Dense Packs Research Articles

Antarctic spring ice-edge blooms observed from space by ICESat-2

Polar ocean ecosystems are experiencing rapid environmental change, but measuring the associated phytoplankton responses is challenging using traditional satellite passive ocean color measurements due to signal contamination from clouds and sea ices and to low solar elevation angles. Active satellite lidar measurements allow retrieval of ocean phytoplankton properties under conditions prohibitive to passive ocean color sensors. The ICESat-2 satellite lidar measurements provide two-dimensional distributions of upper ocean phytoplankton properties. The spring phytoplankton blooms extending about 230 km horizontally from dense packs ice near Antarctic marginal ice zones and 15 m vertically below ocean surface are observed from space for the first time. Our findings highlight the advantages of satellite lidar technology for understanding high latitude plankton ecology and biogeochemistry.

Chemical characterization of pterosaur melanin challenges color inferences in extinct animals

Melanosomes (melanin-bearing organelles) are common in the fossil record occurring as dense packs of globular microbodies. The organic component comprising the melanosome, melanin, is often preserved in fossils, allowing identification of the chemical nature of the constituent pigment. In present-day vertebrates, melanosome morphology correlates with their pigment content in selected melanin-containing structures, and this interdependency is employed in the color reconstruction of extinct animals. The lack of analyses integrating the morphology of fossil melanosomes with the chemical identification of pigments, however, makes these inferences tentative. Here, we chemically characterize the melanin content of the soft tissue headcrest of the pterosaur Tupandactylus imperator by alkaline hydrogen peroxide oxidation followed by high-performance liquid chromatography. Our results demonstrate the unequivocal presence of eumelanin in T. imperator headcrest. Scanning electron microscopy followed by statistical analyses, however, reveal that preserved melanosomes containing eumelanin are undistinguishable to pheomelanin-bearing organelles of extant vertebrates. Based on these new findings, straightforward color inferences based on melanosome morphology may not be valid for all fossil vertebrates, and color reconstructions based on ultrastructure alone should be regarded with caution.

A spatially resolved fluid-solid interaction model for dense granular packs/soft-sand

Fluid flow through dense granular packs or soft sands can be described as a Darcy’ s flow for low injection rates, as the friction between grain-grain and grain-walls dominate the solid system behaviour. For high injection rates, fluid forces can generate grain displacement forming flow channels or “fractures”, which in turn modify local properties within the system, such as permeability and stress distribution. Due to this kind of “self organized” behaviour, a spatially resolved model for these interactions is required to capture the dynamics of these systems. In this work, we present a resolved model based on the approach taken by the CFDEM open source project which uses LIGGGHTS – a discrete elements method (DEM)– to model the granular behaviour and OpenFoam finite volume library for computational fluid dynamics (CFD), to simulate the fluid behaviour. The capabilities provided by the DEM engine allows the properties of the solid phase, such as inter-grain cohesion and solid confinement stress to be controlled. In this work the original solver provided by the CFDEM project was modified so as to deal with dense granular packs more effectively. Advantages of the approach presented are that it does not require external “scaling parameters” to reproduce well known properties of porous materials and that it inherits the performance provided by the CFDEM project. The model is validated by reproducing the well-known properties of static porous materials, such as its permeability as a function of the system porosity, and by calculating the drag coefficient for a sphere resting inside a uniform flow. Finally, we present fracture patterns obtained when modelling water injection into a Hele-Shaw cell, filled with a dense granular pack.

Particula: A simulator tool for computational rock physics of granular media

Granular dynamics simulations provide insights to the contact-scale physics of loose sediments. However, simulations using identical spherical grains do not reflect characteristics observed in natural sediments, such as pack sorting, grading, grain sphericity, and grain roundness. We have developed software to create 3D grain packs of a range of regular and irregular shapes with geologically realistic variations in sorting and grading. An efficient approach is used to create multiple realizations of nonspherical irregularly shaped grains using coherent noise modification of the spherical grain surface. The discrete-element method is used to assemble the grain pack with different depositional styles by letting grains fall under the influence of gravity. Characterization of various parameters of random loose and dense grain packs, and comparison with previous studies, helps to establish the validity, flexibility, and consistency of the simulator. The output of this software is a digital grain pack, including metadata such as contacts and coordinates, that can be studied further using other analysis tools, e.g., by conducting fluid flow, mechanical, or electrical simulations.

