bluuebunny/crossref_metadata_2025 huggingface dataset:
Some New Variational Properties of Hydromagnetic Equilibria
Authors: Grad Harold
Published: December 2004

Earlier variational formulations of problems in hydromagnetic equilibrium are extended by partially relaxing some of the boundary conditions. The resulting natural boundary conditions reflect situations of physical interest. Part of the analysis is done in the large and yields a simple intuitive condition which must be satisfied by an equilibrium which is free to move at its ends (the magnetic lines are not ``tied''). Applications are made to stability theory.
Calculations on Shock Tubes with Current Crowbarring
Authors: Bakken J. A.
Published: December 2004

Current crowbarring in electromagnetic shock tubes is analysed in terms of the snowplough model. The analysis has a bearing upon the crowbar action of secondary discharges which may give rise to decoupled current loops. Expressions relating current sheet position to time, sheet speed and current are given in nondimensional form. It is shown that the speed may continue to increase after crowbarring.
Steady Detonations in Idealized Two-Reaction Systems
Authors: Erpenbeck Jerome J.
Published: December 2004

The investigation of idealized detonations in a two-reaction A⇄B, A⇄C medium is extended to consideration of the one-dimensional piston problem for cases in which strictly steady flows are not possible. On easing the steady-state assumption so as to allow two (steady) waves of different velocity, the weak detonation can no longer be excluded on the basis of ``structural stability''. For the parallel-reaction case in which both reactions are exothermic but the first is at least twice as energetic as the second, the weak detonation solution is proposed when the second reaction is sufficiently slow. Slight overdrive is shown to yield two-wave detonations involving either a diffuse shock or a second detonation wave following the weak detonation. At sufficient overdrive the two waves merge into a strong detonation. The regimes of relative rate and piston velocity appropriate to the various situations are given numerically for several values of the other parameters.
Orbit Stability in a Helically Perturbed Magnetic Field
Authors: Lidsky L. M.
Published: December 2004

The stability of particle orbits in resonant helices (``corkscrews'') is investigated for axial acceleration and deceleration of the injected particles. It is shown that deviations from the unperturbed orbit lead to growing oscillations about that orbit for a decelerating corkscrew. The growth rate is small in devices suitable for injection into mirror fields. The numerical results are in agreement with experimental observations of injection parameters and output properties. Deviations from the equilibrium orbit in an accelerating corkscrew lead to damped oscillations.
Influence of Gyration Radius and Collisions on Hydromagnetic Stability
Authors: Coppi Bruno
Published: December 2004

The stability of low-pressure equilibrium configurations is analyzed considering regimes where the effects of ion Larmor radius and collisions are important. The theory is based on the asymptotic solution of moment equations derived from the Fokker-Planck equation. Purely growing interchange modes are found with growth rates proportional to η∥/a2, where η∥ is the longitudinal electrical resistivity and a the ion gyro-radius. The class of modes of this type which are not localized and a new class, of electrostatic and localized modes, are shown to have the same growth rates. Nonlocalized tearing-type modes are found to be overstable and have growth rates proportional to η∥/a⅔. When the ion gyro-radius effects are relatively large, the presence of a resistivity gradient in the equilibrium is shown not to modify tearing modes nor introduce new localized modes because of thermal conductivity along the magnetic field. For all the instabilities mentioned the width of the region where collisional effects are important is dependent on the ion gyro-radius but not on transport coefficients.
Multiple Instability in a Plasma-Beam System
Authors: Neufeld Jacob , Wright Harvel
Published: December 2004

A ``multiple instability'' in a plasma-beam system (when several waves having different frequencies and aligned in the same direction are simultaneously excited) is investigated for a system in which a cold and uniform ionized medium immersed in a static magnetic field B0 interacts with a beam of protons aligned in the direction of B0. The excited waves are transverse and move in the direction parallel to B0. The multiple instability occurs when the velocity of the proton beam is within a definite velocity range. This range is defined in terms of the parameters of the plasma-beam system. There are three transverse waves excited by the beam within the range of multiple instability. If ω′, ω″, and ω‴ are, respectively, the frequencies of these waves satisfying the inequality ω′ > ω″ > ω‴, then the corresponding rates of growth |Im δ′|, |Im δ″|, and |Im δ‴| for these waves satisfy the inequality |Imδ′| < |Im δ″| < |Im δ‴|. Thus, the wave having lowest frequency has the largest rate of growth and represents the dominant instability. Using the magnetodynamic approximation, it is shown that |Im δ″| increases with the velocity v of the proton beam until it reaches a maximum for v ≈ 0.7c (where c designates the velocity of light) and then decreases for v > 0.7c. Various relationships between the quantities which define a multiple instability and the parameters which describe plasma-beam systems are investigated.
Contribution to the Theory of Brownian Motion
Authors: Zwanzig Robert W.
Published: December 2004

