasma. New terms for Landau damping appear. The local rotational behavior may become prevailing.
Spatial temporal theory of Raman forward scattering
Authors: Decker C. D. , Mori W. B. , Katsouleas T. , Hinkel D. E.
Published: July 2002

Raman forward scattering is examined in regimes relevant to short-pulse (<1ps) high-intensity (I≤1018 W/cm2) laser-plasma interactions. A set of coupled differential equations is derived which describes the spatial-temporal growth of forward and near forward Raman scattering for plane wave pumps. An exact solution for direct forward scattering is obtained for physically relevant initial/boundary conditions. Furthermore, the asymptotic behavior at near forward angles is compared to finite width analyses in which the paraxial ray approximation is made. Noise sources for Raman forward scattering are examined. The consequences of these results to present and near term experiments are discussed.
Theory of drift waves in the presence of parallel and perpendicular flow curvature. I. Slab model
Authors: Sen S. , Sen A.
Published: July 2002

It has recently [S. Sen and M. G. Rusbridge, Phys. Plasmas 2, 2705 (1995)] been shown that, contrary to the usual belief, parallel flow curvature (V∥″) can stabilize drift-like microinstabilities. Here the earlier work is extended to include the effect of the perpendicular flow curvature (V⊥″), which is known to have a stabilizing role on the drift-like microinstabilities. The full analytic stability analysis shows that the ratio of the stabilizing influences of the perpendicular to the parallel flow curvature scales as Ls/2Ln, where Ls and Ln are the magnetic shear and the density variation scale length, respectively. Thus, at the plasma edge (since Ls≫Ln) the perpendicular flow may play a crucial role in stabilizing microinstabilities and turbulence in the improved regimes of confinement [like the high (H) modes]. However, in the core confinement improvement [like the very high (VH) mode] both the parallel and the perpendicular flow curvatures are important, since Ls∼2Ln. Furthermore, as the confinement improvement in the core is usually related to the toroidal velocity and since V∥ coming from the toroidal flow is much more than V⊥ (V⊥∼εV∥, here ε is the inverse aspect ratio), this implies that it is the parallel component of the toroidal flow and not the perpendicular component, as is usually thought, which is responsible for the core confinement improvement.
Quasilinear theory of collisionless electron heating in radio frequency gas discharges
Authors: Aliev Yu. M. , Kaganovich I. D. , Schlüter H.
Published: July 2002

On the basis of quasilinear kinetic theory, the electron heating of rf discharges is treated for characteristic scale lengths of the heating field much shorter than the electron mean free path. The analysis considers plasmas bounded by walls providing specular reflection. The expressions for the coefficients of electron diffusion in energy space are obtained and analyzed for inductively and capacitively coupled plasmas. Accounting for the oscillatory spatial structure of the penetrating electric field in the regime of anomalous skin effect leads to a decrease of the diffusion coefficient for high, and an increase for low, energy electrons. It is shown that the presence of a second boundary in the case of collisionless plasma slabs results in a similar effect. The formation of energy distribution functions in various regimes of collisionless heating is discussed. The diffusion coefficients are presented taking into account ambipolar electric fields.
Kinetic effects in tokamak scrape-off layer plasmas
Authors: Batishchev O. V. , Krasheninnikov S. I. , Catto Peter J. , Batishcheva A. A. , Sigmar D. J. , Xu X. Q. , Byers J. A. , Rognlien T. D. , Cohen R. H. , Shoucri M. M. , Shkarofskii I. P.
Published: July 2002

The short mean-free path expansion used in fluid modeling of scrape-off layer plasmas is often violated for typical discharge parameters, especially by the superthermal particles, which carry most of the heat flux. Thus, the tail of the distribution function can strongly depart from Maxwellian due to nonlocal mean-free path effects, which can modify plasma transport, impurity radiation, and plasma–neutral gas interactions. These nonlocal effects become particularly pronounced for detached plasma conditions that are characterized by sharp gradients in the plasma parameters along the magnetic field. These problems are being addressed by developing one spatial dimension and two velocity variables, fully kinetic, collisional, and time-dependent particle-in-cell code, W1 [Contrib. Plasma Phys. 34, 436 (1994)], and its parallel-computer version, PW1 [Contrib. Plasma Phys. 34, 424 (1996)]. Comparisons are made with the Fokker–Planck code ALLA [Phys. Plasmas 3, 1634 (1996)] and with experimental results. Kinetic effects on probe measurement interpretation, impurity radiation, and parallel heat conductivity due to non-Maxwellian features in scrape-off layer plasmas are considered. Heat conductivity is compared with ad hoc heat flux limit models.
Two-dimensional effects in a tunnel ionized plasma
Authors: Parashar Jetendra , Pandey H. D. , Tripathi V. K.
Published: July 2002

An intense short pulse laser of finite spot size propagating through a gas produces plasma via tunnel ionization on a femtosecond time scale. The radial profile of plasma density is strongly peaked on the axis and has a defocusing property. As electron density grows with time, the trailing part of the laser pulse suffers stronger divergence than the leading front, causing severe temporal distortion of the pulse. A self-consistent paraxial ray theory of electron density evolution and defocusing of the laser reveals that a square (in time) laser pulse, after propagating one Rayleigh length, has an order of magnitude difference in the axial intensity at the front and the tail of the pulse.
The radial electric field dynamics in the neoclassical plasmas
Authors: Novakovskii S. V. , Liu C. S. , Sagdeev R. Z. , Rosenbluth M. N.
Published: July 2002

A numerical simulation and analytical theory of the radial electric field dynamics in low collisional tokamak plasmas are presented. An initial value code “ELECTRIC” has been developed to solve the ion drift kinetic equation with a full collisional operator in the Hirshman–Sigmar–Clarke form together with the Maxwell equations. Different scenarios of relaxation of the radial electric field toward the steady-state in response to sudden and adiabatic changes of the equilibrium temperature gradient are presented. It is shown, that while the relaxation is usually accompanied by the geodesic acoustic oscillations, during the adiabatic change these oscillations are suppressed and only the magnetic pumping remains. Both the collisional damping and the Landau resonance interaction are shown to be important relaxation mechanisms. Scalings of the relaxation rates versus basic plasma parameters are presented.