bluuebunny/crossref_metadata_2025 huggingface dataset:
Negative nonlinearity in selected fluorocarbons
Authors: Cramer M. S.
Published: July 2002

The Martin–Hou equation has been employed to compute the fundamental derivative of gasdynamics for seven commercially available fluorocarbons. Each fluid was found to have a region of negative nonlinearity large enough to include the critical isotherm. Negative nonlinearity twice as strong as that found in previous investigations is reported. In addition, estimates for the fundamental derivative at saturation are given.
Rotating, stratified flow past a shallow ridge
Authors: Chilakamarri Kiran B. , Foster M. R.
Published: July 2002

Streamline patterns in experiments performed by Boyer and Biolley [Philos. Trans. R. Soc. London Ser. A 318, 411 (1986)] on flow past a shallow ridge on the floor of a channel in a rotating, stratified fluid compare very poorly with two-dimensional theory, even though the aspect ratio of the ridge is relatively large at 3. In the present study, it is shown that a fully three-dimensional calculation agrees quite well with the experiments, both in terms of the qualitative form of the streamline patterns, and also with respect to a quantitative measure of streamline deflection. The equations are solved by means of Fourier series across the channel and Fourier integral in the streamwise direction. The resultant double Fourier series is truncated, and the partial sum computed by machine. The form of the solution is relatively easily extended to other obstacle shapes.
Stability of low-speed reacting flows
Authors: Mahalingam S. , Cantwell B. J. , Ferziger J. H.
Published: July 2002

Small perturbation, linear, inviscid stability theory is developed and applied to coflowing chemically reacting jets. A necessary condition for the existence of axisymmetric and azimuthal instabilities is derived. The eigenfunction structure provides considerable insight into the underlying physical mechanism. In nonpremixed cases, high temperature tends to occur in regions where the shear layer tries to roll up; this reduces the available momentum, suppressing the growth of instabilities. It is found that chemical reaction during the growth of instabilities is unimportant, suggesting that this effect could be ignored in a viscous theory. Both nonpremixed and premixed flames in axisymmetric shear layers are generally more stable to small perturbations than in the corresponding cold case. However, while for nonpremixed flames the frequency of the most amplified wave decreases with increasing heat release, the opposite occurs for premixed cases.
Effect of turnaround lines of magnetic flux in two-dimensional MHD channel flow under a traveling sine wave magnetic field
Authors: Ueno Kazuyuki
Published: July 2002

Laminar liquid metal flow fields between two parallel insulator walls under a traveling sine wave magnetic field are numerically solved for a small magnetic Reynolds number. When both average Hartmann number Ha and cubic root of interaction parameter N1/3 are much greater than dimensionless wavelength Λ, a set of turnaround lines of magnetic flux, which exist every half-wavelength, cause a set of recirculating flows. Consequently a nozzlelike converging and diverging flow is formed between them. The throat width of the nozzlelike flow is estimated to be the greater of N−1/4Λ3/4 or Ha−1/2Λ1/2 compared with the channel height.
Neutron sawtooth behavior in the PLT, DIII-D, and TFTR tokamaks
Authors: Lovberg J. A. , Heidbrink W. W. , Strachan J. D. , Zaveryaev V. S.
Published: July 2002

The effect of the sawtooth instability on the 2.5 MeV neutron emission in the PLT [Plasma Physics Controlled Nuclear Fusion Research, 1976 (IAEA, Vienna, 1977), Vol. I, p. 21], DIII-D [Plasma Physics Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 1, p. 159], and TFTR [Plasma Physics Controlled Nuclear Fusion Research, 1984 (IAEA, Vienna, 1985), Vol. I, p. 29] tokamaks is studied. In thermonuclear plasmas, the instability typically results in a 20% reduction in emission. The time evolution of the thermonuclear neutron signal suggests that the sawtooth crash consists of four phases. First, the electron density profile flattens rapidly (in approximately 30 μsec on PLT) but, in some cases, there is little associated change in neutron emission, suggesting that most reacting ions remain confined in the sawtooth region but do not completely mix. After the electron sawtooth, the ions continue to mix, resulting in an approximate 10% reduction in neutron emission in about 0.5 msec. The emission then decays more slowly during the final two phases. Thermalization of reacting ions on an approximate 3τii time scale accounts for only about 20% of the slow drop. Most of the slow drop seems to be caused by loss of ion energy from the mixing region (an ion heat pulse).
The tracking force on a relativistic electron beam in an Ohmic plasma channel
Authors: O’Brien K. J.
Published: July 2002

The linearized tracking force on an electron beam propagating slightly off axis in an Ohmic plasma channel is calculated. Electron inertial effects are included in Ohm’s law. The channel of conductivity (σ), density (ne), and radius (a) is surrounded by a region of ambient plasma with conductivity (σ0), density (ne0), and radius (b). Enclosing the system is a highly conducting metallic tank. The electromagnetic fields are calculated using the frozen-field approximation of the Maxwell field equations. The fields are expanded into monopole and dipole moments linearized in the small parameter (d/a), where (d) is the beam displacement. The electron beam is assumed to be a rigid cylinder of charge propagating at the speed of light through the channel and is not allowed to respond dynamically to the force.
Phase-space diffusion in turbulent plasmas: The random acceleration problem revisited
Authors: Pécseli H. L. , Trulsen J.
Published: July 2002

