Profile blunting and flow blockage in a yield stress fluid: A molecular dynamics study

The flow of a simple glass forming system (a 80:20 binary Lennard-Jones mixture) through a planar channel is studied via molecular dynamics simulations. The flow is driven by an external body force similar to gravity. Previous studies show that the model exhibits both a static [Varnik et al. J. Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich Phys. Rev. B 73, 174209 (2006)] yield stress in the glassy phase. \blue{These observations are corroborated by the present work, where we investigate how the presence of a yield stress may affect the system behavior in a Poiseuille-type flow geometry.} In particular, we observe a blunted velocity profile across the channel: A relatively wide region in the channel center flows with a constant velocity (zero shear rate) followed by a non linear change of the shear rate as the walls are approached. The observed velocity gradients are compared to those obtained from the knowledge of the shear stress across the channel and the flow-curves (stress versus shear rate), the latter being determined in our previous simulations of homogeneous shear flow. Furthermore, using the value of the (dynamic) yield stress known from previous simulations, we estimate the threshold body force for a complete arrest of the flow. Indeed, a blockage is observed as the imposed force falls below this threshold value. Small but finite shear rates are observed at stresses above the dynamic but below the static yield stress. We discuss the possible role of the \blue{stick-slip like motion} for this observation.Comment: 22 pages, 8 figure

Representations of J-central J-Potapov functions in both nondegenerate and degenerate cases

AbstractLet J be an m×m signature matrix (i.e. J∗=J and J2=Im) and let D:={z∈C:|z|<1}. Denote PJ(D) the class of all J-Potapov functions in D, i.e. the set of all meromorphic m×m matrix-valued functions f in D with J-contractive values at all points of D at which f is holomorphic. Further, denote PJ,0(D) the subclass of all f∈PJ(D) which are holomorphic at the origin. Let f∈PJ,0(D), and let f(w)=∑j=0∞Ajwj be the Taylor series representation of f in some neighborhood of 0. Then it was proved in [B. Fritzsche, B. Kirstein, U. Raabe, On the structure of J-Potapov sequences, Linear Algebra Appl., in press] that for each n∈N the matrix An can be described by its position in a matrix ball depending on the sequence (Aj)j=0n-1. The J-Potapov function f is called J-central if there exists some k∈N such that for each integer j⩾k the matrix Aj coincides with the center of the corresponding matrix ball.In this paper, we derive left and right quotient representations of matrix polynomials for J-central J-Potapov functions in D. Moreover, we obtain recurrent formulas for the matrix polynomials involved in these quotient representations

Электропривод насоса перекачки золошлаковой пульпы

Объектом исследования является система автоматического управления насоса перекачки золошлаковой пульпы. Цель работы – разработка и исследование электропривода насоса перекачки золошлаковой пульпы. В выпускной квалификационной работе произведён расчет и выбор приводного двигателя преобразователя частоты для управления электроприводами. Обоснован выбор способа управления, рассчитаны механические и электромеханические характеристики.The object of study is an automatic control system for the pump for transferring ash and slag pulp. The purpose of the work is the development and study of the electric drive of the pump for transferring ash and slag pulp. In the final qualification work, the calculation and selection of the drive motor of the frequency converter for controlling electric drives were made. The choice of control method is justified, mechanical and electromechanical characteristics are calculated

Measurement of the branching ratio for beta-delayed alpha decay of 16N

While the 12C(a,g)16O reaction plays a central role in nuclear astrophysics, the cross section at energies relevant to hydrostatic helium burning is too small to be directly measured in the laboratory. The beta-delayed alpha spectrum of 16N can be used to constrain the extrapolation of the E1 component of the S-factor; however, with this approach the resulting S-factor becomes strongly correlated with the assumed beta-alpha branching ratio. We have remeasured the beta-alpha branching ratio by implanting 16N ions in a segmented Si detector and counting the number of beta-alpha decays relative to the number of implantations. Our result, 1.49(5)e-5, represents a 24% increase compared to the accepted value and implies an increase of 14% in the extrapolated S-factor

Footprint-weighted tile approach for a spruce forest and a nearby patchy clearing using the ACASA model

The ACASA (Advanced Canopy-Atmosphere-Soil Algorithm) model, with a higher-order closure for tall vegetation, has already been successfully tested and validated for homogeneous spruce forests. The aim of this paper is to test the model using a footprint-weighted tile approach for a clearing with a heterogeneous structure of the underlying surface. The comparison with flux data shows a good agreement with a footprint-aggregated tile approach of the model. However, the results of a comparison with a tile approach on the basis of the mean land use classification of the clearing is not significantly different. It is assumed that the footprint model is not accurate enough to separate small-scale heterogeneities. All measured fluxes are corrected by forcing the energy balance closure of the test data either by maintaining the measured Bowen ratio or by the attribution of the residual depending on the fractions of sensible and latent heat flux to the buoyancy flux. The comparison with the model, in which the energy balance is closed, shows that the buoyancy correction for Bowen ratios > 1.5 better fits the measured data. For lower Bowen ratios, the correction probably lies between the two methods, but the amount of available data was too small to make a conclusion. With an assumption of similarity between water and carbon dioxide fluxes, no correction of the net ecosystem exchange is necessary for Bowen ratios > 1.5

