bluuebunny/crossref_metadata_2025 huggingface dataset:
A unified theory of tokamak transport via the generalized Balescu–Lenard collision operator
Authors: Mynick H. E. , Duvall R. E.
Published: July 2002

A unified basis from which to study the transport of tokamaks at low collisionality is provided by specializing the ‘‘generalized Balescu–Lenard’’ collision operator to toroidal geometry. Explicitly evaluating this operator, ripple, turbulent, and neoclassical transport coefficients are obtained, simply by further specializing the single operator to different particular classes of fluctuation wavelength and mode structure. For each class of fluctuations, the operator possesses a diffusive, test-particle contribution D, and in addition a dynamic drag term F, which makes the operator self-consistent, and whose presence is accordingly essential for the resultant fluxes to possess the appropriate conservation laws and symmetries. These properties, well known for axisymmetric transport, are demonstrated for one type of turbulent transport, chosen for definiteness, by explicit evaluation of both the ‘‘anomalous diffusion’’ term arising from D, as well as the closely related ‘‘anomalous pinch’’ term coming from F. The latter term is neglected by test-particle calculations, but is shown to have an important impact on the predicted fluxes.
Electric field due to velocity space particle loss in field-reversed configurations
Authors: Hsiao Ming-Yuan , Staudenmeier Joseph L. , Chiang Pi-Ren
Published: July 2002

Particle confinement criteria for velocity space particle loss are used to calculate the radial and axial electric potential profiles in field-reversed configurations (FRC’s). Assuming Vlasov equilibrium distribution functions for both electrons and ions, the density profile and the potential profile are calculated self-consistently. With rest Maxwellian distribution functions for both species and the Hill’s vortex magnetic configuration, a radial electric field of about 0–70 V/cm and an axial electric field of about 0–4 V/cm, depending on positions, have been found for a typical set of experimental parameters: electron temperature Te=100 eV, ion temperature Ti=200 eV, magnetic field B0=5500 G, separatrix radius rs=12 cm, and elongation factor k=5. The corresponding density profile has behavior similar to profiles observed in the experiments. Cases using rigid-rotor distribution functions with different rigid rotating frequencies are also studied.
The dispersion relation for D(3He) ion-cyclotron resonance heating
Authors: Chow C. , Fuchs V. , Bers A.
Published: July 2002

The fourth-order dispersion relation for D(3He) obtained from a straightforward small k⊥ρD expansion does not properly reproduce the exact kinetic dispersion relation near ion–ion hybrid resonance. It is shown that, in general, whenever the majority is not resonant the expanded dispersion relation may break down. A method to obtain a correct approximate fourth-order dispersion relation has been developed.
Test particle acceleration by small amplitude electromagnetic waves in a uniform magnetic field
Authors: Ginet Gregory P. , Heinemann Michael A.
Published: July 2002

The behavior of a charged test particle in an electromagnetic wave propagating at an oblique angle with respect to a uniform magnetic field is investigated using analytical theory and direct numerical solutions of the equations of motion. From the Hamiltonian a fully relativistic pseudopotential is derived that predicts the maximum particle energy (Umax ) and associated long time scale (many cyclotron orbits) oscillation period (τp ) for motion in small amplitude waves with frequencies satisfying a cyclotron resonance condition, but quite arbitrary polarizations and indices of refraction. Considering only cold initial particles, substantial energy enhancement above the ‘‘quiver energy’’ is found for a broad range of initial phases only when the frequency is near the first or second harmonic of the cyclotron frequency. Furthermore, the largest particle energies occur when the index of refraction η and angle of propagation α are such that η cos α=1. In this regime Umax is independent of wave amplitude and there can be significant acceleration for wave frequencies above the second cyclotron harmonic, but only for very narrow ranges of initial phases. Analytical formulas that give Umax and τp as functions of wave amplitude, index of refraction, and angle of propagation are given and tested against numerical solutions.
Nonlocal electron heat flux revisited
Authors: Sanmartín J. R. , Ramírez J. , Fernández Feria R.
Published: July 2002

A known nonlocal model of electron heat flux, applying for (scale length/thermal ion–electron mean-free path) of order Z1/2i(ε*/T)3/2, ionization number Zi large, and ε*∼6.5 T (the energy of electrons carrying most of the flux), is reconsidered. The large ε*/T ratio simplifies the complete formalism. A simple flux formula, exact for both smooth and steep profiles, is given. Thermoelectric effects and other models are discussed.
Time-dependent phase conjugation in plasmas: Numerical results and interpretation
Authors: Goldman Martin V.
Published: July 2002

The salient features of time-dependent phase conjugation and four-wave mixing in a plasma are summarized and developed numerically in an extension of the work of Goldman and Williams [Phys. Fluids B 3, 751 (1991)]. It is shown that plasma nonuniformity can reduce the decay time of a conjugate wave responding to a delta-function signal without significant reduction in the maximum amplitude of the phase-conjugate wave. A phase-conjugating nonuniform plasma may therefore offer advantages over other media in the tracking or self-targeting of moving objects. The temporal approach to steady-state of an amplified phase-conjugate wave is found numerically for the case of resonant four-wave mixing driven by pumps of unequal intensity; the temporal response above threshold for absolute instability is also studied. Related absolute instabilities pumped by Langmuir waves are briefly described. Effects of nonuniformity are considered in connection with amplified phase conjugation.
Neoclassical poloidal and toroidal rotation in tokamaks
Authors: Kim Y. B. , Diamond P. H. , Groebner R. J.
Published: July 2002

