bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of imaging plates at near saturation for high energy density particles
Authors: Tanimoto Tsuyoshi , Ohta Kazuhide , Habara Hideaki , Yabuuchi Toshinori , Kodama Ryousuke , Tampo Motonobu , Zheng Jian , Tanaka Kazuo A.
Published: November 2008

Since an imaging plate (IP) is sensitive to electron, ion, and x rays, it can be used as a detector for laser plasma experiment using ultraintense laser. Moreover, an IP has the advantageous features such as high sensitivity, wide dynamic range, and high spatial resolution. Even though IP itself has a considerable wide dynamic range up to 105, the IP data have appeared often saturated at an IP reading device. We propose a reading technique by inserting optical density filters so that an apparently saturated IP data can be saved.
The National Ignition Facility Neutron Imaging System
Authors: Wilke Mark D. , Batha Steven H. , Bradley Paul A. , Day Robert D. , Clark David D. , Fatherley Valerie E. , Finch Joshua P. , Gallegos Robert A. , Garcia Felix P. , Grim Gary P. , Jaramillo Steven A. , Montoya Andrew J. , Moran Michael J. , Morgan George L. , Oertel John A. , Ortiz Thomas A. , Payton Jeremy R. , Pazuchanics Peter , Schmidt Derek W. , Valdez Adelaida C. , Wilde Carl H. , Wilson Doug C.
Published: November 2008

The National Ignition Facility (NIF) is scheduled to begin deuterium-tritium (DT) shots possibly in the next several years. One of the important diagnostics in understanding capsule behavior and to guide changes in Hohlraum illumination, capsule design, and geometry will be neutron imaging of both the primary 14 MeV neutrons and the lower-energy downscattered neutrons in the 6–13 MeV range. The neutron imaging system (NIS) described here, which we are currently building for use on NIF, uses a precisely aligned set of apertures near the target to form the neutron images on a segmented scintillator. The images are recorded on a gated, intensified charge coupled device. Although the aperture set may be as close as 20 cm to the target, the imaging camera system will be located at a distance of 28 m from the target. At 28 m the camera system is outside the NIF building. Because of the distance and shielding, the imager will be able to obtain images with little background noise. The imager will be capable of imaging downscattered neutrons from failed capsules with yields Yn>1014 neutrons. The shielding will also permit the NIS to function at neutron yields >1018, which is in contrast to most other diagnostics that may not work at high neutron yields. The following describes the current NIF NIS design and compares the predicted performance with the NIF specifications that must be satisfied to generate images that can be interpreted to understand results of a particular shot. The current design, including the aperture, scintillator, camera system, and reconstruction methods, is briefly described. System modeling of the existing Omega NIS and comparison with the Omega data that guided the NIF design based on our Omega results is described. We will show NIS model calculations of the expected NIF images based on component evaluations at Omega. We will also compare the calculated NIF input images with those unfolded from the NIS images generated from our NIS numerical modeling code.
Development of imaging bolometers for magnetic fusion reactors (invited)
Authors: Peterson Byron J. , Parchamy Homaira , Ashikawa Naoko , Kawashima Hisato , Konoshima Shigeru , Kostryukov Artem Yu. , Miroshnikov Igor V. , Seo Dongcheol , Omori T.
Published: November 2008

Imaging bolometers utilize an infrared (IR) video camera to measure the change in temperature of a thin foil exposed to the plasma radiation, thereby avoiding the risks of conventional resistive bolometers related to electric cabling and vacuum feedthroughs in a reactor environment. A prototype of the IR imaging video bolometer (IRVB) has been installed and operated on the JT-60U tokamak demonstrating its applicability to a reactor environment and its ability to provide two-dimensional measurements of the radiation emissivity in a poloidal cross section. In this paper we review this development and present the first results of an upgraded version of this IRVB on JT-60U. This upgrade utilizes a state-of-the-art IR camera (FLIR/Indigo Phoenix-InSb) (3–5 μm, 256×360 pixels, 345 Hz, 11 mK) mounted in a neutron/gamma/magnetic shield behind a 3.6 m IR periscope consisting of CaF2 optics and an aluminum mirror. The IRVB foil is 7 cm×9 cm×5 μm tantalum. A noise equivalent power density of 300 μW/cm2 is achieved with 40×24 channels and a time response of 10 ms or 23 μW/cm2 for 16×12 channels and a time response of 33 ms, which is 30 times better than the previous version of the IRVB on JT-60U.
In-vessel design of ITER diagnostic neutron activation system
Authors: Cheon M. S. , Pak S. , Lee H. G. , Bertalot L. , Walker C.
Published: November 2008

