bluuebunny/crossref_metadata_2025 huggingface dataset:
The rotational spectrum of single OCS molecules in liquid 4He droplets
Authors: Grebenev Slava , Hartmann Matthias , Havenith Martina , Sartakov Boris , Toennies J. Peter , Vilesov Andrei F.
Published: July 2002

Single OCS molecules have been embedded in large 4He droplets (N̄=1×103–8×103 atoms) and their infrared spectra in the vicinity of the ν3-fundamental at 2062 cm−1 have been studied using coaxial laser depletion spectroscopy. Sharp lines corresponding to the P- and R-branches with a linewidth of 160 MHz or greater are observed. From the line intensities a droplet temperature of 0.37±0.02 K is obtained and from the line positions the rotational constants B for the ground and excited states and an average centrifugal distortion constant D are determined. The former are about a factor of 2.8 smaller and the latter four orders-of-magnitude larger than for the free molecule. The decrease in B is attributed to an attached nonsuperfluid component which has the same effect as a ring of about six 4He atoms in an equatorial plane around the waist of the OCS molecule, which is carried along in the end-over-end rotation. The widths of the individual lines show an interesting asymmetry which is oppositely sloped for the P- and R-branches and increases with the rotational quantum number. Several possible explanations for the line shapes and widths are presented.
A CCSDT study of the effects of higher order correlation on spectroscopic constants. I. First row diatomic hydrides
Authors: Feller David , Sordo Jose A.
Published: July 2002

Spectroscopic constants have been determined for 13 first row diatomic hydrides using coupled cluster theory with explicit inclusion of (iterative) triple excitations (CCSDT). Comparison of the predicted dissociation energies, bond lengths, harmonic frequencies, and anharmonicities was made with experiment and other high-level theoretical treatments. These include complete active space configuration interaction wave functions, coupled cluster theory with perturbative triples [CCSD(T)], and new benchmark full configuration interaction calculations. Excellent overall agreement with experiment was found, even without correcting for small changes due to core/valence and relativistic effects. The intrinsic CCSDT error with respect to experiment for each molecule and property was estimated by extrapolating to the complete basis set limit. Among the various properties examined in this study, no significant differences were found between CCSD(T) and CCSDT. In light of the substantial increase in computational cost associated with the latter method, there appears to be little justification for selecting it over CCSD(T) in studies of first row hydrides. Preliminary results for first row diatomics, e.g., N2, suggest that the impact of CCSDT will increase with the number of electrons.
On the possibility of performing self-calibrated selective π/2 pulses in nuclear-magnetic resonance
Authors: Abergel Daniel , Louis-Joseph Alain , Lallemand Jean-Yves
Published: July 2002

In this paper, a generalization of the concept of selective “slaved” pulses in NMR spectroscopy introduced recently by the authors is presented. The shape of these pulses is slaved to that of a signal produced by the sample by the action of an electronic feedback loop. The theoretical analysis is based on results of the theory of nonlinear dynamical systems, which predicts the possibility of performing 90° self-calibrated radiofrequency pulses, through the simultaneous action of a constant radio-frequency field and of a magnetization dependent field of the kind above. Experimental demonstrations show that rotation of the magnetization onto the xy-plane is achieved, regardless of its initial position. Moreover, it is shown to remain stable for more than 50 ms, without significant loss of intensity.
Algebras of distributions suitable for phase-space quantum mechanics. II. Topologies on the Moyal algebra
Authors: Várilly Joseph C. , Gracia-Bondía José M.
Published: July 2002

The topology of the Moyal *-algebra may be defined in three ways: the algebra may be regarded as an operator algebra over the space of smooth declining functions either on the configuration space or on the phase space itself; or one may construct the *-algebra via a filtration of Hilbert spaces (or other Banach spaces) of distributions. The equivalence of the three topologies thereby obtained is proved. As a consequence, by filtrating the space of tempered distributions by Banach subspaces, new sufficient conditions are given for a phase-space function to correspond to a trace-class operator via the Weyl correspondence rule.
Some remarks on AB-percolation in high dimensions
Authors: Kesten Harry , Su Zhonggen
Published: July 2002

In this paper we consider the AB-percolation model on Z+d and Zd. Let pHalt(Zd) be the critical probability for AB-percolation on Zd. We show that pHalt(Zd)∼1/(2d2). If the probability of a site to be in state A is γ/(2d2) for some fixed γ>1, then the probability that AB-percolation occurs converges as d→∞ to the unique strictly positive solution y(γ) of the equation y=1−exp(−γy). We also find the limit for the analogous quantities for oriented AB-percolation on Z+d. In particular, pHalt(Z+d)∼2/d2. We further obtain a small extension to the two parameter problem in which even vertices of Zd have probability pA of being in state A and odd vertices have probability pB of being in state B (but without relation between pA and pB). The principal tools in the proofs are a method of Penrose (1993) for asymptotics of percolation on graphs with vertices of high degree and the second moment method.
Localization of vibrational defects in one-dimensional structures with a complex unit cell
Authors: Mamalui M. A. , Syrkin E. S. , Feodosyev S. B.
Published: July 2002

