bluuebunny/crossref_metadata_2025 huggingface dataset:
A note on the probability distribution of the dissipation rate in locally isotropic turbulence
Authors: Liu Tianshu
Published: July 2002

A nonlinear random differential equation describing the magnitude of vorticity is given for fine vortex tube in locally isotropic turbulence. From this vortex dynamics model, the asymptotic expressions of the probability density function of the dissipation rate in locally isotropic turbulence are obtained. In particular, several physical mechanisms behind turbulence statistics are explored.
Effect of boundary conditions on axial flow in a concentrated vortex core
Authors: Cohn Richard K. , Koochesfahani Manoochehr M.
Published: July 2002

The character of axial flow in the core of a concentrated line vortex is studied using flow visualization. Results show that axial flow can be initiated when the no-slip boundary condition is imposed on the vortex core over a spatial extent comparable to the core diameter. It is further demonstrated that no-slip is not a necessary condition to generate the axial flow; a region of spanwise shear (i.e., a slip boundary) is sufficient. Two distinct axial flow velocity profiles are observed; they are either jetlike or spatially undulated corresponding to a maximum axial speed occurring at the core center or outer edge, respectively.
A statistical analysis of numerically simulated plasma turbulence
Authors: Pécseli H. L. , Trulsen J.
Published: July 2002

Low-level electrostatic ion acoustic turbulence generated by the ion–ion beam instability was investigated numerically. It is demonstrated that a conditional statistical analysis can reveal the formation, propagation, and decay of negative potential wells which correspond to ion phase-space vortices. While the statistical analysis gives results in terms of averages over a conditionally selected subensemble, individual phase-space structures can be investigated by small clusters of test particles placed at selected positions in phase space. The predictions of the conditional statistical analysis can be tested and their accuracy estimated by this method. A discussion of various aspects of test particle dynamics in turbulent plasmas follows naturally, with particular emphasis on conditioned particle release and relative diffusion of particles. The statistical properties of the fluctuating electric fields sampled along Lagrangian particle orbits is analyzed.
Damping of the fast Alfvén wave in ion-cyclotron resonance heating
Authors: Chow C. , Ram A. K. , Bers A.
Published: July 2002

The conditions are determined under which the damping of the fast Alfvén wave, used in the heating of tokamak plasmas, can be described by the geometrical optics approximations. Also, for these conditions, an analytic formula is derived, which explicitly determines the amount of energy that is damped onto the plasma particles by the fast wave. The conditions for the validity of the geometrical optics approximations cover a wide range of k∥’s (the component of the wave vector parallel to the total magnetic field). There is a narrow range of k∥’s over which this is not valid; this is the regime where mode conversion is dominant. The results from this theory are in good agreement with those obtained from numerical solutions of fourth- and sixth-order differential equations that are commonly used to describe this type of heating. However, this theory has a distinct advantage of lending itself to detailed scaling studies as explicit expressions for the damping of the fast Alfvén wave are obtained.
Space-charge effects in warm ion sheet beams in the Vlasov–Maxwell approximation
Authors: Abraham-Shrauner B. , Anderson O. A.
Published: July 2002

A warm, intense, externally focused ion sheet beam is analyzed: a bounded one-component plasma that not only has practical interest but is well-suited for illustrating the use of some powerful new tools based on the use of Liouville invariants and Lie point group symmetries. The two-dimensional, time-independent Vlasov–Maxwell equations are shown to reduce to a set of one-dimensional, time-dependent, nonlinear Vlasov–Maxwell equations. Special examples of analytic solutions are given for two cases: (1) a uniform, but axially varying, warm ion beam and (2) a nonuniform, axially stationary, warm ion beam.With the spatial dependence of the ion beam density chosen, analytic expressions for the ion distribution function, electric fields, transverse bulk flow velocity, rms beam size, and rms emittance are derived. Some of these results corroborate previous work while others are new. In this derivation the usual Vlasov–Poisson equations for the ion beam are replaced by the self-consistent Vlasov–Maxwell equations; this generalization will be important for extensions of this treatment to asymmetric problems. A careful discussion of boundary conditions and of the simplifications used in the derivation is included.
On the kinetic Alfvén wave
Authors: Spies Günther Otto , Li Jin
Published: July 2002

