A cell-by-cell thermally driven mushy cell tracking algorithm for phase-change problems

A thermally driven mushy cell tracking algorithm for phase-change problems with a moving boundary is presented. The equation used to track the moving boundary is based on energy balance over the mushy cell and is applied to advance a moving front in a cell-by-cell manner. The efficacy of the tracking algorithm is demonstrated on specific problems solved using the finite volume method. An implicit scheme is adopted to ensure that the numerical solution is unconditionally stable in time. A preconditioned conjugated gradient (P-CG) solver is implemented to ensure that solutions converge in a finite number of iterations. Four benchmark cases are used to validate the algorithm including solidification in one dimensional space (two-region problem), melting of pure aluminum in two-dimensional (2D) space, solidification with periodic boundary conditions, and solidification of one-region problem. The results obtained show that the current algorithm is capable of converging to accurate solutions for moving fronts and the numerical predications are in excellent agreement with corresponding analytical solutions.     

A temperature-based finite element solution for phase-change problems

A finite element procedure for solving multidimensional phase change problems is described. The algorithm combines a temperature formulation with a finite element treatment of the differential equation and discontinuous integration within the two-phase elements to avoid the necessity of regularization. A new criterion for the computation of the iteration matrix is proposed. It is based on a quasi-Newton correction of the Jacobian matrix for conduction problems without change of phase. A set of test problems with exact solution is analysed and demonstrates that the procedure can accurately evaluate the front position and temperature history with a reasonable computational effort.     

A numerical simulation for two-dimensional moving boundary problems with a mushy zone

For materials such as alloy, organic phase-change materials and many others, the change of phases may take place over a temperature range. This leads to phase-change problems with the mushy zone in which the solid and liquid phases coexist. The present study introduces a numerical method combining the Laplace transform technique and the control volume method to solve two-dimensional phase-change problems with the mushy zone. The hybrid numerical method involves the control volume formulation for the space domain and the Laplace transform technique for the time domain. The Taylor's series approximation is applied to linearize nonlinear terms in the governing equation. The transfinite mapping method is used to generate control-volume meshes in each region. The growth of the mushy zone is unknown a priori and is predicted by using the least-square iteration scheme. It will be found that the present hybrid numerical method can be efficiently applied to solve two-dimensional phase-change problems with a mushy zone.     

Stability limit for thermally driven acoustic oscillation

The study of oscillatory instabilities occuring in cryogenic systems has a practical importance in the system development. One form of instability, thermally driven acoustic oscillation, is investigated experimentally using U-shaped tubes. The stability limit of oscillation for helium gas predicted by theory agrees well with our experimental results. The temperature ratio of a warm part to a cold one and the ratio of the inner radius of the tube to the Stokes boundary layer thickness are convenient variables for characterizing thermally driven acoustic oscillation.     

A temperature-based formulation for finite element analysis of generalized phase-change problems

A finite element formulation for solving multidimensional phase-change problems is presented. The formulation considers the temperature as the unique state variable, it is conservative in the weak form sense and it preserves the moving interface condition. In this work, an approximate jacobian matrix that preserves numerical convergence and stability is also derived. Furthermore, a comparative analysis with other different phase-change finite element techniques is performed. Finally, several numerical examples are analysed in order to show the performance of the proposed methodology.     

一种基于蝙蝠算法的多目标跟踪数据关联方法

针对多目标跟踪中的数据关联问题,提出了基于蝙蝠算法的数据关联方法。首先将多目标数据关联问题建模成组合优化问题,结合数据关联的特点,对蝙蝠算法的搜索更新规则进行改进,使其可以应用于多目标的数据关联问题,并给出了基于蝙蝠算法的多目标数据关联的详细流程。通过仿真实验和被动声呐实测数据测试表明,基于蝙蝠算法的多目标数据关联方法切实可行并且具有较好的效果。     

A reliability-cost and Makespan driven fault-tolerant scheduling algorithm for distributed systems

为解决异构分布式环境下采用主副版本策略的可靠性调度问题,提出一种基于优先级约束的可靠性代价和Makespan(调度时长)驱动的分布式容错调度算法DRCAMD.该算法可在满足系统可调度性的前提下,以异构分布式环境的节点、通信链路的可靠性与Makespan做为可调节局部目标函数,实现具有较高可靠性及较短执行时间的容错调度策略,避免将任务分配到失效率较高的节点上执行.另外,算法的副版本采用被动和主副重叠方式执行,使得容错调度算法具有较大的灵活性.仿真实验表明,该算法性能优于现有容错算法.     

