Granular Target Research Articles

Access to HIV prevention services in East African cross-border areas: a 2016-2017 cross-sectional bio-behavioural study.

IntroductionEast African cross‐border areas are visited by mobile and vulnerable populations, such as men, female sex workers, men who have sex with men, truck drivers, fisher folks and young women. These groups may not benefit from traditional HIV prevention interventions available at the health facilities where they live, but may benefit from services offered at public venues identified as places where people meet new sexual partners (e.g. bars, nightclubs, transportation hubs and guest houses). The goal of this analysis was to estimate availability, access and uptake of prevention services by populations who visit these venues.MethodsWe collected cross‐sectional data using the Priorities for Local AIDS Control Efforts sampling method at cross‐border locations near or along the land and lake borders of Kenya, Rwanda, Tanzania and Uganda from June 2016–February 2017. This bio‐behavioural survey captured information from a probability sample of 11,428 individuals at 833 venues across all areas. Data were weighted using survey sampling weights and analysed using methods to account for the complex sampling design.ResultsAmong the 85.6% of persons who had access to condoms, 60.5% did not use a condom at their last anal or vaginal sexual encounter. Venues visited by high percentages of persons living with HIV were not more likely than other venues to offer condoms. In 12 of the 22 cross‐border areas, male or female condoms were available at less than 33% of the venues visited by persons having difficulty accessing condoms. In 17 of the 22 cross‐border areas, education outreach visits in the preceding six months occurred at less than 50% of the venues where participants had low effective use of condoms.ConclusionsIndividuals visiting venues in cross‐border areas report poor access to and low effective use of condoms and other prevention services. Availability of HIV prevention services differed by venue and population type and cross‐border area, suggesting opportunities for more granular targeting of HIV prevention interventions and transnational coordination of HIV programming.

Dynamics of oblique impact in a quasi two-dimensional granular medium

When a solid projectile impacts a granular target, it experiences a drag force and abruptly comes to rest as its momentum transfers to the grains. An empirical drag force law successfully describes the force experienced by the projectile, and the corresponding grain-scale mechanisms have been deciphered for normal impacts. However, there is little work exploring non-normal impacts. Accordingly, we extend studies to explore oblique impact, in which a significant horizontal component of the drag force is present. In our experiments, a projectile impacts a quasi-two-dimensional bed of bidisperse photoelastic grains. We use high-speed imaging to measure high-resolution position data of the projectile trajectory and simultaneously visualize particle-scale force propagation in the granular medium. When the impact angle becomes important, the spatial structure of the stress response reveals relatively weak force chain propagation in the horizontal direction. Based on these observations, we describe the decrease of the inertial drag force with impact angle.

Central uplift collapse in acoustically fluidized granular targets: Insights from analog modeling

AbstractDepending on their sizes, impact craters have either simple or complex geometries. Peak‐ring craters such as the Chicxulub impact structure possess a single interior ring of peaks and hills and a flat interior floor. The exact mechanisms leading to the formation of a morphological peak‐ring are still a matter of debate. In this study, analog modeling was used to study the flow field of a collapsing central uplift. A 3‐D‐printed cast was used to bring the analog material in the shape of an overheightened central uplift that was based on numerical modeling. The cast was then quickly removed and the central peak collapsed, forming a flattened broad mound that spread out onto the annular moat of the crater cavity. A subwoofer was used to fluidize the granular target material. The kinematics of the collapse were analyzed with the aid of particle image velocimetry, revealing a downward and outward collapse of the central uplift. This mode of collapse is partly in agreement with numerical models, in particular for the initial and middle phases. The overthrusting of the collapsing central peak onto the inward moving crater floor predicted by numerical modeling was observed, though to a lesser degree. A peak‐ring, however, could not be reproduced since the collapse came to a halt before the central peak was completely leveled. Nevertheless, the method provides qualitative insights into the kinematics of collapse phenomena. This experimental study provides independent support of the theory of acoustic fluidization, in addition to numerical simulations.

