Initial Spatial Distribution Research Articles

Receptivity of plane Poiseuille flow to local micro-vibration disturbance on wall

The receptivity of plane Poiseuille flow to local single-period micro-vibration disturbances with different phases at the top and bottom walls was investigated through direct numerical simulation of three-dimensional incompressible Navier-Stokes equations. Results show that the disturbance presents a symmetrical distribution in the spanwise direction when the micro-vibration on the wall ends, and the initial disturbance velocities and spatial distribution of the disturbance structure are different at the top and bottom walls. The disturbance's velocity, amplitude, and high- and low-speed streaks increase with time, and the amplitude of streamwise disturbance velocity is larger than those of spanwise and vertical disturbance velocities. However, no significant Tollmien-Schlichting wave was found in the flow field. The number of disturbance vortex cores gradually increases with the disturbance area. High-speed disturbance fluid concentrates near the wall and its normal velocity largely points to the wall, while low-speed disturbance fluid largely deviates from the wall. Furthermore, the streamwise velocity profiles near the top and bottom walls both become plump because of the existence of the disturbances, and the streamwise velocity profiles show a trend of evolving into turbulent velocity profiles. The shear stress near the wall increases significantly. The local micro-vibration disturbance on the wall in plane Poiseuille flow can induce the development of a structure similar to turbulent spots.

Evolution of microstructure and texture during recovery and recrystallization in heavily rolled aluminum

The annealing behavior of nanostructured aluminum AA1050 prepared by cold rolling to an ultrahigh strain (εvM = 6.4) has been investigated using both transmission electron microscopy and electron backscatter diffraction techniques, paying particular attention to changes in microstructure and texture during recovery and their influence on subsequent recrystallization. It is found that coarsening of lamellar structures during recovery can occur via triple junction motion, and that this process can modify the proportion of different boundary types and texture components compared to those in the cold rolled material. Additionally, the heavily deformed material is characterized by different textures and different spatial arrangements of rolling texture components in the center and subsurface. It is found that changes in the misorientation distribution and texture during coarsening are greatly affected by the initial spatial distribution of crystallographic orientations. In particular, the reduction in the fraction of high angle boundaries observed during recovery is much more pronounced in the subsurface layers than in the center layer. The initial through-thickness heterogeneity is thus greatly enhanced during recovery, which leads to significant differences in recrystallized microstructure and texture in the different layers.

Implications of Electrical Crosstalk for High Density Aligned Array of Single-Wall Carbon Nanotubes

Single-wall carbon nanotube (SWNT) based devices promise a number of applications in flexible electronics, sensing, and optoelectronics. Specifically, devices based on densely packed aligned array of SWNTs are desirable for high drive current. An unintended consequence of dense packing is a new type of electrostatic crosstalk, where a single broken SWNT-at a critical field-may initiate correlated breakdown (CBD) that produces a nanoscale fissure in the channel, and thereby, defines the density limit of the aligned-array devices. In this paper, self-consistent numerical and analytical transport models (coupled to stochastic Monte Carlo simulation) are developed to explore and interpret electrical crosstalk in aligned-array SWNTs, and guidelines for improved design are proposed. Our analysis suggests that a broken SWNT induces localized voltage drop in the neighboring SWNTs, initiating band-to-band-tunneling/impact-ionization current, and corresponding breakdown electric-field is ~23 V/μm. The fissure profile is defined by an interplay between the initial (spatial) distribution of broken SWNTs and the correlation defined by the electrostatic crosstalk. Results are summarized in a density-field phase diagram that distinguishes regions of safe operation versus random breakdown/CBD. The critical field is ultimately defined by properties of the tubes (i.e., diameter distribution, doping, and size of the breakdown spot) and the substrate (e.g., dielectric and thickness).

Fast ignition when heating the central part of an inertial confinement fusion target by an ion beam

We investigate the ignition and burning of a precompressed laser fusion target when it is rapidly heated by an ion beam with the formation of a temperature peak in the central part of the target. We present the results of our comprehensive numerical simulations of the problem that include the following components: (1) the target compression under the action of a profiled laser pulse, (2) the heating of the compressed target with spatially nonuniform density and temperature distributions by a beam of high-energy ions, and (3) the burning of the target with the initial spatial density distribution formed at the instant of maximum target compression and the initial spatial temperature distribution formed as a result of the compressed-target heating by an ion beam. The dependences of the threshold energies of the igniting ion beam and the thermonuclear gain on the width of the Gaussian beam ion energy spectrum have been established. The peculiarities of fast ignition by an ion beam related to the spatial distribution of parameters for the target precompressed by a laser pulse are discussed.

