bluuebunny/crossref_metadata_2025 huggingface dataset:
Scaling laws for homogeneous turbulent shear flows in a rotating frame
Authors: Speziale Charles G. , Mac Giolla Mhuiris Nessan
Published: July 2002

The scaling properties of plane homogeneous turbulent shear flows in a rotating frame are examined mathematically by a direct analysis of the Navier–Stokes equations. It is proved that two such shear flows are dynamically similar if and only if their initial dimensionless energy spectrum E*(k*,0), initial dimensionless shear rate SK0/ε0, initial Reynolds number K20/νε0, and the ratio of the rotation rate to the shear rate Ω/S are identical. Consequently, if universal equilibrium states exist at high Reynolds numbers, they will only depend on the single parameter Ω/S. The commonly assumed dependence of such equilibrium states on Ω/S through the Richardson number Ri=−2(Ω/S)(1−2Ω/S) is proved to be inconsistent with the full Navier–Stokes equations and to constitute no more than a weak approximation. To be more specific, Richardson number similarity is shown to only rigorously apply to certain low-order truncations of the Navier–Stokes equations (i.e., to certain second-order closure models) wherein closure is achieved at the second-moment level by assuming that the higher-order moments are a small perturbation of their isotropic states. The physical dependence of rotating turbulent shear flows on Ω/S is discussed in detail, along with the implications for turbulence modeling.
The dynamics of low initial disturbance turbulent jets
Authors: Drubka R. E. , Reisenthel P. , Nagib H. M.
Published: July 2002

Recent experimental results in axisymmetric free jets are discussed with an emphasis on the dynamics of their self-forced states. The role played by the initial shear-layer instabilities and their coupling with subsequent jet instabilities is examined to reveal key mechanisms and scaling relations.
Linear-eddy modeling of turbulent transport. Part V: Geometry of scalar interfaces
Authors: Kerstein Alan R.
Published: July 2002

Linear-eddy modeling is a method for simulating molecular mixing on a one-dimensional domain imbedded in a turbulent flow. This method is used to investigate the stochastic geometry of scalar interfaces. Simulated interfaces (i.e., scalar level sets) in a domain with a spatially uniform mean scalar gradient are found to exhibit fractal structure with respect to the absolute level, but not with respect to the level referenced to its local mean value. Implications regarding the interpretation of measured fractal properties are discussed.
Toward a new Reynolds stress model for rotating turbulent flows
Authors: Cambon C. , Jacquin L. , Lubrano J. L.
Published: July 2002

A new approach for modeling turbulent rotating flows is presented. The main objective of this model is to take into account the complex effects of background rotation, which have been elucidated by homogeneous spectral analysis. The model is based on the following elements. First, it is shown, or recalled, that the use of the deviatoric part aij=Rij/Rll−δij/3 of the Reynolds stress tensor Rij as the unique tensorial argument for closing unknown terms (a basic principle in actual models) is no longer valid in the presence of solid body rotation. A decomposition of this tensor into two parts (aij=aeij+aZij), namely aeij (directional dependence) and aZij (polarization), is suggested from pure inviscid and homogeneous rapid distortion theory.

It is shown that the first part aeij is preserved while the second aZij is damped by the pressure-velocity correlations through the action of the Coriolis force. This decomposition appears to provide the minimum amount of information in physical space needed to deal with the complexity of the problem. A system of equations for aeij and aZij is then given for predicting in an ‘‘ad hoc’’ way some phenomena consistent with those identified by spectral models, recent direct numerical simulations (DNS), and experimental results. These phenomena are: (1) damping of any term attached to nonlinear interactions at very low Rossby numbers, in accordance with a convenient modification (for rotation) in the ε (dissipation rate) equation, (2) linear ‘‘rapid’’ reorganization of an initial anisotropy, as shown by pure inviscid rapid distortion theory, and (3) nonlinear ‘‘Taylor–Proudman structuring’’ at intermediate Rossby numbers. Although the present model is restricted to the case of pure solid body rotation, developments that could lead to a more general version that takes into account any kind of mean velocity gradient are discussed at the end of the paper. For this purpose, a consistent closure is provided for the ‘‘rapid’’ pressure rate of the strain correlation tensor Φ2ij=eΦ2ij+ZΦ2ij (involved in the Rij equation), and for the related contribution involved in the equation for Reij=Rllaeij, without introducing any adjustable constant. Finally, we present a general two-tensor model (Rij,Reij;ε) of which we hope will encourage new efforts in second-order modeling.
The rate of coalescence in a suspension of high Reynolds number, low Weber number bubbles
Authors: Kumaran V. , Koch Donald L.
Published: July 2002

