Realistic Structural Model Research Articles

Accelerated First-Principles Exploration of Structure and Reactivity in Graphene Oxide.

Graphene oxide (GO) materials are widely studied, and yet their atomic-scale structures remain to be fully understood. Here we show that the chemical and configurational space of GO can be rapidly explored by advanced machine-learning methods, combining on-the-fly acceleration for first-principles molecular dynamics with message-passing neural-network potentials. The first step allows for the rapid sampling of chemical structures with very little prior knowledge required; the second step affords state-of-the-art accuracy and predictive power. We apply the method to the thermal reduction of GO, which we describe in a realistic (ten-nanometre scale) structural model. Our simulations are consistent with recent experimental findings, including X-ray photoelectron spectroscopy (XPS), and help to rationalise them in atomistic and mechanistic detail. More generally, our work provides a platform for routine, accurate, and predictive simulations of diverse carbonaceous materials.

Accelerated First‐Principles Exploration of Structure and Reactivity in Graphene Oxide

Graphene oxide (GO) materials are widely studied, and yet their atomic‐scale structures remain to be fully understood. Here we show that the chemical and configurational space of GO can be rapidly explored by advanced machine‐learning methods, combining on‐the‐fly acceleration for first‐principles molecular dynamics with message‐passing neural‐network potentials. The first step allows for the rapid sampling of chemical structures with very little prior knowledge required; the second step affords state‐of‐the‐art accuracy and predictive power. We apply the method to the thermal reduction of GO, which we describe in a realistic (ten‐nanometre scale) structural model. Our simulations are consistent with recent experimental findings, including X‐ray photoelectron spectroscopy (XPS), and help to rationalise them in atomistic and mechanistic detail. More generally, our work provides a platform for routine, accurate, and predictive simulations of diverse carbonaceous materials.

Exploring the challenges of interpreting near-surface towed transient electromagnetic data on saline permafrost

We analyze a continuous near-surface transient electromagnetic (TEM) survey on permafrost around Ilulissat in western Greenland, an area characterized by continuous saline permafrost. The TEM data are severely affected by induced polarization (IP) effects, causing a large range of decay shapes. We identify seven unique decay shapes: oversteepened, sign change, all negative, double sign change, no apparent IP, flat spot, and positive nonmonotonic, the last two of which have not previously been identified in the scientific literature. A clear spatial dependency of the decay shapes is observed. Inversion of the data is carried out using a Cole-Cole model that poses a highly nonunique inversion problem with an extreme starting-model dependency. A series of inversion and forward-modeling experiments demonstrate these challenges and indicate that a low-resistivity and highly chargeable layer with time constant ([Formula: see text]) values between [Formula: see text] and [Formula: see text] and frequency exponent ( C) values above 0.74 is needed to fit the data used in the inversion experiment. Forward modeling further indicates that low [Formula: see text] and high C values are needed to reproduce the observed flat-spot and positive-nonmonotonic decay shapes. Based on these observations, we attribute the IP effects to the orientational polarization of ice in the soil column. This mechanism allows for low-resistivity, high-chargeability layers due to partially frozen saline sediments, a combination that is difficult to explain by the IP mechanisms traditionally considered. Based on the forward modeling of a realistic structural model of a layered sediment pack over dipping bedrock, we are able to reproduce all the observed decay shapes. This model provides a consistent framework for qualitative interpretation of the entire data set and evidence for the presence of saline deposits in the central parts of the sedimentary basins.

On the use of frictional dampers for flutter mitigation of a highly flexible wing

The prospect of aeroelastic instability, leading to large-amplitude limit cycle oscillations, is of particular concern for the highly flexible wings of solar high-altitude long-endurance (HALE) aircraft. Among the different strategies of vibration control, passive mitigation through friction dampers stands out as a promising solution in this context. While this concept has been proposed previously, related studies systematically use simple two-degrees-of-freedom airfoil models. In this paper, a more realistic structural model involving a three-dimensional nonlinear beam description of a full wing is considered. A friction damper is installed at a specific location on the wing span, and the response to aerodynamic loads is investigated numerically. Comparisons of critical speed and limit cycle amplitudes with and without the damper suggest improved vibration mitigation with respect to a linear viscous damper.