Laser-equipped gas reaction chamber for probing environmentally sensitive materials at near atomic scale

Numerous metallurgical and materials science applications depend on quantitative atomic-scale characterizations of environmentally-sensitive materials and their transient states. Studying the effect upon materials subjected to thermochemical treatments in specific gaseous atmospheres is of central importance for specifically studying a material’s resistance to certain oxidative or hydrogen environments. It is also important for investigating catalytic materials, direct reduction of an oxide, particular surface science reactions or nanoparticle fabrication routes. This manuscript realizes such experimental protocols upon a thermochemical reaction chamber called the "Reacthub" and allows for transferring treated materials under cryogenic & ultrahigh vacuum (UHV) workflow conditions for characterisation by either atom probe or scanning Xe(+)/electron microscopies. Two examples are discussed in the present study. One protocol was in the deuterium gas charging (25 kPa D(2) at 200°C) of a high-manganese twinning-induced-plasticity (TWIP) steel and characterization of the ingress and trapping of hydrogen at various features (grain boundaries in particular) in efforts to relate this to the steel’s hydrogen embrittlement susceptibility. Deuterium was successfully detected after gas charging but most contrast originated from the complex ion FeOD(+) signal and the feature may be an artefact. The second example considered the direct deuterium reduction (5 kPa D(2) at 700°C) of a single crystal wüstite (FeO) sample, demonstrating that under a standard thermochemical treatment causes rapid reduction upon the nanoscale. In each case, further studies are required for complete confidence about these phenomena, but these experiments successfully demonstrate that how an ex-situ thermochemical treatment can be realised that captures environmentally-sensitive transient states that can be analysed by atomic-scale by atom probe microscope

Beta-delayed deuteron emission from 11Li: decay of the halo

The deuteron-emission channel in the beta-decay of the halo-nucleus 11Li was measured at the ISAC facility at TRIUMF by implanting post-accelerated 11Li ions into a segmented silicon detector. The events of interest were identified by correlating the decays of 11Li with those of the daughter nuclei. This method allowed the energy spectrum of the emitted deuterons to be extracted, free from contributions from other channels, and a precise value for the branching ratio B_d = 1.30(13) x 10-4 to be deduced for E(c.m.) > 200 keV. The results provide the first unambiguous experimental evidence that the decay takes place essentially in the halo of 11Li, and that it proceeds mainly to the 9Li + d continuum, opening up a new means to study of the halo wave function of 11Li.Comment: 4 pages, 3 figure

Study of bound states in 12Be through low-energy 11Be(d,p)-transfer reactions

The bound states of 12Be have been studied through a 11Be(d,p)12Be transfer reaction experiment in inverse kinematics. A 2.8 MeV/u beam of 11Be was produced using the REX-ISOLDE facility at CERN. The outgoing protons were detected with the T-REX silicon detector array. The MINIBALL germanium array was used to detect gamma rays from the excited states in 12Be. The gamma-ray detection enabled a clear identification of the four known bound states in 12Be, and each of the states has been studied individually. Differential cross sections over a large angular range have been extracted. Spectroscopic factors for each of the states have been determined from DWBA calculations and have been compared to previous experimental and theoretical results

Aging induced changes on NEXAFS fingerprints in individual combustion particles

Soot particles can significantly influence the Earth's climate by absorbing and scattering solar radiation as well as by acting as cloud condensation nuclei. However, despite their environmental (as well as economic and political) importance, the way these properties are affected by atmospheric processing of the combustion exhaust gases is still a subject of discussion. In this work, individual soot particles emitted from two different vehicles, a EURO 2 transporter, a EURO 3 passenger car, and a wood stove were investigated on a single-particle basis. The emitted exhaust, including the particulate and the gas phase, was processed in a smog chamber with artificial solar radiation. Single particle specimens of both unprocessed and aged soot were characterized using near edge X-ray absorption fine structure spectroscopy (NEXAFS) and scanning electron microscopy. Comparison of NEXAFS spectra from the unprocessed particles and those resulting from exhaust photooxidation in the chamber revealed changes in the carbon functional group content. For the wood stove emissions, these changes were minor, related to the relatively mild oxidation conditions. For the EURO 2 transporter emissions, the most apparent change was that of carboxylic carbon from oxidized organic compounds condensing on the primary soot particles. For the EURO 3 car emissions oxidation of primary soot particles upon photochemical aging has likely contributed as well. Overall, the changes in the NEXAFS fingerprints were in qualitative agreement with data from an aerosol mass spectrometer. Furthermore, by taking full advantage of our in situ microreactor concept, we show that the soot particles from all three combustion sources changed their ability to take up water under humid conditions upon photochemical aging of the exhaust. Due to the selectivity and sensitivity of the NEXAFS technique for the water mass, also small amounts of water taken up into the internal voids of agglomerated particles could be detected. Because such small amounts of water uptake do not lead to measurable changes in particle diameter, it may remain beyond the limits of volume growth measurements, especially for larger agglomerated particles