3D Localization for Light-Field Microscopy via Convolutional Sparse Coding on Epipolar Images.

Light-field microscopy (LFM) is a type of all-optical imaging system that is able to capture 4D geometric information of light rays and can reconstruct a 3D model from a single snapshot. In this paper, we propose a new 3D localization approach to effectively detect 3D positions of neuronal cells from a single light-field image with high accuracy and outstanding robustness to light scattering. This is achieved by constructing a depth-aware dictionary and by combining it with convolutional sparse coding. Specifically, our approach includes 3 key parts: light-field calibration, depth-aware dictionary construction, and localization based on convolutional sparse coding (CSC). In the first part, an observed raw light-field image is calibrated and then decoded into a two-plane parameterized 4D format which leads to the epi-polar plane image (EPI). The second part involves simulating a set of light-fields using a wave-optics forward model for a ball-shaped volume that is located at different depths. Then, a depth-aware dictionary is constructed where each element is a synthetic EPI associated to a specific depth. Finally, by taking full advantage of the sparsity prior and shift-invariance property of EPI, 3D localization is achieved via convolutional sparse coding on an observed EPI with respect to the depth-aware EPI dictionary. We evaluate our approach on both non-scattering specimen (fluorescent beads suspended in agarose gel) and scattering media (brain tissues of genetically encoded mice). Extensive experiments demonstrate that our approach can reliably detect the 3D positions of granular targets with small Root Mean Square Error (RMSE), high robustness to optical aberration and light scattering in mammalian brain tissues.

Formation of a hidden cavity below droplets impacting on a granular substrate

Droplet impact on a granular layer results in various morphologies of the liquid–grain mixture. Some are concentrated and highly curved, some are extended and flatter. No matter how the morphology looks from the top, it is generally believed that its bottom is tightly connected to the concavely deformed granular target. In this paper we report the discovery of a hidden cavity below a droplet residual, formed upon impact on packings of hydrophilic grains and exposed by X-ray tomography. Its occurrence in the parameter space is explored. We elucidate the mechanism leading to this counterintuitive phenomenon using a dual-curvature model and an energy criterion. This research may shed new light onto the ongoing discussion about the origin of the so-called fossilized raindrop impressions.

Pattern of Impact-induced Ejecta from Granular Targets with Large Inclusions

Abstract We performed impact experiments to observe patterns in an ejecta curtain with targets consisting of small sand particles and large inclusions comparable to or smaller than the size of the projectiles. The spatial intensity distributions in the ejecta at early stages of crater formation depend on the size of the inclusions. Our numerical simulations of radially spreading particles with different sizes support this result. Based on the results, we proposed a procedure for evaluating the subsurface structures of celestial bodies from the images of ejecta curtains obtained from space-impact experiments.

High-temperature tribological behavior of tungsten

The tribological performance of tungsten is of extremely importance for the design and optimum of the gravity-driven tungsten granular target, which is being considered as the most promising high-power target for Accelerator-Driven System. In this work, therefore, we investigated the friction and wear behavior of the rolled tungsten in the temperature range from room temperature to 1000 °C. Results indicated that the friction coefficient of tungsten tends to decrease with increasing time and temperature whereas the wear rate reaches maximum at 250 °C and minimum at RT. Correspondingly, wear mechanisms of tungsten at different temperatures were also investigated in details.

Stopping of porous projectiles in granular targets

ABSTRACT Using granular mechanics, we determine the stopping force acting on spherical granular projectiles impinging on a flat granular bed. We find that the stopping force is proportional to the impact energy, as in Poncelet’s law. For fixed velocity, it is proportional to the projectile cross-sectional area rather than to its volume. These dependences only hold in the early stages of stopping, before the projectile has been strongly fragmented. Analogies to the stopping of atomic clusters in compact matter are pointed out.

Impact-Induced Energy Transfer and Dissipation in Granular Clusters under Microgravity Conditions.