Phase-space diffusion of test particles in turbulent plasmas is studied by an approach based on a conditional statistical analysis of fluctuating electrostatic fields. Analytical relations between relevant conditional averages and higher-order correlations, 〈En(x0,t0) E(x0+x,t0+t)〉, and triple correlations, 〈En(x0,t0) E(x0+x,t0+t) E(x0+y,t0+s)〉, facilitate considerably the practical application of the resulting equations.
Fluid models for relativistic electron beams: An independent derivation
Authors: Pennisi S. , Anile A. M.
Published: July 2002

A generalized fluid model is proposed for a relativistic charged particle beam. The model is constructed by starting with the covariant Vlasov equation, taking the moments, and closing via a suitable ‘‘constitutive law’’ for the third-order moment. The ‘‘constitutive law’’ is uniquely specified by requiring consistency with a supplementary conservation law representing entropy balance. The derivation presented here is independent and also different from a previous one utilizing results obtained within the framework of relativistic extended thermodynamics.
Stability of Alfvén gap modes in burning plasmas
Authors: Betti R. , Freidberg J. P.
Published: July 2002

A stability analysis is carried out for energetic particle-Alfvén gap modes. Three modes have been identified: the toroidicity, ellipticity, and noncircular triangularity induced Alfvén eigenmodes (TAE, EAE, and NAE). In highly elongated plasma cross sections with κ−1∼1, the EAE may be a more robust mode than the TAE and NAE. It is found that electron Landau damping in highly elongated plasmas has a strong stabilizing influence on the n=1 EAE, while ion Landau damping stabilizes the n=1 TAE in high-density regimes. Furthermore, the NAE turns out to be stable for all currently proposed ignition experiments. The stability analysis of a typical burning plasma device, Burning Plasma Experiment (BPX) [Phys. Scr. T16, 89 (1987)] shows that n>1 gap modes can pose a serious threat to the achievement of ignition conditions.
Landau fluid equations for electromagnetic and electrostatic fluctuations
Authors: Hedrick C. L. , Leboeuf J.-N.
Published: July 2002

Closure relations are developed to allow approximate treatment of Landau damping and growth using fluid equations for both electrostatic and electromagnetic modes. The coefficients in these closure relations are related to approximations of the plasma dispersion function by ratios of polynomials. Thirteen different numerical sets of coefficients are given and explicitly related to previous fits to the plasma dispersion function. The application of the techniques presented in this paper is illustrated with the specific example of resistive g modes. Comparisons of full kinetic and approximate results are made for the solutions to the dispersion relation, radially resolved modes in sheared magnetic geometry, and the plasma dispersion function itself.
A gyrokinetic calculation of transmission and reflection of the fast wave in the ion cyclotron range of frequencies
Authors: Lashmore-Davies C. N. , Fuchs V. , Dendy R. O.
Published: July 2002

A full-wave equation has been obtained from the gyrokinetic theory for the fast wave traversing a minority cyclotron resonance [Phys. Fluids B 4, 493 (1992)] with the aid of the fast wave approximation [Phys. Fluids 31, 1614 (1988)]. This theory describes the transmission, reflection, and absorption of the fast wave for arbitrary values of the parallel wave number. For oblique propagation the absorption is due to both ion cyclotron damping by minority ions and mode conversion to the ion Bernstein wave. The results for a 3He minority in a D plasma indicate that for perpendicular propagation and minority temperatures of a few keV the power lost by the fast wave is all mode converted whereas for minority temperatures ∼100 keV∼30% of the incident power is dissipated by the minority ions due to the gyrokinetic correction. The gyrokinetic correction also results in a significant reduction in the reflection coefficient for low field side incidence when kzLB≲1 and the minority and hybrid resonances overlap.
Nonlinear electrical conductivity in hydrogen plasma
Authors: Stobbe Michael , Redmer Ronald , Könies Axel
Published: July 2002

Employing the Zubarev method for the derivation of a nonequilibrium statistical operator, Vlasov–Uehling–Uhlenbeck quantum kinetic equations are derived for the distribution functions of clusters with arbitrary mass number. Considering a partially ionized hydrogen plasma, the electronic distribution function is determined numerically for the homogeneous and stationary case. Restricting to elastic scattering processes between the electrons, ions and atoms, the analytical solution of Schenter and Liboff [Phys. Fluids 30, 1789 (1987)] is reproduced. Furthermore, the influence of inelastic collisions such as ionization is investigated. Using the electronic distribution function, the electrical conductivity is calculated for hydrogen plasma of a given composition at arbitrary field strengths. The nonlinear behavior obtained is dependent on the different expressions applied for the electron–atom and the electron–ion cross sections. The effect of inelastic collisions is, in general, a lowering of the conductivity because such processes lead to a limitation of the energy gain of electrons in an electric field.
Relativistic excitation of plasma waves at the difference frequency of two laser beams in a plasma
Authors: Salimullah Md. , Liu Y. G.
Published: July 2002

