bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman up-scattering in long-scale-length, laser-produced plasmas
Authors: Batha S. H. , Meyerhofer D. D. , Simon A. , Drake R. P.
Published: July 2002

Frequency up-shifted light, produced by the interaction of a 351 nm laser with a plasma large enough to produce efficient down-shifted Raman backscattering, was measured. The plasma was created using low-intensity, 351 nm laser beams striking a 15 μm thick CH foil. During the period of interest, the electron temperature was estimated to be 1 keV, the peak density to be 0.15 of the critical density, and the plasma scale length to be 1500 μm at one-tenth critical density. The most straightforward explanation of the up-shifted light was found to be scattering from externally enhanced plasma waves. Scattering from a plasma mode created by coupling a large ion wave to a stimulated-Raman-scattering-produced plasma wave is also possible. Collisional damping precludes either scattering from a reflected electron-plasma wave produced by SRS or scattering from a plasma wave resulting from the onset of the Langmuir decay instability.
Nonlinear, relativistic return current sheath for an ion-focused beam
Authors: Whittum David H.
Published: July 2002

The plasma equilibrium is analyzed for propagation in the ion-focused regime, of a high-current relativistic electron beam through a radially infinite, collisionless plasma. Beam current variation is assumed adiabatic over an electron plasma period and ion motion is neglected. The magnetohydrodynamic equations are reduced to a nonlinear ordinary differential equation describing the plasma return current sheath and solutions are characterized by the Budker parameter, with length scale set by the plasma skin depth.
Electrode heating in a wire-to-plane arc
Authors: Jog M. A. , Cohen I. M. , Ayyaswamy P. S.
Published: July 2002

A steady wire-to-plane electric discharge has been modeled in a prolate spheroidal coordinate system with the wire shape taken as a hyperboloid of revolution. A set of continuum conservation equations for the charged particle densities and temperatures together with Poisson’s equation for the self-consistent electric potential describe the steady electric discharge process. These equations have been solved numerically to obtain ion and electron densities, temperature distribution, and electrode heat fluxes. Particle densities show the main body of the arc is quasineutral bounded by space charge sheaths at both electrodes. The temperature is greatest in a region around the discharge axis about one-third of the distance from the wire to the plane. Strong electric fields are concentrated in the electrode sheaths. The heat flux to the wire is sharply peaked near the tip but on the plane it decays slowly away from the discharge axis. The knowledge of heat transfer from the arc to the electrodes is useful in determining arc parameters that govern the ball formation process used in wire bonding of microelectronic semiconductor chips as well as welding processes.
A quasilinear description for fast-wave minority heating permitting off-magnetic axis heating in a tokamak
Authors: Catto Peter J. , Myra J. R.
Published: July 2002

By keeping only the essential physical effects the simplest as possible, experimentally relevant quasilinear description of fast-wave minority heating in a tokamak is derived in a tractable form suitable for convenient numerical implementation. The resulting quasilinear operator retains all trapped and passing particle effects and permits the minority heating resonance line to be offset from the magnetic axis. Rather than averaging the infinite, homogeneous quasilinear operator, the derivation is carried out in tokamak geometry to properly treat the merging of distinct rf–particle interaction regions. Merging occurs when the banana tips of trapped particles are in the vicinity of the minority resonance or, for both trapped and passing particles, when the minority resonance is tangent or nearly tangent to a flux surface. A phenomenological model of the influence of collisions on merging is also presented.
Stabilization of sawteeth in tokamaks by sheared poloidal flows
Authors: Kleva Robert G.
Published: July 2002

The resistive internal kink mode in a current profile with a central safety factor q(0)<1 is shown to be stabilized by sheared poloidal flows. The internal kink mode is found to be stabilized when the local gradient in the poloidal rotation frequency ω at the q=1 resonant surface r1 is larger than the local gradient in the shear Alfvén frequency ωA=k∥(r)VA, where VA is the Alfvén velocity and the parallel wave number k∥(r)=R−1[1−1/q(r)] with R the major radius of the torus and q(r) the safety factor profile in the minor radial coordinate r; i.e., ‖dω/dr‖r1≥‖dωA/dr‖r1. The internal kink is stabilized by sub-Alfvénic flows in which the peak velocity is only a very small fraction of the Alfvén velocity. When the shear in the rotation frequency at the resonant surface is too weak to linearly stabilize the internal kink mode, then the kink mode can still be nonlinearly stabilized, resulting in incomplete reconnection, if the shear in the rotation increases just outside the resonant surface at larger radii.
Breakdown voltage characteristic of a pseudospark device
Authors: Rhee M. J. , Ding B. N.
Published: July 2002

The breakdown voltage of a pseudospark device is measured for a wide range of gas pressure and anode–cathode gap distance. The data are analyzed by least-squares-fit methods, and the breakdown voltage is expressed by an empirical function of the gas pressure and the anode–cathode gap distance. It is found that the breakdown voltage is a function of p2d, the product of the pressure squared and anode–cathode gap distance. This is in contrast to that of a parallel-plate system that is described by Paschen’s curve, where the breakdown voltage is a function of pd.
Midplane measurements of charged fusion product diffusion in the Tokamak Fusion Test Reactor
Authors: Boivin R. L. , Zweben S. J.
Published: July 2002

