流体硒的同步辐射结构研究

液体硒是典型的半导体，在高温高压下临界点邻近发生半导体金属—绝缘体（SC—M—I）转变。为了研究高温高压下流体硒在SC—M—I转变过程中结构的变化，利用SPring—8的同步辐射光对高温高压下流体硒进行了能量色散X射线衍射测量和小角X射线散射测量。测量范围包括液态区、超临界区和稠密气态区域。在衍射实验中，我们获得了精确的结构因子S(k)和偶分布函数g(r)。结果证实了在金属液体区域中流体硒仍保持两配位的链结构和共价键轻微地收缩，还发现随着流体硒的金属特性的增强g(r)的第一极小值发生了显著的增加，即Se链的链问相互作用增强。小角散射结果表明在临界密度邻近，密度的涨落显著，相干长度存在极大值。这些结果表明Se链的共价键的特性和Se链问的相互作用是理解流体硒中的SC—M—I转变的机制的关键。     

IMPROVEMENT OF BUBBLE MODEL FOR CAVITATING FLOW SIMULATIONS

In the present research,a bubble dynamics based model for cavitating flow simulations is extended to higher void fraction region for wider range of applications.The present bubble model is based on the so-called Rayleigh-Plesset equation that calculates a temporal bubble radius with the surrounding liquid pressure and is considered to be valid in an area below a certain void fraction.The solution algorithm is modified so that the Rayleigh-Plesset equation is no more solved once the bubble radius(or void fraction)reaches at a certain value till the liquid pressure recovers above the vapor pressure in order to overcome this problem.This procedure is expected to stabilize the numerical calculation.The results of simple two-dimensional flow field are presented compared with the existing bubble model.     

球面壳体表面风压分布特性风洞试验研究

球面壳体是工程中一种常用的屋面结构形式。本文通过刚性模型风洞试验,对不同场地类别下球面壳体表面风压分布进行了同步测量。根据测到的同步风压分布数据,对壳体表面风压场特性进行了分析,包括平均风压系数及脉动风压系数分布、脉动风压的自功率谱及互功率谱分布、风压场的本征正交分解特性、雷诺数对壳体表面风压分布的影响等。结果表明,球面壳体模型表面风压分布受雷诺数的影响明显,且模型的曲面特性使得由风场本征正交分解得到的前几阶特征模态对整个风压分布的贡献增大。该研究为这类结构在抗风分析中风荷载的确定及数值模拟提供试验依据。     

ANALYSIS of the HELICAL SCREW RHEOMETER FOR FLUID FOOD

The Helical Screw Rheometer (HSR) is being evaluated for rheological measurements of fluid food suspensions. the HSR consists of a helical screw enclosed in a tight fitting cylinder. With a closed system the inner screw is rotated creating a linearly increasing pressure differential along the axis of the screw, which is proportional to the viscosity of the fluid. This paper presents theoretical one- and two-dimensional flow analyses for Newtonian fluids using both rectangular and cylindrical coordinates. For non-Newtonian fluids a one-dimensional rectangular analysis was developed using the power law model.     

The evaluation of time-weighted quadratic performance indices for discrete and sampled-data linear systems

Time-weighted quadratic performance indices are evaluated for discrete and sampled-data linear systems. An evaluation in simple form is presented for discrete systems using a direct method based on manipulating the matrix infinite series. The result obtained is applied to sampled-data systems in which the performance index is given by the integral of continuous-data errors. It is shown that the result for sampled-data systems leads to the result for continuous systems when the sampling period approaches zero.     

阶梯形平屋顶及其参数对建筑屋面极值风压的影响

针对阶梯形平屋面的设计风压系数在前人研究及风荷载规范中都鲜有提及的现状,设计一系列刚体模型测压风洞实验,考察阶梯形平屋顶中等高度建筑的屋面极值风压特性以及阶梯高度和形状的影响。在验证数据可靠性基础上,重点讨论点极值风压及其最不利位置和风向角并据此确定阶梯形平屋顶风压分区;计算面积平均极值风压并将其与美国规范(ASCE7)进行比较。结果表明,阶梯形平屋顶的高层屋面极值负压最不利值与普通平屋顶并无差别,而低层屋面的极值正压和负压受阶梯参数影响明显。除附属面积较小时的负压超过规范限值外,其他面积平均风压结果与规范吻合较好。这些结果不仅能为阶梯形平屋顶的屋面系统设计提供参考,也为其将来纳入我国规范积累了基本数据。     

State feedback stabilization of cascaded nonlinear systems with discontinuous connection

In practical engineering, many phenomena are described as a discontinuous function of a state variable,and the discontinuity is usually the main reason for the degradation of the control performance. For example, in the setpoint control problem of mechanical systems, the static friction (described by a sgn function of velocity of the contacting faces) causes undesired positioning error. In this paper, we will investigate the stabilization problem for a class of nonlinear systems that consist of two subsystems with cascaded connection.We will show the basic idea with a special case first, and then the result will be extended to more general cases. Some interesting numerical examples will be given to demonstrate the effectiveness of the proposed design approach.     

Spectroscopic Measurement of Shock Waves in an Arcjet Plasma Expanding Through a Conical Nozzle

A diamond-shaped shock wave was created in a helium arcjet plasma. Visi- ble/ultraviolet emission spectroscopy was used to investigate the condition for the formation of stable shocks and to determine characteristics of the plasma. Dependence of the position of the shock front on the gas pressure in the expansion region was investigated. It was found that the shock wave arises from the collision of plasma particles and residual neutral atoms in that region. Continuum and line spectra of neutral helium were measured, from which the electron temper- atures were derived. The electron density was deduced from the Inglis-Teller limit of the He I 2p 3 P-3d 3 D series. The temperature and density were found to have almost constant values of 0.2 eV and 8.5×10 13 cm 3 , respectively, across the shock front.     

Microstructure formation and grain refinement of Mg-based alloys by electromagnetic vibration technique

The microstructure formation and grains refinement of two Mg-based alloys, i.e. AZ31 and AZ91D, were reported using an electromagnetic vibration （EMV） technique. These two alloys were solidified at various vibration frequencies and the microstructures were observed. The average size of grains was quantitatively measured as a function of vibration frequencies. Moreover, the grain size distribution was outlined versus number fraction. A novel model was proposed to account for the microstructure formation and grain refinement when considering the significant difference of the electrical resistivity properties of the solid and the liquid during EMV processing in the semisolid state. The remarkable difference originates uncoupled movement between the mobile solid and the sluggish liquid, which can activate melt flow. The microstructure evolution can be well explained when the fluid flow intensity versus vibration frequency is taken into account. Moreover, the influence of the static magnetic field on texture formation is also considered, which plays an important role at higher vibration frequencies.