Finite-element model development of a uniformly loaded simply supported beam using strain energy density criterion

The paper examines the use of strain energy density distribution within deformed structures as a criterion for the systematic development of finite element models. This criterion has been used in the study of a simply supported beam with uniform loading to identify regions in the model which require a more comprehensive modelling consideration.     

AquaCrop: FAO's crop water productivity and yield response model

With the help of relatively few conservative crop parameters, AquaCrop simulates final crop yield in four steps that are easy to understand, which makes the modeling approach transparent. The steps consist in the simulation of development of the green crop canopy cover, crop transpiration, above-ground biomass, and final crop yield. Temperature and water stresses directly affect one or more of the above processes. Nutrient deficiencies and salinity effects are simulated indirectly by moderating canopy cover development over the season, and by reducing crop transpiration and the normalized water productivity. The effect of CO₂ concentration on biomass is simulated by altering the normalized water productivity. The model requires a relatively small number of explicit parameter values and mostly intuitive input variables. The paper describes the essence of AquaCrop Version 4.0, applications and parameterization of crops, crop responses to elevated CO₂ concentration, soil fertility and salinity, and further model developments.     

Ultimate strength of eccentrically loaded steel plates and box sections

A finite element method is applied to compute the elastic-plastic post-buckling behavior of initially deflected and eccentrically loaded steel plates and box sections. Comparisons with available theoretical and experimental results reveal that the present finite element analysis gives extremely accurate results even with small numbers of elements. Based on an extensive numerical study, an empirical formula is proposed to predict the ultimate strength of steel plates as well as steel box sections with various supporting conditions.     

The effect of aspect ratio on the elastoplastic response of stiffened plates loaded in uniaxial edge compression

A numerical study of the effect of aspect ratio on the buckling and collapse behaviour of flatbar-stiffened plates in compression is described. The plate equations are expressed in finite difference form and the solution is obtained using a dynamic relaxation algorithm. Initial imperfections and residual stresses are also introduced in the study. It was found that, for aspect ratios between 2 and 3.5, buckling occurs in the elastic regime, with the panels exhibiting significant post-buckling strength prior to collapse. For aspect ratios between 1 and 2 buckling occurs as the plastic zone increases, followed by rapid unloading as the panel collapses suddenly. The ultimate strength of the panel reduces with increasing aspect ratio, remaining practically constant at higher aspect ratios. The latter is attributed to the initial single half-wave distortion profile that prohibits the formation of the preferred buckling mode.     

Finite element model updating of honeycomb sandwich plates using a response surface model and global optimization technique

Honeycomb sandwich plates are used widely in the aerospace industry. Building accurate finite element models of honeycomb sandwich plates is necessary for analyzing and optimizing the microvibration that occurs in spacecraft. This study investigated two types of finite element dynamic models of honeycomb plates: a sandwich shell model and a shell-volume-shell model. Two response surface model-based optimization methods and a particle swarm optimization method were compared for updating the finite element models. Finally, we validated the accuracy of the two optimized honeycomb sandwich plate finite element dynamic models by comparing the results obtained by the frequency response functions with experimental data.     

Elasto-plastic analysis of a plane stress problem using linearized yield surface and mixed hardening rule

The objective of this paper is to present the mathematical formulations in the incremental theory of plasticity, which is based on the mixed hardening rule and a linear yield surface. A three-parameter, uniaxial symmetric, linear yield surface suitable for tension-weak as well as equal tension and compression yield stress material is presented. This yield condition, along with the mixed hardening and associated flow rules is used to formulate the constitutive laws for sides and corners of the yield surface. The formulation is based on incremental plasticity with the assumption of small displacements and is suitable for plane stress problems under monotonie and cyclic loading.

The mixed hardening rule, which is mathematically modeled, could be changed to either kinematic or isotropic hardening by a simple change in the model. This hardening rule could handle different degrees of Bauschinger effect, as opposed to kinematic hardening, which assumes only an ideal Bauschinger effect, or isotropic hardening, which does not account for the effect at all. The theory is applied to a ductile material using the finite element method and cyclic loading.     

Indices for the evaluation of wildfire spread simulations using contemporaneous predictions and observations of burnt area

Methods to objectively evaluate performance are critical for model development. In contrast to recent advances in wildfire simulation, there has been limited attention to evaluating fire model performance. Information to validate fire models is typically limited, commonly to a few perimeter observations at a small number of points in time. We review metrics for comparing two burnt areas at a point in time: observed and predicted. These are compared in an idealised landscape and with a case study evaluating the performance of simulations of an Australian wildfire. We assessed: Shape Deviation Index (SDI), Jaccard's coefficient, F1, Sørensen's Similarity and Area Difference Index (ADI). For decomposing fit into error components (overprediction and underprediction) we assessed the partial indices of SDI and ADI, Precision and Recall. The various metrics were evaluated for their ability to represent error and their suitability for use in model improvement frameworks.     

