Hemispherical Geometry Research Articles

Hemispherical Retina Emulated by Plasmonic Optoelectronic Memristors with All-Optical Modulation for Neuromorphic Stereo Vision.

Binocular stereo vision relies on imaging disparity between two hemispherical retinas, which is essential to acquire image information in three dimensional environment. Therefore, retinomorphic electronics with structural and functional similarities to biological eyes are always highly desired to develop stereo vision perception system. In this work, a hemispherical optoelectronic memristor array based on Ag-TiO2 nanoclusters/sodium alginate film is developed to realize binocular stereo vision. All-optical modulation induced by plasmonic thermal effect and optical excitation in Ag-TiO2 nanoclusters is exploited to realize in-pixel image sensing and storage. Wide field of view (FOV) and spatial angle detection are experimentally demonstrated owing to the device arrangement and incident-angle-dependent characteristics in hemispherical geometry. Furthermore, depth perception and motion detection based on binocular disparity have been realized by constructing two retinomorphic memristive arrays. The results demonstrated in this work provide a promising strategy to develop all-optically controlled memristor and promote the future development of binocular vision system with in-sensor architecture.

Experimental analysis of hemispherical combustion geometry of diesel engine (neem oil) biodiesel with nano additives

Experimental analysis of hemispherical combustion geometry of diesel engine (neem oil) biodiesel with nano additives

Understanding the explosion risk presented by ammonium nitrate and aluminium home-made explosives detonated as surface charges in hexahedral main charge containers

Ammonium nitrate and aluminium (AN–Al) has been used as a typical homemade explosive (HME) by non-state actors since the turn of the century. Despite the regulation applied to ammonium nitrate above 16% nitrogen content and an aluminium particle size below 200 µm, their use has been widespread in Afghanistan, Columbia, Iraq, Syria and Yemen. Containers used to utilise AN–Al as a man-portable improvised explosive device (IED) are typically hexahedral in shape, not the spherical or hemispherical geometries used to theorise risk mitigation. This is particularly important in post-blast investigation where explosives of a non-ideal nature are often used in non-spherical containers.Given the breadth of HME available to criminals, the explosion performance of forty hexahedral containers filled with AN–Al of unknown manufacture is examined. Performance of the AN–Al is determined through the surface detonation of these containers on alluvial soil, with apparent crater volume compared to theoretical calculations for spherical charges of TNT detonated in that same medium. A conversion factor for hexahedral main charges to spherical charges is then established to achieve more accurate predictions of the explosion risk using Kingery-Bulmash and Bowen curves. The paper provides worked examples for practical application and a methodology by which predictions of charge mass in other mediums such as asphalt can be determined.

A hyperelastic approach for modeling the membrane behavior in finite element forming simulation of unidirectional non-crimp fabrics (UD-NCF)

Unidirectional non-crimp fabrics (UD-NCFs) are highly suitable for high-performance components due to their excellent lightweight potential. However, during forming they are prone to wrinkling and gapping compared to woven or biaxial textiles. Macroscopic simulation models can be used to efficiently predict these effects as well as the global forming behavior for complex geometries. Therefore, a new hyperelastic membrane model is proposed to describe the typical deformation of UD-NCFs based on superimposed shear, transverse tension and compression perpendicular to the fiber rovings. The model is parameterized using the forces and different ratios of superimposed strains obtained in experimental off-axis-tension-tests at bias angles of 30°, 45°and 60°. The resulting approach is validated by forming simulations of a hemisphere and tetrahedron geometry in different configurations and quantitative comparison to experimental tests. The model accurately predicts the forming behavior of UD-NCF with a good agreement of the global deformation behavior and local strains.