The interactions between bubbles of nearly equal size (size ratio less than 1.1 at Re=100, and size ratio less than 1.07 at Re=200) in a suspension of bubbles rising due to gravity were analyzed under high Reynolds number, low Weber number conditions. The potential flow interaction between a pair of bubbles was studied using the equations of motion derived by Kumaran and Koch [Phys. Fluids A 5, 1123 (1993)]. If the horizontal distance between the bubbles is smaller than a critical value, the bubbles approach each other along the horizontal plane and collide. Previous studies suggest that in a collision, the bubbles could either coalesce or bounce off each other. If it is assumed that the bubbles bounce when they collide, they continue to collide repeatedly. The amplitude of the oscillations decrease due to viscous drag, and ultimately the bubbles rise as a horizontally aligned pair. In this configuration, scaling arguments indicate the bubbles coalesce in finite time. The coalescence frequency in a suspension of bubbles was calculated by averaging over pair interactions. This procedure is valid for V≪18/Re, where V is the volume fraction of the bubbles. The coalescence frequency is proportional to V Re−2/5 in this limit.
Dynamical simulations of sedimenting spheres
Authors: Ladd Anthony J. C.
Published: July 2002

The sedimentation of monodisperse suspensions of rigid spheres has been studied by dynamical simulation; computational techniques are described and numerical results are reported. It has been found that there is a slow relaxation of the suspension microstructure during sedimentation, so that compared with the initial equilibrium distribution, there is an increased number of pairs of particles near contact; this leads to a 5%–10% increase in the average sedimentation velocity. Individual particle velocities fluctuate about the mean fall speed; these fluctuations are large and persist for long times. The resulting hydrodynamically induced dispersion of the particles can be characterized by strongly anisotropic diffusion coefficients; however, the dispersion process is non-Fickian at high solids concentrations.
Turbulence amplification by a shock wave and rapid distortion theory
Authors: Jacquin L. , Cambon C. , Blin E.
Published: July 2002

Amplification of turbulent kinetic energy in an axial compression is examined in the frame of homogeneous rapid distortion theory (RDT) by using the Craya–Herring formalism. By separating the turbulent field into solenoidal and dilatational modes (Helmholtz decomposition), one can show the dilatational mode is mediated by the parameter Δm0=D0/a0k0, which corresponds to the initial ratio between the acoustic time scale (a0k0)−1 and the compression time scale D0−1, with D0 the compression rate. It is shown here that amplification of total kinetic energy is then limited by two analytical solutions obtained for Δm0=0 (purely solenoidal-acoustical regime) and for Δm0≫1 (‘‘pressure released’’ regime), respectively. The results of the theory are first compared to results of direct numerical simulations (DNS) on homogeneous axial compression. The applicability of this homogeneous approach to the shock wave turbulence interaction, is then discussed. Considering a shock-induced compression at given Mach number, it is shown that the corresponding amplification factors predicted by homogeneous RDT largely differs from that obtained from Ribner’s linear interaction analysis and DNS on the shock problem.
Thermocapillary convection in a multilayer system
Authors: Géoris Ph. , Hennenberg M. , Simanovskii I. B. , Nepomniaschy A. , Wertgeim I. I. , Legros J. C.
Published: July 2002

The Marangoni–Bénard instability for a symmetrical three-layer system is examined theoretically. Linear stability analysis and nonlinear numerical simulations show that the ratio of the heat diffusivities determines the nature of the instability. Monotonic disturbances exist only when this parameter is far enough from one, the motion being driven by one interface. When the heat diffusivity ratio is close to one, oscillatory convection is observed. This is explained on a physical base: the oscillation rests on the coupling of both interfaces, which creates a flip–flop mechanism leading to a double inversion of the vortices rotation during one period of oscillation.
Pressure and potential fluctuation measurements in the interior of a tokamak plasma
Authors: Saadatmand K. , Crowley T. P. , Hickok R. L.
Published: July 2002

Fluctuations in electric potential, electron pressure, and their relative phase have been measured in the RENTOR tokamak [Phys. Rev. Lett. 55, 2417 (1985)] using a heavy ion beam probe. Between 70 and 200 kHz, pressure fluctuation levels of p̃/p∼0.04 and potential fluctuation levels of eφ̃/Te=0.08–3 have been observed. The phase angle between the pressure and potential fluctuations is approximately 25°. The heat transport by the observed fluctuations is not enough to account for the observed heat loss at the plasma edge. The RENTOR results are compared with measurements on the TEXT (J. Forster, Ph.D. thesis, Rensselaer Polytechnic Institute, 1987) and ISX-B [Phys. Rev. Lett. 59, 1301 (1987)] tokamaks and with drift wave and rippling mode theories.
Exact current to a spherical electrode in a collisionless, large-Debye-length magnetoplasma
Authors: Sonmor L. J. , Laframboise J. G.
Published: July 2002

