Constructing efficient substructure-based preconditioners for BEM systems of equations

In this work, a generic substructuring algorithm is employed to construct global block-diagonal preconditioners for BEM systems of equations. In this strategy, the allowable fill-in positions are those on-diagonal block matrices corresponding to each BE subregion. As these subsystems are independently assembled, the preconditioner for a particular BE model, after the LU decomposition of all subsystem matrices, is easily formed. So as to highlight the efficiency of the preconditioning proposed, the Bi-CG solver, which presents a quite erratic convergence behavior, is considered. In the particular applications of this paper, 3D representative volume elements (RVEs) of carbon-nanotube (CNT) composites are analyzed. The models contain up to several tens of thousands of degrees of freedom. The efficiency and relevance of the preconditioning technique is also discussed in the context of developing general (parallel) BE codes.     

A residual-based a posteriori error estimator for a fully-mixed formulation of the Stokes–Darcy coupled problem

In this paper we develop an a posteriori error analysis of a new fully mixed finite element method for the coupling of fluid flow with porous media flow in 2D. Flows are governed by the Stokes and Darcy equations, respectively, and the corresponding transmission conditions are given by mass conservation, balance of normal forces, and the Beavers–Joseph–Saffman law. We consider dual-mixed formulations in both media, which yields the pseudostress and the velocity in the fluid, together with the velocity and the pressure in the porous medium, and the traces of the porous media pressure and the fluid velocity on the interface, as the resulting unknowns. The set of feasible finite element subspaces includes Raviart–Thomas elements of lowest order and piecewise constants for the velocities and pressures, respectively, in both domains, together with continuous piecewise linear elements for the traces. We derive a reliable and efficient residual-based a posteriori error estimator for the coupled problem. The proof of reliability makes use of the global inf–sup condition, Helmholtz decompositions in both media, and local approximation properties of the Clément interpolant and Raviart–Thomas operator. On the other hand, inverse inequalities, the localization technique based on element-bubble and edge-bubble functions, and known results from previous works, are the main tools for proving the efficiency of the estimator. Finally, some numerical results confirming the theoretical properties of this estimator, and illustrating the capability of the corresponding adaptive algorithm to localize the singularities of the solution, are reported.     

Recurrent pattern matching based stereo image coding using linear predictors

The Multidimensional Multiscale Parser (MMP) is a pattern-matching-based generic image encoding solution which has been investigated earlier for the compression of stereo images with successful results. While first MMP-based proposals for stereo image coding employed dictionary-based techniques for disparity compensation, posterior developments have demonstrated the advantage of using predictive methods. In this paper, we focus on recent investigations on the use of predictive methods in the MMP algorithm and propose a new prediction framework for efficient stereo image coding. This framework comprises an advanced intra directional prediction model and a new linear predictive scheme for efficient disparity compensation. The linear prediction model is the main novelty of this work, combining adaptive linear models estimated by least-squares algorithm with fixed linear models provided by the block-matching algorithm. The performance of the proposed intra prediction and disparity compensation methods when applied in an MMP encoder has been evaluated experimentally. Comparisons with the current stereo image coding standards showed that the proposed MMP algorithm significantly outperforms the Stereo High Profile of H.264/AVC standard. In addition, it presents a competitive performance relative to the MV-HEVC standard. These results also suggest that current stereo image coding standards may benefit from the proposed linear prediction scheme for disparity compensation, as an extension to the omnipresent block-matching solution.     

Novel Algorithms for Quadratic Programming by Using Hypergraph Representations

In this paper novel algorithms are introduced for solving NP hard discrete quadratic optimization problems commonly referred to as unconstrained binary quadratic programming. The proposed methods are based on hypergraph representation and recursive reduction of the dimension of the search space. In this way, efficient and fast search can be carried out and high quality suboptimal solutions can be obtained in real-time. The new algorithms can directly be applied to the quadratic problems of present day communication technologies, such as multiuser detection and scheduling providing fast optimization and increasing the performance. In the case of multiuser detection, the achieved bit error rate can approximate the Bayesian optimum and in the case of scheduling better Weighted Tardiness can be achieved by running the proposed algorithms. The methods are also tested on large scale quadratic problems selected from ORLIB and the solutions are compared to the ones obtained by traditional algorithms, such as Devour digest tidy-up, Hopfield neural network, local search, Taboo search and semi definite relaxing. As the corresponding performance analysis reveals the proposed methods can perform better than the traditional ones with similar complexity.     