Uncovering the dual role of dimensionless radius in buckling of spherical shells with random geometric imperfections

Localized deformation and randomly shaped imperfections are salient features of buckling-type instabilities in thin-walled load-bearing structures. However, it is generally agreed that their complex interactions in response to mechanical loading are not yet sufficiently understood, as evidenced by buckling-induced catastrophic failures which continue to today. This study investigates how the intimate coupling between localization mechanisms and geometric imperfections combine to determine the statistics of the pressure required to buckle (the illustrative example of) a hemispherical shell. The geometric imperfections, in the form of a surface, are defined by a random field generated over the nominally hemispherical shell geometry, and the probability distribution of the buckling pressure is computed via stochastic finite element analysis. Monte-Carlo simulations are performed for a wide range of the shell's radius to thickness ratio, as well as the correlation length of the spatial distribution of the imperfection. The results show that over this range, the buckling pressure is captured by the Weibull distribution. In addition, the analyses of the deformation patterns observed during the simulations provide insights into the effects of certain characteristic lengths on the local buckling that triggers global instability. In light of the simulation results, a probabilistic model is developed for the statistics of the buckling load that reveals how the dimensionless radius plays a dual role which remained hidden in previous deterministic analyses. The implications of the present model for reliability-based design of shell structures are discussed.

Density scaling approximation for Monte-Carlo simulations of radioactive plumes

The release of radioactive gas into the atmosphere can diffuse into large volumes of air downwind from the point of release. The extent of radioactivity can cover thousands of cubic meters of air. For such large volumes, the weather models used to predict the down-wind distribution of the plume and the radiation transport models used to predict the radiation reaching ground-level from the plume can take tens of hours of computer time on multi-node institutional High-Performance Computing facilities.In this paper we focus on the radiation transport aspect of plume modeling. We describe a phenomenological method for approximating the amounts of radiation that reach ground level from large volumes of a static radioactive plume that can be calculated on a stand-alone personal computer in much shorter computation times than those usually needed for such large volume evaluations. We refer to this method as the Density Scaling Approximation (DSA). Its ability to approximate ground-level count rates of large plumes comes from using a small-plume volume with a scaled-up value of air density to simulate the same number of scatterings that occur during transport in larger plume volumes at normal air density.We demonstrate the DSA by using a 100 m-diameter air-filled hemispherical dome geometry with a uniform volumetric activity of 135Xe gas throughout the air-filled volume. The DSA for a larger dome diameter is obtained by evaluating the 100 m dome with an air density scaled up by the linear ratio of the larger diameter to the 100 m diameter. We find that this approximation works well for dome diameters up to 1200 m – the largest diameter studied and a size more than sufficient for accounting for all the radiation from 135Xe. Moreover, most of our DSA results can be calculated over 500 times faster than corresponding full-sized geometry with normal air density.To help evaluate the accuracy of the DSA and gain insight into how well it can reproduce different regions of the spectra, we use three, easily understood regions of interest to compare the DSA results to the full-sized geometry at normal air density results. These regions are the full-energy peak, the region of single-Compton scattering, and the region of multiple-Compton scattering. We show how the dominance of the Compton scattering mechanism determines this division and thus provides insight into how Compton scattering is manifested in spectra from photon scattering through air in general, and how well the DSA approximation works.

Direct numerical simulation of internally heated natural convection in a hemispherical geometry

Internally heated (IH) natural convection can be found in nature, industrial processes, or during a severe accident in a light water reactor. In this accident scenario, the nuclear reactor core and some internal structures can melt down and relocate to the lower head of the reactor pressure vessel (RPV) and interact with the remaining coolant. Subsequent re-heating and re-melting under decay and oxidation heat creates a transition from a debris bed to a molten pool. The molten pool, which can involve more than hundred tons of dangerously superheated oxidic and metallic liquids, imposes thermo-mechanical loads on the vessel wall that can lead to a thermal and/or structural failure of the vessel and subsequent release of radioactive materials to the reactor pit, and can possibly make its way to the environment. This study uses Direct Numerical Simulation (DNS) to investigate homogeneous IH molten pool convection in a hemispherical domain using Nek5000, an open-source spectral element code. With a Rayleigh number of 1.6 × 1011, the highest reached through DNS in this confined hemispherical geometry, and a Prandtl number of 0.5, which corresponds to a prototypic corium, the study provides detailed information on the thermo-fluid behavior. The results show a turbulent flow with three distinct regions, consistent with the general flow observations from the BALI experiments. The study also presents detailed information on turbulence, such as turbulent kinetic energy (TKE), turbulent heat flux (THF), and temperature variance. Additionally, the study provides 3D heat flux distributions along the boundaries. The heat fluxes along the top boundary fluctuate due to the turbulent eddies in the vicinity, while along the curved boundary the heat fluxes increase nonlinearly from the bottom to the top.

