A Lagrangian model for the evolution of turbulent magnetic and passive scalar fields

In this paper we present an extension of the \emph{Recent Fluid Deformation (RFD)} closure introduced by Chevillard and Meneveau (2006) which was developed for modeling the time evolution of Lagrangian fluctuations in incompressible Navier-Stokes turbulence. We apply the RFD closure to study the evolution of magnetic and passive scalar fluctuations. This comparison is especially interesting since the stretching term for the magnetic field and for the gradient of the passive scalar are similar but differ by a sign such that the effect of stretching and compression by the turbulent velocity field is reversed. Probability density functions (PDFs) of magnetic fluctuations and fluctuations of the gradient of the passive scalar obtained from the RFD closure are compared against PDFs obtained from direct numerical simulations

Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks

Planetesimals in gaseous protoplanetary disks may grow by collecting dust particles. Hydrodynamical studies show that small particles generally avoid collisions with the planetesimals because they are entrained by the flow around them. This occurs when $St$, the Stokes number, defined as the ratio of the dust stopping time to the planetesimal crossing time, becomes much smaller than unity. However, these studies have been limited to the laminar case, whereas these disks are believed to be turbulent. We want to estimate the influence of gas turbulence on the dust-planetesimal collision rate and on the impact speeds. We used three-dimensional direct numerical simulations of a fixed sphere (planetesimal) facing a laminar and turbulent flow seeded with small inertial particles (dust) subject to a Stokes drag. A no-slip boundary condition on the planetesimal surface is modeled via a penalty method. We find that turbulence can significantly increase the collision rate of dust particles with planetesimals. For a high turbulence case (when the amplitude of turbulent fluctuations is similar to the headwind velocity), we find that the collision probability remains equal to the geometrical rate or even higher for $St\geq 0.1$, i.e., for dust sizes an order of magnitude smaller than in the laminar case. We derive expressions to calculate impact probabilities as a function of dust and planetesimal size and turbulent intensity

Tuning excitability by alloying: the Rh(111)/Ni/H2 + O2 system

The dynamic behavior of the O2 + H2 reaction on a Rh(111) surface alloyed with Ni has been studied in the 10(-5) mbar range using photoemission electron microscopy (PEEM) as a spatial resolving method. For T = 773 K and p(O2) = 5 x 10(-5) mbar the bifurcation diagram has been mapped out as a function of the Ni coverage in a range of 0 ML /= 1.3 ML. A critical Ni coverage of Theta(Ni,crit) = 0.13 monolayers (ML) is required for excitability. In the excitable parameter range pulse trains and irregular chemical wave patterns are found. Whereas the propagation speed of the pulses exhibits no clear-cut dependence on the Ni coverage, the frequency of the local PEEM intensity oscillations increases linearly with Ni coverage in the range from Theta(Ni) = 0.13 ML to Theta(Ni) = 1.3 ML.DF

Dispersion Relations for Thermally Excited Waves in Plasma Crystals

Thermally excited waves in a Plasma crystal were numerically simulated using a Box_Tree code. The code is a Barnes_Hut tree code proven effective in modeling systems composed of large numbers of particles. Interaction between individual particles was assumed to conform to a Yukawa potential. Particle charge, mass, density, Debye length and output data intervals are all adjustable parameters in the code. Employing a Fourier transform on the output data, dispersion relations for both longitudinal and transverse wave modes were determined. These were compared with the dispersion relations obtained from experiment as well as a theory based on a harmonic approximation to the potential. They were found to agree over a range of 0.9<k<5, where k is the shielding parameter, defined by the ratio between interparticle distance a and dust Debye length lD. This is an improvement over experimental data as current experiments can only verify the theory up to k = 1.5.Comment: 8 pages, Presented at COSPAR '0

Nudging sustainability transitions in Central Mozambique

An Agricultural Innovation Platform (AIP) facilitates and accelerates transition of farmers from subsistence farming to socially inclusive, environmentally sound and market-oriented farming. AIPs adapt interventions around interlinked levers in the food value chains, with entry points based on a good understanding of farming systems and input/output market requirements. Learning activities around these entry points strengthen stakeholder networks. Capacitating farmers through stakeholder networks, using mutual learning and additional technical training sessions, improves their self-organization and helps them become entrepreneurs, with the private sector as business mentors and government and extension as facilitators and support. Prospects for change in complex farming systems often appear few and uncertain, leaving farmers feeling helpless in the face of challenges. AIPs demonstrate that small interventions around significant leverage points will have far-reaching benefits. With time, stakeholders facilitate multiple changes through AIPs, magnifying the impact and paving the way for initiatives beyond a project’s lifespan. The ‘AIP experience’ changes their self-perception, resilience, motivation and empowerment forever

β-TrCP is dispensable for Vpu's ability to overcome the CD317/Tetherin-imposed restriction to HIV-1 release

