bluuebunny/crossref_metadata_2025 huggingface dataset:
Rapid initiation in condensed phases through resonant nonlinear acoustics
Authors: Almgren Robert , Majda Andrew , Rosales Rodolfo R.
Published: July 2002

Experiments indicate that small-scale inhomogeneities have a prominent role in accelerating the global heat-release rate in a variety of condensed-phase liquid and solid materials. In this paper, simplified asymptotic equations are studied that incorporate small-scale inhomogeneities, their interaction with each other, and the interaction of the microstructure with the global reaction scale. A variety of chemical-acoustic resonant mechanisms with the microstructure are developed and documented for both temperature-sensitive and pressure-sensitive reaction rates. It is demonstrated that these specific resonant mechanisms lead to dramatic shortening of the local induction time for regimes of equations of state and chemical reactions appropriate for condensed phases. These mechanisms are completely different than the conventional mechanism of simple thermal explosion of hot spots.
Three-dimensional vorticity modes in the wake of a flat plate
Authors: Lasheras J. C. , Meiburg E.
Published: July 2002

The three-dimensional development of the wake behind a flat plate separating two laminar streams of equal velocity, subjected to perturbations in its initial conditions, is studied experimentally and numerically at moderate Reynolds numbers. The effect of single waves as well as subharmonic streamwise forcing is discussed. For the case of a single wave, streamwise forcing combined with a sinusoidal spanwise perturbation, the two distinct three-dimensional vorticity modes found in an early study [J. Fluid Mech. 190, 1 (1988)], are further studied and their symmetry properties analyzed. Depending on the orientation of the spanwise perturbation, the counter-rotating pairs of streamwise vortex tubes forming in the braids are shown to induce undulations in the cores of the Karman-like vortices, resulting in either an in-phase or a varicose configuration. Furthermore, when the wake is subjected to subharmonic streamwise forcing, additional modes of the topology of the vorticity field are shown to exist. The evolution of these subharmonic three-dimensional modes is analyzed as a function of the initial conditions.
A linear analysis of g-jitter effects on viscous cylindrical liquid bridges
Authors: Meseguer J. , Perales J. M.
Published: July 2002

This paper deals with the dynamics of isothermal, axisymmetric, cylindrical liquid columns held by capillary forces between two circular, concentric, solid disks; in particular, it deals with the dynamic response of the bridge to an excitation consisting of a small change in the value of the microgravity level. The problem has been solved by using a linearized one-dimensional Cosserat model, which includes viscosity effects, and with the axial velocity considered as constant in each section of the liquid bridge. The analysis has been performed by using the Laplace transform, and the time variation of both the axial velocity field and the liquid bridge interface have been obtained.
Influence of forcing on the composition of mixed fluid in a two-stream shear layer
Authors: Koochesfahani M. M. , MacKinnon C. G.
Published: July 2002

The probability density function (pdf) of mixed-fluid composition in nonreacting forced liquid mixing layers is measured using laser-induced fluorescence diagnostics. Results show that the composition distribution of mixed fluid in forced shear layers is essentially uniform across the width of the shear layer similar to previous results in natural shear layers. For the cases studied, forcing results in an increase of the total amount of mixed fluid (integrated across the layer width) and a shift of the predominant mixed-fluid concentration to larger values. The increase of the amount of mixing in the forced layer appears to be mostly due to the increase in the layer width and not so much the result of improved small-scale mixing. It is shown that the details of the forcing waveform shape can have a significant influence on the structure of the flow and the mixing field.
Effect of suction on controlling the secondary instability of boundary layers
Authors: El-Hady Nabil M.
Published: July 2002

The effect of suction on controlling the three-dimensional secondary instability is investigated for a boundary layer with a pressure gradient in the presence of a small but finite-amplitude Tollmien–Schlichting wave. The focus is on the principal parametric resonance responsible for the strong growth of subharmonics in a low disturbance environment. Calculations are presented for the effect of suction on the onset and amplification of the secondary instability in Blasius and Falkner–Skan flows, as well as its effect on controlling the production of the vortical structure.
Stability experiment of flow between a stationary and a rotating disk
Authors: Sirivat Anuvat
Published: July 2002

Three modes of instability are observed experimentally in the flow between a stationary and a rotating disk. Mode I instability appears as steady-state, circular cells, whereas mode II instability consists of moving spiral vortices. Mode III instability is of a boundary layer type as the vortices are stationary and confined within the rotating boundary layer. The critical cell sizes are found to be comparable to the disk gap size, with a distinct value for each mode. The axis-orientation angles of these cells or vortices for all three modes are equal or less than zero, in contrast to class B waves observed in the one-disk flow. The three modes of instability observed originate from different basic velocity fields: the torsional Couette flow, the Batchelor-type velocity field, and the self-similar velocity field. These basic flows can be realized in the laboratory as the basic flow parameter 1/Ek=Ωh2/ν is increased.
Turbulent mixing and transport in a thermally stratified interfacial layer in decaying grid turbulence
Authors: Jayesh  , Yoon Kyunghwan , Warhaft Z.
Published: July 2002