Latent variable based model predictive control: Ensuring validity of predictions

This paper presents a methodology to constrain the optimisation problem in LV-MPC so that validity of predictions can be ascertained. LV-MPC is a model-based predictive control methodology implemented in the space of the latent variables and is based on a linear predictor. Provided real processes are non-linear, there is model-process mismatch, and under tight control, the predictor can be used for extrapolation. Extrapolation leads to bad predictions which deteriorates control performance, hence the interest in validity of predictions. In the proposed approach first two validity indicators on predictions are defined. The novelty in the two indicators proposed is they neglect past data, and so validity of predictions is ascertained in terms of future moves which are actually the degrees of freedom in the optimisation. Second, the indicators are introduced in the optimisation as constraints. Provided the indicators are quadratic, recursive optimisation with linearised constraints is implemented. A MIMO example shows how ensuring validity of predictions neglecting past data can improve closed-loop performance, specially under tight control outside the identification region.     

The dynamics of HIV spread: a computer simulation model

Mathematical modeling of the AIDS pandemic has been limited by the difficulty of satisfactorily representing the marked behavioral heterogeneity that characterizes the various populations at risk. We propose an approach which models the spread of infection as a discrete-event simulation using SIMSCRIPT, a powerful simulation language. The program developed provides sufficient flexibility to adequately represent and study a wide range of risk-group dynamics. Using this tool we have verified the May-Anderson prediction relating contact rate heterogeneity to the rate of HIV spread. We have also been able to assess the sensitivity of the model to the particular choice of distribution for contact rates, disease stage durations, and intercontact intervals. It is thought that this approach will permit the empirical testing of hypotheses which do not lend themselves to a purely mathematical treatment.     

Spectral response of rice crop and its relation to yield and yield attributes

Ground-based radiometric measurements were conducted on six varieties of rice crop during an entire growth cycle using a hand-held seven-band radiometer. Concomitant measurements of some of the yield attributes were also made. Spectral data were also collected on a single variety grown under 12 different fertilizer treatments. Spectral data have been correlated with leaf area index, total wet biomass, total dry biomass, plant water content and final grain, straw and total yield. The results show similar temporal spectral responses of all six varieties and a strong correlation of agronomic parameters with spectral parameters derived from the near-infrared and red radiances. Red and near-infrared radiance ratio and normalized differences were found sensitive to the N fertilizer application but not to the P and K fertilizers. Linear correlations were observed between spectral parameters and final grain, straw and total yield     

Optimality model of unsupervised spike-timing-dependent plasticity: synaptic memory and weight distribution

We studied the hypothesis that synaptic dynamics is controlled by three basic principles: (1) synapses adapt their weights so that neurons can effectively transmit information, (2) homeostatic processes stabilize the mean firing rate of the postsynaptic neuron, and (3) weak synapses adapt more slowly than strong ones, while maintenance of strong synapses is costly. Our results show that a synaptic update rule derived from these principles shares features, with spike-timing-dependent plasticity, is sensitive to correlations in the input and is useful for synaptic memory. Moreover, input selectivity (sharply tuned receptive fields) of postsynaptic neurons develops only if stimuli with strong features are presented. Sharply tuned neurons can coexist with unselective ones, and the distribution of synaptic weights can be unimodal or bimodal. The formulation of synaptic dynamics through an optimality criterion provides a simple graphical argument for the stability of synapses, necessary for synaptic memory.     

Combined use of equivalent center mass and stiffness factors to better estimate frequencies of mass loaded plates

This paper is concerned with the frequency analysis of vibrating plates carrying multiple masses at various positions. By virtue of an equivalent center method, the frequencies of the plates carrying multiple masses can be predicted by using merely the data obtained earlier for the same plate with each corresponding mass component. Further accounting the change of the strain energy into the model, by introducing respective stiffness ratios, enables one to obtain a quick and better estimation of the loaded plates with various boundary conditions. The proposed model is validated through experimentation of a rectangular clamped plate carrying concentrated masses. Analytical and experimental results for the plate system are compared and discussed. It is found that the change in the strain energy should be incorporated in the model, especially for cases of large masses placed on a thin plate, in order to predict well the natural frequency of the amalgamated system from those of the component systems. The excellent performance of the proposed model is due to the fact that the effects of both the different masses and their locations on the frequency have been accounted for implicitly in the respective equivalent center mass and stiffness factors. Although the model considered is rather simple, the proposed methodology can be extended to plates with other geometry and configurations.     

Electronic community factories: the model and its application in the tourism sector

The electronic community factory (ECF) is proposed as an archetype for value creating cross-organization virtual communities of practice in which members engage actively and through domain-specific tools in the practice the community is about. In an ECF the reason for building the community is to appropriate the benefits of virtual networking for developing new products of added-value and collective ownership. In terms of engineering an ECF, the normative perspective adopted is that community management is distinctively different from engaging in the practice the community is about. Respectively, the tools needed for each function should embody this philosophy and be designed accordingly. The paper elaborates on the concept of the ECF and demonstrates its value through a pilot in regional tourism. Virtual ethnographic studies provide insights to operational aspects of the ECF and how it fosters community and practice management. Collectively, the results reported in the paper constitute an alternative model for community-based electronic commerce suitable for value-creating cross organization virtual communities of practice engaging in high quality product assembly in information based industries.     