A neutron activation system utilizing an encapsulated metal sample technique will be implemented to evaluate total fusion power from ITER plasma. The positions of seven irradiation ends for each poloidal section are determined to optimize the measurements from asymmetric, elongated plasma with minimum uncertainty. To determine the exact location of the irradiation end, some physical and technical aspects for each location are investigated. Monte Carlo neutronic calculations are performed to assess the physical characteristics and optimize the design of each irradiation end.
Diagnostics hardening for harsh environment in Laser Mégajoule (invited)
Authors: Bourgade J. L. , Marmoret R. , Darbon S. , Rosch R. , Troussel P. , Villette B. , Glebov V. , Shmayda W. T. , Gomme J. C. , Le Tonqueze Y. , Aubard F. , Baggio J. , Bazzoli S. , Bonneau F. , Boutin J. Y. , Caillaud T. , Chollet C. , Combis P. , Disdier L. , Gazave J. , Girard S. , Gontier D. , Jaanimagi P. , Jacquet H. P. , Jadaud J. P. , Landoas O. , Legendre J. , Leray J. L. , Maroni R. , Meyerhofer D. D. , Miquel J. L. , Marshall F. J. , Masclet-Gobin I. , Pien G. , Raimbourg J. , Reverdin C. , Richard A. , Rubin de Cervens D. , Sangster C. T. , Seaux J. P. , Soullie G. , Stoeckl C. , Thfoin I. , Videau L. , Zuber C.
Published: November 2008

The diagnostic designs for the Laser Mégajoule (LMJ) will require components to operate in environments far more severe than those encountered in present facilities. This harsh environment will be induced by fluxes of neutrons, gamma rays, energetic ions, electromagnetic radiations, and, in some cases, debris and shrapnel, at levels several orders of magnitude higher than those experienced today on existing facilities. The lessons learned about the vulnerabilities of present diagnostic parts fielded mainly on OMEGA for many years, have been very useful guide for the design of future LMJ diagnostics. The present and future LMJ diagnostic designs including this vulnerability approach and their main mitigation techniques will be presented together with the main characteristics of the LMJ facility that provide for diagnostic protection.
Tenfold improved sensitivity using high refractive-index substrates for surface plasmon sensing
Authors: Micheletto Ruggero , Hamamoto Katsumi , Fujii Takashi , Kawakami Yoichi
Published: November 2008

Surface plasmon resonance sensors exploit the high sensitivity to local perturbations of plasma waves in a thin metal layer. These devices have a wide range of applications as biomedical, environmental, industrial, and homeland security. We concentrate on the theoretical aspects of the sensing principle. By calculations at various indexes of refraction we proved that using substrate material of higher index, sensitivity and dynamics range improve conspicuously. Finally, we show experimental data taken using a special transparent ceramic material of exceptionally high index of refraction n=2.04. Tests demonstrate sensitivity about one order of magnitude better than those obtained with conventional BK7 glass.
Metal modulation epitaxy growth for extremely high hole concentrations above 1019cm−3 in GaN
Authors: Namkoong Gon , Trybus Elaissa , Lee Kyung Keun , Moseley Michael , Doolittle W. Alan , Look David C.
Published: November 2008

The free hole carriers in GaN have been limited to concentrations in the low 1018cm−3 range due to the deep activation energy, lower solubility, and compensation from defects, therefore, limiting doping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to ∼10% in GaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrations grown by periodically modulating Ga atoms and Mg dopants were over ∼1.5×1019cm−3.
Modeling the pressure Hessian and viscous Laplacian in turbulence: Comparisons with direct numerical simulation and implications on velocity gradient dynamics
Authors: Chevillard L. , Meneveau C. , Biferale L. , Toschi F.
Published: November 2008

Modeling the velocity gradient tensor A=∇u along Lagrangian trajectories in turbulent flow requires closures for the pressure Hessian and viscous Laplacian of A. Based on an Eulerian–Lagrangian change in variables and the so-called recent fluid deformation closure, such models were proposed recently [Chevillard and Meneveau, Phys. Rev. Lett. 97, 174501 (2006)]. The resulting stochastic model was shown to reproduce many geometric and anomalous scaling properties of turbulence. In this work, direct comparisons between model predictions and direct numerical simulation (DNS) data are presented. First, statistical properties of A are described using conditional averages of strain skewness, enstrophy production, energy transfer, and vorticity alignments, conditioned upon invariants of the velocity gradient. These conditionally averaged quantities are found to be described accurately by the stochastic model. More detailed comparisons that focus directly on the terms being modeled in the closures are also presented. Specifically, conditional statistics associated with the pressure Hessian and the viscous Laplacian are measured from the model and are compared with DNS. Good agreement is found in strain-dominated regions. However, some features of the pressure Hessian linked to rotation-dominated regions are not reproduced accurately by the model. Geometric properties such as vorticity alignment with respect to principal axes of the pressure Hessian are mostly predicted well. In particular, the model predicts that an eigenvector of the rate of strain will be also an eigenvector of the pressure Hessian, in accord with basic properties of the Euler equations. The analysis identifies under what conditions the Eulerian–Lagrangian change in variables with the recent fluid deformation closure works well, and in which flow regimes it requires further improvements.
On streak breakdown in bypass transition
Authors: Schlatter Philipp , Brandt Luca , de Lange H. C. , Henningson Dan S.
Published: November 2008

