Discrete Structural Models Research Articles

Multi-objective structural profile optimization of ships based on improved Artificial Bee Colony Algorithm and structural component library

This paper addresses the multi-objective, design variables discrete problem of river boat profile structure optimization. The study considers not only the economic performance but also the stability of the boat to a certain extent when selecting the optimization objectives. First, the standard component library of the vessels is established and the discrete structural model is converted into a continuous optimization model based on the component library index. Second, to balance the global search and local search capability of the optimization algorithm, an adaptive Artificial Bee Colony Algorithm (I-ABC) is proposed. I-ABC uses a modified chaotic mapping to improve the diversity of nectar sources, and adaptive search and following mechanisms are adopted for the leading and following bee stages, respectively. Then, a Dominating Artificial Bee Colony Algorithm (D-ABC) is proposed for multi-objective optimization by introducing Pareto domination and considering the degree of individual crowding in terms of hypersphere neighborhood. Then, numerical simulations and comparative analysis of the proposed algorithm are performed using different test functions, and the results show that I-ABC and D-ABC have excellent performance in both single-objective and multi-objective optimization. Finally, using the component library established in this paper and the proposed improved Bee Colony Algorithm to optimize the profile structure of a chemical-liquid tanker, the optimization results show that the maximum reduction of the ship profile area is 5.08% and the maximum reduction of the vertical center height is 7.427%. The proposed optimization algorithm is also applicable to other river boats.

Optimal Design of Funicular Arches Under Equally Spaced Point Loads for Maximum Buckling Load

This paper presents the optimal designs of pinned supported funicular arches under equally spaced point loads for maximum in-plane buckling load. Under such loading conditions, the funicular arch shapes comprise straight arch members between the point loads, that is, following the shape of the bending moment diagram of an equivalent simply supported beam under the same loading condition. Two classes of funicular arch optimization problems are considered herein. The first class of funicular arches imposes a constraint on the cross-sectional area to be uniform throughout the entire arch length. The second class of funicular arches allows the cross-sectional area to be different from one straight arch member to another member. To facilitate the buckling analysis, the Hencky bar-chain model (HBM) is adopted. This discrete structural model simplifies the optimization process as the decision variables are the HBM rotational spring stiffnesses that define the cross-sectional areas and the horizontal force that controls the arch shape. Presented herein are new optimal funicular arch shapes under various numbers of equally spaced point loads. By increasing the number of point loads, the optimal solution approaches the solution of a parabolic arch under a uniformly distributed load.

Stiffening and softening in the power-law rheological behaviors of cells

Living cells are a complex soft material with fascinating mechanical properties. Confusingly, experiments have shown that cells exhibit stiffening and more solid-like behaviors under uniaxial stretches or shear, while they present softening and more fluid-like behaviors under biaxial stretches. For both of these seemingly paradoxical stiffening and softening rheological behaviors, cells often exhibit a robust power-law rheological characteristic. Here, based on the structural features, we propose a cellular structural model to investigate these rheological behaviors of cells under different loading conditions. It is found that this structural model can naturally capture the stiffening and softening behaviors in the power-law rheological responses of cells, depending on the loading conditions. Both stiffening and softening of cells originate from changes in the configuration of the discrete cytoskeleton: stiffening from the rotation of the microtubules to the loading direction and softening from the elastic buckling of individual microtubules. Moreover, for both stiffening and softening in the rheological behaviors of cells, there exists a unified relationship that the power-law exponent decreases linearly with the cellular stiffness in a semi-logarithmic coordination. We further present that a self-similar hierarchical model can be used to analyze this unified relationship. This study not only provides a discrete cellular structural model to capture the essential mechanisms of cellular rheology, but also suggests that the scaling rheological exponent may be treated as a mechanical marker for monitoring cellular healthy states.

