bluuebunny/crossref_metadata_2025 huggingface dataset:
Single and multiple helicity Ohmic states in reversed-field pinches
Authors: Finn John M. , Nebel Rick , Bathke Charles
Published: July 2002

The properties of single helicity and multiple helicity Ohmic states in reversed-field pinches (RFP’s) are investigated by a combination of analytic and numerical methods. The single helicity results show that toroidal field reversal can be provided in a helical Ohmic equilibrium driven by a toroidal loop voltage, and that reversal in helical symmetry is related to stellarator transform. Nevertheless, a constant λ≡j∥/B state cannot be sustained because λ must reverse at the toroidal field reversal surface. This helical equilibrium can be thought of as the saturated state of an unstable tearing mode. For a force-free plasma, helical reversal can be maintained by a relatively small value of δB/B because it is accompanied by an inward paramagnetic pinch velocity. Conversely, in models with △⋅v=0, a large outward diffusive velocity must build up to balance the inward paramagnetic pinch velocity, requiring poloidal beta to be of order unity. It is shown that in three dimensions no multihelical Ohmic equilibrium can exist if there is an area of destroyed flux surfaces. Numerical simulations in three dimensions (3D) indeed show that Ohmic states exist, but with fields varying on a resistive time scale. Also, a bifurcation is found between two distinct classes of solutions in 3D. The first is a broad spectrum multihelical state with almost no flux surfaces. The other class consists of narrow spectrum nearly single helicity states with a large fraction of the plasma volume occupied by good flux surfaces. The existence of this second class of states indicates the possibility of operating RFP’s in a single helicity mode with good flux surfaces.
Effect of nonaxisymmetric perturbations on the structure of a tokamak poloidal divertor
Authors: Pomphrey N. , Reiman A.
Published: July 2002

The effects of n=1 nonaxisymmetric perturbations on a tokamak poloidal divertor are described. Despite the existence of a region of chaotic field line trajectories outside the last closed flux surface, the footprint of the trajectories on the divertor plates is found to be largely coherent, forming a spiral structure. At a fixed toroidal angle, the footprint exhibits a bifurcation similar to that seen experimentally on DIII-D [Nucl. Fusion 28, 902 (1988)]. For field errors of the magnitude that exist in present-day tokamaks, the width of the nonaxisymmetric structure in the divertor footprint is comparable to the width of the scrape-off layer.
Energy transfer dynamics of dissipative trapped ion convective cell turbulence
Authors: Newman D. E. , Terry P. W. , Diamond P. H.
Published: July 2002

The properties of the spectral energy transfer for a two-dimensional fluid representation of dissipative trapped ion convective cell turbulence are studied numerically using a spectral method. It is established that the spectral energy flow is from long to short wavelength, as governed (under the dynamics of the E×B nonlinearity) by a single quadratic invariant, the energy. This flow is correctly predicted by equilibrium statistical mechanics, as is the equilibrium spectrum. Examining the locality of energy flow, strong nonlocal energy transfer is observed, a process that efficiently transfers the energy of a mode across the spectrum in a correlation time. This transfer process deviates dramatically from the canonical self-similar cascade dynamics of Kolmogorov that typifies the cascade of two- and three-dimensional Navier–Stokes and Hasegawa–Mima drift wave turbulence. Anisotropy of the spectral transfer dynamics is also observed.
Simulation study of stepwise relaxation in a spheromak plasma
Authors: Horiuchi Ritoku , Sato Tetsuya , Uchida Masaya
Published: July 2002

The energy relaxation process of a spheromak plasma in a flux conserver is investigated by means of a three-dimensional magnetohydrodynamic simulation. The resistive decay of an initial force-free profile brings the spheromak plasma to an m=1/n=2 ideal kink unstable region. It is found that the energy relaxation takes place in two steps; namely, the relaxation consists of two physically distinguished phases, and there exists an intermediate phase in between, during which the relaxation becomes inactive temporarily. The first relaxation corresponds to the transition from an axially symmetric force-free state to a helically symmetric one with an n=2 crescent magnetic island structure via the helical kink instability. The n=2 helical structure is nonlinearly sustained in the intermediate phase. The helical twisting of the flux tube creates a reconnection current in the vicinity of the geometrical axis. The second relaxation is triggered by the rapid growth of the n=1 mode when the reconnection current exceeds a critical value. The helical twisting relaxes through magnetic reconnection toward an axially symmetric force-free state. It is also found that the poloidal flux reduces during the helical twisting in the first relaxation and the generation of the toroidal flux occurs through the magnetic reconnection process in the second relaxation.
Observation of loss boundaries and ion flux spectra of end-loss ions in the tandem mirror GAMMA 10
Authors: Ishii K. , Maeda Y. , Tsumori K. , Shimouchi Y. , Katanuma I. , Itakura A. , Yatsu K. , Miyoshi S.
Published: July 2002