A stably stratified mixing layer, sandwiched in between regions of neutral turbulence, was studied in decaying grid turbulence. The layer, which was shearless, was formed by heating the upper half of the flow by means of elements placed at the entrance to the plenum of a large, open circuit low speed wind tunnel 0.91×0.91 m2 in cross section and 9.14 m in length. The hot air above mixed with the cold below forming the stratified layer in between. As the flow evolved and the turbulence decayed, the buoyancy forces increased relative to the inertial forces (i.e., the Richardson number increased) causing the heat flux to collapse. This resulted in a thinning of the mixing layer with downstream distance (rather than growth which occurs for the passive case). Inside the layer the vertical velocity variance diminished and the vertical heat flux correlation coefficient was reduced to zero. Smoke wire photographs showed a wavylike damped region inside the layer, surrounded by the normal, more energetic turbulence outside. Second-order turbulence quantities scaled in the same way with the local Richardson number both along the layer and across it. The two stably stratified cases studied had centerline Froude numbers of 95 and 65 at 40 mesh lengths from the grid. The results are compared to a passive thermal mixing layer and are contrasted with recent experiments concerning a constant temperature gradient in grid turbulence.
Evidence against the Kolmogorov refined similarity hypothesis
Authors: Hosokawa I. , Yamamoto K.
Published: July 2002

It is found that the well-known ‘‘refined similarity hypothesis,’’ proposed by Kolmogorov and Obukhov in 1962, fails in a typical direct numerical simulation of three-dimensional turbulence: If r is an inertial-range scale, the velocity difference across a domain of linear dimension r is almost statistically independent of the average dissipation within the domain. This result casts doubt on the often-used relation, ζp = p/3 − μp/3, between scaling indices ζp for velocity structure functions and scaling indices μq for moments of locally averaged dissipation. It seems questionable that a single turbulence model can predict both sets of exponents successfully. Other aspects of the treatment of intermittency in isotropic turbulence may also need reconsideration.
A proper velocity scale for modeling subgrid-scale eddy viscosities in large eddy simulation
Authors: Horiuti Kiyosi
Published: July 2002

The limitations of the commonly used Smagorinsky subgrid-scale (SGS) eddy viscosity model in large eddy simulation (LES) of turbulent flows are that the model’s eddy viscosity constant must be optimized in different flows, and also that a damping function must be used to account for near-wall effects. A new SGS model which mitigates these drawbacks is proposed, i.e., a more proper eddy viscosity velocity scale was determined by utilizing the third-order terms in an anisotropic representation model of the Reynolds stresses [K. Horiuti, Phys. Fluids A 2, 1708 (1990)]. This method utilizes the direct numerical simulation (DNS) database for fully developed turbulent channel flow to show these drawbacks to be inherent in the use of an improper velocity scale, i.e., the total SGS energy component adopted in the Smagorinsky model. As a result, the SGS normal shear stress was alternatively employed as the velocity scale, thereby significantly improving the correlation with DNS data. Methods to correlate the SGS normal shear stress to the grid scale quantities are proposed and compared, and the resultant high accuracy of the scale-similarity model to represent the SGS turbulence fluctuations is shown. The proposed SGS model was also tested in actual LES computations of turbulent channel flow, where it was found that the SGS eddy viscosity in the near-wall region similarly acted as the conventionally used Van Driest damping function. This result is consistent with previous reports which assert that in the Reynolds averaged models, the rapid reduction of the Reynolds shear stress as the wall is approached is due to the preferential damping of the normal shear stress. It is shown that three eddy viscosity parameters contained in the proposed SGS model can be practically reduced to a single parameter, which is subsequently shown to be more universal and independent of the flow field than the Smagorinsky model constant. A qualitative interpretation for the variance of the Smagorinsky model constant in different flows is also provided via a correlation with the anisotropy of SGS turbulence intensities.
Resistive ballooning modes in the second region of stability
Authors: Kim Jin-Yong , Choi Duk-In
Published: July 2002

A recent study suggested that the resistive ballooning modes are largely stable in the second region of the ideal ballooning stability, unlike in the first region. This problem is reexamined here by numerically solving the full resistive mode equation. It is shown that in the second region there are unstable resistive ballooning modes with larger growth rates compared to those in the first region. This behavior is shown to be caused by the fact that the curvature force significantly influences the dynamics in the resistive region, and the mode is mainly driven unstable from the resistive region and not from the ideal region through Δ′.
Nonlinear self-sustainment of magnetic islands
Authors: White R. B. , Romanelli F.
Published: July 2002

A mechanism is proposed for a nonlinear self-sustainment of magnetic islands that relies on the presence of a chaotic region. Particles diffusing in the stochastic magnetic field produce a local increase of current at X points on a rational surface, which in turn sustains the original perturbation if the equilibrium profile is concave. It is shown that the new equilibrium is stable. In a tokamak the mechanism is operative in the region outside the q=2 surface, in agreement with the observation of strong electron confinement degradation in this region.
Ballooning stability of axisymmetric plasmas with sheared equilibrium flows
Authors: Bhattacharjee A. , Iacono R. , Milovich J. L. , Paranicas C.
Published: July 2002