A radially movable detector located just below the outer midplane of the Tokamak Fusion Test Reactor (TFTR) [in Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 27] has been used to study the confinement of charged fusion products (CFP’s). The scrape-off length of escaping CFP flux was measured using perturbative techniques based on shadowing the detector with different obstacles. Experimental results indicate the presence of a diffusive process for trapped particles. Derived diffusion step sizes were found to be in excellent agreement with those expected from toroidal magnetic field (TF) stochastic ripple diffusion.
Relativistic electron nonlinear orbit for a combined plasma wave wiggler and an axial guide magnetic field near magnetoresonance
Authors: Yu Junsheng , Gao Qingdi , Lu Bo
Published: July 2002

The relativistic electron nonlinear orbit is investigated for a combined electrostatic plasma wave wiggler and an axial guide magnetic field free electron laser near magnetoresonance. Through simplification, the perpendicular orbit equation is reduced to be identical to the wave-driven nonlinear oscillator equation [R. C. Davidson, Phys. Lett. A 126, 21 (1987)] but with different expressions of parameters. It is found that the maximum perpendicular orbit excursion Δxmax is in proportion to (aw/γ0)1/3, which is largely determined by the nonlinear effect, where aw is the wiggler strength and γ0 is the initial relativistic factor. The analytical results are in good agreement with the numerical calculation for small ratio of aw to γ0.
Absorption of nearly perpendicularly propagating waves in the second harmonic layer
Authors: Imre Kaya , Weitzner Harold
Published: July 2002

Propagation of waves nearly perpendicular to the equilibrium magnetic field and incident to the second electron-cyclotron harmonic layer is investigated in an inhomogeneous weakly relativistic plasma. The resonance region is treated by a boundary layer analysis, and the solutions are matched to the geometrical optics solutions outside the layer. This approach allows one to investigate the transmission, mode coupling, reflection, absorption, and the effects of relativistic broadening. This work extends a previous investigation with the purely perpendicular propagation. It is shown that the mode conversion and reflection rapidly cease to be of importance outside a narrow propagation cone as the doppler broadening becomes predominant. The geometrical optics approach, which breaks down in the purely perpendicular propagation, then becomes valid. It is also shown that the transmission coefficient and the reflection from the high-field side incidence are not altered within this cone.
Statistical theories of Langmuir turbulence II: Subsonic to sonic transition
Authors: DuBois D. F. , Rose Harvey A. , Nicholson D. R.
Published: July 2002

The subsonic limit of the quadratic direct interaction approximation (DIA) applied to the Zakharov equations is compared with the cubic DIA applied to the nonlinear Schrödinger equation, which is the subsonic limit of the Zakharov equations. Comparisons with Monte Carlo simulations of a truncated system show that the first theory more accurately describes the regime of stationary turbulence, while the second theory more accurately describes the subsonic evolution of the modulational instability. The weak turbulence limits of the two theories describe the sonic and subsonic regimes, respectively. The addition of vertex corrections to the DIA leads to a hybrid weak turbulence theory that smoothly interpolates between the sonic and subsonic regimes.
Nonambipolar radial particle transport in a tandem mirror
Authors: Hooper E. B. , Cohen R. H. , Correll D. L. , Gilmore J. M. , Grubb D. P.
Published: July 2002

Nonambipolar transport has been measured in the tandem mirror TMX-U [Phys. Rev. Lett. 53, 783 (1984)] by applying charge conservation to the measured electron currents to the end walls. The resulting confinement time τ⊥ is found to depend upon the central-cell potential φ approximately as τ⊥(msec) =3φ(kV)−2. The transport rate, deduced from the data, agrees to within a factor of 1–5 with resonant-transport theory applied to the measured plasma parameters. Attempts to include radial effects by modeling the plasma self-consistently using resonant transport are less successful; near the axis the transport coefficients become too small to explain the equilibrium. Modeling using an ad hoc φ−2 law for the transport coefficients is more successful.
Theory of energetic trapped particle-induced resistive interchange-ballooning modes
Authors: Biglari Hamid , Chen Liu
Published: July 2002

A theory describing the influence of energetic trapped particles on resistive-interchange-ballooning modes in tokamaks is presented. It is shown that a population of hot particles trapped in the region of adverse curvature can resonantly interact with and destabilize the resistive interchange mode, which is stable in their absence because of favorable average curvature. The mode is different from the usual resistive interchange mode not only in its destablization mechanism, but also in that it has a real component to its frequency comparable to the precessional drift frequency of the rapidly circulating energetic species. Corresponding growth rate and threshold conditions for this trapped-particle-driven instability are derived and finite banana width effects are shown to have a stabilizing effect on the mode. Finally, the ballooning/tearing dispersion relation is generalized to include hot particles, so that both the ideal and the resistive modes are derivable in the appropriate limits.
Finite beta in reversed field pinches
Authors: Strauss H. R.
Published: July 2002