Uncertainty quantification of geologic model parameters in 3D gravity inversion by Hessian-informed Markov chain Monte Carlo

Geologic modeling has been widely adopted to investigate underground structures. However, modeling processes inevitably have uncertainties due to scarcity of data, measurement errors, and simplification of the modeling method. Recent developments in geomodeling methods have introduced a Bayesian framework to constrain the model uncertainties by considering the additional geophysical data in the modeling procedure. Markov chain Monte Carlo (MCMC) methods are normally used as tools to solve the Bayesian inference problem. To achieve a more efficient posterior exploration, advances in MCMC methods use derivative information. Hence, we introduce an approach to efficiently evaluate second-order derivatives in geologic modeling and adopt a Hessian-informed MCMC method, the generalized preconditioned Crank-Nicolson (gpCN), as a tool to solve the 3D model-based gravity Bayesian inversion problem. The result is compared with two other widely applied MCMC methods, random-walk Metropolis–Hastings and Hamiltonian Monte Carlo, on a synthetic geologic model and a realistic structural model of the Kevitsa deposit. Our experiment demonstrates that superior performance is achieved by the gpCN compared with the other two state-of-the-art sampling methods. This indicates the potential of the proposed method to be generalized to more complex models.

Basin analysis palaeo‐landscape modelling: Testing the critical controls using experimental design constrained by a real 3D geological model, Gippsland Basin, Australia

AbstractThe main controlling variables for palaeo‐landscape evolution are investigated to assess their relative importance using the Gippsland Basin geological history. Palaeo‐landscape reconstruction is a complicated process controlled and affected by multiple variables, including tectonic, palaeo‐environment, sea‐level change, rainfall, sediment erosion, transportation, deposition, etc. The Basin and Landscape Dynamics software (Badlands) software was used with an efficient experimental design (ED) to guide the selected scenarios, process the results, and generate the multi‐variate equations that define and identify the important controlling variables. The ED was used to test and identify the main uncertainties and their possible ranges, based on actual field data, while at the same time ensuring that the full multi‐dimensional space for those variables was covered to enable the computation of multivariate equations from the minimum number of scenario runs. A full suite of 3D forward palaeo‐landscape models of the Gippsland Basin was built to reconstruct the basin history from its formation to the present (Early Cretaceous to Holocene, 137‐0 Ma). The models are compared to the corresponding full 3D realistic structural and stratigraphic model of the basin that has been built in Petrel (Schlumberger software). This constrains the sedimentary, stratigraphic, burial and thermal histories to the relative subsidence rates and basin‐fill for each geological sequence by using the model isopachs input to the Badlands modelling. The ED required only 22 scenarios to fit 12 identified variables and test for possible interactions with each other. The most significant variables are those that control sediment supply including non‐marine erodibility, rainfall, (Rainfall × Area) exponent m, Slope and critical slope while maximum % Marine Deposition and marine dispersal are also required to fill the marine accommodation space. Sea Level and subsidence only become significant when rapid enough to outpace sediment supply. The controlling factors change over time with basin development from rift to post‐rift phases and interactions are highly significant.