Mechanical behaviour of hybrid FFRP/aluminium honeycomb sandwich structures

The combination of lightweight properties and high energy absorption capacity makes sandwich structures suitable for applications in transportation industry, such as automotive, aerospace, and shipbuilding industry. In these fields, the gas emissions reduction, fuel saving, vehicle safety and load capacity increase are currently important requirements. This research deals with the study of the mechanical behaviour of a “green” hybrid honeycomb sandwich structure made by an aluminium honeycomb core and flax fibres reinforced skins. Specific interest has been addressed to their mechanical behaviour in terms of bending properties and low-velocity impact response. For the impact tests, two indenters having hemispherical geometry with a diameter of 10 mm (H10) and 20 mm (H20) were selected. The results were compared with other hybrid structures analysed in previous studies. The static three-point bending tests produced different collapse modes, depending on the support span distance. Non-destructive evaluation was carried out for failure mode analysis by optical microscopy and digital radiography (DR). In the aluminium-flax hybrid sandwich structures it is found an increase in the impact strength of 25 % compared to the unreinforced aluminium honeycomb sandwich structures. In terms of specific energy absorption (SEA), it is interesting to underline an increase of 50 % compared to traditional AHS structures.

Study of Unsteady Surface Flowfields on and Around Turrets with Different Protrusions

Wind-tunnel experiments were conducted to measure the unsteady surface pressure field on and around a hemisphere-on-cylinder turret of varying protrusion in subsonic flow. These measurements were obtained using fast-response pressure-sensitive paint coupled with pressure transducers. The surface pressure field data resulting from Mach 0.5 flow () over a partially submerged hemisphere, a hemisphere, and a hemisphere on a cylinder were analyzed using proper orthogonal decomposition modal analysis, as well as a variant of this approach referred to as joint proper orthogonal decomposition. The results showed that decreased turret protrusion into the freestream flow reduced the prevalence of antisymmetric surface pressure field fluctuations caused by antisymmetrical vortex shedding. The frequency associated with this fluctuation was found to be around . When a partial hemispherical turret geometry was used, it was shown that the antisymmetric mode was greatly suppressed; and the wake was dominated by a symmetric mode with a broadband spectral peak at a higher frequency of . This suggests that there is a “mode switching” as the protrusion is changed from the hemisphere to the partial hemisphere configuration. An optical flow approach was used to find the convective velocity field in the wake, from which topological flow features could be identified. The size of the wake separation region was found to grow smaller with the decreased protrusion while keeping a similar shape.

Reprocessing Models for the Optical Light Curves of Hypervariable Quasars from the Sloan Digital Sky Survey Reverberation Mapping Project

We explore reprocessing models for a sample of 17 hypervariable quasars, taken from the Sloan Digital Sky Survey Reverberation Mapping project, which all show coordinated optical luminosity hypervariability with amplitudes of factors ≳2 between 2014 and 2020. We develop and apply reprocessing models for quasar light curves in simple geometries that are likely to be representative of quasar inner environments. In addition to the commonly investigated thin-disk model, we include the thick-disk and hemisphere geometries. The thick-disk geometry could, for instance, represent a magnetically elevated disk, whereas the hemisphere model can be interpreted as a first-order approximation for any optically thick out-of-plane material caused by outflows/winds, warped/tilted disks, and so on. Of the 17 quasars in our sample, 11 are best-fitted by a hemisphere geometry, five are classified as thick disks, and both models fail for just one object. We highlight the successes and shortcomings of our thermal reprocessing models in case studies of four quasars that are representative of the sample. While reprocessing is unlikely to explain all of the variability that we observe in quasars, we present our classification scheme as a starting point for revealing the likely geometries of reprocessing for quasars in our sample and hypervariable quasars in general.