A nonlinear relativistic excitation of longitudinal electrostatic electron plasma wave at the beat frequency of two laser beams has been studied in a hot, collisionless, and homogeneous plasma. The nonlinearity has been taken through the ponderomotive force on electrons in the relativistic Vlasov equation. It is observed that the power conversion efficiency of the excited electron plasma wave is much higher for the relativistic calculation than that for the nonrelativistic calculations.
Electron-cyclotron heating of a tokamak reactor at down-shifted frequencies
Authors: Fidone I. , Giruzzi G. , Mazzucato E.
Published: July 2002

The absorption of electron cyclotron waves in a hot, dense tokamak plasma is investigated for the case of the extraordinary mode for outside launching. It is shown that, for electron temperatures Te≥5 keV, strong absorption occurs for oblique propagation at frequencies significantly below the electron gyrofrequency at the plasma center. A new density dependence of the wave absorption is found which is more favorable for plasma heating than the familiar n−1e scaling.
An unsteady momentum integral calculation for the turbulent spot
Authors: Mautner T. S. , Van Atta C. W.
Published: July 2002

Velocity and wall shear stress measurements were made along the centerline of turbulent spots generated in a laminar boundary layer on a flat plate. A momentum integral calculation method based on two-dimensional boundary layer theory and the assumption of potential flow above the spot was developed by transforming the unsteady momentum equation into nondimensional similarity coordinates. The centerline wall shear stress, calculated using ensemble-averaged data for the velocity field, shows fairly good agreement with the measured values.
Nonlinear circularly polarized electromagnetic waves in a warm magnetoplasma
Authors: Shukla P. K. , Rao N. N. , Anderson D. , Lisak M. , Wilhelmsson H.
Published: July 2002

This paper considers the nonlinear interaction of a background warm plasma with intense, circularly polarized radiation along an external magnetic field. It is shown that the couplings of the adiabatic particle as well as the hydrodynamic plasma motions with the radiation field can lead to localized wave-envelope packets. The resulting self-consistent equilibria consist of finite-amplitude electron density perturbations which are nonlinearly driven by electromagnetic fields.
Steady-state discharges
Authors: Miller Guthrie
Published: July 2002

The properties of steady-state discharges with stochastic magnetic fields are considered using a fluid model. Magnetic field stochasticity provides a simple explanation of the phenomenon of relaxation of a discharge, defined by J. B. Taylor.
Steady detonation waves with losses
Authors: Huerta Manuel A.
Published: July 2002

The Euler equations for a reacting polytropic gas with model losses, applied to unsupported steady detonation waves, lead to fast and slow detonation speeds for a given loss. The reaction rate is taken to have the Arrhenius form. The differential equations for the Mach number squared m and the reaction progress variable λ are integrated numerically using a fourth-order Runge–Kutta–Fehlberg method. The calculations are presented as plots of trajectories in the λ-log (m) plane. The main parameters that are varied are the activation temperature, the heat of reaction, the order of the reaction, and the polytropic exponent γ. The onset of detonation failure, and a continuum of solutions that appears in some parameter ranges, are explored in detail. The results are summarized in plots of the detonation Mach number squared m0 versus the logarithm of the loss parameter.
Pinch dynamics in thin foil relativistic electron beam diodes
Authors: Nardi E. , Zinamon Z.
Published: July 2002

Time-resolved Kα emission from targets struck by relativistic electrons is used as a diagnostic of the electron flow in pinch relativistic electron beam diodes. Pinch dynamics in thin foil anode diodes are studied. A transition from a regime of reflexing pinch to a regime of flow dominated by ion emission from the anode is observed. Current fluctuations in the pinch region are also detected.
Two-dimensional turbulent boundary layers over rigid and moving swept wavy surfaces
Authors: Lekoudis S. G. , Sengupta T. K.
Published: July 2002

The two-dimensional, incompressible, turbulent boundary layer over a flexible wall is analyzed. The wall consists of sinusoidal waves that are swept with respect to the flow direction. The mean flow is a boundary layer with wave-induced stresses. These stresses are evaluated from the solution of the linear problem. It is found that the small mean skin friction reduction, observed for the case of zero sweep, persists. The pressure drag reduces for swept wavy wall from its value for no sweep. There is a small drag reduction for the moving wall cases even for the lowest phase speed considered. Within the assumptions of the turbulence model used, a possible drag reduction mechanism is discussed.
Relaxation of toroidal plasmas with finite pressure
Authors: Bhattacharjee A. , Kwok Y-C.
Published: July 2002

Fast relaxation of toroidal plasmas with finite pressure is investigated, using an energy principle with global invariants. Relaxed states, which are not generally force-free and are stable according to Mercier’s criterion, are constructed by minimizing the potential energy of the plasma subject to a small set of global invariants. Application of the method to stellarators, in which perfect flux surfaces do not usually exist, is suggested.
The electron/electron acoustic instability
Authors: Gary S. Peter
Published: July 2002