Task-Related Edge Density (TED)-A New Method for Revealing Dynamic Network Formation in fMRI Data of the Human Brain.

The formation of transient networks in response to external stimuli or as a reflection of internal cognitive processes is a hallmark of human brain function. However, its identification in fMRI data of the human brain is notoriously difficult. Here we propose a new method of fMRI data analysis that tackles this problem by considering large-scale, task-related synchronisation networks. Networks consist of nodes and edges connecting them, where nodes correspond to voxels in fMRI data, and the weight of an edge is determined via task-related changes in dynamic synchronisation between their respective times series. Based on these definitions, we developed a new data analysis algorithm that identifies edges that show differing levels of synchrony between two distinct task conditions and that occur in dense packs with similar characteristics. Hence, we call this approach “Task-related Edge Density” (TED). TED proved to be a very strong marker for dynamic network formation that easily lends itself to statistical analysis using large scale statistical inference. A major advantage of TED compared to other methods is that it does not depend on any specific hemodynamic response model, and it also does not require a presegmentation of the data for dimensionality reduction as it can handle large networks consisting of tens of thousands of voxels. We applied TED to fMRI data of a fingertapping and an emotion processing task provided by the Human Connectome Project. TED revealed network-based involvement of a large number of brain areas that evaded detection using traditional GLM-based analysis. We show that our proposed method provides an entirely new window into the immense complexity of human brain function.

Rapid Generation of Particle Packs at High Packing Ratios for DEM Simulations of Granular Compacts

In this work we present a simple, fast technique for generating particle packs at high packing ratios aiming at the simulation of granular compacts via the discrete element method (DEM). We start from a random sequence addition particle generation algorithm to generate a layer of non-overlapping spherical particles that are let to evolve dynamically in time under the action of compacting or jamming pseudo forces. A layer-by-layer approach is then followed to generate multiple layers on top of each other. In the end, very dense packs with pre-defined bulk shapes and sizes (e.g. rectangles in two dimensions and prisms in three dimensions) are achieved. The influence of rolling motion (with particle rotation and spin) along with inter-particle friction on the density and ordering of the generated packs is assessed. Both congruent and inhomogeneous packs (with respect to particle sizes) are created and their packing properties evaluated. We believe that simple techniques for fast generation of particle packs at high packing ratios are essential tools for the DEM simulation of granular compacts.

Use of immunohistochemical markers for the diagnosis of fibrosarcoma skin in bovines

Use of immunohistochemical markers for the diagnosis of fibrosarcoma skin in bovines

Pore scale coupling of fluid displacement and unconsolidated sediment mechanics

In unconsolidated sediments, capillary pressure in the pore space may be large enough to move grains apart during drainage. This motion alters the pore throat sizes which control subsequent displacement in the sediment. We present two grain scale models for this coupled process. In both cases, we assume fluid interfaces are controlled by capillary forces, and determine the detailed geometry of those interfaces. We compute the net force exerted on each grain by capillary pressure, including cohesion at grain contacts supporting pendular rings. We combine those forces to determine the movement of grains using a a kinematic model b a rigorous model where mechanical stress and elastic properties of grains are included via a discrete element method. This is the first coupling in 3D, albeit in samples of limited size. Preliminary results in disordered dense sphere packs suggest the development of high permeability gas channels rather than planar structures.

Elastic weakening of a dense granular pack by acoustic fluidization: Slipping, compaction, and aging

Sound velocity measurements in dense glass bead packs reveal significant softening effect at large amplitudes, due to the frictional nonlinearity at the grain contacts. Beyond a certain amplitude, the sound-matter interaction becomes irreversible, leaving the medium in a weakened and slightly compacted state. A slow recovery of the initial elastic modulus is observed after acoustic perturbation, revealing the plastic creep growth of microcontacts. The cross-correlation function of configuration-specific acoustic speckles highlights the relationship between the macroscopic elastic weakening and the local change of the contact networks, induced by strong sound vibration, in the absence of appreciable grain motion.