High velocity impact response of sandwich structures with auxetic tetrachiral cores: Analytical model, finite element simulations and experiments

This paper investigates the energy absorption and deformation characteristics of sandwich structures with auxetic tetrachiral core under high velocity projectile impact. The finite elementmodel of the structure is developed using the Johnson-Cook material model and is used to studythe effects of projectile impacts forvelocities between150 and 450 m/s. TheFE model is validated usingin-houseexperiments on 3D printed PLA sandwich with tetrachiral core under hemispherical projectile impact and also using experimental datareported in the literature. Further, a semi-analytical model is developed to estimate the residual velocities and energy absorption of sandwich structure under blunt projectile impact, which shows good agreement with the FE results with an error of less than 8%. Finally, the performance of the sandwich structure with tetrachiral cores is studied under projectile impact for velocities ranging from 150 to 350 m/s and compared with sandwich structures with re-entrant and double arrowhead cores. The results show that for velocities above the ballistic limit, the energy absorbed by the facesheets, and core are nearly constant and is greatest for blunt projectile impact, followed by hemispherical, ogive, and conical geometries. Furthermore, sandwich structures with auxetic cores exhibit better energy absorption characteristics compared to those with hexagonal honeycomb cores.

Water Phantom Characterization of a Novel Optical Fiber Sensor for LDR Brachytherapy

This work considers the feasibility of using a novel optical fiber-based sensor, employing a terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) inorganic scintillator, as a real-time in vivo dosimetry solution for applications in low-dose-rate (LDR) prostate brachytherapy (BT). This study specifically considers the influence of scintillator geometry (hemisphere tip versus cylindrical cavity), polymethyl methacrylate (PMMA) fiber core diameter (0.5 versus 1.0 mm), and sensor housing material (stainless steel versus plastic) on the measured scintillation signal. Characterization measurements were performed using a silicon photon-multiplier (SiPM) detector and a commercial water phantom system, integrated with custom 3-D printed components to allow for precise positioning of the LDR BT radiation source with respect to the optical fiber sensor (OFS). Significant differences in the rate of fall-off in the scintillation signal, with distance from the source, were observed between the different scintillator geometries considered. The hemisphere tip geometry was shown to be the most accurate, tracking with the expected fall-off in dose-rate, within measurement uncertainty. Reducing the fiber core diameter from 1.0 to 0.5 mm resulted in a sixfold reduction in the detected scintillation signal. A further 57% reduction was observed when housing the 0.5-mm fiber within a stainless steel LDR BT needle applicator. Initial results demonstrate the feasibility of employing an OFS, for applications in LDR BT, given the excellent agreement of measurements with theoretical expectations. Furthermore, a calibration process has been described for converting the detected scintillation signal into absorbed dose/dose rate, using our water phantom-based experimental setup.

Performance evaluation of VRAIN: a brain-dedicated PET with a hemispherical detector arrangement

Objective. For PET imaging systems, a smaller detector ring enables less intrinsic spatial resolution loss due to the photon non-collinearity effect as well as better balance between production cost and sensitivity, and a hemispherical detector arrangement is more appropriate for brain imaging than a conventional cylindrical arrangement. Therefore, we have developed a brain-dedicated PET system with a hemispherical detector arrangement, which has been commercialized in Japan under the product name of VRAINTM. In this study, we evaluated imaging performance of VRAIN. Approach. The VRAIN used 54 detectors to form the main hemispherical unit and an additional half-ring behind the neck. Each detector was composed of a 12 × 12 array of lutetium fine silicate crystals (4.1 × 4.1 × 10 mm3) and a 12 × 12 array of silicon photomultipliers (4 × 4 mm2 active area) with the one-to-one coupling. We evaluated the physical performance of VRAIN according to the NEMA NU 2–2018 standards. Some measurements were modified so as to fit the hemispherical geometry. In addition, we performed 18F-FDG imaging in a healthy volunteer. Main results. In the phantom study, the VRAIN showed high resolution for separating 2.2 mm rods, 229 ps TOF resolution and 19% scatter fraction. With the TOF gain for a 20 cm diameter object (an assumed head diameter), the peak noise-equivalent count rate was 144 kcps at 9.8 kBq ml−1 and the sensitivity was 25 kcps MBq−1. Overall, the VRAIN provided excellent image quality in phantom and human studies. In the human FDG images, small brain nuclei and gray matter structures were clearly visualized with high contrast and low noise. Significance. We demonstrated the excellent imaging performance of VRAIN, which supported the advantages of the hemispherical detector arrangement.