Optical frequency comb generation based on dual-parallel Mach–Zehnder modulator and intensity modulator with RF frequency multiplication circuit

A new method of generating optical frequency comb by cascading a single dual-parallel Mach–Zehnder modulator and two intensity modulators is proposed. The intensity modulators are modulated by a frequency multiplication circuit of radio frequency. Theoretical model of the system is derived, and optical frequency comb with ultra-flatness and high side-mode suppression ratio can be obtained. The study results show that 72-line optical frequency comb with spectrum power variation of 0.26 dB, SMSR of 21 dB and frequency space of 4 GHz is gotten easily by adjusting the amplitude of radio frequency signal and the direct current bias of the modulators. The simulation result is in good agreement with the theoretical analysis model.     

Air Processing of Thick and Semitransparent Laminated Polymer:Non-Fullerene Acceptor Blends Introduces Asymmetric Current–Voltage Characteristics

Non-fullerene acceptors have recently revolutionized indoor organic photovoltaics (OPVs) with power conversion efficiencies exceeding 30% in laboratory scale. Nevertheless, transferring their superior performance to larger-scale prototyping, i.e., air-processing via roll-to-roll compatible techniques, still shows severe challenges. Herein, the industrial potential of the PM6:IO4Cl blend, which is one of the most successful indoor OPV photoactive layers (PALs), is thoroughly investigated. The corresponding thick and semitransparent laminated devices are fabricated entirely in air, by blade and slot-die coating. Their current–voltage (J–V) characteristics show anomalous features depending on the illumination side, with the cathode side generally outperforming the anode counterpart. Electrical and optical modeling reveal that a plausible cause of such a phenomenon is a dead layer that forms at the PAL/anode contact interface that does not contribute to the photocurrent. Said layer becomes undetectable when the PALs are made thin enough (<35 nm each) leading to symmetric J–V curves and improved light utilization efficiency. By screening the photovoltaic performance of multiple donor:acceptor blends, certain all-polymer and polymer:fullerene PALs are identified as adequately symmetric candidates for thick device up-scaling. Finally, ternary blends based on PM6:IO4Cl:fullerene may constitute a viable route to mitigate the electrical asymmetry detected on conventional binary blends.     

Vitamins as Active Agents for Highly Emissive and Stable Nanostructured Halide Perovskite Inks and 3D Composites Fabricated by Additive Manufacturing

The use of non-toxic and low-cost vitamins like α-tocopherol (α-TCP, vitamin E) to improve the photophysical properties and stability of perovskite nanocrystals (PNCs), through post-synthetic ligand surface passivation, is demonstrated for the first time. Especially interesting is its effect on CsPbI₃ the most unstable inorganic PNC. Adding α-TCP produces that the photoluminescence quantum yield (PLQY) of freshly prepared and aged PNCs achieves values of ≈98% and 100%, respectively. After storing 2 months under ambient air and 60% relative humidity, PLQY is maintained at 85% and 67%, respectively. α-TCP restores the PL features of aged CsPbI₃ PNCs, and mediates the radiative recombination channels by reducing surface defects. In addition, the combination of α-TCP and PNCs facilitates the chemical formulation to prepare PNCs-acrylic polymer composites processable by additive manufacturing. This enables the development of complex shaped parts with improved luminescent features and long-term stability for 4 months, which is not possible for non-modified PNCs. A PLQY ≈92% is reached in the 3D printed polymer/PNC composite, the highest value obtained for a red-emitting composite solid until now as far as it is known. The passivation shell provided by α-TCP makes that PNCs inks do not suffer any degradation process avoiding the contact with the environment and preserve their properties after reacting with polar monomers during composite polymerization.     

一种用于电视系统中的图像清晰度测量算法及实现

本文介绍了数字电视中用于增强图像画质的一种清晰度测量的方法,该方法使用图像中的频率信息、细节信息和局部图像的动态范围来判断当前输入的图像是一幅真实的高清图像还是在信号传输过程中被放大成高清信号的伪高清图像信号。从而利用得出的清晰度测量结果对真实高清图像信号和伪高清图像信号进行不同的清晰度和对比度调整。该方法的设计应用填补了数字电视图像后处理中对于原始信号检测的一个空白。本文介绍了该方法的基本原理和具体实现的算法,并讨论了该算法的硬件实现方案和代价。