Radical polymerization of tetrafluoroethylene that forms a colloid solution and gel of oligomers

As the concentration of tetrafluoroethylene (TFE) in the solvent increases, its polymerization is shown to be accompanied by a transformation of the resulting homogeneous solution of oligomers into a colloid one, which then transforms into gel. The micron-sized colloid particles consist of oligomers and solvent molecules, whose number per TFE unit in the oligomer decreases from 15–20 at the threshold of dense phase formation to 4–6 in a gel. A kinetic model of polymerization was suggested. In this model, the chain propagation is limited by monomer diffusion to the growing macroradical and the chain transfer (oligomer formation and regeneration of the prime radical) is limited by the reaction of the growing macroradical with the solvent molecule. The model that allows for the decrease in the diffusion coefficients with increasing of the oligomer chain length and their local concentration leads to a spatially inhomogeneous molecular mass distribution, with long oligomers accumulated in the vicinity of the macroradical and short ones in the distant zone. The formation of colloid particles and gel occurs when the concentration of long oligomers shaped as rigid rods exceeds the critical value for the dense phase formation and the percolation threshold, respectively. The transition from the initial spatial distribution to the equilibrium distribution described by the phase diagram was considered using the model of viscoelastic separation so that phases retain spatial continuity.

A numerical model of nonuniform solar cell stability

We model degradation in extended two-dimensional solar cells. The model couples electrical and thermal physics, so that voltage and temperature can vary over a device. Degradation is modeled phenomenologically, such that local open circuit voltage degradation is dependent on local temperature. Initial device nonuniformity is introduced with an initial spatial distribution of random shunt conductances. A comparison of pre- to post-stress performance shows variations, despite the deterministic nature of the degradation model, which is interpreted as a consequence of temperature variations.

Exploiting intrinsic triangular geometry in relativistic (3)He+Au collisions to disentangle medium properties.

Recent results in d+Au and p+Pb collisions at RHIC and the LHC provide evidence for collective expansion and flow of the created medium. We propose a control set of experiments to directly compare particle emission patterns from p+Pb, d+Au, and ^{3}He+Au or t+Au collisions at the same sqrt[s_{NN}] . Using a MonteCarlo Glauber simulation we find that a ^{3}He or triton projectile, with a realistic wave function description, induces a significant intrinsic triangular shape to the initial medium. If the system lives long enough, this survives into a significant third-order flow moment v_{3} even with viscous damping. By comparing systems with one, two, and three initial hot spots, one could disentangle the effects from the initial spatial distribution of the deposited energy and viscous damping. These are key tools for answering the question of how small a droplet of matter is necessary to form a quark-gluon plasma described by nearly inviscid hydrodynamics.

Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3.

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on SrTiO(3) consists of nondegenerate t(2g) states with different dimensional characters. While the d(xy) bands have quasi-2D dispersions with weak k(z) dependence, the lifted d(xz)/d(yz) bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the d(xz)/d(yz) Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO(3) in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O 1s core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-E(F) electrons in the surface region.

The analysis of the optimal two dimensional pump energy distribution for narrower electro-optically Q-switched pulse׳s width

The pump energy distribution in the cross section of the gain medium of a LDA side pumped laser and the effect of the initial gain spatial distribution on the electro-optically (EO) Q-switched pulse׳s width are numerical studied in this paper. The simulation results show that the relatively high energy distribution at the edge of the gain medium will lead to a longer pulse width. And two ideal pump distributions, top-hat distribution and Super Gaussian distribution at part of the cross section are also analyzed. It shows that a uniform pump region at the medium center with around 50% of the medium׳s cross section area leads to narrower pulse width than other pump distributions with the same max initial population inversion density.