Explicit expressions for the neoclassical poloidal and toroidal rotation speeds of primary ion and impurity species are derived via the Hirshman and Sigmar moment approach. The rotation speeds of the primary ion can be significantly different from those of impurities in various interesting cases. The rapid increase of impurity poloidal rotation in the edge region of H-mode discharges in tokamaks can be explained by a rapid steepening of the primary ion pressure gradient. Depending on ion collisionality, the poloidal rotation speed of the primary ions at the edge can be quite small and the flow direction may be opposite to that of the impurities. This may cast considerable doubts on current L to H bifurcation models based on primary ion poloidal rotation only. Also, the difference between the toroidal rotation velocities of primary ions and impurities is not negligible in various cases. In Ohmic plasmas, the parallel electric field induces a large impurity toroidal rotation close to the magnetic axis, which seems to agree with experimental observations. In the ion banana and plateau regime, there can be non-negligible disparities between primary ion and impurity toroidal rotation velocities due to the ion density and temperature gradients. Detailed analytic expressions for the primary ion and impurity rotation speeds are presented, and the methodology for generalization to the case of several impurity species is also presented for future numerical evaluation.
Scaling laws for the cyclotron resonance laser accelerator
Authors: Chen Chiping
Published: July 2002

The nonlinear interaction of a tenuous continuous electron beam with a traveling electromagnetic wave in the cyclotron resonance laser accelerator is analyzed using a single-particle model and multiparticle simulation. It is found that the maximum beam energy gain and acceleration distance obey certain scaling laws for optimized systems, that the maximum energy gain is limited primarily by the degree of wave dispersion, and is almost independent of the wave amplitude.
Poloidal beta scaling for a bootstrapped tokamak
Authors: Iacono R. , Bhattacharjee A.
Published: July 2002

A new scaling of the poloidal plasma beta with the bootstrap fraction is derived for a steady-state tokamak. In this derivation, the seed current profile is specified, but no assumptions are made regarding the functional form of the total current profile. The scaling, which holds for a wide class of density and temperature profiles, exhibits a quadratic dependence of the poloidal beta on the bootstrap fraction. The effects of the ionic charge and unequal electron and ion temperatures are considered.
Matter acceleration in laser-irradiated multifoil systems
Authors: Bolotin V. A. , Gavrilov V. V. , Gol’berg S. M. , Gol’tsov A. Yu. , Kondrashov V. N. , Kovalsky N. G. , Velikovich A. L. , Zavyalets S. V.
Published: July 2002

Experimental and theoretical studies of collisions of thin planar aluminum foils and Mylar films accelerated to high velocities by powerful laser radiation (λ=1.054 μm, I=(5–7)×1013 W/cm2, pulse duration 2 nsec) are reported. The ‘‘induced opacity’’ diagnostic technique developed in the present work in order to estimate compactness of accelerated plasmoids is described. The experimental data and numerical simulation results presented indicate feasibility of a ‘‘soft’’ low-entropy mechanism of matter acceleration in a condensed state to tens of kilometers per second by a high-velocity plasmoid impacting a thin-film target.
Inverse problem for bremsstrahlung radiation
Authors: Voss K. E. , Fisch N. J.
Published: July 2002

Information about the momentum distribution of electrons in a plasma is often of great importance, for example, when superthermal electrons are created by noninductive means. For such predominantly one-dimensional distribution functions, an analytic inversion has been found that yields the velocity distribution of superthermal electrons given their bremsstrahlung radiation.
Comment on ‘‘Modification of stimulated Brillouin, saturated Raman scattering and strong Langmuir turbulence by nonlocal heat transport’’ [Phys. Fluids B 4, 1394 (1992)]
Authors: Epperlein E. M. , Short R. W.
Published: July 2002

The purpose of this commont is to clarify the general applicability of nonlocal heat transport formulas, and especially their applicability to stimulated Brillouin scattering. (AIP)
Limitations of quasilinear transport theory
Authors: Balescu R.
Published: July 2002

The anomalous fluxes are evaluated in the simplest possible geometric situation: drift waves in a shearless slab geometry, in the presence of density and temperature gradients. It is shown that, within the strict quasilinear framework, the linear transport equations relating the fluxes to the thermodynamic forces have serious limitations. Such a linear relation does not even exist for the ion energy flux. For all the fluxes, the first ‘‘correction’’ has a singularity whose location depends on the relative value of the density gradient and of the ion temperature gradient: its existence seriously restricts the domain of validity of the quasilinear transport theory. The ‘‘semiempirical quasilinear’’ formulas used in the comparisons with experiments are also discussed.
Effects of secondary and thermionic electron emission on the collector and source sheaths of a finite ion temperature plasma using kinetic theory and numerical simulation
Authors: Schwager L. A.
Published: July 2002

