Integrating structure and control design to achieve mixed H2/H performance

Present technology in structure design (smart structures, civil structures and aerospace structures) includes the use of feedback control. While retrofitting such active elements can be useful in existing structures, future designs will require something more than retrofitting technology. Future technology will certainly require a more systematic integration of the design of a structure and its active elements. This paper provides a step in that direction. We seek to integrate the design of the structure with its active elements to achieve mixed H2/H performance for the controlled structural system (closed loop system). More specificaly, the approach presented here solves a mixed passive control (structure design) and active control (feedback control law design) problems with performance characterized by system norms such that H performance bounds are guaranteed with les active energy. This approach al ows us to answer the question 'what is an optimal distribution of mass, stiffness, damping and control energy throughout a structure?' The main conclusion drew in this paper is that control design tools can be useful for structure design.     

Large deflection analysis of plates and shells by discrete energy method

The discrete energy method—a special form of finite difference energy approach—is presented as a suitable alternative to the finite element method for the large deflection elastic analysis of plates and shallow shells of constant thickness. Strain displacement relations are derived for the calculation of various linear and nonlinear element stiffness matrices for two types of elements into which the structure is discretized for considering separately energy due to extension and bending and energy due to shear and twisting. Large deflection analyses of plates with various edge and loading conditions and of a shallow cylindrical shell are carried out using the proposed method and the results compared with finite element solutions. The computational efforts required are also indicated.     

夹层板系统碰撞性能数值仿真分析技术

为促进夹层板系统（Sandwich Plate System,SPS）在船舶耐撞结构设计中的应用,用Abaqus分析SPS在碰撞载荷下的数值模型化技术,包括夹芯层和面板的建模方式、连接形式和网格尺寸．根据该技术研究SPS在碰撞冲击载荷作用下的力学性能,如结构损伤变形、碰撞力和结构吸能等．结果表明,SPS建模采用壳一体混合模型（即上、下面板采用壳单元,夹芯层采用体单元）较合理;夹芯层与面板之间采用绑定连接较合理;SPS具有良好的耐撞性能．     

Discrete energy method for the analysis of cylindrical shells

This paper presents an investigation into the use of the closely associated finite difference technique for the analysis of shell structures as a feasible alternative to the finite element method. The method discretises the total energy of the structure into energy due to extension and bending and that due to shear and twisting, contributed by two separate sets of rectangular elements formed by a suitable finite difference network. The derivatives in the corresponding energy expressions are replaced by their finite difference forms and the nodal displacements then constitute the undetermined parameters in the variational formulation. The formulation is also extended to a cylindrical shell element of rectangular planform. The results obtained by DEM are compared with existing results and they show excellent agreement.     

Discontinuous Hamiltonian Finite Element Method for Linear Hyperbolic Systems

We develop a Hamiltonian discontinuous finite element discretization of a generalized Hamiltonian system for linear hyperbolic systems, which include the rotating shallow water equations, the acoustic and Maxwell equations. These equations have a Hamiltonian structure with a bilinear Poisson bracket, and as a consequence the phase-space structure, “mass” and energy are preserved. We discretize the bilinear Poisson bracket in each element with discontinuous elements and introduce numerical fluxes via integration by parts while preserving the skew-symmetry of the bracket. This automatically results in a mass and energy conservative discretization. When combined with a symplectic time integration method, energy is approximately conserved and shows no drift. For comparison, the discontinuous Galerkin method for this problem is also used. A variety numerical examples is shown to illustrate the accuracy and capability of the new method.     

Partitioning elements of the Periodic Table via fuzzy clustering technique

In this paper we present fuzzy clustering results of the elements of the Periodic Table according to several physical properties related directly to their electronic structure (ionization potential, cohesion energy, conductivity, atomic number). We find that the resulting classes reproduce the Periods of the Periodic Table. Using additional physical properties not directly related to the electronic structure of the elements (compressibility, density) in the fuzzy clustering procedure, we find that the elements of the Periodic Table are partitioned in another way.The authors are grateful to the referees for their valuable suggestions that contributed to improve the presentation of their work.     

Polarization independent broadband metamaterial absorber for microwave applications

A new polarization independent broadband metamaterial absorber (MA) structure based on split ring resonators which are loaded with lumped elements and via connection lines is proposed. The designed structure shows a perfect absorption between 4 and 16 GHz which is validated by simulation studies. Experimental study is only made for the structure that has no via connections and no the lumped element resistors to show the importance of these entities in the proposed metamaterial structure. Both numerical and experimental study results show that broadband MA property depends on the resistors and via connections on the proposed structure. By having high absorption in a wideband range which is numerically demonstrated, the proposed structure can be used in energy harvesting or wireless power transfer applications with higher efficiencies.     