Exact analytic expressions are obtained for Green’s functions of a linear chain with a diatomic unit cell containing an isotopic substitutional impurity. An infinite chain as well as a semi-infinite chain with an impurity at the free end are considered. The total change in the phonon spectrum and the spectral densities of vibrations of the impurity and its nearest neighbors are investigated. The conditions for the formation and characteristics of local and gap vibrations are analyzed. A general method of obtaining exact solutions by using Jacobi matrices is proposed for a wide class of one-dimensional systems.
The elliptical model of two-dimensional vortex dynamics. II: Disturbance equations
Authors: Dritschel David G. , Legras Bernard
Published: July 2002

In Part I [Phys. Fluids A 3, ▪▪▪ (1991)] approximate equations were developed describing the basic evolution of vortices in a general strain field. These equations take the form of a set of coupled, nonlinear ordinary differential equations describing the time evolution of the centroids, aspect ratios, and orientations of a nested set of elliptical contours representing each vortex. Here, in Part II, the model is extended to include disturbances to the elliptical shape of each contour, disturbances that are excited naturally by the interaction with other vortices. This interaction is worked out explicitly for the first time. The final equations obtained decouple into sets of equations for each mode symmetry, allowing for a very simple description of the disturbance evolution. Numerical tests show remarkable agreement between the elliptical model and the full equations of motion in four problems: (1) the equilibrium contour shapes of a multicontour family of vortices, (2) the linear stability of this family, (3) the equilibrium, nonelliptical shapes of two corotating vortex patches, and (4) the interaction between two symmetrical vortex patches, including merging.
Numerical test of weak turbulence theory
Authors: Payne G. L. , Nicholson D. R. , Shen Mei-Mei
Published: July 2002

The analytic theory of weak Langmuir turbulence is well known, but very little has previously been done to compare its predictions with numerical solutions of the basic dynamical evolution equations. In this paper, numerical solutions of the statistical weak turbulence theory are compared with numerical solutions of the Zakharov model of Langmuir turbulence, and good agreement in certain regimes of very weak field strength is found.
Congruent reduction in parametrically unstable space- and time-varying plasmas
Authors: Goldner G. , Friedland L.
Published: July 2002

A recently developed congruent reduction technique is applied to the parametric amplification problem in space- and time-varying plasmas. The systematic reduction procedure yields a second-order system of partial differential equations describing mode coupling in the presence of a strong pump wave. This system is similar in form to that characterizing the general multidimensional mode conversion problem so that the solution in both cases proceeds similarly. The method allows a consistent derivation of the expression for the parametric amplification coefficient in general geometry. An unmagnetized plasma case is considered as an example.
Hydrogenlike magnesium x-ray laser design
Authors: Eder D. C.
Published: July 2002

Design calculations for a hydrogenlike magnesium recombination x-ray laser driven by a 20 psec optical laser are presented. Results for narrow free-standing magnesium and magnesium fluoride strip targets and the more realizable situation of a lasant strip supported on a thin plastic backing are compared. Accurate modeling of the hydrodynamic expansion of the composite targets is achieved by the use of distinct zoning in the plastic and the lasant strip. The effect of the backing is shown to be largest for strips that are narrow and thin. The gain coefficient for the 3d5/2−2p3/2 transition in pure magnesium is calculated to be approximately a factor of 2.5 larger than for magnesium fluoride strips without photon trapping. In contrast, including photon trapping causes the gains to be comparable leading to the possibility of an experiment to test trapping. General principles of target design are discussed, as well as specific targets suitable for the Nova two-beam laser [Rev. Sci. Instrum. 57, 2101 (1986)].
Vacuum–plasma boundary helicity injection
Authors: Barrow B. J. , Goldenbaum G. C.
Published: July 2002

The magnetic helicity inside a volume enclosed by a conducting boundary with electrodes inserted through insulator gaps has been measured. By comparing the measured time dependence of the helicity with predictions of a numerical simulation and analytic theory, the importance of the insulator gap, and injection across a vacuum–plasma boundary inside the volume has been shown.
Self-modulation of magnetic electron drift waves
Authors: Salahuddin M. , Saleem H. , Shukla P. K.
Published: July 2002

The self-modulation of a large-amplitude magnetic electron drift (MED) wave is considered. It is found that the dynamics of the modulated wave packet is governed by the cubic Schrödinger equation. Since, in the latter, the product of the group dispersion and the nonlinear frequency shift is negative, it turns out that the one-dimensional nonlinear modulated MED wave packet propagates in the form of a rarefactional pulse. The present nonlinear structure can possibly be associated with the magnetic cavity observed in the vicinity of the magnetopause.
A simulation for diffusions of charged particles in space correlation electric field
Authors: Hu X. W.
Published: July 2002

A Monte Carlo method is used to simulate a turbulent electrostatic field that could process any given space correlation functions or corresponding power spectra. Then the diffusions of charged particles could be obtained by integrating the particle motion equations in this electric field. The simulation relative diffusion (without a magnetic field) shows good agreement with the analytic predictions on the extent of both quality and semiquantity. The simulation results in a strong magnetic field also agree qualitatively with both analytic theories and simulation results based on drift motion equation except for the parameter scaling law of diffusion coefficients. But the transverse relative diffusions in weak and middle magnetic fields are quite different from the analytic results. Two analytic discussions are provided for explaining these discrepancies.
Criteria for excitation of parametric instability in the ionosphere by an obliquely incident high-frequency heater
Authors: Kuo S. P.
Published: July 2002