The impact-induced energy transfer and dissipation in granular targets without any confining walls are studied by microgravity experiments. A solid projectile impacts into a granular target at low impact speed (0.045≤v_{p}≤1.6 m s^{-1}) in a laboratory drop tower. Granular clusters consisting of soft or hard particles are used as targets. Porous dust agglomerates and glass beads are used for soft and hard particles, respectively. The expansion of the granular target cluster is recorded by a high-speed camera. Using the experimental data, we find that (i)a simple energy scaling can explain the energy transfer in both soft-particle and hard-particle granular targets, (ii)the kinetic impact energy is isotropically transferred to the target from the impact point, and (iii)the transferred kinetic energy is 2%-7% of the projectile's initial kinetic energy. The dissipative-diffusion model of energy transfer can quantitatively explain these behaviors.

DEM/FEM simulation for impact response of binary granular target and projectile

Three-dimensional dynamic discrete element method (DEM) and finite element method (FEM) simulation using LS-DYNA were, respectively, applied to the randomly distributed binary granular material and to the cylindrical projectiles in order to clarify the effect of size distribution of target particles on the dynamic behaviour of low density granular material. It was found that the peak resistance force of projectile during penetration depends on the packing density of the granular materials and on the impact velocity of projectile. The change of resistance force was well understood in connection with the propagation and the reflection of stress wave in projectile. The variety of particle size almost did not affect the resistance force of projectile. Densified region in granular material was generated ahead of projectile after impact. The densified region propagated in the depth direction and its propagation speed depended on the packing density of target granular material and impact velocity of projectile. That is, projectile impact behaviours of granular materials can be uniformly handled by impact velocity and packing density of the granular material, not depending on the variety of the particle size.

The optimization on neutronic performance of the granular spallation target by using low-density porous tungsten

The concept of gravity-driven Dense Granular Target (DGT) has been proposed and tungsten granule is one of the candidate target materials. With the increase of DGT diameter, the source efficiency is significantly reduced, which is not conducive to the design of high-power. Low density porous tungsten particles are proposed as the target material of DGT instead of the natural tungsten. First, the neutronics of the bare target have been studied with porous tungsten granule density using Monte Carlo code. An ADS model was then set up to evaluate the neutron sources performance caused by density variation of porous tungsten granule. The results show that there is an optimal combination of tungsten granular density and target diameter to make the utmost of leakage neutrons. In the ADS driven by tungsten DGT, an appropriate density porous tungsten as the target material can effectively increase the source efficiency, reduce the beam requirement and the thermal deposition on the target.

Numerical study on the inner temperature measurement for the target in CiADS

Abstract The gravity-driven dense granular target (DGT) was proposed as a new concept for a high-power spallation target in China initiative Accelerator Driven System (CiADS). A DGT has several attractive features, coexisting with some new challenges such as the inner temperature measurement for the target. It has been analyzed and proved that a traditional method cannot be used for the inner temperature measurement. A natural viewpoint is that measuring the temperature by means of the other physical quantities. In this paper, the minimum velocity distribution along the radial direction was firstly gained by investigating the relationship between the velocity and energy deposited in grains. Then, the relationship between the temperature increment and flow rate was studied based on the velocity and flow characteristics of grains in the target. At the same time, the temperature field at a specific flow rate was also studied. The method was finally proved feasible and practical theoretically. The trial of the research thought is new and also suitable for other cases similar with the one in this paper.

Energy deposition and neutron flux study in a gravity-driven dense granular target (DGT) with GEANT4 toolkit

