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81.
A High-Order Finite Difference Scheme for 3D Unsteady Convection Diffusion Reaction Equations北大核心CSCD
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针对三维非稳态对流扩散反应方程,构造了一种高精度紧致有限差分格式,对空间的离散采用四阶紧致差分方法,对时间的离散采用Taylor级数展开和余项修正技术,所提格式在时间上的精度为二阶、在空间上的精度为四阶。利用Fourier稳定性分析法证明了该格式是无条件稳定的。最后给出数值算例验证了理论结果。 相似文献
82.
超大型航天结构具有超大柔性、超低固有频率的特点,空间机器人在轨组装时应尽可能避免激起超大型结构的柔性振动.空间机器人组装超大型结构模块的过程分成抓捕阶段、位姿调整与稳定阶段、安装阶段和爬行阶段.通过对安装阶段的动力学与控制研究,提出共线安装的轨迹规划方法,有效避免了柔性结构振动.首先,采用自然坐标法和绝对节点坐标法建立主结构-空间机器人-待组装结构的在轨组装系统动力学模型.然后,将共线安装的要求转化为空间机器人的轨迹规划约束,要求空间机器人质心到主结构/待组装结构的距离保持不变,实现共线安装的轨迹规划.数值仿真表明:提出的组装方法在组装过程中可有效避免超大型结构的横向运动,降低夹持力矩.最后,分析了系统参数对组装过程动力学响应的影响,为超大型航天器的在轨组装提供了参考. 相似文献
83.
Shaul K. Bar-Lev Daoud Bshouty Peter Enis Gérard Letac I-Li Lu Donald Richards 《Journal of Theoretical Probability》1994,7(4):883-929
A natural exponential family (NEF)F in ? n ,n>1, is said to be diagonal if there existn functions,a 1,...,a n , on some intervals of ?, such that the covariance matrixV F (m) ofF has diagonal (a 1(m 1),...,a n (m n )), for allm=(m 1,...,m n ) in the mean domain ofF. The familyF is also said to be irreducible if it is not the product of two independent NEFs in ? k and ? n-k , for somek=1,...,n?1. This paper shows that there are only six types of irreducible diagonal NEFs in ? n , that we call normal, Poisson, multinomial, negative multinomial, gamma, and hybrid. These types, with the exception of the latter two, correspond to distributions well established in the literature. This study is motivated by the following question: IfF is an NEF in ? n , under what conditions is its projectionp(F) in ? k , underp(x 1,...,x n )∶=(x 1,...,x k ),k=1,...,n?1, still an NEF in ? k ? The answer turns out to be rather predictable. It is the case if, and only if, the principalk×k submatrix ofV F (m 1,...,m n ) does not depend on (m k+1,...,m n ). 相似文献
84.
B. Kaminski 《Proceedings of the American Mathematical Society》1996,124(5):1533-1537
It is shown that if an abelian countable group is such that is a finite group and every aperiodic positive entropy action of on a Lebesgue probability space has a countable Haar spectrum in the subspace , where denotes the Pinsker -
algebra of , then every aperiodic positive entropy action of on has the same property. A positive answer to the question of J.P. Thouvenot is obtained as a corollary.
algebra of , then every aperiodic positive entropy action of on has the same property. A positive answer to the question of J.P. Thouvenot is obtained as a corollary.
85.
Jean Barge 《K-Theory》1993,7(1):9-16
In this paper we construct a canonical 2-cocycle on the groupP SL(2,k) with values in the Witt groupW(k) of the fieldk. This allows us to produce anatural homomorphism :H
2(SL(2,k); Z)I
2(k), whereI
2(k) is the square of the fundamental ideal. We prove that this homomorphism is in fact a lift of Milnor's symbol. 相似文献
86.
87.
A convergent selective route to (+)-brefeldin A (BFA) and 7-epi-BFA was developed, with the crucial C-4/C-5 stereogenic centers were established using Crimmins asymmetric aldolization. 相似文献
88.
The convection in atmosphere discussed in ref. [1] is rigorously treated by considering the variation of environmental temperature
with the height. This represents an example of applications of the elementary catastrophe theory in Hamiltonian systems. 相似文献
89.
E. M. Chudnovsky 《Journal of statistical physics》1985,41(5-6):877-897
It is shown that biological-natural-selection-like behavior can occur, as a general type of time evolution, in a statistical system where detailed balance is violated owing to the presence of metastable energy states. A model of a non-equilibrium phase transition corresponding to the spontaneous origin of self-reproduction in the system is suggested. After a phase transition, the system passes from one quasistationary distribution of self-reproducing subsystems to another, with an increase in the total organization, as long as the growth of the energy flow through the system or a reduction of energy dissipation in the system is possible. The entropy production is calculated for this process in terms of selective values of Eigen's theory for self-organization in autocatalytic systems. Correspondence of the extremal principle of Eigen's theory with the criterion of evolution in Prigogine's thermodynamics is established. 相似文献
90.
A new pulse sequence is proposed for the determination of scalar coupling correlation in small- and medium-sized organic compounds. The method uses a combination of the double pulsed field gradient spin-echo (DPFGSE) and the selective population transfer (SPT) techniques and is shown to be useful in the analysis of complex spectra with many overlapped signals. The usefulness of this method in the structural elucidation of natural substances is demonstrated using strychnine and digitoxin as examples. 相似文献