The region between a Maxwellian plasma source and an absorbing surface that emits cool electrons is modeled numerically with dynamic, electrostatic particle simulation and theoretically with a static, kinetic plasma-sheath model. Steady-state emission results are applied specifically to secondary electrons that are induced by either incident ions or electrons, but are also valid for thermionic and photoelectrons. The ratio of the emitted electron current to incident electron current is varied up to and beyond the critical emission coefficient (ratio) that causes electric field reversal at the collector. Results from these models agree very well over the range from zero to five times the critical emission coefficient. Increasing the secondary emission coefficient is found to reduce the collector potential and decrease the ion energy deposited, yet increase the total energy flux to the collector. In the simulation, some heating of the secondary electron stream is observed to gradually evolve over many Debye lengths, possibly because of a beam–plasma interaction. This heating increases potential fluctuations but causes only small deviations from the predictions with static theory.
Brillouin-enhanced four-wave mixing and phase conjugation in a plasma*
Authors: Domier C. W. , Luhmann N. C.
Published: July 2002

Detailed experimental investigations of Brillouin-enhanced four-wave mixing (BEFWM) and phase conjugation of microwaves in an unmagnetized plasma have been performed. Through the use of plasma probes and energy analyzers together with a millimeter-wave interferometer/scattering system, both the transient and steady-state responses of the plasma and the phase conjugate wave have been investigated. Low-power, low-density operation is shown to agree well with two-fluid plasma theory. Strong suppression of the BEFWM interaction is observed at an electron density ne≊nc/4, indicative of the 2ωpe instability. High-power, high-density operation is seen to deviate from simple theoretical predictions; slight amounts of plasma pushed out of the interaction region by the ponderomotive force of the high-power pump waves are observed to disrupt the relative phases of the electromagnetic waves involved in the BEFWM interaction.
Effect of rigid rotation on the stability to external kink modes
Authors: Bhattacharyya S. N. , Sen A.
Published: July 2002

The effect of rigid toroidal rotation on the stability of a tokamak to external kink modes is examined. The effect of rigid rotation is modeled by a centrifugal force field. The problem is studied in the cylindrical limit for which this reduces to a uniform force field acting in a direction transverse to the plasma column. The effect of a transverse force field on the limit on the safety factor q for stability to external kink modes is studied. For values of q above this, limits on the plasma beta, denoted β, are obtained. A study of the equilibrium of a toroidal plasma undergoing rigid toroidal rotation yields another limit on β as a function of q. Combining the results of cylindrical stability with that of toroidal equilibrium provides approximate estimates of the maximum β that could be obtained. The effect of rigid rotation on this maximum β is also studied.
Exact solutions for helical magnetohydrodynamic equilibria
Authors: Villata Massimo , Tsinganos Kanaris
Published: July 2002

Three novel classes of exact solutions of the generalized Grad–Shafranov equation for helically symmetric magnetohydrodynamic (MHD) equilibria are presented. The first two classes may be applied to helical MHD equilibria for plasma confined between two coaxial cylinders, while the third one to the modeling of helicoidal magnetic fields and flows in several recently observed astrophysical jets. The same solutions can be also used for the testing of sophisticated numerical codes. It is also shown that all helically symmetric MHD equilibria can be treated by the same general method which is employed to generate exact MHD solutions for systems possessing an ignorable coordinate in a system of three orthogonal basis vectors, although in the case of helical symmetry an orthogonal ignorable coordinate does not exist, contrary to what happens in the well-known cases of axial and translational symmetries.
Thermal filamentation in plasmas with nonlocal thermal conductivity
Authors: Schmitt Andrew J.
Published: July 2002

Thermal filamentation and self-focusing of laser light has been shown to be significantly altered in typical inertial confinement fusion (ICF)-type laser plasmas because of heat flux inhibition associated with nonlocal electron thermal conduction. The conductivity becomes appreciably nonlocal when the temperature scale length is less than about 100 electron mean-free paths. In contrast to previous flux-limiter models, this mechanism is also important for small temperature perturbations such as those associated with thermal filamentation, where the heat flux can be orders of magnitude less than the free-streaming value. Using a three-dimensional time-dependent laser–plasma propagation code modified to include nonlocal electron thermal conductivity, the scaling and behavior of nonlocal thermal filamentation are investigated. Both unsmoothed and optically smoothed laser beams are studied. The simulation results are contrasted with earlier results generated with the classical Spitzer–Härm conductivity, and compared to a perturbation analysis. The filamentation length predicted by the perturbation analysis for unsmoothed and smoothed beams compares favorably to the simulations. This theory is used to predict the behavior of optically smoothed beams in large inhomogeneous plasmas that cannot be adequately resolved by simulation.
A consistency condition for Reynolds stress closures
Authors: Lele Sanjiva K.
Published: July 2002

A new consistency condition is derived for the Reynolds stress turbulent closures. Recommended values of model constants used with the k-ε model and with various Reynolds stress closures do not satisfy this condition exactly. It is shown that a slight adjustment of certain computer-optimized constants would make some of these models internally consistent.
The stability of buoyancy-driven rolls aligned with a shear flow when the temperature gradient is nonlinear
Authors: Jones A. D. W.
Published: July 2002