Optimal design of functionally graded lattice structures using Hencky bar-grid model and topology optimization

Presented herein is a novel design framework for obtaining the optimal design of functionally graded lattice (FGL) structures that involve using a physical discrete structural model called the Hencky bar-grid model (HBM) and topology optimization (TO). The continuous FGL structure is discretized by HBM comprising rigid bars, frictionless hinges, frictionless pulleys, elastic primary and secondary axial springs, and torsional springs. A penalty function is introduced to each of the HBM spring’s stiffnesses to model non-uniform material properties. The gradient-based TO method is applied to find the stiffest structure via minimizing the compliance or elastic strain energy by adjusting the HBM spring stiffnesses subjected to prescribed design constraints. The optimal design of FGL structures is constructed based on the optimal spring stiffnesses of the HBM. The proposed design framework is simple to implement and for obtaining optimal FGL structures as it involves a relatively small number of design variables such as the spring stiffnesses of each grid cell. As illustration of the HBM-TO method, some optimization problems of FGL structures are considered and their optimal solutions obtained. The solutions are shown to converge after a small number of iterations. A Python code is given in the Appendix for interested readers who wish to reproduce the results.

Discrete optimization design for a cabin-skeleton coupling structure of blended-wing-body underwater glider

Objectives The blended-wing-body underwater glider cabin-skeleton coupling structure is optimized by using a data-driven discrete optimization idea. Methods Firstly, a Kriging-assisted discrete global optimization algorithm (KDGO) is proposed for computationally intensive black-box problems. The KDGO uses a novel infill-sampling strategy to capture discrete sample points with better performance. In the infill-sampling strategy, a multi-start method with data mining strategy is introduced, including multi-start optimization, projection, sampling, and selection. Secondly, the parametric skeleton-cabin coupling structure model was established and was analyzed by finite element in the lifting deformation and deep-water pressure conditions. Then, take the float-to-weight ratio and the strength and stability of the skeleton-cabin structure as the as the goal and constraints, respectively. Meanwhile, considering the interference between shape and cabin and the coupling relationship between skeleton and cabin, then establish a discrete optimization mathematical model of the overall coupling structure. Finally, the discrete optimization algorithm and coupling structure simulation are combined to build an overall optimization framework. Results By using KDGO to conduct 200 function evaluations and comparing the optimal feasible point in DoE with the global optimal feasible point after optimization, it was found that the optimized float-to-weight ratio of the coupling structure increased by nearly 40%, and satisfactory results were obtained. Conclusion The research results can provide a reference for the cabin-skeleton coupling structure design of blended-wing-body underwater glider.

A numerical methodology for thermo-fluid dynamic modelling of tyre inner chamber: towards real time applications

The characterization and reproduction of tyre behaviour for vehicle modelling is a topic of particular interest both for real-time driver in the loop simulations and for offline performance optimization algorithms. Since the accuracy of the tyre forces and moments can be achieved by the accurate physical modelling of all the phenomena concerning the tyre-road interaction, the link between the tyre thermal state and the tyre frictional performance turns into a crucial factor. An integrated numerical methodology, allowing to couple the full 3D CFD (Computational Fluid Dynamics) flux within the internal chamber of the tyre with an equivalent discrete 3D structure model, is proposed with the aim to completely represent the tyre thermodynamic convective behaviour in the steady-state operating conditions. 3D CFD model enables the evaluation of the internal distribution of the gas temperature and of the thermal powers exchanged at each sub-wall in detail. This allows to increase the reliability of the tyre thermodynamic modelling with a particular reference to the proper managing of the aero-thermal flow of the brake disc impact on the rim temperature and therefore on the internal gas dynamics in terms of temperature and pressure, being able to optimize the tyre overall dynamic performance in both warm-up and stabilized thermal conditions. The steady RANS (Reynolds Averaged Navier–Stokes) simulations have been performed employing the 3D CFD model in a wide range of angular velocities with the aim to calculate the convective thermal flux distributions upon rim and inner liner surfaces. The simulation results have been then exploited to derive the convective heat transfer coefficients per each sub domain to be employed within the real-time tyre physical thermal model, with the peculiar advantage of an enhanced model reliability for thermal characteristics. To validate the proposed methodology, the tyre thermal model outputs, in terms of temperatures of internal and external layers, have been validated towards the acquired ones within the specific routine performed on tyre force and moment test bench, confirming an excellent agreement with the experimental data in the entire range of operating conditions explored.