Exact calculations of the steady-state current drawn from a collisionless, Maxwellian plasma in a uniform magnetic field by a spherical, perfectly absorbing electrode are presented for a range of dimensionless electrode potentials and magnetic-field strengths. These calculations are valid in the limit of large Debye length. The results are compared with the theory of Rubinstein and Laframboise, which gives upper and lower bounds for both the attracted-species and the repelled-species current. It is found that as the electrode potential increases from space potential with magnetic-field strength fixed, the electron (i.e., attracted-species) current decreases, but not as quickly as the adiabatic-limit (effectively lower-bound) current. The ion current also diverges immediately from the adiabatic-limit current. As the electrode potential increases further, the electron current rises and moves monotonically toward the canonical upper bound, which is the warm-plasma generalization of the well-known Parker and Murphy upper bound. It is unclear whether the current approaches the upper bound asymptotically as the electrode potential becomes large, or instead a constant proportion of the upper bound which varies with magnetic-field strength. The dependence on magnetic-field strength is more complicated. As expected for small fixed electrode potentials, the attracted-species current approaches the adiabatic-limit current monotonically as the magnetic-field strength increases. However, for large electrode potentials this pattern reverses: the current approaches the canonical upper bound monotonically as the magnetic-field strength increases. These patterns are expected to persist when the Debye length is finite. Interpretation of these results leads to an inference that for large electrode potentials, the effect of decreasing the Debye length may be to reduce the current, as in the nonmagnetic case.
Implosion of a two-layer shell driven by a shaped pressure pulse
Authors: Piriz A. R. , Wouchuk J. G.
Published: July 2002

An analytical model for the implosion of a two-layer spherical shell target driven by a two-step pressure pulse is developed. The process of formation of the central spark is approximately described and the effect of the prepulse on the final stage of fuel and pusher is studied. The size of the central spark is determined by the thermal conduction, and scaling laws relating the temperature and density of the spark with the parameters of the fuel and the pulse are found. These scaling laws can be useful in the design of high-gain targets for inertial confinement fusion.
Stochastic forcing of magnetized particles: A kinematic mechanism for particle energization
Authors: Newman Alice L. , Newman William I.
Published: July 2002

Charged particles in a uniform magnetized field respond both to Larmor and electric forces. The kinematic behavior for a smoothly varying electric force is simply determined and may generally be described as having gyrational motion superimposed upon a continuously evolving ‘‘guiding center’’ or drift motion. When the electric force is stochastic (i.e., the time scale over which the electric field changes is much shorter than the gyroperiod) but has stationary statistical properties, the kinematics are a variant of an Ornstein–Uhlenbeck process and the asymptotic behavior of the particle motion and the resulting Fokker–Planck solution can be exactly obtained. The random walk character of the particle trajectories results in drift velocities whose rms value is preserved while the rms gyration speed increases linearly with time, providing a relatively efficient mechanism for introducing thermal energy into an ensemble of particles. The stochastic behavior, therefore, is demonstrably nonadiabatic, and there are no ‘‘adiabatic invariants’’ describing the kinematics. An illustrative example where the electric force varies according to discontinuous instantaneous jumps with random orientation at random times is considered. This example represents a potential energization mechanism for space and laboratory plasmas.
Measurements of long-wavelength density fluctuations in TFTR
Authors: Paul S. F. , Bretz N. , Durst R. D. , Fonck R. J. , Kim Y. J. , Mazzucato E. , Nazikian R.
Published: July 2002

Several experiments have been devised to measure plasma fluctuations in an effort to help elucidate a possible connection between plasma microturbulence and anomalous transport. Results from microwave scattering on the Tokamak Fusion Test Reactor (TFTR) [Nucl. Fusion 18, 1089 (1978)] show that the level of plasma fluctuations increases toward the long-wavelength region (k⊥ρs≤0.2), at which point the fluctuations cannot be spatially resolved. The desire to measure long-wavelength fluctuations has motivated the development of two fluctuation diagnostics, beam emission spectroscopy (BES), and microwave reflectometry on TFTR. BES measures long-wavelength density fluctuations (k⊥≤2 cm−1) by observing the fluorescence emitted from collisionally excited atoms in a TFTR heating beam. In L-mode discharges with relatively flat density profiles, the fluctuations measured with BES are concentrated in the low-frequency region (≤30 kHz). In the laboratory frame, the fluctuations have a poloidal propagation velocity that is approximately equal to that of the plasma rotation, and the frequency spectra are broadened by Doppler effects. Measured fluctuation levels are 5%–10% at the edge of the plasma. In the core, the level falls to less than 1%, which is comparable to observations made with microwave reflectometry and scattering. The fluctuation level in the core (r/a=0.7) is seen to increase with injected beam power, as is observed with microwave scattering at shorter wavelengths (k⊥≊2–10 cm−1). In contrast, the fluctuation level in the edge region does not change significantly with neutral beam power.
Fluid model treatment of surface plasma structures
Authors: Montierth L. M. , Neuman W. A. , Morse R. L.
Published: July 2002

A fluid model is presented for the purpose of calculating numerically the structures of surface plasmas with neutrals returning from the surface, in collision-dominated parameter regimes. Limiting corrections to thermal conduction and viscous pressure are obtained through comparisons with previous Fokker–Planck transport calculations. The model includes removal by pumping, as well as by ionization, of some of the returning neutrals, and solutions are obtained for different relative strengths of pumping. Increasing velocities of plasma flow toward the surface and increasing plasma temperatures near the surface are seen with increased pumping. In the asymptotic region, far from the surface, agreement is found between these families of numerical model solutions and two classes of analytic solutions. Applications to other fundamental and applied problems are discussed.
An optimization of beta in the DIII-D tokamak
Authors: Lazarus E. A. , Lao L. L. , Osborne T. H. , Taylor T. S. , Turnbull A. D. , Chu M. S. , Kellman A. G. , Strait E. J. , Ferron J. R. , Groebner R. J. , Heidbrink W. W. , Carlstrom T. , Helton F. J. , Hsieh C. L. , Lippmann S. , Schissel D. , Snider R. , Wroblewski D.
Published: July 2002