The electron acoustic wave becomes a normal mode of an unmagnetized collisionless plasma in the presence of two electron components with similar densities, but strongly disparate temperatures. The characteristic frequency of this mode is the plasma frequency of the cooler electron component. If these two electron components have a relative drift speed several times the thermal speed of the cooler component, the electron/electron acoustic instability may arise. This paper describes the parametric dependences of the threshold drift speed and maximum growth rate of this instability, and compares these with the same properties of the electron/ion acoustic instability. Under the condition of zero current, the electron/ion acoustic instability typically has the lower threshold drift speed, so that observation of the electron/electron acoustic instability is a strong indication of the presence of an electrical current in the plasma.
Relation between the Kolmogorov and Batchelor constants
Authors: Yakhot Victor , Orszag Steven A.
Published: July 2002

It is shown that the renormalization group theory of turbulence leads to the relation Ba=CKPt between the turbulent Prandtl number Pt, the Kolmogorov constant CK, and the Batchelor constant Ba.
The second-harmonic layer of an obliquely propagating extraordinary wave in a tokamak
Authors: Matsuda Kyoko
Published: July 2002

Differences in dispersion curves around the second-harmonic electron-cyclotron frequency obtained for a complex k⊥ and for a complex ω are investigated for an obliquely propagating extraordinary wave. The differences found in this study indicate the need for a full wave theory to predict the angular dependence of heating efficiency.
Two-fluid Chapman–Enskog theory for binary gas mixtures
Authors: Fernandez de la Mora J. , Fernandez-Feria R.
Published: July 2002

As a result of the increasing inefficiency in the transfer of energy in collisions between species with a decreasing ratio of molecular masses, the Knudsen number range of validity of the Chapman–Enskog (CE) theory for binary gas mixtures decreases linearly with the molecular mass ratio. To remedy the situation, a two-fluid CE theory uniformly valid in the molecular mass ratio is constructed here. The analysis extends previous two-fluid theories to allow for arbitrary potentials of intermolecular interaction and arbitrary mass ratios. The treatment differs from the CE formulation in that the mean velocities and temperatures of the two gases are not required to be identical to lowest order. To first order, the streaming terms of the Boltzmann equation are thus computed in terms of the derivatives of the two-fluid hydrodynamic quantities, rather than the mean mixture properties as in the CE theory. As a result, associated with the nonconservation of momentum and energy for each species alone, two new ‘‘driving forces’’ appear in the first-order integral equations. The amount of momentum and energy transferred per unit time between the species appear in the theory as free constants, which allow satisfying the constraint that all hydrodynamic information be contained within the lowest-order two-fluid Maxwellians. Simultaneously, this constraint fixes the rate of momentum and energy interchange in terms of the two-fluid hydrodynamic quantities and their gradients. The driving force d12 of the CE theory is directly related to the rate of interspecies momentum transfer, and the corresponding CE functions D1 and D2 appear here unmodified.

But the physical interpretation of d12 is very different in the two pictures. On the CE side there is only one momentum equation, while d12 provides constitutive information fixing the diffusion flux (velocity differences) in the mass conservation equation. Here, the similar constitutive information associated to d12 is used to couple two different momentum equations. Although the CE theory captures some of the two-velocity aspects of the problem, no CE analog exists with the functions E1 and E2 associated here with temperature differences, which now require solving new integral equations. Finally, the presence of two velocities and two temperatures leads to four coefficients of viscosity and of thermal conductivity for the two stress tensors and heat flux vectors. Also, two thermal diffusion factors enter now into the expression for d12. Although all these new coefficients arise as portions of the overall CE transport coefficients, their independent optimal determination requires new developments. The corresponding variational formulation is presented here and used to first order to obtain explicit expressions for all two-fluid transport coefficients by means of Sonine polynomials as trial functions.
Numerical studies of minority ion heating current drive
Authors: Wong K. L. , Ishihara O. , Gahl J. , Hagler M. , Kristiansen M.
Published: July 2002

Current generation by asymmetrical heating of minority ions without the injection of net momentum into a plasma is studied numerically by solving the Fokker–Planck equation in which the quasilinear diffusion term for cyclotron absorption is added to the Coulomb collision terms. The results show that current drive efficiency is better in a H–D plasma than in a 3He–D plasma. The applicability of asymmetrical minority ion heating current drive in a hydrogen tokamak plasma with the existence of impurity ions is also investigated. The practicality of this current drive process for fusion reactors is discussed.
Electron temperature and ionization degree dependence of the electrical conductivity in diatomic gases
Authors: Erwin Daniel A. , Kunc Joseph A.
Published: July 2002

An efficient method for determination of the electrical conductivity of plasmas generated from diatomic gases is presented. The approach is the same as that used recently by the authors [Phys. Fluids 28, 3349 (1985)] for monatomic gases; the method is generalized here to take into account molecular dissociation and the consequent presence of several neutral species in the plasma. Examples of calculations are given in the form of plots of the dc conductivity of H2 and O2 gases in the range of electron temperature 300≤Te≤30 000 K and of ionization degree 10−8≤x≤1 with no applied magnetic field, using the thermochemical approach for calculation of the dissociation degree.
The effects of capillarity on the contraction coefficient of a jet emanating from a slot
Authors: Ackerberg R. C. , Liu Ta-Jo
Published: July 2002