The buoyancy driven instability of rolls aligned with a shear flow of a Boussinesq fluid is considered. It is supposed that the temperature varies not only with height but also in the direction of the flow thus allowing the vertical temperature profile to be nonlinear. Conditions are derived for the neglect of the horizontal temperature gradient in the perturbation equations. These are (i) that the vertical temperature gradient varies sufficiently slowly in the direction of the flow and (ii) that Pr h/d≫1, where Pr is the Prandtl number and h and d are the length scales for the vertical variation of velocity and temperature, respectively. Under these conditions, stability is governed by the equation for Bénard convection. The principle of exchange of stabilities is proved for an arbitrary temperature gradient profile, with isothermal, free-surface boundary conditions, and the equation is solved numerically and by an asymptotic method for a model thermal boundary layer. The analysis is then applied to an experimental study of spoke patterns, observed in the growth of electronic materials. This work also indicates that a previous investigation of the stability of stratified shear flow has a restricted range of validity.
A four-field model for tokamak plasma dynamics
Authors: Hazeltine R. D. , Kotschenreuther M. , Morrison P. J.
Published: July 2002

A generalization of reduced magnetohydrodynamics is constructed from moments of the Fokker–Planck equation. The new model uses familiar aspect-ratio approximations but allows for (i) evolution as slow as the diamagnetic drift frequency, thereby including certain finite Larmor radius effects, (ii) pressure gradient terms in a generalized Ohm’s law, thus making accessible the adiabatic electron limit, and (iii) plasma compressibility, including the divergence of both parallel and perpendicular flows. The system is isothermal and surprisingly simple, involving only one additional field variable, i.e., four independent fields replace the three fields of reduced magnetohydrodynamics. It possesses a conserved energy. The model’s equilibrium limit is shown to reproduce not only the large-aspect-ratio Grad–Shafranov equation, but also such finite Larmor radius effects as the equilibrium ion parallel flow. Its linearized version reproduces, among other things, crucial physics of the long mean-free-path electron response. Nonlinearly, the four-field model is shown to describe diffusion in stochastic magnetic fields with good qualitative accuracy.
Kinetic description of the sideband instability in a helical-wiggler free-electron laser
Authors: Davidson Ronald C.
Published: July 2002

The Vlasov–Maxwell equations are used to investigate properties of the sideband instability for a tenuous, relativistic electron beam propagating in the z direction through a constant-amplitude helical-wiggler magnetic field Bw(x)=B̂w(cos k0 zêx+sin k0 zêy). The theoretical model neglects longitudinal perturbations (δφ≂0) and transverse spatial variations (∂/∂x=0 =∂/∂y). Moreover, the analysis is carried out for perturbations about an equilibrium BGK state in which the distribution of beam electrons Gs(γ′) and the wiggler magnetic field (normalized amplitude aw=eB̂w/mc2k0) coexist in quasisteady equilibrium with a finite-amplitude, circularly polarized, primary electromagnetic wave (ωs,ks) with normalized amplitude as=eB̂s/mc2ks. Transforming the linearized Vlasov–Maxwell equations to the ponderomotive frame moving with velocity vp=ωs/(ks+k0), a formal dispersion relation is derived in the diagonal approximation, and the trapped- and untrapped-electron susceptibilities are calculated in terms of the exact electron trajectories in the ponderomotive frame, including the full influence of the finite-amplitude primary electromagnetic wave (ωs,ks).

Detailed properties of the sideband instability are investigated for the fundamental (n=1) mode in circumstances where ω′2≊ω̂2B(γ′) and the distribution of trapped electrons GTs(γ′) is localized near the bottom of the ponderomotive potential. Here ω′ and k′ denote the frequency and wavenumber in the ponderomotive frame, ω̂B(γ′) =(awasc2k′2p/γ′2)1/2 is the bounce frequency near the bottom of the potential, and kp=(ks+k0)/γp, where γp=(1−v2p/c2)−1/2. The growth rate and oscillation frequency of the sideband instability are calculated for several choices of trapped-electron distribution function GTs(γ′), ranging from a uniform distribution, to a monoenergetic distribution, to a monotonic decreasing distribution. A striking feature of the stability analysis is that the detailed stability properties are relatively insensitive to the form of GTs(γ′), at least in circumstances where the trapped electrons are localized near the bottom of the ponderomotive potential.
Global vortex pattern in a rotating plasma
Authors: Nycander J. , Pavlenko V. P.
Published: July 2002

A set of nonlinear fluid equations that include the effect of finite ion Larmor radius is derived to describe the dynamics of flute modes in a rotating inhomogeneous plasma with a strong axial magnetic field. It is shown that these equations possess a solution in the form of a global pattern of two-dimensional vortices, different from the solitary vortex structure found earlier for the nonlinear Rossby waves.
Magnetic reconnection induced by perturbation on boundaries
Authors: Shivamoggi Bhimsen K.
Published: July 2002

The magnetic field reconnection is considered in a plasma induced by perturbing the boundaries of a slab of incompressible plasma with a magnetic neutral surface inside. It is assumed that the boundaries of the plasma slab are perturbed at both fast and slow rates compared with the hydromagnetic evolution rate, and the ensuing adjustments in the plasma and the magnetic field threading through it are investigated.
Ensemble-mean modeling of two-equation type in magnetohydrodynamic turbulent shear flows
Authors: Yoshizawa Akira
Published: July 2002

