Set Of Snapshots Research Articles

Interface spaces for the Multiscale Robin Coupled Method in reservoir simulation

The Multiscale Robin Coupled Method (MRCM) is a recent multiscale numerical method based on a non-overlapping domain decomposition procedure. One of its hallmarks is that the MRCM allows for the independent definition of interface spaces for pressure and flux over the skeleton of the decomposition. The accuracy of the MRCM depends on the choice of these interface spaces, as well as on an algorithmic parameter β in the Robin interface conditions imposed at the subdomain boundaries. This work presents an extensive numerical assessment of the MRCM in both of these aspects. Two types of interface spaces are implemented: usual piecewise polynomial spaces and informed spaces, the latter obtained from sets of snapshots by dimensionality reduction. Different distributions of the unknowns between pressure and flux are explored. Two non-dimensionalizations of β are tested. The assessment is conducted on realistic, high contrast, channelized permeability fields from a SPE benchmark database. The results show that β, suitably non-dimensionalized, can be fixed to unity to avoid any indeterminacy in the method. Further, with both types of spaces it is observed that a balanced distribution of the interface unknowns between pressure and flux renders the MRCM quite attractive both in accuracy and in computational cost, competitive with other multiscale methods from the literature

Novel Molecular-Dynamics-Based Protocols for Phase Space Sampling in Complex Systems.

The adequate exploration of the phase space of a chromophore is a fundamental necessity for the simulation of their optical and photophysical properties, taking into account the effects of vibrational motion and, most importantly, the coupling with a (non-homogeneous) molecular environment. A representative set of conformational snapshots around the Franck-Condon region is also required to perform non-adiabatic molecular dynamics, for instance in the framework of surface hopping. Indeed, in the latter case one needs to prepare a set of initial conditions providing a meaningful and complete statistical base for the subsequent trajectory propagation. In this contribution, we propose two new protocols for molecular dynamics-based phase space sampling, called “local temperature adjustment” and “individual QM/MM-based relaxation.” These protocols are intended for situations in which the popular Wigner distribution sampling procedure is not applicable—as it is the case when anharmonic or nonlinear vibrations are present—and where regular molecular dynamics sampling might suffer from an inaccurate distribution of internal energy or from inaccurate force fields. The new protocols are applied to the case of phase space sampling of [Re(CO)3(Im)(Phen)]+ (im, imidazole; phen, phenanthroline) in aqueous solution, showing the advantages and limitations of regular Wigner and molecular dynamics sampling as well as the strengths of the new protocols.

Reduced order modeling for nonlinear structural analysis using Gaussian process regression

A non-intrusive reduced basis (RB) method is proposed for parametrized nonlinear structural analysis undergoing large deformations and with elasto-plastic constitutive relations. In this method, a reduced basis is constructed from a set of full-order snapshots by the proper orthogonal decomposition (POD), and the Gaussian process regression (GPR) is used to approximate the projection coefficients. The GPR is carried out in the offline stage with active data selection, and the outputs for new parameter values can be obtained rapidly as probabilistic distributions during the online stage. Due to the complete decoupling of the offline and online stages, the proposed non-intrusive RB method provides a powerful tool to efficiently solve parametrized nonlinear problems with various engineering applications requiring multi-query or real-time evaluations. With both geometric and material nonlinearities taken into account, numerical results are presented for typical 1D and 3D examples, illustrating the accuracy and efficiency of the proposed method.

Reading Ourselves: Placing Critical Literacies in Contemporary Language Education

ABSTRACT This article problematizes the place of critical literacies (CLs) in contemporary language education. In doing so, we ask: Where do we place critique within the curriculum in neoconservative times? What is left to teachers in their commitment to educate critical citizens? Do critical literacies suffice? To respond to these questions, we bring a set of contemporary snapshots, unveiling all the anguish brought up by the complex politics of “us” versus “them”. Some understandings of CLs within the field are then reviewed, preparing the terrain for the reading of ourselves in relation to our theories and practices. To (in)conclude, we outline a few orientations which seek to relocate CLs beyond the dichotomic view of the micro versus macro as a formative strategy in dealing with our frustrations in such dark times.