Parametric excitation of lower-hybrid decay mode together with upper-hybrid sideband in the ionosphere by an obliquely incident high-frequency pump is studied. The conditions for the instability, including the frequency range, the elevation angle, and the threshold field intensity of the pump are determined.
Radial control of the electrostatic potential in a tandem mirror with quadrupole end cells
Authors: Severn G. D. , Hershkowitz N.
Published: July 2002

It is demonstrated that the radial electrostatic potential in the Phaedrus tandem mirror [Plasma Physics and Controlled Fusion Research, 1984 (International Atomic Energy Agency, Vienna, 1985), Vol. 2, p. 265] can be altered using plasma potential control rings (PPC rings) situated at each end of the device. With the PPC rings grounded, the radial electric field in the central cell was directed outward and peaked near the plasma edge. By externally controlling the potentials of the PPC rings, the peak value of the radial electric field was varied by over a factor of 2, obtaining plasmas with a maximum value of the radial electric field E(r)≤15 V/cm. A simple calculation to model the change in the central cell potential due to a bias voltage applied to the PPC rings is presented.
A class of conservative tunneling problems
Authors: Friedland L. , Cohen D.
Published: July 2002

A conservative generalization of the Budden wave tunneling problem in an inhomogeneous medium is considered. The model describes the interaction between an electromagnetic wave and a ‘‘ballistic,’’ wavelike perturbation mediating between the incident and reflected electromagnetic wave channels. The transmission and reflection coefficients of the electromagnetic wave passing the interaction region are the same as in the classical Budden problem, while the ballistic mode carries the remainder of the energy flux. The theory is illustrated in the case of a whistler mode passing a local cyclotron resonance by propagating along a magnetized electron beam. The theory is also generalized to include other types of couplings, as those occurring in wave transformation problems at ion-cyclotron resonances, when the magnitude of the magnetic field has gradients along the field itself. The minority ion cyclotron heating problem for arbitrary direction of the gradient of the cyclotron frequency is studied as an example of such a generalized problem.
Effects of neutral beam injection on poloidal rotation and energy transport in tokamaks
Authors: Hinton F. L. , Kim Y.-B.
Published: July 2002

The poloidal mass flow and ion energy transport driven by friction between thermal ions and fast ions resulting from neutral beam injection are investigated. Transport coefficients are derived which relate the poloidal mass flow and the radial ion heat flux to the fast ion friction and the radial temperature gradient. An Onsager symmetry condition shows that the coefficient of the fast ion friction in the radial heat flux is the negative of the coefficient of the temperature gradient in the poloidal mass flow. Numerical results are given for the banana regime. The driven poloidal mass flow can be significantly larger than the standard neoclassical poloidal rotation driven by the ion temperature gradient. The resulting viscous heating of the ions can be larger than the standard neoclassical ion heat conduction term in the ion energy equation. The driven ion energy flux contains a convective contribution, due to the diffusional mixing of the fast ions and thermal ions. This energy flux is inward (a heat pinch) in the case of coinjection, and can be significantly larger than the standard neoclassical energy flux.
Linear analysis of a resistive compressible plasma flow around a sharp corner
Authors: Chanty Jean-Marc G. , Martinez-Sanchez Manuel
Published: July 2002

In this article the authors describe a perturbation method used to calculate the flow of a compressible magnetized plasma around a sharp corner with a small turning angle. The main purpose for this analysis is the study of the flow in the exit region of a plasma accelerator in the regime of high magnetic Reynolds number. The physical model is based on a one-fluid resistive magnetohydrodynamic model (continuum model). The analysis predicts the existence of an acoustic expansion fan near the sharp corner, and a magnetoacoustic expansion fan in the far field. The current lines refract across the acoustic fan. Near an anode there is a strong mass depletion downstream of the corner. Along an anode, some of the current lines reattach far downstream, creating a magnetic boundary layer along the electrode. Near a cathode there is a region of high density and high temperature ahead of the corner. Along a cathode most of the current attaches itself ahead of the corner. Finally, the current refraction predicted by this theory correlates qualitatively with the experimental measurements in the regime of high magnetic Reynolds number.
A closure theory of intermittency of turbulence
Authors: Qian J.
Published: July 2002

The variational approach to the closure problem of turbulence theory, proposed in an earlier article [Phys. Fluids 26, 2098 (1983); 27, 2229 (1984)], is extended to evaluate the flatness factor, which indicates the degree of intermittency of turbulence. Since the flatness factor is related to the fourth moment of a turbulent velocity field, the corresponding higher-order terms in the perturbation solution of the Liouville equation have to be considered. Most closure methods discard these higher-order terms and fail to explain the intermittency phenomenon. The computed flatness factor of the idealized model of infinite isotropic turbulence ranges from 9 to 15 and has the same order of magnitude as the experimental data of real turbulent flows. The intermittency phenomenon does not necessarily negate the Kolmogorov k−5/3 inertial range spectrum. The Kolmogorov k−5/3 law and the high degree of intermittency can coexist as two consistent consequences of the closure theory of turbulence. The Kolmogorov 1941 theory [J. Fluid Mech. 62, 305 (1974)] cannot be disqualified merely because the energy dissipation rate fluctuates.
Dynamics of a virtual cathode oscillator driven by a pinched diode
Authors: Sze H. , Benford J. , Woo W. , Harteneck B.
Published: July 2002

