An improved practice to manufacture Al–Ti–B master alloys by reacting halide salts with molten aluminium

The well-established “halide salt” route was employed in the present work to produce Al–Ti–B grain refiner alloys with consistent, good properties. The holding step in the production cycle was revised, however, to avoid oxidation of the molten alloy which is believed to be responsible for the relatively low Ti recoveries and thus for the inadequate and inconsistent grain refining efficiency. Stirring during holding was found to degrade the grain refining properties when molten potassium aluminium fluride salt was left on the molten alloy to avoid excessive oxidation. Likewise, holding temperatures higher than 800 °C and holding times longer than 30 min both had an undesirable effect on the grain refining performance. The experimental Al–5Ti–1B grain refiner alloy produced according to the present method provided consistent and better overall grain refining performance.     

Compressed sensing of superimposed chirps with adaptive dictionary refinement

The compressed sensing （CS） theory shows that accurate signal reconstruction depends on presetting an appropriate signal sparsifying dictionary. For CS of superimposed chirps, this dictionary is typically taken to be a waveform-matched dictionary formed by blindly diseretizing the frequency-chirp rate plane. However, since practical target parameters do not lie exactly on gridding points of the assumed dictionary, there is always mismatch between the assumed and the actual sparsifying dictionaries, which will cause the performance of conventional CS reconstruction methods to degrade considerably. To address this, we model the waveform- matched sparsifying dictionary as a parameterized one by treating its sampled frequency-chirp rate grid points as the underlying parameters. As a consequence, the sparsifying dictionary becomes refinable and its refinement can be achieved by optimizing the underlying parameters. Based on this, we develop a novel reconstruction algorithm for CS of superimposed chirps by utilizing the variational expectation-maximization （EM） algorithm. By alternating between steps of sparse coefficients estimation and dictionary parameters optimization, the algorithm integrates the process for dictionary refinement into that of signal reconstruction, and thus can achieve sparse reconstruction and dictionary optimization simultaneously. Experimental results demonstrate that the algorithm effectively deals with the performance degradation incurred by dictionary mismatch, and also outperforms the state-of-the-art CS reconstruction methods both in compressing the signal measurements and in suppressing the measurement noise.     

Test frame updating in CPM testing of Prolog programs

Category Partition Method (CPM) is a general approach to specification-based program testing, where test frame reduction and refinement are two important issues. Test frame reduction is necessary since too many test frames may be produced, and test frame refinement is important since during CPM testing new information about test frame generation may be achieved and considered incrementally. Besides the information provided by testers or users, implementation related knowledge offers alternative information for reducing and refining CPM test frames. This paper explores the idea by proposing a call patterns semantics based test frame updating method for Prolog programs, in which a call patterns analysis is used to collect information about the way in which procedures are used in a program. The updated test frames will be represented as constraints. The effect of our test frame updating is two-fold. On one hand, it removes “uncared” data from the original set of test frames; on the other hand, it refines the test frames to which we should pay more attention. The first effect makes the input domain on which a procedure must be tested a subset of the procedure’s input domain, and the latter makes testers stand more chance to find out the faults that are more likely to show their presence in the use of the program under consideration. Our test frame updating method preserves the effectiveness of CPM testing with respect to the detection of faults we care. The test case generation from the updated set of test frames is also discussed. In order to show the applicability of our method an approximation call patterns semantics is proposed, and the test frame updating on the semantics is illustrated by an example.

Compositional CSP Traces Refinement Checking

Compositional verification using assume-guarantee reasoning has recently seen an uprise due to the introduction of automatic techniques for learning assumptions. In this paper, we transfer this technique to a setting with CSP as modelling and property specification language, and present an approach to compositional traces refinement checking. The approach has been implemented using the CSP model checker FDR as teacher during learning. The implementation shows that the compositional approach can both drastically outperform as well as underperform FDR's performance, depending on the example at hand.     