System Architecture and Signal Processing for Frequency-Modulated Continuous-Wave Radar Using Active Backscatter Tags

In this paper, we study a frequency-modulated continuous-wave based radio-frequency identification system, which is composed of a reader with single transmitter and multiple receiving antennas as well as multiple active backscatter tags. In particular, our contribution is twofold in this paper. First, we construct a general architecture for achieving the functionality of wireless positioning based on the principle of the switched injection-locked oscillator (SILO). Unlike the previous SILO-based schemes, some limitations on the pulsed waveform generated by the tags are overcome. In addition, only one reader set up with multiple receiving antennas is used to precisely localize tags based on the delay and direction-of-arrival estimates. Second, the developed algorithms make use of a shift invariant property among the baseband signal snapshots of intra- and/or intermodulation intervals, i.e., snapshots in one modulation period and/or sets of snapshots in multiple modulation periods. A new joint singular value decomposition based ESPRIT algorithm is developed for determining the number of multiple active tags and their positions by implementing the matrix reconstruction and stacking techniques. The proposed method has its basic foundation on the processing of received data with global fusion scheme. In this way, this method can improve the performance of parameter estimates in terms of accuracy and resolution without suffering from the pair matching problem. System simulations and experimental validations are carried out to verify the proposed system architecture associated with the signal processing technique.

Binding site for coenzyme A revealed in the structure of pyruvate:ferredoxin oxidoreductase from Moorella thermoacetica

Pyruvate:ferredoxin oxidoreductase (PFOR) is a microbial enzyme that uses thiamine pyrophosphate (TPP), three [4Fe-4S] clusters, and coenzyme A (CoA) in the reversible oxidation of pyruvate to generate acetyl-CoA and carbon dioxide. The two electrons that are generated as a result of pyruvate decarboxylation are used in the reduction of low potential ferredoxins, which provide reducing equivalents for central metabolism, including the Wood-Ljungdahl pathway. PFOR is a member of the 2-oxoacid:ferredoxin oxidoreductase (OFOR) superfamily, which plays major roles in both microbial redox reactions and carbon dioxide fixation. Here, we present a set of crystallographic snapshots of the best-studied member of this superfamily, the PFOR from Moorella thermoacetica (MtPFOR). These snapshots include the native structure, those of lactyl-TPP and acetyl-TPP reaction intermediates, and the first of an OFOR with CoA bound. These structural data reveal the binding site of CoA as domain III, the function of which in OFORs was previously unknown, and establish sequence motifs for CoA binding in the OFOR superfamily. MtPFOR structures further show that domain III undergoes a conformational change upon CoA binding that seals off the active site and positions the thiolate of CoA directly adjacent to the TPP cofactor. These structural findings provide a molecular basis for the experimental observation that CoA binding accelerates catalysis by 105-fold.

Thermal performance of reciprocating two-phase thermosyphon with nozzle

Flow images and thermal performances of the two-phase reciprocating thermosyphons (RT) with an internal nozzle at 50% volumetric filling ratio (FR) were measured. Two sets of flow snapshots and thermal performance data detected from two similar RTs with and without nozzle were compared. Above the critical reciprocating frequencies (fcr), the circulation routes of working fluid in the RTs with and without nozzle were reversed to cause disparities in thermal performances. At the reciprocating frequencies (f) of 1.75, 1.83, 1.92 and 2 Hz with four different heating and cooling duties for each fixed f, the time-mean local and regionally averaged Nusselt numbers (Nu) along the evaporator/condenser centerlines of the RTs with and without nozzle were dominated by reciprocation number (Reci) but subject to the coupled effects attributed from the heating and cooling duties indexed by boiling number (Bo) and dimensionless condenser thermal resistance (Rth,con). Local Nu were raised by increasing Reci, Bo and/or Rth,con for both RTs with and without nozzle. Due to the inversed liquid film circulation and jet impingement, the overall thermal resistances (Rth) of present RT with nozzle were reduced by 40.06–59.96% from those of RT without nozzle. To add engineering applications, evaporator Nu correlations were devised for present RT with nozzle.

A reduced-basis element method for pin-by-pin reactor core calculations in diffusion and [formula omitted] approximations

The reduced order model (ROM) methods allow to significantly improve the computation time and memory usage. Therefore these methods are very useful for real-time or many-query reactor core simulation analysis. The ROM approach is based on the proper orthogonal decomposition methods which generate an optimal truncated subspace from a given set of test solutions or snapshots. In the case of high-fidelity real-size problems the calculation of one snapshot might be too expensive or not possible due to memory limitations. To circumvent this problem we apply the reduced basis element method (RBEM). This method is based on a spatial decomposition of the core domain and on the application of the ROM approach on each subdomain. In this work the RBEM is developed for diffusion and SP3 equations and the results are illustrated with several two-dimensional reactor core calculations.