The Alfvén root of the dispersion relation for plane waves in a collisionless magnetized plasma with Maxwellian distribution functions is discussed for wave vectors nearly perpendicular to the magnetic field and for wavelengths comparable to the ion gyroradius. When measured in units of the Alfvén frequency, this root is a function of the ratio κ⊥ of the ion gyroradius and the perpendicular wavelength, of the ratio β of ion and magnetic pressures, and of the ratio τ of electron and ion temperatures. It equals unity for κ⊥=0 (the fluid Alfvén wave obtains in the limit of vanishing gyroradii), and its deviation from this is O(κ2⊥) for small k⊥, provided β is not large. If β is large, the root is a function only of the product κ2⊥ β1/2 (not of τ), and its deviation from unity is O(κ2⊥ β1/2), thus becoming finite already for values of κ2⊥ as small as κ2⊥∼β−1/2 for which the ions are still adiabatic. If τ≪1 and/or β≫1, the electric field is nearly perpendicular to the magnetic field, implying that the Vlasov-fluid model (which assumes E∥=0 in addition to cold massless adiabatic electrons) is valid not only for τ≪1 (as originally stipulated), but even for all values of τ, provided β≫1. An application to the problem of gyroradius stabilization of short wavelength magnetohydrodynamic instabilities localized in the high beta region near the center of a Z pinch is outlined.
Control of plasma shape and performance of the PBX-M tokamak experiment in high-βt /high-βp regimes
Authors: Bell R. E. , Asakura N. , Bernabei S. , Chance M. S. , Duperrex P.-A. , Fonck R. J. , Gammel G. M. , Greene G. , Hatcher R. E. , Holland A. , Jardin S. C. , Jiang T.-W. , Kaita R. , Kaye S. M. , Kessel C. E. , Kugel H. W. , LeBlanc B. , Levinton F. M. , Okabayashi M. , Ono M. , Paul S. F. , Powell E. T. , Qin Y. , Roberts D. W. , Sauthoff N. R. , Sesnic S. , Takahashi H.
Published: July 2002

The highly indented plasmas of the PBX-M tokamak experiment [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1989), Vol. 1, p. 97] have reached plasma regimes of both high volume-averaged beta (βt), and high-beta poloidal (βp), and show evidence of the suppression of external surface modes by the passive stabilizing system. Values of βt up to 4.0 I/aB (% MA/m T) with Ti(0)≊4 keV have been obtained. A magnetohydrodynamic analysis of plasmas with βp=2.0 indicates that these plasmas are near the threshold of the second stability regime. A value of βt of 6.8% has been reached with Ti(0)>5 keV and an indentation of 28%. Control of plasma shape is accomplished with a feedback system that uses a moment expansion about a single equilibrium and is augmented by time-dependent waveforms to redefine plasma shape. Diagnostics to measure the safety factor q have been developed and used to make accurate measurements of q(r) and to verify changes made in q(0).
Space plasma and its origin at the sun
Authors: Parker E. N.
Published: July 2002

This paper provides a concise review of the large picture of space plasmas, concentrating on the heliosphere and the origin of the x-ray corona and coronal hole at the sun. It is tentatively suggested that the active x-ray corona is heated primarily by the dissipation of magnetic energy at the spontaneous tangential discontinuities (current sheets) in the confining bipolar magnetic fields of active regions. The discontinuities arise because of the continuous mapping of the footpoints of the field by the photospheric convection. The coronal holes appear to be heated by a combination of Alfvén waves, providing the high-speed streams in the solar wind, and the microflaring in the small-scale network fields.
Dynamos and angular momentum transport in accretion disks
Authors: Vishniac Ethan T. , Jin Liping , Diamond P. H.
Published: July 2002

The transport of angular momentum in astrophysical disks is one of the major issues in modern astrophysics. Here, recent work [Astrophys. J. 347, 435 (1989); 365, 648 (1990)] will be reviewed that suggests that internal waves, analogous to deep ocean waves, play a critical role in transporting angular momentum in neutral disks and generating a magnetic dynamo in ionized disks. Previously, it was shown that low-frequency, slightly nonaxisymmetric (‖m‖=1) waves in thin accretion disks could penetrate to small radii with a unique amplitude because of nonlinear saturation. Here, the ability of these waves to drive an α-Ω dynamo in a disk of thickness H and radius r and keplerian rotational frequency Ω(r)∝r−3/2 is examined. The asymmetry in the wave distribution that creates a nonzero helicity follows from the fact that the fundamental waves all have a positive angular momentum flux. As a result, there will be a large-scale magnetic field driven by an α-Ω dynamo. It is also likely that small-scale fields, driven by higher-order wave modes, will contribute significantly to the local value of BrBφ. It is argued that the magnetic field saturates when its pressure is comparable to the thermal pressure and a crude model of the nonlinear transfer of power to small-scale turbulence is presented. The dynamo process creates a large-scale, axisymmetric toroidal field with Br∼(H/r)3/2Bφ. Smaller-scale waves create small-scale fields with a maximum brbφ∼(H/r)6/5P. In this model, viscous and thermal instabilities in radiation pressure dominated, and electron scattering regions in accretion disks appear to be substantially suppressed.
Quenching of Einstein A coefficients in plasmas and lasers
Authors: Suckewer S.
Published: July 2002

The coefficient of spontaneous emission (Einstein A coefficient) is considered to be one of the basic constants of a given transition in an atom or ion. The formula for the Einstein A coefficient was derived in the pioneering works of Weisskopf and Wigner (WW) [Z. Phys. 63, 54 (1930)] based on Dirac’s theory of light. More recently, however, it was noted in several papers that the rate of spontaneous radiative decay can deviate significantly from the WW expression in certain conditions, for example, in a laser cavity. A different type of change in A coefficients was inferred from measurements of changes in the intensity branching ratio of spectral lines in a plasma. A change of branching ratio of up to a factor of 10 was observed in Civ for 3p-3s (580.1–581.2 nm) and 3p-2s (31.2 nm) transitions when the electron density changed from approximately Ne≊1×1018 to 5×1018 cm−3. This effect was also observed in Ciii and Nv. An initial theoretical approach to the problem based on the integration of the Schrödinger equation with the ion Coulomb potential modified by the electron cloud within the Debye radius was unsuccessful in predicting the experimental observations. The effect of quenching of spontaneous emission coefficients was observed also in an Ar-ion laser as a function of the intracavity power density (photon density) for lines originating from the same upper level as the lasing line. Measurements of these line profiles, absorption for different lasing conditions, and related discussions are also presented.
Stability of long wavelength modes in tokamaks
Authors: Hirose A.
Published: July 2002