刚柔耦合接触动态粘合算法研究

建立一种动态粘合算法,用于多体系统中刚体与弹性体或弹性体与弹性体接触问题的仿真。该算法通过装配矩阵和界面柔度矩阵构建接触界面粘合矩阵,用来传递接触力和位移,当接触位置改变时界面柔度矩阵和装配矩阵随之变化,粘合矩阵需要动态更新。利用该方法可以将刚柔耦合系统中相互接触的结构分成独立的子系统,分别在独立的程序或平台中进行分布式仿真。本文使用该方法进行刚体与弹性体接触仿真,和统一方程的刚柔耦合仿真结果基本一致。     

A multi-grid enhanced GMRES algorithm for elasto-plastic problems

A combination of both GMRES and multi-grid (MG) methods is presented in this paper for solving large-scale two- and three-dimensional elasto-plastic problems, in which each MG iteration cycle serves as the preconditioning step for the GMRES procedure. A particular multi-grid approach, termed the Galerkin multi-grid scheme, is considered and the main effort is devoted to the implementation aspects of the proposed algorithm. Numerical examples, characterised by large-scale (up to 82145 DOF), strong non-linearity (nearly plastic limit state, necking and localization) and severe ill-conditioned states (presence of loading limit points), and also involving symmetric and unsymmetric as well as SPD and indefinite system matrices, are provided. The numerical results illustrate that the proposed method exhibits a remarkable performance in terms of efficiency and robustness in all circumstances. © 1998 John Wiley & Sons, Ltd.     

A simultaneous iteration algorithm for symmetric eigenvalue problems

A FORTRAN IV algorithm is presented for determining sets of dominant eigenvalues and corresponding eigenvectors of symmetric matrices. It is also extended to the solution of the equations of natural vibration of a structure for which symmetric stiffness and mass matrices are available. The matrices are stored and processed in variable bandwidth form, thus enabling advantage to be gained from sparseness in the equations. Some of the procedures may also be used to solve symmetric positive definite equations such as those arising from the static analysis of structures loaded within the elastic range.     

A point-iterative algorithm for three-dimensional magnetic vector problems

Magnetostatic field problems are solved in three dimensions by applying a variational method that employs finite elements. Formulation through a partial differential equation allows solution for the magnetic vector potential given an inhomogeneous, orthotropic medium and a distributed current source. Three vector boundary conditions are discussed and interior sheet currents are allowed within the medium. In addition, the Lorentz condition is enforced by including a penalty term in the energy functional. A point-iterative algorithm is used to solve the set of equations resulting from finite element discretization. This method is particularily suitable for regions with regular geometry and a moderate (1,000 to 10,000) number of unknowns.     

A two-phase heuristic algorithm for cell formation problems considering alternative part routes and machine sequences

In this paper, we deal with the multi-objective machine cell formation problem. This problem is characterized as determining part route families and machine cells such that the total sum of inter-cell part movements and maximum machine workload imbalance are simultaneously minimized. Together with the objective function, alternative part routes and the machine sequences of part routes are considered in grouping part route families. Also, it is assumed that the number of machine cells is not pre-defined. Owing to the complexity of the problem, a two-phase heuristic algorithm is proposed. Computational experiments were conducted to verify the performance of the algorithm. Throughout the computational experiments, we verified that the two-phase heuristic algorithm is effective for large-scale machine cell formation problems.     

基于被动式传感器的目标跟踪算法研究

仅有角度量测的被动式单传感器跟踪搜索系统有广泛的应用前景,但现有的针对性跟踪算法并不能很好的处理实际问题,失跟现象常常发生。本文对现有算法所存在的问题进行了深入研究,并提出了相应的改进算法。通过对机动目标运动分析,总结出角度向量在时间域中的变化规律,通过引入新的计算向量,对原算法残差计算进行了修正。它对基于单个被动传感器的纯方位跟踪问题的处理提供了新的思路。仿真实验及实际应用已证明了该算法有效性。     

极坐标-直角坐标下水下目标跟踪算法及试验研究

在实际水下目标跟踪系统中,由声纳获得的速度量测是一个相对径向速度,并且量测噪声是非白的,为此提出了基于扩展量测和扩展状态的混合坐标系下水下目标跟踪算法。该方法把径向速度量测引入观测模型,提高了目标的状态估计精度和收敛时间;引入了扩充向量,克服了实际水下目标观测噪声非白的影响。提出了水下多目标跟踪水池试验方法。该方法应用静态长线阵和多目标信号源在水池实现了多目标和跟踪体之间的相对运动的模拟。水池试验结果表明所提出的方法性能明显优于基于量测转换跟踪方法(如MC方法),具有重大工程应用价值。     

