14-moment maximum-entropy modelling of collisionless ions for Hall thruster discharges

Ions in Hall thruster devices are often characterized by a low collisionality. In the presence of acceleration fields and azimuthal electric field waves, this results in strong deviations from thermodynamic equilibrium, introducing kinetic effects. This work investigates the application of the 14-moment maximum-entropy model to this problem. This method consists in a set of 14 PDEs for the density, momentum, pressure tensor components, heat flux and fourth-order moment associated to the particle velocity distribution function. The model is applied to the study of collisionless ion dynamics in a Hall thruster-like configuration, and its accuracy is assessed against different models, including the kinetic solution. Three test cases are considered: a purely axial acceleration problem, the problem of ion-wave trapping and finally the evolution of ions in the axial-azimuthal plane. Most of this work considers ions only, and the coupling with electrons is removed by prescribing reasonable values of the electric field. This allows us to obtain a direct comparison among different ion models. However, the possibility to run self-consistent plasma simulations is also briefly discussed, considering quasi-neutral or multi-fluid models. The maximum-entropy system appears to be a robust and accurate option for the considered test cases. The accuracy is improved over the simpler pressureless gas model (cold ions) and the Euler equations for gas dynamics, while the computational cost shows to remain much lower than direct kinetic simulations

What do they know? The effects of outside director acquisition experience on firm acquisition performance

This article contributes to the literature on board effectiveness by being perhaps the first to systematically examine how the nature of outside directors' prior experience, and resulting expertise, will influence the performance of a focal firm's strategic initiatives. Our theoretical model is grounded in the psychological literature on expertise and its role in group decision making effectiveness. We focus on outside director expertise in acquisition decision making, and its implications for the performance of the acquisitions of a focal firm. Our conceptual framework indicates that directors will develop expertise in making particular kinds of acquisition decisions (e.g., related or unrelated acquisitions or acquisitions in specific industries or product markets) through their past experiences at other firms with decisions about those specific types of acquisitions, and we predict that this experience and expertise will have positive effects on the performance of a focal firm's acquisitions. We extend our theoretical model to consider the conditions under which relevant director experience will prove most beneficial. Our model predicts that outside director acquisition expertise will deliver the greatest benefits when the focal firm's board is independent from management. We find empirical support for all of our hypotheses. In considering how and when director experience and resulting expertise may influence the performance of corporate acquisitions, our theory and results help to highlight a potential second main focus for research on the long-standing question of what factors render boards of directors effective. Copyright © 2008 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61235/1/704_ftp.pd

The surface-state of the topological insulator Bi2_2Se3_3 revealed by cyclotron resonance

To date transport measurements of topological insulators have been dominated by the conductivity of the bulk, leading to substantial difficulties in resolving the properties of the surface. To this end, we use high magnetic field, rf- and microwave-spectroscopy to selectively couple to the surface conductivity of Bi$_2$Se$_3$ at high frequency. In the frequency range of a few GHz we observe a crossover from quantum oscillations indicative of a small 3D Fermi surface, to cyclotron resonance indicative of a 2D surface state

Investigating the impact of temperature on growth rate of the root rot fungus, Gymnopus fusipes

Gymnopus fusipes is an understudied root rot pathogen associated with multiple tree species and is linked to episodes of oak decline across the United Kingdom and Europe. Although the reported distribution of G. fusipes is broad, many observations rely solely on visual identification of fruiting bodies, which can be unreliable, and lack confirmation by molecular and/or isolation data to verify this broad ecological range. Given the paucity of information regarding the true ecological distribution of G. fusipes, it is difficult to predict and model the potential distribution of the species under both current and future climate scenarios. In this study, to determine the growth capabilities of G. fusipes across a range of ecologically relevant temperatures, five geographically diverse isolates of G. fusipes were grown at five different temperatures ranging from 4–37°C, to determine the optimal temperature for G. fusipes growth, and to establish whether geographically diverse isolates exhibit local adaptation to temperature tolerance. Incubation temperature had a significant effect on G. fusipes growth rate, with 25°C representing the optimum (P&lt;0.001). Isolates had differing growth rates at each of the temperatures, with an isolate from the UK having the highest overall growth rate across all five temperatures tested (P&lt;0.001), and at the optimum, increased by a mean value of over 4915 mm2. Local adaptation to temperature tolerance was not found in the isolates tested. These data demonstrate the optimal incubation temperature for future laboratory studies on G. fusipes and provide the first data on the growth rate of this pathogen across ecologically relevant climate ranges that may inform land managers, modellers, and policy makers in predicting the current and potentially future geographical limits of this widespread root rot pathogen.<br/

Numerical simulation of rarefied supersonic flows using a fourth-order maximum-entropy moment method with interpolative closure

Maximum-entropy moment methods allow for the modelling of gases from the continuum regime to strongly rarefied conditions. The development of approximated solutions to the entropy maximization problem has made these methods computationally affordable. In this work, we apply a fourth-order maximum-entropy moment method to the study of supersonic rarefied flows. For such conditions, we compare the maximum-entropy solutions to results obtained from the kinetic theory of gases at different Knudsen numbers. The analysis is performed for both a simplified model of a gas with a single translational degree of freedom (5-moment system) and for a typical gas with three degrees of freedom (14-moment system). The maximum-entropy method is applied to the study of the Sod shock-tube problem at various rarefaction levels, and to the simulation of two-dimensional low-collisional crossed supersonic jets. We show that, in rarefied supersonic conditions, it is important to employ accurate estimates of the wave speeds. Since analytical expressions are not presently available, we propose an approximation, valid for the 14-moment system. In these conditions, the solution of the maximum-entropy system is shown to realize large degrees of non-equilibrium and to approach the Junk subspace, yet provides a good overall accuracy and agreement with the kinetic theory. Numerical procedures for reaching second-order accurate discretizations are discussed, as well as the implementation of the 14-moment solver on Graphics Processing Units (GPUs)