Abstract China initiative Accelerator Driven System (CiADS) has been approved as a strategic plan to build an ADS demonstration facility in the next few years. It proposed a new concept for a high-power spallation target: the gravity-driven dense granular target (DGT). As the same with a monolithic target (MT), both solid and liquid target, energy deposition and neutron flux are two critical issues. In this paper, we focus on these two issues and long for some valuable results for the project. Unlike a solid target, the internal geometry structure of a DGT is very complicated. To be as much as closer with the reality, we designed an algorithm and firstly packed the grains randomly in a cylindrical container in GEANT4 software. The packing result was in great agreement with the experimentally measured results. It shows that the algorithm is practicable. In the next step, all the simulations about energy deposition and neutron flux of a DGT were performed with the GEANT4 codes, and the results were compared with the data of a MT. Compared to a MT, a DGT has inarguable advantages in both terms of energy deposition and neutron flux. In addition, the simulations with different radius of grains were also performed. Finally, we found that both the energy deposition and neutron flux are nearly irrelevant to the radius of the grains in the range of 0.5 mm–5 mm when the packing density is same by analyzing the results meticulously.

Numerical analysis of the granular flow and heat transfer in the ADS granular spallation target

The Accelerator-Driven System (ADS) is one of the most effective tools to deal with the spent fuels by nuclear transmutation. In the ADS, the neutrons could be produced by bombarding a proper spallation target with a concentrated beam of high-energy accelerator protons. Subsequently, the produced neutrons drive the nuclear transmutation in the sub-critical reactor. Therefore, the spallation target is an essential part of the ADS. Recently researchers proposed a gravity-driven dense granular target, which has both solid and fluid target characteristics.This study developed the FOCUS code to simulate the granular flow and heat transfer behaviors involved in the gravity-driven dense granular target. A solid sedimentation experiment was performed and simulated by FOCUS code. The simulation results were consistent with the findings in the experiment, which proved FOCUS to be an appropriate code to simulate the granular flow. Then the sensitivity analyses were performed to investigate the influence of the inlet flow rate and the target area shape on the state of the granular flow in the ADS granular spallation target. The simulation results showed that the velocity of each granular spallation target was related with the height of piled-up particles in the targets area. The geometry size of the target area obviously affected the flow state of targets. Meanwhile the inlet flow rate had a slight influence on the stable flow, unless the inlet flow was so large that targets overflow the target area. Furthermore, the study also calculated the heat transfer between each target and found that heat conduction between two contacted granular spallation targets was the main process in the targets area. The present work would be instructive for the ADS system design.

Validation of the idea of granular flow target: A beam coupling test

The concept of granular flow target was proposed very recently and there are many potential advantages of the target on flow constancy, energy deposition and heat removal, etc. In this work first loop of granular flow target is built and an electron beam coupling experiment is reported, which aims to test the flow constancy, energy deposition and heat removal of the granular materials. The results show a good constancy of the dense granular flow and indicate granular flow as a candidate for energy removal. Moreover, there is a consistence in flow rate and temperature rise between experimental and numerical studies. This work supports the idea of granular target and is valuable for the following ADS facilities and granular flow target design.

Toward high-efficiency and detailed Monte Carlo simulation study of the granular flow spallation target

The dense granular flow spallation target is a new target concept chosen for the Accelerator-Driven Subcritical (ADS) project in China. For the R&D of this kind of target concept, a dedicated Monte Carlo (MC) program named GMT was developed to perform the simulation study of the beam-target interaction. Owing to the complexities of the target geometry, the computational cost of the MC simulation of particle tracks is highly expensive. Thus, improvement of computational efficiency will be essential for the detailed MC simulation studies of the dense granular target. Here we present the special design of the GMT program and its high efficiency performance. In addition, the speedup potential of the GPU-accelerated spallation models is discussed.

Neutronics analysis of uranium compounds spallation target using Monte Carlo simulation

In this paper, we investigated the neutronics performance of uranium carbide (UC), uranium nitride (UN) and uranium dioxide (UO2) monolith targets and granular targets by using GPU-based Monte Carlo particle transport program (GMT). Effects of the target thickness and diameter on leaked neutron yield of monolith targets irradiated by 1GeV protons were studied; the leaked neutron yield and neutron spectra for the dense granular uranium compounds and UC-kernel TRISO type targets were presented. Results show that, with an optimal dimension ϕ25×50cm, uranium compounds target can produce 40% more leaked neutron yield than the tungsten target. Results also show that by 1GeV protons incident on, our designed carbon and SiC layers coated UC-kernel TRISO type pellets target with about 70% kernel volume fraction can produce about 30% more leaked neutron yield than the granular tungsten target in their optimal diameters.

