Numerical simulations of the flow and sediment transport regimes surrounding a short cylinder

Author Posting. © IEEE, 2007. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 32 (2007): 249-259, doi:10.1109/JOE.2007.890986.The 3-D flow field and bed stress surrounding a short cylinder in response to combined wave and mean-flow forcing events is examined. Model simulations are performed with a 3-D nonhydrostatic computational fluid dynamics model, FLOW-3D. The model is forced with a range of characteristic tidal and wave velocities as observed in 12–15 m of water at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The 2.4-m-long and 0.5-m diameter cylinder is buried 10% of the diameter on a flat, fixed bed. Regions of incipient motion are identified through local estimates of the Shields parameter exceeding the critical value. Potential areas of sediment deposition are identified with local estimates of the Rouse parameter exceeding ten. The model predictions of sediment response are in general in agreement with field observations of seabed morphology obtained over a one-week period during the 2003–2004 MVCO mine burial experiment. Both observations and simulations show potential transport occurring at the ends of the mine in wave-dominated events. Mean flows greater than 10 cm/s lead to the formation of larger scour pits upstream of the cylinder. Deposition in both cases tends to occur along the sides, near the center of mass of the mine. However, the fixed-bed assumption prohibits the prediction of full perimeter scour as is observed in nature. Predicted scour and burial regimes for a range of wave and mean-flow combinations are established.This work was supported in part by the U.S. Office of Naval Research under the mine burial project N00014-00-1-0570. The work of K. A. Hatton was supported by the National Science Foundation Graduate Research Fellowship

Charting the free energy landscape of metastable topological magnetic objects

Chiral magnets with Dzyaloshinskii-Moriya interactions feature a rich phase diagram with a variety of thermodynamical phases. These include helical and conical spin arrangements and topologically charged objects such as (anti)Skyrmions. Crucially, due to hysteresis effects, the thermodynamical phases can coexist at any given temperature and external magnetic field, typically leading to metastability of, e.g., the material’s topological phase. In this paper, we use Monte Carlo simulations to study these effects. We compute the relative free energies of coexisting states, enabling us to determine the ground state at all values of the external parameters. We also introduce a method to estimate the activation energy, i.e., the height of the energy barrier that separates the topological phase from the ground state. This is one of the key ingredients for the determination of the Skyrmion lifetime, which is relevant for technological applications. Finally, we prescribe predicting the system’s evolution through any path in the space of external parameters. This can serve as a guideline to prepare the magnetic material in any desired phase or even trigger a phase transition in an experimental setup

Computer-controlled apparatus for automated development of continuous flow methods

An automated apparatus to assist in the development of analytical continuous flow methods is described. The system is capable of controlling and monitoring a variety of pumps, valves, and detectors through an IBM PC-AT compatible computer. System components consist of two types of peristaltic pumps (including a multiple pump unit), syringe pumps, electrically and pneumatically actuated valves, and an assortment of spectrophotometric and electrochemical detectors. Details of the interface circuitry are given where appropriate. To demonstrate the utility of the system, an automatically generated response surface is presented for the flow injection determination of iron(II) by its reaction with 1,10-phenanthroline

Inverse Melting of an Electronic Liquid Crystal

Inverse melting refers to the rare thermodynamic phenomenon in which a solid melts into a liquid upon cooling, a transition that can occur only when the ordered (solid) phase has more entropy than the disordered (liquid) phase, and that has so far only been observed in a handful of systems. Here we report the first experimental observation for the inverse melting of an electronic liquid crystalline order in strontium-doped lanthanum nickelate, a compound isostructural with the superconducting cuprates, with a hole doping concentration of 1/3. Using x-ray scattering, we demonstrate that the isotropic charge modulation is driven to nematic order by fluctuating spins and shows an inverse melting transition. Using a phenomenological Landau theory, we show that this inverse melting transition is due to the interlayer coupling between the charge and spin orders. This discovery points to the importance of the interlayer correlations in the system, and provides a new perspective to study the intricate nature of the electronic liquid crystal phases in strongly correlated electronic systems, including possibly the Cu- and Fe-based high-Tc superconductors.Comment: 7 pages, 7 figure

Inverse order-disorder transition of charge stripes

We report an unusual transition behavior of charge stripes in La1.67Sr0.33NiO4 using x-ray scattering. The segregated holes in La1.67Sr0.33NiO4 are observed to form anisotropic stripes in the a × b plane of the crystal space below the transition temperature T 238 K, and at the same time, display an unusual inverse order-disorder transition along the c axis. Using a phenomenological Landau theory, we show that this inverse transition is due to the interlayer coupling between the charge and spin orders. This discovery points to the importance of the interlayer correlations in the strongly correlated electrons system

Confinement of Skyrmions in Nanoscale FeGe Device-like Structures

Skyrmion-based devices have been proposed as a promising solution for low-energy data storage. These devices include racetrack or logic structures and require skyrmions to be confined in regions with dimensions comparable to the size of a single skyrmion. Here we examine skyrmions in FeGe device shapes using Lorentz transmission electron microscopy to reveal the consequences of skyrmion confinement in a device-like structure. Dumbbell-shaped elements were created by focused ion beam milling to provide regions where single skyrmions are confined adjacent to areas containing a skyrmion lattice. Simple block shapes of equivalent dimensions were also prepared to allow a direct comparison with skyrmion formation in a less complex, yet still confined, device geometry. The impact of applying a magnetic field and varying the temperature on the formation of skyrmions within the shapes was examined. This revealed that it is not just confinement within a small device structure that controls the position and number of skyrmions but that a complex device geometry changes the skyrmion behavior, including allowing skyrmions to form at lower applied magnetic fields than in simple shapes. The impact of edges in complex shapes is observed to be significant in changing the behavior of the magnetic textures formed. This could allow methods to be developed to control both the position and number of skyrmions within device structures

In vitro activity of an aqueous allicin extract and a novel allicin topical gel formulation against Lancefield group B streptococci

Background: Studies have shown the efficacy of intra-partum antibiotics in preventing early-onset group B streptococcal sepsis. This approach results in a high intra-partum antibiotic use. Worryingly, the same antibiotics used in prophylaxis are also first-line treatment for neonatal sepsis, and antibiotic exposure in the peri-natal period has been shown to be a risk factor for late-onset serious bacterial infections and allergic disease. Antibiotic exposure in the peri-natal period is becoming a major public health issue; alternative strategies are needed. Garlic has been traditionally used to treat vaginal infections. Allicin is the main antibacterial agent isolated from garlic. Objectives: The aim of the study was to investigate the in vitro activity of a novel allicin extract in aqueous and gel formulation against 76 clinical isolates of Lancefield group B streptococci (GBS). Methods: MICs and MBCs of allicin were determined for 76 GBS isolates by agar dilution and microtitre plate methods. Killing kinetics were determined for a selected 16 of the 76 strains. Agar diffusion tests were compared for allicin liquid and gel (500 mg/L). Results and conclusions: MICs and MBCs of allicin liquid were 35 to 95 mg/L and 75 to 315 mg/L, respectively. Time/dose kill curves produced a 2–3 log reduction in cfu/mL within 3 h and no detectable growth at 8 and 24 h. A novel 500 mg/L allicin gel produced an average zone size of 23+-6 mm compared with 21+-6 mm for allicin in water. Aqueous allicin is bactericidal against GBS isolates and maintains activity in a novel gel formulation