Loss boundaries and ion flux spectra of end-loss ions have been observed by means of a new diagnostic device in the tandem mirror GAMMA 10 [Nucl. Fusion Suppl. 1, 429 (1983)]. In consequence of measuring simultaneously both the pitch angle profile and energy of the end-loss ions, the loss boundaries were directly ascertained to be composed of two hyperbolic lines characterized by the electrostatic plug potential in the velocity space, and also high outer mirror throat potential was found. The flux spectra were compared with the two types of theoretical results, and discussed from a viewpoint of Coulomb collision.
The absorption mechanisms of whistler waves in cool, dense, cylindrically bounded plasmas
Authors: Harvey B. M. , Lashmore-Davies C. N.
Published: July 2002

It can be shown that whistler waves with low parallel phase velocity can be subject to strong cyclotron damping, even when the wave frequency is well below the cyclotron frequency. This resonance arises as a result of the Doppler effect which is proportional to the plasma density and increases the frequency experienced by electrons moving in the opposite direction to the wave. In this paper the dispersion relation of whistler waves in a cylindrically bounded plasma is obtained and then solved. This geometry is desirable for its relevance to experiment and also for comparison with theoretical results by other authors. In addition to cyclotron damping, Landau damping as well as electron–ion, electron–electron, and electron–neutral collisions are all included thus enabling the relative importance of these damping mechanisms to be evaluated. The dispersion relation that is obtained is used to explore the transition from Landau dominated damping to cyclotron dominated damping.
Properties of transit-time interactions in magnetized plasmas: Analytic and numerical results
Authors: Melatos A. , Robinson P. A.
Published: July 2002

The recently developed perturbation theory of transit-time interactions between particles and coherent wave packets in magnetized plasmas is applied to particular field structures. Limits of validity are determined by comparison with test-particle simulations, showing that the theory is accurate everywhere except near certain well-determined resonances, for wave fields exceeding a characteristic threshold, and for particles below a particular velocity. The properties of transit-time interactions in magnetized plasmas are investigated in detail to determine their dependence on the fields and parameters of the particle motion. Resonant particle scattering is found to occur at low particle velocities when the frequency of the coherent wave packet is an integer multiple of the gyrofrequency. Two different types of resonant transit-time dissipation are also observed: one arises from transient cyclotron acceleration in the localized wave packet, the other from beating between the gyration of the particles and the oscillation of the wave packet field. Both effects involve an interplay between the field geometry and resonant oscillations.
A new formulation of the resistive tearing mode stability criterion
Authors: Chu M. S. , Dewar R. L. , Greene J. M. , Pletzer A.
Published: July 2002

A new formulation of the toroidal, finite beta, resistive tearing stability problem is presented. As in standard tearing mode theory, the mode structure throughout most of the configuration is determined by an ideal, inertia-free model. Thus, it is very closely related to that obtained from standard ideal magnetohydrodynamic (MHD) numerical stability programs that depend on an energy principle. The effects of inertia, resistivity, and any other plasma properties are important only in thin layers enclosing resonant surfaces. These surfaces are distinguished by the fact that they are composed of closed field lines. Instability growth rates are obtained from the condition of matching between the inner and outer regions. The data needed from the outer region for matching are conventionally reduced to a quantity Δ′, but in toroidally coupled axisymmetric systems the relevant quantity is a matrix. A previous paper [Pletzer and Dewar, J. Plasma Phys. 45, 427 (1991)] presented a relation between an extension of the ideal energy and the information from the outer region that is needed in matching to the inner layers. Here, this is used to construct a relation for the tearing mode growth rates directly in terms of an extension of the ideal energy matrix. This demonstrates a convenient way to extend the numerical programs for ideal stability to include stability against tearing modes.
A pressure-gradient-driven tokamak ‘‘resistive magnetohydrodynamic’’ instability in the banana-plateau collisionality regime
Authors: Callen J. D. , Shaing K. C.
Published: July 2002

The moment equation approach to neoclassical processes is used to derive the linearized electrostatic perturbed flows, currents, and resistive MHD-like equations for a tokamak plasma. The new features of the resultant ‘‘neoclassical magnetohydrodynamics,’’ which requires a multiple length scale analysis for the parallel eigenfunction, but is valid in the experimentally relevant banana-plateau regime of collisionality, are: (1) a global Ohm’s law that includes a fluctuating bootstrap current resulting from the ‘‘parallel’’ electron viscous damping (at rate μe) of the poloidal flow due to the perturbed radial pressure gradient; (2) reduction of the curvature effects to their flux surface average because Pfirsch–Schlüter currents cancel out the lowest-order geodesic curvature effects: (3) an increased polarization drift contribution with B−2, replaced by B−2Θ where BΘ is the poloidal magnetic field component. An electrostatic eigenmode equation is determined from ∇⋅J̃=0. For the unstable fluid-like eigenmodes, the new viscous damping effects dominate (by ε−3/2) over the curvature effects, but the growth rates still scale roughly like resistive-g or resistive-ballooning modes, γμτA∼n2/3S1/3Nβ2/3T ×( μe/νe)1/3. Diamagnetic drift frequency corrections to these new modes are also discussed.
Conductivity of rf-heated plasma
Authors: Fisch Nathaniel J.
Published: July 2002