Beta is computed self-consistently in reversed field pinches, using reduced magnetohydrodynamics with transport. Qualitative agreement with the OHTE experiment [Bull. Am. Phys. Soc. 29, 330 (1984)] is found. A derivation of the finite beta reduced equations is presented. The relation of the dynamo effect to anomalous transport is described. The scaling of the δB/B with S is obtained.
Finite pressure ballooning mode stability in toroidal equilibriums
Authors: Llobet X. , Berk H. L. , Rosenbluth M. N.
Published: July 2002

In this paper the effect of finite pressure on the ballooning instability in toroidal magnetohydrodynamics (MHD) equilibria of steep boundary stellarators and tokamaks is examined. Ballooning modes tend to arise near the place where the local shear vanishes and the normal curvature (the curvature component perpendicular to the flux surface, pointing away from the magnetic axis) is negative. It is shown how the pressure gradient determines the position of the shearless points, and demonstrated in detail how this effect explains the existence of second stability in tokamaks. For large aspect ratio circular cross-section tokamaks the second stability condition is found to scale as α=const S1.25. Stellarators are inherently more stable because of the negative vacuum shear, which at moderate pressure gradients allows the zero shear point to localize on the inner side of the flux surface. However, at high pressure gradients the Pfirsch–Schlüter current produces a positive mean shear when the total toroidal current on a flux surface is zero. This causes the zero shear point to localize on the outer edge near the vertical extremes of the flux surface. This effect, together with helical contributions to the helical curvature, allows for ballooning instability to arise. At higher pressure gradients, with zero net toroidal current, an unstable ballooning mode which localizes to within a helical period always arises where the normal curvature is unfavorable.
Magnetic drift induced ion-cyclotron maser instability
Authors: Tsang K. T. , Hafizi B.
Published: July 2002

It is shown that cyclotron maser-type instabilities are possible for ions. Unlike the electron-cyclotron maser instability, the azimuthal bunching mechanism is caused by the magnetic drift rather than relativistic effects. For a delta function distribution, it is found that the extraordinary modes over a wide range of cyclotron harmonic numbers can be destabilized with growth rates comparable to the ion-cyclotron frequency.
The unsteady characteristics of laminar juncture flow
Authors: Thomas Andrew S. W.
Published: July 2002

The popular view that juncture flows are dominated by an array of steady horseshoe vortices is not universally correct. Visualizations are presented that show that vortices do form but that they undergo a periodic generation and shedding motion. The temporal scale of this cycle does not depend strongly on the characteristics of the incoming boundary layer. Also, the vortices exhibit an unusual pinching-off process in the wake of the cylinder leading to new horseshoe-vortex filaments existing behind the cylinder, which are then convected downstream.
Numerical study of drift-resistive modes in general toroidal geometry
Authors: Kim Jin-Yong , Cho Guang-Sup , Choi Duk-In
Published: July 2002

The linearized resistive boundary layer equations, including electron diamagnetic drift terms in general toroidal geometry, are solved numerically to obtain the dispersion relations of the resistive ballooning modes as well as the resistive interchange and tearing modes in both highly collisional and semicollisional regimes. Numerical solutions with high accuracy are obtained using two different numerical methods. Various physical effects, such as electron diamagnetic drift, parallel and perpendicular compression, and average curvature, are investigated in a wide range of equilibrium parameters. The present work is believed to be the first numerical study of the resistive ballooning modes with finite electron diamagnetic drift effects. These numerical solutions cover a wider range of parameter spaces than thus far available from analytical and other numerical methods. The new solutions contain previously known results as special cases. From the present study it is found that for the resistive ballooning and tearing modes, which have similar forms of dispersion relation, ion sound compression gives a substantial stabilizing effect, while ion-polarization compression gives a destabilizing effect and the stabilizing effect of favorable average curvature is significantly reduced as the diamagnetic drift effect increases. For the resistive interchange modes, these numerical results are shown to be in good agreement with previous results.
Energy transport equations for toroidal plasmas
Authors: Shaing K. C. , Nakamura Y. , Wakatani M.
Published: July 2002

The differences in the energy transport equations for tokamaks and stellarators associated with ΓrEr are discussed, where Γr is the radial particle flux and Er is the radial electric field. The energy exchange quantities are evaluated in terms of poloidal plasma flow, pressure gradient, and Er. It is found that for stellarators the radial flux Γr in ΓrEr should be Γr =Γna, the nonaxisymmetric flux, instead of the total particle flux.
Ion confinement in a simple mirror stabilized by surface line tying
Authors: Good Timothy N. , Thompson Harold R. , Rynn Nathan
Published: July 2002

Experiments were conducted using a barium plasma in a simple axisymmetric mirror machine that was stabilized against the magnetic curvature-driven interchange mode by surface line tying. Ion cyclotron resonance heating was applied to heat ions, producing a mirror-trapped ion population. The ion-velocity distribution function was diagnosed nonperturbatively by using laser-induced fluorescence techniques. The rf heated distribution was a non-Maxwellian in fI(v⊥), and a sloshing distribution in fI(v∥). When the plasma source was cut off, the plasma decay was studied to determine the confinement properties of the plasma afterglow and the effects of the stabilizing blanket. Mirror-trapped ions were lost axially at the classical ion–ion collision rate. Analysis of the temporal evolution of radial profiles of ion-energy density, in the afterglow, indicates that axial loss rates dominanted over radial losses.
Reply to the comments of Stangeby
Authors: Hutchinson I. H.
Published: July 2002