This study is concerned with the effects of surface tension on the steady potential flow of a liquid issuing from a two-dimensional slot with inclined walls. The mathematical problem, which is characterized by a nonlinear boundary condition along the free streamline, is solved using a novel numerical technique based on the fast direct Poisson solver. Numerical solutions were obtained for various Weber numbers and slot angles, and the results are compared with the free streamline shapes and contraction coefficients of the corresponding classical potential solutions without surface tension, and with some available theoretical results. When the numerical grid size is reduced or when the Weber number is sufficiently small, undesirable standing waves appear on the free surface that violate a causality condition. The cause of these waves was examined. A criterion was found for the occurrence of the waves.
Magnetic field behavior beyond the laser spot
Authors: Goldman S. R. , Schmalz R. F.
Published: July 2002

A self-consistent, analytic solution for the two-dimensional, rotationally symmetric, time varying problem of the interaction of a laser plasma with its self-generated magnetic field has been determined at lateral locations away from the laser spot. The plasma is described by a two-fluid model, with the magnitude of the electron velocity much greater than the ion velocity. Increased spatial gradients in the density and velocity around the magnetic field maximum have been found. This leads to a double humped ion-velocity spectrum, which could be interpreted in terms of a two-temperature electron distribution, in qualitative agreement with experiment.
Why are these disks dark? The optics of Rankine vortices
Authors: Sterling M. H. , Gorman M. , Widmann P. J. , Coffman S. C. , Strozier J. , Kiehn R. M.
Published: July 2002

Under uniform illumination from above, a Rankine-like vortex, shed from partially submerged objects moving through a tank of water, produces an image, at any depth, which is a dark disk surrounded by a bright ring. The observed features of this image can be explained by applying Snell’s law to a Rankine-like surface with a parabolic core and an outer region of negative curvature. The dark disk is caused by a parabolic shape at the center of the vortex and the bright ring is caused by the focusing of rays by the surface of negative curvature at larger r. The analysis of the image can provide qualitative and quantitative information about the shape of the vortex at the surface of the water.
A new first-harmonic wave in an incompressible two-core vortex
Authors: Dodgson N.
Published: July 2002

The two-dimensional oscillations of an incompressible two-core vortex are reexamined with particular reference to the first harmonic. A new, nontrivial, analytic solution, representing a stable disturbance propagating entirely within the cores of the vortex, is constructed for the first harmonic.
Rarefaction shock in the near wake
Authors: Diebold D. , Hershkowitz N. , Eliezer S.
Published: July 2002

Laboratory experiments and fluid theory find a stationary rarefaction shock in the near wake of an electrically grounded obstacle placed in a steady state, supersonic plasma flow. The shock is only found when two electron temperatures, differing by at least an order of magnitude, are present. These shocks are analogous to rarefaction shocks in plasma free expansions.
Volterra’s series solutions of free boundary plasma equilibria
Authors: De Menna L. , Miano G. , Rubinacci G.
Published: July 2002

In this paper the nonlinear problem of the free boundary equilibrium of a plasma inside a conducting circular shell has been solved analytically in the high beta limit. The unknown function describing the plasma shape has been expanded in a Volterra functional series, the functional analogous to the Taylor series. The hierarchy of the linear integral equations obtained from the expansion is, at least in principle, analytically solvable, so that the solution of each equation can be given in a closed form. The analytical computations have been carried out up to the fourth order and the results compared with numerical computations.
The effect of a moving sink on selective withdrawal from a discretely stratified fluid
Authors: Munson B. R. , Ingber M. S. , Razifard Peyman
Published: July 2002

The effect of a moving sink on the selective withdrawal process from a discretely stratified fluid is studied experimentally. The critical, draw-down flow rate can be increased significantly by employing a moving sink. The distortion of the interface between the two layers caused by the moving sink gives rise to a secondary effect, which is dependent on whether the sink speed is less than or greater than the interfacial wave speed.
Helicity injection with moving vacuum–plasma boundary with arbitrary flux surfaces
Authors: Bellan P. M.
Published: July 2002

If a toroidal plasma has arbitrary nested magnetic flux surfaces and a moving plasma–vacuum interface, then any helicity injected by modulating the magnetic fields is simply consumed by an increase in helicity dissipation due to the modulated fields.
Electron beam equilibria with self-fields for a free electron laser with a planar wiggler
Authors: Weitzner Harold , Fruchtman Amnon
Published: July 2002

A general formalism for non-neutral cold relativistic planar steady flows is developed and applied to the study of the equilibrium of a sheet electron beam in a planar wiggler free electron laser. The full transverse dependence of the wiggler field as well as the equilibrium self-fields of the beam are included. In particular, the betatron oscillations in the presence of self-fields are studied. For a thick beam equilibrium with a particular density profile it is shown that the betatron oscillations are eliminated. For a thin beam configuration the paraxial approximation is employed and it is also shown that for some critical density there are no betatron oscillations. If the density is larger than this critical density the beam oscillates with the betatron frequency but there are no trajectory crossings and the beam preserves its cold fluid nature. The single-particle equations of motion are also considered in the presence of both planar wiggler and planar self-fields. It is shown that in some cases the particles oscillate with a reduced betatron frequency, in contrast to the previous case of cold fluid motion where the self-fields do not change the betatron frequency. For the study of the betatron oscillations in the thick beam equilibrium a two-space scale method is employed. For the thin beam within the paraxial approximation the Floquet theory for equations with periodic coefficients is used.
Analysis of the interaction terms of the motion of a shock wave due to an intense explosion
Authors: Yousaf M.
Published: July 2002