The electron velocity distribution of rf-heated plasma may be so far from Maxwellian that Spitzer conductivity no longer holds. A new conductivity for such plasmas is derived, and the result can be put in a general form. The new expression should be of practical value in examining schemes for current ramp-up in tokamaks by means of lower-hybrid or other waves.
Asymptotic theory of ion conic distributions
Authors: Crew G. B. , Chang T. S.
Published: July 2002

The formation of ion conics is discussed in terms of a model which permits an asymptotic determination of the ion distribution function. This model is applicable to a class of observed ion conic events where the ion energization is attributable to lower hybrid wave activity. A two-stage process is considered wherein the upward moving component of the ambient ion population is first subjected to a region of weak turbulence and is then allowed to drift adiabatically up the geomagnetic field. Identification of the ratio of ion thermal speed to mean wave speed as a small parameter leads to a uniformly valid solution of the quasilinear diffusion equation. The resulting analytic form for the ion conic is then available for further analytic work. Implications and limitations of the model will be discussed.
Collisional effects on trapped particle modes in tandem mirrors
Authors: Lane Barton
Published: July 2002

The effects of collisions on trapped particle modes in tandem mirrors are analyzed. Two regimes are considered, a low collisionality regime ω∼ωi*>νe and a high collisionality regime νi<ω<νe. The magnetic geometry of the equilibrium is left arbitrary and a pitch angle scattering operator is used to model the effects of collisions. For ω>νe electron collisions are found to destabilize an otherwise stable negative energy wave. Because of a boundary layer phenomenon, the growth rate scales as (νe‖ω*‖)1/2. For νi<ω<νe two modes are obtained: (a) a flute mode whose stability is determined by the flux tube integral of the beta-weighted curvature drive, and (b) a dissipative trapped ion mode driven unstable by the difference in collisionality between electrons and ions. To first order in (ω/νe), (νi/ω), (ωd/ω), and k2⊥ ρ2i the stability and eigenfrequency of the flute mode is independent of νi while the dissipative trapped ion mode is damped by increasing ion collisionality.
Electron beam hollowing instability threshold
Authors: Godfrey Brendan B.
Published: July 2002

High-current, fast-rise-time electron beams in air often are subject to powerful hollowing instabilities. Previous work indicates that the instability is triggered by avalanche ionization of the air, which occurs for a ratio of inductive electric field strength to air density exceeding 130 kV/cm-atm. Here, computer simulations are used to recast this threshold criterion in terms of the beam current rise rate, which is more readily accessible experimentally. The threshold dependence on beam radius and peak current is much weaker. The current rise rate tends to increase during propagation caused by erosion of the beam front, and this behavior is explored with additional simulations. Evidence for hollowing stabilization by electron energy spread also is presented. Numerical algorithms employed in these calculations are included for completeness.
Weakly relativistic dispersion of Bernstein waves
Authors: Robinson P. A.
Published: July 2002

Weakly relativistic effects on the dispersion of Bernstein waves are investigated for waves propagating nearly perpendicular to a uniform magnetic field in a Maxwellian plasma. Attention is focused on those large-wave-vector branches that are either weakly damped or join continuously onto weakly damped branches since these are the modes of most interest in applications. The transition between dispersion at perpendicular and oblique propagation is examined and major weakly relativistic effects on dispersion at perpendicular propagation are found to persist at other angles; these effects can dominate even in low-temperature plasmas. A number of simple analytic criteria are obtained which delimit the ranges of harmonic number and propagation angle within which various types of weakly damped Bernstein modes can exist.
Study of resistive drift and resistive interchange modes in a cylindrical plasma with magnetic shear
Authors: Sugama Hideo , Wakatani Masahiro , Hasegawa Akira
Published: July 2002

The resistive drift mode and resistive interchange mode in the presence of magnetic curvature and shear are studied theoretically as well as computationally, based on the electrostatic model equations of isothermal electrons and two-dimensional cold ions in a cylindrical geometry. There are two branches of unstable modes: one has characteristics similar to the resistive drift wave instability with a real frequency ωr of the order of a diamagnetic frequency ω*, and the other is a curvature driven mode (the resistive interchange instability) with ‖ωr‖=‖ωi‖ localized in the neighborhood of the resonant surface. The growth rates of both of these instabilities are proportional to ν1/2ei in the small νei limit, where νei is the electron–ion collision frequency. The resistive interchange instability is dominant even when the curvature drift frequency is a fraction of ω*. Nonlinear evolution of these instabilities with multihelicity modes based on the model equations demonstrates a global spectrum condensation at the m=0 mode, where m denotes the poloidal mode number. The saturated state generates a region of zonal flows in the azimuthal direction. Since particle transport across the constant φ surface vanishes, the zonal flow is expected to improve global confinement.
Morphology of magnetic fields generated in laser-produced plasmas
Authors: Boyd T. J. M. , Cooke D.
Published: July 2002