While the exact value of viscosity is indeed open to question, additional clarifying discussion, with reference to subsequent work, shows that zero viscosity is a singular case, qualitatively different from nonzero. Unity remains arguably the most plausible order of magnitude for the viscosity/diffusivity ratio. A new two-dimensional model calculation shows that even nondiffusive transport caused by coherent flows past the probe is well modeled by the viscous calculation, but not by the inviscid.
Discrete shedding modes of the cylinder wake in a jet with a homogeneous core
Authors: Brede M. , Eckelmann H. , König M. , Noack B. R.
Published: July 2002

The von Kármán vortex street behind a circular cylinder in a laminar homogeneous jet core is experimentally investigated. The Strouhal–Reynolds number relationships are measured for various shear-layer thicknesses and aspect ratios. The experimental Strouhal number values are found to collapse with the discrete vortex shedding modes, which were observed for boundary-layer end conditions. The results indicate that the shedding modes are independent from the end conditions, but are an intrinsic feature of the shedding process of an infinitely long cylinder. The experimentally assumed shedding mode, however, is strongly dependent of the geometric parameters, like the shear-layer thickness and the aspect ratio. The observed tendencies can be made physically plausible.
Convection effects on radial segregation and crystal–melt interface in vertical Bridgman growth
Authors: Tanveer S.
Published: July 2002

The influence of convection caused by horizontal heat transfer through the sides of a vertical Bridgman apparatus is studied analytically. The case when the heat transfer across the side walls is small is considered, so that the resulting interfacial deformation and fluid velocities are also small. This allows one to linearize the Navier–Stokes equations and express the interfacial conditions about a planar interface through a Taylor expansion. Using a no-tangential stress condition on the side walls, asymptotic expressions for both the interfacial slope and radial segregation at the crystal–melt interface are obtained in closed form in the limit of large thermal Rayleigh number. It is suggested that these can be reduced by appropriately controlling a specific heat transfer property at the edge of the insulation zone on the solid side.
Capillary wave scattering from a surfactant domain
Authors: Chou T. , Lucas S. K. , Stone H. A.
Published: July 2002

The study of capillary wave scattering by a circular region with different interfacial properties from the rest of an otherwise homogeneous interface is motivated by experiments on wave attenuation at a monolayer-covered air–water interface where domains of one surface phase are dispersed in a second surface phase. Here the scattering function is calculated for an incident wave of frequency ω (wavevector k0) scattering from an isolated circular domain of radius a with surface tension σ1 which is imbedded in an otherwise infinite interface of surface tension σ0. The underlying fluid is treated as irrotational and the three-dimensional flow problem coupling the heterogeneous surface to the underlying liquid is reduced to a set of dual integral equations, which are solved numerically. With this solution the scattering amplitudes and the total scattering cross sections are calculated as a function of the surface tension ratio σ0/σ1 and incident wavenumber k0a. The analogous problem of a discontinuous change in bending rigidity is also considered and the solution to the complete viscous problem is outlined in the Appendix. Experimental implications of these results are discussed.
Quantitative molecular-mixing measurements in a round jet with tabs
Authors: Zhang Shucheng , Schneider Steven P.
Published: July 2002

The near field of chemically reacting round jets was studied at Reynolds numbers between 5.2×103 and 3.2×105. The measured chemical product was used to infer molecular mixing. The effects of mixing tabs and Reynolds number were determined. Four tabs enhance molecular mixing more than two tabs do, although two tabs cause the velocity profile to decay faster than four tabs do. The product formation rate is nearly constant with respect to streamwise distance, for turbulent jets with zero, two, and four tabs. The amount of chemical product is determined by both the molecular mixing and the equivalence ratio. The chemical product and the effects of the tabs are independent of Reynolds number, once the jet is turbulent. The effects of the tabs are smaller in a transitional jet than in a turbulent jet.
Onset and end of drop formation along the surface of turbulent liquid jets in still gases
Authors: Wu P.-K. , Faeth G. M.
Published: July 2002

The onset and end of drop formation along the surface of turbulent liquid jets in still gases was studied for liquid/gas density ratios greater than 500 where aerodynamic effects are small. Measurements were correlated using a phenomenological turbulent breakup theory based on the interactions between the turbulence energy spectrum and the energy requirements for drop formation. The onset and end of drop formation along the surface generally was associated with turbulent eddies in the internal and large-eddy subranges of the turbulence spectrum, respectively, although breakup of the entire liquid column ends breakup along the surface in some instances, as well.
On the streak spacing and vortex roll size in a turbulent channel flow
Authors: Rajaee M. , Karlsson S. , Sirovich L.
Published: July 2002

