An efficient traffic control scheme for store‐and‐forward video‐on‐demand services

According to the MPEG‐1/2 standard, full motion video can be compressed and stored in an information warehouse. Multiplexing with normal voice calls, it is retrieved and delivered to the customer's local BISDN central office via 155 Mb/s trunks. These voice calls have higher priority than video‐on‐demand (VOD) so that normal voice services will not be influenced by VOD transmission. The number of voice calls always fluctuates in real‐time. Thus, an efficient traffic control scheme is highly desired to guarantee a given level of performance and achieve as great as possible use of available bandwidth. Based on the fact that the number of real‐time voice calls is variable, and the use of available capacity is less than 100 per cent, this paper will present an efficient traffic control scheme for store‐and‐forward VOD services, and demonstrate the scheme by various simulation results. Copyright © 2001 John Wiley & Sons, Ltd.     

Periodical shedding of cloud cavitation from a single hydrofoil in high-speed cryogenic channel flow

In order to explain criteria for periodical shedding of the cloud cavitation, flow patterns of cavitation around a plano-convex hydrofoil were observed using a cryogenic cavitation tunnel of a blowdown type. Two hydrofoils of similarity of 20 and 60 mm in chord length with two test sections of 20 and 60 mm in width were prepared. Working fluids were water at ambient temperature, hot water and liquid nitrogen. The parameter range was varied between 0.3 and 1.4 for cavitation number, 9 and 17 m/sec for inlet flow velocity, and −8° and 8° for the flow incidence angle, respectively. At incidence angle 8°, that is, the convex surface being suction surface, periodical shedding of the whole cloud cavitation was observed on the convex surface under the specific condition with cavitation number and inlet flow velocity, respectively, 0.5, 9 m/sec for liquid nitrogen at 192°C and 1.4, 11 m/sec for water at 88°C, whereas under the supercavitation condition, it was not observable. Periodical shedding of cloud cavitation occurs only in the case that there are both the adverse pressure gradient and the slow flow region on the hydrofoil.     

Ultrasonic absorption and visco-relaxation study of poly(vinyl alcohol) aqueous solution. (A method for the determination of molecular weight)

Ultrasonic absorption and velocity measurements are reported on aqueous solutions of poly(vinyl alcohol) (PV-OH) of different molecular weights and concentrations obtained using a pulsed ultrasonic apparatus operating at 2 MHz and 303 K. The variations in these parameters as a function of concentration are interpreted in terms of changes in the magnitude of the contribution due to relaxation of the backbone and of the side chain acetyl groups. The results show an increase in velocity, density and viscosity with increase in molecular weight and concentration of PV-OH. In contrast the attenuation values decrease with increase in molecular weight. This suggests that interaction is occurring between PV-OH and water molecules. Deviation of the variation of the attenuation with concentration from the ‘ideal’ dilute solution behaviour at high concentration is ascribed to the effects of polymer-polymer interaction. The magnitude of the volume change associated with polymer-solvent interactions is estimated and discussed. A mathematical equation correlating the relaxation amplitude and the molecular weight of the polymer is suggested. This approach was applied to the calculation of the molecular weight of four unknown samples of PV-OH from their measured relaxation amplitudes. The results obtained agree well with those obtained from osmometry.     

Diffusion cloud around and downstream of active sphere immersed in granular bed through which fluid flows

The concentration distribution in the wake of (and around) a soluble or fast reacting sphere, immersed in a granular bed of inert particles through which liquid or gas flows, with “uniform velocity”, has been obtained numerically, for solute transport by both advection and diffusion.Fluid flow in the granular bed around the sphere was assumed to follow Darcy's law and the elliptic PDE equation, resulting from a differential material balance on the solute in an elementary control volume, was solved numerically over the “whole range” of values of the Peclet number. The solution gives the steady state concentration contour plots around the sphere and, for each concentration level, the downstream extent and approximate maximum width of the corresponding contour surface were determined. General expressions are presented (as Eqs. (20) and (21)) to relate those variables with the Peclet number and this provides simple formulae to compute the approximate size of the diffusion wake downstream of the active sphere.     

风频参数的估算方法与拟合误差

在风场确定之后，以对数正态风频模型为例，系统地研究了几种估算参数的方法，并分别对各种参数估算方法与拟合误差之间的关系进行了讨论。为了更准确地判断拟合状况，作为拟合标准，引入一个新的拟合误差。理论结果用实测风况资料检验，结论令人满意。     

Thermal conductivity of normal pentane in the temperature range 306–360 K at pressures up to 0.5 GPa

This paper reports the results of new, absolute measurements of the thermal conductivity of normal pentane in the temperature range 306 to 360 K at pressures up to 0.50 GPa. The experimental data have an estimated uncertainty of ±0.3%. The density dependence of the thermal conductivity along all of the isotherms cannot be represented by a common equation within its estimated uncertainty. Nevertheless, such a universal equation does provide a simple method of correlating the complete set of data with an error of no more than ±2.5%.     

ICS TRUSTED系统在炼油厂压缩机调速中的应用

介绍ICS TRUSTED系统在炼油厂压缩机调速中的应用,重点介绍ICS TRUSTED强大的软件功能以及在压缩机调速系统中运用的可靠性、适用性和灵活性。     

The Limiting Density of Unit Root Test Statistics: A Unifying Technique

In this note we introduce a simple principle to derive a constructive expression for the density of the limiting distribution, under the null hypothesis, of unit root statistics for an AR(1)-process in a variety of situations. We consider the case of unknown mean and reconsider the well-known situation where the mean is zero. For long-range dependent errors we indicate how the principle might apply again. We also show that in principle the method also works for a near unit root case. Weak convergence and subsequent Karhunen-Loeve expansion of the weak limit of the partial sum process of the errors plays an important role, along with the evaluation of a certain normal type integral with complex mean and variance. For independent and long range dependent errors this weak limit is ordinary and fractional Brownian motion respectively.
AMS 1991 subject classification. Primary 62M10; secondary 62E20.     

含缺陷球形压力容器的简化、常规评定

对某化工厂含缺陷球罐的其中4处较严重的埋藏缺陷分别用简化(一级)、常规(二级)评定方法进行评定.结果发现,两种评定结果相互矛盾,出现所谓一级评定能通过,二级评定不能通过的反转现象,因此建议改进常规评定方法.     

直链饱和烃及其衍生物性质的拓扑化学研究

在分析Wiener指数W的基础上,充分考虑直链饱和烃及其衍生物的结构特征,提出了一个新的拓扑指数M,并研究了M与直链饱和烃及其衍生物的一系列物理化学性质的关系,找出了P与M的普遍关系式P=aM+bM~(1/2)+c 对于不同系列的同系物和性质,常数a,b,c的值不同。