Recent theoretical, numerical, and experimental investigations performed at the Department of Mechanics, KTH Stockholm, and the Department of Mechanical Engineering, Eindhoven University of Technology, are reviewed, and new material is presented to clarify the role of the boundary-layer streaks and their instability with respect to turbulent breakdown in bypass transition in a boundary layer subject to free-stream turbulence. The importance of the streak secondary-instability process for the generation of turbulent spots is clearly shown. The secondary instability manifests itself as a growing wave packet located on the low-speed streak, increasing in amplitude as it is dispersing in the streamwise direction. In particular, qualitative and quantitative data pertaining to temporal sinuous secondary instability of a steady streak, impulse responses both on a parallel and a spatially developing streak, a model problem of bypass transition, and full simulations and experiments of bypass transition itself are collected and compared. In all the flow cases considered, similar characteristics in terms of not only growth rates, group velocity, and wavelengths but also three-dimensional visualizations of the streak breakdown have been found. The wavelength of the instability is about an order of magnitude larger than the local boundary-layer displacement thickness δ∗, the group velocity about 0.8 of the free-stream velocity U∞, and the growth rate on the order of a few percent of U∞/δ∗. The characteristic structures at the breakdown are quasistreamwise vortices, located on the flanks of the low-speed region arranged in a staggered pattern.
Direct numerical simulation of the Ekman layer: A step in Reynolds number, and cautious support for a log law with a shifted origin
Authors: Spalart Philippe R. , Coleman Gary N. , Johnstone Roderick
Published: November 2008

Results at Ekman Reynolds numbers Re ranging from 1000 to 2828 expand the direct numerical simulation (DNS) contribution to the theory of wall-bounded turbulence. An established spectral method is used, with rules for domain size and grid resolution at each Reynolds number derived from the theory. The Re increase is made possible by better computers and by optimizing the grid in relation to the wall shear-stress direction. The boundary-layer thickness in wall units δ+ varies here by a factor of about 5.3, and reaches values near 5000, or 22 times the minimum at which turbulence has been sustained. An equivalent channel Reynolds number, based on the pressure gradient in wall units, would reach about Reτ=1250. The principal goal of the analysis, the impartial identification of a log law, is summarized in the local “Karman measure” d(ln z+)/dU+. The outcome differs from that for Hoyas and Jiménez [Phys. Fluids18, 011702 (2006)] and for Hu [AIAA J.44, 1541 (2006)] in channel-flow DNS at similar Reynolds numbers, for reasons unknown: Here, the law of the wall is gradually established up to a z+ around 400, with little statistical scatter. To leading order, it is consistent with the experiments of Österlund [Phys. Fluids12, 1 (2000)] in boundary layers. With the traditional expression, a logarithmic law is not present, in that the Karman measure drifts from about 0.41 at z+≈70 to the 0.37–0.38 range for z+≈500, with Re=2828. However, if a virtual origin is introduced with a shift of a+=7.5 wall units, the data support a long logarithmic layer with κ=0.38 a good fit to d(ln[z++a+])/dU+. A determination of the Karman constant from the variation of the skin-friction coefficients with Reynolds numbers also yields values near 0.38. The uncertainty is about ±0.01. These values are close to the boundary-layer experiments, but well below the accepted range of [0.40,0.41] and the experimental pipe-flow results near 0.42. The virtual-origin concept is also controversial, although nonessential at transportation or atmospheric Reynolds numbers. Yet, this series may reflect some success in verifying the law of the wall and investigating the logarithmic law by DNS, redundantly and with tools more impartial than the visual fit of a straight line to a velocity profile.

At the request of the authors and editor, this article is being retracted effective 28 October 2009.
Control and system identification of a separated flow
Authors: Huang Shao-Ching , Kim John
Published: November 2008

A procedure to construct linear optimal control for separated flows is presented. Unlike previous works in which a system model is derived from the linearized Navier–Stokes equations, we use an approximate linear model for the flow system generated by a system identification method based on input-output data sequences from numerical solutions of the Navier–Stokes equations. The approximate model is used in linear quadratic Gaussian synthesis to compute feedback control laws. Various properties of the identified model are tested and discussed. The closed-loop control is applied to a two-dimensional separated boundary layer, aiming at reducing its separation bubble size.
Ab initio electronic and magnetic properties of half-metallic NiCrSi and NiMnSi Heusler alloys: The role of defects and interfaces
Authors: Galanakis I. , Özdoğan K. , Şaşıoğlu E.
Published: November 2008

Using state-of-the-art first-principles calculations, we study the properties of the ferromagnetic Heusler compounds NiYSi, where Y stands for V, Cr, or Mn. NiCrSi and NiMnSi contrary to NiVSi are half-metallic at their equilibrium lattice constant exhibiting integer values of the total spin magnetic moment, and thus we concentrate on these two alloys. The minority-spin gap has the same characteristics as for the well-known NiMnSb alloy being around ∼1 eV. Upon tetragonalization, the gap is present in the density of states even for expansion or contraction of the out-of-plane lattice parameter by 5%. The Cr–Cr and Mn–Mn interactions make ferromagnetism extremely stable and the Curie temperature exceeds 1000 K for NiMnSi. Surface and interfaces with GaP, ZnS, and Si semiconductors are not half-metallic but, in the case of NiCrSi, the Ni-based contacts present spin polarization at the Fermi level around 90%. Finally, we show that there are two cases of defects and atomic-swaps. The first ones, which involve the Cr(Mn) and Si atoms, induce states at the edges of the gap, which persists for a moderate concentration of defects. Defects involving Ni atoms induce states localized within the gap completely destroying the half-metallicity. Based on single-impurity calculations, we associate these states to the symmetry of the crystal.
Theoretical study of multiferroic BiFeO3 nanoparticles
Authors: Wesselinowa J. M. , Apostolova I.
Published: November 2008