Streamwise high vorticity rolls and streaks in the turbulent channel flows have been the subject of many studies due to their important role in turbulence production, as a result of sweeping, ejection, and bursting of these structures. Understanding the physics of these streamwise structures is important in controlling drag producing events. Investigations of the average streak spacing of the low-speed streaks have resulted in the generally accepted range of λ+=λ̄uτ/ν=100±20, where λ̄ is the mean spanwise spacing between streaks, normalized to the viscous length ν/uτ. It is also reported, for y+≤30, that the streak spacing grows nearly linearly with distance from the wall. The previous studies mostly have focused on distances close to the wall. Here we report on correlation measurements extended into the log layer, which show that the linear growth of the vortex diameter and the streak spacing extends well in the log layer. Arguments are presented to distinguish these two measures.
Convection rolls and their instabilities in the presence of a nearly insulating upper boundary
Authors: Clever R. M. , Busse F. H.
Published: July 2002

The problem of steady convection rolls and their instabilities in a fluid layer heated from below is studied numerically in the case of a highly conducting, rigid lower and a nearly insulating, stress-free upper boundary. A Galerkin method is used to obtain two-dimensional solutions in dependence on the Rayleigh number R and the wave number α for different values of the Prandtl number P. Their stability is analyzed through the superposition of general three-dimensional infinitesimal disturbances. Most instabilities correspond qualitatively to those found in the case of symmetric highly conducting, rigid boundaries. A new instability, the subharmonic varicose instability, is found, however, which restricts the region of stable rolls toward a higher Rayleigh number in the case of moderate Prandtl numbers.
Experimental study of rotating disk instability. I. Natural flow
Authors: Jarre S. , Le Gal P. , Chauve M. P.
Published: July 2002

This article is devoted to the study of the rotating disk flow instability. This inflectional-type instability, called a cross-flow instability, exemplary of the transition to turbulence in three-dimensional boundary layers. We first present the experimental marginal stability curve of unstable waves obtained by hot-film probe measurements and compare it with the theoretical results available in the literature. The experiment is in accordance with different theoretical determinations of the linear threshold, but we note a difference between experimental and theoretical critical wave-number values. The unstable wave dynamics is then investigated by means of experimental dispersion curves (linking frequencies to the wave-number vector components) determined by two-probe measurements. The results show, in particular, the existence of traveling dispersive waves in the boundary layer of the rotating disk. Finally, we show that the emergence of nonlinear effects occurs very early in the system, far from the transition point.
Nonlinear Tollmien–Schlichting waves for a Blasius flow over compliant coatings
Authors: Ehrenstein Uwe , Rossi Maurice
Published: July 2002

In a previous work, we considered the influence of a compliant wall on finite-amplitude Tollmien–Schlichting (TS) waves in plane Poiseuille flow. In the present investigation we extend this analysis to the more relevant Blasius boundary-layer profile. Similarly to W. Koch [J. Fluid Mech. 243, 319 (1992)], who studied finite-amplitude TS waves over a rigid wall, we use a parallel flow assumption and expand the flow quantities in Fourier and Chebyshev modes. Our coating is of Kramer’s type. We present a model that takes into account the nonlinear fluid/structure interaction all over the flow field. The problem to be numerically feasible, we mainly focus on moderately flexible walls, being stable (at finite Reynolds numbers) with respect to flow-induced surface instability (FISI) waves. Our computations of nonlinear TS instability waves show that for a certain range of wall parameters two-dimensional finite-amplitude traveling waves exist well below critical Reynolds numbers predicted by linear theory, typical for a subcritical bifurcation behavior. Computations of the flow structure and skin friction of the finite-amplitude TS waves are provided as well.
Stability analysis of the generalized Rosenhead point vortex approximation
Authors: Nachbin André
Published: July 2002

A new desingularization procedure was recently used in the point vortex approximation. This numerical scheme (the generalized Rosenhead point vortex approximation) leads to a novel type of roll-up of the interface, which eventually intersects itself. A linear stability analysis is used to show the regularizing mechanism built into the method.
On the log-Poisson statistics of the energy dissipation field and related problems of developed turbulence
Authors: Gledzer E. , Villermaux E. , Kahalerras H. , Gagne Y.
Published: July 2002

An energy cascading model of intermittency involving rare localized regions of both large and/or weak energy dissipation (dynamical intermittency) is considered and compared to the case of intermittency arising from a large number of regions with nearly equal dissipation rates (space intermittency). The latter leads to the log-normal statistics of the dissipation rate while the first scenario leads to shifted log-Poisson distributions either for a large or for weak energy dissipation. The only difference between these two cases is that small values of dissipation (with respect to the maximum of PDF) are more probable for intermittency of the regions with weak dissipation than for intermittency of the regions with large values of dissipation. Some consequences are derived which show that Novikov’s inequalities are valid for intermittency with rare regions of a weak dissipation only. Different experimental data of probability distributions of dissipation are presented and compared to theoretical predictions. Some experimental evidences of quasi-two-dimensional vortical structures with weak dissipation are discussed. They suggest that the scenario involving dynamical intermittency with holes of dissipation could apply to a real world turbulence.
Growth of large scale structures in two-dimensional mixing layers
Authors: De Young David S.
Published: July 2002