3D thermo-hydro-mechanical coupled discrete beam lattice model of saturated poro-plastic medium

In this paper, we present a 3D thermo-hydro-mechanical coupled discrete beam lattice model of structure built of the nonisothermal saturated poro-plastic medium subjected to mechanical loads and nonstationary heat transfer conditions. The proposed model is based on Voronoi cell representation of the domain with cohesive links represented as inelastic Timoshenko beam finite elements enhanced with additional kinematics in terms of embedded strong discontinuities in axial and both transverse directions. The enhanced Timoshenko beam finite element is capable of modeling crack formation in mode I, mode II and mode III. Mode I relates to crack opening, mode II relates to in-plane crack sliding, and mode III relates to the out-of-plane shear sliding. The pore fluid flow and heat flow in the proposed model are governed by Darcy\'s law and Fourier\'s law for heat conduction, respectively. The pore pressure field and temperature field are approximated with linear tetrahedral finite elements. By exploiting nodal point quadrature rule for numerical integration on tetrahedral finite elements and duality property between Voronoi diagram and Delaunay tetrahedralization, the numerical implementation of the coupling results with additional pore pressure and temperature degrees of freedom placed at each node of a Timoshenko beam finite element. The results of several numerical simulations are presented and discussed.

Generalized inverses of Boolean tensors via the Einstein product

Applications of the theory and computations of Boolean matrices are of fundamental importance to study a variety of discrete structural models. But the increasing ability of data collection systems to store huge volumes of multidimensional data, the Boolean matrix representation of data analysis is not enough to represent all the information content of the multiway data in different fields. From this perspective, it is appropriate to develop an infrastructure that supports reasoning about the theory and computations. In this paper, we discuss the generalized inverses of the Boolean tensors with the Einstein product. Further, we elaborate on this theory by producing a few characterizations of different generalized inverses and several equivalence results on Boolean tensors. We explore the space decomposition of the Boolean tensors and present reflexive generalized inverses through it. In addition to this, we address rank and the weight for the Boolean tensor.

Структурные динамические модели управляемого выпрямителя

A discrete structural model of a controlled rectifier operating with an intermittent current is substantiated. The model has been obtained using the linearization method known from the theory of automatic control of discrete systems. The developed model differs from the known ones in having an additional negative feedback circuit for rectified current, which is implemented through an additional ideal impulse element. The availability of this feedback explains why the transfer gain is essentially lower in the intermittent current mode than it is during operation with continuous current. By disconnecting the above-mentioned feedback, the considered model is transformed into the well-known dynamic model of a controlled rectifier operating with continuous current. The model was subjected to analytical manipulations as a result of which is was transformed to the form known from other scientific publications on this subject.

Tension-Compression and Shear of Plane Test Specimens from Laminated Composites with the [90°]s Structure. Initial Stress-Strain State

Using the previously constructed equations of the theory of the plane multi-layer beams with a thin-layer structure in thickness, based on a discrete-structural model, a linear problem on the formation of the initial (subcritical) stress-strain state in test specimens of a unidirectional laminated composites with the [90°]s structure was given in shear tensile-compression tests. A numerical method was developed for solving the formulated problem, based on the reduction of the original problem to a system of integro-algebraic equations and the construction of their solution by the method of finite sums. The properties of the constructed equations are investigated and numerical experiments are carried out on the basis of the developed method. It is shown that in the test specimens of the laminated composite of the considered structure at their tension (compression) in the direction across the fibers and in shear, predominantly normal tensile (compression) stresses are formed, which are constant along the length and width of the specimen, and the tangential stresses in these directions are variable.

Computational simulation of fracture behaviour in auxetic cellular structure by multiaxial loading

A computational simulation of fracture behaviour in auxetic cellular structure, subjected to multiaxial loading is presented in this paper. A fracture behaviour of the 3D (three-dimensional) chiral auxetic structure under multiaxial loading conditions was studied. The computational models were used to study the geometry effect of the unit cell on the Poisson’s ratio and fracture behaviour of the analysed chiral auxetic structure. A 3D computational model was built using FEM-code LS DYNA. The discrete computational model of chiral auxetic structure was built using beam finite elements. The lattice model of the analysed auxetic structure was positioned between rigid plates and assembled in a way to simulate a hydro-compression loading conditions. Between the contacting surfaces interactions in normal (contact) and tangential direction (friction) with the node-to-surface approach were simulated. A developed computational model offers insight in the fracture behaviour of considered auxetic cellular structure and helps to better understanding their crushing behaviour under impact multiaxial loading.