A new method is demonstrated for extracting radiation from a virtual cathode oscillator in transverse electric (TE) waveguide modes. The dominant radiation mechanism occurs in the region of the virtual cathode, and is not due to reflexing of electrons. Microwave radiation occurs simultaneously or just after beam pinching in the diode. Electrostatic signals show simultaneous occurrence of the virtual cathode and microwave radiation. At the same time, the electron population divides into a beam population and a reflexing electron population. Inhibition of pinching by an axial guide field suppresses microwave radiation.
Coherent structures in the far field of a turbulent wake
Authors: Browne L. W. B. , Antonia R. A. , Bisset D. K.
Published: July 2002

The topology of a typical group of large-scale coherent structures in the far-field of a slightly heated cylinder wake is clearly delineated by using averages of the velocity vector components conditioned on the occurrence of spatially coherent temperature fronts. Velocities are determined relative to a frame of reference traveling with the approximate convection velocity of the structures. When the results for a number of positions across the wake are assembled a clear topology picture of a typical group of structures in the plane of the main shear is obtained. The contributions of the coherent and random motions of all structures to the momentum and heat transports have been determined. In all cases the contribution that results from the random motion is larger than that of the coherent motion. This difference is more pronounced for the momentum transport than for the lateral heat transport.
Influence of interfacial properties on the modes of propagation of a gravity wave on the surface of a liquid
Authors: Benticha H’maǐed , Dudeck Michel
Published: July 2002

The evolution of monochromatic waves in an interfacial medium separating a liquid and a gas is investigated. Two viscosity effects are found and studied: a dispersive effect and a damping effect. The introduction of a constant tension tends to eliminate the viscous effects. Finally, if the interface is similar to a material film and is characterized by an elastic-type behavior, the effects of inertia and elasticity film are presented.
Conditional eddies in plasma turbulence
Authors: Johnsen H. , Pécseli H. L. , Trulsen J.
Published: July 2002

Low-frequency electrostatic turbulence generated by the ion–ion beam instability was investigated experimentally in a double-plasma device. Real time signals were recorded and examined by a conditional statistical analysis. Conditionally averaged potential distributions reveal the formation and propagation of structures with a relatively long lifetime. Various methods for making a conditional analysis are discussed and compared. The results are discussed with reference to ion phase space vortices and clump formation in collisionless plasmas.
A perturbation solution for long internal waves
Authors: Jou Wen-Huei , Weissman Michael A.
Published: July 2002

The propagation of a long internal wave along a stratified shear layer is examined using the method of inner–outer expansions. In the limit that the thickness parameter μ=kh→0, where k is the wavenumber and h the thickness of the layer, the outer system is reduced to the two-layer model of Kelvin–Helmholtz flow for an arbitrary wave amplitude. The discontinuity of the outer solution at the ‘‘interface’’ is removed by an inner solution, which makes use of an inner coordinate aligned with the shear layer. The matching conditions for the higher-order outer problem modify the kinematic and dynamic conditions across the layer. These modifications give rise to the finite-thickness correction to the lowest-order Kelvin–Helmholtz dispersion relation. The double series expansion in the amplitude parameter ε=ak, where a is the amplitude, and the thickness parameter μ enables differentiation of the physical mechanisms associated with the finite-amplitude and finite-thickness effects. Thus the restriction μ≫ε, the usual requirement for the validity of the Taylor–Goldstein equation, does not apply. The present investigation demonstrates the relationship between the discontinuous Kelvin–Helmholtz system and finite-amplitude internal waves propagating in a continuously stratified shear flow.
Comment on ‘‘Richardson criteria for stratified vortex motions under gravity’’ [Phys. Fluids 2
9, 368 (1986)]
Authors: Lin S. J. , Pierrehumbert R. T.
Published: July 2002

It is shown that the sufficient conditions for stability established by Fung [Phys. Fluids 29, 368 (1986)] for the stratified vortex flows with velocity and density distribution varying in both the radial and axial directions can never be satisfied except for a trivial case, i.e., the solid body rotation. However, an estimate of the growth rate can still be made based on the technique developed by Fung.
The effect of asymmetric current-supporting electron velocity distributions on second harmonic electron cyclotron resonance heating: A ray-tracing treatment
Authors: Dendy R. O. , Montes A. , Leite J. P.
Published: July 2002