The classical theory of Brownian motion of a periodic system is generalized to include the case where the period of the system is very short compared with times characteristic of its interaction with the environment. The system is described in terms of action and phase variables, which are constants of the motion in the absence of interactions. The probability density of the system, averaged over a time which is very long compared with a period of the motion, and long enough to include many interactions, is shown to be a solution of a Fokker-Planck equation in action-phase variables. Conditions for this are that the interaction is sufficiently weak and that the environment remains in thermal equilibrium. Explicit expressions for the friction coefficients are obtained. When the probability density of the system is independent of its phase, its irreversible behavior can be described as a random walk in action space. This is a reasonable classical analog to the quantum-statistical description by means of the Pauli equation. The properties of a harmonic oscillator with a special interaction are considered in detail; it is shown that the friction coefficients are proportional to the spectral density of a fluctuating force associated with the interaction, evaluated at the frequency of the oscillator.
Formal Solution of the Equations of Statistical Equilibrium
Authors: Zumino Bruno
Published: December 2004

A closed expression is given for the distribution functions in the classical theory of statistical equilibrium. This expression, when suitably expanded, permits one to obtain rather simply the general term in the virial expansion of the distribution functions and of the equation of state.
Propagation of Disturbances at High Frequencies in Gases, Liquids, and Plasmas
Authors: Green H. S.
Published: December 2004

A method is developed for investigating the propagation of disturbances at small amplitude in fluids from the molecular or ionic standpoint. Adopting as variables the displacements of the particles from a configuration of thermal equilibrium, the equations of motion are derived, in statistical form, for both classical and quantum mechanics.

It is shown how to apply this theory in the high-frequency region by neglecting the correlation between the displacements of neighboring particles. Relaxation frequencies are defined, and it is shown that anomalous absorption and dispersion will be found in their neighborhood. Beyond the relaxation frequency, the medium will show elastic or dispersive properties, according to whether the thermal velocities are below or exceed the velocity of propagation. A completely ionized system will have no relaxation frequencies.
Remarks on Chandrasekhar's Results Relating to Heisenberg's Theory of Turbulence
Authors: Blanch Gertrude , Ferguson Harry
Published: December 2004

Improved numerical results are obtained for the spectrum of turbulence based on Heisenberg's elementary theory during the stage of decay in a nonstationary process, which is discussed in Chandrasekhar's 1949 paper. The choice of the form of the differential equation is examined from the viewpoint of its sensitivity in numerical computations to variations of the basic parameter.
Propagation of Weak Hydromagnetic Discontinuities
Authors: Bazer J. , Fleischman O.
Published: December 2004

A theoretical discussion is given of the propagation of weak hydromagnetic discontinuities (e.g., weak shocks) in an infinitely conducting, perfect, compressible fluid. The undisturbed flow is assumed to be steady and isentropic. As in geometrical optics, the wave fronts evolving from a given initial manifold-there will be six of them, in general-are constructed by means of rays. In addition, formulas are derived describing the variation of the discontinuity ``strength'' of a given propagating mode along rays associated with that mode. These results are employed to obtain an explicit solution of a simple mixed initial, boundary-value problem that has been designed to illustrate, (1) a method of producing hydromagnetic disturbances, and (2) the fact that an initial disturbance gives rise, in general, to several propagating waves.
Peculiarity of Shock Impingement on Area Convergence
Authors: Oppenheim A. K. , Urtiew P. A. , Stern R. A.
Published: December 2004

General study of wave systems which can result from the interaction of plane shock with an area convergence has led to the discovery of an interesting regime, specified in terms of functional relationships between shock Mach numbers and area ratios, where two or three solutions exist at the same time, all satisfying the dynamic boundary conditions imposed by the conservation laws. The ambiguity is resolved by invoking the extremum principle of irreversible thermodynamics.
Relaxation Processes in Multistate Systems
Authors: Shuler Kurt E.
Published: December 2004

An analysis is made of the relaxation of ``multistate'' systems, i.e., systems with more than two quantum states or two different chemical species, for linearized processes described by the ``Master Equation'' of the theory of transport processes. These results are compared with those obtained from the analysis of two-state relaxation processes and the concept of ``relaxation time'' is discussed in this framework. A discussion is presented of the transformation of microscopic to macroscopic relaxation equations. The existence of periodic and/or aperiodic oscillatory solution of the linear multistate relaxation equation is investigated. It is shown that the multistate relaxation equations admit of aperiodic oscillatory solutions.
Condensation of an Imperfect Boson Gas
Authors: Kraichnan Robert H.
Published: December 2004

The nature of the Einstein-Bose condensation in an imperfect boson gas is discussed. It is argued that no matter how low the temperature, provided it is not zero, macroscopic second-sound waves should be weakly excited as thermal fluctuations about equilibrium. This implies that the single-particle momentum distribution must be smeared so that the state of zero momentum cannot contain a finite fraction of the particles. A microscopic mechanism which independently leads to a momentum spread is discussed. It is suggested that the actual Einstein-Bose condensation of an imperfect gas takes place in energy rather than momentum but that even in terms of this variable it becomes perfectly sharp only at zero temperature. This hypothesis is formulated analytically in terms of correlation functions of the second-quantized boson field.
Steady Extension of Incompressible Simple Fluids
Authors: Coleman Bernard D. , Noll Walter
Published: December 2004