Accurate equilibrium reconstruction and detailed stability analysis of a strongly shaped, double-null, βT=11% discharge shows that the plasma core is in the second stable regime to ideal ballooning modes. The equilibrium reconstruction using all the available data (coil currents, poloidal magnetic loops, motional Stark effect data, the kinetic pressure profile, the magnetic axis location, and the location of the two q=1 surfaces) shows a region of negative magnetic shear near the magnetic axis, an outer positive shear region, and a low shear region connecting the two. The inner negative shear region allows a large positive shear region near the boundary, even at low q (q95=2.6), permitting a large outer region pressure gradient to be first regime stable. The inner region is in the second stable regime, consistent with the observed axial beta [βT(0)=44%]. In the low shear region p′ vanishes, consistent with Mercier stability. This is one way to extend the ballooning limit in shaped plasmas while maintaining stability against external kinks. The n=1 analysis shows that the plasma is unstable to an ideal internal mode, consistent with the experimental observations of a saturated internal m/n=1/1 mode. The core plasma pressure, not being limited by ballooning stability, appears to be reaching a local equilibrium limit at the magnetic axis.
An experimental shock wave study of aerosol droplet evaporation in the transition regime
Authors: Roth Paul , Fischer Rüdiger
Published: July 2002

The evaporation rate of submicron aerosol droplets suspended in argon was studied behind incident shock waves at initial droplet Knudsen numbers 0.1≤KnG≤0.9. In this case the Knudsen number is formulated with the mean free path of the surrounding carrier gas and the droplet radius. The reduction in the size of the particles in the post-shock relaxation zone was measured by laser light scattering, simultaneously recorded under four fixed angles. Mie theory was applied to determine the time-dependent droplet size during the evaporation process. By comparison with an analytical description of the transfer process of a single droplet with the surrounding gas, a physical interpretation of the measurements could be established. The experimental results are consistent with analytical first-order correction terms of the mass flux in the transition regime.
Dissipative trapped particle modes in tandem mirrors
Authors: Berk H. L. , Rosenbluth M. N. , Cohen R. H. , Nevins W. M.
Published: July 2002

Dissipative trapped particle modes are studied in tandem mirrors by including electron collisions and ion Landau damping. A variational approach is used to obtain a collisional response in terms of the collisionless result plus a collisional term. The collisional term is then self-consistently solved in all collision frequency regimes. When ν/ω≪1 (ν≡electron collision frequency) and the dissipationless mode is stable through a positive (negative) charge uncovering mechanism, there are two stable waves with phase velocities in the ion (electron) diamagnetic direction. The higher frequency wave with ω∼ω*i(ω*e) is destabilized by ion (electron) dissipation while the lower frequency wave is destabilized by electron (ion) dissipation. At high collision frequency (ν>ω*e), the only unstable trapped particle wave has ω∼ω*e, with electron collisions being destabilizing.
Temperature field induced around a sphere in a uniform flow of a rarefied gas
Authors: Aoki Kazuo , Sone Yoshio
Published: July 2002

The temperature variation induced around a sphere with a uniform temperature in a slow uniform flow of a rarefied gas with the same temperature as the sphere is analyzed on the basis of the linearized Boltzmann–Krook–Welander equation with diffuse or specular reflection as the boundary condition. The analytic expressions of the temperature field are obtained for small and large Knudsen numbers.
Nonlinear analysis of instability modes in the Taylor–Dean system
Authors: Laure Patrice , Mutabazi Innocent
Published: July 2002

The linear and weakly nonlinear stability of flow in the Taylor–Dean system is investigated. The base flow far from the boundaries, is a superposition of circular Couette and curved channel Poiseuille flows. The computations provide for a finite gap system, critical values of Taylor numbers, wave numbers and wave speeds for the primary transitions. Moreover, comparisons are made with results obtained in the small gap approximation. It is shown that the occurrence of oscillatory nonaxisymmetric modes depends on the ‘‘anisotropy’’ coefficient in the dispersion relation, and that the critical Taylor number changes slightly with the azimuthal wave number for large absolute values of rotation ratio. The weakly nonlinear analysis is made in the framework of the Ginzburg–Landau equations for anisotropic systems. The primary bifurcation towards stationary or traveling rolls is supercritical when Poiseuille component of the base flow is produced by a partial filling. An external pumping can induce a subcritical bifurcation for a finite range of rotation ratio. Special attention is also given to the influence of anisotropy properties on the phase dynamics of bifurcated solution (Eckhaus and Benjamin–Feir conditions).
The unexpected response of near-critical fluids to low-frequency vibrations
Authors: Carlès P. , Zappoli B.
Published: July 2002