Magnetic fields in the megagauss range have been measured in experiments on plasmas generated by irradiating targets with high power lasers. A study of the morphology of these self-generated fields is important not only for its intrinsic interest but for possible implications in laser–target physics. In this paper work on the numerical modeling of large magnetic fields generated in target experiments is reported. The results show generally satisfactory agreement with the fields measured experimentally both in terms of the magnitude of the peak fields and their morphology. In the numerical model the contribution from the Hall term in describing the evolution of the magnetic field is shown to be important especially in short pulse (≂100 psec) experiments.
An improvement of the nonlocal heat flux formula
Authors: Bendib A. , Luciani J. F. , Matte J. P.
Published: July 2002

A new heat flux formula valid in strong inhomogeneous laser-produced plasmas has been derived from the Fokker–Planck equation. The nonlocal treatment of Luciani et al. [Phys. Rev. Lett. 51, 1664 (1983); Phys. Fluids 28, 835 (1985)] is improved by taking into account the electric potential effect. A simple and reliable phenomenological formula is proposed, which is in good agreement with numerical Fokker–Planck computations.
Interchange stabilization of a mirror plasma using radio-frequency waves below the ion cyclotron frequency
Authors: Browning J. J. , Majeski R. , Intrator T. , Hershkowitz N. , Meassick S.
Published: July 2002

It is demonstrated that radio-frequency (rf) waves applied below the ion cyclotron frequency (ω/Ωi≊0.75) can stabilize a mirror plasma against the interchange instability. A set of phased antennas in the Phaedrus-B tandem mirror central cell [Phys. Rev. Lett. 59, 206 (1987)] is used to select the rf azimuthal mode number (m). When the m=−1 mode is selected, the ponderomotive force from the left-hand polarized wave fields is sufficient to stabilize the plasma. Measurements of the rf mode number and of the wave polarization indicate strong excitation of m=−1 modes with much of the wave having a left-hand polarization.
Ion temperature gradient mode in the weak density gradient limit
Authors: Dominguez R. R. , Waltz R. E.
Published: July 2002

The toroidal ion temperature gradient mode (ITG mode) is analyzed in the fluid limit for flat density profiles, typical of many H-mode plasmas. It is shown that the stability boundary is not Ln/LTi=const ≊O(1 ■2) (Ln and LTi are the density and ion temperature gradient scale lengths, respectively), as typically quoted. Rather, for large Ln/R the stability threshold is characterized by LTi/R =const≊O(0.1) with R the major radius. This result could substantially reduce the impact of ITG-mode turbulence, allowing a reconciliation of present drift wave transport models with improved H-mode energy confinement.
Second viscosity enhancement in turbulent nonequilibrium flow
Authors: Orou Jean Chabi , Johnson Joseph A.
Published: July 2002

Using a zeroth-order entropy production model, a relationship between the second viscosity, the relaxation time for a nonequilibrium process, and turbulence is determined from the macroscopic entropy rate equation. For turbulence generated reaction rate distortion, enhanced second viscosities can be predicted for turbulent fluid systems with long relaxation times and nonzero velocity divergence.
Effect of suction on the near-wall anisotropy of a turbulent boundary layer
Authors: Antonia R. A. , Spalart P. R. , Mariani P.
Published: July 2002

Direct numerical simulation data obtained for a zero pressure gradient turbulent boundary layer at a moderate Reynolds number indicate a significant increase in the anisotropy of the near-wall region when suction is applied uniformly at the wall. This anisotropy is reflected in the approach of the Reynolds stress tensor towards a one-component state.
Shear flow over a plane wall with an axisymmetric cavity or a circular orifice of finite thickness
Authors: Pozrikidis C.
Published: July 2002

Shear flow over a plane wall that contains an axisymmetric depression or pore is studied using a new boundary integral method which is suitable for computing three-dimensional Stokes flow within axisymmetric domains. Numerical results are presented for cavities in the shape of a section of a sphere or a circular cylinder of finite length, and for a family of pores or orifices with finite thickness. The results illustrate the distribution of shear stresses over the plane wall and inside the cavities or pores. It is found that in most cases, the distribution of shear stresses over the plane wall, around the depressions, is well approximated with that for flow over an orifice of infinitesimal thickness for which an exact solution is available. The kinematic structure of the flow is discussed with reference to eddy formation and three-dimensional flow reversal. It is shown that the thickness of a circular orifice or depth of a pore play an important role in determining the kinematical structure of the flow underneath the orifice in the lower half-space.
Nonstirring of an inviscid fluid by a point vortex in a rectangle
Authors: Meleshko V. V.
Published: July 2002

Numerical evidence is presented that two-dimensional motion of a large region of incompressible, inviscid fluid due to the motion of a point vortex in a rectangular container leaves does not stir certain finite regions. Such regions are formed by means of the Poincaré mapping in the regular domain for a nonintegrable Hamiltonian system of fluid particle motion.
Turbulence suppression in a jet by means of a fine ring
Authors: Tong Chenning , Warhaft Z.
Published: July 2002