A WKB theory is formulated for large-n ballooning modes in axisymmetric, toroidal plasmas with sheared equilibrium flows. The validity of the standard ballooning representation is severely restricted in the presence of sheared toroidal flow, despite the fact that to leading order in (1/n), where n is the azimuthal number, the eigenmode equation contains only derivatives along a field line. Necessary and sufficient conditions for stability are obtained in a high-beta ordering for rigid toroidal rotation as well as field-aligned flows.
Kelvin–Helmholtz vortex formation and particle transport in a cross-field plasma sheath. II. Steady state
Authors: Theilhaber K. , Birdsall C. K.
Published: July 2002

The steady-state behavior of the magnetized plasma–wall sheath has been studied through two-dimensional particle simulations, which have shown that the sheath maintains itself in a strongly nonlinear, turbulent equilibrium, continuously driven by the edge Kelvin–Helmholtz instability. The sheath assumes a thickness of order lx∼5ρi, and maintains large, long-lived vortices, with amplitudes δφ∼2.5Ti/e, which drift parallel to the wall at half the ion thermal velocity. The sheath also maintains a large, spatially averaged potential drop from the wall to the plasma with Δφ≊−2Ti/e, opposite in sign to that of the unmagnetized sheath. Accompanying the long-wavelength vortices are shorter-wavelength fluctuations, which induce an anomalous cross-field transport, scaling in accordance to Bohm diffusion when ωpi≥2ωci. At lower densities, ωpi<2ωci, the diffusion coefficient has an additional factor, proportional to the density. These results permit the modeling of the cross-field sheath by a simple effective boundary condition.
Resonance broadening in drift wave turbulence
Authors: Ishihara O. , Grabowski C. , Hirose A.
Published: July 2002

Turbulent diffusion of electrons in drift modes in a sheared magnetic field is studied by using a test particle numerical experiment. Electrons diffuse across the magnetic field over mode rational surfaces, where electrons interact with waves in resonance. A spatial diffusion coefficient, which describes resonance broadening, is found to be time dependent and departs from quasilinear predictions in stronger turbulence even well before the time when particles hit the resonance boundary of the rational surfaces.
Observation of nonthermal electron tails in an rf excited argon magnetoplasma
Authors: Zhu Peiyuan , Boswell R. W.
Published: July 2002

Nonthermal beamlike electron tails have been observed in an argon magnetoplasma excited by rf without electron beam injecting. The plasma is highly ionized (∼100%) with central density ∼1014 cm−3, and is based on the excitation of helicon waves. Nonthermal electron tails are observed at the beginning of the plasma pulse and last for about 1 msec. There is a maximum in the electron energy distribution at 30–80 eV and a minimum at 20–30 eV. The mechanism responsible for driving this beamlike tail is not yet known.
Transit-time damping and the arrest of wave collapse
Authors: Robinson P. A.
Published: July 2002

Power dissipation by transit-time damping is investigated analytically and numerically using a perturbation expansion and a test-particle code, respectively. Excellent agreement between the two methods is found for both one-dimensional and multidimensional systems. It is shown that the local power dissipation can take on positive or negative values depending on position, implying that particles not only carry off energy from localized fields, but redistribute it within them. The results are applied to estimate the arrest scales of the collapsing wave packets found in strongly turbulent plasmas. Arrest scales in the ranges (14–23)λD and (16–26)λD are found for two- and three-dimensional wave collapse, respectively. These estimates are consistent with results from particle-in-cell simulations, which yielded arrest scales of ∼14λD in 2D and ∼20λD in 3D, and with experimental results that implied arrest at scales of (17–30)λD in 3D. The previously problematical outcome that 3-D collapse is arrested at a longer scale than in 2D, despite its stronger nature, results because the larger fraction of high-velocity particles in the 3-D plasma velocity distribution leads to stronger transit-time damping than in the corresponding 2-D system. It is argued that transit-time dissipation will arrest the collapse of particularly intense wave packets at even longer scales due to the increase in the local Debye length caused by localized heating near the center of these wave packets and due to the formation of high-velocity tails by transit-time acceleration.
Kinetic theory of the ion-temperature-gradient-driven mode in the long wavelength limit
Authors: Romanelli F. , Chen Liu , Briguglio S.
Published: July 2002

The stability of the ion-temperature-gradient-driven mode in the long wavelength limit is investigated by retaining the kinetic effects associated with the ion transit frequency. The complete trapped electron dynamics is also considered. The toroidal branch is weakly affected by the trapped electron dynamics while the slablike branch is more strongly affected by trapped electrons. The implications of these results for the understanding of the improved confinement regimes observed in the presence of peaked density profiles is also discussed.
Bifurcations and chaos in a current-carrying ion sheath
Authors: Komori A. , Kono M. , Norimine T. , Kawai Y.
Published: July 2002

Cascading bifurcations to chaos are investigated experimentally and theoretically in a current-carrying stable plasma. A dc plasma current is required to produce an electron-depleted thick sheath on a grid, which obeys the Child–Langmuir law of space-charge-limited current in a diode. Bifurcation cascade and chaotic behavior are exhibited when an external periodic oscillation is applied to the grid, and are in good agreement for the first time with a theory, which describes ion dynamics in the Child–Langmuir sheath and is represented by the differential equation with three independent variables. A fractal dimension predicted by the theory is verified by the experiment.
Kinetic theory of rf current drive and helicity injection
Authors: Mett R. R.
Published: July 2002