Constraints fromv2fluctuations for the initial-state geometry of heavy-ion collisions

The ability to accurately compute the series of coefficients $v_n$ characterizing the momentum space anisotropies of particle production in ultrarelativistic heavy ion collisions as a function of centrality is widely regarded as a triumph of fluid dynamics as description of the bulk matter evolution. A key ingredient to fluid dynamical modeling is however the initial spatial distribution of matter as created by a yet not completely understood equilibration process. A measurement directly sensitive to this initial state geometry is therefore of high value for constraining models of pre-equilibrium dynamics. Recently, it has been shown that such a measurement is indeed possible in terms of the event by event probability distribution of the normalized $v_n$ distribution as a function of centrality, which is to high accuracy independent on the details of the subsequent fluid dynamical evolution and hence directly reflects the primary distribution of spatial eccentricities. We present a study of this observable using a variety of Glauber-based models and argue that the experimental data place very tight constraints on the initial distribution of matter and rule out all simple Glauber-based models.

Effect of changing the initial excitation spatial profile on the radiation emitted from a slab of two-level atoms

For the purpose of investigating the effect of the initial spatial distribution of the excited atoms in a slab on its emission characteristics, I compute, for different configurations of a 1D system consisting initially of equal number of atoms in the excited and ground states, the emitted field at both the front-end and back-end of the slab. I show that depending on the spatial configuration of the excited atoms/ground states atoms in the slab, the system may be at later times in a metastable state, emits equally from both end faces or emits preferentially from only one of these. These qualitatively distinct states of the system are due to the different spatial coherence present in each case, and/or the difference in the strong inter-modal coupling present and resulting from the nonlinearity in the Maxwell–Bloch equations.

Uneven abundance of the invasive sun coral over habitat patches of different orientation: An outcome of larval or later benthic processes?

Larval behavior in the water column and preference among natural benthic habitats are known to determine initial spatial distribution patterns in several sessile marine invertebrates. Such larval attributes can be adaptive, promoting adult benthic distributions which maximize their fitness. Further benthic processes may, however, substantially change initial distribution of settlers. In this study, we first characterized spatial distributions of adult colonies and single-polyp recruits of the invasive azooxanthellate coral Tubastraea coccinea over substrates of different orientation, and evaluated their consistency at both small (several tens of meters) and intermediate (a few km) spatial scales. We then assessed, through field and laboratory experiments, larval preferences and relative settlement and recruitment rates on surfaces with different orientations to determine whether processes taking place during the larval and early post-larval stages could help explain the distribution patterns of recruits and adult colonies. Results suggest that larval passive buoyancy and active larval behavior, unrelated to light conditions, determine a clear settlement distribution pattern, in which the density of settlers is highest at undersurfaces and almost nil at upward facing horizontal substrates. Except for an almost absence of settlers, recruits and adult individuals on upward facing horizontal habitat, there is substantial mismatch between the distribution of settlers and that of recruits and adult colonies. The latter were also common in vertical substrate in the field. We speculate that coastal runoff at the study area and subsequent sedimentation may inhibit coral development on flat upward facing habitat, and that competitive interference and pre-emptive interactions with other azooxanthellate corals could constrain abundance of T. coccinea in underface horizontal habitat.

High uncertainty in 21st century runoff projections from glacierized basins

Glacier response to a changing climate and its impact on runoff is understood in general terms, but model-based projections are affected by considerable uncertainties. They originate from the driving climate model, input data quality, and simplifications in the glacio-hydrological model and hamper the reliability of the simulations. Here, an integrative assessment of the uncertainty in 21st century glacier runoff is provided based on experiments using the Glacier Evolution Runoff Model (GERM) applied to the catchment of Findelengletscher, Switzerland. GERM is calibrated and validated in a multi-objective approach and is run using nine Regional Climate Models (RCMs) until 2100. Among others, the hydrological impacts of the RCM downscaling procedure, the winter snow accumulation, the surface albedo and the calculation of ice melt and glacier retreat are investigated. All experiments indicate rapid glacier wastage and a transient runoff increase followed by reduced melt season discharge. However, major uncertainties in, e.g., glacier area loss (−100% to −63%) and the change in annual runoff (−57% to +25% relative to today) by 2100 are found. The impact of model assumptions on changes in August runoff is even higher (−94% to −5%). The spread in RCM results accounts for 20–50% of the overall uncertainty in modeled discharge. Initial ice thickness, the amount and spatial distribution of winter snow and the glacier retreat model have the largest effect on the projections, whereas the RCM downscaling procedure, calibration data quality and the melt model (energy balance vs. degree-day approach) are of secondary importance.