``Steady extension'' is a flow possible in every incompressible simple fluid (Newtonian, non-Newtonian, or viscoelastic) without neglect of inertia. A special example of this flow is realized by considering a cylindrical fluid mass which is being continually elongated (or shortened) at a rate proportional to its length. The behavior of a general simple fluid in steady extension is determined by material functions other than the viscosity and normal-stress functions which govern viscometric flows.
Effect of Dissociation on the Rate of Vibrational Relaxation
Authors: Treanor Charles E. , Marrone Paul V.
Published: December 2004

The rate of dissociation behind strong shock waves in N2 and O2 is calculated using a revised model for the coupling of vibration and dissociation. In previous calculations a model which coupled the rate of dissociation to the degree of vibrational excitation was used. The present model adds to the coupled vibration-dissociation model the fact that the rate of vibrational excitation is in part determined by the rate of dissociation. Since the average energy of molecules which are dissociated is greater than the average energy of the remaining molecules, this coupling results in a drain on the average vibrational energy. It is shown that this coupling reduces the strong overshoot in vibrational energy that was previously obtained, and decreases the rate of dissociation behind strong shocks.
Experimental Study of Rayleigh-Taylor Instability in Plasma
Authors: Dickinson H. , Bostick W. H. , DiMarco J. N. , Koslov S.
Published: December 2004

A clear experimental observation of Rayleigh-Taylor, flute-type instability in a high-temperature plasma expanding across a magnetic field has been made with time-delay, Kerr cell, sequence photography. The wavelength and growth rate of the instability are measured as a function of time for various values of external magnetic field and background gas pressure. With interface decelerations of about 109 m/sec2 the instability develops in 1–4 μsec with wavelengths of about 10−2 m. After 1–4 μsec the flutes grow with constant velocity.
Rayleigh-Taylor Instabilities of a Collapsing Cylindrical Shell in a Magnetic Field
Authors: Harris E. G.
Published: December 2004

The instability of a radially accelerated cylindrical shell in a magnetic field has been investigated. It was assumed that the shell was of infinitesimal thickness. For perturbations which do not bend the lines of the magnetic field, the growth rate was found to be w = (gk)½, where g is the acceleration of the shell and k is the wavenumber. This growth rate is independent of the shell thickness. Perturbations which do bend the lines of the field were also found to be unstable. From a supplementary calculation, it was concluded that these instabilities were effective only for wavelengths greater than 8πa where 2a is the shell thickness.
Dispersion Relations in Rarefied Gas Dynamics
Authors: Sirovich Lawrence
Published: December 2004

The one-dimensional initial-value problem of a monatomic single component gas is considered. Using the linearized Boltzmann equation the dispersion relation is studied. In addition to the usual gas-dynamic sound waves, one finds an infinity of decaying propagating waves. The phenomenon exhibits itself as a sequence of epochs, the last state of which is hydrodynamic. With reference to the same problem, macroscopic equations such as Euler, Navier-Stokes, Burnett, moments equations, etc., are considered. In addition, the recently considered ``kinetic models'' of Gross et al. are applied to the problem. These various formulations are critically analyzed and compared with each other and with the Boltzmann analysis. Lastly, several modifications are offered which remedy some of the shortcomings which appear in the approximate theories.
Limits of Collective Description and Their Consequences for Landau Damping
Authors: Ecker G. , Hölling J.
Published: December 2004

By application of the concept of collective description to a fully ionized gas, Vlasov's equation follows from Liouville's theorem. We want to investigate to what extent the statistical collective procedure describes correctly the behavior of the single system under consideration. Therefore a criterion for the validity of the collective approach is deduced by comparison of the Vlasov equation with the exact individual description. The result is exemplified by an application to Landau's theory of damping of plasma oscillations.
Some Properties of the Two-Stream Instability at Large Amplitudes
Authors: Kellogg Paul J.
Published: December 2004

The behavior of the electric two-stream instability for electrons streaming through ions is investigated by treating the relevant Vlasov equations as partial differential equations and integrating them on a computer. As was shown by Buneman, the growth of the instability provides a friction between the electrons and ions. It is shown that the friction is proportional to the mean square electric field, approximately, and the coefficient is derived analytically and checked by the computer calculations. The friction converts the relative motion into heat, and the ratio between the heating of the electrons and the ions is shown to be of the order of the square root of the mass ratio.
Oscillations and Stability of a Plasma Jet
Authors: Sundaram A. K.
Published: December 2004

The longitudinal oscillations of a cylindrical plasma jet which moves with the lines of force of an externally impressed magnetic field (aligned to the jet) is investigated under the usual normal mode technique. A generalized dispersion relation is derived using the appropriate boundary conditions. Formal criteria for the cylindrical vortex sheet to be stable are obtained for long and short wavelength perturbations. Certain special cases in which the conditions for stability can be simplified, are discussed in detail. It is found that the cylindrical current-vortex sheet is stable to supersonic disturbances (whose wavelengths are assumed to be large) for strong magnetic interaction and equal uniform density distribution in the stationary configuration, in either of the media. For the incompressible case, the stability conditions are rederived and it is shown that the magnetic field stabilises the current-vortex sheet, irrespective of the strength of the magnetic field.
Linearized Analysis of Diffusing, Relaxing Gas Flow
Authors: Sforza Pasquale M. , Bloom Martin H.
Published: December 2004