A simple analytic model based on experimental data is presented for the growth of large scale structures and the mass inflow into them in two-dimensional incompressible mixing layers. The model draws on the experimental evidence for persistent large scale structures in the nonlinear phase of mixing layer development. The results are compared with previous work and with time dependent numerical simulations of mixing layer development using an array of point vortices. Both the simulations and previous results provide reasonable agreement with the model.
Potential/complex-lamellar descriptions of incompressible viscous flow
Authors: Yokota Jeffrey W.
Published: July 2002

In this manuscript a taxonomy of potential/complex-lamellar decompositions will be developed for describing the flow of an incompressible viscous fluid. Since these decompositions are not unique, they are best defined by their governing equations and not by their individual components. Within this framework we will construct a number of decompositions, both classical and contemporary, inviscid and viscous. Finally, we will use these results to derive the conditions under which an unsteady flow is circulation preserving. First, using a decomposition based on the Lighthill–Darwin–Hawthorne drift function, we will show that a flow ceases to be circulation preserving when the cross product of its circulation and drift function gradients are not harmonic. Then we will show that an unsteady flow can be circulation preserving only when it is an irrotational one as well.
Wavy rolls in the Taylor–Bénard problem
Authors: Auer M. , Busse F. H.
Published: July 2002

The Taylor–Bénard problem is realized in the narrow gap limit of fluid flow between differentially rotating coaxial cylinders which are kept at different temperatures. When the outer cylinder is heated and the centrifugal force by far exceeds gravity, buoyancy gives rise to the same axisymmetric vortices that are also realized in the isothermal Taylor–Couette system. The mathematical identity of the axisymmetric motions provides the basis for the analysis of nonaxisymmetric motions in the form of wavy vortices. It is shown that wavy convection rolls in a Rayleigh–Bénard layer and wavy Taylor vortices are special cases of the wavy rolls found as secondary bifurcation in the Taylor–Bénard problem.
Analysis of spatial patterns in secondary flows generated by torsionally oscillating spheres in linearly stratified fluids
Authors: Folse R. F. , Piker C. W.
Published: July 2002

Measurements have been made of the cellular flow patterns observed by Folse [Phys. Fluids 6, 537 (1994)] in the secondary flow field of torsionally oscillating spheres in stratified fluids. Shadowgraphs were made of the secondary flow field of spheres, radii between 2.52 and 5.08 cm, torsionally oscillating (frequencies in the range 1.0 rad/s⩽ω⩽13.5 rad/s and amplitudes θ0⩽1 rad) in linearly stratified fluids, with Brunt–Väisälä frequencies between 0.8 rad/s⩽N⩽2.1 rad/s. The spatial pattern was determined by measuring the vertical size, d, and radial position, r, for each cell. Dimensionless cell size, d/r, was found to depend on the parameters (ω/N), θ0, and the angular position on the sphere φ measured from the pole. A correlation of the dimensionless cell size is found, independent of sphere radius, given by: dn/r̄n=0.22(ω/N)θ02 cot(φ̄n), where r̄n and φ̄n are the average radial and angular positions of the nth cell. This correlation results in a discrepancy between measurements and expectations based on a heuristic energy argument.
Bifurcation and stability analyses for a two-phase Rayleigh–Benard problem in a cavity
Authors: Lan C. W. , Liang M. C. , Chen M. K.
Published: July 2002

Stability and bifurcation analyses of a partially melted or solidified material heated from below and cooled from above in a cavity, the so-called two-phase Rayleigh–Benard problem, are conducted by a finite-volume/Newton’s method. Bifurcation analysis techniques using a numerical Jacobian and an iterative matrix solver suitable to this large complicated system are adopted. The onset and evolution of melt flows coupling with the heat conduction in the solid and a deformable melt/solid interface are illustrated through detailed bifurcation diagrams, and the linear stability of each flow family is carefully examined. Some comparison with the one-phase system is performed. Results are presented for a variety of parameters of interest, including the Rayleigh number, aspect ratio, and tilt angle. Although most calculations are presented for the melt with a Prandtl number of one, the effects of Prandtl number on the onset of cellular convection and the sensitivity of symmetry breaking by tilting are examined. Furthermore, the dynamic responses of an unstable static state to stable solutions after small disturbances are illustrated, and the effect of heat of fusion is discussed.
The modeling of chemical reactions and thermochemical nonequilibrium in particle simulation computations
Authors: Gallis Michael A. , Harvey John K.
Published: July 2002

The treatment of chemical reactions and nonequilibrium energy exchange in Direct Simulation Monte Carlo calculations is examined. Details of a Maximum Entropy chemical reaction model are presented that is based on the classical scheme devised by Levine and Bernstein. Data are given for all of the significant reactions that occur in hypersonic reentry flight into the atmospheres of the Earth, Mars, and Venus. The method is an extension of that described and used previously by the authors (Gallis and Harvey [J. Fluid Mech. 312, 149 (1996); AIAA J. 34(7), 1378 (1996)]) and now includes carbon dioxide/nitrogen and ionic reactions. The model allows an appropriate dependence of each reaction on its controlling energy mode and avoids inappropriate use of equilibrium distributions to determine the reaction probabilities and post-collision energy reallocation. Sample flow solutions are given and comparisons are made with results obtained using continuum solvers.
Non-Boussinesq effect: Asymmetric velocity profiles in thermal convection
Authors: Zhang Jun , Childress Stephen , Libchaber Albert
Published: July 2002