Studied are the effects on the velocity field of a fine circular wire ring placed axisymmerically downstream of an air jet of Reynolds number 1.4×104. When placed close to the jet exit (x/Dj<3 where Dj is the jet diameter) the ring caused a pronounced reduction in the rms longitudinal velocity fluctuations, the maximum reduction (of approximately 30% based on the peak value) occurring when the ring is very close to the jet (x/Dj=0.07) and in the shear layer although rings of smaller diameter located in the core also produced the same qualitative effect. The ring also caused a shift in the virtual origin of the mean flow and an increase of the momentum thickness of the shear layer. There resulted a reduction of the magnitude of the fundamental roll-up frequency, and it saturated later than for the undisturbed flow. The spatial growth rate of the disturbances in the shear layer were also drastically reduced. Spectra showed that vortex pairing was inhibited by the ring and there was an earlier transition to fully developed turbulence. These effects are attributed to the ring causing a change in the form of the mean velocity profile in the shear layer. The effects of vortex shedding by the ring were discounted by showing that the results did not qualitatively change when the ring wire Reynolds number, Rew, was varied from sub to super critical values (30<Rew<120). These findings are compared with previous results, using acoustic excitation to reduce the turbulence intensity in a jet, with which they have much in common.
Drag coefficient of a gas bubble in an axisymmetric shear flow
Authors: Takagi S. , Prosperetti A. , Matsumoto Y.
Published: July 2002

The drag coefficient of a gas bubble in steady motion in an unbounded axisymmetric shear flow is obtained numerically for different values of the Reynolds and Weber numbers.
On pressure minima in two-dimensional vortex flows
Authors: Lugt Hans J. , Copeland G. Scott
Published: July 2002

For the present, a vortex will be defined as a two-dimensional region containing nested closed streamlines. Such a vortex need not contain an extremal point of vorticity nor a minimal point of pressure. In the Stokes-flow limit, a pressure minimum is not possible. A local criterion for the existence of a pressure minimum within a vortex is derived, leading to a transition Reynolds number above which the vortex contains a pressure minimum. In the limit of infinite Reynolds number, a pressure minimum must exist within the vortex. Specific data for a cavity flow and the flow past a circular cylinder are presented.
On long-lived vortices in 2-D viscous flows, most probable states of inviscid 2-D flows and a soliton equation
Authors: Pasmanter Rubén A.
Published: July 2002

This paper considers, in contraposition to the most probable states of quasi-inviscid theories, the status of the vorticity(ω)–streamfunction(ψ) relations satisfied by the long-lived vortices observed in some numerical simulations of decaying two-dimensional turbulence and in experiments in stratified fluids. For the case ω=−Δψ=sinh ψ, the circular solutions can be expressed in terms of the IIIrd Painlevé transcendent and dipolar solutions can be constructed by means of a Bäcklund transform.
Flow visualization of the three-dimensional, time-evolving structure of a turbulent boundary layer
Authors: Goldstein José E. , Smits Alexander J.
Published: July 2002

Experiments were performed using volumetric flow visualization to study the evolution of three-dimensional boundary layer turbulence, and the interactions between inner and outer layer coherent structures. In the experiment, the inner and outer layers of a low Reynolds number (Rθ=705) turbulent boundary layer in air were seeded with smoke. A section of the boundary layer was rapidly scanned with a high intensity pulsed laser sheet, and the individual images were recorded using a high-speed video camera. The video images were digitized and assembled into three-dimensional volumes. A variety of image processing techniques and conditional sampling and statistical methods were then used to extract quantitative information from the three-dimensional, time-evolving visualization data.
Characteristics of the linearized equations of motion for a compressible fluid
Authors: Papamoschou Dimitri
Published: July 2002

The characteristics and bicharacteristics of the linearized equations of motion for compressible fluid are identical to the wave fronts and rays, respectively, calculated by Fermat’s least-time principle of geometric acoustics. The bicharacteristics emanating from the middle of a compressible shear layer curve so as to hinder communication in the axial direction.
On stationary equivalent modons in an eastward flow
Authors: Swaters Gordon E.
Published: July 2002

Modons correspond to isolated dipole vortex solutions of the quasigeostrophic equations. They have been proposed as prototype models for some geophysical (and plasma) vortices. The classical modon solution on a β plane does not permit a Rossby wave field in the exterior or far-field region of the modon. However, it is qualitatively known that the gravest mode associated with a normal mode decomposition of a stationary modon in a continuously stratified fluid of finite depth necessarily contains a Rossby wave tail in the downstream region if the background flow is eastward. The same effect can be formally recreated in an equivalent-barotropic model of a stationary modon embedded in a constant eastward zonal flow. An analytical solution to this problem satisfying the correct upstream radiation condition is presented and its dynamical characteristics are discussed.
The characteristics and evolution of large-scale structures in compressible mixing layers
Authors: Elliott Gregory S. , Samimy Mo , Arnette Stephen A.
Published: July 2002