A multi-stage approach of simulating turbulence-induced vibrations in a wire-wrapped tube bundle for fretting wear prediction

MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) is a prototype of a Generation IV reactor that will be constructed in Mol, Belgium. It is a liquid metal fast reactor, using lead-bismuth eutectic (LBE) as a coolant. Flow-induced vibrations resulting in fretting wear could be of concern due to the high density of LBE. Additionally, a wire is wrapped around each fuel rod to preserve their mutual distance in the hexagonal array, which affects the flow pattern.In this research turbulence-induced vibrations are investigated. Large-eddy simulation is a sound technique to acquire turbulent loads, but it is computationally demanding to combine this in a two-way coupled fluid–structure interaction (FSI) simulation. However, because accurate prediction of the vibration of cylinders subject to liquid flow typically requires a two-way simulation due to the added-mass effect, a methodology consisting of multiple steps is proposed. First, the modal characteristics of the pin are determined in a fully coupled FSI simulation of a bare bundle. The material properties used in this model are modified to take into account the effect of the wire, which was determined experimentally beforehand. The resulting characteristics are then used to construct a realistic structural model, employed in a one-way simulation with turbulent loads from a large-eddy simulation of a wire-wrapped bundle.In this structural model contact with neighboring rods and the hexagonal duct is detected, providing realistic boundary conditions to the rod and allowing to extract contact forces. These forces are further post-processed together with the sliding displacement to calculate work rates, which is an important parameter for fretting wear prediction. Two pins of a 19-pin bundle were investigated, the central pin and a pin located in one of the corners. The simulation is repeated for multiple friction coefficients, and its impeding effect of friction on sliding motions was confirmed to moderate fretting.

Deep-learning seismic full-waveform inversion for realistic structural models

Velocity model inversion is one of the most important tasks in seismic exploration. Full-waveform inversion (FWI) can obtain the highest resolution in traditional velocity inversion methods, but it heavily depends on initial models and is computationally expensive. In recent years, a large number of deep-learning (DL)-based velocity model inversion methods have been proposed. One critical component in those DL-based methods is a large training set containing different velocity models. We have developed a method to construct a realistic structural model for the DL network. Our compressional-wave velocity model building method for creating dense-layer/fault/salt body models can automatically construct a large number of models without much human effort, which is very meaningful for DL networks. Moreover, to improve the inversion result on these realistic structural models, instead of only using the common-shot gather, we also extract features from the common-receiver gather as well. Through a large number of realistic structural models, reasonable data acquisition methods, and appropriate network setups, a more generalized result can be obtained through our proposed inversion framework, which has been demonstrated to be effective on the independent testing data set. The results of dense-layer models, fault models, and salt body models that we compared and analyzed demonstrate the reliability of our method and also provide practical guidelines for choosing optimal inversion strategies in realistic situations.

Numerical study on the mechanical behavior of ultrahigh performance concrete using a three‐phase discrete element model

AbstractThis paper deals with numerical analysis on the mechanical behavior of ultrahigh performance concrete (UHPC) under uniaxial compression. For the modeling of the mechanical behavior of UHPC, the mesh‐free discrete element method was applied. To calibrate the model parameters and validate the numerical simulation results, a set of experimental investigations including mechanical tests, microscopy and tomography analysis, were performed. The scanning electron microscopy and polarized light microscopy were used to examine cross sections of UHPC as well as to characterize interfacial transition zone and aggregate in detail. X‐ray microtomography analysis was used to obtain information about the nonspherical shape of aggregate and to generate a realistic structural model. Simulation results have shown that the developed model predict stiffness reliably, strength, and breakage pattern of UHPC and shows good agreement with experimental results. Finally, the model has been applied to analyze the main crack initiation in static failure and to investigate the influence of different parameters such as aggregate content as well as aggregate and binder stiffness on the mechanical behavior of UHPC.