A reduced-order DG formulation based on POD method for the time-domain Maxwell’s equations in dispersive media

In this work, a proper orthogonal decomposition (POD) method is applied to time-domain Maxwell’s equations coupled to a Drude dispersion model, which are discretized in space by a discontinuous Galerkin (DG) method. An auxiliary differential equation (ADE) method is used to represent the constitutive relation for the dispersive medium. A POD–DGTD formulation with lower dimension and sufficiently high accuracy is established, together with the description of the POD reduced-order basis, its construction from a snapshot set, and its application to the solution of the time-domain Maxwell’s equations. The overall goal is to reduce the computational time while maintaining an acceptable level of accuracy, in order to obtain an efficient time-domain solver to be used as a starting-point for an optimization strategy. We provide results from numerical experiments for two-dimensional problems that illustrate the capabilities of the proposed POD–DGTD formulation and assess its efficiency.

Multiscale model reduction for fluid infiltration simulation through dual-continuum porous media with localized uncertainties

Here, we present some Reduced Basis (RB) methods for fluid infiltration problems through certain porous media modeled as dual-continuum with localized uncertainties. We apply dimension reduction techniques to construct a reduced order model. In the RB methods, to perform the offline–online computation decomposition, the model inputs need to be affinely dependent on the uncertainties. We develop a Proper Orthogonal Decomposition and Greedy (POD-Greedy) RB method for stochastic dual-continuum models. In the POD-Greedy RB framework, for heterogeneous porous media, we need to solve the stochastic dual-continuum models many times using very fine grid to construct a set of snapshots for building optimal reduced basis. This offline computation may be very expensive. To improve the offline computational efficiency, we further develop a local–global RB method, which integrates the coupled multiscale and multicontinuum approach using Generalized Multiscale Finite Element Method (GMsFEM) to the POD-Greedy RB method. To illustrate the efficiency of the proposed methods, we present two numerical examples for stochastic dual-continuum models. Our numerical results show that both the POD-Greedy RB method and the local–global RB method greatly improve the computation efficiency with high approximation accuracy.

Ionic Solution: What Goes Right and Wrong with Continuum Solvation Modeling.

Solvent-mediated electrostatic interactions were well recognized to be important in the structure and function of molecular systems. Ionic interaction is an important component in electrostatic interactions, especially in highly charged molecules, such as nucleic acids. Here, we focus on the quality of the widely used Poisson-Boltzmann surface area (PBSA) continuum models in modeling ionic interactions by comparing with both explicit solvent simulations and the experiment. In this work, the molality-dependent chemical potentials for sodium chloride (NaCl) electrolyte were first simulated in the SPC/E explicit solvent. Our high-quality simulation agrees well with both the previous study and the experiment. Given the free-energy simulations in SPC/E as the benchmark, we used the same sets of snapshots collected in the SPC/E solvent model for PBSA free-energy calculations in the hope to achieve the maximum consistency between the two solvent models. Our comparative analysis shows that the molality-dependent chemical potentials of NaCl were reproduced well with both linear PB and nonlinear PB methods, although nonlinear PB agrees better with SPC/E and the experiment. Our free-energy simulations also show that the presence of salt increases the hydrophobic effect in a nonlinear fashion, in qualitative agreement with previous theoretical studies of Onsager and Samaras. However, the lack of molality-dependency in the nonelectrostatics continuum models dramatically reduces the overall quality of PBSA methods in modeling salt-dependent energetics. These analyses point to further improvements needed for more robust modeling of solvent-mediated interactions by the continuum solvation frameworks.

Model reduction of dynamical systems by proper orthogonal decomposition: Error bounds and comparison of methods using snapshots from the solution and the time derivatives

Abstract We consider two proper orthogonal decomposition (POD) methods for dimension reduction of dynamical systems. The first method (M1) uses only time snapshots of the solution, while the second method (M2) augments the snapshot set with time-derivative snapshots. The goal of the paper is to analyze and compare the approximation errors resulting from the two methods by using error bounds. We derive several new bounds of the error from POD model reduction by each of the two methods. The new error bounds involve a multiplicative factor depending on the time steps between the snapshots. For method M1 the factor depends on the second power of the time step, while for method 2 the dependence is on the fourth power of the time step, suggesting that method M2 can be more accurate for small between-snapshot intervals. However, three other factors also affect the size of the error bounds. These include (i) the norm of the second (for M1) and fourth derivatives (M2); (ii) the first neglected singular value and (iii) the spectral properties of the projection of the system’s Jacobian in the reduced space. Because of the interplay of these factors neither method is more accurate than the other in all cases. Finally, we present numerical examples demonstrating that when the number of collected snapshots is small and the first neglected singular value has a value of zero, method M2 results in a better approximation.