Single- and double-pulsed visualizations were employed to study the characteristics and evolution of large-scale structures in compressible mixing layers with convective Mach numbers (Mc) of 0.51 and 0.86. Instantaneous images and spatial correlations based on large ensembles of images show that large-scale structures which span the entire mixing layer thickness are a more dominant feature at Mc=0.51 than at Mc=0.86. Double-pulsed images in the Mc=0.51 developing region show the structures’ formation to proceed as the roll-up of a wavy mixing region, in agreement with the Kelvin–Helmholtz picture of structure formation derived from studies of incompressible mixing layers. However, little indication of roll-up was present for Mc=0.86. In the Mc=0.51 fully developed region, the large-scale structures evolve similarly to those of incompressible mixing layers, even undergoing pairing processes. Similar structure evolution processes were not encountered in Mc=0.86; instead, it was more difficult to even track large-scale motions between the initial and delayed images. Convective velocities derived from space-time correlations of ensembles of double-pulsed images are in good agreement with the theoretical convective velocity at the center of the mixing layer, but are higher than the theoretical value toward the high-speed side of the mixing layer and lower toward the low-speed side.
Nonlinear spin-up in a circular cylinder
Authors: van de Konijnenberg J. A. , van Heijst G. J. F.
Published: July 2002

Nonlinear spin-up in a circular cylindrical tank is investigated experimentally and compared with the Wedemeyer model. The experiments were performed with water, using tracer particles floating at the free surface in order to visualize the flow field. The experimentally determined vorticity profiles show differences from the Wedemeyer model that indicate the need for an improved estimation of the Ekman pumping on a finite domain. In particular, the Wedemeyer model appears to be inaccurate in the region close to the sidewall. The vorticity field in a spin-down experiment can be reproduced very well by using numerical data of Rogers and Lance for the Ekman suction of an unbounded rotating fluid over a nonrotating plate. However, a more general use of the data of Rogers and Lance on a bounded domain is shown to be inadequate because this would lead to a violation of mass conservation of the Ekman layer.
Instabilities in three-dimensional differentially-heated cavities with adiabatic horizontal walls
Authors: Janssen R. J. A. , Henkes R. A. W. M.
Published: July 2002

Considered are the transitional instabilities of the flow inside three-dimensional rectangular cavities that are differentially heated over two opposing vertical walls. The horizontal and lateral walls are adiabatic. Emphasis is on (though not restricted to) the air-filled, cubical cavity. For this configuration, it was found that the occurrence of unsteady oscillations in the flow was preceded by a steady instability (i.e. an instability resulting in a steady solution of the Navier–Stokes equations for large time) which originated in an internal, stratified shear layer that separates from the adiabatic horizontal walls of the cavity. This instability is inherently three-dimensional and characterized by the presence of streamwise-oriented, counterrotating vortices. It is probably caused by centrifugal forces. The subsequent, low-frequency, unsteady instability is strongly influenced by this steady instability and as a result its frequency differs strongly from its counterpart in the two-dimensional, square cavity. For larger Prandtl numbers, however, the frequencies in the two- and three-dimensional cavities are almost equal since no prior steady instability occurs. The instability mechanism responsible for the unsteady instability is therefore the same in both configurations even though the instability in the three-dimensional cavity shows a distinct wave-like modulation in the third direction.
On the existence of a Generalized Langevin model representation for second-moment closures
Authors: Wouters H. A. , Peeters T. W. J. , Roekaerts D.
Published: July 2002

The Generalized Langevin model representations of two second-moment closure models for the rapid pressure-strain term, proposed by Fu and Launder and by Jones and Musonge, are obtained. This representation makes it possible to use these models in PDF calculations of turbulent flows. The implications of three realizability constraints for the relationship between Langevin models and these second-moment closures are discussed. A Generalized Langevin model representation exists only if the rapid pressure-strain model satisfies realizability at the 2D turbulence limit.
Granular dynamics of inelastic spheres in Couette flow
Authors: Lun Cliff K. K.
Published: July 2002