Multiferroics, which exhibit both ferroelectricity and ferromagnetism, are currently under intensive investigation. Their spontaneous polarization and saturation magnetization are very low in comparison to many standard ferroelectrics and ferromagnets. In order to explain the experimentally observed enhanced polarization and magnetization in multiferroic nanoparticles, we investigate the influence of surface and particle size on ferromagnetic and ferroelectric properties. The studies are based on two microscopic models: the modified Heisenberg and the transverse Ising model and a multiferroic coupling term. A Green’s function technique beyond the random phase approximation allows the calculation of static and dynamic properties in dependence of temperature, particle size, and different model parameters. It is demonstrated that magnetization, polarization, phase transition temperatures, spin-wave energies, and their damping are very sensitive to the exchange interaction constants on the surface and to the electromagnetic coupling constant. We obtain that the magnetization and the polarization are enhanced in BiFeO3 nanoparticles due to surface and size effects. The results compare very well with the experimental data.
Determination of the tip temperature in laser assisted atom-probe tomography using charge state distributions
Authors: Marquis Emmanuelle A. , Gault Baptiste
Published: November 2008

The method suggested by Kellogg [J. Appl. Phys. 52, 5320 (1981)] to estimate the temperature of a field emitter under laser pulsing irradiation is reconsidered in the case of a W-Re alloy. It is shown that the temperature obtained using this method is not the absolute temperature, but, if properly calibrated, a value that could be considered as a good approximation of the average temperature reached by the tip when illuminated by picosecond laser pulses.
Molecular effects on boundary condition in micro/nanoliquid flows
Authors: Ulmanella Umberto , Ho Chih-Ming
Published: November 2008

We experimentally investigated molecular effects of the slip/no-slip boundary condition of Newtonian liquids in micro- and nanochannels as small as 350 nm. The slip was measurable for channels smaller than approximately 2μm. The amount of slip is found to be independent of the channel size, but is a function of the shear rate, the type of liquid (polar or nonpolar molecular structure), and the morphology of the solid surface (molecular-level smoothness).
Stability of a channel flow subject to wall blowing and suction in the form of a traveling wave
Authors: Lee Changhoon , Min Taegee , Kim John
Published: November 2008

Inspired by the recent finding by Min et al. [J. Fluid Mech. 558, 309 (2006)], the stability of a channel flow subject to wall blowing and suction in the form of a traveling wave is investigated by combined use of the Floquet analysis, direct numerical simulation, and singular value decomposition analysis. Results show that stability highly depends on the phase speed of the traveling wave; most disturbances become highly unstable when the phase speed is around 40% of the centerline velocity, while streamwise streak-type three-dimensional disturbances become stabilized with transient growth suppressed when the phase speed exceeds the centerline velocity for both subcritical and supercritical Reynolds numbers. This destabilization is interpreted by investigation of wave interactions. An upstream-traveling wave, which reduces mean drag, does not stabilize the flow.
Turbulent dispersion from line sources in grid turbulence
Authors: Viswanathan Sharadha , Pope Stephen B.
Published: November 2008

Probability density function (PDF) calculations are reported for the dispersion from line sources in decaying grid turbulence. The calculations are performed using a modified form of the interaction by exchange with the conditional mean (IECM) mixing model. These flows pose a significant challenge to statistical models because the scalar length scale (of the initial plume) is much smaller than the turbulence integral scale. Consequently, this necessitates incorporating the effects of molecular diffusion in order to model laboratory experiments. Previously, Sawford [Flow Turb. Combust. 72, 133 (2004)] performed PDF calculations in conjunction with the IECM mixing model, modeling the effects of molecular diffusion as a random walk in physical space and using a mixing time scale empirically fit to the experimental data of Warhaft [J. Fluid Mech. 144, 363 (1984)]. The resulting transport equation for the scalar variance contains a spurious production term. In the present work, the effects of molecular diffusion are instead modeled by adding a conditional mean scalar drift term, thus avoiding the spurious production of scalar variance. A laminar wake model is used to obtain an analytic expression for the mixing time scale at small times, and this is used as part of a general specification of the mixing time scale. Based on this modeling, PDF calculations are performed, and comparison is made primarily with the experimental data of Warhaft on single and multiple line sources and with the previous calculations of Sawford. A heated mandoline is also considered with comparison to the experimental data of Warhaft and Lumley [J. Fluid Mech. 88, 659 (1978)]. This establishes the validity of the proposed model and the significant effect of molecular diffusion on the decay of scalar fluctuations. The following are the significant predictions of the model. For the line source, the effect of the source size is limited to early times and can be completely accounted for by simple transformations. The peak centerline ratio of the rms to the mean of the scalar increases with the Reynolds number (approximately as Rλ1/3), whereas this ratio tends to a constant (approximately 0.4) at large times independent of Rλ. In addition, the model yields a universal long-time decay exponent for the temperature variance.
Discrete conservation principles in large-eddy simulation with application to separation control over an airfoil
Authors: You Donghyun , Ham Frank , Moin Parviz
Published: November 2008