Effects of collisional trapped electron dynamics on the incompressible ballooning mode and electromagnetic corrections to the trapped ion modes have been investigated in terms of a semilocal kinetic dispersion relation. The ballooning mode is subject to strong collisional stabilization. The trapped ion modes are suppressed when the plasma β exceeds the ideal ballooning limit.
Lagrangian dynamics of a charged particle in a tokamak magnetic field
Authors: Dendy R. O.
Published: July 2002

The Euler–Lagrange equations for a charged particle in a tokamak magnetic field, in the limit of large aspect ratio, are obtained in terms of toroidal coordinates (r,θ,φ,ṙ,θ̇,φ̇) from the exact Lagrangian, which is expressed in terms of the toroidal and poloidal magnetic fluxes. It differs fundamentally from the standard guiding-center approach, where the local magnetic-field direction defines a spatial axis, and where there is no direct link between the velocity coordinates and the time derivatives of the spatial coordinates, so that application to the particular magnetic geometry of the tokamak is often difficult. In contrast, the tokamak magnetic field is specified at the outset in the present approach. For example, cyclotron motion in the combined poloidal and toroidal fields appears as a libration in real space, superimposed on motion composed of linear nonoscillatory and averaged quadratic oscillatory terms. The primary application that is considered concerns the plasma response to high-frequency waves, using both cold plasma and kinetic treatments. A kinetic expression for cyclotron resonance is obtained that agrees with the gyrokinetic result [see, for example, C. N. Lashmore-Davies and R. O. Dendy, Phys. Fluids B 1, 1565 (1989) and references therein], which differs from the standard expression.
Boundary Larmor radius effect on electrostatic waves
Authors: Li Jin , Scheffel Jan
Published: July 2002

The linearized Vlasov–Poisson equations, which combine to an integrodifferential equation for the perturbed electric potential, are used to investigate the effect of finite plasma size on the stability of electrostatic waves in a homogeneous plasma slab. The distortion of the gyromotion of the particles at the plasma boundary influences wave stability, a phenomenon termed the boundary Larmor radius (BLR) effect. The integrodifferential equation, treated as an eigenvalue problem, is discretized into a matrix dispersion equation by use of the Galerkin method and is then solved numerically. It is found that the ion Bernstein wave, which is undamped in an infinite homogeneous plasma, now becomes damped with a maximum damping rate of 0.35 ωci at rG/L (ion Larmor radius over wall distance)≊0.15. In general, the damping is less pronounced at shorter perpendicular wavelengths. It implies a necessity to take into account the BLR effect in the kinetic stability studies for sufficiently large ion Larmor radius in comparison to the characteristic dimension.
Current generation by alpha particles interacting with lower-hybrid waves in tokamaks
Authors: Anderson D. , Belikov V. S. , Kolesnichenko Ya. I. , Lisak M.
Published: July 2002

The problem of the influence of fusion-generated alpha particles on lower-hybrid-wave current drive is examined. Analysis is based on a new equation for the lower-hybrid-wave-fast ion interaction, which is derived by taking into consideration the nonzero value of the longitudinal wave number. The steady-state velocity distribution function for high-energy alpha particles is found. The alpha current driven by LH-waves as well as the rf power absorbed by alpha particles are calculated.
Coupling of the fringing fields of an ion-Bernstein wave antenna
Authors: Chiu S. C. , Chan V. S. , Perkins F. W. , Puri S.
Published: July 2002

It is proposed that the fringing field of the Faraday screen of an ion–Bernstein wave antenna can couple to a short wavelength electrostatic mode and cause significant parasitic loading. The mixed boundary value problem is solved analytically and numerically. The characteristics of the resulting parasitic loading are described and relevance to experiments is described.
Strong interaction of solitons and their trailing oscillations with resonant beam ions
Authors: Honzawa Tadao , Arakawa Masazo , Singh Sanjay
Published: July 2002

Strong interaction of ion-acoustic solitons and their trailing oscillations with resonant beam ions in a plasma is experimentally studied. Here, an effective soliton amplification is observed to occur when beam ions with an average energy of about 2 eV collide with the soliton from behind. Furthermore, amplitude dependence of the soliton velocity and width is confirmed to be almost unchanged by such strong interaction with beam ions. In addition, small oscillations behind the soliton are also observed to evolve into a large-amplitude wave train under the influence of resonant beam ions. Finally, some tentative interpretations for these observed phenomena are presented.
Charge-state equilibrium and nonequilibrium modeling of the carbon-pellet plasma interaction
Authors: ElCashlan A. G. , Gerdin G. A. , Vahala L. L. , Parks P. B.
Published: July 2002