Ab initio lattice stability in comparison with CALPHAD lattice stability

A systematic first-principles calculation for the total energies of 78 pure elemental solids has been performed at zero Kelvin using the projector augmented-wave method within the generalized gradient approximation. The total energy differences, i.e. lattice stabilities, among the face-centered-cubic (fcc), body-centered-cubic (bcc), and hexagonal-close-packed (hcp) crystal structures are studied and compared with the Scientific Group Thermodata Europe (SGTE) database developed by the CALPHAD method. For non-transitional elements, favorable comparison is observed, while for the majority of transition elements, particularly the V, Cr, Mn, Fe, and Co group elements, significant discrepancies exist. The Bain/tetragonal distortion analysis between fcc and bcc structures shows that when one structure is stable, the other is unstable, and the higher the energy of the unstable structure, the larger the discrepancy. Through analysis of the alloying effect in binary systems, we conclude that the lattice stability of unstable structures obtained through extrapolation of first-principles calculations in binary systems is close to the SGTE lattice stability obtained by the CALPHAD method.     

Metastable lattice stabilities for the elements

Lattice stabilities for the metastable FCC (Al), BCC (A2) and CPH (A3) allotropes of 43 elements have been evaluated. The results are based on (1) Assessed stable phase data; (2) Phase boundary extrapolations from binary alloy, and elemental pressure-temperature, phase diagrams; (3) A relationship between the entropy of fusion, crystal structure and melting point; (4) Stacking fault energies; (5) Periodic and group trends and (6) First principle electronic energy calculations. Qualitative trends proposed by previous thermochemical evaluations for the transition metals are to a large extent confirmed. However, the evaluated energy differences between the different crystal allotropes are substantially higher and can be closer in magnitude to those predicted by ab-initio electron energy calculations, although particular discrepancies, for example concerning Cr(FCC), still remain. Many of the changes proposed here arise from the reassessment of stable phase data, particularly with respect to recently measured heats of fusion of the high melting point elements.     

流程工业能耗系统多维子模型及其开放性集成框架

流程工业组成因素多,运行结构复杂,针对单一模型无法合理、全面地描述其能源消耗系统各因素及其关联关系的问题,提出了流程工业能耗系统多维子模型及其集成化构建方法。首先,综合设备、能量、信息、人员等多方面因素,从静态结构描述、能源消耗的动态行为以及企业能耗系统运作的目的性3个层面建立流程工业能耗系统多维子模型;然后,分析了不同视角子模型关联关系;最后,详细研究了能耗系统多维模型的开放性集成框架,从而为流程工业能耗系统建立集信息流、能源流和物料流为一体,并同时反映内、外部关键因素的多维集成化模型提供了方法支持。这种内外关联、高度集成的流程工业能耗系统模型的构建将为流程工业综合性能效评估奠定基础。     

基于 Web 的二元化合物结构推演自动流程实现

结构原型是通过任一定义的具有代表性的条件筛选出来的具有相同条件的结构类型。结构推演是一种理论上预测新化合物的方法。首先用特定元素替换大量的结构原型中的元素,得到很多化合物结构;然后,利用第一性原理对这些结构进行结构优化和能量计算,得到每个结构的能量值、形成焓等物理量;最终以形成焓为依据预测这些结构是否有可能形成稳定化合物。这一系列的处理和计算就是一次结构推演。结构推演是一种典型的高通量筛选,涉及到多通道,多任务,高并发的计算。MatCloud 材料计算平台是一个国内自主研发的高通量材料计算平台,其中一个主要功能就是实现了这些多通道,多任务,高并发作业的自动计算流程。为了更为有效及高效地开展结构推演,本论文的工作是在 MatCloud 材料计算平台已有的工作基础上,开发出一个基于 Web 的结构推演自动流程计算模块。     

基于 DNA 组装的纳米光电技术新进展

新型纳米光电器件需要在纳米尺度实现对光学元件进行精确和可控的空间排列。DNA具有独特的碱基配对识别机制和双螺旋结构,以及制备简单,形状和尺寸均可程序化设计,并可精确定点修饰等优势,因此已经作为模板组装各类超分子广泛应用于纳米光电和纳米医学等领域。DNA组装的光子阵列是将一系列的发色官能团根据设计需要精确定位在DNA模板上,通过多步荧光共振能量转移,完成光子的传递过程。文章主要综述了近年来基于DNA组装的光子阵列取得的新进展,并对以后的发展进行展望。     

An optimization method for dynamics of structures with repetitive component patterns

The design obtained from a topology optimization problem can be sensitive to the type and details of the ground structure used. A new type of ground structure containing hinged beam elements is described that increases the types of elements in the ground structure available to the optimizer. In addition to the pin-ended truss and rigidly-connected beam elements that have traditionally been used, two new types of elements are introduced: (1) a beam with a hinge on one end and a solid connection on the other end, (2) a beam with hinges on both ends. These elements have different deflection characteristics than those of the typical truss or beam elements, and can be used in ground structure-based compliant mechanism design. Given a reduced ground structure, these new elements effectively increase the design space. Pin-ended members with lengths larger than those of the ground structure cell size are permitted to develop, reducing the sensitivity of solutions to the cell density of the reduced ground structure. Furthermore, because these longer members do not require intermediate lateral support to provide stability, as series connections of shorter pin-ended members do, fewer ancillary members are required. Two compliant mechanism problems are solved to demonstrate the effectiveness of these new elements.     