Dynamics of Excess Electronic Charge in Aliphatic Ionic Liquids Containing the Bis(trifluoromethylsulfonyl)amide Anion

In a recent article (J. Am. Chem. Soc. 2011, 133, 20186) we investigated the initial spatial distribution of dry excess electrons in a series of room-temperature ionic liquids (RTILs). Perhaps unexpectedly, we found that in some alkylammonium-based systems the excess negative charge resided on anions and not on the positive cations. Following on these results, in the current paper we describe the time evolution of an excess electronic charge introduced in alkylammonium- and pyrrolidinium-based ionic liquids coupled with the bis(trifluoromethylsulfonyl)amide ([Tf2N(-)]) anion. We find that on a 50 fs time scale an initially delocalized excess electron localizes on a single [Tf2N(-)] anion which begins a fragmentation process. Low-energy transitions have a very different physical origin on the several femtoseconds time scale when compared to what occurs on the picosecond time scale. At time zero, these are intraband transitions of the excess electron. However after 40 fs when the excess electronic charge localizes on a single anion, these transitions disappear, and the spectrum is dominated by electron-transfer transitions between the fragments of the doubly charged breaking anion.

The Role of Scaffold Architecture and Composition on the Bone Formation by Adipose-Derived Stem Cells

Scaffold architecture and composition are crucial parameters determining the initial cell spatial distribution and consequently bone tissue formation. Three-dimensional poly-ε-caprolactone (PCL) scaffolds with a 0/90° lay-down pattern were plotted and subjected to (1) an oxygen plasma (PCL O) or (2) a postargon plasma modification with gelatin and fibronectin (PCL Fn). These scaffolds with an open pore structure were compared with more compact scaffolds fabricated by conventional processing techniques: oxidized polylactic acid (LA O) and collagen (COL) scaffolds. Human adipose tissue-derived stem cell/scaffold interactions were studied. The study revealed that the biomimetic surface modification of plotted scaffolds did not increase the seeding efficiency. The proliferation and colonization was superior for PCL Fn in comparison with PCL O. The plotted PCL Fn was completely colonized throughout the scaffold, whereas conventional scaffolds only at the edge. Protein-based scaffolds (PCL Fn and COL) enhanced the differentiation, although plotted scaffolds showed a delay in their differentiation compared with compact scaffolds. In conclusion, protein modification of plotted PCL scaffolds enhances uniform tissue formation, but shows a delayed differentiation in comparison with compact scaffolds. The present study demonstrates that biomimetic PCL scaffolds could serve as a guiding template to obtain a uniform bone tissue formation in vivo.

Stochastic effects in plasticity in small volumes

Recent studies of micro- and nano-scale metallic structures have exposed considerable statistical distribution, in addition to significant size dependencies, in the yield strength. This intrinsic statistical variation is particularly evident in the micro-compression and microtension thin film tests. This work investigates the relationship between the initial dislocation density, the heterogeneous initial spatial dislocation distribution, and the resulting localized deformation with multiscale discrete dislocation dynamics simulations. This relationship is examined separately from commonly reported external factors affecting observed strength, such as variations in specimen geometry and base support. Towards this end, we performed multiscale dislocation dynamics simulations of geometries commonly employed in micro-scale testing techniques, including micro-pillar compression, microtensile thin film, and microbulge tests. The statistical variation of yield strengths from all three simulation geometries is in agreement with experimental data from the corresponding loading techniques. We show that the onset of plasticity is stochastic in small volumes containing a small density of dislocations: a contrast to classical deterministic plasticity theory. The yield stress in these small volumes is stochastic, not deterministic, because of statistical variation of the initial dislocation content. The numerical results exhibit a localized deformation process and demonstrate a strong dependence of the yield stress on the initial dislocation density, the initial dislocation spatial distribution, and the specimen geometry size. Leveraging nucleation theory, a stochastic model for the onset of plasticity in micro- and nano-scale structures is developed based on these results.