A linearized model of an ideal diatomic gas with diffusion processes and rate reaction is applied to one-dimensional unsteady motion generated by initial nonuniformities of state and velocity. The time-dependent system resembles that of steady free-mixing of jet or wake type, with two space coordinates. The temporal and spatial character of the profiles of flow variables is studied analytically under two sets of conditions. First considered is the case of unit Lewis number and for this case the investigation is subdivided as follows: (i) Zero mass motion with state nonuniformities for planar, cylindrical, and spherical configurations; and (ii) nonzero mass motion with state nonuniformities for the planar and cylindrical configurations. Secondly, the case of arbitrary, but constant, Lewis number is considered. In this instance, the investigation is limited to nonzero mass motion with state nonuniformities in the planar geometry. In all cases the Prandtl number is assumed to be an arbitrary constant. Analytic solutions are obtained for all cases and various parametric effects discussed.
Scattering of a Test-Particle by Enhanced Plasma Fluctuations
Authors: Tidman Derek A. , Eviatar Aharon
Published: December 2004

The scattering of a test-particle by enhanced electric field fluctuations in a plasma containing nonthermal electrons is considered, together with some characteristic times for wave-particle scattering in such a plasma. The calculations are based on the Balescu-Lenard equation.
Cylindrical Imploding Shock Waves
Authors: Lee John H. , Lee Benedict H. K.
Published: December 2004

A technique for producing imploding cylindrical shock waves through imploding detonation waves is described. It is found that using the present method of initiation, symmetrical cylindrical imploding detonation waves can be produced in equimolar acetylene-oxygen mixtures at initial pressures above 150 mm Hg. Due to the effect of area convergence, the detonation wave becomes increasingly over-driven as it implodes towards the center. In the limit the highly overdriven detonation wave approaches a strong shock wave. Pressure measurements at different radii from the center of symmetry indicate this self-amplifying characteristic of imploding detonation waves. Close agreement between the experimentally measured detonation pressures and those calculated by the Chester-Chisnell-Whitham method is obtained.
Alfvén Waves in an Incompressible Medium
Authors: Hoult David P.
Published: December 2004

The theory of Alfvén wave propagation in an incompressible, inviscid, infinitely conducting medium is presented. The theory is presented in terms of canonical equations of motion. Topics included are the generation of arbitrary small amplitude waves, the propagation of finite amplitude waves on a nonuniform magnetic field, and the reflection of finite amplitude waves from an idealized surface of the sun. A nonlinear amplification process associated with reflection is discussed.
Adiabatic Time Development of Plasma Sheaths
Authors: Morse Richard L.
Published: December 2004

The formation of the electromagnetic sheath separating a field free Maxwellian plasma from a confining field is treated by an adiabatic approximation. It is found that the condition imposed by the approximation is not severe. Solutions are shown for several values of the ion-electron temperature ratio. The problem is done in plane symmetry and with the collisionless approximation. The question of stability is not considered.
Maximum Plasma Pressure for Stability in Magnetic Fields with Finite Minima
Authors: Taylor J. B. , Hastie R. J.
Published: December 2004

The maximum plasma pressure is discussed for stable containment in magnetic fields that possess a nonzero minimum in field strength, (minimum-B fields or magnetic wells). The basic limitations are ones on the pressure gradient and are calculated exactly for a special class of equilibria and more generally by an expansion procedure based on a shallow-well approximation. Transcribed into estimates of the critical pressure itself, these results indicate a maximum pressure equal to the depth of the magnetic well. If B1 is the field strength at the largest closed |B| contour and B2 the field strength at the lowest point of the well, then p⊥max ≃ ½(B12 − B22).
Perpendicular Temperature of the C-Stellarator Plasma
Authors: Uo Koji
Published: December 2004

The perpendicular temperature of the current-carrying plasma of the C Stellarator is measured by means of a diamagnetic coil. The perpendicular temperature and the resistivity temperature are in substantial agreement and a particle velocity distribution that is essentially isotropic. For the discharge having a long pulse duration and a higher temperature, the distribution differs from isotropic to a small extent, and the discrepancy becomes larger for the period at the end of the discharge. This suggests the production of runaway electrons, and the resistivity temperature would then appear higher than the true temperature. For the period at the end of the discharge, the plasma density becomes very low, while the Ohmic heating current is forced to be constant by the external electronic feedback control system. Thus the probability of the generation of the runaway electrons increases.
Problem of Steady-State Shear-Flow Turbulence
Authors: Deissler Robert G.
Published: December 2004