Se intercalation between PtSe2 and the Pt surface during synthesis of PtSe2 by direct selenization of Pt(111)

Using the first principles calculations, we analyze the structural and electronic properties of a ${\mathrm{PtSe}}_{2}$ monolayer on Pt substrate, obtained by direct selenization of the Pt(111) surface [Wang et al., Nano Lett. 15, 4013 (2015)]. We demonstrate that in order to reproduce the experimental result that ${\mathrm{PtSe}}_{2}$ is physisorbed on the surface, the surface must be passivated. We propose that this passivation is most likely due to intercalation of Se atoms between ${\mathrm{PtSe}}_{2}$ and Pt surface during the selenization process. In this case the mean distance between the Se-passivated surface and ${\mathrm{PtSe}}_{2}$ is found to be $3.24\phantom{\rule{4pt}{0ex}}\text{\AA{}}$, which is consistent with the distance that can be extracted from the scanning transmission electron microscopy image of the hybrid system, and the adsorption energy is found to fall into physisorption range. Therefore, our findings provide an insight into the synthesis of ${\mathrm{PtSe}}_{2}$ by direct selenization: A realistic structural model should include a Se-passivated surface and not a clean one.

Detailed analysis of photonic structure in the wing scale of Rajah Brooke's birdwing butterfly.

The male Rajah Brooke's birdwing butterfly, Trogonoptera brookiana, has black wings with bright green stripes, and the unique microstructure in the wing scales causes wavelength-selective reflection. It has been reported that the reflectance spectrum has several peaks in the visible wavelength range. However, there has been little progress in the interpretation of the spectral shape, and questions remain unanswered. For example, what are the physical origins of the observed reflectance peaks, and how are their wavelengths determined? To answer these questions, we performed a detailed analysis of the photonic structure of the wing scale of Trogonopterabrookiana. The reflectance spectrum also shows strong polarization dependence. This paper describes the analysis for TM polarization, which is perpendicular to the longitudinal ridges on the scale. We first constructed a realistic structural model that reproduced the experimentally determined reflectance spectrum. We then simplified the model and calculated the reflectance spectrum while varying several structural parameters. For three of the four observed spectral peaks, our calculations revealed the reflection paths for constructive interference to explain the peak wavelengths. A possible origin of the fourth peak is discussed. Such detailed understanding of natural photonic structures can inspire optical component design.

Multi-Wave Vibration Caused by Flutter Instability and Nonlinear Tip-Shroud Friction

Abstract Measurements revealed the contribution of multiple traveling waves to the flutter vibrations of bladed disks. Saturated flutter vibration, whether in this multi-wave or in its better-understood single-wave form, is a nonlinear phenomenon. However, it is still not understood of what physical origin the relevant nonlinearities are, and under what conditions single-wave or multi-wave flutter vibration occurs. Recent theoretical work suggests that multi-wave flutter vibration can be explained by strongly nonlinear frictional interblade coupling. The verity of this hypothesis is strictly limited by the simplicity of the considered model, namely, a cyclic chain of seven oscillators with frictional coupling and rather unrealistic aeroelastic behavior. In this work, it is demonstrated that nonlinear dynamical contact interactions at tip-shrouds are a likely cause for the observed multi-wave flutter vibration. To this end, a more realistic structural turbine blade row model with a more realistic aeroelastic behavior is considered. Some insight into its intriguing dynamics, dependence of limit states on initial conditions, and eigenvalue placement is provided. For instance, it is shown that there is an intimate relation between internal combination resonance conditions of certain traveling wave modes and the spectral content of single- and multi-wave flutter oscillations.

Gippsland Basin 3D forward modelling in Badlands

The Gippsland basin geological history is modelled using the Badlands software constrained by a realistic 3D structural and stratigraphic model built in Petrel. The aim is to assess and calibrate t...

Local atomic structure and lattice defect analysis in heavily Co-doped ZnS thin films using X-ray absorption fine structure spectroscopy

