Dispersion Problem Research Articles

Homogeneous silver nanoparticles decorating 3D carbon nanotube sponges as flexible high-performance electromagnetic shielding composite materials

Nowadays flexible high-performance electromagnetic interference (EMI) shielding materials are greatly demanded in military and daily purpose. To yield high shielding effectiveness (SE), the traditional metallic EMI materials always require a large mass of loading in a composite matrix, which is uneconomic and harmful to the mechanical property of the materials. Herein the homogeneous decoration of Ag nanoparticles on a light-weight and robust carbon nanotube sponge (CNTS) based 3D conductive scaffold is introduced. The flexible Ag/CNTS/PDMS composites exhibit an extremely high and tunable EMI shielding performance with improved mechanical strength and high conductivity. The maximum EMI SE is measured to be even greater than 90 dB in the X-band (8–12 GHz) with a low loading of Ag (3%, wt %) and CNTS (4%, wt %). In addition, the 3D inter-connected pore structure enables EM waves to reflect multiple times inside the Ag/CNTS hybrids which results in a strong absorption ability. The total EMI shielding property can be further conveniently controlled by tuning the Ag-deposition parameters as well as the thickness of the sample. The current approach successfully avoids the dispersion problem of the fillers and will be of great importance in development of high-performance EMI shielding materials.

On the Frequency Dispersion in DBF SAR and Digital Scalloped Beamforming

Digital beamforming (DBF) with Scan-On-Receive (SCORE) in elevation is a powerful technique for spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide-swath (HRWS) images. DBF in spaceborne SAR should be implemented in real-time to reduce the volume of the downloaded data. Current digital beamformers in SAR systems usually use phase shift instead of a time delay network to ensure real-time implementation. However, conventional phase shift DBF SAR with narrow array bandwidth is not applicable for broadband signals. The beam steering deviation as a function of frequency is obvious for signals with large fractional bandwidth, which appears as a phenomenon of frequency dispersion. If the signal bandwidth is wider than the array bandwidth, frequency dispersion can significantly deteriorate the signal-to-noise ratio (SNR) and must be well relieved. This article analyzes the frequency dispersion in DBF SAR and proposes an innovative DBF-SCORE scheme with specially designed scalloped beam. The proposed digital scalloped beamforming (DSBF) scheme is effective in relieving the influence of frequency dispersion in DBF SAR. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme. The proposed DSBF scheme gives practical solution to the problem of frequency dispersion in broadband DBF SAR, which makes DBF-SCORE a more practical technique for future spaceborne HRWS SAR.

Enhancement of the dispersion and catalytic performances of copper in the hydrogenation of cinnamaldehyde by incorporation of aluminium into mesoporous SBA-15 silica

Copper is active in hydrogenation reactions, but it is a base metal difficult to disperse inside mesostructured supports, with adverse consequences on the catalytic properties. Incorporating aluminium into mesoporous SBA-15 silicas allows solving both problems of Cu dispersion and activity. The incorporation of 20 wt.% Al2O3 in the SBA-15 host structure leads to an increase of the copper metallic surface area by a factor 6 compared to the pure siliceous SBA-15-supported catalyst. The support also proves to be stable upon exposure to the impregnation aqueous solution used for copper introduction. The incorporation of Al provides Lewis acidic sites that favour the transformation of cinnamaldehyde into cinnamyl alcohol, a feature usually assigned to larger metal particles. As a consequence, the catalytic activity of Cu catalysts is enhanced compared to silica-supported Cu catalysts, while the chemoselectivity towards the unsaturated alcohol appears to be similar to that of a mesoporous alumina-supported system.

Non-invasive respiratory support for patients with novel coronavirus pneumonia: clinical efficacy and reduction in risk of infection transmission.

Non-invasive respiratory support for patients with novel coronavirus pneumonia: clinical efficacy and reduction in risk of infection transmission.

Heuristics for the capacitated dispersion problem

AbstractIn this paper, we investigate the adaptation of the greedy randomized adaptive search procedure (GRASP) and variable neighborhood descent (VND) methodologies to the capacitated dispersion problem. Dispersion and diversity problems arise in the placement of undesirable facilities, workforce management, and social media, among others. Maximizing diversity deals with selecting a subset of elements from a given set in such a way that the distance among the selected elements is maximized. We target here a realistic variant with capacity constraints for which a heuristic with a performance guarantee was previously introduced. In particular, we propose a hybridization of GRASP and VND implementing within the strategic oscillation framework. To evaluate the performance of our heuristic, we perform extensive experimentation to first set key search parameters, and then compare the final method with the previous heuristic. Additionally, we propose a mathematical model to obtain optimal solutions for small‐sized instances, and compare our solutions with the well‐known LocalSolver software.