A numerical program has been developed to simulate an assembly of inelastic, frictional hard spheres inside a control volume undergoing a steady-state rapid Couette flow induced by the top and bottom bumpy walls. The bumpy walls are made of hemispheric particles fixed onto flat plates. The flow particles can collide with the wall particles and the exposed flat areas of the walls. The macroscopic flow properties are found to depend on a number of material and geometric properties of the granules, the bumpy walls, and the control volume. These properties include the overall solids fraction of the system, the height of the shear gap, the wall-particle concentration, the wall-particle distribution, the diameter ratio of the wall particle to the flow particle, the coefficients of restitution, the friction coefficients, and the sticking tangential restitution coefficients between the flow particles, the wall particles, and the flat walls. A parametric study is undertaken to examine the effect of some of the interesting factors identified above. A new definition for the slip velocity yields positive values consistently, and it represents a significant improvement over the previous ones. By exposing the flat areas of the bumpy walls for collisions, the transfer of energy and momentum from the driving surfaces to the flow medium can be enhanced. Depending on the wall-particle distribution, there exist optimal wall-particle concentrations at which the stresses may be maximized or the slip velocities may be minimized. For hemispheric wall particles arranged in an equilateral triangular lattice, the optimal wall-particle area fraction for maximizing the stresses is about 0.44 while the one for minimizing the slip velocity is about 0.36. The simulation results also show that there exists for gravity-free Couette flow of inelastic, frictional spheres a critical solids fraction of about 0.5 beyond which the stresses are found to decrease with increasing solids concentration. In general, there is reasonable agreement between the simulation results for stresses and the experimental measurements.
Hierarchy of transverse structure functions
Authors: Camussi R. , Benzi R.
Published: July 2002

An experimental validation of a recent statistical model [Phys. Rev. Lett. 72, 336 (1994)] is reported. The analysis is focused on the transverse components of the velocity structure functions, and extend previous tests conducted only on longitudinal velocity components.
On the problem of natural convection in liquid phase thermotransport coefficients measurements
Authors: Garandet J. P. , Praizey J. P. , Van Vaerenbergh S. , Alboussiere T.
Published: July 2002

We focus in this paper on the effect of natural convection in thermodiffusion coefficients measurements in liquid metal alloys both for normal and microgravity conditions. Our previous experimental results are briefly recalled, with a special emphasis on the data recently obtained from the EURECA space mission. With respect to the ground based values, it is seen that the solutal separation is always significantly higher in microgravity, even in systems where solutal stabilization of the flow has an effect. Simple scaling analysis arguments show that the error induced by additional convective transport scales with the square of the fluid velocity. Such a result compares favorably with existing three dimensional (3D) numerical data. The theory also accounts qualitatively for the reduced separation observed experimentally in ground based set-ups. We conclude that it is in principle possible to perform accurate measurements in space, but that the size of the capillaries used in the experiments should always be limited to roughly two millimeters. On Earth on the other hand, the risk of convective interference cannot be avoided.
Two-phase modeling of DDT: Structure of the velocity-relaxation zone
Authors: Kapila A. K. , Son S. F. , Bdzil J. B. , Menikoff R. , Stewart D. S.
Published: July 2002

The structure of the velocity relaxation zone in a hyperbolic, nonconservative, two-phase model is examined in the limit of large drag, and in the context of the problem of deflagration-to-detonation transition in a granular explosive. The primary motivation for the study is the desire to relate the end states across the relaxation zone, which can then be treated as a discontinuity in a reduced, equivelocity model, that is computationally more efficient than its parent. In contrast to a conservative system, where end states across thin zones of rapid variation are determined principally by algebraic statements of conservation, the nonconservative character of the present system requires an explicit consideration of the structure. Starting with the minimum admissible wave speed, the structure is mapped out as the wave speed increases. Several critical wave speeds corresponding to changes in the structure are identified. The archetypal structure is partly dispersed, monotonic, and involves conventional hydrodynamic shocks in one or both phases. The picture is reminiscent of, but more complex than, what is observed in such (simpler) two-phase media as a dusty gas.
Different intermittency for longitudinal and transversal turbulent fluctuations
Authors: Grossmann Siegfried , Lohse Detlef , Reeh Achim
Published: July 2002

Scaling exponents of the longitudinal and transversal velocity structure functions in numerical Navier–Stokes turbulence simulations with Taylor–Reynolds numbers up to Reλ=110 are determined by the extended self similarity method. We find significant differences in the degree of intermittency: For the sixth moments the scaling corrections to the classical Kolmogorov expectations are δξ6L=−0.21±0.01 and δξ6T=−0.43±0.01, respectively, independent of Reλ. Also the generalized extended self similarity exponents ρp,q=δξp/δξq differ significantly for the longitudinal and transversal structure functions. Within the She–Leveque model this means that longitudinal and transversal fluctuations obey different types of hierarchies of the moments. Moreover, the She–Leveque model hierarchy parameters βL and βT show small but significant dependences on the order of the moment.
Convective turbulence in mercury: Scaling laws and spectra
Authors: Camussi R. , Verzicco R.
Published: July 2002

The turbulent regime of a Rayleigh–Bénard convective flow is studied through the analysis of temporal series and three-dimensional data fields obtained by direct numerical simulations. Numerical probes are positioned in the homogeneous (in the center) and non–homogeneous (close to the boundaries) turbulence regions of the cell and the corresponding data are obtained at low Prandtl (Pr=0.022) for a range of Rayleigh numbers 5×104⩽Ra⩽106. We discuss the dependence upon the Rayleigh number of some quantities characterizing the turbulence in the low and high frequency range such as length scales, dissipation and flow isotropy. The analysis of the velocity and vorticity spectra indicates that, in the center of the cell, the turbulence remains locally isotropic for the whole range of Ra considered. Furthermore, the dynamics of the large convective cell and the turbulence parameters, depend upon Ra in a manner consistent with experimental results at higher Ra. In conclusion, none of the indicators considered supports the idea of a transition in the turbulent state (often referred to as the soft–to–hard turbulence transition).
Spatial correlations in dilute granular flows: A kinetic model study
Authors: Brey J. Javier , Moreno F. , Ruiz-Montero M. J.
Published: July 2002