The response of a slab-shaped container filled with a near-critical fluid and subjected to mechanical vibrations is studied by means of matched asymptotic expansions applied to the Navier–Stokes equations. The different characteristic regimes of the fluid’s vibration are explored and described. In particular, a specific low-frequency regime is highlighted, in which the mechanical response couples with thermo-acoustic convection (piston effect). In this particular regime, the bulk part of the fluid behaves almost like a solid which bounces back and forth between two highly compressible thermal boundary layers. Such a response is entirely specific to near-critical fluids and is never witnessed in perfect gases.
Partition functions and equilibrium measures in two-dimensional and quasi-three-dimensional turbulence
Authors: Chorin Alexandre J.
Published: July 2002

An attempt is made to construct numerically equilibrium measures for the Euler equations by first examining measures for discretized approximate systems and then searching on the computer for the limit of vanishing discretization. First the partition function is evaluated for two-dimensional discretized incompressible fields with a hydrodynamical energy function and an infinite number of invariants; the behavior of the partition functions is examined as the discretization is refined. The results are contrasted with those of recent mean-field theories, which are seen to be reasonable approximations only at moderate temperatures. The two-dimensional vortex system has no phase transitions and no states invariant under refinement of the discretization, except at zero temperature. Finite-temperature equilibrium measures may appear if a simple representation of vortex stretching is added to the system, in agreement with recent work on three-dimensional turbulence, where these equilibrium measures are used as key building blocks.
On the instability of pipe Poiseuille flow
Authors: Zikanov Oleg Yu.
Published: July 2002

Stability of the flow of incompressible viscous fluid in a circular pipe is studied numerically. A perturbation consisting of finite-amplitude two-dimensional and infinitesimal three-dimensional parts is imposed on the basic flow. The temporal evolution of the perturbation is analyzed by direct numerical calculation of the Navier–Stokes equations. The two-dimensional disturbances are independent of the streamwise coordinate and initially take the form of streamwise rolls. It is shown that the nonlinear development of two-dimensional perturbations results in substantial spanwise modulation of the streamwise velocity component manifesting itself as a formation of streaks and the occurrence of inflection points. The modulated mean flow is found to be highly unstable to the three-dimensional perturbations which are localized spatially near these points. An instability mechanism that includes the modulation of the flow by growing two-dimensional disturbances and the inflectional instability of the modulated flow to three-dimensional perturbations is proposed.
Probability density function in the log-law region of low Reynolds number turbulent boundary layer
Authors: Tsuji Yoshiyuki , Nakamura Ikuo
Published: July 2002

The logarithmic velocity region is considered in zero-pressure-gradient turbulent boundary layers. We propose a new definition of the log-law region using the probability profiles of streamwise velocity fluctuation. The log-law profile is extracted readily from the experimental data as well as from the probability density function (pdf) equation. The measure called Kullback Leibler divergence is applied for distinguishing the probability profiles. If the logarithmic profile, U+=A⋅log y++B, is a good representation of experimental data, our results show that A is independent of the Reynolds number while B depends on it. The ratio of boundary layer thickness δ to the upper end of log-law extent, δL, is not constant but approaches the value δL/δ=0.2 as the Reynolds number increases.
Numerical simulation of the Rayleigh–Bénard convection of air in a box of a large aspect ratio
Authors: Tomita Hirofumi , Abe Kanji
Published: July 2002

The Rayleigh–Bénard convection in a box of a large aspect ratio is investigated using direct numerical simulation. The range of the Rayleigh number (Ra) is 2500⩽Ra⩽10000. When Ra≲6000, nearly straight roll convection is observed, and the average roll diameter increases linearly with Ra. When Ra≳6000, traveling waves are superimposed on the convection rolls. The gradient of increase in the average roll diameter for Ra≳6000 is smaller than that for Ra≲6000. The average curvature of the roll axis increases rapidly when Ra≳6000.
Spatio-temporal development of the long and short-wave vortex-pair instabilities
Authors: Fabre David , Cossu Carlo , Jacquin Laurent
Published: July 2002

We consider the spatio-temporal development of the long-wave and short-wave instabilities in a pair of counter-rotating vortices in the presence of a uniform axial advection velocity. The stability properties depend upon the aspect ratio a/b of the vortex pair, where a is the core radius of the vortices and b their separation, and upon W0/U0 the ratio between the self-induced velocity of the pair and the axial advection velocity. For sufficiently small W0/U0, the instabilities are convective, but an increase of W0/U0 may lead to an absolute instability. Near the absolute instability threshold, spatial growth rates are larger than those predicted by temporal stability theory. Considering aeronautical applications, it is shown that instabilities of the type considered in this communication cannot become absolute in farfield wakes of high aspect ratio wings.
An experimental study of density waves in a hypersonic shock layer on a flat plate
Authors: Mironov S. G. , Maslov A. A.
Published: July 2002

The characteristics of density waves in a shock layer on a flat plate have been measured by the method of electron beam fluorescence of nitrogen for the free-stream Mach number M∞=21 and the local Reynolds number Rex=2×104–3×105. The measurements have been performed for natural perturbations. The data on spatial distribution of fluctuation spectra and phase velocities of the waves in the longitudinal and transverse directions have been obtained. The propagation angles and the increments of density waves have been determined. The characteristics of fluctuation coherence have been measured. The experimental equipment, techniques of measurements, and methods for the reconstruction of the mean density and density fluctuations are described in the paper. The limitations imposed upon the measurement method are discussed.
Decay of velocity fluctuations in a stably stratified suspension
Authors: Luke Jonathan H. C.
Published: July 2002