In this paper various interactions caused by different types of disturbances behind the shock are examined. The similarity solution of Sedov [Prikl. Mat. Mekh. 10, 241 (1946)] is used to find the five interaction terms at all points of the flow.
Stability of swirling gas flows
Authors: Hultgren Lennart S.
Published: July 2002

The stability of inviscid swirling gas flows to small nonaxisymmetric perturbations is considered. For small Brunt–Väisälä frequencies, the problem reduces to the classical Sturm–Liouville form and the oscillation theorem can be applied. The resulting necessary and sufficient stability condition is compared to various criteria in the literature and a limited numerical study of isothermal rigidly rotating Poiseuille flow. For given azimuthal and axial wavenumbers, it is found numerically that the higher inertial modes become unstable for successively lower Rossby numbers and that this sequence of critical values approaches the theoretical value from above.
Dispersion relations for the lower hybrid frequency range
Authors: Cho Suwon , Swanson D. G.
Published: July 2002

The hot plasma electrostatic dispersion relation for the lower hybrid frequency range has been cast into a form without any sums using the method of steepest descents. This new form of the dispersion relation with the exact resonance term, which is valid for general complex wavenumber and each term of which is identified according to its role of representing physical waves, is shown to be accurate and to be reducible to an expression obtained by Brambilla [Plasma Phys. 18, 699 (1976)] when some approximations are taken. A very simple dispersion relation is also obtained without singular terms near the high ion cyclotron harmonics that are encountered by lower hybrid waves propagating in inhomogeneous magnetic fields. Finally, the damping rate in space is numerically calculated using the equation derived and compared with the result from the unmagnetized ion dispersion relation.
Dynamics of nonlinearly excited plasma waves
Authors: Miano Giovanni
Published: July 2002

In an underdense plasma a large-amplitude plasma oscillation may be produced by the beating of two electromagnetic waves with a frequency difference approximately equal to the plasma frequency. In the spatially one-dimensional, cold, and collisionless plasma the large-amplitude plasma oscillation is limited by the nonlinearity caused by relativistic effects. In this paper a simple nonlinear equation, resembling the original equation of Rosenbluth and Liu [Phys. Rev. Lett. 29, 701 (1972)], is derived in the weak beat power limit from the fully relativistic fluid model proposed by Sprangle, Sudan, and Tang [Appl. Phys. Lett. 45, 375 (1984); Phys. Fluids 28, 1974 (1985)]. This equation also contains the effects of the relativistic transverse motion. Its analytical solution, describing the plasma oscillation dynamics, is given in a closed form by using Jacobian elliptic functions. The analytical computations are compared with numerical computations. Finally the fully relativistic equation, describing free plasma oscillations, is studied analytically.
Ion front and ablation in a freely expanding two-ion, two-temperature, noncollisional plasma
Authors: Srivastava M. K. , Sinha B. K. , Lawande S. V.
Published: July 2002

The equations for the free expansion of a two-ion, two-temperature, noncollisional, isothermal, and quasineutral plasma are derived and solved using a self-similar technique. The hydrodynamic variables are determined by four independent parameters, viz., α, the relative charge to mass ratio of the two ions; N10, the initial concentration of the main ion; τ, the temperature ratio of the hot and cold electrons; and ρ, the initial concentration ratio of the cold and hot electrons. The formation of an ion front in the numerically dominant species is predicted for the first time even under the quasineutral and isothermal assumptions. A necessary condition for the breakdown of the self-similar solution is derived. The location of the ion front and the point where the solution breaks down are tabulated as a function of the four parameters and the results are discussed. The expression for the normalized ion current, taking into account the flux limitation under the free-streaming limit, is derived. The expressions for the mass, momentum, and energy ablation are also derived and the relevant data are discussed with reference to laser–plasma interaction.
Relativistic solitary-wave solutions of the beat-wave equations
Authors: McKinstrie C. J.
Published: July 2002

In the beat-wave accelerator [Phys. Rev. Lett. 43, 267 (1979)], a large-amplitude Langmuir wave is produced by the beating of two laser beams whose frequencies differ by approximately the plasma frequency. The relativistic equations governing this three-wave interaction are shown to admit two types of solitary-wave solutions. Temporal solitary waves propagate at speeds greater than the speed of light and carry no information. Spatial solitary waves propagate at speeds less than the speed of light and do carry information. Analytic expressions are obtained for the envelopes of these waves and for the relationship between their speed and maximum amplitude. For the limit in which the propagation speed of the solitary wave is equal to the speed of light, there is no distinction between a temporal solitary wave and a spatial solitary wave. However, it can be shown that the solitary wave is unstable in this limit. The potential of the spatial solitary waves for particle acceleration [Phys. Rev. Lett. 57, 1421 (1986)] is studied. Although the spatial solitary waves are capable of accelerating particles to high energy, for typical beat-wave parameters the laser–plasma coupling efficiency is too low for this scheme to be practical.
Free-electron laser–electrostatic electron cyclotron resonance maser hybrids
Authors: Shenggang Liu , Zhengbiao Ouyang
Published: July 2002