Current drive and helicity injection by plasma waves are examined with the use of kinetic theory. The Vlasov equation yields a general current drive formula that contains resonant and nonresonant (ponderomotivelike) contributions. Standard quasilinear current drive is described by the former, while helicity current drive may be contained in the latter. Since direct analytical comparison of the sizes of the two terms is, in general, difficult, a new approach is taken. Solution of the drift-kinetic equation shows that the standard Landau damping/transit time magnetic pumping quasilinear diffusion coefficient is the only contribution to steady-state current drive to leading order in ε=ρL/l, where ρL is the Larmor radius and l is the inhomogeneity scale length. All nonresonant contributions, including the helicity, appear at higher order, after averages are taken over a flux surface, over azimuth, and over time. Consequently, at wave frequencies well below the electron cyclotron frequency, a wave helicity flux perpendicular to the magnetic field does not influence the parallel motion of electrons to leading order and therefore will not drive a significant current. Any current associated with a wave helicity flux is then either ion current (and thus inefficient) or electron current stemming from effects not included in the drift-kinetic treatment, such as cyclotron, collisional, or nonlinear (i.e., not quasilinear).
Stability of the toroidicity-induced Alfvén eigenmode in axisymmetric toroidal equilibria
Authors: Fu G. Y. , Cheng C. Z. , Wong K. L.
Published: July 2002

The stability of toroidicity-induced Alfvén eigenmodes (TAE) is investigated in general tokamak equilibria with finite aspect ratio and finite plasma beta. The finite orbit width of the hot particles and the collisional damping of the trapped electrons are included. For the trapped hot particles, the finite orbit width is found to be stabilizing. For the circulating hot particles, the finite orbit width effect is stabilizing for larger values of vh/vA (≳1) and destabilizing for smaller values of vh/vA (<1), where vh is the hot particle speed and vA is the Alfvén speed. The collisional damping of the trapped electrons is found to have a much weaker dependence on the collision frequency than the previous analytic results. The contribution of the curvature term to the trapped electron collisional damping is negligible compared to that of the parallel electric field term for typical parameters. The calculated critical hot particle beta values for the TAE instability are consistent with the experimental measurements.
Fluid analysis of the collisionless magnetohydrodynamic ballooning mode branch in tokamaks
Authors: Nordman H. , Jhowry B. , Weiland J.
Published: July 2002

The stability of electromagnetic ballooning modes is investigated using a toroidal two-fluid model allowing for arbitrary Ln/LB (the characteristic scale lengths of density and magnetic-field inhomogeneities). The ballooning mode equation is solved numerically and the two-fluid and magnetohydrodynamic stability properties are discussed and compared. The perpendicular compressibility strongly reduces the growth rate and for Ln/LB∼Ln/LT∼1, the mode is completely stabilized.
Neoclassical theory of the plasma edge
Authors: Stacey W. M.
Published: July 2002

Tokamak neoclassical theory is extended to the plasma edge region, where the gradient scale lengths of density, temperature, and rotation speed are comparable to the ion poloidal gyroradius, and where vθBφ/vthBθ∼O(1). A new drift due to viscosity is found to be important. A self-consistent set of equations for determining particle flows within and across flux surfaces, asymmetries over the flux surface in densities and in the electrostatic potential, and the radial electric field in the plasma edge of tokamaks is presented.
Linear stability of a viscous–inviscid interface
Authors: Hogan J. M. , Ayyaswamy P. S.
Published: July 2002

In this paper the stability of the interface separating fluids of widely differing viscosities has been examined. It is shown that a viscous–inviscid (V–I) model offers a consistent zeroth-order approximation to the stability problem. The zeroth-order solution is obtained by neglecting the smallest-order effect, viz., viscosity on the less viscous side of the interface. In this sense, the V–I model significantly differs from the Kelvin–Helmholtz (K–H) approach where both the viscosities are dropped in a single step. A closed form solution for the stability criterion governing the V–I model has been obtained, and a novel instability mechanism is described. It is shown that the V–I model is also a consistent zeroth-order approximation for the Rayleigh–Taylor problem of a viscous–viscous, nonflowing interface when the viscosity ratio tends to zero. For the interface separating two viscous, nonflowing, incompressible fluids, exact solutions for the velocities, pressures, and interface displacement for a disturbance of a given wavelength have been provided for the stable (lighter fluid on top) wave motion. By discussing the roles played by the dynamic and kinematic viscosities, it is made clear why neither the V–I nor the K–H model should apply to the air–water interface. The results of the V–I model compare well with experimental observations. The V–I model serves as an excellent basis for comparison in detailed numerical studies of the viscous–viscous interface.
Plasma kinetic theory in action-angle variables
Authors: Mahajan S. M. , Chen C. Y.
Published: July 2002

An appropriate canonical perturbation theory to correctly deal with general electromagnetic field perturbation is developed and is used to set up plasma kinetic theory in action-angle variables. A variety of test problems are solved to show the unifying power of the method. Basic linear, quaislinear, and nonlinear equations, which can serve as the starting point for a whole range of plasma problems, are derived.
Stimulated Brillouin scattering of whistler waves off the kinetic Alfvén waves in plasmas
Authors: Sharma R. P. , Rozmus W. , Offenberger A. A.
Published: July 2002