An algorithm for automatically tracking ablating boundaries

An algorithm has been developed which automatically calculates the time-dependent positions of points on the ablating boundaries of two-dimensional continuum structures with geometrially complex shapes. The structural boundary may consist of ablating and non-ablating parts. The initial ablating and non-ablating boundaries are defined by the disjoint union of piecewise linear arcs passing through a finite set of nodal points on the boundary, each defined by a pair of rectangular Cartesian co-ordinates. For a specified ablation rate, the algorithm calculates successive positions of the boundary points at times specifiec by the user. The algorithm is designed such that it may be easily incorporated, along with an automated mesh generation procedure, into existing finite element codes for transient thermal or stress analysis of structures with ablating boundaries. Two examples are presented from the field of solid rocket analysis.     

A multi-population genetic algorithm approach for solving ill-posed problems

In this paper we develop a multi-population genetic algorithm (GA) for regularizing nonlinear ill-posed problems. Real coded genetic algorithms are used for calculating the minimizers of the Tikhonov functional. The algorithm is based on evolving separate populations for various values of the regularization parameter. The rate of convergence of the algorithm is substantially increased by exchanging information between neighbouring populations by the process of migration.     

A reformulated Arnoldi algorithm for non-classically damped eigenvalue problems

In applying Arnoldi method to non-symmetric eigenvalue problems for damped structures, a structure of the projected upper Hessenberg matrix is obtained in this paper. By exploiting the structure of the upper Hessenberg matrix and taking advantages of the block properties of system matrices, the Arnoldi reduction algorithm is reformulated for less computation and higher accuracy. In conjunction with the reformulated Arnoldi algorithm, real Schur decomposition instead of Jordan decomposition is adopted aiming at non-complex arithmetic, non-discriminative processing of defective and non-defective systems and numeric stability. A concise reduction algorithm for eigenproblems for undamped gyroscopic systems is obtained by directly degenerating from the reformulated Arnoldi algorithm. For safely solving engineering problems without omitting eigenvalues, a restart reduction procedure is proposed in terms of the reformulated reduction algorithm with deflation developed in this paper. Numerical examples once solved with algorithms originated from Lanczos methods were re-solved. In addition, the non-symmetric eigenvalue problem for a shear wall by BEM modeling and a damped gyroscopic system with eigenvalues of high multiplicity were also used to demonstrate the efficacy of the presented methods. © 1997 John Wiley & Sons, Ltd.     

时间异步无线传感器网络的目标跟踪动态成簇算法

为了解决时间异步无线传感器网络在目标跟踪时的节点协作管理和跟踪时间配准问题,提出了一种适用于时间异步条件下目标跟踪的动态成簇算法。该方法通过分析目标的无线信号强度和各节点至目标的距离来动态组建跟踪簇,然后依据目标及簇头的通信距离对簇头射频信号的覆盖区域进行功能划分,实现节点对目标的协作跟踪,同时以簇为跟踪时间的计算单元,通过簇内计时和簇间贯序传递的方法实现跟踪时间的配准。仿真实验表明,该算法进行目标跟踪时能有效均衡网络能耗,且具有较好的跟踪精度和系统鲁棒性。     

A free-front tracking algorithm for a control-volume-based Hele–Shaw method

With free surface flow problems the position of the free front at a particular time step needs to be predicted. When simulating the flow of hot molten polymer in injection moulds this position is essential for the prediction of the position of the weld lines on the final product. Weld lines are important in that they indicate positions of diminished strength and spoil the aesthetics of the product. A code using a finite volume approach on the thin gap Hele–Shaw flow equations to simulate the filling stage of the injection moulding process has been developed which requires a suitable algorithm for the free-front tracking. The Volume of Fluid (VOF) method was implemented in this study for the tracking of the free front. The discretization and various formulations of the VOF equation are presented as well as the results obtained. The impact on the existing equations is also investigated as well as the development of a packing routine that is used to eliminate the diffusive nature of the VOF equation. © 1998 John Wiley & Sons, Ltd.     

一种纯方位跟踪中的在线式伪线性算法

针对伪线性参数估计不能进行实时状态跟踪的问题，讨论了一种新的在线式的伪线性跟踪算法．对算法的实现过程进行了推导，并应用于纯方位目标的跟踪问题中．仿真结果表明，与传统累积形式的伪线性跟踪算法相比，该算法在保证一定精度的同时，能够降低每个估计时刻的计算量，减少计算占用的资源，是一种有效的算法．