A limited asymmetric distortion of the distribution of electron velocities parallel to the magnetic field, sufficient to support the plasma current, is considered. Its effects on second harmonic electron cyclotron absorption of the extraordinary mode propagating away from normal incidence are quantified using a fully three-dimensional ray-tracing code. Significant deviations from the zero current power deposition profiles occur, both in real space and in velocity space. At higher densities, the effects of the extraordinary mode cutoff are softened. These considerations should be taken into account in future studies of temperature and current profile modification using second harmonic electron cyclotron absorption on COMPASS [Proceedings of the Course and Workshop on Applications of rf Waves to Tokamak Plasmas, Varenna (Monotypia Franchi, Perugia, 1985), Vol. II, p. 797].
Neoclassical flux-friction relations in arbitrary closed-end plasmas
Authors: Lau Y. T.
Published: July 2002

By following the moment approach of neoclassical transport theory and introducing a vector field D determined by the magnetic field, flux-friction relations in arbitrary closed-end plasmas are derived. Unlike the conventional ones, which are based on the flux coordinates, the new formulas can be used with any coordinate system. They are therefore more convenient for systems with complicated magnetic axes, such as DRAKON’s [in Controlled Fusion and Plasma Physics, 1981, Proceedings of the 10th European Conference, Moscow (European Physical Society, Budapest, 1981), Vol. 1, paper E-8; in Plasma Physics and Controlled Nuclear Fusion Research, 1982, Proceedings of the 9th International Conference, Baltimore (IAEA, Vienna, 1983), Vol. 3, p. 159], for which coordinates that follow the magnetic axes are more natural than the flux coordinates. As a simple application, the Pfirsch–Schlüter fluxes are obtained for arbitrary closed-end plasmas. In particular, the Pfirsch–Schlüter fluxes of nonaxisymmetric toroidal systems and DRAKON’s are calculated, showing that the Pfirsch–Schlüter fluxes of DRAKON’s are in general smaller than those of toroidal systems. The ambipolar potential and the parallel flows in the Pfirsch–Schlüter regime are also determined for arbitrary closed-end plasmas. The fluxes associated with the parallel flows are generally negligible in this regime.
A hydrodynamic description of disk galaxies
Authors: Sygnet J. F. , Pellat R. , Tagger M.
Published: July 2002

A consistent hydrodynamic model is established to describe the evolution of thin disk galaxies. A previous result showing that the heat flow vanishes in the limit of small epicyclic motion is generalized and simplified, providing a closure relation for the hierarchy of fluid equations. Its systematic use at first allows the obtainment of a new invariant of the fluid motion, governing the evolution of the anisotropic pressure (stress) tensor. This might represent a step toward explicit adiabatic laws relating the pressure and the density of the stellar gas. The same closure relation is then used to derive a powerful variational principle, describing linearized perturbations of the system, which takes the pressure anisotropy fully into account. This principle is independent of the usual WKB (‘‘tightly wound’’) approximation, which is easily recovered in the asymptotic study of spiral waves and spiral modes.
Nonlinear stability of Taylor’s vortex array
Authors: Lin S. P. , Tobak M.
Published: July 2002

It is proved that the two-dimensional Taylor vortex array, which is an exact unsteady solution of the Navier–Stokes equation, is globally and asymptotically stable in the mean with respect to three-dimensional periodic disturbances. A time-dependent bound on the decay rate of the kinetic energy of disturbances is obtained.
The three-dimensional boundary layer in the entry region of curved pipes with finite curvature ratio
Authors: Yao L. S. , Berger S. A.
Published: July 2002

The major flow development in the region within a distance O((aR)1/2) from the entrance of a curved pipe occurs near the pipe wall, where a is the radius of the pipe cross section, assumed circular, and R is the radius of curvature of the central axis of the pipe. A three-dimensional boundary-layer solution is obtained for elucidating the physics of this developing flow; in particular, the effect of nonzero curvature ratio α=a/R on the geometric similarity of the flow. The numerical results show that the series solution in terms of α is valid only when α≤0.1 and s≤0.1 (aR)1/2, where s is the distance from the inlet along the pipe axis. The crossover of the axial wall shear is purely a geometric property and its location is a strong function of α. It is also demonstrated that (aR)1/2 is the proper length scale by showing that the solution of the first region, s∼O(a), is included in that of the second, s∼O(aR)1/2.
Couette flow of a binary gas mixture
Authors: Valougeorgis Dimitris
Published: July 2002

The linearized binary model described by Hamel [Phys. Fluids 8, 418 (1964)] is used to obtain a set of kinetic equations and boundary conditions for the Couette flow problem. The derived set of two coupled integrodifferential equations is solved by iteration implementing standard discretization techniques. Highly accurate numerical results are presented for the mass velocity distribution and the total shear stress of the binary gas system.
Particle motion in vorticity-conserving, two-dimensional incompressible flows
Authors: Brown Michael G. , Samelson Roger M.
Published: July 2002

It is shown that particle motion is integrable in any vorticity-conserving, two-dimensional incompressible flow if the vorticity is a differentiable function whose gradient never vanishes. More generally, the result is true if any Lagrangian invariant replaces the vorticity.
Vortex stripping and the erosion of coherent structures in two-dimensional flows
Authors: Mariotti Annarita , Legras Bernard , Dritschel David G.
Published: July 2002