In this paper an investigation of the parametric process is presented in which a high-power whistler wave decays into another whistler wave and a low-frequency kinetic Alfvén wave (mixed mode), viz., the stimulated Brillouin scattering of a whistler. The dominant coupling in the equation for the scattered whistler arises from the electron motion along the static magnetic field in the low-frequency wave and the partially electrostatic nature of the kinetic Alfvén wave. On the other hand, interaction of two whistler waves leads to a component of the ponderomotive force along the static magnetic field. This component is responsible for the electrostatic part of the kinetic Alfvén wave. Explicit expressions for the growth rate and threshold power are given. At a pump power of ∼1 kW, the growth time of this parametric process comes out to be ∼5 μsec for the parameters of the mirror device. For the magnetospheric parameters, the convective threshold electric field is ∼0.3 mV/m.
An experimental determination of the hot electron ring geometry in a bumpy torus and its implications for bumpy torus stability
Authors: Hillis D. L. , Wilgen J. B. , Bigelow T. S. , Jaeger E. F. , Swain D. W. , Hankins O. E. , Juhala R. E.
Published: July 2002

The hot electron rings of the ELMO Bumpy Torus (EBT) [Plasma Physics and Controlled Nuclear Fusion (IAEA, Vienna, 1975), Vol. II, p. 141] are formed by electron-cyclotron resonance heating (ECRH) and have an electron temperature of 350–500 keV. The original intention of these hot electron rings was to provide a local minimum in the magnetic field and, thereby, stabilize the simple interchange and flute modes, which are inherent in a closed field line bumpy torus. To evaluate the electron energy density of the EBT rings and determine if enough stored energy is present to provide a local minimum in the magnetic field, a detailed understanding of the spatial distribution of the rings is imperative. The purpose of this paper is to measure the ring thickness and investigate its implications for bumpy torus stability. The spatial location and radial profile of the hot electron ring is measured with a unique metal ball pellet injector, which injects small metallic balls into the EBT ring plasma. From these measurements the radial extent (or ring thickness) is about 5–7 cm full-width at half-maximum for typical EBT operation, which is much larger than previously expected. These measurements and recent modeling of the EBT plasma indicate that the hot electron ring’s stored energy may not be sufficient to produce a local minimum in the magnetic field.
Three-dimensional numerical study of convection in a cylindrical thermal diffusion cell: Its influence on the separation of constituents
Authors: Henry D. , Roux B.
Published: July 2002

The convection arising in a cylindrical container, heated from the ends and designed for a measurement of Soret coefficient, is examined. A three-dimensional method using a pseudostationary scheme with a finite differences technique is used. Attention is focused on a horizontal cylinder with an aspect ratio of 6, a Prandtl number of 0.6, and a Schmidt number of 60. The influence of convection on the separation and on the mass fraction profiles is examined for moderate Grashof numbers (0.01≤GrH≤10) and realistic Soret parameters (−0.75≤S≤1). A domain is found where the flow has no influence on the separation, corresponding to a ‘‘separation’’ regime. Extensions of the results to different Prandtl and Schmidt numbers and to larger aspect ratios are proposed.
The return of strongly anisotropic turbulence to isotropy
Authors: Nakauchi Norihiko , Oshima Hiroshi
Published: July 2002

Energy spectrum equations for homogeneous axisymmetric turbulence are formulated using the eddy-damped quasinormal Markovian (EDQNM) approximation. The equations, which consist of two spectrum functions of the two variables k (the magnitude of the wavevector k) and μ (the cosine of the angle between k and the direction of axisymmetry), are computed directly, without any reduction, under initial conditions for different degrees of anisotropy. In this paper, the results for cases of medium anisotropic and nearly two-dimensional initial conditions are reported. A remarkable result was that in the nearly two-dimensional case, some characteristic properties of two-dimensional isotropic turbulence, such as inverse energy cascades, are observed. Also, particular attention is paid to the effects of pressure-strain correlation and inertial energy transfer on the return of anisotropic turbulence to isotropy.
Identification of cross-wave regimes in the vicinity of a cut-off frequency
Authors: Shemer Lev , Lichter Seth
Published: July 2002

Experiments were performed in a long rectangular water tank equipped with a plane wavemaker. Cross waves were generated in the vicinity of the cutoff frequency of the fifth mode. Four different wave regimes were identified as a function of forcing frequency and amplitude. Long-time modulation of the wave field was observed to be either quasiperiodic or chaotic. Under certain experimental conditions, envelope solitons were generated nearly periodically at the wavemaker. Propagation velocity of these solitons is also presented.
Spectra of velocity derivatives in a turbulent wake
Authors: Antonia R. A. , Shah D. A. , Browne L. W. B.
Published: July 2002

One-dimensional spectra have been measured in the self-preserving wake of a circular cylinder for each of the nine major velocity derivatives that feature in the average dissipation of turbulent kinetic energy. The comparison between measured spectra and isotropic calculations indicates that isotropy is approached only at high wavenumbers, especially for spectra of derivatives with respect to lateral directions of lateral velocity fluctuations. This result is discussed in the context of the organized large scale motion.
Experimental study of nonlinear M=1 modes in the Tara tandem mirror
Authors: Irby J. H. , Lane B. G. , Casey J. A. , Brau K. , Golovato S. N. , Guss W. C. , Horne S. F. , Kesner J. , Post R. S. , Sevillano E. , Sullivan J. D. , Smith D. K.
Published: July 2002