An unstructured-grid large-eddy simulation (LES) technique is used to investigate the turbulent flow separation over an airfoil with and without synthetic-jet control. Numerical accuracy and stability on arbitrary shaped mesh elements at high Reynolds numbers are achieved using a finite-volume discretization of the incompressible Navier–Stokes equations based on higher-order conservation principles—i.e., in addition to mass and momentum conservation, kinetic energy conservation in the inviscid limit is used to guide the selection of the discrete operators and solution algorithm. Two different stall configurations, which consist of flow over a NACA 0015 airfoil at 16.6° and 20° angles of attack, are simulated at Reynolds number of 896 000 based on the airfoil chord length and freestream velocity. In the case of 16.6° angle of attack where flow separates around a midchord location, LES results show excellent agreement with the experimental data for both uncontrolled and controlled cases. LES confirms the experimental finding that synthetic jets, which are produced through a slot across the entire span on suction surface at 12% chord location, effectively delay the onset of flow separation and cause a significant increase in the lift coefficient. In the case of 20° angle of attack where flow separates near the leading edge, LES predicts reasonable results comparable to experimental data when grid resolution is sufficient to predict the separated shear layer. In this case, the synthetic-jet actuation at 12% chord location is found marginally effective in controlling leading-edge separation.
Torus-shaped dust clouds trapped in a magnetized anodic plasma
Authors: Pilch Iris , Reichstein Torben , Piel Alexander
Published: November 2008

Dust particles confined in a magnetized anodic plasma can form a torus-shaped cloud with a dust-free region (void) in the center. Most of the dust particles perform a rotational motion about the major axis of the torus. The torus-shaped dust cloud and the velocity of the particles are studied by varying the external plasma parameters like magnetic field strength and rf-power of the source plasma. Two-dimensional potential contours are measured with an emissive probe. The results are used to discuss the force balance between electric field force and ion drag acting on the dust particles that determines the void size.
Multiscale features of density and frequency spectra from nonlinear gyrokinetics
Authors: Görler T. , Jenko F.
Published: November 2008

Gyrokinetic turbulence simulations covering both electron and ion spatio-temporal scales self-consistently are presented. It is found that for experimentally realistic transport levels at long wavelengths, electron temperature gradient modes may yield substantial or even dominant high-wavenumber contributions to the electron heat flux. It is investigated in which way this behavior is reflected in several experimentally accessible quantities as, for instance, density or frequency spectra.
Time evolving simulations as a tentative reproduction of the Reynolds experiments on flow transition in circular pipes
Authors: Orlandi P.
Published: November 2008

Time developing numerical simulations of the Navier–Stokes equations in circular pipes can be performed to make an attempt to reproduce Reynolds’ 1883 experiments [Philos. Trans. R. Soc. London 174, 935 (1883)]. However, it should be demonstrated that these simulations are equivalent to space developing simulations. For Reynolds it was rather difficult to estimate exactly the inlet conditions. With high probability these were different from a Poiseuille profile with superimposed clean disturbances, the conditions often assigned in stability and transitional studies. To be close to the Reynolds experiment, in the present study, a turbulent field has been assigned as the initial condition. The lowest transition Reynolds numbers have been evaluated by decreasing the Reynolds numbers, and determining, by pressure gradient time history, and flow visualizations when the turbulent flow survives. This paper, in honor of John Kim, who used direct numerical simulation to understand the role of the near wall structures in turbulent plane channels, shows that the flow remains turbulent only when the near wall structures survive.
Fluid and kinetic models of the low temperature H2 plasma produced by a radio-frequency reactor
Authors: Diomede P. , Michau A. , Redolfi M. , Morscheidt W. , Hassouni K. , Longo S. , Capitelli M.
Published: November 2008

A comparative study of two models, a kinetic and a fluid one, for the numerical simulation of H2 plasmas in parallel plate capacitively coupled rf discharges in one space dimension is presented. Both models adopt a multicomponent plasma description including several ionic species, take into account the self-consistent electric field by solving the Poisson equation and include a self-consistent coupling with the vibrational kinetics of the electronic ground state of H2 molecules. The peculiarities of this particular test case for model comparison with respect to previous studies are highlighted. The merits and the limitations of both approaches are discussed and the results are compared, in particular the steady-state density of charged species, the space-time variation of the electron energy, the vibrational distribution, and the atomic density.
Direct intervention of hairpin structures for turbulent boundary-layer control
Authors: Kang Yong-Duck , Choi Kwing-So , Chun Ho Hwan
Published: November 2008

Direct intervention of large-scale, outer-layer structures of a turbulent boundary layer has been carried out by counteracting the upwash motion of hairpin vortices with jets issued from a nozzle placed outside the boundary layer. The methodology of this turbulent boundary-layer control is similar in concept to the opposition control of near-wall turbulence, where the induced velocity field of vortical motion during the turbulence activities is opposed by suction and blowing at the wall. Unlike wall-based turbulence control techniques whose time and length scales reduce with an increase in the Reynolds number, scales of the proposed control are those of the outer layer, making this control technique highly practical. Here we show some results from a direct intervention of hairpin structures in a turbulent boundary layer, demonstrating that this is a promising technique for turbulence control.
Variations of von Kármán coefficient in canonical flows
Authors: Nagib Hassan M. , Chauhan Kapil A.
Published: November 2008