This paper studies the erosion of a monotonically distributed vortex by the joint action of inviscid stripping, induced by an externally imposed adverse shear, and viscous diffusion, either in the form of Newtonian viscosity or hyperviscosity. It is shown that vortex erosion is greatly amplified by the presence of diffusion; abrupt vortex breakup or gradual quasi-equilibrium evolution depend crucially on the strain to peak vorticity ratio and on the Reynolds number. Peculiar, unexpected effects are observed when hyperviscosity is used in place of Newtonian viscosity.
Acoustic energy exchange in compressible turbulence
Authors: Miura Hideaki , Kida Shigeo
Published: July 2002

The mechanism of exchange of kinetic energy and internal energy in a three-dimensional forced compressible turbulence is investigated by the analysis of a direct numerical simulation data. The Helmholtz decomposition of ρrlxu (ρ is the density of a fluid and u is the velocity), which yields positive–definite spectra of the compressive and rotational kinetic energies, is employed for exploring the relative importance of various terms in the kinetic energy equation. It is shown that the pressure–dilatation term makes a dominant contribution to the exchange of compressive kinetic energy and internal energy. This exchange occurs periodically at all wave numbers, the period of which is inversely proportional to the wave number. This indicates that acoustic waves take part in the exchange of energy.
Instability of a deflagration wave propagating with finite Mach number
Authors: Kadowaki Satoshi
Published: July 2002

The hydrodynamic instability of a deflagration wave propagating with finite Mach number has been investigated. This paper deals with a wave front of deflagration as a surface of hydrodynamic discontinuity, and considers the pressure change through the wave front. The instability of the deflagration wave with respect to infinitesimal fluctuations is analyzed, and the relation between growth rates of fluctuations and their wave numbers (dispersion relation) is obtained. The obtained dispersion relation is consistent with the Darrieus–Landau solution when the Mach number is sufficiently small. Increased value of the Mach number causes larger pressure differences, and causes high growth rates compared with the results of Darrieus and Landau. Therefore, one should take account of the pressure change in the stability analysis of the deflagration wave propagating with considerably fast velocity.
The effect of three-dimensional freestream disturbances on the supersonic flow past a wedge
Authors: Duck Peter W. , Lasseigne D. Glenn , Hussaini M. Y.
Published: July 2002

The interaction between a shock wave (attached to a wedge) and small amplitude, three-dimensional disturbances of a uniform, supersonic, freestream flow are investigated. The paper extends the two-dimensional study of Duck et al. [P W. Duck, D. G. Lasseigne, and M. Y. Hussaini, “On the interaction between the shock wave attached to a wedge and freestream disturbances,” Theor. Comput. Fluid Dyn. 7, 119 (1995) (also ICASE Report No. 93-61)] through the use of vector potentials, which render the problem tractable by the same techniques as in the two-dimensional case, in particular by expansion of the solution by means of a Fourier-Bessel series, in appropriately chosen coordinates. Results are presented for specific classes of freestream disturbances, and the study shows conclusively that the shock is stable to all classes of disturbances (i.e., time periodic perturbations to the shock do not grow downstream), provided the flow downstream of the shock is supersonic (loosely corresponding to the weak shock solution). This is shown from our numerical results and also by asymptotic analysis of the Fourier-Bessel series, valid far downstream of the shock.
Gas-liquid heat transfer in a bubble collapsing near a wall
Authors: Yuan H. , Prosperetti A.
Published: July 2002

The collapse of a gas bubble near a solid wall is studied numerically by assuming the liquid to be incompressible and the Mach number of the gas flow to be small. The liquid motion is simulated by a boundary integral method and the gas thermo-fluid dynamics by finite differences on a boundary-fitted grid. With the physical properties of a liquid monopropellant, it is found that the liquid heating is essentially localized in the microjet, but is probably not sufficient to cause spontaneous ignition. The reasons for this conclusion — that, while being in general agreement with available experimental evidence, is at variance with deductions from previous spherical collapse calculations — are elucidated.
Model dynamics and vertical collapse in decaying strongly stratified flows
Authors: Majda Andrew J. , Grote Marcus J.
Published: July 2002

Recent laboratory experiments with decaying strongly stratified grid turbulence at moderate Reynolds numbers reveal remarkable behavior. These experiments document the evolution of an initial sea of columnar dipole vortex pairs with dominant vertical vorticity to stratified “pancake” vortex sheets with dominant horizontal vorticity, together with a concurrent dominance of vertical dissipation of kinetic energy as compared with horizontal dissipation. Here we build exact solutions of the equations for low Froude number limiting dynamics, which capture basic qualitative features observed in the above experiments. Unlike the actual turbulent experiments, these exact solutions are laminar and do not involve a cascade of many scales in the horizontal. The exact solutions of the limiting dynamics involve a periodic array of dipole vortices in a weakly vertically sheared horizontal flow. The effect of finite Rossby numbers on the collapse of these exact solutions is also described here. At moderately large Rossby numbers, the effect of rotation is to inhibit the vertical collapse process.
Image of absolute instability in a liquid jet
Authors: Vihinen I. , Honohan A. M. , Lin S. P.
Published: July 2002