The nature of a rigid, flutelike M=1 instability as seen in the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, 1985), Vol. 2, p. 285] is discussed. Radial density and light emission profiles obtained by inverting chord measurements are compared to end loss radial profiles during the evolution of the mode to its nonlinear saturated state. This final state is characterized by a coherent, flutelike motion of the plasma as a whole about the machine axis.
Spherical expansion of a plasma into an underdense plasma
Authors: Seyhoonzadeh Amir , Forsling Peter , Bailey Mark , Rosario Rachel , Beardsley Kelly , Tao Jin , Lonngren Karl E.
Published: July 2002

Experiments on the spherical expansion of a plasma into an underdense plasma are described. The initial spherical plasmas are stable anode spots that expand into the ambient region. The anode spots are formed in low pressure noble gas plasmas that have been impregnated with acetone. The formation mechanism is interpreted in terms of electron attachment on the acetone molecules.
A covariant formalism for wave propagation applied to stimulated Raman scattering
Authors: McKinstrie C. J. , DuBois D. F.
Published: July 2002

The governing equations for stimulated Raman scattering are derived in a Lorentz frame moving with arbitrary velocity relative to the background plasma. These equations are fundamental to the study of relativistic beat-wave solitary waves, which have recently been proposed for particle acceleration by Mima et al. [Phys. Rev. Lett. 57, 1421 (1986)]. An averaged Lagrangian density is constructed for this three-wave interaction. This results in a natural definition for the action flux density four-vector of each wave and the combined stress–energy tensor. It also follows from the Lagrangian structure of the system that the Manley–Rowe relations are satisfied. The covariant formalism presented here can also be used to study wave propagation in a multicomponent plasma, in which each plasma species moves with arbitrary velocity relative to the frame of observation. As a specific example, the dispersion relation for Langmuir-wave propagation in two warm relativistic electron beams is derived for the first time.
On temperature spectra in grid turbulence
Authors: Jayesh  , Tong Chenning , Warhaft Z.
Published: July 2002

This paper reports wind tunnel measurements of passive temperature spectra in decaying grid generated turbulence both with and without a mean transverse temperature gradient. The measurements cover a turbulence Reynolds number range 60<Rel<1200. It is shown that for a linear temperature profile the temperature spectrum has a scaling exponent close to −5/3 and that its dilation is consistent with Kolmogorov–Obukhov–Corrsin (KOC) scaling, i.e., the width of the scaling region increases approximately as Re3/4l. The remarkably low Reynolds number onset (Rel∼70) of Kolmogorov–Obukhov–Corrsin scaling in isotropic grid turbulence is contrasted to the case of scalars in (anisotropic) shear flows where KOC scaling only appears at very high-Reynolds numbers (Rel∼105). It is also shown that when the temperature fluctuations are inserted very close to the grid in the absence of a gradient (by means of a mandoline), the temperature spectrum behaves in a similar way to the linear gradient case, i.e., a spectrum with a scaling exponent close to −5/3 is observed, a result noted earlier in heated grid experiments. However, when the scalar is inserted farther downstream of the grid (in the fully developed turbulence), the spectrum has a scaling region of −1.3 and its dilation with Re is less well defined than for the other cases. The velocity spectrum is also shown to have a scaling region, of slope −1.3, and its onset occurs at higher Reynolds number than for the case of the scalar experiments that exhibit the KOC scaling.
Stress relation for three-dimensional turbulent flows
Authors: Taulbee Dale B. , Sonnenmeier James R. , Wall Kenneth M.
Published: July 2002

In this Brief Communication, the nonlinear stress–strain model for three-dimensional turbulent flows, as given by Taulbee [Phys. Fluids A 4, 11 (1992)], is expanded upon. That relation represents a closed form solution to the algebraic Reynolds stress model equation set which is obtained from the modeled transport equation for the Reynolds stress. The parameter values, which appear in the linear pressure–strain closure, that were suggested by Taulbee to obtain a simplified stress relation for three dimensions, are justified in this Brief Communication. A stress solution is also presented for a wider range of pressure–strain model parameter values.
Shocks in direct numerical simulation of the confined three-dimensional mixing layer
Authors: Vreman Bert , Kuerten Hans , Geurts Bernard
Published: July 2002

The occurrence of shocks in the confined three-dimensional turbulent mixing layer at convective Mach number 1.2 is established by means of direct numerical simulations. The shocks are generated by the turbulent motions in the flow. Consequently, they can have different shapes and orientations, while they persist for a relatively short time. Furthermore, they are created by different types of turbulent vortices. The shocks do not strongly contribute to the turbulent dissipation. Even at the time when the largest shocks occur, the fraction of the turbulent dissipation due to the shocks is less than 10%.
Cavity flow induced by a fluctuating acceleration field
Authors: Thomson J. Ross , Casademunt Jaume , Viñals Jorge
Published: July 2002