Being Black in a White World: Understanding Racism in British Universities

Este artículo examina críticamente las experiencias de racismo de académicos que trabajan en universidades británicas. El marco conceptual en el que se apoya son la teoría crítica de la raza y el feminismo postcolonial. Estas perspectivas teóricas se entrelazan con el fin de presentar una serie de imágenes que documentan los diversos retos y barreras que se encuentran académicos británicos de color y de diversidad en la educación superior. La base empírica de este trabajo es cualitativa, habiéndose realizado una serie de entrevistas en profundidad y semi-estructuradas con académicos de color y de diversidad, y que en su mayoría trabajan en el ámbito de las ciencias sociales y las humanidades. Las personas entrevistadas provienen de una amplia gama de orígenes raciales, religiosos y etno-nacionales, y ocupan diferentes posiciones académicas dentro de la estructura universitaria. A través de un trabajo minucioso sobre el material empírico, este texto analiza los efectos de la naturaleza estructural y sistémica del racismo tal y como es experienciada por las personas entrevistadas. El artículo se focaliza en tres temas clave, incluyendo microagresiones y racismo institucional, docencia, y ascenso y apoyo

Cultural snapshots: Theory and method

AbstractCausal influences of culture on cognition are challenging to examine scientifically. We here introduce a method to address this challenge.Cultural snapshotsenable scientists to (a) characterize the cultural information commonly and frequently encountered by a collective, (b) examine how such cultural information influences the cognitions of individuals, and (c) draw conclusions about the emergence of shared cognition. Specifically, cultural snapshots are recorded samples of public environments commonly encountered by many people. Television scenes, photographs of public spaces, magazine pages, and social media conversations are all examples of cultural snapshots. Representative sets of cultural snapshots can be coded to index the systematic patterns of information encountered by a collective (i.e.,cultural patterns). These same materials can be used to experimentally manipulate those cultural patterns, allowing scientists to examine cultural influences on cognition and behavior. We here review and provide guidelines for cultural snapshots research, trace cultural snapshots to classic theories of culture, and describe how cultural snapshots balance the constraints of representative design (Brunswik,) with those of causal inference. We then illustrate how this approach is used to address (a) questions of causality in cultural psychology and (b) questions of applicability in social cognition research. We conclude by evaluating the strengths and weaknesses of the approach.

On POD-based Deflation Vectors for DPCG applied to porous media problems

We study fast and robust iterative solvers for large systems of linear equations resulting from simulation of flow through strongly heterogeneous porous media. We propose the use of preconditioning and deflation techniques, based on information obtained from the system, to reduce the time spent in the solution of the linear system. An important question when using deflation techniques is how to find good deflation vectors, which lead to a decrease in the number of iterations and a small increase in the required computing time per iteration. In this paper, we propose the use of deflation vectors based on a POD-reduced set of snapshots. We investigate convergence and the properties of the resulting methods. Finally, we illustrate these theoretical results with numerical experiments. We consider compressible and incompressible single-phase flow in a layered model with variations in the permeability layers up to 1 0 3 and the SPE 10 benchmark model with a contrast in permeability coefficients of 1 0 7 . Using deflation for the incompressible problem, we reduce the number of iterations to 1 or 2 iterations. With deflation, for the compressible problem, we reduce up to ∼ 80 % the number of iterations when compared with the only-preconditioned solver.

Temporal localized nonlinear model reduction with a priori error estimate

This work presents a model reduction framework using a temporal localized basis approach to efficiently reduce the simulation time for nonlinear dynamical systems with rapid changes over a short time period, and proposes a corresponding a priori error bound. This framework employs the proper orthogonal decomposition (POD) to construct localized basis sets from different temporal subdomains, which can be used in the Galerkin projection to accurately capture the important local dynamics of the system. The discrete empirical interpolation method (DEIM) with the corresponding temporal localized basis sets is then applied to efficiently compute the projected nonlinear terms. A heuristic procedure for subdividing snapshots over the temporal domain is proposed. This procedure first partitions the set of snapshots where there are possible significant changes in system dynamics, and then uses the notion of distance between subspaces to later remove unnecessary partitioning. An a priori error bound is derived to confirm the convergence of this framework and to explain how the propagated errors from the localized reduced systems affect the overall accuracy. Numerical experiments demonstrate the accuracy improvement of the temporal localized framework through a parametrized nonlinear miscible flow simulation. The results show the applicability of the proposed approach to various parameter values that are not necessary used for generating the POD and DEIM localized basis sets.