The evolution of velocity fluctuations in a stably stratified suspension settling under the influence of gravity is studied. Based on a continuum model for sedimentation dynamics, decay of long-wave fluctuations in the particle density is found. Substantial reduction in velocity fluctuations is an important consequence. Specifically, the evolution of the particle density in a stably stratified suspension immediately leads to particle distributions, where the variance in the velocity is bounded as the container size grows to infinity. This result is a possible explanation for why velocity fluctuations have been observed to depend on container size in numerical simulations but generally not in physical experiments.
Plasma kinetic effects in stochastic helical field
Authors: Chechkin A. V. , Yanovsky V. V. , Tur A. V.
Published: July 2002

The kinetic equation for a plasma in a stochastic electromagnetic helical field is constructed and studied. The kinetic equation contains an additional term, proportional to the helicity. The evolution of the main moments of the distribution function is considered. A solution describing the evolution of the distribution function for arbitrary initial conditions is found and possible consequences of presence of the helicity term are discussed. A model example is considered when a single-momentum electron beam with an inhomogeneous velocity profile creates a new particle flow in the transverse direction, so that the resulting flow also possesses the helicity.
Stimulated Raman scattering in inhomogeneous collisional laser-produced plasmas
Authors: Barr H. C. , Boyd T. J. M. , Mackwood A. P.
Published: July 2002

Convective gain and absolute growth of stimulated Raman scattering (SRS) are determined for a hot inhomogeneous collisional plasma using an integrated global model that includes the effects of wave propagation and both collisional and Landau damping in a density ramp. This unifies the time asymptotic theory of SRS occurring near the quarter-critical density of the laser light and that occurring at lower densities. The level of collisions has a striking effect on the convective behavior of the instability. When collisions are weak, no significant regime of convective gain exists. In consequence, absolute resonances are very sensitive and susceptible to detuning, particularly so near the quarter-critical density. This is shown to be a key factor in understanding the Raman ‘‘gap’’ seen in experiments. No such gap appears in highly collisional regimes in which the absolute resonances are insensitive and significant convective gain occurs over the whole range of wavelengths.
Critical neutral density for high-mode bifurcation in tokamaks
Authors: Shaing K. C. , Hsu C. T.
Published: July 2002

The effects of a neutral population are included in the high-mode (H-mode) bifurcation theory [Phys. Rev. Lett. 63, 2369 (1989)]. It is shown that, for a given set of parameters, there exist critical values of neutral density above which H-mode bifurcation cannot occur. However, if the neutral density is fixed, for a given ion collisionality, this implies the existence of a critical ion temperature below which H-mode bifurcation cannot occur. These critical values can be tested experimentally.
Theoretical transport modeling of Ohmic cold pulse experiments
Authors: Kinsey J. E. , Waltz R. E. , St. John H. E.
Published: July 2002

The response of several theory-based transport models in Ohmically heated tokamak discharges to rapid edge cooling due to trace impurity injection is studied. Results are presented for the Institute for Fusion Studies—Princeton Plasma Physics Laboratory (IFS/PPPL), gyro-Landau-fluid (GLF23), Multi-mode (MM), and the Itoh–Itoh–Fukuyama (IIF) transport models with an emphasis on results from the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Technol./Fusion 1, 479 (1981)]. It is found that critical gradient models containing a strong ion and electron temperature ratio dependence can exhibit behavior that is qualitatively consistent with experimental observation while depending solely on local parameters. The IFS/PPPL model yields the strongest response and demonstrates both rapid radial pulse propagation and a noticeable increase in the central electron temperature following a cold edge temperature pulse (amplitude reversal). Furthermore, the amplitude reversal effect is predicted to diminish with increasing electron density and auxiliary heating in agreement with experimental data. An Ohmic pulse heating effect due to rearrangement of the current profile is shown to contribute to the rise in the core electron temperature in TEXT, but not in the Joint European Tokamak (JET) [A. Tanga and the JET Team, in Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 65] and the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk, V. Arunsalam, M. G. Bell et al., in Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 51]. While this phenomenon is not necessarily a unique signature of a critical gradient, there is sufficient evidence suggesting that the apparent plasma response to edge cooling may not require any underlying nonlocal mechanism and may be explained within the context of the intrinsic properties of electrostatic drift wave-based models.
Surface sensitive Mössbauer spectroscopy by the combination of total external reflection and conversion electron detection
Authors: Frost J. C. , Cowie B. C. C. , Chapman S. N. , Marshall J. F.
Published: July 2002

The combination of a grazing incidence total external reflection geometry with conversion electron detection is shown to produce surface sensitive Mössbauer spectra, which are selective for the first few nanometers of the solid. This approach is a practical and simply implemented method of obtaining the Mössbauer spectra of surfaces.
Staggered band alignments in AlGaAs heterojunctions and the determination of valence-band offsets
Authors: Dawson P. , Wilson B. A. , Tu C. W. , Miller R. C.
Published: July 2002