Buoyancy driven convection induced by a fluctuating acceleration field is studied in a two dimensional square cavity. This is a simplified model of, for example, fluid flow in a directional solidification cell subject to external accelerations, such as those encountered in a typical microgravity environment (g-jitter). The effect of both deterministic and stochastic acceleration modulations normal to the initial density gradient are considered. In the latter case, the acceleration field is modeled by narrow band noise defined by a characteristic frequency Ω, a correlation time τ, and an intensity G2. If the fluid is quiescent at t=0 when the acceleration field is initiated, the ensemble average of the vorticity at the center of the cavity remains zero for all times. The mean squared vorticity 〈ξ2〉, however, is seen to exhibit two distinct regimes: For t≪τ, 〈ξ2〉 oscillates in time with frequency Ω. For t≫τ, 〈ξ2〉 grows linearly in time with an amplitude equal to R2Pr/(1+(Ωτ))2, where R is a new dimensionless number which reduces to the Rayleigh number in the case of a constant gravity, and Pr is Prandtl number. At yet later times, viscous dissipation at the walls of the cavity leads to saturation, with 〈ξ2〉sat={(Pr τ+1)R2/[(Pr τ+1)2+Ω2τ2]}.
Monte Carlo simulation of the Boltzmann equation for uniform shear flow
Authors: Montanero J. M. , Santos A. , Garzó V.
Published: July 2002

The Boltzmann equation is numerically solved by means of the Direct Simulation Monte Carlo method for molecules interacting via repulsive r−μ-potentials under uniform shear flow far from equilibrium. The non-Newtonian shear viscosity is found to be rather insensitive to the interaction model. The results show that the divergence of velocity moments of degree equal to or larger than four obtained from an exact solution for Maxwell molecules [Phys. Rev. Lett. 71, 3971 (1993)] is also present for other interaction potentials.
Steady states and oscillatory instability of swirling flow in a cylinder with rotating top and bottom
Authors: Gelfgat Alexander Yu. , Bar-Yoseph Pinhas Z. , Solan Alexander
Published: July 2002

In this study we present a numerical investigation of steady states, onset of oscillatory instability, and slightly supercritical oscillatory states of an axisymmetric swirling flow of a Newtonian incompressible fluid in a cylinder, with independently rotating top and bottom. The first part of the study is devoted to the influence of co- and counter-rotation of the bottom on the steady vortex breakdown, which takes place in the well-known problem of flow in a cylinder with a rotating top. It is shown that weak counter-rotation of the bottom may suppress the vortex breakdown. Stronger counter-rotation may induce a stable steady vortex breakdown at relatively large Reynolds numbers for which a vortex breakdown does not appear in the case of the stationary bottom. Weak corotation may promote the vortex breakdown at lower Reynolds numbers than in the cylinder with the stationary bottom. Stronger corotation leads to the detachment of the recirculation zone from the axis and the formation of an additional vortex ring. The second part of the study is devoted to the investigation of the onset of oscillatory instability of steady flows. It is shown that the oscillatory instability sets in due to a Hopf bifurcation. The critical Reynolds number and the critical frequency of oscillations were calculated as a function of the rotation ratio (ξ=Ωbottom/Ωtop) for a fixed value of the aspect ratio γ (height/radius) of the cylinder γ=1.5. The stability analysis showed that there are several most unstable linear modes of the perturbation that become successively dominant with a continuous change of ξ. It is shown that the oscillatory instability may lead to an appearance and coexistence of more than one oscillating separation vortex bubble.
A dynamical model for turbulence. IV. Buoyancy-driven flows
Authors: Canuto V. M. , Dubovikov M. S. , Dienstfrey A.
Published: July 2002

We apply a recent model of turbulence to turbulent convection at high Rayleigh number Ra and compare the results with new laboratory and DNS data. We derive a closed set of equations for the total turbulent kinetic energy, turbulent kinetic energy in the z direction, temperature variance, and convective flux. The equations are coupled, time dependent, and nonlocal. We solve the equations both analytically and numerically. In the first case, we neglect diffusion and derive the relation Nu=Nu(σ,Ra), where Nu is the Nusselt number and σ is the Prandtl number. For σ≫1, Nu becomes independent of σ; for σ≪1, Nu is proportional to σ1/3; for 0.025 (mercury)⩽σ⩽0.7 (helium), Nu is proportional to σ2/7. The numerical solution (with diffusion) yields (a) Nusselt number Nu, 〈θ2〉w, 〈θ2〉c (temperature variance near the wall and at the center), λT (thermal boundary layer thickness), and Pe (Peclet number) versus Ra, (b) the z profile of mean temperature T, 〈θ2〉, horizontal, and vertical Peclet numbers, (c) spectra versus kh (horizontal wave number) of total kinetic energy, vertical kinetic energy, temperature variance, and temperature flux; (d) dependence of the Nu vs Ra relation on the Prandtl number. For large aspect ratios, the agreement with both laboratory and DNS data is satisfactory. The model contains no free parameters.
Spectral anisotropy in forced two-dimensional turbulence on a rotating sphere
Authors: Nozawa Toru , Yoden Shigeo
Published: July 2002