Self-consistent equilibrium and nonequilibrium charge-state models are formulated for the spherical expansion of low-Z pellet vapor as an inviscid perfect gas of constant ratio of specific heats being heated volumetrically by the incident electrons of a thermonuclear plasma. The two models are found to be in agreement in the region where the ratio of the ionization length ζj to pellet radius rp is less than unity, but a single parameter, such as the magnitude of this ratio on the sonic surface, is insufficient to determine whether an equilibrium model will be valid for all regions of the ablatant for carbon pellets. Thus a nonequilibrium model is necessary to model the outer regions of the ablatant cloud even for thermonuclear plasma conditions when the cloud is very dense. Also, the effect of the ionization of the ablatant by the incident plasma electrons is found to be 10% or less for even the C+3 region in the thermonuclear regime. Finally, although the model used for the healing of the ablatant by the plasma electrons is that for a neutral carbon ablatant, it is shown that the differences in heating by the plasma electrons between this model and that for an ionized ablatant are small.
The role of self-consistent Lagrangian chaos in Bénard convection in an annulus
Authors: Finn John M. , Hermiz Keith
Published: July 2002

The nonlinear behavior of the two-dimensional Bénard problem with periodic boundary conditions in the horizontal direction is studied with particular emphasis on the role of self-consistent chaotic advection. The results show a complex interplay between vortices driven by the Bénard (Rayleigh–Taylor) instability and shear flow, which is driven by the vortices [J. Drake et al., Phys. Fluids B 4, 4881 (1992)] and which causes their decay. Chaotic advection occurs in the transition from the low Rayleigh number (Ra) regime to the high Ra regime [J. Finn, Phys. Fluids B 5, 415 (1993)]. For the former, vortex flow and shear flow coexist, possibly with slow relaxation oscillations. In the high Ra regime there are vortices localized near the upper and lower boundaries with a shear flow in between. As Ra is decreased from the high Ra regime, these vortices broaden, eventually overlapping, causing self-consistent Lagrangian chaos. This onset of chaos is responsible for several properties of the transition state between the low Ra and the high Ra regimes, most notably the damping of the relaxation oscillations involving vortex and shear flow. It is also observed that the Nusselt number Nu has a peak with respect to Ra in this transition regime characterized by Lagrangian chaos. In the low Ra regime, on the other hand, the relaxation oscillations are on a much slower time scale than the eddy turnover time and the Lagrangian behavior is described by separatrix crossing.
Diocotron instability in curved magnetic field
Authors: Pradhan Subrata , Avinash K.
Published: July 2002

The diocotron instability is studied in a plasma confined in between two concentric cylinders by an azimuthal magnetic field. In this geometry the perturbations are compressible while equilibrium is incompressible. The Rayleigh criterion for compressible perturbations is shown to be modified. Marginal stability curves (Im ω=0) for different mode numbers are plotted. The quasilinear evolution of this instability is associated with a mean inward motion of the plasma column (inward pinch) apart from the usual diffusion to the marginal stability.
Statistics of strongly turbulent electric fields
Authors: Benford Gregory , Zhai X.-L.
Published: July 2002

A frequently observed exponential distribution for the probability distribution of strongly turbulent Langmuir fields, P(E,t), may arise from dissipative processes. Experiments show that the distribution is time stationary within the 100 nsec observed resolution, and exhibits exponential time decay after the beam driver ceases. Microwave emission also ceases, compatibly with a beam origin. Return current measures imply that ion waves are much weaker than Langmuir turbulence. Interactions at ion sound wave speeds can mediate the observed μ sec decay, and may also adjust P(E) when the beam driver is on.
Strongly nonlinear evolution of low-frequency wave packets in a dispersive plasma
Authors: Vasquez Bernard J.
Published: July 2002

The evolution of strongly nonlinear, strongly modulated wave packets is investigated in a dispersive plasma using a hybrid numerical code. These wave packets have amplitudes exceeding the strength of the external magnetic field, along which they propagate. Alfvén (left helicity) wave packets show strong steepening for β<1, while fast (right helicity) wave packets hardly steepen for any β. Substantial regions of opposite helicity form on the leading side of steepened Alfvén wave packets. This behavior differs qualitatively from that exhibited by the solutions to the derivative nonlinear Schrödinger (DNLS) equation.
On the Murakami density limit in tokamaks and reversed-field pinches
Authors: Perkins F. W. , Hulse R. A.
Published: July 2002

A theoretical upper limit for the density in an Ohmically heated tokamak discharge follows from the requirement that the Ohmic heating power deposited in the current-carrying channel exceed the impurity radiative cooling in this critical region. A compact summary of our results gives this limit nM for the central density as nM =[Ze/(Ze−1)]1/2 ne0 (BT /1T) (1m/R), where ne0 depends strongly on the impurity species and is remarkably independent of the central electron temperature Te(0). For Te(0) ∼1 keV, ne0 =1.5×1014 cm−3 for beryllium, ne0 =5.5×1013 cm−3 for oxygen, ne0 =1.0×1013 cm−3 for iron and, ne0 =0.5×1013 cm−3 for tungsten. The results agree quantitatively with Murakami’s original observations. A similar density limit, known as the I/N limit, exists for reversed-field pinch devices and this limit has also been evaluated for a variety of impurity species.
Guiding-center autonomy theorem
Authors: Newcomb William A.
Published: July 2002