Ensemble-mean two-equation modeling is studied in magnetohydrodynamic turbulent shear flows. This model consists of a system of equations governing the mean velocity, the mean magnetic field, the total turbulent energy (the sum of turbulent kinetic and magnetic energy), and its dissipation rate. The turbulence effect in the Navier–Stokes equation is written in the form of an eddy-viscosity representation, whereas the counterpart in the mean magnetic induction equation is expressed in the combination of alpha dynamo terms and an eddy-magnetic diffusivity representation. The implication of the present model in reversed-field pinches of plasma in controlled nuclear fusion is also discussed.
Magnetic flux compression by dynamic plasmas. I. Subsonic self-similar compression of a magnetized plasma-filled liner
Authors: Felber F. S. , Liberman M. A. , Velikovich A. L.
Published: July 2002

New self-similar solutions describe the subsonic compression of a plasma with an entrained axial magnetic field by a thin cylindrical liner. Effects of Ohmic dissipation, thermal conductivity, and plasma turbulence are included. The self-similar solutions, obtained in an explicit analytic form, demonstrate that magnetic flux can be compressed by an externally driven annular plasma shell with small losses.
Slowing down tail enhanced, neoclassical and classical alpha particle fluxes in tokamak reactors
Authors: Catto Peter J. , Tessarotto Massimo
Published: July 2002

The classical and neoclassical particle and energy fluxes associated with a slowing down tail, alpha particle distribution function are evaluated for arbitrary aspect ratio ε−1, cross section, and poloidal magnetic field. The retention of both electron and ion drag and pitch angle scattering by the background ions results in a large diffusive neoclassical heat flux in the plasma core. This flux remains substantial at larger radii only if the characteristic speed associated with pitch angle scattering, vb, is close enough to the alpha birth speed v0 so that ε(v0/vb)3 remains less than some order unity critical value which is not determined by the methods herein. The enhanced neoclassical losses would only have a serious impact on ignition if the critical value of ε(v0/vb)3 is found to be somewhat larger than unity.
Note on the Kelvin–Helmholtz instability of stratified fluids
Authors: Triantafyllou George S.
Published: July 2002

In this paper the conditions under which the Kelvin–Helmholtz instability in stratified fluids is absolute or convective are investigated. It is first shown that at transition from convective to absolute instability two double roots of the dispersion relation coalesce. Based on this property an analytical condition for absolute instability is derived. It is found that the instability is absolute for almost all values of the flow velocity and stratification. Convective instability is found only for a narrow range of flow velocities over the instability threshold when the density ratio parameter, r=ρ1/(ρ1+ρ2), lies in the range 1/3<r<1/2. The different behavior near the instability threshold can be related to the signs of the group velocities of the two waves which coalesce to create the instability: For r<1/3, the group velocities of the two waves have opposite signs, and the resulting instability is absolute, whereas for 1/3<r<1/2, the two waves have group velocities with the same sign, and the instability is convective. This result is also shown to be reflected in the form of the amplitude equation at the instability threshold, which is the linearly unstable Klein–Gordon equation.
Absolute and convective instabilities in compressible confined mixing layers
Authors: Peroomian Oshin , Kelly R. E.
Published: July 2002

The local two-dimensional stability characteristics of a confined compressible mixing layer with a hyperbolic tangent velocity profile are studied on an inviscid basis. By examining the temporal growth rate at the saddle point in the wave number space, the flow is characterized as being either absolutely unstable or convectively unstable. Results are presented, showing the amount of backflow necessary to have this type of transition for a range of primary flow Mach number, M1, up to 3.0. The transition curve for M1 between zero and 1.0 and between 1.35 and 3.0 is defined by a single curve. However, for M1 between 1.0 and 1.35, multiple transition curves exist, due to the fact that multiple modes exist when M1 is greater than unity. In this region the mode that becomes absolutely unstable depends on the magnitude of the back flow.
The stability of a slightly eccentric straight vortex filament in a circular tube
Authors: Rusak Z. , Seginer A.
Published: July 2002

The three-dimensional long-wave linear stability of a straight vortex, that runs down a circular tube, parallel with its axis but slightly displaced from it, is analyzed. The vortex flow is modeled by an infinite concentrated, potential vortex filament. The filament is immersed in an axial uniform flow, and autorotates around the tube axis due to its small eccentricity. The tube walls are modeled by a surface distribution of potential sources. Results show that the vortex develops a dynamic instability in a narrow range of wave numbers as a direct result of its small eccentricity. The vortex also tends to diverge statically at certain wave numbers, only when it is in a ‘‘subcritical’’ state. Finally, it is shown that a time-dependent streamwise variation of the tube radius can be used as a controller to actively control the dynamic stability of an eccentric vortex in a tube.
The plane wake of a cylinder: An estimate of the pressure–strain rate tensor
Authors: Aronson D. , Löfdahl L.
Published: July 2002