Photoluminescence spectra from Al0.37Ga0.63As/AlAs multiple quantum well structures with staggered band alignments are presented which provide the first direct optical measure of the valence-band offset at a semiconductor heterojunction. The experiment takes advantage of the crossover occurring at a critical aluminum concentration above which the indirect X minima in the AlAs become the lowest energy conduction bands in the system, and recombination occurs across the interface. The resulting emission fixes the valence-band offset at ΔEv =342±4 meV for this structure.
Structure of lattice-strained InxGa1−xAs/GaAs layers studied by transmission electron microscopy
Authors: Yao J. Y. , Andersson T. G. , Dunlop G. L.
Published: July 2002

The defect structures of strained InxGa1−xAs/GaAs quantum wells with 0.28≤x≤1.00, which were grown by molecular beam epitaxy on GaAs (001), have been investigated by transmission electron microscopy. It was found that with increasing thicknesses of the layers a transition occurred from homogeneous lattice strain to a ‘‘periodic’’ inhomogeneous lattice strain. This effect was more pronounced with increasing indium content. For thicker layers, misfit dislocations were created which existed together with the periodic inhomogeneous strain. For even thicker layers, dislocation complexes arose which primarily consisted of intersecting stacking faults.
Truncation of the class II beta-chain cytoplasmic domain influences the level of class II/invariant chain-derived peptide complexes.
Authors: Smiley S T , Rudensky A Y , Glimcher L H , Grusby M J
Published: July 2002

Previous studies have established that antigen presenting cells (APC) expressing major histocompatibility complex class II beta chains with truncated cytoplasmic domains are impaired in their capacity to activate T cells. While it had been widely accepted that this impairment is due to a defect in class II cytoplasmic domain-dependent signal transduction, we recently generated transgenic mice expressing only truncated class II beta chains, and functional analyses of APC from these mice revealed signaling-independent defects in antigen presentation. Here, we demonstrate that T cells primed on such transgenic APC respond better to stimulation by APC expressing truncated beta chains than by wild-type APC. This finding suggests that APC expressing truncated class II beta chains are not inherently defective in their antigen presenting capacity but, rather, may differ from wild-type APC in the peptide antigens that they present. Indeed, analysis of the peptides bound to class II molecules isolated from normal and transgenic spleen cells revealed clear differences. Most notably, the level of class II-associated invariant chain-derived peptides (CLIP) is significantly reduced in cells expressing only truncated beta chains. Prior studies have established that CLIP and antigenic peptides compete for binding to class II molecules. Thus, our results suggest that the cytoplasmic domain of the class II beta chain affects antigen presentation by influencing the level of CLIP/class II complexes.
Binding of the sigma 70 protein to the core subunits of Escherichia coli RNA polymerase, studied by iron-EDTA protein footprinting.
Authors: Greiner D P , Hughes K A , Gunasekera A H , Meares C F
Published: July 2002

We have used a nonspecific protein cleaving reagent to map the interactions between subunits of the multisubunit enzyme RNA polymerase (Escherichia coli). We developed suitable conditions for using an untethered Fe-EDTA reagent, which does not bind significantly to proteins. Comparison of the cleaved fragments of the subunits from the core enzyme (alpha 2 beta beta') and the holoenzyme (core+sigma 70) shows that absence of the sigma 70 subunit is associated with the appearance of several cleavage sites on the subunits beta (within 10 residues of sequence positions 745, 764, 795, and 812) and beta' (within 10 residues of sequence positions 581, 613, and 728). A cleavage site near beta residue 604 is present in the holoenzyme but absent in the core, demonstrating that a conformational change occurs when sigma 70 binds. No differences are observed for the alpha subunit.
A novel method for simultaneous high resolution identification of HLA-A, HLA-B, and HLA-Cw alleles.
Authors: Arguello R , Avakian H , Goldman J M , Madrigal J A
Published: July 2002

We describe a novel high resolution DNA based typing approach for HLA class I alleles, which identifies the recombinational motifs present in exons 2 and 3 of the HLA class I genes. Unique identification patterns for 201 known HLA-A, HLA-B, and HLA-Cw alleles were generated by the use of only 40 probes, which were targeted at these common motifs. The unambiguous identification of the alleles was achieved by the development of a new and powerful allelic separation technique that allows isolation of single alleles after amplification. To validate the method, we have used locus-specific primers to amplify exons 2 and 3 of HLA-A, HLA-B, and HLA-Cw loci from 22 heterozygous and 41 homozygous cell lines. After amplification, the allelic fragments from each locus were separated, blotted, and hybridized with the 40 probes. In all cases, the allelic products could be separated and 81 different class I alleles, 33 HLA-A, 30 HLA-B, and 18 HLA-Cw, were identified according to the predicted probe hybridization patterns.
Fas (CD95) expression and death-mediating function are induced by CD4 cross-linking on CD4+ T cells.
Authors: Desbarats J , Freed J H , Campbell P A , Newell M K
Published: July 2002