Using level sets for creating virtual random packs of non-spherical convex shapes

Random packs of spheres have been used to model heterogeneous and porous material morphologies during simulations of physical processes such as burning of coal char, convective burning in porous explosives, and regression of solid rocket propellant. Sphere packs have also been used to predict thermo-mechanical properties, permeability, packing density, and dissolution characteristics of various materials. In this work, we have extended the Lubachevsky-Stillinger (LS) sphere packing algorithm to create polydisperse packs of non-spherical shapes for modeling heterogeneity in complex energetic materials such as HMX and pressed gun propellants. In the method, we represent the various particle shapes using level sets. The LS framework requires estimates of inter-particle collision times, and we predict these times by numerically solving a minimization problem. We have obtained results for dense random packs of various convex shapes such as cylinders, spherocylinders, and polyhedra, and we show results with these various particles packed together in a single pack to high packing fraction.

Health-dependent vulnerability to predation affects escape responses of unguarded chinstrap penguin chicks

Predators may select more often to attack the more vulnerable prey or those with an inferior health status. Thus, prey should be able to assess their own vulnerability to predation and modify their antipredatory behavior accordingly. When approached by predator skuas, unguarded penguin chicks flee short distances, and usually aggregate in dense packs, but there is a clear interindividual variability in their responses under similar conditions. We hypothesized that this variability in escape responses might be related to the perceived vulnerability to predation of each individual chick. We simulated predator attacks to chinstrap penguin chicks and analyzed the sources of variation in their escape response, such as the presence of adults or the density of other chicks, and the sex, age, body condition, and health status of responding chicks. Chicks allowed shorter approach distances when they had a better health condition (i.e., a greater T-cell-mediated immunity, CMI), when they were younger, and when the density of adults around was higher. Sex and density of other chicks were not important. Similarly, chicks fled from the experimenter to longer distances when they had a lower CMI and when the density of adults was lower. Therefore, escape characteristics of chicks depended on the presence of adults that can deter predators and on the health-dependent vulnerability of chicks.

Haliangium ochraceum gen. nov., sp. nov. and Haliangium tepidum sp. nov.: novel moderately halophilic myxobacteria isolated from coastal saline environments.

Phenotypic and phylogenetic studies were performed on two myxobacterial strains, SMP-2 and SMP-10, isolated from coastal regions. The two strains are morphologically similar, in that both produce yellow fruiting bodies, comprising several sessile sporangioles in dense packs. They are differentiated from known terrestrial myxobacteria on the basis of salt requirements (2-3% NaCl) and the presence of anteiso-branched fatty acids. Comparative 16S rRNA gene sequencing studies revealed that SMP-2 and SMP-10 are genetically related, and constitute a new cluster within the myxobacteria group, together with the Polyangium vitellinum Pl vt1 strain as the closest neighbor. The sequence similarity between the two strains is 95.6%. Based on phenotypic and phylogenetic evidence, it is proposed that these two strains be assigned to a new genus, Haliangium gen. nov., with SMP-2 designated as Haliangium ochraceum sp. nov. (= JCM 11303(T) = DSM 14365(T)), and SMP-10 as Haliangium tepidum sp. nov. (= JCM 11304(T)= DSM 14436(T)).

Development of shear bands in annular shear granular flows

ABSTRACTUsing hard-disk simulations of relatively dense packs of mono-sized system in an annular Couette geometry the formation of dilute regions inside the granular media, namely shear bands, are investigated. The results represent the influence of entire system characteristics such as solid area fraction and shear rate on the development of shear bands as well as the local properties of grains that cause them to participate in the formation of a shear band. Moreover, simulations have been performed for binary-sized system, which revealed that the formation of such diluted shear bands is unlikely.

Electron-impact inner-shell excitation of Fe XVI

We report R-matrix calculations of electron-impact collision strengths for excitation from the inner L-shell into doubly excited states of Fe XVI. The basic close-coupling expansion used includes 59 LS terms of the target ion dominated by the configurations 2p63s, 2p63p, 2p63d, 2p53s2, 2p53s3p, 2p53s3d, 2p53p2 and 2p53p3d. We investigate the convergence of the collision strengths with the number of partial waves included in the expansion, the contributions from relativistic effects and the convergence of the close-coupling expansion by using pseudo-orbitals that try to account for the effect of the infinite number of levels that lie between the bound and doubly excited states. The present collision strengths exhibit dense packs of high resonances that enhance the effective collision strengths by up to three orders of magnitude. Also important are relativistic effects which alter both the resonance structures and background cross sections, changing the effective collision strengths by up to several orders of magnitude. In addition, a large number of carefully optimized pseudo-orbitals are necessary to achieve convergence in the target expansion. By taking into account the bound states not explicitly included in the expansion, the collision strengths into the doubly excited levels are reduced by up to more than a decade in some cases.