The modeling of the pressure strain rate terms is an important issue in the improvement of the generality of closure models for the Reynolds stress transport (RST) equations. A part of these efforts is to provide accurate experimental information on the pressure–strain rate tensor, which in turn requires equally accurate information on the dissipation and the diffusion tensors. Here the far wake of a cylinder was studied in order to enable the required balances of the RST equations. The experimental results indicate a nonisotropic dissipation tensor, and show the energy redistribution between the different components as described by the pressure–strain rate correlations. Comparisons are made between the experimentally determined pressure–strain rate distributions and the corresponding distributions predicted by closure models.
Far-field condition of vortex methods on an impulsively started two-dimensional circular cylinder with rotation
Authors: Kida Teruhiko , Nagata Toshimi , Nakajima Tomoya
Published: July 2002

The far-field condition in the problem of the steady two-dimensional flow past a rotating circular cylinder is well known, but this condition is not clear in the problem of the two-dimensional unsteady flow past a rotating circular cylinder which is impulsively started. In the latter problem, the circulation around a contour of sufficient large radius surrounding the cylinder is dependent on initial conditions: (1) The circular cylinder is impulsively started with rotation from rest and (2) the rotating cylinder is impulsively started. The hybrid vortex method combined with a panel method is used for the flow of a slightly viscous, incompressible fluid around the rotating circular cylinder in the two initial conditions mentioned above. The numerical accuracy of this hybrid method is tested on the flow around an impulsively started cylinder without rotation. The detailed results of the flow around the rotating cylinder with a constant clockwise angular velocity show: (1) The initial shedding vortex is anticlockwise in the former initial case, but it is clockwise in the latter initial case. (2) The growth of the wake depends on the initial condition. (3) The time history of the aerodynamic force is dependent on the initial condition at the early stages of the development but it is almost independent of the initial condition after the early stages.
Application of the dynamic subgrid-scale model to axisymmetric transitional boundary layer at high speed
Authors: El-Hady Nabil M. , Zang Thomas A. , Piomelli Ugo
Published: July 2002

The subgrid scales are modeled dynamically in a large-eddy simulation of transitional boundary-layer flow along a cylinder at a Mach number of 4.5. The behavior of the dynamic-model coefficients, which is determined from local information in the resolved field, is investigated through both an a priori test with direct numerical simulation data for the same case and a complete large-eddy simulation. Both contractions proposed by Germano et al. and Lilly are used for the unique determination of the coefficients of the dynamic model, and their results are compared and assessed. The behavior, as well as the energy cascade of the subgrid-scale field structure, is investigated at various stages of the transition process.
The decay of modons due to Rossby wave radiation
Authors: Flierl G. R. , Haines K.
Published: July 2002

A weakly radiating theory is developed to describe the decay of a westward propagating dipole modon on a β plane in the case of small β. The barotropic vorticity equation is expanded about an ‘‘f-plane’’ (β=0) modon using the expansion parameter ε=βL2/U. The theory also assumes shape preservation of the modon. An expression is found for the energy loss from the dipole due to Rossby wave radiation. Two alternative constraints, one involving global enstrophy conservation and the other assuming conservation of the maximal vorticity values in the center of the dipole vortices, are used to derive expressions for the decay rate of the dipole. In both cases, the predicted decay is algebraic in time. These theories are tested in a numerical model for various values of c. It is found that enstrophy conservation is not a good constraint because fluid leaks from the dipole in filaments whose small scales ensure loss through numerical dissipation. The theory based on the conservation of peak vorticity works well for small ε=0.2, and gives reasonable results for much larger values, up to ε=1.0. The theory is used to estimate the decay time of atmospheric blocking due to this radiation phenomenon, and the implied effect is significant.
Spiral streaklines in pre-vortex breakdown regions of axisymmetric swirling flows
Authors: Hourigan K. , Graham L. J. W. , Thompson M. C.
Published: July 2002

In steady swirling flows in closed cylinders, it has been common to observe the transition to spirals of otherwise straight dye streaklines. This occurs in the regions where bubble type breakdown occurs but at a slightly lower Reynolds number. These regions are of particular interest for those seeking to explain the origins of vortex breakdown. The hitherto unexplained occurrence of the spiral streaklines, postulated previously to be due to non-axisymmetry of the flow, is found to be due to small offsets of the dye injection from the central axis. The important implications of this finding are that (i) non-axisymmetry is not a necessary route to bubble-type vortex breakdown, and (ii) that flows displaying spiral streaklines may be still sufficiently axisymmetrical for comparison with numerical and theoretical treatments of the breakdown phenomenon.
Inertial lift on a moving sphere in contact with a plane wall in a shear flow
Authors: Krishnan Gokul P. , Leighton David T.
Published: July 2002