Investigating the impact of temperature on growth rate of the root rot fungus, Gymnopus fusipes

Gymnopus fusipes is an understudied root rot pathogen associated with multiple tree species and is linked to episodes of oak decline across the United Kingdom and Europe. Although the reported distribution of G. fusipes is broad, many observations rely solely on visual identification of fruiting bodies, which can be unreliable, and lack confirmation by molecular and/or isolation data to verify this broad ecological range. Given the paucity of information regarding the true ecological distribution of G. fusipes, it is difficult to predict and model the potential distribution of the species under both current and future climate scenarios. In this study, to determine the growth capabilities of G. fusipes across a range of ecologically relevant temperatures, five geographically diverse isolates of G. fusipes were grown at five different temperatures ranging from 4–37°C, to determine the optimal temperature for G. fusipes growth, and to establish whether geographically diverse isolates exhibit local adaptation to temperature tolerance. Incubation temperature had a significant effect on G. fusipes growth rate, with 25°C representing the optimum (P&lt;0.001). Isolates had differing growth rates at each of the temperatures, with an isolate from the UK having the highest overall growth rate across all five temperatures tested (P&lt;0.001), and at the optimum, increased by a mean value of over 4915 mm2. Local adaptation to temperature tolerance was not found in the isolates tested. These data demonstrate the optimal incubation temperature for future laboratory studies on G. fusipes and provide the first data on the growth rate of this pathogen across ecologically relevant climate ranges that may inform land managers, modellers, and policy makers in predicting the current and potentially future geographical limits of this widespread root rot pathogen.<br/

454-Pyrosequencing: A Molecular Battiscope for Freshwater Viral Ecology

Viruses, the most abundant biological entities on the planet, are capable of infecting organisms from all three branches of life, although the majority infect bacteria where the greatest degree of cellular diversity lies. However, the characterization and assessment of viral diversity in natural environments is only beginning to become a possibility. Through the development of a novel technique for the harvest of viral DNA and the application of 454 pyrosequencing, a snapshot of the diversity of the DNA viruses harvested from a standing pond on a cattle farm has been obtained. A high abundance of viral genotypes (785) were present within the virome. The absolute numbers of lambdoid and Shiga toxin (Stx) encoding phages detected suggested that the depth of sequencing had enabled recovery of only ca. 8% of the total virus population, numbers that agreed within less than an order of magnitude with predictions made by rarefaction analysis. The most abundant viral genotypes in the pond were bacteriophages (93.7%). The predominant viral genotypes infecting higher life forms found in association with the farm were pathogens that cause disease in cattle and humans, e.g. members of the Herpesviridae. The techniques and analysis described here provide a fresh approach to the monitoring of viral populations in the aquatic environment, with the potential to become integral to the development of risk analysis tools for monitoring the dissemination of viral agents of animal, plant and human diseases

Lignocellulose-Degrading Microbial Communities in Landfill Sites Represent a Repository of Unexplored Biomass- Degrading Diversity Emma

The microbial conversion of lignocellulosic biomass for biofuel production represents a renewable alternative to fossil fuels. However, the discovery of new microbial enzymes with high activity is critical for improving biomass conversion processes. While attempts to identify superior lignocellulose-degrading enzymes have focused predominantly on the animal gut, biomass-degrading communities in landfill sites represent an unexplored resource of hydrolytic enzymes for biomass conversion. Here, to address the paucity of information on biomass-degrading microbial diversity beyond the gastrointestinal tract, cellulose (cotton) “baits” were incubated in landfill leachate microcosms to enrich the landfill cellulolytic microbial community for taxonomic and functional characterization. Metagenome and 16S rRNA gene amplicon sequencing demonstrated the dominance of Firmicutes, Bacteroidetes, Spirochaetes, and Fibrobacteres in the landfill cellulolytic community. Functional metagenome analysis revealed 8,371 carbohydrate active enzymes (CAZymes) belonging to 244 CAZyme families. In addition to observing biomass-degrading enzymes of anaerobic bacterial “cellulosome” systems of members of the Firmicutes, we report the first detection of the Fibrobacter cellulase system and the Bacteroidetes polysaccharide utilization locus (PUL) in landfill sites. These data provide evidence for the presence of multiple mechanisms of biomass degradation in the landfill microbiome and highlight the extraordinary functional diversity of landfill microorganisms as a rich source of biomass-degrading enzymes of potential biotechnological significance

RNA-viromics reveals diverse communities of soil RNA viruses with the potential to affect grassland ecosystems across multiple trophic levels

The distribution and diversity of RNA viruses in soil ecosystems are largely unknown, despite their significant impact on public health, ecosystem functions, and food security. Here, we characterise soil RNA viral communities along an altitudinal productivity gradient of peat, managed grassland and coastal soils. We identified 3462 viral contigs in RNA viromes from purified virus-like-particles in five soil-types and assessed their spatial distribution, phylogenetic diversity and potential host ranges. Soil types exhibited minimal similarity in viral community composition, but with >10-fold more viral contigs shared between managed grassland soils when compared with peat or coastal soils. Phylogenetic analyses predicted soil RNA viral communities are formed from viruses of bacteria, plants, fungi, vertebrates and invertebrates, with only 12% of viral contigs belonging to the bacteria-infecting Leviviricetes class. 11% of viral contigs were found to be most closely related to members of the Ourmiavirus genus, suggesting that members of this clade of plant viruses may be far more widely distributed and diverse than previously thought. These results contrast with soil DNA viromes which are typically dominated by bacteriophages. RNA viral communities, therefore, have the potential to exert influence on inter-kingdom interactions across terrestrial biomes