A study is made of the guiding-center motion of charged particles in a collisionless magnetoplasma in the standard paraxial limit, with finite-orbit ordering. As a preliminary, a simple one-fluid representation of the macroscopic plasma motion is set up, and the guiding-center equations are then referred to a Lagrangian coordinate network comoving with the representative fluid. They are then found to split themselves off from the fluid equations as an autonomous subsystem. It is concluded that the fluid motion (or the network motion) has no effect on the guiding-center trajectories relative to the network.
The generation of noise in impinging vortex motion past a step
Authors: Conlisk A. T. , Veley Duane
Published: July 2002

The noise field generated by the motion of one, two, or several rectilinear vortices in an inviscid, irrotational mean flow past a step is considered for low Mach number. The streakline patterns of an impinging vortex are compared with flow visualization studies, and qualitative agreement between the present analytical results and the experiments is achieved. Both acoustic pressure and intensity are strong functions of the vortex motion near the step which, because of the complexity of the geometry, must be computed numerically. Results for the acoustic pressure and intensity are presented for a variety of incident vortex arrays.
Self-similar expansion of dense matter due to heat transfer by nonlinear conduction
Authors: Pakula R. , Sigel R.
Published: July 2002

The hydrodynamic flow, called the ablative heat wave, from a dense body which is suddenly brought into contact with a thermal bath with a temperature of the order of 105–108°K is investigated theoretically. It is shown that the hydrodynamic equations with nonlinear conduction admit a self-similar solution of the asymptotic type, i.e., approached by the flow for t→∞. Self-similarity is a consequence of the fact that the solid may be considered as infinitely dense in this limit, even for a bath temperature varying in time according to T∼tτ (within certain limits for τ). The self-similar solution is calculated in detail, and scaling relations for the hydrodynamic variables are given. Numerical values are obtained for the radiative heating of gold. The range of validity of the solution is discussed.
Influence of equilibrium flows on viscous tearing modes
Authors: Sandal Leif Kristoffer
Published: July 2002

Earlier investigations of resistive instability have shown that an equilibrium flow, including a flow of the order of the natural diffusion, has a profound influence on the stability properties of the tearing mode. It has been reported that a perpendicular flow of the same order as the natural diffusion (η/a) makes a finite shift in the stability threshold as η→0, and only the sign of the velocity matters in this limit. Here the primary concern is with how a moderate or small magnetic Prandtl number changes the stability threshold of a magnetofluid in the presence of a perpendicular flow: it is found that the threshold is again continuous at v=0. The flow is still stabilizing or destabilizing according to the direction of the flow. The viscosity plays important different roles in the static and nonstatic cases. The stability of a static plasma is independent of the magnetic Prandtl number, but the growth rate is not. The stability depends on this number in the nonstatic cases. It is found that the magnitude of the shift in the stability threshold decreases with increasing magnetic Prandtl number. This effect tends to oppose the stabilizing or destabilizing effect of the flow. An increase in the viscosity may in fact destabilize modes when the flow is diffusive. The boundary in parameter space between the inertial and viscous regime is estimated. High-beta devices are well within the viscous regime. It has been possible to demonstrate that the correction caused by viscosity is far more important than those corrections caused by a perpendicular or a shear parallel flow for modes of finite growth rate.
Flow field effects on nucleation in a reacting mixing layer
Authors: Kennedy I. M.
Published: July 2002

Chemical nucleation has been studied numerically in a stagnation point mixing layer in which reactants in two counter-flowing streams form a condensable monomer. The response of the subsequent nucleation kinetics to the velocity gradient in the flow is described in terms of a Damkohler number. Two limiting cases have been established. First, if the Damkohler number for monomer production is small, i.e., the rate of monomer production is slow, then the nucleation of particles can be strongly affected by the flow field in a manner which is equivalent to the effect of supersaturation in a uniform vapor. Second, if the Damkohler numbers for cluster growth are small (because of a small accommodation factor for monomer–cluster interactions), the concentrations of clusters do not achieve equilibrium levels. This can result in the suppression of particle formation over a critical range of Damkohler numbers. In this case the behavior of the nucleation kinetics is analogous to the transient phase of nucleation in a uniform vapor.
A reconsideration of thermal diffusion in ionized gases: Quantal and dynamic shielding effects
Authors: Monchick L. , Mason E. A.
Published: July 2002

The theory of thermal diffusion in ionized gases is reconsidered taking into account quantal effects and effects caused by dynamic shielding. Quantal effects arise because it is only in a small range of intermediate energies that the de Broglie wavelength is negligible relative to the effective collision diameter. Dynamic shielding effects are caused by the lack of instant response to particle motion by the charged particle sea surrounding them and are important at low temperatures and high electron densities. The errors incurred by the omission of these effects can usually be ignored in planetary ionospheres or in the solar corona, but are by no means negligible under conditions characteristic of the solar interior. Similar, but not identical, results were found for calculations of the binary diffusion coefficient.
Reply to comments of J. A. Krommes
Authors: Terry P. W. , Diamond P. H.
Published: July 2002