X-ray absorption fine structure (XAFS) spectroscopy was used to address both the local atomic structure and the local electronic structure around Zn, Co, and S in Co-doped ZnS thin films. X-ray absorption near-edge spectroscopy (XANES) revealed changes in the strong pre-edge feature of the S K-edge spectra that depend on the Co concentration and are related to atomic bound-state transitions through hybridization between 3d transition metal and sp host semiconductor electronic states. These changes reveal intrinsic effects, such as S vacancies and dopant effects. From the Co K-edge XANES spectra, substitution of Co2+ into ZnS is observed. The local atomic structure around the dopant, obtained by XAFS spectroscopy, indicates a reduction of the Co–S bond length. Optical analysis shows deep absorption centers or intermediate states in the bandgap, ascribed to d-d transitions in the Co atoms that modify the electronic band structure and cause a decrease in the bandgap as a function of Co concentration. This work provides direct evidence of dopant effects in heavily Co-doped ZnS, including bond distances and Debye-Waller factors, providing key elements to generate a realistic atomic structural model, which is crucial to understand the observed electronic properties of this material.

A stochastic simulation of skeletal muscle calcium transients in a structurally realistic sarcomere model using MCell.

Skeletal muscle contraction is initiated when an action potential triggers the release of Ca2+ into the sarcomere in a process referred to as excitation-contraction coupling. The speed and scale of this process makes direct observation very challenging and invasive. To determine how the concentration of Ca2+ changes within the myofibril during a single activation, several simulation models have been developed. These models follow a common pattern; divide the half sarcomere into a series of compartments, then use ordinary differential equations to solve reactions occurring within and between the compartments. To further develop this type of simulation, we have created a realistic structural model of a skeletal muscle myofibrillar half-sarcomere using MCell software that incorporates the myofilament lattice structure. Using this simulation model, we were successful in reproducing the averaged calcium transient during a single activation consistent with both the experimental and previous simulation results. In addition, our simulation demonstrated that the inclusion of the myofilament lattice within our model produced an asymmetric distribution of Ca2+, with more Ca2+ accumulating near the Z-disk and less Ca2+ reaching the m-line. This asymmetric distribution of Ca2+ is also apparent when we examine how the Ca2+ are bound to the troponin-C proteins along the actin filaments. Our simulation model also allowed us to produce advanced visualizations of this process, including two simulation animations, allowing us to view Ca2+ release, diffusion, binding and uptake within the myofibrillar half-sarcomere.

EXCHANGE RATE PASS‐THROUGH IN A SMALL OPEN ECONOMY: A STRUCTURAL VAR APPROACH

ABSTRACTStudying exchange rate pass‐through in Turkey for the period of 2006m1‐2015m6, we first show that the commonly used recursive VAR model generates unrealistic dynamics like effects of domestic variables on external variables in small open economies and biased estimates. Bias comes from unrealistic decline in energy prices in response to depreciation of currency. However, a more realistic structural VAR model suitable for small open economies generates more sensible dynamics and suggests a higher pass‐through than the recursive VAR model. Overall, our analysis demonstrates the importance of using a more realistic model setup and checking the relationships across variables when estimating ERPT in small open economies.

Nonlinear analysis of a riverine platform under earthquake and environmental loads

A realistic FEM structural model is developed to predict the behavior, load transfer, force distribution and performance of a riverine platform under earthquake and environmental loads. The interaction between the transfer plate and the piles supporting the platform is investigated. Transfer plate structures have the ability to redistribute the loads from the superstructure above to piles group below, to provide safe transits of loads to piles group and thus to the soil, without failure of soil or structural elements. The distribution of piles affects the distribution of stress on both soil and platform. A materially nonlinear earthquake response spectrum analysis was performed on this riverine platform subjected to earthquake and environmental loads. A fixed connection between the piles and the platform is better in the design of the piles and the prospect of piles collapse is low while a hinged connection makes the prospect of damage high because of the larger displacements. A fixed connection between the piles and the platform is the most demanding case in the design of the platform slab (transfer plate) because of the high stress values developed.