The existence of absolute instability in a liquid jet has been predicted for some time. The disturbance grows in time and propagates both upstream and downstream in an absolutely unstable liquid jet. The image of absolute instability is captured in the NASA 2.2 sec drop tower, and is reported here.
Lagrangian dynamics in high-dimensional point-vortex systems
Authors: Weiss Jeffrey B. , Provenzale Antonello , McWilliams James C.
Published: July 2002

We study the Lagrangian dynamics of systems of N point vortices and passive particles in a two-dimensional, doubly periodic domain. The probability distribution function of vortex velocity, pN, has a slow-velocity Gaussian component and a significant high-velocity tail caused by close vortex pairs. In the limit for N→∞, pN tends to a Gaussian. However, the form of the single-vortex velocity causes very slow convergence with N; for N≈106 the non-Gaussian high-velocity tails still play a significant role. At finite N, the Gaussian component is well modeled by an Ornstein-Uhlenbeck (OU) stochastic process with variance σN=N ln N/2π. Considering in detail the case N=100, we show that at short times the velocity autocorrelation is dominated by the Gaussian component and displays an exponential decay with a short Lagrangian decorrelation time. The close pairs have a long correlation time and cause nonergodicity over at least the time of the integration. Due to close vortex dipoles the absolute dispersion differs significantly from the OU prediction, and shows evidence of long-time anomalous dispersion. We discuss the mathematical form of a new stochastic model for the Lagrangian dynamics, consisting of an OU model combined with long-lived close same-sign vortices engaged in rapid rotation and long-lived close dipoles engaged in ballistic motion. From a dynamical-systems perspective this work indicates that systems of dimension O(100) can have behavior which is a combination of both low-dimensional behavior, i.e., close pairs, and extremely high-dimensional behavior described by traditional stochastic processes.
A numerical study of periodic disturbances on two-layer Couette flow
Authors: Li Jie , Renardy Yuriko Y. , Renardy Michael
Published: July 2002

The flow of two viscous liquids is investigated numerically with a volume of fluid scheme. The scheme incorporates a semi-implicit Stokes solver to enable computations at low Reynolds numbers, and a second-order velocity interpolation. The code is validated against linear theory for the stability of two-layer Couette flow, and weakly nonlinear theory for a Hopf bifurcation. Examples of long-time wave saturation are shown. The formation of fingers for relatively small initial amplitudes as well as larger amplitudes are presented in two and three dimensions as initial-value problems. Fluids of different viscosity and density are considered, with an emphasis on the effect of the viscosity difference. Results at low Reynolds numbers show elongated fingers in two dimensions that break in three dimensions to form drops, while different topological changes take place at higher Reynolds numbers.
Short-wavelength instability of a vortex in a multipolar strain field
Authors: Le Dizès Stéphane , Eloy Christophe
Published: July 2002

The extension of the elliptical instability to a rotational flow with an n-fold symmetry is considered in this Brief Communication. Based on the geometrical optics approach, the short-wavelength stability analysis of the multipolar strained vortex described by the streamfunction (in polar coordinates) Ψn=r2/2−(p/n)rn cos(nθ) is carried out for n=3,4,5 and p⩾0. Both the growth rate and the wavevector properties of the most unstable wave are computed as a function of the real parameter εn=p(2nC/(n−2))(n−2)/2 which characterizes the geometry of the streamline Ψn=C. For almost circular flows, i.e., in the small εn limit, exact estimates are also obtained by perturbation methods.
Three-dimensional centrifugal-type instabilities of two-dimensional flows in rotating systems
Authors: Sipp Denis , Jacquin Laurent
Published: July 2002

This paper deals with the stability of incompressible inviscid planar basic flows in a rotating frame. We give a sufficient condition for such flows to undergo three-dimensional shortwave centrifugal-type instabilities. This criterion reduces to the Bradshaw–Richardson (1969) or Pedley (1969) criterion in the specific case of parallel shear flows subject to rotation, to Rayleigh’s centrifugal criterion (1916) in the case of axisymmetric vortices in inertial frames, to the Kloosterziel and van Heijst (1991) criterion in the case of axisymmetric vortices subject to rotation and to Bayly’s criterion (1988) in the case of general two-dimensional flows in inertial frames. The criterion states that a steady 2D basic flow subject to rotation Ω is unstable if there exists a streamline for which at each point 2(V/R+Ω)(W+2Ω)<0 where W is the vorticity of the streamline, ℛ is the local algebraic radius of curvature of the streamline and V is the local norm of the velocity. If this condition is satisfied then the flow is unstable according to the geometrical optics method introduced by Lifschitz and Hameiri (1991), which consists in following wave packets along the flow trajectories using a Wentzel–Kramers–Brillouin formalism. When the streamlines are closed, it is further shown that a localized unstable normal mode can be constructed in the vicinity of a streamline. As an application, this new criterion is used to study the centrifugal-type instabilities in the Stuart vortices, which is a family of exact solutions describing a row of periodic co-rotating eddies. For each solution of that family and for each rotation parameter f=2Ω, we give the unstable streamline interval, according to the criterion of instability. This criterion gives only a sufficient condition of centrifugal instability. The equations of the geometrical optics method are therefore numerically solved to obtain the true centrifugally unstable streamline intervals. It turns out that our criterion gives excellent results for highly concentrated vortices, i.e., the two approaches yield the same unstable streamline intervals. In less concentrated vortices, some streamlines undergo centrifugal instability although our criterion is not fulfilled. From these numerical results, another criterion of centrifugal instability for a flow with closed streamlines is conjectured which reduces to the change of sign of the absolute vorticity W+2Ω somewhere in the flow.
Flow structure and heat transfer in a horizontal converging channel heated from below
Authors: Chiu Wilson K. S. , Richards Cristy J. , Jaluria Yogesh
Published: July 2002