Effects of parental leave policies on female career and fertility choices

This paper constructs and estimates a dynamic discrete choice structural model of female employment and fertility decisions that incorporates job protection and cash benefits of parental leave legislation. The structural model is used for ex ante evaluation of policies that change the duration of job protection and/or the arrangement for cash benefits. Counterfactual simulations indicate that introducing an initial 1‐year job protection policy increases maternal employment significantly, but extending the existing job protection period from 1 to 3 years has little effect. In addition, the employment effects of cash benefits seem modest. Overall, parental leave policies have little effect on fertility.

Расчет переходных процессов и обоснование динамических моделей резонансного преобразователя постоянного напряжения с фазовым регулированием в режиме прерывистого тока

The article presents the results of ongoing investigations into the static and dynamic characteristics of a phase controlled resonance DC voltage converter (DCVC). For accurate calculation of transients, the fitting method in vector-matrix form was implemented. However, this method is rather cumbersome for practical applications and involves difficulties in obtaining the generalized parameters characterizing the DCVC dynamic properties that are necessary for development of DCVC closed-loop control systems. The article proposes a technique for analyzing the dynamic characteristics of a resonance DCVC operating in an intermittent current mode (ICM). The proposed technique is based on separating the processes into fast (in the resonance circuit) and slow ones (in the circuit of the output filter with the load) in combination with the averaging method. The article also proposes a simplified procedure for calculating the processes in the DCVC, according to which the mean value of current in the LC-circuit for a half switching cycle is determined, which is then used to calculate the voltage across the LC circuit capacitor at the half cycle end with the known value of this voltage at the half-cycle beginning, and also for calculating the process in the DCVC output circuit. The DCVC dynamic model is linearized, as a result of which discrete structural models suitable for synthesizing the DCVC control system and also a simple continuous model are obtained. The validity of the results is verified by determining the steady-state operation parameters according to the proposed procedure and comparing them with the previously obtained results.

Algorithm of Rainwater Accumulation Load Estimation


 
 The paper presents methodology of estimating rainwater load accumulation for a discrete-structural model. According to it an algorithm for estimating rainwater accumulation loading has been developed. This algorithm evaluates the geometrically nonlinear structural model deformation process. The geometrically nonlinear variation of the structural model is based on iterative calculations of the discrete model, considering the deformations of this model due to the effect of rainwater accumulation. A discrete model that conforms the calculation of the first loading iteration was provided. In addition, after the iterative calculation, a secondary model that takes into account the change of initial structure’s geometry was given.
 

A continuum mechanics-based musculo-mechanical model for esophageal transport

In this work, we extend our previous esophageal transport model using an immersed boundary (IB) method with discrete fiber-based structural model, to one using a continuum mechanics-based model that is approximated based on finite elements (IB-FE). To deal with the leakage of flow when the Lagrangian mesh becomes coarser than the fluid mesh, we employ adaptive interaction quadrature points to deal with Lagrangian–Eulerian interaction equations based on a previous work (Griffith and Luo [1]). In particular, we introduce a new anisotropic adaptive interaction quadrature rule. The new rule permits us to vary the interaction quadrature points not only at each time-step and element but also at different orientations per element. This helps to avoid the leakage issue without sacrificing the computational efficiency and accuracy in dealing with the interaction equations. For the material model, we extend our previous fiber-based model to a continuum-based model. We present formulations for general fiber-reinforced material models in the IB-FE framework. The new material model can handle non-linear elasticity and fiber-matrix interactions, and thus permits us to consider more realistic material behavior of biological tissues. To validate our method, we first study a case in which a three-dimensional short tube is dilated. Results on the pressure-displacement relationship and the stress distribution matches very well with those obtained from the implicit FE method. We remark that in our IB-FE case, the three-dimensional tube undergoes a very large deformation and the Lagrangian mesh-size becomes about 6 times of Eulerian mesh-size in the circumferential orientation. To validate the performance of the method in handling fiber-matrix material models, we perform a second study on dilating a long fiber-reinforced tube. Errors are small when we compare numerical solutions with analytical solutions. The technique is then applied to the problem of esophageal transport. We use two fiber-reinforced models for the esophageal tissue: a bi-linear model and an exponential model. We present three cases on esophageal transport that differ in the material model and the muscle fiber architecture. The overall transport features are consistent with those observed from the previous model. We remark that the continuum-based model can handle more realistic and complicated material behavior. This is demonstrated in our third case where a spatially varying fiber architecture is included based on experimental study. We find that this unique muscle fiber architecture could generate a so-called pressure transition zone, which is a luminal pressure pattern that is of clinical interest. This suggests an important role of muscle fiber architecture in esophageal transport.