Numerical simulation of detonation using an adaptive Cartesian cut-cell method combined with a cell-merging technique

A second-order accurate scheme for the Cartesian cut-cell method developed previously by the authors [Ji H, Lien F-S, Yee E. Comput. Methods Appl. Mech. Eng. 198 (2008), 432] is generalized for application to both two- and three-dimensional inviscid compressible flow problems. A cell-merging approach is used to address the so-called “small cell” problem that has plagued Cartesian cut-cell methods. In the present cell-merging approach, the conservative variables are stored at the cut-cell centroid (including the non-merged and merged cut-cells) rather than at the Cartesian cell center. Although this approach results in a more complicated search algorithm for the determination of the neighbor cells (required for the computation of the spatial gradients of the conservative variables), this approach enables the straightforward formulation of a higher than first-order accurate discretization scheme in the vicinity of the (complex and irregular) internal boundaries of the flow domain. Six test cases (including detonation problems) are used to demonstrate the accuracy and capability of the adaptive cut-cell method, for which both mesh refinement and derefinement techniques are employed in the case of an unsteady shock diffraction problem.     

基于完备抽象解释的模型检验CTL公式研究

在模型检验中,抽象是解决状态空间爆炸问题的重要方法之一.给定具体Kripke结构和时序描述语言CTL,基于抽象解释框架以及完备抽象解释和性质强保留之间的关系,抽象模型最小精化使得CTL性质强保留,可转换为抽象解释中抽象域的最小完备精化,并且总是存在抽象域的最小完备精化.根据状态标签函数确定初始抽象域,然后通过不动点求解,获得对CTL标准算子完备的最小抽象域,并依据此抽象域求得CTL性质强保留的最优抽象状态划分,最后构造出CTL性质强保留且最优的抽象状态转换系统.并指出了抽象域对CTL标准算子是完备的当且仅当抽象域对补集和标准前向转换是完备的.     

卫星通信系统QOS管理的策略提炼

针对基于策略的DVB—RCS卫星通信系统服务质量管理,提出一种策略提炼模型,结合实例给出目标提炼过程。该模型以线性时态逻辑为基础,在提炼模式的辅助下从高层目标中提取出低层策略,可以有效保证策略提炼过程的正确性、合理性及一致性。在系统设计阶段,该模型用于指导系统功能规划与设计,在系统运行阶段,该模型为网络管理人员提供高层策略向系统设备行为的映射方法。     

Three dimensional discontinuous Galerkin methods for Euler equations on adaptive conforming meshes

In the numerical simulation of three dimensional fluid dynamical equations, the huge computational quantity is a main challenge. In this paper, the discontinuous Galerkin (DG) finite element method combined with the adaptive mesh refinement (AMR) is studied to solve the three dimensional Euler equations based on conforming unstructured tetrahedron meshes, that is according the equation solution variation to refine and coarsen grids so as to decrease total mesh number. The four space adaptive strategies are given and analyzed their advantages and disadvantages. The numerical examples show the validity of our methods.     

Multiple levels of cell by cell Adaptive Mesh Refinement in the Lagrangian step of the ALE code CORVUS

A multi-level cell by cell Adaptive Mesh Refinement (AMR) hydrodynamics capability has been developed for the Lagrangian step of the 2D Arbitrary Lagrangian Eulerian (ALE) code CORVUS. Connectivity arrays are used rather than a hierarchical data structure. We solve only on the dynamic unstructured mesh rather than on every level of a refinement hierarchy. The strength, artificial viscosity, slide and programmed burn methods have been generalised to work with AMR. Results are presented for a range of Lagrangian test problems.     

基于动态网格细分的烟雾模拟

提出了一种基于动态网格细分的烟雾模拟方法,该方法主要采取图形设备上的动态网格管理对烟雾进行并行处理以达到泊松方程的迭代求解。为了实现高性能,利用高速缓存以提高存取权限和适应硬件的能力。实验结果表明,该方法能够实现比较快速的模拟,结果比较令人满意。