Constant-Momentum Acceleration Time-of-Flight Mass Spectrometry with Energy Focusing

Fundamental aspects of constant-momentum acceleration time-of-flight mass spectrometry (CMA-TOFMS) are explored as a means to improve mass resolution. By accelerating all ions to the same momentum rather than to the same energy, the effects of the initial ion spatial and energy distributions upon the total ion flight time are decoupled. This decoupling permits the initial spatial distribution of ions in the acceleration region to be optimized independently, and energy focus, including ion turn-around-time error, to be accomplished with a linear-field reflectron. Constant-momentum acceleration also linearly disperses ions across time according to mass-to-charge (m/z) ratio, instead of the quadratic relationship between flight time and m/z found in conventional TOFMS. Here, CMA-TOFMS is shown to achieve simultaneous spatial and energy focusing over a selected portion of the mass spectrum. An orthogonal-acceleration time-of-flight system outfitted with a reduced-pressure DC glow discharge (GD) ionization source is used to demonstrate CMA-TOFMS with atomic ions. The influence of experimental parameters such as the amplitude and width of the time-dependent CMA pulse on mass resolution is investigated, and a useful CMA-TOFMS focusing window of 2 to 18Da is found for GD-CMA-TOFMS.

Understanding the nuclear initial state with an electron ion collider

In these proceedings I describe how a future electron-ion collider will allow us to directly measure the initial spatial distribution of gluons in heavy ions, as well as its variance ("lumpiness") in exclusive diffraction. I show the feasibility of such a measurement by means of simulated data from the novel event generator Sartre.

Cyanobacterial calcification in modern microbialites at the submicrometer scale

Abstract. The search for microfossils in the geological record has been a long-term challenge. Part of the problem comes from the difficulty of identifying such microfossils unambiguously, since they can be morphologically confused with abiotic biomorphs. One route to improve our ability to correctly identify microfossils involves studying fossilization processes affecting bacteria in modern settings. We studied the initial stages of fossilization of cyanobacterial cells in modern microbialites from Lake Alchichica (Mexico), a Mg-rich hyperalkaline crater lake (pH 8.9) hosting currently growing stromatolites composed of aragonite [CaCO3] and hydromagnesite [Mg5(CO3)4(OH)2 · 4(H2O)]. Most of the biomass associated with the microbialites is composed of cyanobacteria. Scanning electron microscopy analyses coupled with confocal laser scanning microscopy observations were conducted to co-localize cyanobacterial cells and associated minerals. These observations showed that cyanobacterial cells affiliated with the order Pleurocapsales become specifically encrusted within aragonite with an apparent preservation of cell morphology. Encrustation gradients from non-encrusted to totally encrusted cells spanning distances of a few hundred micrometers were observed. Cells exhibiting increased levels of encrustation along this gradient were studied down to the nm scale using a combination of focused ion beam (FIB) milling, transmission electron microscopy (TEM) and scanning transmission x-ray microscopy (STXM) at the C, O and N K-edges. Two different types of aragonite crystals were observed: one type was composed of needle-shaped nano-crystals growing outward from the cell body with a crystallographic orientation perpendicular to the cell wall, and another type was composed of larger crystals that progressively filled the cell interior. Exopolymeric substances (EPS), initially co-localized with the cells, decreased in concentration and dispersed away from the cells while crystal growth occurred. As encrustation developed, EPS progressively disappeared, but remaining EPS showed the same spectroscopic signature. In the most advanced stages of fossilization, only the textural organization of the two types of aragonite recorded the initial cell morphology and spatial distribution.

Filamentation of femtosecond laser pulses governed by variable wavefront distortions via a deformable mirror

Filamentation of focused UV and IR femtosecond laser pulses and plasma channel formation governed by variable wavefront distortions was experimentally and numerically studied. A deformable mirror was used to control the plasma channel length by introducing a spherical aberration into the initial transverse spatial distribution of a femtosecond laser pulse. An at least double increase of the plasma channel length was observed with increasing deformation of the mirror. Numerical calculations show that the hat-like phase shape of the aberration ensures that the energy of the initial laser pulse remains confined for a longer distance within the limited transverse size of the filament.