Dissipative drift-wave turbulence, the type of turbulence studied by Terry and Diamond [Phys. Fluids 28, 1419 (1985)], is shown to differ significantly from homogeneous Navier–Stokes turbulence, the type of turbulence from which Krommes draws the intuition and results used in his comparison. These differences make extrapolations from one case to the other unjustified and misleading. In its own right, clump theory is shown to compare well with laboratory and simulation data representing turbulence relevant to the theoretical model.
Electrostatic instabilities of velocity-space-shell distributions in magnetized plasmas
Authors: Sentman D. D. , Leboeuf J. N. , Katsouleas T. , Huff R. W. , Dawson J. M.
Published: July 2002

Electron instabilities of magnetized spherical shell distributions in velocity space with a colder Maxwellian background are investigated analytically with simulations using electrostatic particle codes. The resonant and nonresonant instabilities observed in the particle simulations are in agreement with zeros of the dielectric function, as found from the resonant approximation for waves with an electric field component along the magnetic field or by computation with a root solver code in the case of perpendicular propagation. Saturation of the instabilities is by nonlinear cyclotron resonance with the cold background in the resonant case or by nonstochastic cyclotron harmonic damping by the cold background in the nonresonant case. Instabilities invariably lead to perpendicular acceleration and heating of the cold background to velocities sometimes exceeding the shell velocity.
Effects of diffusion in the asymptotics of perturbations in stratified shear flow
Authors: Criminale W. O. , Cordova J. Q.
Published: July 2002

Analysis of perturbations in unbounded stratified shear flow is investigated both by integral transforms and by a more general solution given by Lord Kelvin [Philos. Mag. 24(5), 188 (1887)] that is possible for a constant mean shear flow. The latter solution is also valid for the full nonlinear problem and represents an exact solution of the Navier–Stokes equations. Diffusion of momentum and mass or heat are included subject to the Boussinesq approximation and it is concluded that (a) the asymptotic time behavior in the inviscid problem is only valid immediately after the initial moment, (b) diffusion of heat can cause destabilization whereas mass diffusion is stabilizing, and (c) viscosity, no matter how small, dominates the final period with decay ultimately being exponential and independent of the Richardson number.
Statistical properties and correlation functions for drift waves
Authors: Horton Wendell
Published: July 2002

The dissipative one-field drift wave equation is solved using the pseudospectral method to generate steady-state fluctuations. The fluctuations are analyzed in terms of space-time correlation functions and modal probability distributions. Nearly Gaussian statistics and exponential decay of the two-time correlation functions occur in the presence of electron dissipation, while in the absence of electron dissipation long-lived vortical structures occur. Formulas from renormalized, Markovianized statistical turbulence theory are given in a local approximation to interpret the dissipative turbulence.
Internal solitary waves and their head-on collision. II
Authors: Mirie Rida M. , Su C. H.
Published: July 2002

The head-on collision between two modified Korteweg–de Vries (MKdV) solitary waves is investigated where cubic and quadratic nonlinearities balance dispersion. These waves propagate at the interface of an inviscid two-fluid system where the ratio of the fluids densities is comparable to the square of the ratio of depths. The third-order perturbation solution is obtained and it is found that the collision is inelastic because of a dispersive wave train generated behind each emergent solitary wave.
A kinetic full wave theory of strong spatial damping of electron cyclotron waves in nearly parallel stratified plasmas
Authors: Fruchtman Amnon , Riedel Kurt , Weitzner H. , Batchelor D. B.
Published: July 2002

Propagation of extraordinary mode waves in nearly parallel stratified plasmas (magnetic beach geometry) is investigated. Since these waves are very heavily damped WKB theory is unreliable and kinetic effects make the problem inherently nonlocal. The resonance region is treated by a boundary layer expansion which reduces the problem to an integrodifferential system in one dimension. It is proved analytically that for moderate to high density, waves incident from the high field side are totally absorbed with no reflected wave. At very low density some transmission is possible, where the transmission coefficients are being given correctly by cold plasma theory. Numerical solution of the integrodifferential system shows that the power deposition profile can differ significantly from that predicted from a local WKB theory.
Neoclassical kernel for electron cyclotron wave driven current in tokamaks
Authors: Chan V. S.
Published: July 2002

When electron cyclotron wave propagation can be treated by geometric optics, the local current drive efficiency can be expressed as an integral. A kernel for the integral that takes into account important neoclassical effects such as particle trapping and orbit modification is derived using a momentum-conserving collision operator. For absorption at the low field side of a tokamak, trapping effects reduce the efficiency significantly, while for absorption at the high field side, modification of passing particle orbits also diminishes the driven current, although to a lesser extent.
Convection in rotating binary mixtures. II. Küppers–Lortz instability
Authors: Bhattacharjee Jayanta K.
Published: July 2002

Nonlinear effects on the stationary convection in rotating binary mixtures are studied. It is found that the nonequilibrium tricritical point is unaffected by the rotation. The threshold for the Küppers–Lortz instability is studied as a function of the separation parameter (ψ). It is seen that the threshold has a minimum when ψ∼L2, where L is the Lewis number. These results have been obtained for idealized boundary conditions.
Induced absorption of extraordinary (Z-mode) waves via electron pumping
Authors: Borovsky Joseph E.
Published: July 2002