In this paper we calculate the lift force on a smooth sphere rotating and translating in a simple shear flow in contact with a rigid wall. The calculation involves only known creeping flow solutions and is presented in terms of six different coefficients, each arising as a result of a pairwise combination of the translational velocity, rotational velocity, and the imposed shear flow. The results obtained agree well with those of Cherukat and McLaughlin [J. Fluid Mech. 263, 1 (1994a); and (personal communication, 1994b)], extrapolated for the case of zero separation distance. The calculated lift is further integrated into a force and torque balance on a non-neutrally buoyant rough sphere moving in contact with a plane. It is found that if the shear Reynolds number Re is sufficiently large, the lift force exceeds the gravitational force and the sphere separates from the plane. The increased separation is accompanied by an increase in the translational velocity U of the sphere and a corresponding decrease in the lift force due to the negative shear-translation coefficient, ultimately resulting in the sphere acquiring some steady separation distance. The equilibrium separation distance and velocity are plotted as a function of the parameter Re2/Res, where Res is the sedimentation Reynolds number.
Inappropriateness of the heat-conduction equation for description of a temperature field of a stationary gas in the continuum limit: Examination by asymptotic analysis and numerical computation of the Boltzmann equation
Authors: Sone Yoshio , Aoki Kazuo , Takata Shigeru , Sugimoto Hiroshi , Bobylev A. V.
Published: July 2002

It is shown analytically and numerically on the basis of the kinetic theory that the heat-conduction equation is not suitable for describing the temperature field of a gas in the continuum limit around bodies at rest in a closed domain or in an infinite domain without flow at infinity, where the flow vanishes in this limit. The behavior of the temperature field is first discussed by asymptotic analysis of the time-independent boundary-value problem of the Boltzmann equation for small Knudsen numbers. Then, simple examples are studied numerically: as the Knudsen number of the system approaches zero, the temperature field obtained by the kinetic equation approaches that obtained by the asymptotic theory and not that of the heat-conduction equation, although the velocity of the gas vanishes.
Experimental evidence for power-law wave number spectra of fractal tracer distributions in a complicated surface flow
Authors: Sommerer John C.
Published: July 2002

Fractal tracer distributions, experimentally observed in surface flows, have previously been successfully modeled as the snapshot attractors of a random dynamical system. The accompanying paper by Namenson, Antonsen, and Ott [Phys. Fluids 8, 2426 (1996)] suggests that the wave number spectra of such snapshot attractors should exhibit power law dependence on wave number, where the power is given in terms of the correlation dimension of the fractal spatial pattern. Their theoretical and numerical results, while supporting the power-law result, also caution that the power law may be obscured in an experimental situation. This paper demonstrates that, with proper averaging, the predicted power-law scaling can be observed in a real experiment.
Analogy between the Navier–Stokes equations and Maxwell’s equations: Application to turbulence
Authors: Marmanis Haralambos
Published: July 2002

A new theory of turbulence is initiated, based on the analogy between electromagnetism and turbulent hydrodynamics, for the purpose of describing the dynamical behavior of averaged flow quantities in incompressible fluid flows of high Reynolds numbers. The starting point is the recognition that the vorticity (w=∇×u) and the Lamb vector (l=w×u) should be taken as the kernel of a dynamical theory of turbulence. The governing equations for these fields can be obtained by the Navier–Stokes equations, which underlie the whole evolution. Then whatever parts are not explicitly expressed as a function of w or l only are gathered and treated as source terms. This is done by introducing the concepts of turbulent charge and turbulent current. Thus we are led to a closed set of linear equations for the averaged field quantities. The premise is that the earlier introduced sources will be apt for modeling, in the sense that their distribution will depend only on the geometry and the total energetics of the flow. The dynamics described in the preceding manner is what we call the metafluid dynamics.
Entropy-layer instabilities over a blunted flat plate in supersonic flow
Authors: Dietz G. , Hein S.
Published: July 2002

Entropy-layer instabilities in the supersonic flow over a blunted flat plate with a nose radius of Rn=2.5 mm at a Mach number M=2.5 and a unit Reynolds number Re/l=9.9×106/m are investigated. A type of entropy-layer instability most amplified at zero wave angle is found. Its streamwise wave number and the wall-normal phase distribution of the numerically obtained density eigenfunctions agree with flow-field visualization using the Schlieren technique. The frequencies of this entropy-layer instability made nondimensional on the basis of an appropriate entropy-layer length scale matches documented experimental observations at a blunted cone.
Shock waves in nonuniform gas
Authors: Ishii Ryuji , Fujimoto Hitoshi , Hatta Natsuo , Umeda Yoshikuni , Yuhi Masatoshi
Published: July 2002

Shock waves propagating through a stratified gas are investigated numerically and analytically. A shock wave is produced by a piston that begins to move from rest abruptly at some constant velocity in a two-dimensional horizontal duct. Initially the gas in the duct has a temperature or density distribution only along the vertical axis at a constant pressure. The initial density distribution, which is assumed to change monotonically, has a zero spatial gradient at the upper and lower walls and then has a single inflection point. It is confirmed that at least three types of shock patterns can be realized asymptotically. The first is a single curved shock, the second is a shock with a Mach stem (Mach reflection), and the last is a shock with a reflected branch (regular reflection). The first is substantially steady but the latter two are essentially unsteady. The time evolution of the induced flow field is investigated in detail. Based on this information, an analytical solution for the substantially steady curved shock is obtained in the coordinate system fixed on the shock. The shock profile as well as the induced flow field is investigated in detail with this solution. It is shown that the analytical results can predict quite well the numerical results. Finally, the flow instability of this shock-induced flow is investigated, because the induced flow has a nonuniform horizontal velocity distribution along the vertical axis at a constant pressure.
Velocity fluctuations in a bidisperse sedimenting suspension
Authors: Peysson Yannick , Guazzelli Élisabeth
Published: July 2002

