THREE-DIMENSIONAL VISCOUS FLOW THROUGH A ROTATING CHANNEL: A PSEUDOSPECTRAL MATRIX METHOD APPROACH

A Fourier–Chebyshev pseudospectral method is used for the numerical simulation of incompressible flows in a three-dimensio nal channel of square cross-section with rotation. Realistic, non-periodic boundary conditions that impose no-slip conditions in two directions (spanwis e and vertical directions) are used. The Navier–Stokes equations are integrated in time using a fractional step method. The Poisson equations for pressure and the Helmholtz equation for velocity are solved using a matrix diagonalization (eigenfunction decomposition) method, through which we are able to reduce a three-dimensional matrix problem to a simple algebraic vector equation. This results in signficant savings in computer storage requirement, particularly for large-scale computations. Verification of the numerical algorithm and code is carried out by comparing with a limiting case of an exact steady state solution for a one-dimensional channel flow and also with a two-dimensional rotating channel case. Two-cell and four-cell two-dimensional flow patterns are observed in the numerical experiment. It is found that the four-cell flow pattern is stable to symmetri cal disturbances but unstable to asymmetrical disturbances.     

Beyond Simpson's paradox: A descriptive approach

In considering the strength of association of particular variables, we cannot ignore the effects of confounding factors that cause Simpson's paradox. Many methods for adjusting these effects have been proposed, and a great deal of effort has been devoted to statistical tests. Apart from the statistical tests, the aim of the present study is to examine the strength of association of two categorical variables without reference to any explicit confounding factors. In other words, our aim is to specify the conditions under which Simpson's paradox does not occur, where the idea of classifying the original universe into groups is adopted. Let us begin by focusing our attention on a 2 × 2 contingency table (cross-classification table) and considering the association of X with Y, where X and Y denote dichotomous variables with classes A and B for X and classes + and − for Y. To examine the strength of association between these variables, the index k = q/p is used, where p denotes the proportion of A + in A and q denotes that of B + in B. Using the maximum and minimum values of the index k obtained by numerical calculation, the strength of association is examined. The results are discussed and examples given. © 1996 John Wiley & Sons, Ltd.     

源于G.I.Taylor工作的现代经典物理学

一位科学家的工作提供了一门完整课程的素材,涉及从流体动力学稳定性、湍流到流体电动力学、微生物的运动．     

Period stabilization in the Busse–Heikes model of the Küppers–Lortz instability

The Busse–Heikes dynamical model is described in terms of relaxational and non-relaxational dynamics. Within this dynamical picture a diverging alternating period is calculated in a reduced dynamics given by a time-dependent Hamiltonian with decreasing energy. A mean period is calculated which results from noise stabilization of a mean energy. The consideration of spatial-dependent amplitudes leads to vertex formation. The competition of front motion around the vertices and the Küppers–Lortz instability in determining an alternating period is discussed.     

指派问题的表上作业解法

解极小化的指派问题常用匈牙利解法,但我们从指派问题的数学模型的特征中发现,它还可以采用解运输问题的表上作业法去求解,中通过实例说明其算法,并且可以看出这种解法与匈牙利方法一样简单方便。     

Continued Fraction Approach to the Solution of a Trapped Ultracold Ion Interacting with a Standing Wave Laser without Rotating Wave Approximation

In coherent state representation and in the absence of rotating wave approximation, we present an approach to solve the interaction of a trapped ultracold ion with a standing wave laser by a continued fraction, within the Lamb-Dicke limit and the low excitation regime. It is shown that, the eigen-energies as well as the coefficients of the series of the eigenfunction can be obtained rapidly from the continued fraction.     

国际化学元素周期表年纪念邮票及其教育价值

2019年为国际化学元素周期表年,世界范围内举行了丰富多彩的庆祝活动,邮票发行成为各国参与国际年活动的内容之一。在对世界12个国家发行的国际化学元素周期表年邮票进行收集的基础上,解读了邮票图案中的化学信息,阐述了这些化学信息在普及元素知识、拓展教学情境素材、培养科学态度方面的教育价值。     

灰色理论在旋转机械故障诊断与预报中的应用

利用灰色理论的关联度分析实现了旋转机械故障的精密诊断 ,利用灰色模型( GM)实现了旋转机械的状态预测及故障预报 ,并用 C语言编制了一套完整的旋转机械灰色故障诊断、状态预测与故障预报软件。同时本文对某厂透平机进行了全面的计算机诊断与预报 ,通过现场对该透平机检查发现故障与本软件诊断结果一致 ,进而揭示了利用灰色理论开发旋转机械故障诊断和预报系统的可行性。本软件为旋转机械故障诊断专家系统的开发提供了新的借鉴     

砖石古塔模型振动台试验的离散元模拟分析

砖石古塔块体间黏结强度较低,受地震作用易产生裂缝后发生破坏,为研究砖石古塔在地震作用下开裂及裂缝发展机制,以玄奘塔1/8缩尺模型结构为对象,建立离散元模型,计算了地震波激励下结构的加速度及位移反应,与振动台试验结果进行对比,分析了塔体开裂破坏全过程。结果表明,数值计算所得结构的加速度及位移反应与振动台试验结果一致,当地震烈度较低时,两者顶层位移变化曲线基本一致;烈度提高后,塔体开裂导致结构动力响应的计算值与试验结果出现差异,但两者的变化规律相同。在地震波激励下,塔体首层先开裂,随地震烈度的提高延伸至中部塔层,块体第2层出现阶梯状错动,模型损伤过程与试验裂缝发展状况基本吻合。研究结果为砖石古塔地震损伤及破坏过程分析提供了参考。     

Electrospinning P(LLA-CL) Nanofiber: A Tubular Scaffold Fabrication with Circumferential Alignment

In this paper a synthetic Poly(L-lactic-co-ϵ-caprolactone) [P(LLA-CL)] (75:25) copolymer has been fabricated into a nanofibrous structure by electrospinning. The polymer crystal structure has been investigated by DSC and x-ray diffraction method. During electrospinning at room temperature, a crystallization of LLA sequence in the P(LLA-CL) copolymer could not form, while a relatively regular arrangement of CL sequence was observed. In order to obtain a tubular scaffold, a rotating mandrel was designed to collect the fiber, so that the tubular scaffold can be retrieved from the mandrel with an inner diameter same as that of the outer diameter of the mandrel. An auxiliary electrode with a sharp edge and a negative charge was set under the mandrel to guide the fiber deposition on the mandrel. When the sharp edge bar was vertical to the rotating axle of the mandrel and just beneath the spinning nozzle, nanofibers with circumferential alignment were obtained. With this method it is possible to obtain a tubular scaffold with suitable fiber alignment for blood vessel tissue engineering.