Some steady-state solutions of two-point nonlinear correlation equations for shear flow turbulence are obtained by expansion in power series in the space variables. The correlation equations, which are for inhomogeneous turbulence, are constructed from the Navier-Stokes equations. To make the problem determinate, the weak-turbulence approximation is used. Because only the low-order terms are retained in the power series used in the expansions, the solutions are accurate only for small values of the space variables. The forms of the solutions show that critical values of a parameter (similar to Reynolds number) exist below which the turbulent fluctuations are zero. The presence of pressure-velocity correlations and of nonlinear production terms in the equations was found to be indispensable if steady-state shear-flow turbulence is to exist. It is concluded that a system of correlation equations for turbulent shear flow which is closed by neglecting the highest order correlations can yield reasonable steady-state solutions.
Expansion Solutions and Wave Propagation in Partly Ionized Gases
Authors: Kahn David
Published: December 2004

Kinetic theory solutions of the Boltzmann equations for a partly ionized gas are obtained for the study of wave propagation in a three-fluid plasma. The following simplifying assumptions are made: The interactions between charged particles are divided into long-range Coulomb forces which effectively act as an external electric field, and a short-range Coulomb force which is cut off at the Debye length; the interactions between other pairs of particles are assumed to obey an inverse fifth-power law. Further, the gas is assumed to be sufficiently close to equilibrium for the usual expansions of the velocity distribution functions around the equilibrium distribution functions to be used. Under these assumptions, the general sets of coefficient equations are obtained, and a 13 and 20 coefficient approximation (which include the effects of self-collisions or viscosity and thermal conductivity) are given explicitly. For a five-coefficient approximation, corresponding to employment of the conservation equations, a dispersion relation is obtained and solved for the phase velocities and attenuation of longitudinal waves as functions of wave and collision frequencies. It is found that the solutions which correspond to waves of high phase velocity are exactly those obtained when the adiabatic law is assumed to hold. Solutions corresponding to low phase velocity waves, however, are found to be more highly attenuated than their counterparts in the adiabatic theory.
Hydromagnetic Instability of Fluid between Two Coaxial Cylinders
Authors: Chow Chuen-Yen , Uberoi Mahinder S.
Published: December 2004

When a uniform electric current is passed axially through a stationary fluid between two coaxial nonconducting cylinders with an electric current flowing along the axis, the fluid may become unstable due to the nonuniform electromagnetic body force field. By introducing an adjoint system of equations, the characteristic value problem is solved without the restriction of small spacing between the cylinders. Numerical results are given for the case when the ratio of the radii of the two cylinders is one-half. It shows that the wavenumber of the disturbance appearing in the fluid is a constant independent of the magnitudes of applied electric currents, which agrees with the result obtained under the narrow-gap assumption.
Numerical Integration of Kinetic Equations
Authors: Dolinsky A.
Published: December 2004

The problem of the relaxation to a Maxwellian of the Balescu-Lenard equation is solved numerically as an initial value problem for isotropic distributions of electrons situated in a neutralizing, uniformly smeared out background of positive charge. Several different forms for the initial distribution function are chosen: a Gaussian peaked at about 0.28 of the electron thermal speed, a resonance function, and a Maxwellian coexisting with a sharply peaked Gaussian (the peak of the Gaussian being located at two thermal speeds of electrons). The Fokker-Planck equation with the Rosenbluth-MacDonald-Judd collision term is also solved numerically under the same restrictions and with the same initial distribution functions. Comparison of the solutions of the two kinetic equations shows that the difference between them is very small, and a probable reason for this is advanced. The evolution of the distribution functions with time is studied, and a relaxation time is defined.
High-Temperature Transport Properties of Alkali Metal Vapors
Authors: Davies R. H. , Mason E. A. , Munn R. J.
Published: December 2004

Estimates of the potential-energy curves for the interaction of ground-state alkali metal atoms have been obtained from available spectroscopic data and a semi-empirical scaling procedure. From these curves the transport collision integrals for monatomic alkali-metal vapors have been calculated over the temperature range 1000° to 10 000°K. Collision integrals for the diffusion of alkali-metal ions through their parent monatomic vapors have been calculated over the same range from estimates of the charge-exchange cross sections, obtained by a scaling procedure based on the experimental measurements for cesium. Comparison with limited existing experimental data on second virial coefficients, viscosities, thermal conductivities, and charge exchange has been made, and the collision integrals estimated to be uncertain by about 25%.
Magneto-Atmospheric Waves in a Horizontally Stratified Conducting Medium
Authors: Yu C. P.
Published: December 2004

The propagation of magneto-atmospheric waves in an inviscid, compressible and perfectly electrically conducting medium in the presence of a vertical gravitational field and a horizontal magnetic field is investigated. If the medium is isothermal and its magnetic pressure is proportional to its mechanical pressure, it is found that the convective instability arises when the ratio of the Alfvén speed to the sound speed exceeds [2(1 — 1/γ)]½ where γ is the adiabatic index. When the medium is found to be stable, a disturbance in the medium will travel as waves in three different modes. For small wavenumbers, two of these modes are similar to the acoustic and gravity modes found in the nonmagnetic case. For very large wavenumbers, the acoustic mode becomes the modified sound wave found in the case of a uniform medium, while the gravity wave behaves as the corresponding modified hydromagnetic wave unless the component of the wavenumber in the direction of the magnetic lines is sufficiently small. The third mode which does not appear in the nonmagnetic case behaves nearly as a pure hydromagnetic wave; it is influenced only to a minor extent by compressibility and gravity. The damping of each mode due to viscosity, finite electrical and thermal conductivity is discussed.
Hydrodynamical Theory of Diffusive Separation of Mixtures in a Free Jet
Authors: Sherman Frederick S.
Published: December 2004

