Prediction of gas flow directions in gas assisted injection molding when cavities and runners are involved

In the prediction of gas flow-direction for gas-assisted injection molding (GAIM), the statement“Gas goes to the direction of the last area to fill with resin”, has been accepted as a correct one. When there exists more than one area to fill with resin, a mold design engineer for GAIM technologies has to determine to which direction gas goes that commercial software for GAIM (e.g., Moldflow) may be utilized for its mold design. However commercial software is generally expensive and is sometimes hard to become familiar with. As a rule of thumb, it is suggested that the resistance to the initial velocity of melt polymer at the nearest geometry to a gas injection point should be used as its criterion since gas goes in the direction of least resistance to initial resin-velocity. Neither the flow rate nor the resistance to flow rate can be a criterion in the prediction. Thus the statement“Gas goes to the direction of the least resistance to flow rates”, should be corrected to“More flow rate goes to the direction of the least resistance to flow rates.”, The rule of thumb suggested in this paper was verified by using commercial software, Moldflow, in the prediction of gas flow directions in GAIM under geometries where cavities and runners were involved. When the ratio of initial resin-velocity is so close to unity it is proposed as the adapted rule of thumb to calculate new emerging resin-velocities and resistances to resin-velocity at the first coming change of diameters in series of pipes and to compare those for upper and lower sides each other to predict the gas direction. Thus the judgment as to which point is the point where gas starts to choose a preferred direction is very important in the prediction.     

荫罩式PDP单元放电过程中电极电流的模拟

本文采用二维流体模型研究荫罩式PDP(SMPDP)结构放电单元内的放电过程,并在此基础上与表面电荷法结合模拟各电极上的电流.给出了寻址周期和维持周期各电极上的电流变化过程.研究了在不同寻址电压下各电极电流变化情况,分析了电流峰值的大小和电压的关系及到达电流峰值的时间和电压的关系.得到的SMPDP放电单元各电极上电流变化的规律,可用于优化驱动电压波形和改进单元结构.     

Field and Laboratory Validation of High-Flow Air Bubbler Mechanics

Recent physical model studies have refined designs for high-flow air diffusers for managing accumulations of broken ice at navigation projects. Although these solutions are successful in the model, implementing them in the field can be difficult because of uncertainties in airflow scaling. This study uses field and laboratory data to test theoretical relationships between airflow from the diffuser and the resulting near-surface water velocity. In the experiments, water velocities were measured adjacent to bubbler plumes for depths ranging from 0.52?to?6.5?m and airflow rates ranging from 0.015?to?0.68 standard cubic meters per minute/meter. The observed vertical and horizontal water velocity data compared moderately well to theoretical curves based on the equations of Kobus and Ashton. In addition, a reasonably linear relationship was found between the average velocity of the horizontal, near-surface flow field V and unit airflow from the diffuser Qa.     

Numerical Simulation of Street Canyon Flows with Simple Building Geometries

The velocity and pressure fields of the flow over street canyons formed by groups of buildings are studied numerically. The flow fields are computed by solving the time-dependent incompressible Navier–Stokes equations using the fractional step method. The numerical model is validated by simulating flows over a square cylinder at different Reynolds numbers. The Strouhal numbers, which reflect the dynamic flow characteristics, agree well with published experimental data over a wide range of Reynolds numbers. The wind field model is then applied to two street canyon configurations. First, flows inside street canyons formed by four identical buildings are simulated. The incidental flow is raised by the most upstream building and becomes parallel to the ground at the rooftop level of the fourth building downstream, resulting in a clockwise rotating vortex in downstream street canyons with an inflow from left to right. Second, flows inside street canyons formed by two identical buildings are simulated. In this case, a primary eddy that is counterclockwise rotating may be formed due to flow separation at the front corner of the upstream building. A clockwise rotating primary eddy is formed in the wake area of the separate zone above the street canyon, which drives the counterclockwise rotating eddy in the street canyon. The result indicates that rooftop level flows cannot be assumed parallel to the ground as some modelers have done in their studies. Studies also show that flow regimes in the street canyon will remain unchanged while the inflow velocity is greatly increased from 0.1 to 6.0?m/s. In addition, the wind velocities in the street canyon have a linear response to the inflow velocity.     

Flow Upstream of Orifices and Sluice Gates

Potential flow solutions of a point/line sink are extended to study the velocity field upstream of a finite-size orifice and sluice gate. It is found that, in the “near field” zones, the iso-velocity surfaces appear to be semiellipsoidal; while in the “far field” zones they become hemispheres. The shape and size of the orifice/sluice gate were found to be of no effect on the flow behavior beyond a certain distance. The development of velocity profile away from the orifice and sluice gate is examined, and the effects of water depth are studied. The results of this study compare very well with other numerical and experimental studies, and provide a general understanding of the flow field upstream of orifices and sluice gates.     

Two Methods for the Computation of Commercial Pipe Friction Factors

Two methods are proposed for the computation of friction factors of commercial pipes. The first method applies the mean value of the zero velocity point (MZVP) to a theoretical friction factor equation, and the other directly computes the mean friction factor (MFF) by averaging the friction factor of both the smooth and rough walls while considering their relative contribution. The MFF method is preferred, because it is simple but covers all the flow characteristics of commercial pipes. Both MFF and MZVP methods consider two parts of a wall with different roughness heights: One part is rough and the other is smooth. A regression analysis was performed to determine optimum values of the roughness height and probability of encountering each part, using several sets of field data, including galvanized iron, wrought iron, cast iron, concrete, riveted steel, and concrete. The analysis showed that both the roughness height and the relative contribution of the rough part are strongly dependent on the pipe diameter. The MFF method gave an average error of less than 3%, whereas the traditional Colebrook–White equation gave an average error of more than 11% when compared with Colebrook’s data.     

一个基于虚拟现实技术的车辆仿真系统

自主车辆的研制过程中，应用仿真的方法，对控制器进行分析设计是一个重要阶段。在对控制器性能进行评价时，控制对象的选择是一个关键的问题，可视化的控制对象，有形象直观，易于理解的优点，可以简化评价的过程。该文以汽车理论为基础，结合自主车辆的研制，应用虚拟现实技术，实现了一个车辆动力学模型的仿真系统，重点介绍了车辆的纵向、侧向动力学模型，在这些模型的基础上，实现了车辆动力学模型与几何模型的结合。文章最后介绍了作者利用该仿真系统进行的一些研究工作。     

基于Matlab的液压节流调速性能仿真研究

节流调速回路在小功率液压系统中得到广泛应用，但其速度稳定性不好，该文通过建立几种调速回路的数学模型，用MAatlab数学软件对其速度负载特性进行仿真，从而讨论其在不同负载下的速度稳定性和速度刚性。     

压水堆受热冲击三通构件中流速比的选取

从传热学的角度分析了压水堆中的三通构件所受到的热冲击作用，探讨了流速比对于构件所受热冲击的影响，实际运行中，为降低构件受到的热冲击，最佳流速比范围应在0．04－0．1之间。