In thermal convection at high Rayleigh numbers, in the hard turbulent regime, a large scale flow is present. When the viscosity of the fluid strongly depends on temperature, the top–bottom symmetry is broken. In addition to the asymmetric temperature profile across the convection cell, the velocity profiles near the plate boundaries show dramatic difference from the symmetric case. We report here that the second derivative of the velocity profiles are of opposite signs in the thermal sublayers, through measurements derived from the power spectrum of temperature time-series. As a result, the stress rate applied at the plates is maintained constant within a factor of 3, while the viscosity changes by a factor of 53, in qualitative agreement with previous theory.
Conditional sampling of the turbulent vorticity field near a smooth corner
Authors: Gavrilakis S.
Published: July 2002

The turbulence source terms in the equation for the mean vorticity near the walls of a smooth corner are examined through correlations involving the fluctuating vorticity field. A manifestation of the corner effect is the specific asymmetries of the estimated probability density functions for the production terms. These can be used to extract the type of three-dimensional structures that contribute in the production of mean vorticity.
Rarefaction–undercompressive fronts in driven films
Authors: Münch A. , Bertozzi A. L.
Published: July 2002

We consider experiments by Ludviksson and Lightfoot [AIChE J. 17, 1166 (1971)] on thin liquid films driven up a vertical plate by a thermally induced surface tension gradient with a counteracting gravitational force, and revisit their theoretical analysis, which neglects the effects of curvature, for predicting the climbing rate of the front. We present a new theory for the lubrication model with curvature effects, and get rising rates that depend on the microscopic length scale at the contact line. The predictions are, in general, in better agreement with the experiment.
Experimental investigation of the motion of a bubble in a gradient of turbulence
Authors: Risso Frédéric
Published: July 2002

The role of the turbulence-intensity gradient upon the motion of a bubble is investigated. Isolated bubbles are put into a turbulence field, which is isotropic and decreases exponentially in intensity in the z-direction. Experiments are carried out under microgravity conditions, so that the buoyancy is negligible. Bubbles are observed to move in the z-direction. Their mean velocity is found to result from a balance between the mean-pressure gradient and the Reynolds-stress gradient.
Asymptotic estimates for two-dimensional sloshing modes
Authors: Davis Anthony M. J. , Weidman Patrick D.
Published: July 2002

Estimates for the natural frequencies of linear two-dimensional sloshing modes in channels composed of two planar walls, either opening or closing at the bottom, are derived using conformal transformation techniques. The results are asymptotic in the sense that the wavelength of surface waves are assumed small in comparison to the horizontal extent of the quiescent free surface. An experiment was constructed to test the asymptotic theory for odd sloshing modes in two symmetric and three asymmetric containers. Good corroboration between measurement and theory is obtained when the increase in frequency due to surface tension, not accounted for in the theoretical analysis, is estimated and removed from the experimental data.
Nonlinear waves in hot magnetized plasma
Authors: Gedalin M.
Published: July 2002

Nonlinear low-frequency long wavelength waves are studied in the framework of the proposed method of the direct kinetic equation expansion in the inverse gyrofrequency power series. Nonlinear two-dimensional equations are found for perpendicular magnetosonic waves with subsequent reduction to the Korteweg–de Vries equation for quasistationary one-dimensional (QS1D) perpendicular magnetosonic wave. The dispersion is determined by the ion gyroradius ρi. Equations for nonlinear QS1D parallel Alfvén waves and QS1D oblique fast magnetosonic waves are found. It is shown that the form of the equations coincides with that one obtained from hydrodynamics but the dispersion lengths and coefficients at nonlinear terms are different. Dispersion of quasiparallel fast waves is positive and determined by the ion inertial length c/ωpi, while quasiperpendicular waves have negative dispersion determined by the ion gyroradius ρi.
Ergodic mixing for turbulent drift motion in an inhomogeneous magnetic field
Authors: Isichenko M. B. , Petviashvili N. V.
Published: July 2002

The turbulent E×B drift of a test particle in an inhomogeneous magnetic field is not reducible to a simple diffusion, but rather leads to a biased diffusion producing an inhomogeneous density distribution (pinch effect). The statistical properties of the long-time chaotic two-dimensional drift motion of a charged particle in the magnetic field B(x,y) and the time-dependent electrostatic potential φ(x,y,t) are studied by numerical symplectic integration. For a conditionally periodic potential with two or more incommensurate frequencies, an ergodic behavior is demonstrated in which the probability density of the particle position is proportional to the magnetic field B. The accuracy of this prediction is found to be independent of the number Nω of the incommensurate frequencies for Nω≥2. The relation of this result with the Kolmogorov-Arnold-Moser theory is discussed.
Alpha currents driven by ion cyclotron resonance heating in a tokamak fusion reactor
Authors: Chang C. S.
Published: July 2002