Horizontal Fracture Design Based on Propped Fracture Area

Abstract Present fracture design procedures are based on the total fracture area created. A method to distinguish between total area and the propped or effective fracture area has not been available. This paper presents a solution to this problem, applicable to horizontal fractures. The differences between effective fracture area and total area are demonstrated in example calculations. This work is based on experimentally determined transport efficiencies of solids in sand-liquid slurries. Newtonian and non-Newtonian systems are considered. Introduction Fifteen years after commercial introduction, hydraulic fracturing remains the most successful stimulation technique in the oil field. This success is primarily due to ability of induced fractures to penetrate and alter permeabilities deep within formations. Many fields producing today could not have been developed without the hydraulic fracturing process. Because of wide usage, fracture-treatment design has received a great deal of engineering and research effort. This work, resulting in improved equipment and materials, has increased the benefits from fracture treatments as well as the applicability of the process. A major contribution was the development of fluid-loss additives. Necessarily, the number of parameters to be considered in treatment design has steadily increased, resulting in more complicated design techniques. Almost all present design procedures are based on the precepts set forth by Howard and Fast. Relating the fluid volume lost into the formation, the volume required in extending the fracture, and the total slurry volume injected. They developed an expression for the total fracture area created in terms of pertinent treatment parameters. Fluid loss during treatment was expressed as a function of time for three flow mechanisms. Although modifications of fluid loss equations have been made, the total fracture area concept has remained unaltered. A vast amount of field data indicate that induced fractures must be propped and held open to be effective. A notable exception is the Mesa Verde formation in the San Juan basin. However, analysis of these treatments shows that improved well productivities are obtained when propping agents are incorporated in the treating fluid. Although propped fracture area has been recognized as an important design parameter, a method to distinguish between total area and effective fracture area has not been available. The necessary information on slurry-sand transport in fractures has been lacking. Interest in the propped region of induced fractures is not restricted to areal extent alone. The distribution of sand within fractures is important from the standpoint of fracture flow capacities. Flow capacity affects the increase in well productivity after stimulation. The work of Huitt and Darin shows that partial monolayers of sand have large flow capacities compared to thick, dense sand packs. It has been postulated that gelled fluids have the ability to transport sand within the fractures at the desired low concentrations. An early contribution in the area of sand placement in fractures was made by Kern et al. They studied sand movement in a transparent vertical fracture model. It was observed that the sand tended to settle out in the bottom of the model before moving very far. When the fluid velocity exceeded a certain critical value, all of the sand injected began moving through the crack even though it settled to the bottom. This critical velocity was determined under several flow conditions. Some work on sand movement in horizontal fractures has been reported in Russian publications. Sand movement was studied by Izyumova and Shan'gin using a transparent "pie-shaped" flow model to simulate a horizontal radial flow system. However, the data were limited, especially in a quantitative sense. Dorozhkin, Zheltov and Zheltov studied the behavior of sand-liquid slurries in a horizontal linear flow model. The quantitative data were restricted primarily to the thickness of sand deposits formed at the bottom of the fracture. An earlier paper provided basic data on the flow of sand in horizontal fractures. This study was designed to yield specific quantitative information on rate of advance of sand particles and pressure behavior under various flow conditions. A comprehensive photographic study was undertaken in a 10-ft windowed flow cell to provide the necessary qualitative and quantitative data. Since the number of potential variables far exceeded the capacity of the initial study, emphasis was placed on the effects of sand concentration, oil viscosity and oil flow rate. A detailed description of these experiments and the results are described in Ref. 9. However, the implications of this work on the fracture design calculations were not discussed. An analysis of these data as well as new data is provided in the following sections. EXPERIMENTAL RESULTS The primary objective of the experimental investigation was to provide information on the rate of advance of the solids in sand-liquid slurries. JPT P. 723ˆ