The static molar concentration and fractional molar flow rates of the components of a binary mixture are computed to order (Re)−1 on the centerline of an axially symmetric free jet, under conditions of nearly inviscid flow. These computed results confirm the conclusions of Reis and Fenn regarding the mechanism underlying measured separation effects in jets at relatively high Reynolds number. Some qualitative comments about probe-induced separation are given.
Velocity Distributions in Molecular Beams from Nozzle Sources
Authors: Anderson J. B. , Fenn J. B.
Published: December 2004

The velocity distributions in molecular beams of argon produced by the Kantrowitz-Grey supersonic nozzle technique have been investigated using a time-of-flight method. It is shown that the transition from continuum to free molecular flow in free jet expansion limits the minimum width of the velocity distribution which may be obtained. The Mach number corresponding to the axial velocity distribution of the expanding gas approaches a limit which varies inversely with the two-fifths power of the Knudsen number at the nozzle. With argon at room temperature the narrowest velocity distribution obtained corresponds to a Mach number of 23 (90% of the molecules within 5.4% of the most probable velocity).
Unsteady Boundary Layers on Vibrating Spheres in a Uniform Stream
Authors: Yeh Hsu-Chieh , Yang Wen-Jei
Published: December 2004

The influence of radial vibrations on forced convection from a sphere placed in a uniform stream of an incompressible fluid is studied mathematically. Consideration is given to first-order perturbations of the harmonic vibrations of the spherical surface for both high and low frequencies. Theoretical results are obtained by means of a Kármán-Pohlhausen method for velocity and temperature distributions, skin friction, heat-transfer rate and the shift in the separation point of the boundary layer. Numerical calculations are carried out for boundary-layer thickness and velocity profiles in the fluctuating boundary layer.
Stability of Two-Layer Viscous Stratified Flow Down an Inclined Plane
Authors: Kao Timothy W.
Published: December 2004

The stability of flow down an inclined plane is investigated for the case of a stratified fluid system consisting of two layers of viscous fluids of different densities. This problem is an extension of the works of Benjamin and Yih for a homogeneous fluid; thus their results are a special case of the solution for this more general problem. Reynolds numbers for the bifurcation point of the neutral curve are found for various ratios of density and depth of the two layers. For the purpose of comparing the relative stability between different configurations, a stability index is defined. It is found that the two-layer flow is more stable or unstable than the homogeneous case of equal total depth, depending on whether the upper fluid is lighter or heavier than the lower one.
Electromagnetic Wake of a Charged-Particle Pulse in a Plasma
Authors: Yadavalli S. V.
Published: December 2004

Employing the hydrodynamic approach, the electromagnetic wake of a charged-particle pulse passing through a plasma is investigated. The perturbation in the magnetic field amplitude Bθ(1) which is of interest in the study of `hose' instability of the beam is evaluated. The effect of charge density profile and different velocity regimes of the beam pulse on the behavior of Bθ(1) is considered. Finally, numerical results relevant to a typical example are presented.
Hydromagnetic Waves in a Plasma with Finite Larmor Radius
Authors: Talwar S. P.
Published: December 2004

The effect of finite Larmor radius and Larmor frequency on hydromagnetic waves in a plasma is investigated. It is concluded that the finite Larmor radius has considerable influence. A uniform rotation is also included in view of its astrophysical importance.
Observed Dwells in Current through a Pulsed Argon Plasma
Authors: Manka C. K. , Zinke O. H.
Published: December 2004

Current dwells of durations from 1 to 38μsec have been observed with capicator-discharge, argon plasmas pulsed in vacuo. The current during dwell is zero with a measuring error of 1 A in contrast to large dwell currents observed in plasma of exploded wires.
Influence of Viscosity on Shock Waves Structured by Radiation
Authors: Cohen Ira M. , Clarke Joseph H.
Published: December 2004

The structure of normal shock waves with radiation, viscosity, and thermal conductivity is treated in the physically relevant limit of large radiation absorption length (or photon mean free path) compared to molecular transport length (or molecular mean free path). If radiation is not too intense or the shock too weak, then a viscous, conducting shock is found embedded in a radiation structured inviscid shock. Under very general conditions, this viscous ``shock within a shock'' has precisely the same structure as a classical (radiationless) viscous, conducting shock when reinterpreted as to initial and final states, to lowest order in an expansion based on the ratio of the two transport lengths. This lowest order solution is no longer trivial when the optically thin region considered is embedded in a much thicker region. The next order corresponds to the usual optically thin case but the well known formula (accounting for emission but not absorption by the gas) does not apply; the divergence of the radiation flux is actually given by the emission minus absorption of the radiation from the outer region. Since the relative importance of these two effects changes markedly through the inner viscous shock, the usual formula is meaningless here.
Kelvin-Helmholtz Instability in an Anisotropic Plasma
Authors: Talwar S. P.
Published: December 2004