When induced by pitch-angle scattering, waves of the middle branch of the extraordinary mode will give up their energy to a plasma by magnetically pumping electrons. The damping decrement for small-amplitude extraordinary waves is calculated and the decrement for finite-amplitude waves is estimated. Because heating a plasma makes it a better absorber of extraordinary waves, such waves can produce localized hot cavities in plasmas. The evolution of electron distribution functions f(v) is examined and it is found that magnetic-pumping-mediated absorption leads to f∝v−3 tails.
Electric field profile in the presence of sawtooth activity in a tokamak
Authors: Alladio F. , Vlad G.
Published: July 2002

The two coupled diffusive equations for the perturbed electron temperature and poloidal magnetic field have been solved in the framework of the Kadomtsev model for the sawtooth activity. It is found that the time averaged toroidal electric field profile is radially nonuniform. A simple formula that describes the radial behavior of the toroidal electric field is proposed, and the effect on the power balance is presented.
Neoclassical tearing modes in a tokamak
Authors: Hahm T. S.
Published: July 2002

Linear tearing instability is studied in the banana collisionality regime in tokamak geometry. Neoclassical effects produce significant modifications of Ohm’s law and the vorticity equation, so that the growth rate of tearing modes driven by Δ′ is dramatically reduced compared to the usual resistive magnetohydrodynamic values. Consequences of this result, regarding the presence of pressure-gradient-driven neoclassical resistive interchange instabilities and the evolution of magnetic islands in the Rutherford regime, are discussed.
Discrete mappings and resonant ripple transport in a tokamak
Authors: Albert Jay M. , Boozer Allen H.
Published: July 2002

Discrete mappings are used to analyze bounce-averaged resonant ripple transport in a tokamak. The banana-shaped orbits of toroidally trapped test particles are subject to ripple-induced radial drift, collisionality, and toroidal precession, linearized about zero mod 2π for the case of resonance. The bounce-averaged mapping resembles a Taylor–Chirikov standard mapping, augmented by random collisions in two distinct ways. Each considered by itself gives a form of the standard map with noise that has been treated previously, in different contexts; the joint effect of both noise terms on the particle diffusion coefficient is discussed.
Nonlinear stability of counter-rotating vortices
Authors: Dauxois Thierry
Published: July 2002

Recently, Mallier and Maslowe [Phys. Fluids A 5, 1074 (1993)] found an exact nonlinear solution of the inviscid, incompressible, two-dimensional Navier–Stokes equations, representing an infinite row of counter-rotating vortices, which extended the previous Kelvin–Stuart vortices. The aim of this work is to establish explicit sufficient conditions for the nonlinear stability of this solution. The result is derived with the energy-Casimir stability method as a function of the parameters of the solution and the domain size. The size of the domain over which the street of vortices is unstable is exhibited.
Numerical simulation of turbulent flow over a moving wavy boundary: Norris and Reynolds extended
Authors: Harris J. A. , Street R. L.
Published: July 2002

A numerical model of fully developed turbulent channel flow over a moving wavy boundary is presented. The model is of the linear-stability type, employs a transformed coordinate system, and uses the finite difference method for solution. Both the time-independent mean flow and the time-dependent perturbation to the mean flow caused by the traveling wavy boundary are computed. A low-turbulence-Reynolds-number form of the k-epsilon turbulence closure model is employed and three levels of sophistication in modeling the wave-induced Reynolds stresses (in the transformed coordinate system) are introduced. These models are the viscous quasilaminar (VQL) model, the mean eddy viscosity (EV) model, and the full perturbation (FP) model. The model is applied to simulate previous experimental investigations of the flow over wavy boundaries (including a water wave) under a variety of conditions. The solutions generated using both the EV and FP Reynolds stress closure models generally demonstrate very good agreement with the available pressure, stress, velocity, and Reynolds stress experimental data. In contrast, due to its neglect of the perturbation Reynolds stresses, the simple VQL closure model has limited predictive ability, especially for downstream running waves whose wave speed exceeds about half the channel centerline velocity and for cases with low inverse Reynolds numbers (based on the shear velocity and wavelength). For the water wave case, the component of the wave growth rate due to the pressure mechanism predicted by the EV and FP models can be up to three times those of the VQL model in the gravity wave growth range.
Scale resolved intermittency in turbulence
Authors: Grossmann Siegfried , Lohse Detlef
Published: July 2002

The deviations δζm (‘‘intermittency corrections’’) from classical (‘‘K41’’) scaling ζm=m/3 of the mth moments 〈‖u(p)‖m〉 in high Reynolds number turbulence are calculated, extending a method to approximately solve the Navier–Stokes equation described earlier. It is suggested to introduce the notion of scale resolved intermittency corrections δζm(p), because these δζm(p) are found to be large in the viscous subrange, moderate in the nonuniversal stirring subrange but, surprisingly, extremely small if not zero in the inertial subrange. If ISR intermittency corrections persisted in experiment up to the large Reynolds number limit, it would show by calculation that this could be due to the opening of phase space for larger wave vectors. In the higher order velocity moments 〈‖u(p)‖m〉 the crossover between inertial and viscous subrange is (10ηm/2)−1, thus the inertial subrange is smaller for higher order moments.
Pressure in lattice Boltzmann simulations of flow around a cylinder
Authors: Wagner Lukas
Published: July 2002