Combustion characteristics of trimethyl borate, diesel, and trimethyl borate-diesel blend droplets

In this study, droplet tests were carried out for pure and blended forms of diesel fuel, which is the traditional fuel, and trimethyl borate fuel, which is new generation fuels. In this context, the evolution of fuel droplet diameter, flame structure, and flame temperature over time was observed using a high-speed camera and a thermal camera simultaneously. Measurements were made for the addition of 20%, 40%, 60%, and 80% trimethyl borate fuel to diesel fuel in fuel blends. The curves of the time-dependent change of the dimensionless square of the droplet diameter (D/D0)2 of the fuel droplets considered in the study, in general, have been determined to comply with the D2-law. Also, as the amount of diesel rose, there were high changes in droplet distortion and high deviations from the hemispherical geometry, and also elongation in the dominant shape droplet shape changed times. While the highest flame temperature was monitored by the thermal camera in the flame formed by the trimethyl borate droplet, as the amount of diesel fuel in the mixture increased, the maximum flame temperature decreased, but the burning time of the droplet was prolonged. On the other hand, the shortest ignition delay was measured for trimethyl borate, while the longest ignition delay time was detected for diesel fuel droplet. In general, it has been observed that the addition of trimethyl borate reduces the ignition delay, shortens the extinction time, and increases the temperature during combustion compared to diesel.

Ultraflexible and Ultrasensitive Near‐Infrared Organic Phototransistors for Hemispherical Biomimetic Eyes

AbstractThe diverse biological eyes vision systems found in nature can provide attractive design inspiration for near‐infrared (NIR) imaging devices. Outstanding features of the retina are its concave hemisphere geometry and high‐sensitivity image acquisition, which simplify the optical complexity of the eye and improve the imaging quality of visual perception. However, developing high‐performance NIR biomimetic imaging systems with these characteristics is extremely challenging due to the limitations of their hemisphere‐like structures, constituent materials, and conventional imaging modules. Here, a hemispherical biomimetic eyes imaging system based on ultraflexible all‐polymer heterojunction NIR phototransistors is presented. The device features a self‐supporting ultrathin (≈659 nm) NIR imaging structure that can maintain stable photoelectric performance while adhering to the human body or arbitrary‐shaped objects and have extraordinary photosensitivity (≈106) under dim light conditions (0.038 mW cm−2, 808 nm). Based on the versatile NIR device array, hemispherical ultrasensitive NIR biomimetic eyes are successfully achieved and higher‐resolution imaging is realized. Results demonstrated in this work provide a new strategy for constructing ultraflexible and ultrasensitive NIR photodetectors, showing remarkable application potential in next‐generation visual prosthetics and intelligent bioimaging systems.

Prediction of shear deformation and thickness gradients in deep-draw forming of ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) composite armor offers a high level of lightweight protection. In the manufacturing of complex curvature UHMWPE based protective materials, UHMWPE composites are transformed from flat sheet stock to a near net shape complex curvature preform through deep-draw forming. The development of a deep-draw forming process model is critical for predicting UHMWPE sheet deformation during processing and knowing the fiber orientations at each location in the entire final part. This work introduces a robust methodology to model the deep-draw forming process. The model is validated by comparing critical output with experimental results for multiple UHMWPE materials and over a range of part thicknesses. A hemispherical geometry is used to demonstrate the methodology. A homogenization strategy for grouping sheets is presented and validated, which is critical for keeping computational times manageable. The influence of the material in-plane shear response, a critical model input, is investigated. The model developed in this work represents a valuable tool for selecting deep-draw forming parameters and accelerates the evaluation of new and emerging UHMWPE materials for application in manufacturing complex curvature protective armor.