The free-electron laser (FEL)–electrostatic electron cyclotron resonance maser (EECRM) hybrids, a new kind of FEL comprised of a relativistic electron beam and an electrostatic centrifugal field, are presented and studied in this paper. In a fluid dynamical linear theory, the dispersion equation for transverse electromagnetic (TEM) mode operation is derived, analyzed, and numerically solved. It is found both analytically and numerically that FEL instability occurs near beam resonance [(ωl−klv∥)2−ω2p (γ−2∥ −β2∥β2⊥φ̇20/ Ω2l)=0]. However, FEL instability near cyclotron resonance (ωs−ksv∥ =±(2−β2⊥)1/2φ̇0) is not found in the TEM mode analysis.
Entropy gradient-driven electron inertial instabilities
Authors: Amendt Peter , Rahman H. U. , Strauss M.
Published: July 2002

In the vicinity of a density transition region or surface, electron temperature fluctuations are known to drive low frequency electromagnetic surface waves which may be connected with the self-generation of magnetic fields in the laser–plasma interaction. Two methods of destabilization are considered. In the first approach an accelerating plasma surface is considered and this leads to instability only when an accompanying temperature gradient scale length is at most as large as the density gradient scale length. In the second approach the introduction of current is also found to cause instability near the plasma surface provided an identical temperature gradient threshold is exceeded. In both cases the growth rates for each instability are enhanced by the square root of the ion to electron mass ratio over the associated growth rates for ion-inertial-type instability.
The stability of a two-dimensional rising bubble
Authors: Nie Qing , Tanveer Saleh
Published: July 2002

The stability of an inviscid two-dimensional bubble subject to two-dimensional disturbances is considered and the bubbles are found to be linearly stable for all Weber numbers, for which a steady solution is known. Certain aspects of the nonlinear initial value problem are also studied. An initial condition that consists of a superposition of a suitable symmetric eigenmode (of the linear stability operator) on a steady state is found to result in pinching of the bubble neck as it tends to oscillate about the steady state. An estimate of the threshold amplitude of such a disturbance needed to cause breakup of a large aspect ratio bubble is obtained. The presence of gravity appears to inhibit this pinching process.
Particle behavior in the turbulent boundary layer. II. Velocity and distribution profiles
Authors: Kaftori D. , Hetsroni G. , Banerjee S.
Published: July 2002

The velocity, flux, and concentration distribution of solid particles in a turbulent boundary layer of a horizontal water flume were investigated experimentally by means of LDA and visualization techniques. The particles were of polystyrene (specific density ∼1.05). Results show that coherent wall structures are responsible for most of the characteristics of particle behavior throughout the boundary layer. Particles are often concentrated in regions of low velocity, associated with wall structures, and as a result the average particle velocity is lower than the fluid’s. This was also noted previously by Rashidi et al., but not explained. The actual relative velocity between the particles and the surrounding fluid is often small. In addition, the data suggest that as the shear rate increases, the particle flux profiles asymptotically approach a shape where a strong gradient of particle flux exists in the lower part of the boundary layer (y+≤250), while it is relatively constant at higher elevations. This phenomenon may also be attributable to interactions with the wall structures.
Effect of concentrated wall suction on a turbulent boundary layer
Authors: Antonia R. A. , Zhu Y. , Sokolov M.
Published: July 2002

The effect of suction, applied through a short porous wall strip, on a low Reynolds number self-preserving turbulent boundary layer has been quantified by measuring the local wall shear stress and the main Reynolds stresses downstream of the strip. When the suction rate is sufficiently high, pseudo-relaminarization occurs almost immediately downstream of the strip. Farther downstream, transition occurs followed by a slow return to a fully turbulent self-preserving state. During relaminarization, the measured skin friction coefficient cf falls below the level corresponding to the no suction value, reaching a minimum where transition begins. An empirical cf distribution is proposed that groups together results obtained at different streamwise stations and different suction rates. Of all the measured Reynolds stresses, the longitudinal turbulence intensity recovers relatively quickly from the change in boundary conditions while the wall-normal turbulence intensity and the Reynolds shear stress are significantly affected by the suction. The Reynolds shear stress, which is negligible during relaminarization, has the slowest recovery.
Reynolds stress analysis of EMHD-controlled wall turbulence. Part I. Streamwise forcing
Authors: Crawford Catherine H. , Karniadakis George Em
Published: July 2002