<p>Abstract</p> <p>Background</p> <p>The cellular transmembrane protein CD317/BST-2/HM1.24/Tetherin restricts HIV-1 infection by physically tethering mature virions to the surface of infected cells. HIV-1 counteracts this restriction by expressing the accessory protein Vpu, yet the mechanism of this antagonism is incompletely understood. β-TrCP is the substrate recognition domain of an E3 ubiquitin ligase complex that interacts with the di-serine motif S52/S56 in the cytoplasmic tail of Vpu to target the CD4 receptor for proteasomal degradation. Recently, it has been suggested that β-TrCP is also critically involved in Vpu's ability to overcome the CD317-mediated virion release block.</p> <p>Results</p> <p>To test this model, we analyzed the consequences of several experimental strategies to interfere with the Vpu-β-TrCP protein-protein interaction. Under these conditions, we studied effects of Vpu on expression and localization of CD317 and CD4, as well as on its ability to promote HIV-1 release. Our results demonstrate a strict requirement for Vpu's di-serine motif for degradation of CD4 and also CD317, reduction of cell surface exposure of CD317, and HIV-1 release enhancement. We further show a critical role of β-TrCP2, but not of the structurally related β-TrCP1 isoform, for Vpu-mediated degradation of both receptors. Most importantly, Vpu remained active in downregulating CD317 from the cell surface and in overcoming the HIV-1 release restriction in β-TrCP-depleted cells.</p> <p>Conclusions</p> <p>These results demonstrate that β-TrCP is not strictly required for Vpu's ability to counteract the CD317-imposed virion release block and support the relevance of cell surface down-modulation of the restriction factor as a central mechanism of Vpu antagonism. Moreover, we propose the existence of a critical, yet to be identified cellular factor that interacts with Vpu via its di-serine motif to alter the trafficking of the restriction factor.</p

Coupling of morphological instability and kinetic instability: Chemical waves in hydrogen oxidation on a bimetallic Ni/Rh(111) surface

The oxidation and reduction of a bimetallic Ni/Rh model catalyst during the water forming O2+H2 reaction is studied with low-energy electron microscopy, microspot-low-energy electron diffraction, and x-ray photoemission electron microscopy. Oxidation of a submonolayer Ni film results in the formation of three-dimensional (3D) NiO nanoparticles. Reduction of 3D-NiO in H2 produces a dispersed two-dimensional film of metallic Ni. Chemical waves during the O2+H2 reaction involve a cyclic transformation between 3D-NiO and 2D-NiO

Empowering Women in Integrated Crop-Livestock Farming through Innovation Platforms: Experience in Semi-arid Zimbabwe

Empowering Women in Integrated Crop-Livestock Farming through Innovation Platforms has drawn upon the experience gained during the ZimCLIFS project in semi-arid Zimbabwe, funded by the Australian Centre for International Agricultural Research, to introduce legume fodder crops like Mucuna as a better livestock feed, to improve market linkages for farmers to benefit from higher livestock productivity and to enhance knowledge and skills among support services enabling upscaling of the impacts of the integrated crop-livestock systems. In particular, it looks at how agricultural innovation platforms of commodities important for women farmers (goat, groundnut) can empower women in this rural context. In rural Zimbabwe, women farmers face multiple challenges such as degraded soil, lack of labor, inaccessible/far-off markets to sell their produce/livestock, inadequate supplies of resources (seeds/fertilizers), as also climate change-related issues such as frequent droughts and dry spells. Innovation platforms that support integrated crop-and-livestock systems and particularly increase women farmers’ ability to grow and monetize high-value crops and livestock can significantly mitigate the challenges faced by them and improve their livelihoods. By encouraging women to participate in processes of market development, capacity building and infrastructure improvement, a gender-sensitive framework can be implemented and maintained sustainably..

Goat production and marketing: Baseline information for semi-arid Zimbabwe

The role of livestock in rural communities is changing rapidly. Goats are increasingly used to augment cash income and enhance food security, thus serving as an important component in household’s livelihood strategies, particularly in drought-prone areas. While much has been done to improve agricultural production in the small-scale sector, little is known and documented about the current state of goat production and marketing in the communal areas of Zimbabwe. This report aims to set a baseline of the current status of goat production and marketing in southwestern Zimbabwe. The report illustrates the main functions of goats. Basic statistics describe goat ownership patterns with regard to the socioeconomic profi les of goat keepers, and the responsibilities of different household members in day-to-day goat management and marketing. The current productivity of goat fl ocks and seasonal trends are illustrated, indicating major challenges that farmers face in goat production and marketing. The report then provides a detailed description of the current management practices and existing marketing systems, and illustrates differences between districts, proximity to markets, flock sizes, levels of education and gender. Goat mortality has been found to be the most important constraint. Farmers with few goats are unable to sustain their flocks, whereas those with larger flocks do not realize the potential benefits from goats due to high mortality rates. Poor access to animal health support, dry season feed shortages and inadequate housing are the most important immediate factors contributing to high mortalities and can generally be ascribed to a lack of information and poor service structures, both resulting from limited support given to the small stock sector by government and NGO support services. The study also shows that although many farmers attempt to sell goats, markets are underdeveloped, infrastructure is inadequate and market information is not readily available. This results in poor confidence in markets, high transaction costs and low prices for goats. It is hypothesized that improved market access will act as an incentive for farmers to invest more in goat production. Market development is thus singled out as the next important step in further developing the goat industry in Zimbabwe. Practical options to enhance the contribution of goats to food security and income growth are discussed, and priority interventions are recommended to service providers, development agents and policymakers in Zimbabwe