The instability arising due to a tangential discontinuity of velocity in an unbounded, anisotropic plasma subject to a uniform magnetic field is investigated using the Chew, Goldberger, and Low approximation. Formal conditions for monotonic instability and the growing-wave instability are written. It is found that monotonic instability is possible, for a range of relative speeds, only if the plasma is characterized by the condition 2p∥ > p⊥. For 2p∥ < p⊥ the configuration is, however, overstable for relative speeds less than a certain critical value. The bearing of the results on the stability of the magnetospheric boundary and the coronal streamers is discussed and it is suggested that a possible fine structure in the solar wind may satisfy the requirement for instability and thus produce irregularities of plasma properties and the magnetic field in the medium.
Finite Larmor Radius Equations with Nonuniform Electric Fields and Velocities
Authors: Rosenbluth Marshall N. , Simon Albert
Published: December 2004

A low-β collisionless plasma cylinder confined in a uniform magnetic field is considered. The well-known flute instabilities occur due to plasma rotation or radial gravity which simulates field line curvature. The effects of finite Larmor radius and nonuniform rotation are included deriving a proper set of hydromagnetic equations and variational principles for determining stability. In most cases these are merely sufficient but in a variety of cases may be both necessary and sufficient for stability.
Ground Flow Induced by a Moving Cylinder
Authors: Joseph Daniel D. , Tao L. N.
Published: December 2004

Ground flow induced by rotation and translation of a solid cylinder is investigated in the context of a coupled boundary-value problem in which the Stokes flow outside porous regions and the Darcy flow inside porous regions are connected by the continuity requirements on the pressure and normal component of velocity. A principal effect of the porous flow on the coupled motion is a diminution of pressure differences which would develop if all solids were impermeable. Corresponding changes in shear stress (which are largely induced by the Couette motion) are slight, and this can markedly alter the character of force resultants. Thus classical results which give (a) zero force on a rotating cylinder near a stationary non-porous ground and (b) zero torque on a nonrotating cylinder near a moving solid wall are annulled as a consequence of ground flow.
Cooperative Effects in a Tenuous Energetic Plasma Contained by a Magnetic Mirror Field
Authors: Damm C. C. , Foote J. H. , Futch A. H. , Gardner A. L. , Gordon F. J. , Hunt A. L. , Post R. F.
Published: December 2004

The formation and characteristics of a steady-state hydrogen plasma contained in a magnetic mirror field are described. The mean ion energy is 20 keV. The plasma is formed by ionizing and trapping a portion of a beam of energetic hydrogen atoms passing through the confining field. The methods of measurement used to determine the plasma properties are described. Measurements of the radial and azimuthal trapped-ion distributions, the average ion and electron densities, and the plasma potential are compared with the predictions of simple theory, neglecting cooperative plasma effects. The observed deviations from these simple predictions show that the plasma properties are dominated by cooperative phenomena. The plasma density is found to be limited to a low value (∼4 × 107 ions/cm3) by a flute or drift instability. This instability is characterized by a low frequency rotation of the plasma at a frequency typically close to the ▿B precession frequency of a 20-keV proton in the nonuniform mirror field. The plasma density at the unstable limit is observed to be related to the value of plasma potential. This relationship can be understood quantitatively by an extension of an earlier theoretical treatment of the drift instability to the case of unequal ion and electron densities. The flute oscillation frequency is also derived from this theory. Oscillations at the ion gyrofrequency are observed, marking the presence of an electrostatic resonance type of instability. Coupling of these modes with the flute oscillations is described, but detailed identification of the electrostatic resonance modes cannot yet be made.
Radiation at the Plasma Frequency and its Harmonic from a Turbulent Plasma
Authors: Sturrock P. A. , Ball R. H. , Baldwin D. E.
Published: December 2004

Any mechanism which couples electrostatic waves (plasma oscillations) and electromagnetic waves in a plasma will lead to radiation from plasma oscillations. The most important consequence of nonlinearity of the equations is that two electrostatic waves are coupled to one electromagnetic wave; this process is therefore responsible for radiation at the second harmonic (2ωp). Low-frequency or static inhomogeneities of the plasma couple one electrostatic wave to one electromagnetic wave, and therefore, produce radiation at (or near) the plasma frequency. Two types of inhomogeneity are considered: ``isotropic'' inhomogeneities, which are fluctuations in the plasma density, and ``anisotropic inhomogeneities,'' which comprise an inhomogeneous distribution of currents and magnetic fields. These radiation mechanisms are analyzed by means of a set of fluid equations for the electron component, which should provide an adequate description for plasma oscillations with wavenumbers small compared with the Debye wavenumber. The radiation field is evaluated by a Green function method, ignoring re-absorption, which one expects to be a very weak process. For each radiation mechanism, a formula is obtained for the volume emissivity of a turbulent plasma representing the plasma oscillations and the inhomogeneities each by a correlation function and an energy spectrum. Useful approximate forms of these formulas are given.
Stability of Finite Amplitude Ion Cyclotron Waves in Plasma
Authors: Jukes J. D.
Published: December 2004