The overlap parameters for the logarithmic law are obtained for available turbulent pipe and channel flow data using composite profiles fitted to the mean velocity. The composite profile incorporates κ, B, and Π as the varying parameters and their resulting behavior with Reynolds number is examined for these flows and compared to results from boundary layers. The von Kármán coefficient in channel flow is smaller than the well-established value for zero pressure gradient turbulent boundary layers of 0.384, while in pipe flows it is consistently higher. In contrast, the estimates of the wake parameter Π are the smallest for channel flows and largest for boundary layers. Further, the Superpipe data are reanalyzed to reveal that κ=0.41 is a better value for the von Kármán constant in pipe flow. The collective behavior of κ in boundary layers, pipes, and channels reveals that the von Kármán coefficient is not universal and exhibits dependence not only on the pressure gradient but also on the flow geometry.
Subsonic jet noise reduction by fluidic control: The interaction region and the global effect
Authors: Laurendeau E. , Jordan P. , Bonnet J. P. , Delville J. , Parnaudeau P. , Lamballais E.
Published: November 2008

A microjet arrangement comprising both penetration (or immersion) and convergence (jets oriented such that two jets of a pair interact with one another) is used to control a subsonic turbulent jet with a view to noise reduction. The acoustic effect of the so-called fluidevron system is comparable to chevrons and nonconverging microjets as far as the noise reduction is concerned. Detailed experimental measurements are performed for a main jet with Mach and Reynolds numbers of 0.3 and 310 000, respectively. A direct numerical simulation study is performed for a model, plane mixing-layer problem using the immersed-boundary method, in order to help understand the topological features of the fluidevron–mixing-layer interaction. In terms of modifications produced in the flow, two relatively distinct regions are identified: the near-nozzle region, 0<(x/D)<1, where the dynamics are dominated by the fluidevron–main-jet interaction, and the region (x/D)>1, where the jet recovers many of the uncontrolled-jet flow characteristics, but with globally reduced turbulence levels and a longer potential core. The flow structure produced in the near-nozzle region is found to comprise an ejection of fluid from the main jet; the ejection process leads to very high fluctuation levels. This highly turbulent fluid, on being reassimilated by the mixing-layer downstream of the interaction point, has a spectacular local impact on turbulent kinetic energy production and on the entrainment: the former is reduced by 70%, and the latter boosted by 30% over the range 0.2<(x/D)<3. The impact of the flow control on the integral scales of the turbulence is assessed, as these are central to acoustic-analogy-based source models. A significant reduction is found in the radial integral scales, and these are then weighted by the local fluctuation energy in order to assess the impact of the control on the source mechanisms of the flow (considered in the context of Lighthill’s formulation of the problem). Considerable reductions are shown between the base line and controlled flows in terms of these energy-weighted space scales.
Monte Carlo simulation of ferroelectricity induced by collinear magnetic order in Ising spin chain
Authors: Yao Xiaoyan , Lo Veng Cheong
Published: November 2008

To understand the collinear-magnetism-driven ferroelectricity in the frustrated Ising spin chain, the fascinating magnetoelectric behavior is investigated by using Monte Carlo simulation based on one dimensional elastic Ising model. Our simulation quantitatively reproduces the experimental results of the complicated electric and magnetic behaviors as functions of temperature observed in Ca3CoMnO6 compound [Phys. Rev. Lett. 100, 047601 (2008)]. Moreover, the short-ranged up-up-down-down magnetic ordering is confirmed to be the origin of electric polarization, and the curves of energy components dependent on temperature provide a reasonable explanation for the unconventional magnetoelectric phase transition revealed in the Ising chain magnet.
Weyl quantization of fractional derivatives
Authors: Tarasov Vasily E.
Published: November 2008

The quantum analogs of the derivatives with respect to coordinates qk and momenta pk are commutators with operators Pk and Qk. We consider quantum analogs of fractional Riemann–Liouville and Liouville derivatives. To obtain the quantum analogs of fractional Riemann–Liouville derivatives, which are defined on a finite interval of the real axis, we use a representation of these derivatives for analytic functions. To define a quantum analog of the fractional Liouville derivative, which is defined on the real axis, we can use the representation of the Weyl quantization by the Fourier transformation.
In situ verification of single-domain III-V on Si(100) growth via metal-organic vapor phase epitaxy
Authors: Döscher Henning , Hannappel Thomas , Kunert Bernardette , Beyer Andreas , Volz Kerstin , Stolz Wolfgang
Published: November 2008

Reflectance anisotropy spectroscopy (RAS) was used in situ for the quantification of antiphase domains on surfaces of thin GaP films deposited onto Si(100) by metal-organic vapor phase epitaxy (MOVPE). The preparation of a single-domain GaP∕Si(100) surface was determined via the analysis of RAS peak intensities in reference to the well-known P-rich surface reconstruction of homoepitaxially grown GaP(100). Both preprocessed Si(100) substrates and MOVPE as-grown GaP∕Si(100) films were also characterized ex situ by atomic force microscopy to identify the formation of mono- and diatomic surface steps and to analyze of the domain distribution, respectively.
First-principles calculation of the effect of stress on the chemical activity of graphene
Authors: de Andres P. L. , Vergés J. A.
Published: November 2008

Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects σ and π bonds and the resulting change in the chemical activity. Stress affects more strongly π bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing 120° and 90°, an intermediate state between sp2 and sp3 bonding. We use ab initio density functional theory to study the adsorption of hydrogen on large clusters and two-dimensional periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed.
Dependency of threshold switching on density of localized states of Ge2Sb2Te5 thin films for phase change random access memory
Authors: Ryu Seung Wook , Lee Jong Ho , Ahn Yong Bae , Kim Choon Hwan , Choi Byung Joon , Hwang Cheol Seong , Kim Hyeong Joon
Published: November 2008

The threshold switching of Ge2Sb2Te5 (GST) films for phase change random access memory applications was investigated by measuring the variation in the threshold voltage (VT) with the crystallinity of the GST films and photon energy absorption spectra. As the GST film was amorphized, VT increased to approximately 1 V and its electrical resistance increased. The optical band gap and Urbach edge of the GST increased from 0.66 to 0.97 eV and from 12 to 65 meV, respectively, upon its amorphization. It was experimentally confirmed that the threshold switching is associated with the density of localized states of the GST.
All-angle zero reflection at metamaterial surfaces
Authors: Li Xin , Liang Zixian , Liu Xiaohan , Jiang Xunya , Zi Jian
Published: November 2008

The authors study theoretically reflection on the surface of a metamaterial with a hyperbolic dispersion. It is found that reflection is strongly dependent on how the surface is terminated with respect to the asymptote of the hyperbolic dispersion. For a surface terminated normally to the asymptote, zero reflection occurs for all incident angles. It is exemplified by a metamaterial made of a periodic metal-dielectric layered structure with its surface properly cut through numerical simulations.
Reduction of efficiency droop in InGaN light emitting diodes by coupled quantum wells
Authors: Ni Xianfeng , Fan Qian , Shimada Ryoko , Özgür Ümit , Morkoç Hadis
Published: November 2008

Light emitting diodes (LEDs) based on InGaN suffer from efficiency droop at current injection levels as low as 50 A cm−2. We investigated multiple quantum well InGaN LEDs with varying InGaN barrier thicknesses (3–12 nm) emitting at ∼400–410 nm to investigate the effect of hole mass and also to find out possible solutions to prevent the efficiency droop. In LEDs with electron blocking layers, when we reduced the InGaN barriers from 12 to 3 nm, the current density for the peak or saturation of external quantum efficiency increased from 200 to 1100 A cm−2 under pulsed injection conditions, which eliminates the heating effects to a large extent. Our calculations show that such reduction in the barrier thickness makes the hole distribution more uniform among the wells. These results suggest that the inferior low hole transport through the barriers exacerbated by large hole effective mass and low hole injection due to relatively low hole concentration and the consequent electron leakage are responsible for the efficiency droop at high current injection levels.
Spatially resolved electronic properties of MgO/GaAs(001) tunnel barrier studied by ballistic electron emission microscopy
Authors: Guézo S. , Turban P. , Lallaizon C. , Le Breton J. C. , Schieffer P. , Lépine B. , Jézéquel G.
Published: November 2008

The spatially resolved electronic structure of the epitaxial Au/MgO/GaAs(001) tunnel junction has been studied by ballistic electron emission microscopy. The Schottky barrier height of Au on the MgO/GaAs heterostructure is determined to be 3.90 eV, in good agreement with spatially averaged x-ray photoelectron spectroscopy measurements. Locally, two well-defined conduction channels are observed for electrons energies of 2.5 and 3.8 eV, i.e., below the conduction band minimum of the oxide layer. These conduction channels are attributed to band of defect states in the band-gap of the tunnel barrier related to oxygen vacancies in the MgO layer. These defect states are responsible for the low barrier height measured on magnetic tunnel junctions with epitaxial MgO(001) tunnel barriers.
Measurement of temperature after hypervelocity collision of microparticles in the range from 10 to 40 km/s
Authors: Miyachi Takashi , Fujii Masayuki , Hasebe Nobuyuki , Miyajima Mitsuhiro , Okudaira Osamu , Takechi Seiji , Onishi Toshiyuki , Minami Shigeyuki , Shibata Hiromi , Ohashi Hideo , Iwai Takeo , Nogami Ken-ichi , Sasaki Sho , Grün Eberhard , Srama Ralf , Okada Nagaya
Published: November 2008

The temperature recorded immediately after hypervelocity collision of microparticles comprising iron and nickel with a silver-coated piezoelectric plate was analyzed using photomultipliers of different spectral response characteristics. The conversion rate between the velocity and temperature is estimated to be ∼900 K/km∕s in the velocity range of 10–40 km/s. This rate is greater than that reported earlier.
Application of carbon nanotubes as a source of priming electrons in ac plasma display panels
Authors: Okada Takeru , Yoshioka Toshihiro
Published: November 2008