Effect of carbon nanotubes sonication on mechanical properties of cement pastes

Abstract The excellent mechanical properties of carbon nanotubes (CNTs) make them a highly attractive material for use as reinforcement in cement composites. However, it is established that this nanomaterial presents a severe dispersion problem in cement matrices owing to its high specific surface area. An alternative method for dispersing the CNTs is the use of sonication in the presence of a superplasticizer admixture. However, high energy sonication can severely damage CNTs, which can affect their behavior in cement pastes. Thus, it is necessary to optimize the CNTs sonication parameters to optimize the performance of CNT-cement pastes. After sonication with different energies of CNTs water suspensions, the CNTs dispersion was evaluated through UV-Vis spectroscopy and the eventual CNTs damage observed by TEM. Workability, compressive and flexural strengths of the CNTs-cement pastes were also measured. The results revealed that the sonication parameters can be optimized to enhance the effect of CNTs on cement paste behavior.

A Novel Outdoor Positioning Technique Using LTE Network Fingerprints.

In recent years, wireless-based fingerprint positioning has attracted increasing research attention owing to its position-related features and applications in the Internet of Things (IoT). In this paper, by leveraging long-term evolution (LTE) signals, a novel deep-learning-based fingerprint positioning approach is proposed to solve the problem of outdoor positioning. Considering the outstanding performance of deep learning in image classification, LTE signal measurements are converted into location grayscale images to form a fingerprint database. In order to deal with the instability of LTE signals, prevent the gradient dispersion problem, and increase the robustness of the proposed deep neural network (DNN), the following methods are adopted: First, cross-entropy is used as the loss function of the DNN. Second, the learning rate of the proposed DNN is dynamically adjusted. Third, this paper adopted several data enhancement techniques. To find the best positioning fingerprint and method, three types of fingerprint and five positioning models are compared. Finally, by using a deep residual network (Resnet) and transfer learning, a hierarchical structure training method is proposed. The proposed Resnet is used to train with the united fingerprint image database to obtain a positioning model called a coarse localizer. By using the prior knowledge of the pretrained Resnet, feed-forward neural network (FFNN)-based transfer learning is used to train with the united fingerprint database to obtain a better positioning model, called a fine localizer. The experimental results convincingly show that the proposed DNN can automatically learn the location features of LTE signals and achieve satisfactory positioning accuracy in outdoor environments.

The nickel based composite coating fabricated by pulse electroplating through graft between nano-TiN and graphene oxide

In the present study, inspired by the natural phenomenon of insects stuck by spider web, nano TiN particles were grafted onto graphene oxide (GO) with the help of polydopamine (PDA) by the reaction between amine in PDA and carboxyl in GO. The results of FTIR and XPS confirm that nano TiN particles are grafted onto GO successfully, which solves the dispersion problem of micro-content nano TiN particles in the fabrication of composite coatings by pulse electroplating. The introduction of nano TiN particles results in the grain refinement of the coating. Due to the synergistic effect of grain refinement and nano TiN particles, the hardness and wear resistance of the coating are enhanced successfully.

Max–min dispersion on a line

Given a set P of n locations on which facilities can be placed and an integer k, we want to place k facilities on some locations so that a designated objective function is maximized. The problem is called the k-dispersion problem. For instance it is desirable to locate fire departments far away each other. In this paper we give a simple $$O((2k^2)^k n)$$ time algorithm to solve the max–min version of the k-dispersion problem if P is a set of points on a line. If k is a constant then this is an O(n) time algorithm. This is the first O(n) time algorithm to solve the max–min k-dispersion problem for the set of “unsorted” points on a line. If P is a set of sorted points on a line, and the input is given as an array in which the coordinates of the points are stored in the sorted order, then by slightly modifying the algorithm above one can solve the dispersion problem in $$O(\log n)$$ time. This is the first sublinear time algorithm to solve the max–min k-dispersion problem for the set of sorted points on a line.

A General and Programmable Synthesis of Graphene-Based Composite Aerogels by a Melamine-Sponge-Templated Hydrothermal Process

Three-dimensional (3D) graphene networks are performance boosters for functional nanostructures in energy-related fields. Although tremendous intriguing nanostructures-decorated 3D graphene network...