A circularly polarized, shear Alfvén wave of finite amplitude is shown to become rapidly unstable as its frequency ω1 approaches ion cyclotron resonance Ω. A particular mode of decay is into two perturbations consisting of a shear Alfvén wave of slightly lower frequency and an acoustic wave of much lower frequency. The decay rate is proportional to S½/(1 − ω1/Ω) where S is the power flux.
Combining Cues While Avoiding Perceptual Conflicts
Authors: Hogervorst Maarten A , Brenner Eli
Published: December 2004

A common assumption in cue combination models is that small discrepancies between cues are due to the limited resolution of the individual cues. Whenever this assumption holds, information from the separate cues can best be combined to give a single, more accurate estimate of the property of interest. We examined whether information about the discrepancy itself is lost when this is done. In our experiments, subjects were required to combine cues to match certain properties while avoiding perceptual conflicts. In part 1, they combined expansion and change in disparity to estimate motion in depth; and in part 2, they combined perspective and binocular disparities to estimate slant. We compared the pattern in the way that subjects set the two cues with the patterns predicted by models of cue combination with and without a loss of information about the discrepancy. From this comparison we conclude that little information about the discrepancies between cues is lost when the cues are combined.
Perception of Scene Layout from Optical Contact, Shadows, and Motion
Authors: Ni Rui , Braunstein Myron L , Andersen George J
Published: December 2004

Kersten et al (1997 Perception26 171−192) found that the perceived motion of an object in a 3-D scene was determined by the motion of a shadow. In the present study, we compared the effect of a shadow to that of a second object on the ground in determining the perceived position in depth of a floating object in both dynamic and stationary scenes. Changing the second (lower) object from textured to dark increased the influence of the second object on the judged position of the first object. Giving the second object zero thickness had this effect only if it was also dark. Variations in the height of the floating object were important with a second object but not with a shadow, in motion scenes. With alternative shadows present, the position of the floating object was determined primarily by matching speeds, with matching sizes as a secondary factor. These results show some similarities but important differences between the effect of a second object and that of a shadow.
Removing Eyebrows Impairs Recognition of Famous Faces, or Doesn't, Depending on How the Eyebrows are Removed
Authors: White Murray
Published: December 2004

Matching photos of famous people's faces with their names was as fast with photos that had eyebrows masked by adhesive plaster as with unretouched original photos. But matching was slower with photos that had eyebrows erased and replaced with adjacent skin texture and colour than with original photos. The conjecture that erasure impairs recognition because it alters a face's configuration was examined by repeating the experiment with the photos shown upside down, the rationale being that because configural information is difficult to encode from inverted faces the erasure effect should diminish. With inverted faces, matching was no different for original, eyebrows-masked, and eyebrows-erased photos. Eyebrows appear to be less important for face recognition as informative parts and features than as sources of information about a face's configuration.
Water loss physiology and the evolution within the Tasmanian conifer genus Athrotaxis (Cupressaceae)
Authors: Jordan Gregory J. , Brodribb Timothy J. , Loney Prue E.
Published: December 2004

The Tasmanian montane and rainforest conifer genus Athrotaxis provides a system for investigating the relationship between leaf form and function and its adaptive significance. The two species differ markedly in leaf size, shape, degree of imbricacy and stomatal distribution, whereas natural hybrid swarms and glasshouse-grown hybrid progeny are highly variable for these traits. In glasshouse-grown plants of the true species and a diverse hybrid progeny, stomatal conductance and density were strongly correlated, and varied by approximately 400% among individuals. Hybrids displayed lower stomatal densities and less discrimination of 13C than the true species, leading to a negative relationship between stomatal density and δ13C. In contrast with the highly variable stomatal densities and δ13C in glasshouse plants, field-grown plants were highly conservative in both characters. This, combined with relatively low stomatal density and high water-use efficiency in field-grown plants suggests optimisation of the trade-off between assimilation and water loss. Foliar conductance in the light for the hybrids and A.selaginoides was only 4–6 times as great as, and was strongly correlated with, conductance in the dark, suggesting incomplete stomatal closure or high cuticular conductance. Athrotaxis cupressoides was less ‘leaky’. This may reflect adaptation to its more exposed habitat.
A revision of Cassinia (Asteraceae: Gnaphalieae) in Australia.3. Section Leptocephalae
Authors: Orchard A. E.
Published: December 2004

The species of Cassinia sect. Leptocephalae are keyed, described and illustrated. New taxa described are Cassinia leptocephala subsp. everettiae Orchard, Cassina decipiens Orchard, C. macrocephala Orchard subsp. macrocephala, C. macrocephala subsp. tenuis Orchard, C. telfordii Orchard and C. accipitrum Orchard; C. longifolia var. straminea Benth. is raised to species level as C. straminea (Benth.) Orchard.