A detailed experimental study is carried out to investigate the flow structure and heat transfer in a converging horizontal channel with localized heating on the bottom surface. This configuration arises in typical chemical vapor deposition (CVD) systems and the impact of the transport mechanisms on the deposition process is of particular interest. Experimental observations of various flow circumstances indicate the appearance of longitudinal and transverse roll structures. These flow structures have direct implications with respect to CVD processing where deposition rates and film quality are critically dependent on reactive species and byproduct transport onto the deposition surface. Different flow regimes are observed for typical operating conditions, and their existence is described using regime maps. Regime boundaries are correlated in terms of important parameters that describe the flow rate, heating rate, and tilt angle. A regime transition from longitudinal rolls to transverse rolls occurs at around Gr/Re2=6000, while the critical Rayleigh number is found to be dependent on the flow. Heat transfer correlations are also derived and it is seen that the heating rate is the dominant effect on the susceptor temperature, while the effects of flow rate and tilt angle are small. The implication of heat transfer results on mass transfer are discussed for horizontal CVD reactors. The flow structure and heat transfer phenomena observed in this study yield valuable insight into the basic buoyancy and forced-flow-induced transport in horizontal channels as well as guidelines for the modeling and design of CVD systems.
Excitation and propagation of electrostatic ion-cyclotron waves in plasma with structured transverse flow
Authors: Koepke M. E. , Carroll J. J. , Zintl M. W.
Published: July 2002

Attention has been drawn to the recently discovered inhomogeneous energy-density driven (IEDD) instability because of its implications for plasmas in the laboratory and in space. To be excited, the IEDD waves require structured transverse flow, and a range of suitably inhomogeneous transverse-velocity profiles are possible in the West Virginia University Q Machine. This paper describes experimentally measured aspects of this equilibrium essential to the wave growth, the three-dimensional mode structure of the propagating waves, and a transition in the magnetic-field-aligned phase velocity that reveals a new aspect of the instability mechanism.
Thermostatted δf
Authors: Krommes John A.
Published: July 2002

The δf simulation method is revisited. Statistical coarse graining is used to rigorously derive the equation for the fluctuation δf in the particle distribution. It is argued that completely collisionless simulation is incompatible with the achievement of true statistically steady states with nonzero turbulent fluxes because the variance W of the particle weights w grows with time. To ensure such steady states, it is shown that for dynamically collisionless situations a generalized thermostat or “W stat” may be used in lieu of a full collision operator to absorb the flow of entropy to unresolved fine scales in velocity space. The simplest W stat can be implemented as a self-consistently determined, time-dependent damping applied to w. A precise kinematic analogy to thermostatted nonequilibrium molecular dynamics is pointed out, and the justification of W stats for simulations of turbulence is discussed. An extrapolation procedure is proposed such that the long-time, steady-state, collisionless flux can be deduced from several short W-statted runs with large effective collisionality, and a numerical demonstration is given.
Effect of conduction-band nonparabolicity on quantized energy levels of a quantum well
Authors: Hiroshima T. , Lang R.
Published: July 2002

The effects of conduction-band nonparabolicity on quantized energy levels of an In0.5Ga0.5As /In0.5Al0.5As quantum well have been analyzed using the original Luttinger–Kohn ‘‘effective mass’’ equation, which is, in principle, valid as long as the perturbation to the periodic lattice potential can be regarded as slowly varying. The results differ substantially from those previously reported which employed the energy-dependent effective mass approach.
Observation of boron acceptor neutralization in silicon produced by CF4 reactive ion etching or Ar ion beam etching
Authors: Mu X. C. , Fonash S. J. , Singh R.
Published: July 2002

It is demonstrated, using two very different techniques (viz., CF4 reactive ion etching and Ar ion beam etching), that dry etching exposure causes extensive boron acceptor neutralization in silicon. This boron neutralization is observed to occur as far as microns below the etched surface with etching exposures of only ≂1 min. Neutralization occurs if the temperature does not exceed about 180 °C during etching; it may be annealed out by subsequent exposure to such temperatures. Adsorbed water vapor or, alternatively, hydrogen inherent in the silicon is proposed to provide hydrogen-related species which cause this neutralization. Protons created in the plasma from these sources, or hydrogen-related species directly liberated from their sources on or in the solid, may be injected into the silicon during the energetic ion impact constantly present in dry etching. This observation of boron neutralization deep below the etched surface demonstrates that dry etching exposure causes extensive permeation of foreign species into the etched material.