Acoustic and elastic finite-difference modeling by optimal variable-length spatial operators

Time-space domain finite-difference modeling has always had the problem of spatial and temporal dispersion. High-order finite-difference methods are commonly used to suppress spatial dispersion. Recently developed time-dispersion transforms can effectively eliminate temporal dispersion from seismograms produced by the conventional modeling of high-order spatial and second-order temporal finite differences. To improve the efficiency of the conventional modeling, I have developed optimal variable-length spatial finite differences to efficiently compute spatial derivatives involved in acoustic and elastic wave equations. First, considering that temporal dispersion can be removed, I prove that minimizing the relative error of the phase velocity can be approximately implemented by minimizing that of the spatial dispersion. Considering that the latter minimization depends on the wavelength that is dependent on the velocity, in this sense, this minimization is indirectly related to the velocity, and thus leads to variation of the spatial finite-difference operator with velocity for a heterogeneous model. Second, I use the Remez exchange algorithm to obtain finite-difference coefficients with the lowest spatial dispersion error over the largest possible wavenumber range. Then, dispersion analysis indicates the validity of the approximation and the algorithm. Finally, I use modeling examples to determine that the optimal variable-length spatial finite difference can greatly increase the modeling efficiency, compared to the conventional fixed-length one. Stability analysis and modeling experiments also indicate that the variable-length finite difference can adopt a larger time step to perform stable modeling than the fixed-length one for inhomogeneous models.

Investigation of interunit dispersion in 2D street canyons: A scaled outdoor experiment

Interunit dispersion problems have been studied previously mainly through on-site measurements, wind tunnel tests, and CFD simulations. In this study, a scaled outdoor experiment was conducted to examine the interunit dispersion characteristics in consecutive two-dimensional street canyons. Tracer gas (CO2) was continuously released to simulate the pollutant dispersion routes between the rooms in street canyons. The wind velocity, wind direction, air temperature, and tracer gas concentrations were monitored simultaneously. Two important parameters, the air exchange rate and reentry ratio, were analyzed to reveal the ventilation performance and interunit dispersion of the rooms in the street canyons. Based on the real-time weather conditions, it was found that the ventilation performance of the source room varied according to the room location. The air exchange rate distribution of the leeward-side room was more stable than that of the windward side. The tracer gas was mainly transported in the vortex direction inside the street canyon, and the highest reentry ratio was observed at the room nearest to the source room along the transportation route. In addition, under real weather conditions, the rooms in the street canyon have a high probability of experiencing a high reentry ratio based on the maximum reentry ratio of each room. This study provides authentic airflow and pollutant dispersion information in the street canyons in an urban environment. The dataset of this experiment can be used to validate further numerical simulations.

A Chelyshkov polynomial based algorithm to analyze the transport dynamics and anomalous diffusion in fractional model

The evolution equations with fractional or variable order derivatives can deliver a proper mathematical modeling to define the transport dynamics and anomalous diffusion in complex dynamical structures. Herein, a hybrid method based on operational matrices of derivative is proposed and successfully applied to explore the solution of mobile–immobile advection–dispersion problem of variable order. The variable order of the model is considered as function of space and time. The operational matrices of derivative named exact and approximate are constructed with the aid of two different approaches and related theorems are available to support the mathematical justification. The error bound and convergence analysis is presented to validate the mathematical formulation of the computational algorithm. A comparative study is enclosed in our investigation which endorses the credibility of the exact operational matrix of derivative. The numerical simulations for various problems are encountered and set of graphs are presented. The numerical examples are endorsing that the proposed mathematical algorithm is computationally effective and efficient tool and one can extend it to other physical problems of fractional or variable order.

An efficient low complexity algorithm for box-constrained weighted maximin dispersion problem

<p style='text-indent:20px;'>The box-constrained weighted maximin dispersion problem is to find a point in an <inline-formula><tex-math id="M1">\begin{document}$ n $\end{document}</tex-math></inline-formula>-dimensional box such that the minimum of the weighted Euclidean distance from given <inline-formula><tex-math id="M2">\begin{document}$ m $\end{document}</tex-math></inline-formula> points is maximized. In this paper, we first propose a two-phase method to solve it. In the first phase, we adopt a block successive upper bound minimization (BSUM) algorithm framework and choose a special piecewise linear upper bound function for the weighted maximin dispersion problem. The per-iteration complexity of our algorithm is very low, since the subproblem is a one-dimensional piecewise linear minimax problem over the box constraints, or eqivalently, a two-dimensional linear programming problem which can be solved in at most <inline-formula><tex-math id="M3">\begin{document}$ O(m) $\end{document}</tex-math></inline-formula> time by existing algorithms. In the second phase, a useful rounding is employed to enhance the solution. Moreover, we propose another strengthened two-phase algorithm, which employs a maximum improvement successive upper-bound minimization (MISUM) algorithm instead of BSUM algorithm in the first phase. At each step, only the block that provides the maximum improvement of the upper bound function is updated. Then, it can be proved that every limit point of the iterate generated by this strengthened algorithm is a stationary point. Numerical results show that the proposed algorithms are efficient.