LUPOD: Collocation in POD via LU decomposition

A collocation method is developed for the (truncated) POD of a set of snapshots. In other words, POD computations are performed using only a set of collocation points, whose number is comparable to the number of retained modes, in a similar fashion as in collocation spectral methods. Intending to rely on simple ideas which, moreover, are consistent with the essence of POD, collocation points are computed via the LU decomposition with pivoting of the snapshot matrix. The new method is illustrated in simple applications in which POD is used as a data-processing method. The performance of the method is tested in the computationally efficient construction of reduced order models based on POD plus Galerkin projection for the complex Ginzburg–Landau equation in one and two space dimensions.

An Empirical Study on the Temporal Structural Characteristics of VANETs on a Taxi GPS Dataset

In recent years, vehicular ad hoc networks (VANETs) have received significant interests from both academia and industry because of the ubiquitous communication capabilities they provide. VANETs are expected to support a wide range of applications that may influence our daily life, ranging from road safety applications to entertainment ones. A key to the development of protocols and algorithms for intervehicle communication and services lies in the knowledge of the structural properties of the VANET communication graph. Nonetheless, previous researches represented the VANET communication graph as a set of snapshots and tracked the evolution of some metrics on snapshots at certain time intervals. They neglected to take into account the temporal property of VANETs, which in many real scenarios plays a pivotal role. In this paper, we mainly explore the temporal structural characteristics of VANETs. A time-extended model is first presented to capture the temporal property of VANETs. Based on the model, some key metrics are defined to describe the structural characteristics of VANETs in various aspects. We then employ a real and large-scale urban taxi GPS dataset to provide a comprehensive study of the temporal structural characteristics involved with time-ordered paths, reachability, and connectivity of VANETs. These results are helpful in designing better protocols and algorithms to achieve reliable and low-latency communications.

Telangiectasia Detection in Wireless Capsule Endoscopy Using the Color Slicing Technique.

Wireless capsule endoscopy represents a color imaging technology in the field of medical endoscopy that is extensively used to detect lesions of the human digestive tract. It is the golden standard in evaluating small bowel lesions, offering a set of digital snapshots difficult to get using other investigation methods. Its major drawbacks are the time consumed for image analysis and the burden for the physicians that must spot and classify lesions within more than 55000 images. This paper carries out a study on the detection of telangiectasia in the small bowel, based on an adapted color slicing technique applied not only on unique frames, but on series of successive frames, performing a global analysis suitable on partial sequences or entire wireless capsule endoscopy movies. We have quantified the extracted features and determined a weighting algorithm to find telangiectasia lesions. For frames containing potential lesions, we have determined features not only for the global image, but also for the normal mucosa surrounding the lesion extracted from the image. This approach allows the physician to see variations of parameters within a frame or a sequence that contains lesions. Experimental results prove that the algorithm is effective in detecting telangiectasia patterns of different images, with an accuracy of 93.88%, reducing thus the time spent for the analysis of the images acquired by wireless capsule endoscopy.

Reduced-order proper orthogonal decomposition extrapolating finite volume element format for two-dimensional hyperbolic equations

This paper is concerned with establishing a reduced-order extrapolating finite volume element (FVE) format based on proper orthogonal decomposition (POD) for two-dimensional (2D) hyperbolic equations. For this purpose, a semi discrete variational format relative time and a fully discrete FVE format for the 2D hyperbolic equations are built, and a set of snapshots from the very few FVE solutions are extracted on the first very short time interval. Then, the POD basis from the snapshots is formulated, and the reduced-order POD extrapolating FVE format containing very few degrees of freedom but holding sufficiently high accuracy is built. Next, the error estimates of the reduced-order solutions and the algorithm procedure for solving the reduced-order format are furnished. Finally, a numerical example is shown to confirm the correctness of theoretical conclusions. This means that the format is efficient and feasible to solve the 2D hyperbolic equations.