Symmetries of Energy-Momentum Tensor： Some Basic Facts

It has been pointed out by Hall et al. [Gen. Rel. Gray. 28 （1996） 299.] that matter collineations can be defined by using three different methods. But there arises the question whether one studies matter collineations by using LεTab=0, or LεT^ab = 0 or LεT^b a=0. These alternative conditions are, of. course, not generally equivalent. This problem has been explored by applying these three definitions to general static spherically symmetric spacetimes. We compare the results with each definition.     

Lattice Boltzmann Simulation for the Optimized Surface Pattern in a Micro-Channel

Based on the Bhatangar-Gross-Krook （BGK） models, numerical simulation using the lattice Boltzmann model is performed to investigate the optimized surface pattern in a micro-channel. In order to simulate the practical situation correctly, a slip/no-slip boundary condition is applied with making several assumptions. To assess the validity and efficiency of the model, one benchmark problem with considering the surface patterns is studied. Numerical results show the value of rms velocity Vrms increases with the increasing ratio β and larger Reynolds number Re, higher fluctuation of the rms oscillating velocity. Furthermore, the results show that a good mixing effect can be obtained when Re is large enough and the ratio β is about 1.618,which is the appropriate choice, i.e. the well known golden section phenomenon.     

Spatial coherences of the sound pressure and the particle velocity in underwater ambient noise

The spatial coherences were investigated between the sound pressure and the three orthogonal components of the particle velocity in underwater ambient noise. Based on the ray theory, integral expression was derived for the spatial coherence matrix of the sound pressure and the particle velocity in a stratified ocean with dipole noise sources homogenously distributed on the surface. The integrand includes a multiplying factor of the vertical directivity of the noise intensity, and the layered ocean environment affects the spatial coherences via this directivity factor. For a shallow water environment and a semi-infinite homogenous medium, the coherence calculation results were given. It was showed that the sound speed profile and the sea bottom could not be neglected in determining the spatial coherences of the ambient noise vector field.     

引力波探测展望

早在1916年，爱因斯坦在发表广义相对论时就预言引力辐射的存在．但在初期，大多数人都怀疑引力波（Gravitational　Waves）仅仅是爱因斯坦提出的引力场方程的一个近似解…而没有实质的物理效应．直到1950年代末期，从理论上证明引力波是携带能量的并可以被探测到，引力波的存在才在理论上得到了充分的确认．广义相对论预言引力波的主要特性有：在真空中以光速传播；     

直线、平行线、垂直线

首先,让我们在纸上点两点,然后把这两点连成一条线,而且尽可能把线画直。这条线就叫做直线。     

捷联惯导系统改进回溯快速对准方法（英文）

快速性是捷联惯导系统初始对准的一项重要指标。回溯算法通过存储一段时间的陀螺仪和加速度计的数据,并反复加以利用的方式,有效缩短了对准的时间。为了进一步提高初始对准的快速性,设计了一种快速回溯对准方法(FBA)。优化对准方法被用于算法的粗对准阶段,为回溯罗经法精对准提供较为准确的初始姿态,进而提高罗经回路的收敛速度。分析了回溯算法的基本原理,设计了快速回溯对准算法的实施过程。实验结果表明,提出的算法能够降低回溯次数,大大减少对准所需的数据量,使得对准的快速性进一步提高。     

模糊信息处理在舰艇等级评估中的应用

采用模糊信息综合处理的方法，对舰艇主要设备技术状态作综合处理，评估舰艇的技术状态等级，促进舰艇等级评估趋向科学化、规范化。     

高中数学基础练习(四)

练习31 1.在复数集内因式分解: 答: 2.当实数a为何值时,复数(a~2-1) (a~2 a-2)i是纯虚数。答:a=-1。 3.求i~(66) i~(77) i~(88) i~(99)的值。答:0.     

硒—矾体系中锡的催化波测定

水中锡的各种测定方法报导不少,但这些方法受干扰元素影响较大。高小霞等提出钒体系中锡催化极谱法测定,可提高测定的灵敏度。我们发现,在硒—钒催化体系中测定痕量的锡,可较明显地提高分析的灵敏度。当锡浓度较高时,很少量的硒