The expression for the pressure in lattice Boltzmann realizations of the Navier–Stokes equations involves compressibility effects. The pressure in two-dimensional steady and unsteady cylinder flow is obtained numerically in a lattice Boltzmann scheme for the first time and compared with experimental and finite-difference results.
Interaction of adjacent bursts in the wall region
Authors: Coller B. D. , Holmes P. , Lumley J. L.
Published: July 2002

Motivated by a desire to control the turbulent boundary layer via feedback, the effect on a naturally occuring streamwise vortex pair of a perturbing pair produced by an actuator is examined. It is shown how the resulting velocity field modifies coefficients in the low-dimensional models of Aubry et al. [J. Fluid Mech. 192, 115 (1988)], and a control strategy is sketched that reduces the bursting rate and hence the turbulent drag in the model problem.
On the breakup of viscous liquid threads
Authors: Papageorgiou Demetrios T.
Published: July 2002

A one-dimensional model evolution equation is used to describe the nonlinear dynamics that can lead to the breakup of a cylindrical thread of Newtonian fluid when capillary forces drive the motion. The model is derived from the Stokes equations by use of rational asymptotic expansions and under a slender jet approximation. The equations are solved numerically and the jet radius is found to vanish after a finite time yielding breakup. The slender jet approximation is valid throughout the evolution leading to pinching. The model admits self-similar pinching solutions that yield symmetric shapes at breakup. These solutions are shown to be the ones selected by the initial boundary value problem, for general initial conditions. Furthermore, the terminal state of the model equation is shown to be identical to that predicted by a theory which looks for singular pinching solutions directly from the Stokes equations without invoking the slender jet approximation throughout the evolution. It is shown quantitatively, therefore, that the one-dimensional model gives a consistent terminal state, with the jet shape being locally symmetric at breakup. The asymptotic expansion scheme is also extended to include unsteady and inertial forces in the momentum equations to derive an evolution system modeling the breakup of Navier–Stokes jets. The model is employed in extensive simulations to compute breakup times for different initial conditions; satellite drop formation is also supported by the model and the dependence of satellite drop volumes on initial conditions is studied.
Interaction of laminar far wake with a free surface
Authors: Chan Andy T. , Chwang Allen T.
Published: July 2002

Wave disturbances caused by the uniform translatory motion of a submerged body on or beneath the free surface of a viscous fluid are investigated analytically. The submerged body is idealized as an Oseenlet or an Oseen doublet, and exact solutions in closed integral forms are obtained. Based on these exact solutions, asymptotic representations of the wave amplitude for large Reynolds numbers based on the deep-water wavelength at large distances downstream of the body are derived. The results obtained show explicitly the effect of the laminar wake on the amplitude and the phase of the surface waves thus created.
The structure of variable property, compressible mixing layers in binary gas mixtures
Authors: Kozusko F. , Grosch C. E. , Jackson T. L. , Kennedy Christopher A. , Gatski Thomas B.
Published: July 2002

We present the results of a study of the structure of a parallel compressible mixing layer in a binary mixture of gases. The gases included in this study are hydrogen (H2), helium (He), nitrogen (N2), oxygen (O2), neon (Ne) and argon (Ar). Profiles of the variation of the Lewis and Prandtl numbers across the mixing layer for all 30 combinations of gases are given. It is shown that the Lewis number can vary by as much as a factor of 8 and the Prandtl number by a factor of 2 across the mixing layer. Thus assuming constant values for the Lewis and Prandtl numbers of a binary gas mixture in the shear layer, as is done in many theoretical studies, is a poor approximation. We also present profiles of the velocity, mass fraction, temperature and density for representative binary gas mixtures at zero and supersonic Mach numbers. We show that the shape of these profiles is strongly dependent on which gases are in the mixture as well as on whether the denser gas is in the fast stream or the slow stream.
Three-dimensional instability of the shear layer over a circular cylinder
Authors: Mihailovic J. , Corke T. C.
Published: July 2002

The instability of the shear layer separated from a circular cylinder was investigated for Reynolds numbers, based on cylinder diameter, from 3500 to 53 000. For this, the most amplified instability frequencies were found and compared against available results for this ReD range. Based on this initial set of measurements, ReD=3500 was chosen for a detailed study of the instability of the separated shear layer to 3-D disturbances. This involved the excitation of plane Kelvin–Helmholtz waves using an acoustic source, and pairs of oblique waves of equal but opposite angles produced by roughness which was distributed evenly along the cylinder span at the separation line. The sensitivity of the shear layer instability to the initial 2-D disturbance amplitude and 3-D disturbance wave angle was documented. The results showed that there exists a most amplified 3-D wave angle (spanwise wave length) for the separated shear layer. The sensitivity to the wave angle agreed well with the linear 3-D stability analysis of a Stuart vortex by Pierrehumbert and Widnall and the numerical simulations of a free shear flow by Rogers and Moser.