Generation of plasma current by magnetic trapping of passing alpha particles through ion cyclotron resonance heating (ICRH) is studied. It is found that the efficiency of the ICRH-driven alpha current can be comparable to the known efficiency of the usual fast-wave-driven electron current (e-FWCD). However, the radial dependence can be quite different from the known behavior of e-FWCD. Since a substantial fraction of the fast wave energy can be absorbed by the fusion-born alpha particles through ICRH, the fast wave current drive in a deuterium-tritium reactor plasma may be substantially modified by the synergistic alpha current effect. The present study suggests that, without a careful choice of the wave properties, the synergism may yield a destructive fast-wave-driven radial current profile for plasma equilibrium. However, it also suggests that a proper choice can produce a more favorable result than the e-FWCD can alone.
Detuning effect of negative ions on oscillating-two-stream instabilities of electromagnetic waves
Authors: Cho Guan Sik , Park Jong Rak , Lee Jae Yong , Kong H. J.
Published: July 2002

It is proved analytically in the three-fluid theory of a plasma that the effect of negative ions on the oscillating-two-stream instability results in a change of the frequency mismatch between the electromagnetic pump and the electron plasma wave. Ion thermal and collision terms are assumed to be negligible. This negative ion effect can completely detune and suppress the instability, if the number density ratio of negative to positive ions is greater than 2(v20/v2th), where v0 is the oscillation velocity amplitude of electrons under the pump and vth is the electron thermal speed.
Instability of stagnation points in magnetohydrodynamic equilibria
Authors: Núñez Manuel
Published: July 2002

Instabilities associated with streamlines connecting stagnation points in an equilibrium with plane incompressible flow are studied. It is shown that they belong to two different types: If the velocity of fluid absorption at the endpoint is greater than the velocity of fluid emission at the initial point, only a discrete spectrum of unstable standing waves is possible; otherwise, Alfvénic waves accumulate to form a continuum of unstable frequencies. These are present no matter how small the equilibrium field or the mode number of the magnetic oscillations.
Comment on ‘‘Electron acoustic surface waves in a two-electron component plasma’’ [Phys. Fluids B 5, 4502 (1993)]
Authors: Lee Hee J.
Published: July 2002

The transverse magnetic mode surface wave dispersion relation for a two-electron component plasma which was derived incorrectly in Ref. [Phys. Fluids B 5, 4502 (1993)] [Eq. (5)] is rederived. It is also shown that the transverse attenuation constant as derived in Ref. [Phys. Plasmas 1, 3716 (1994)] should be used.
Finite-β and alpha particle effects on kinetic Alfvén wave in a fusion tokamak plasma
Authors: Ding N. , Huang L. , Qiu X. M.
Published: July 2002

In this paper, a model for the kinetic Alfvén wave (KAW) in the presence of fusion alphas is established. The finite-β (relative to low β) and alpha particle effects on KAW are investigated. In this model, ion sound, transit time magnetic pumping (TTMP), the response of alpha particles (alphas), and those effects considered by preceding authors are included. In cylindrical geometry, a set of three second-order differential equations in r for the perturbed fields Er, E⊥, and E∥ is numerically solved. A dispersion relation of the Alfvén wave in the fusion tokamak plasma is derived. The mode conversion and the energy deposition are qualitatively discussed on the basis of this relation. Both the analytical and numerical analyses indicate that (i) no matter whether m (poloidal mode number) is positive [N. Ding et al., Phys. Plasmas 2, 1529 (1995)] or negative (mainly studied in the present paper), the alphas do not affect the compressional Alfvén wave, but they do affect the KAW evidently; (ii) for m<0, it is preferable to choose the frequency ω of the injected wave so that the inequality ω≳(Pm±1/Rm±1)−1ω*αm holds for optimal power absorption. The energy deposition at the resonant position close to the interior of the fusion tokamak plasma in taking account of the effects of ion sound and TTMP is less than that without taking account of these effects. But for the same β value, at the position adjacent to the edge the contrary is the case. For a certain resonant position, as the β value increases, the energy deposition decreases.
The stochastic diffusion process in reversed-field pinch
Authors: D’Angelo F. , Paccagnella R.
Published: July 2002

The stochastic diffusion process has been carefully analysed for the Reverse Field Pinch (RFP) equilibrium configuration. Three different approaches to the problem have been used, i.e. the direct calculation of the stochastic diffusion coefficient (Dst) from the radial evolution of an ensemble of field lines, an indirect determination using a Gaussian-fit to the field line probability distribution function and finally the relationship of Dst with the maximum Lyapunov exponent of the system. Different perturbation spectra have been also considered and compared and the quasi-linear approximation has been tested.
Toroidal stability boundaries for tearing modes in tokamaks
Authors: Xie H. X. , Bondeson A.
Published: July 2002

The linear stability boundaries for resistive and ideal external modes in circular cross section tokamaks are studied by means of toroidal magnetohydrodynamical (MHD) computations. The resistive MHD prediction at zero pressure is clearly more unstable than experimental results. For Lundquist numbers S=τr/τA of the order 107 or larger, the resistive stability boundaries for moderate beta are rather close to the ideal ones. These boundaries are in reasonable agreement with experimental results. The stabilizing effect of a conducting wall is studied. At high S, the lower limit to the inductance tends to be set by resistive wall modes rather than by tearing modes.