In this work we investigate numerically turbulent flow of low electrical conductivity fluid subject to electro-magnetic (EMHD) forcing. The configuration is similar to the one considered in the experimental work of Henoch and Stace [Phys. Fluids 7, 1371 (1995)] but in a channel geometry. The lower wall of the channel is covered with alternating streamwise electrodes and magnets to create a Lorentz force in the positive streamwise direction. Two cases are considered in detail corresponding to interaction parameter values of 0.4 (case 1) and 0.1 (case 2). The effect of switching off and on the electrodes is also studied for the two cases. At the Reynolds number considered (Reτ≈200), a drag increase was obtained for all cases, in agreement with the experiments of Henoch and Stace. A Reynolds stress analysis was performed based on a new decomposition of the gradients normal to the wall of the Reynolds stress −u′v′¯. It was found that the vortex stretching term w′w2′¯ and the spanwise variation of the stress component u′w′¯ are responsible for the drag increase. More specifically, the term ∂(u′w′)¯/∂x3 is associated with secondary vortical motions in the near-wall and becomes large and positive for large shear stress in regions where fluid is moving toward the wall. In contrast, negative values are associated with regions of lower shear where fluid is being lifted away from the wall. Unlike the unperturbed flow, in the controlled flow high speed near-wall streamwise jets are present (case 1) even in the time-averaged fields. Other changes in turbulence structure are quantified using streak spacing, vortex lines, vorticity quadrant analysis, and plots of the rms value of the vorticity angle.
A new dynamic two-parameter mixed model for large-eddy simulation
Authors: Horiuti Kiyosi
Published: July 2002

A new dynamic subgrid-scale (SGS) mixed model is proposed for large-eddy simulation of turbulent flows. This model is based on the decomposition of the SGS stress terms into the modified Leonard, modified cross and modified SGS Reynolds stress terms. In this model, the modified Leonard term is computed explicitly. The modified cross term and modified SGS Reynolds stress are assumed to be proportional to a new term, the form of which is comparable to the generalized central moment, derived as an extension of the filtered Bardina model proposed by Horiuti [J. Phys. Soc. Jpn. 66, 91 (1997)]. Using a linear combination of this new term with the Smagorinsky model for the modified SGS Reynolds stress, the proposed model contains two model parameters, which are computed dynamically. Two formulations for the test-filtered SGS stress reported by Zang et al. [Phys. Fluids A 5, 3186 (1993)] and Vreman et al. [Phys. Fluids 6, 4057 (1994)] are compared, and the compatibility of the SGS models with the standard dynamic SGS model procedure is discussed. The proposed model is assessed for incompressible channel and mixing layer flows, in comparison with the dynamic Smagorinsky model of Germano et al. [Phys. Fluids A 3, 1760 (1991)], the dynamic mixed model of Zang et al. and the dynamic two-parameter mixed model of Salvetti and Banerjee [Phys. Fluids 7, 2831 (1995)]. In the “a priori” test, the proposed model gave the closest agreement with the modified cross term as well as the modified SGS Reynolds stress term. It is shown that the proposed term represents a more general model of the SGS stress than the modified Leonard term and yields a more accurate approximation. These SGS models are tested further in actual LES of channel and mixing layer flows (“a posteriori” test). The results were consistent with those of the “a priori” tests; the proposed model yielded the most accurate results. In the proposed model, the SGS quantities were predominantly represented using the new term, and the contribution of the Smagorinsky model was minimal. The two parameters contained in the model were determined locally in space on a point-by-point basis.
Behavior of naturally unstable and periodically forced axisymmetric buoyant plumes of helium and helium–air mixtures
Authors: Cetegen Baki M.
Published: July 2002

Effects of forcing on the behavior of naturally unstable buoyant plumes of helium and helium/air mixtures are investigated experimentally. Axisymmetric buoyant plumes originating from a 10 cm diam nozzle are perturbed in a periodic manner by an upstream loudspeaker introducing a sinusoidal streamwise velocity oscillation with a peak-to-peak velocity magnitude of about 3% of the mean velocity at the nozzle exit. The experimental conditions in this study corresponded to Richardson number, Ri=[(ρ∞−ρp)gd]/ρ∞V02, of 42. Video images of the naturally unstable plumes as well as the forced plumes were analyzed to study the features of the resulting flow oscillations and the vortical structures. It was found that the plume responds readily to the imposed oscillations with the toroidal vortices forming at the forcing frequency. Plume images indicate a more complicated and turbulent state of the flow within the large-scale toroidal vortices as they convect downstream. These vortical structures have lateral dimensions of the order of the nozzle diameter at low oscillation frequencies, but the vortex length scales become smaller at higher forcing frequencies. Additionally, mushroom-shaped smaller-scale vortex pairs appear in the very early part of the forced plumes that are absent in the unforced case. Frequency spectrum of the plume centerline velocity fluctuations, detected by two total pressure probes located at one-half and two diameter heights above the nozzle exit, show that the plume predominantly responds to the imposed flow excitation. However, as the flow evolves downstream, frequency spectra exhibit a broader range of frequencies some of which do not coincide with the imposed one and its harmonics. The streamwise convection velocities of the large-scale vortical structures are not influenced by the imposed oscillation frequency. As the excitation frequency approaches that of the natural frequency of the flow, a more chaotic behavior of the large-scale vortices is observed in terms of their convection velocity. This is in contrast to the behavior of momentum dominated shear flows (jets and mixing layers), where a large degree of spatial and temporal coherence of the flow is attained when the flow is excited at its natural frequency. To complement these experiments and probe into the nature of the axisymmetric plume instabilities, we also visualized the transitional behavior of these buoyant plumes from their initially non-oscillatory state to the final periodic state. This is important as far as identifying the nature of the instability as either absolute or convective. Our findings show that the experiments exhibit features that point toward a convective-type instability.