Clare Island by John Feehan

61 BOOK REVIEW Clare Island John Feehan Royal Irish Academy, 2019 i-x + 309pp. RRP €40 ISBN: 978–1–911479–13–0 In this mighty tome on Clare Island, John Feehan digests, analyses and interprets in a comprehensive but readable style the results of undoubtedly one of the most intensive scientific and cultural surveys ever undertaken of a location in Ireland. Feehan, a brilliant academic and teacher of natural sciences and agriculture, and an accomplished author, is one of the few who could have taken on such a daunting task with apparent ease (and he generously acknowledges the support he received from his many colleagues and friends, including the people of Clare Island). The Original Clare Island Survey, coordinated by the renowned naturalist Robert Lloyd Praeger, was undertaken largely between 1909 and 1911. The results were published in 68 parts (one part was not published) in Volume XXXI of the Proceedings of the Royal Irish Academy (1911–15). Praeger assembled a team of the best experts available to address two main objectives:(i) the detailed biological study of a chosen area in western Ireland with the desire of adding to our knowledge of the flora and fauna of the extreme verge of the European continent; and (ii) the examination of an insular area, with a view of throwing light on the question of island life and the problems of dispersal (Praeger, Part 68, General Summary). On the first objective, the results greatly exceeded expectations, with numerous additions to the flora and fauna of Ireland and the British Isles as well as many species new to science. On the second objective,the proximity of Clare Island to the mainland proved a factor that diminished its value for the investigation of insular problems. The idea of a New Survey of Clare Island was first mooted by the Royal Irish Academy in 1988 and was eventually launched in 1992. Like its predecessor , a team of best available experts was assembled with the objective of establishing a new baseline using modern ecological techniques and approaches that would be invaluable for future environmental monitoring.The New Survey would also allow comparison and evaluation of change over the century between the two surveys. With the New Survey adopting a more ecological focus rather than taxonomic, the results are published according to broad themes in ten volumes by the Royal Irish Academy (1999 onwards, with two volumes yet to be published).As with the Original Survey,the New Survey can be considered an outstanding success in providing a new ecological and cultural baseline for the island. However, John Feehan rightly points out that it has not entirely succeeded in its aim of making and evaluating detailed comparison between the island of a hundred years ago and today.This is due in part to differences in survey methodology and intensity of effort between the two but also to numerous taxonomic changes and revisions to species concepts. Indeed, Praeger and his colleagues could not have imagined the modern techniques available, such as Lidar which uses remote sensing technology to show topographical detail of incredible precision. The book comprises a foreword by Michael Viney, ten main chapters, a comprehensive bibliography , a general index and a taxonomic index (prepared by Martin Steer). It is exceptionally well illustrated, with colour photographs (many by John Feehan and other surveyors but also usage of online imagery), high quality reproductions of photographs from the Original Survey (R.H.Welch) as well as from the Charles Browne collection, various Lidar images, a multitude of maps including the 1838 Ordnance Survey Fair Plan maps, and many other drawings including various taxonomic figures from the Original Survey.The quality and reproduction of the illustrations is excellent throughout but the wraparound cover photograph of Clare Island at sunset (by P.J. Rankin) can only be described as stunning. Each chapter is supported by notes which follow the main text—these vary from references to short paragraphs of explanatory information. The Introduction (Chapter 1) gives the background to the two Clare Island surveys as well as a short biography of R. Ll. Praeger. Chapters 2 and 3 cover the geology (Geology:the island’s rock framework ) and geomorphology (The legacy...

Firth bias correction for estimating variance components of logistics linear mixed model using penalized quasi likelihood method

Firth bias correction originally was applied to correct bias of the variance components estimator that obtained by the maximum likelihood method. Extensive research has shown that Firth bias correction is powerful to reduce bias for normal distributed response model. Questions have been raised about the use of Firth bias correction in binomial distributed response model which has under dispersion problem. The motivation of this study is giving contribution to exploring the Firth bias correction for binomial distributed response model. The binomial distributed response model which is estimated by the maximum likelihood method obtain an under-dispersion estimator. Therefore, the Penalized Quasi-Likelihood (PQL) is used as alternative numerical method to estimate the model. This paper aims to investigate whether the Firth method can reduce bias of the variance components using the PQL technique in longitudinal data.