全局与局部模型对QSAR/QSPR预报能力比较

针对两组数据进行了比较讨论,试图说明在QSAR／QSPR研究中经常碰到的一个基本问题。第一组为一散布度(diver- sity)很大分子结构多样化的大样本数据;第二组则是按照分子结构相似度筛选出来的散布度较小结构相似的小样本数据。对于第一组数据,因数据集分散,全局模型难以完全描述物质结构特征与其性质之间的关系,所得回归结果很差(检验集相关系数Q2=0．68、平均预报偏差(RMSEP)=40．65)。试采用新近提出的局部懒惰回归(Local lazy regression,LLR)对其进行改善,但实际结果是局部模型的效果更差(Q2=0．60、RMSEP=45．05)。继对散布度较小且相对均匀(结构相似)的数据集用LLR方法建立局部模型,此时得到的预报结果(Q2=0．90、RMSEP=24．66)却明显优于全局模型(Q2=O．86、RMSEP=29．37)。     

Unsupervised formation of vocalization-sensitive neurons: a cortical model based on short-term and homeostatic plasticity

The discrimination of complex auditory stimuli relies on the spatiotemporal structure of spike patterns arriving in the cortex. While recordings from auditory areas reveal that many neurons are highly selective to specific spatiotemporal stimuli, the mechanisms underlying this selectivity are unknown. Using computer simulations, we show that selectivity can emerge in neurons in an entirely unsupervised manner. The model is based on recurrently connected spiking neurons and synapses that exhibit short-term synaptic plasticity. During a developmental stage, spoken digits were presented to the network; the only type of long-term plasticity present was a form of homeostatic synaptic plasticity. From an initially unresponsive state, training generated a high percentage of neurons that responded selectively to individual digits. Furthermore, units within the network exhibited a cardinal feature of vocalization-sensitive neurons in vivo: differential responses between forward and reverse stimulus presentations. Direction selectivity deteriorated significantly, however, if short-term synaptic plasticity was removed. These results establish that a simple form of homeostatic plasticity is capable of guiding recurrent networks into regimes in which complex stimuli can be discriminated. In addition, one computational function of short-term synaptic plasticity may be to provide an inherent temporal asymmetry, thus contributing to the characteristic forward-reverse selectivity.     

Nonlinear static and dynamic damped response of isotropic, elastic circular plates

Nonlinear static and dynamic response analysis of clamped-in and simply supported boundary conditions, and immovably constrained and stress-free edge conditions for circular plates of isotropic elastic material with damping, subjected to step pressure pulse excitation are presented. The Von Karman relations are used which are reduced to coupled nonlinear partial differential equations and solved by a one-term solution, applying the Ritz-Galerkin technique to the deflection equation. This yields an ordinary nonlinear differential equation in time. The nonlinear dynamic damped response is obtained by applying the ultraspherical polynomial approximation technique. Plots of static deflection to thickness ratio versus non-dimensional load for different boundary and edge conditions, and the effect of damping on the nonlinear dynamic response for different values of non-dimensional damping factor are presented.     

基于平均化骨骼模型的接骨板优化设计

目前系列化接骨板在材料节约和应力分散上不合理。为快捷设计出理想的接骨板,利用平均化骨骼模型,提出通过语义参数编辑优化设计接骨板的方法。首先,为系列化接骨板在数量与尺寸上的分布合理,构建了一种带权值的平均化骨骼模型;然后,基于平均化骨骼模型设计出具有通用性的参数化接骨板,便于后期编辑修改与优化;最后,利用有限元分析,用基于二分策略调整厚度参数来优化接骨板,在满足应力条件下使接骨板材料尽可能少。实验结果显示,对三叶型接骨板和股骨远端Ⅲ型接骨板优化后体积分别减小2.7%和12.2%,最大应力分别减小56.9%和24.4%,表明所提方法能够节省接骨板材料,分散接骨板应力,是一种有效的接骨板优化设计方法。     

基于混沌扩频的多用户应急通信系统模型设计与仿真分析

针对应急通信中信道噪声干扰严重影响通信质量的实际问题,研究了应急通信系统信道噪声干扰的来源与本质特征,提出了应急通信信道模型;论述了混沌扩频应急通信系统干扰噪声抑制原理,设计了多用户混沌扩频应急通信系统仿真模型,并结合典型信道干扰噪声环境对其BER进行了仿真与分析。仿真实验结果表明,基于混沌扩频的应急通信系统能够有效抑制强噪声干扰,显著改善应急通信系统误码性能。