The sedimentation of a dilute bidisperse sedimenting suspension is studied experimentally under creeping flow conditions. The mean velocity and velocity fluctuations of each particle species are measured in different sedimentation zones.
Effects of uniform blowing or suction from a spanwise slot on a turbulent boundary layer flow
Authors: Park Jeongyoung , Choi Haecheon
Published: July 2002

Effects of uniform blowing or suction from a spanwise slot on a turbulent boundary layer flow are investigated using the direct numerical simulation technique. The magnitudes of blowing or suction are less than 10% of the free-stream velocity. The skin friction and near-wall turbulence intensities are significantly changed by blowing and suction. In the case of uniform blowing, the skin friction on the slot rapidly decreases. The streamwise vortices above the wall are lifted up by blowing, and thus the interaction of the vortices with the wall becomes weaker. Accordingly, the lifted vortices become stronger in the downstream due to less viscous diffusion (above the slot) and more tilting and stretching (downstream of the slot), resulting in the increase of the turbulence intensities as well as the skin friction downstream of the slot. The opposite is observed in the case of uniform suction. The velocity fluctuations and Reynolds shear stress react quickly to blowing but recover slowly in the downstream, as compared to the case of suction. For both cases of blowing and suction, the streamwise turbulence intensity recovers quickly from blowing or suction, while other components of the turbulence intensities and Reynolds shear stress recover in a longer downstream distance.
First operation of a wiggler-focused, sheet beam free electron laser amplifier*
Authors: Destler W. W. , Cheng S. , Zhang Z. X. , Antonsen T. M. , Granatstein V. L. , Levush B. , Rodgers J.
Published: July 2002

A wiggler-focused, sheet beam free electron laser (FEL) amplifier utilizing a short-period wiggler magnet has been proposed as a millimeter-wave source for current profile modification and/or electron cyclotron resonance heating of tokamak plasmas. As proposed, such an amplifier would operate at a frequency of approximately 100–200 GHz with an output power of 1–10 MW CW. Electron beam energy would be in the range 500–1000 keV. To test important aspects of this concept, an initial sheet beam FEL amplifier experiment has been performed using a 1 mm×2 cm sheet beam produced by a pulse line accelerator with a pulse duration of 100 ns. The 500–570 keV, 4–18 A sheet beam is propagated through a 56 period uniform wiggler (λw=9.6 mm) with a peak wiggler amplitude of 2–5 kG. Linear amplification of a 5–10 W, 94 GHz signal injected in the TE01 rectangular mode is observed. All features of the amplified signal, including pulse shape and duration, are in accordance with the predictions of numerical simulation. Amplified signal gain has been measured as a function of injected beam energy, current, and wiggler field amplitude and is also in good agreement with simulation results. Continuation of this experiment will involve studying nonlinear amplifier operation and adding a section of tapered wiggler.
Sheared rotation effects, ion- and electron-temperature fluctuations, and generalized form factors on drift waves
Authors: Katou K. , Mamada H.
Published: July 2002

The effect of inhomogeneous azimuthal flows in cylindrical geometry on drift waves with non-Boltzmann density response is investigated. Stabilization of dissipative drift waves by the inhomogeneous flow is also analyzed. Convective cells are shown to be destabilized by the discontinuous flow. Next, ion- and electron-temperature fluctuations in a resistive plasma are investigated in the slab approximation, allowing for the self-consistently stationary system parameters (electron- and ion-temperature inhomogeneities, magnetic shear, and plasma currents). Their magnitude, their propagation directions, and the dependence of their stability characteristics on the system parameters are identified, distinguishing between the weakly and the strongly collisional regions. Finally, the generalized form factor for density fluctuations in ion-temperature gradient modes is calculated. The resulting broad frequency spectrum is shown to stem from the fact that the linear growth rate is comparable to the real wave frequency.
Self-consistent plasma profile steepening in presence of laser light with mixed polarization
Authors: Hüller S. , Ahedo E.
Published: July 2002

Ponderomotive profile steepening and resonance absorption of a flowing stratified laser produced plasma are investigated self-consistently when both cases of linear polarization with respect to the plane of incidence (s and p polarization) are combined. This analysis enables a representation of ‘‘mixed’’ polarized light, desirable for application in advanced codes modeling laser plasmas. Profile steepening forms (i) a layer around the turning point of the light, and, embedded therein, (ii) a thinner sublayer at the critical surface, where plasma waves are excited. The structure and the density transition of both layers are determined as a function of dimensionless parameters, i.e., the angle of incidence and the light intensities corresponding to both polarizations (related to plasma temperature and light wavelength). The plasma flow changes from sub- to supersonic in the steepened region; the sound speed depends on the intensity of the longitudinal electric field. In contrast to the purely polarized cases, one finds that (i) a closed analytical solution is not available, (ii) the sonic point does not coincide with extrema of the light-field components, and (iii) the oscillations of the electromagnetic field and the density rippling in the underdense plasma are not exactly periodic, due to the different dispersion of the p- and s-polarized components. It is found that resonance absorption is maximum for angles of incidence about 15°–20°.