Carbon nanotubes (CNTs) are coated over the MgO protective film in an ac plasma display panel for the stabilization of discharges. The emission of priming electrons from CNTs was evaluated by analyzing the time, voltage, and temperature dependence of the statistical discharge delay. By employing CNTs as a source of priming electrons, discharges were stabilized drastically, and the statistical discharge delay remained almost constant for the measuring time (∼1 s) and temperature (from −50 to 80 °C) range. The emission from CNTs can be explained by field electron emission.
Optimum nitride concentration in multiband III-N–V alloys for high efficiency ideal solar cells
Authors: Cánovas E. , Martí A. , Luque A. , Walukiewicz W.
Published: November 2008

III-Nx–V1−x highly mismatched alloys (HMAs) have been proposed as promising material candidates for the development of high efficiency solar cells. According to the band anticrossing model, these alloys present a multiband character with an intermediate band within the otherwise fundamental bandgap that gives them the ability of improving the efficiency by means of below-bandgap photon absorption. The efficiency of GaNxAs1−x, GaNxP1−x, and their quaternaries InyGa1−yNxAs1−x and GaNxP1−x−yAsy is estimated theoretically versus nitrogen content in this letter. Low nitrogen content in the range of 1%–3.5% in the HMAs analyzed leads to theoretical efficiencies above 60%.
Tonic inhibition of chemotaxis in human plasma
Authors: Malawista Stephen E. , de Boisfleury Chevance Anne , van Damme Jo , Serhan Charles N.
Published: November 2008

We found exaggerated chemotaxis in plasma treated with EDTA and thought that the EDTA might itself be inhibiting a tonic inhibitor(s) of chemotaxis. Our plasma fractionations suggested that evidence should be sought for a lipid moiety carrying this activity, and on spectrometry (LC-MS-MS together with GC-MS analyses), the biologically active but not the inactive fraction contained oleic and arachidonic acids. Because fatty acids are largely protein bound, we flooded plasma preparations with delipidated albumin, reasoning that it would bind enough fatty acids, including inhibitory ones, to counter their tonic inhibition. Indeed, we observed dramatic increases in chemotaxis. Hence, adding delipidated albumin to plasma has a similar effect to that of adding EDTA—amplification of the chemotactic response. Oleic acid in physiologic concentrations diminishes the magnifying effects of both EDTA and of delipidated albumin, and in fact diminishes the chemotactic response even without the presence of the amplifiers of chemotaxis. In contrast, arachidonic acid amplifies further the effect of EDTA but not of delipidated albumin, and this augmentation appears to be caused by an EDTA-dependent enrichment of the chemotactic gradient with leukotriene B4 (LTB4). We conclude that oleic acid, the blood levels of which vary among individuals, is at least one tonic inhibitor of chemotaxis in plasma.
T lymphocytes potentiate endogenous neuroprotective inflammation in a mouse model of ALS
Authors: Chiu Isaac M. , Chen Adam , Zheng Yi , Kosaras Bela , Tsiftsoglou Stefanos A. , Vartanian Timothy K. , Brown Robert H. , Carroll Michael C.
Published: November 2008

Amyotrophic Lateral Sclerosis (ALS) is an adult-onset, progressive, motor neuron degenerative disease, in which the role of inflammation is not well established. Innate and adaptive immunity were investigated in the CNS of the Superoxide Dismutase 1 (SOD1)
G93A
transgenic mouse model of ALS. CD4
+
and CD8+ T cells infiltrated SOD1
G93A
spinal cords during disease progression. Cell-specific flow cytometry and gene expression profiling showed significant phenotypic changes in microglia, including dendritic cell receptor acquisition, and expression of genes linked to neuroprotection, cholesterol metabolism and tissue remodeling. Microglia dramatically up-regulated IGF-1 and down-regulated IL-6 expression. When mutant SOD1 mice were bred onto a TCRβ deficient background, disease progression was significantly accelerated at the symptomatic stage. In addition, microglia reactivity and IGF-1 levels were reduced in spinal cords of SOD1
G93A
(TCRβ−/−) mice. These results indicate that T cells play an endogenous neuroprotective role in ALS by modulating a beneficial inflammatory response to neuronal injury.
Multiple receptors involved in human rhinovirus attachment to live cells
Authors: Rankl Christian , Kienberger Ferry , Wildling Linda , Wruss Jürgen , Gruber Hermann J. , Blaas Dieter , Hinterdorfer Peter
Published: November 2008

Minor group human rhinoviruses (HRVs) attach to members of the low-density lipoprotein receptor family and are internalized via receptor-mediated endocytosis. The attachment of HRV2 to the cell surface, the first step in infection, was characterized at the single-molecule level by atomic force spectroscopy. Sequential binding of multiple receptors was evident from recordings of characteristic quantized force spectra, which suggests that multiple receptors bound to the virus in a timely manner. Unbinding forces required to detach the virus from the cell membrane increased within a time frame of several hundred milliseconds. The number of receptors involved in virus binding was determined, and estimates for on-rate, off-rate, and equilibrium binding constant of the interaction between HRV2 and plasma membrane-anchored receptors were obtained.