Numerical Simulation of the Liquid-Gas Interface Shape in the Shot Sleeve of Cold Chamber Die Casting Machine

A mathematical model has been developed to simulate the flow pattern of molten metal and to predict the liquid-gas interface shape in the shot sleeve of a cold chamber die casting machine during the injection stage. The flow pattern in the shot sleeve is known to be closely related to the extent of gas entrapment of molten metal in the sleeve during the injection operation. In this study, a Variable Spacing Even Mesh (VSEM) method is proposed to incorporate with a computational fluid dynamics technique, named SOLA-MAC, to simulate the flow pattern in the shot sleeve. SOLA-MAC can deal with free surface flow problems while the VSEM method is used to handle the problem where the space in the shot sleeve keeps decreasing as the plunger moves to push the molten metal. The model is then tested on the shot sleeve of a cold chamber die casting machine. Four plunger speeds are tested to demonstrate the effects on the flow pattern of molten metal in the shot sleeve. The critical speed found in this study is 38 cm/s and it is close to the reported critical speed under the conditions that the space between the plunger and the sleeve end is 5 cm in diameter and 30 cm in length, and the fill ratio is 50%. As the plunger speed is slower than the critical speed, the wave front propagates along the sleeve faster than the plunger and reflects against the end wall of the sleeve. The remaining air in the shot sleeve is entrained as the wave front enters to the gate. As the plunger speed is higher than the suggested critical speed, the melt is immediately pushed higher in front of the plunger and forms a surge. The surge traps air in the early stage of the injection process.     

Influence of DMF on the polymerization of tert‐butyl acrylate initiated by 4‐oxo‐TEMPO‐capped polystyrene macroinitiator

BACKGROUND: In a number of studies it has been shown that 2,2,6,6‐tetramethylpiperidinooxy (TEMPO)‐mediated polymerization of acrylates is not facile. Therefore, the object of the study reported here was to prepare poly[styrene‐block‐(tert‐butyl acrylate)] (PS‐b‐PtBA) block copolymers using 4‐oxo‐TEMPO‐capped polystyrene macroinitiator as an initiator, in the presence of small amounts of N,N‐dimethylformamide (DMF). The kinetic analysis and the effect of DMF on the reaction mechanism are also discussed. RESULTS: PS‐b‐PtBA block copolymer was prepared through polymerization of tert‐butyl acrylate (tBA) initiated by 4‐oxo‐TEMPO‐capped polystyrene macroinitiator at 135 °C. The polymerization rate of tBA could be increased by adding a small amount of DMF, and the number average molecular weight of the PtBA block in PS‐b‐PtBA reached 10 000 g mol^?1 with narrow polydispersity. The activation rate constant k_act?tBA of alkoxyamine increased and the recombination rate constant k_rec?tBA decreased with increasing DMF concentration. CONCLUSION: DMF was shown to be a rate‐enhancing additive for the polymerization of tBA using a 4‐oxo‐TEMPO‐capped polystyrene macroinitiator. From the kinetic analysis, it was concluded that the improvement of polymerization with the addition of DMF was due to an increase in k_act?tBA and a decrease in k_rec?tBA. Copyright © 2008 Society of Chemical Industry     

Novel Design of Multi-sectional Two-phase Linear Brushless Machine

In this paper, a novel drive system of a new two-phase linear brushless machine which combines the power electronic switching technology is presented. A new multi-sectional winding mode is also presented. The operation principle, the influence of mechanical parameters, the speed characteristics, the single-chip switching circuit and analysis of system efficiency are discussed. Some simulation and experimental results are given to validate the theoretical analysis.     

Creating data resources for designing usercentric frontends for query-by-humming systems

Advances in music retrieval research greatly depend on appropriate database resources and their meaningful organization. In this paper we describe data collection efforts related to the design of query-by-humming (QBH) systems. We also provide a statistical analysis for categorizing the collected data, especially focusing on intersubject variability issues. In total, 100 people participated in our experiment, resulting in around 2000 humming samples drawn from a predefined melody list consisting of 22 different well-known music pieces and over 500 samples of melodies that were chosen spontaneously by our subjects. These data are being made available to the research community. The data from each subject were compared to the expected melody features, and an objective measure was derived to quantify the statistical deviation from the baseline. The results showed that the uncertainty in human humming varies depending on the musical structure of the melodies and the musical background of the subjects. Such details are important for designing robust QBH systems. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, or republish, to post on servers, or to redistribute to lists requires prior specific permission and/or a fee.     