Childcare Assistance: Are Subsidies or Tax Credits Better?

AbstractWe evaluate price subsidies and tax credits for childcare. We focus on partnered women's labour supply, household income and welfare, demand for childcare and government expenditure. Using Australian data, we estimate a joint, discrete structural model of labour supply and childcare demand. We introduce two methodological innovations – a more flexible quantity constraint that total formal and informal childcare hours are at least as large as the mother's labour supply and the explicit inclusion of maternal childcare in the utility function as a proxy for child development. We find that tax credits are more effective than subsidies in terms of increasing average hours worked and household income. However, tax credits disproportionately benefit wealthier and more educated women. Price subsidies, while less efficient, have positive redistributional effects.

Discrete variable structure model reference adaptive control using only input-output measurements

Designing an adequate controller for a plant with an arbitrary relative degree is still an active area of research. In this paper, a discrete variable structure model reference adaptive control using only input-output measurements (DVS-MRAC-IO) for not strictly positive real systems with a relative degree of two is proposed. In order to show the effectiveness of the proposed controller, a detailed stability analysis is studied using Lyapunov theory. Further, a straightforward generalization of DVS-MRAC-IO for systems with arbitrary relative degree is presented. Numerical results are used to show the effectiveness of the proposed methods.

Effects of Parental Leave Policies on Female Career and Fertility Choices

This paper constructs and estimates a dynamic discrete choice structural model of female employment and fertility decisions that incorporates job protection and cash benefits of parental leave legislation. The estimated structural model is used for ex ante evaluation of policy reforms that change the duration of job protection and/or the arrangement for cash benefits. Counterfactual simulations indicate that introducing an initial one-year job protection policy increases maternal employment significantly, but extending the existing job protection period from one to three years has little effect. The employment effects of cash benefits also seem modest. Overall, parental leave policies have little effect on fertility.

Book Reviews

Benjamin Williams of George Washington University reviews “Structural Econometric Models”, by Eugene Choo and Matthew Shum. The Econlit abstract of this book begins: “Twelve papers explore recent developments in the use of structural econometric models in empirical economics. Papers discuss Euler equations for the estimation of dynamic discrete choice structural models; approximating high-dimensional dynamic models—sieve value function iteration; identifying dynamic games with serially correlated unobservables; partial identification in two-sided matching models; identification of matching complementarities—a geometric viewpoint; comparative static and computational methods for an empirical one-to-one transferable utility matching model; a test for monotone comparative statics; estimating supermodular games using rationalizable strategies; estimation of the loan spread equation with endogenous bank-firm matching; the collective marriage matching model— identification, estimation, and testing; deflation in durable goods markets—an empirical model of the Tokyo condominium market; and a dynamic analysis of the U.S. cigarette market and antismoking policies. Choo is with the Department of Economics at the University of Calgary. Shum is with the Division of Humanities and Social Sciences at the California Institute of Technology.”

Dynamic Effects of Parental Leave Policy on Female Labor Market Outcomes

This paper constructs and estimates a dynamic discrete choice structural model of labor supply, occupational choice, and fertility in the presence of parental leave legislation. The estimated structural model is used for an ex ante evaluation of parental leave expansions that change the duration of job protection and/or the replacement rate of the cash benefit. Counterfactual simulation results indicate that a one-year job protection significantly increased maternal employment and earnings, but extending it from one to three years and offering cash benefits have little effect. Overall, parental leave policies have little effect on fertility. I also find that policy effects are stronger for younger cohorts who observe a policy change several years before childbearing, because they adjust their career paths accordingly as soon as the policy change.