Integrals of Legendre polynomials over half range and their relation to the electrostatic potential in hemispherical geometry

Spherical symmetry is often encountered in many problems in mathematics and physical sciences. On the other hand, cases that involve hemispherical geometry lead to much less studied mathematical situations. In this work, we consider a case study that arises from potential theory where absence of a full spherical symmetry gives rise to expressions involving Legendre polynomials constrained in an interval that is different from the usual range in which they are orthogonal. We use exact mathematical results for the integrals of Legendre polynomials of arbitrary order over half range to obtain analytic expressions for the electrostatic potential created by a solid hemisphere with uniform volume charge density.

Influence of catalytic wall on the effective radius of a blunt body geometry in a nonequilibrium hypersonic flow

The effect of the surface catalycity on the effective radius of a blunt body geometry in hypersonic flow is numerically investigated. The blunt body geometry is defined in terms of a body radius with variation in both nose radius and the corner radius. Five different types of blunt body configurations are considered, that range from a flat-faced to a complete hemisphere geometry. The chosen blunt body configurations are investigated for four flow conditions with different levels of flow enthalpy. The effective radius is obtained by comparing the stagnation heat transfer of the blunt body geometry to that of the spherical geometry. To compute the stagnation heat transfer in the given blunt body configuration, a two-dimensional computational fluid dynamics (CFD) solver with thermochemical non-equilibrium is utilized. Both non-catalytic and fully-catalytic wall boundary conditions are considered in the computation. The obtained effective radius is additionally compared with the original data provided in the literature. The results show that surface catalycity influences the effective radius for the given blunt body configuration.

Tailoring the crystallographic texture of pure copper through control of the scanning strategy in Electron Powder Bed Fusion

There has been increasing interest over the past few years to evaluate how additive manufacturing can be used to tailor the microstructure of metallic materials. The objective of this work is to demonstrate through a few illustrative examples the possibility to tailor the crystallographic texture of pure copper processed by Electron Powder Bed Fusion (PBF-EB) by manipulating the molten pool morphology via the scanning strategy. We illustrate schematically how changes in molten pool geometry via modifications of the scanning strategies can affect texture development. Different processing conditions are selected to produce dense samples by PBF-EB while altering the crystallographic texture. The samples produced are examined by optical microscopy and Electron Back-Scattered Diffraction (EBSD) to characterize the molten pool geometry and the inherited microstructure to experimentally validate the pragmatic approach suggested in this work. Manipulations of the scanning strategies are shown to be efficient to produce a strong Cube texture or Goss texture or alternatively, a mixture between the two. A Cube texture is generated by promoting the formation of a melt pool with a flat geometry (its width being much larger than its depth). A Goss texture results from melt pools having a more hemispherical geometry with a large overlap between adjacent tracks. Adjusting the hatch spacing is a good way to achieve a mixture between columnar grains having a Cube texture and others exhibiting a Goss one. The results of this work could be considered as a guideline to tailor the microstructure of other cubic materials processed by PBF-EB.

Natural polyelectrolyte-based ultraflexible photoelectric synaptic transistors for hemispherical high-sensitive neuromorphic imaging system

Outstanding features of the human retina are its concave hemispherical geometry, high-sensitive image acquisition, and neuromorphic visual processing, which not only simplifies the optical complexity for the eye but also enhances imaging quality for the visual perception. However, the development of a high-performance bioinspired imaging system with these features is extremely challenging because of the restriction of its composing materials, device structures, and conventional imaging modules. Here, we present a novel hemispherical high-sensitive neuromorphic imaging system that leverages a natural acidic polyelectrolyte as dielectric layer in the photoelectric synaptic transistors. The devices have an extraordinary photosensitivity (~104) under a dim light condition (56 μW/cm2) and the light-tunable neuromorphic characteristics, as well as a substrate-free and ultrathin (~475 nm) structure. These advantages enable our system to perform high-quality imaging like the human retina and highly simulate various functions of the human visual system, such as image enhancement and erasure, and visual preprocessing.