基于RBF神经网络的自动驾驶仪故障诊断

介绍了自动驾驶仪的基本工作原理和径向基函数(RBF)神经网络基本结构,提出了基于径向基函数神经网络在某型自动驾驶仪故障诊断中的应用方法,并运用Matlab神经网络工具包对该神经网络模型进行训练和仿真测试,结果表明RBF网络有较高诊断正确率,且能满足实时诊断的要求.     

Video server scheduling using random early request migration

Video request migration among servers to achieve effective video-on-demand (VoD) services is investigated in this work. Our study is focused on the design and analysis of a random early migration (REM) scheme for user requests. When a new request is dispatched to a video server, the REM-based scheduler decides whether request migration is needed with a certain probability, which is a function of the service load. To analyze the request migration process, we introduce a state matrix representation that stores the service load information of each video server and plays an important role in the determination of migration paths. Based on this representation, we develop two methods to calculate performance metrics: the service failure rate and the system delay in service migration. Simulation results show that the REM scheme outperforms both the DASD dancing algorithm [1] and the traditional migration scheme adopted in [2,3] with shorter service delay and lower failure rates. It is also confirmed that our theoretical results match well with experimental results. Revised: 24 October 2004, Published online: 8 April 2005     

线性调频信号的相关检测性能分析

针对线性调频信号相关检测副本信号与待检测信号可能不完全一致的情况,给出2种信号之间存在偏差时的互模糊函数与互相关函数,分析相位差、频偏、调频率偏差对相关检测性能的影响,得出相位差对检测性能没有影响,检测概率会随着频偏与调频率偏差的增加而降低的结论。     

Movie story intensity representation through audiovisual tempo analysis

A comprehensive method for movie abstraction is developed in this research for applications in fast movie content exploring, indexing, browsing, and skimming, Most current approaches rely heavily on specific domain knowledge or models to identify and extract the determining scenes of a given movie; however, the segments extracted are often isolated, presenting a fragmented outline of the original. Our proposed method fuses simple audiovisual features, and measures the “tempos” of a movie directly, especially that of long-term ones. These tempos form a curve that catches the high-level semantics of a movie, indicating the events of interests named as “story intensity.” Through tempo, the proposed algorithm provides a natural way that segments a movie into manageable parts. As our experimental results demonstrate, the condensed skimming clips efficiently extract semantic content that contains the most interesting and informative parts of the original movie.

基于用户意图识别的查询推荐研究

信息检索的效果很大程度上取决于用户能否输入恰当的查询来描述自身信息需求。很多查询通常简短而模糊,甚至包含噪音。查询推荐技术可以帮助用户提炼查询、准确描述信息需求。为了获得高质量的查询推荐,在大规模“查询-链接”二部图上采用随机漫步方法产生候选集合。利用摘要点击信息对候选列表进行重排序,使得体现用户意图的查询排在比较高的位置。最终采用基于学习的算法对推荐查询中可能存在的噪声进行过滤。基于真实用户行为数据的实验表明该方法取得了较好的效果。     

Demonstration of submicron depletion-mode GaAs MOSFETs withnegligible drain current drift and hysteresis

We successfully fabricated submicron depletion-mode GaAs MOSFETs with negligible hysteresis and drift in drain current using Ga₂ O₃(Gd₂O₃) as the gate oxide. The 0.8-μm gate-length device shows a maximum drain current density of 450 mA/mm and a peak extrinsic transconductance of 130 mS/mm. A short-circuit current gain cutoff frequency (f_T) of 17 GHz and a maximum oscillation frequency (f_max) of 60 GHz were obtained from the 0.8 μm×60 μm device. The absence of drain current drift and hysteresis along with excellent characteristics in the submicron devices is a significant advance toward the manufacture of commercially useful GaAs MOSFETs