Neutronics performance and activation calculation of dense tungsten granular target for China-ADS

Abstract Spallation target, which constitutes the physical and functional interface between the high power accelerator and the subcritical core, is one of the most important components in Accelerator Driven Subcritical System (ADS). In this paper, we investigated the neutronics performance, the radiation damage and the activation of dense tungsten granular flow spallation target by using the Monte Carlo programs GMT and FLUKA at the proton energy of 250 MeV with a beam current of 10 mA . First, the leaking neutron yield, leaking neutron energy spectrum and laterally leaking neutron distribution at several time nodes and with different target parameters are explored. After that, the displacement per atom (DPA) and the helium/hydrogen production for tungsten grains and structural materials with stainless steel 316L are estimated. Finally, the radioactivity, residual dose rate and afterheat of granular target are presented. Results indicate that granule diameter below 1 cm and the beam profile diameter have negligible impact on neutronics performance, while the target diameter and volume fraction of grain have notable influence. The maximum DPA for target vessel (beam tube) is about 1.0 (1.6) DPA/year in bare target, and increased to 2.6 (2.8) DPA/year in fission environment. Average DPA for tungsten grains is relatively low. The decline rate of radioactivity and afterheat with cooling time grows with the decrease of the irradiation time.

Numerical study on cooling characteristics in granular and liquid spallation targets

Accelerator Driven System (ADS) has been developed to provide external neutron source for transmutation and multiplication to handle issues of short nuclear fuel supply and increased nuclear waste. With heavy nuclides bombarded by high-energy proton beam, neutron yields in spallation material which is accompanied by formidable energy deposition. Due to little survival of solid target from high power density and many unresolved issues in liquid target, granular target is motivated with higher sustainable power density. Although many investigations into cooling capability in spallation targets have been conducted, little literature is available on granular target which merits further study.In the present study, investigation is focused on cooling characteristics of granular target compared with liquid target based on the same target model. Due to combination of granules and fluid, coupled CFD-DEM method is employed to simulate movement and heat exchange for granular target, but only CFD technique for liquid target. Directed against same studied domain, comparison and analysis are carried out for two types of targets in the present study. Our research indicates that the granular target owns higher safety margin to withstand severe conditions especially under high energy deposition. The present findings may help to clarify and confirm the safe and reliable spallation target for some specific applications.

Low-velocity impact cratering experiments in granular slopes

Low-velocity impact cratering experiments are conducted in sloped granular targets to study the effect of the slope angle θ on the crater shape and its scales. We use two types of granular matter, sand and glass beads, former of which has a larger friction coefficient μs=tanθr, where θr is the angle of repose. Experiments show that as θ increases, the crater becomes shallower and elongated in the direction of the slope. Furthermore the crater floor steepens in the upslope side and a thick rim forms in the downslope side, thus forming an asymmetric profile. High-speed images show that these features are results of ejecta being dispersed farther towards the downslope side and the subsequent avalanche which buries much of the crater floor. Such asymmetric ejecta dispersal can be explained by combining the Z-model and a ballistic model. Using the topographic maps of the craters, we classify crater shape regimes I-III, which transition with increasing θ : a full-rim crater (I), a broken-rim crater (II), and a depression (III). The critical θ for the regime transitions are larger for sand compared to glass beads, but collapse to close values when we use a normalized slope θ^=tanθ/tanθr. Similarly we derive θ^-dependences of the scaled crater depth, length, width and their ratios which collapse the results for different targets and impact energies. We compare the crater profiles formed in our experiments with deep craters on asteroid Vesta and find that some of the scaled profiles nearly overlap and many have similar depth / length ratios. This suggests that these Vestan craters may also have formed in the gravity regime and that the formation process can be approximated by a granular flow with a similar effective friction coefficient.