The CD4 receptor contributes to T-cell activation by coligating major histocompatibility complex class II on antigen presenting cells with the T-cell receptor (TCR)/CD3 complex, and triggering a cascade of signaling events including tyrosine phosphorylation of intracellular proteins. Paradoxically, CD3 cross-linking prior to TCR stimulation results in apoptotic cell death, as does injection of anti-CD4 antibodies in vivo of CD4 ligation by HIV glycoprotein (gp) 120. In this report we investigate the mechanism by which CD4 cross-linking induces cell death. We have found that CD4 cross-linking results in a small but rapid increase in levels of cell surface Fas, a member of the tumor necrosis factor receptor family implicated in apoptotic death and maintenance of immune homeostasis. Importantly, CD4 cross-linking triggered the ability of Fas to function as a death molecule. Subsequent to CD4 cross-linking, CD4+ splenocytes cultured overnight became sensitive to Fas-mediated death. Death was Fas-dependent, as demonstrated by cell survival in the absence of plate-bound anti-Fas antibody, and by the lack of CD4-induced death in cells from Fas-defective lymphoproliferative (lpr) mice. We demonstrate here that CD4 regulates the ability of Fas to induce cell death in Cd4+ T cells.
Observation via one-dimensional 13Calpha NMR of local conformational substates in thermal unfolding equilibria of a synthetic analog of the GCN4 leucine zipper.
Authors: Lovett E G , D'Avignon D A , Holtzer M E , Braswell E H , Zhu D , Holtzer A
Published: July 2002

Synthesis of a 33-residue, capped leucine zipper analogous to that in GCN4 is reported. Histidine and arginine residues are mutated to lysine to reduce the unfolding temperature. CD and ultracentrifugation studies indicate that the molecule is a two-stranded coiled coil under benign conditions. Versions of the same peptide are made with 99% 13Calpha at selected sites. One-dimensional 13C NMR spectra are assigned by inspection and used to study thermal unfolding equilibria over the entire transition from 8 to 73 degrees C. Spectra at the temperature extremes establish the approximate chemical shifts for folded and unfolded forms at each labeled site. Resonances for each amino acid appear at both locations at intermediate T, indicating that folded and unfolded forms interconvert slowly (> >2 ms) on the NMR time scale. Moreover, near room temperature, the structured form's resonance is double at several, but not all, sites, indicating at least two slowly interconverting, structured, local conformational substates. Analysis of the dynamics is possible. For example, near room temperature at the Val-9, Ala-24, and Gly-31 positions, the equilibrium constant for interconversion of the two structured forms is near unity and the time scale is > or= 10-20 ms.
A polygenic mouse model of psoriasiform skin disease in CD18-deficient mice.
Authors: Bullard D C , Scharffetter-Kochanek K , McArthur M J , Chosay J G , McBride M E , Montgomery C A , Beaudet A L
Published: July 2002

Previously, a hypomorphic mutation in CD18 was generated by gene targeting, with homozygous mice displaying increased circulating neutrophil counts, defects in the response to chemically induced peritonitis, and delays in transplantation rejection. When this mutation was backcrossed onto the PL/J inbred strain, virtually all homozygous mice developed a chronic inflammatory skin disease with a mean age of onset of 11 weeks after birth. The disease was characterized by erythema, hair loss, and the development of scales and crusts. The histopathology revealed hyperplasia of the epidermis, subcorneal microabscesses, orthohyperkeratosis, parakeratosis, and lymphocyte exocytosis, which are features in common with human psoriasis and other hyperproliferative inflammatory skin disorders. Repetitive cultures failed to demonstrate bacterial or fungal organisms potentially involved in the pathogenesis of this disease, and the dermatitis resolved rapidly after subcutaneous administration of dexamethasone. Homozygous mutant mice on a (PL/J x C57BL/6J)F1 background did not develop the disease and backcross experiments suggest that a small number of genes (perhaps as few as one), in addition to CD18, determine susceptibility to the disorder. This phenotype provides a model for inflammatory skin disorders, may have general relevance to polygenic human inflammatory diseases, and should help to identify genes that interact with the beta2 integrins in inflammatory processes.
Low-calcium-induced enhancement of chemical synaptic transmission from photoreceptors to horizontal cells in the vertebrate retina.
Authors: Piccolino M , Byzov A L , Kurennyi D E , Pignatelli A , Sappia F , Wilkinson M , Barnes S
Published: July 2002

According to the classical calcium hypothesis of synaptic transmission, the release of neurotransmitter from presynaptic terminals occurs through an exocytotic process triggered by depolarization-induced presynaptic calcium influx. However, evidence has been accumulating in the last two decades indicating that, in many preparations, synaptic transmitter release can persist or even increase when calcium is omitted from the perfusing saline, leading to the notion of a "calcium-independent release" mechanism. Our study shows that the enhancement of synaptic transmission between photoreceptors and horizontal cells of the vertebrate retina induced by low-calcium media is caused by an increase of calcium influx into presynaptic terminals. This paradoxical effect is accounted for by modifications of surface potential on the photoreceptor membrane. Since lowering extracellular calcium concentration may likewise enhance calcium influx into other nerve cells, other experimental observations of "calcium-independent" release may be reaccommodated within the framework of the classical calcium hypothesis without invoking unconventional processes.