MODELING NORMAL WAVES IN FUNCTIONALLY GRADED LAYERS BASED ON THE UNIFIED HIERARCHICAL FORMULATION OF HIGHER-ORDER PLATE THEORIES

The normal wave dispersion in functionally graded plates investigated by implying the hierarchical theory of plates−improving contribution to the Vekua-Amosov theory. The combined use of the Lagrangian formalism of the analytical dynamics of continuum, dimensional reduction method, and of the biorthogonal expansion technique allows one to obtain the hierarchical traditional, as well as semianalytical finite element models under the unified general variational formulation. The comparative analysis of the convergence of approximate solutions of the wave dispersion problem for the isotropic homogeneous plate is performed for the locking phase frequencies. The wave dispersion in power graded plates with symmetric through-thickness structure is considered, the convergence of solutions is analyzed for both orthogonal polynomial and finite element approximation, and the dependence of the locking frequencies on the power index is studied.

Operator splitting method for numerical solving the atmospheric pollutant dispersion problem

The paper discusses the numerical modeling of transport and diffusion of air pollutants in the atmospheric boundary layer. There was developed a mathematical model of industrial emissions spread in the atmosphere taking into account the deposition velocity of fine particles. The model is described by multidimensional partial differential equations with appropriate initial and boundary conditions. The basic laws of hydrothermodynamics were used in deriving the model. In order to obtain the numerical solution of the problem, we used one of the splitting methods according to physical processes involved (transport, diffusion and absorption), as well as a second-order implicit finite-difference scheme in time. Analysis of numerical results showed that the developed computational algorithm provides sufficient accuracy of the problem solution compared with field measurement data and it has a certain advantage over other numerical methods. In the course of computational experiments, there was determined the degree of influence of such parameters as wind speed and direction, absorption coefficient and physicomechanical properties of particles on the process of atmospheric air pollutants dispersion.

Solute transport in porous media studied by lattice Boltzmann simulations at pore scale and x-ray tomography experiments.

With the aid of nondestructive microfocus x-ray computed tomography (CT), we performed three-dimensional (3D) tracer dispersion experiments on randomly unconsolidated packed beds. Plumes of nonreactive sodium iodide solution were point injected into a sodium chloride solvent as a tracer for the evaluation of the dispersion process. The asymptotic dispersion coefficient was obtainable within the experimental scale and was summarized over Péclet numbers from 11.7 to ∼860. Then, the lattice Boltzmann method and moment propagation method were used to elucidate the mechanisms embedded in the dispersion phenomenon. The methods were rigorously verified against the classical Taylor dispersion problem and extended to simulate fluid flow and tracer dispersion in high-resolution 3D digital porous structures from CT. The method of moments, Lagrangian velocity correction function, and dilution index were thoroughly analyzed to evaluate the dispersion behaviors. Numerical simulations revealed ballistic and superdiffusive regimes at the transient times, whereas asymptotic dispersion behaviors appear at longer characteristic times. Besides, the observed transient times unanimously persist over convective length scales of around 12 particles transversely and 16 particles longitudinally. The estimated dispersion coefficients from simulation are in consistence with the experimental result. Furthermore, the simulation also enabled the identification of regimes, including diffusive, power law, and mechanical dispersion. Thus, the proposed experimental and computational schemes are of practical means to study dispersion behaviors by direct pore scale imaging and modeling.

On a New Method for the Rapid Determination of the Strength of Industrial Emissions of a Pollutant

A method has been developed for the operational determination of the strength of industrial emissions of a pollutant by an APS without direct measurements of concentrations and parameters in it, which relates to industrial ecology and can be used in the state environmental control or monitoring system to increase its effectiveness and responsiveness. The essence of the method is to use the information provided by public authorities. An information base is being created for all organized APS located in the observed urbanized area, which contains information about the sources of pollutant emissions and their characteristics; establishes the fact of unauthorized pollutant release by detecting elevated concentrations of various pollutants in the observed area according to the results of quantitative chemical analysis of atmospheric air samples and determines their ratio; the inverse dispersion problem is solved and the area of the location of sources of unauthorized pollutant emissions is identified, the source of unauthorized pollutant emissions is determined, and then the strength of this pollutant emission is estimated (Mi, g/s).