Local atomic structure and analysis of secondary phases in non-stoichiometric Cu2ZnSnS4 using X-ray absorption fine structure spectroscopy

The local atomic structure in non-stoichiometric Cu2ZnSnS4 (CZTS) thin films was investigated using X-ray absorption fine structure spectroscopy (XAFS). From the XAFS spectra we infer the presence of disorder in the Cu-Zn plane suggesting that this characteristic might be intrinsic of non-stoichiometric CZTS. This disorder increases with the Cu/(Zn+Sn) ratio. Structural parameters derived from fits to the data show a contraction in the Cu-S bond length and an expansion in the Sn-S bond length, compared to distances obtained from diffraction, for all samples. X-ray absorption near edge spectroscopy (XANES) indicates presence of secondary phases not observed by X-ray diffraction. In Cu-poor samples, ZnS and SnS were identified as secondary phases. The amount of those phases correlates with the Cu deficit and were quantified by a linear combination fit of the XANES of standard materials. This work provides direct evidence of intrinsic disorder in CZTS, which must be considered to generate a realistic atomic structural model, in addition to lattice relaxations and secondary phases.

PARAMETERS AFFECTING THE STRUCTURAL ANALYSIS OF A TUNNEL STRUCTURE EXPOSED TO FIRE

Behaviour of cut-and-cover tunnels exposed to fire should be analysed by using a realistic structural model that takes account of mechanical and thermal effects on the structure. This has been performed with the aid of Finite Element (FE) software package called SOFiSTiK in parallel, for two types of elements as a scope of research project financed by the German Bundesanstalt für Straßenwesen BAST. Since the stiffness of the structure at elevated temperatures is highly affected, a realistic model of structural behaviour of the tunnel could be only achieved by considering the nonlinear analysis of the structure. This has been performed for a 2–cell cut and cover tunnel by taking account of simultaneous reduction of stiffness and strength and the time-dependent increasing indirect effects due to axial constraints and temperature gradients induced by elevated temperatures. The thermal analyses have been performed and the effects were implemented into the structural model by the multi-layered strain model. The stress–strain model proposed by EN 1992-1-2 is implemented for the elevated temperature. Since there was sufficient amount of Polypropylene fibres in the concrete mixtures, modelling of spalling was excluded from the analysis. The critical corresponding stresses and material behaviour are compared and interpreted at different time stages. The main parameters affecting the accuracy and convergence of the results of structural analysis for the used model are identified: defining a realistic fire action, using concrete material model fulfilling the requirements of fire situation in tunnels, defining appropriate time intervals for load implementations. These parameters along with other parameters, which influence the results to a lesser degree, are identified and investigated in this paper.

Computational Material Analysis of Structural and Hemodynamic Model of Coronary Stent by CFD/FEA in Computer Aided Mechanical Engineering Approach

With the development of new technologies, it is very popular to use a coronary stent that is a small mesh tubeshaped medical device deployed to treat narrow or weak arteries as part of a procedure called percutaneous coronary intervention. Several aspects, such as stent design, stent wire type, mechanical and material characteristics of stent have different influences on stent intervention. It has not been reported about what impacts on stent struts by the hemodynamic behavior on stent material and very few numerical studies have considered both the mechanical and hemodynamic impact of stent implementation. Computational simulation method for realization of realistic structural and hemodynamic micro environment model in this research provided valuable results of long-term functional knowledge of stent material behavior that are time consuming and expensive to determine otherwise. Computational fluid dynamics and finite element analysis simulation models were investigated and developed to evaluate engineering properties that affect stent functional attributes. These characteristics are dependences of material properties on blood flow conditions such as structural load, shear-strain rate, radial strength, and wall shear stresses, which need to be scientifically explored. To understand the material (Fe-18Cr-14Ni-2.5Mo as stainless steel 316LVM) mechanical performance of the stent, a finite element analysis simulation model was established when exposed to pulsatile blood pressure. In this study, computational fluid dynamics model was generated to calculate the wall shear stresses and strain distribution in stented vessel carrying blood to heart. The analytical analysis of mechanical and hemodynamic conduct of a stent in this investigation may help for better designs of stent, and provide deeper comprehension to support clinical cardiovascular surgeons and guide potential therapeutic strategies.