Bifurcation, pre- and post-buckling analysis of frame structures

A procedure is developed for investigating the stability of complex structures that consist of an assembly of stiffened rectangular panels and three-dimensional beam elements. Such panels often form one of the basic structural components of an aircraft or ship structure. In the present study, the stiffeners are treated as beam elements, and the panels between them as thin rectangular plate elements, which may be subject to membrane and/or bending and twisting actions.

The main objective of the study is the determination of the critical buckling loads and the generation of the complete force-deformation behavior of such structures within a specified load range, based on the use of a computer program developed for this purpose. The present formulation can trace through the postbuckling or post limit behavior whether it is of an ascending or descending type. A limit load extrapolation technique is automatically initiated within the computer program, when the stability analysis of an imperfect or laterally loaded structure is being carried out.

The general approach to the solution of the problem is based on the finite element method and incremental numerical solution techniques. Initially, nonlinear strain-displacement relations together with the assumed displacement functions are utilized to generate the geometric stiffness matrices for the beam and plate elements. Based on energy methods and variational principles, the basic expressions governing the behavior of the structure are then obtained. In the incremental solution process, the stiffness properties of the structure are continuously updated in order to properly account for large changes in the geometry of the structure.

The computer program developed during the course of this study is referred at as GWU-SAP, or the George Washington University Stability Analysis Program.     

Energy shaping,interconnection and damping assignment,and integral control in the bond graph domain

This paper presents a methodology to perform energy shaping and interconnection and damping assignment in the bond graph domain, and addresses a new result on integral action control which improves the robustness of these passivity based control methods, which were first introduced on the port-controlled Hamiltonian systems with dissipation formalism. The methods perform expressing the desired closed-loop energy, interconnection and damping properties on a so-called target bond graph, which is built as follows on the plant bond graph: (i) adding virtual storage elements enforces a minimum on the target bond graph energy at a prespecified stable closed-loop equilibrium state, (ii) changing the R-field and its interconnection with the rest of the graph assigns a new dissipation function, and (iii) suitably inserting of power conserving elements (bonds and other structural BG-elements) among junctions yields the desired power conserving interconnection structure. The control law is then determined developing physically based heuristics and formal techniques on the target and plant bond graphs, which deliver a set of partial differential equations to be solved. The method to achieve integral control consists in adding to the target bond graph virtual elements representing the integral action and a change of variables, and then computing the integral control law with standard BG equation-reading procedures. These BG heuristics and prototyping allow to provide integral action on outputs of relative degree greater than one, a contribution of this work not previously available in the literature. This shows that the physical properties of bond graphs are beneficial not only to expediently perform methods contributed by control theory, but also to derive new theoretical results.     

Average-preserving symmetries and energy equipartition in linear Hamiltonian systems

This paper analyzes energy equipartition in linear Hamiltonian systems in a deterministic setting. We consider the group of phase space symmetries of a stable linear Hamiltonian system, and characterize the subgroup of symmetries whose elements preserve the time averages of quadratic functions along the trajectories of the system. As a corollary, we show that if the system has simple eigenvalues, then every symmetry preserves averages of quadratic functions. As an application of our results to linear undamped lumped-parameter systems, we provide a novel proof of the virial theorem, which states that the total energy is equipartitioned on the average between the kinetic energy and the potential energy. We also show that under the assumption of distinct natural frequencies, the time-averaged energies of two identical substructures of a linear undamped structure are equal. Examples are provided to illustrate the results.     

Raising natural frequencies of a structure via surface-grooving technique

Structural dynamics modification (SDM) is a very effective technique to improve a structure’s dynamic characteristics by adding or removing auxiliary structures, changing material properties and shape of structure. Among the SDM techniques, changing or modifying structure shape to raise its natural frequencies has been mostly relied on engineer’s experience and time-consuming trial-and-error process. To develop a systematic method to modify structure shape, surface-grooving technique is studied. In this work, the shape of base structure is modified to improve its dynamic characteristics such as natural frequencies via surface-grooving technique. Grooving shape is formed by merging the neighboring small embossed elements after analyzing frequency increment sensitivities of all the surrounding embossed elements. All this process is targeted to pack in a software to get an optimum grooving shape automatically. In this package, the initial grooving position starts from the element having the highest modal strain energy then it expands into neighboring elements. The range of grooving area for checking its frequency sensitivities is restricted only to their surrounding elements to reduce its computation effort. The developed algorithm was tested with an L-shaped plate and hard disk drive (HDD) cover to raise its natural frequency by giving some groove on its surface. Also, the grooved HDD cover design was manufactured using rapid prototyping and tested to prove the effectiveness of the surface grooving as a SDM tool.