The development of spatial correlations in the hydrodynamic fields of a freely evolving granular fluid is studied. The analysis is based on a model kinetic theory for dilute granular gases, which includes an equation for the pair correlation function. For small inelasticity, the results agree with those derived previously from fluctuating hydrodynamics, and also with preliminary calculations using ring kinetic theory for inelastic hard spheres. A critical discussion of the domain of validity of a perturbative calculation of the hydrodynamic modes is given.
Instability and spatial correlations in a dilute granular gas
Authors: Javier Brey J. , Ruiz-Montero M. J. , Moreno F.
Published: July 2002

Direct Monte Carlo simulation is used to investigate the stability of a dilute freely evolving granular gas of hard disks. The boundary between stability and instability in the plane (α,L), where α is the restitution coefficient and L the size of the system, has been delineated. Instability is associated with the buildup of spatial correlations, which describes the formation of velocity vortices in the system. The simulation results are compared with theoretical predictions presented recently, and a good agreement is found.
Large vortex structures behind a maneuvering body in stratified fluids
Authors: Voropayev S. I. , McEachern G. B. , Fernando H. J. S. , Boyer D. L.
Published: July 2002

When a submerged self-propelled body makes a maneuver, e.g., accelerates, significant momentum is transported to the surrounding fluid. Our experiments show that in a stratified fluid this may lead to the formation of large vortex structures, much larger and different from those produced in the late wake during steady motion. Estimates also show that when an oceanic submerged vehicle changes its velocity by as little as 10% or its direction of motion by 5 degrees, large structures of the size of 1–2 km and with decay times of several days may be expected. Such effects may have potentially important applications and have not been studied previously.
Iterated stretching of viscoelastic jets
Authors: Chang Hsueh-Chia , Demekhin Evgeny A. , Kalaidin Evgeny
Published: July 2002

We examine, with asymptotic analysis and numerical simulation, the iterated stretching dynamics of FENE and Oldroyd-B jets of initial radius r0, shear viscosity ν, Weissenberg number We, retardation number S, and capillary number Ca. The usual Rayleigh instability stretches the local uniaxial extensional flow region near a minimum in jet radius into a primary filament of radius [Ca(1−S)/We]1/2r0 between two beads. The strain-rate within the filament remains constant while its radius (elastic stress) decreases (increases) exponentially in time with a long elastic relaxation time 3We(r02/ν). Instabilities convected from the bead relieve the tension at the necks during this slow elastic drainage and trigger a filament recoil. Secondary filaments then form at the necks from the resulting stretching. This iterated stretching is predicted to occur successively to generate high-generation filaments of radius rn, (rn/r0)=√(rn−1/r0)3/2 until finite-extensibility effects set in.
On the thermal offset in turbulent rotating convection
Authors: Hart J. E. , Ohlsen D. R.
Published: July 2002

A simple mechanistic model to explain the thermal offset observed in highly turbulent rotating convection experiments is presented. The experiments indicate that for sufficiently high Taylor number the system develops an asymmetry that is counter to the expected behavior of a Boussinesq fluid: The temperature at the midplane of the experiment is higher than the average of the top and bottom isothermal boundaries. The magnitude of this bias increases with the basic rotation and with the density difference between the top and bottom boundaries. Our model considers this to be a result of a mean circulation induced by centrifugal buoyancy. An analytical calculation of this motion field, postulated to be a laminar perturbation to the turbulence-induced mean stratification, generates a reasonably accurate description of the observed thermal offset.
Intermittency of a passive scalar advected by a quasifrozen velocity field
Authors: Ching Emily S. C. , Pang C. S. , Tsang Y. K. , Wang X. H.
Published: July 2002

We use a two-dimensional lattice model to study the intermittency problem of a passive scalar advected by a velocity field of finite correlation time. The stream function generating the incompressible velocity field is modeled by a random Gaussian noise that is identically independently distributed at each lattice point and is updated every certain finite time interval. A fixed scalar difference is maintained across one direction of the lattice. There are three time scales in the problem: the correlation or update time of the velocity field τc, the diffusion time of the scalar τdiff, and the advection time of the velocity field τadv. Interesting behavior is observed when τdiff<τc. In this regime the passive scalar field is found to be intermittent while its dynamics between the updates of the velocity field is dominated by diffusion. The intermittency can be described by log-Poisson statistics and is independent of the ratio τc/τadv. On the other hand, the passive scalar field exhibits dissipative scaling and is thus nonintermittent when τdiff⩾τc.