The datasets on the forced two-dimensional turbulence on a rotating sphere obtained in our recent direct numerical simulations were analyzed to study the spectral anisotropy due to the rotation of the sphere. The results were also compared with those previously obtained in some β-plane experiments to assess the β-plane approximation of the rotating sphere. Owing to the effect of rotation the upward energy cascade ceases around a characteristic total wavenumber nβ at which the linear “β-term” is comparable to the nonlinear Jacobian term. The energy density of zonal components (m=0) is dominant in the range of n≲nβ while the energy is very small in an airfoil-shaped region at the lower edge in the wavenumber space (m,n). Anisotropic distribution of the energy is also found in the high wavenumber region n≳nβ; the energy density decreases as the zonal wavenumber m increases. The flow field in the spherical geometry is projected on some tangential planes from the equator to the poles to compare the spherical results directory with previous β-plane experiments. The energy distribution becomes anisotropic to have dominant zonal components as the local “β-effect” increases (or the tangential plane is put closer to the equator). In the case on equatorial tangential planes, the region in which the energy density is very small shows a dumbbell shape indicating strong anisotropy; this is the first confirmation of the recent finding by Vallis and Maltrud in full spherical geometry.
Study of a high-velocity liquid jet stressed by an electric field
Authors: Artana G. , Romat H. , Touchard G.
Published: July 2002

In this work we report an experimental study dealing with the modifications of the breakup phenomena of a high-velocity jet when stressed by an electric field. We considered an arrangement consisting of a cylindrical electrode coaxial with the jet imposing an electric field at the jet exit. With this arrangement we analyzed the behavior of the electrified and nonelectrified jet. We tested a water liquid jet and measured the droplet size and velocity ratio of the droplets of the spray at different positions by means of a Phase Doppler Particle Analyzer. Our results indicate that the electric field increases monotonically the ratio of the components of the velocity in the radial and axial direction and promotes an earlier detachment of droplets from the jet. No significant change in droplet size could be detected. With the aid of a previously reported linear stability analysis we have analyzed the experimental data. This theory reveals itself as a good first approach to compare some aspects of the breakup phenomena of electrified and nonelectrified jets of conducting liquids flowing at high velocities.
Microscopic motion of particles flowing through a porous medium
Authors: Lee Jysoo , Koplik Joel
Published: July 2002

Stokesian dynamics simulations are used to study the microscopic motion of particles suspended in fluids passing through porous media. Model porous media with fixed spherical particles are constructed, and mobile ones move through this fixed bed under the action of an ambient velocity field. The pore scale motion of individual suspended particles at pore junctions are first considered. The relative particle flux into different possible directions exiting from a single pore, for two- and three-dimensional model porous media is found to approximately equal the corresponding fractional channel width or area. Next the waiting time distribution for particles which are delayed in a junction due to a stagnation point caused by a flow bifurcation is considered. The waiting times are found to be controlled by two-particle interactions, and the distributions take the same form in model porous media as in two-particle systems. A simple theoretical estimate of the waiting time is consistent with the simulations. It is found that perturbing such a slow-moving particle by another nearby one leads to rather complicated behavior. Finally, the stability of geometrically trapped particles is studied. For simple model traps, it is found that particles passing nearby can “relaunch” the trapped particle through its hydrodynamic interaction, although the conditions for relaunching depend sensitively on the details of the trap and its surroundings.
Routes to chaos in wide-gap spherical Couette flow
Authors: Wulf P. , Egbers C. , Rath H. J.
Published: July 2002

The dynamical behavior of wide-gap instabilities in spherical Couette flow is investigated experimentally with chaos analyzing techniques applied on time series from Laser-Doppler-Velocimetry (LDV) measurements. With an increasing Reynolds number of the rotating inner sphere, the flow undergoes two Hopf bifurcations and several mode changes. The transition scenarios for two different gap widths investigated can be described primarily as the Ruelle–Takens–Newhouse type and show a strong dependence on the meridional coordinate. The transition processes are accompanied by phenomena such as a locked torus in reconstructed phase space and periodic long wave modulations of chaotic flows. Although the investigated gap widths are all classified as wide, a comparison exhibits significant differences in the development of chaotic motion.
A spectral theory for fingering on a prewetted plane
Authors: Ye Yi , Chang Hsueh-Chia
Published: July 2002

A spectral theory is formulated to quantify the dynamic sensitivity and stability of a liquid front spreading over a prewetted inclined plane. Sensitivity of the front to surface heterogeneity is captured by the stable continuous essential spectrum of a non-normal linear operator over a practically infinite domain. Small-amplitude disturbances on the prewetted film are convected by this mechanism to the front and focused into a sharp but transient spike with a greatly magnified amplitude. A specific transverse wavelength is selected during this transient excitation. However, this amplification can only lead to fingering and select the transverse wavelength if a point eigenvalue of the discrete spectrum with the same wavelength is unstable. This discrete mode is resonantly excited by the disturbance convected and amplified by the stable essential spectrum. The preferentially excited point eigenvalue is nearly neutral due to the translational invariance of the unfingered front and has a much smaller growth rate and much shorter transverse wavelength than the most unstable discrete mode. For a range of inclination angles, it is stable for thick fronts but destabilizes far downstream when the thinning film reaches a critical thickness and a bump appears on the front. Experiments and numerical simulation of fingering on a prewetted plane verify these distinct roles of the essential and discrete spectra in this supercritical fingering instability and quantitatively confirm the predicted fingering position.