Experimental investigation of natural convection from a horizontal ice surface melting in pure water

A study on natural convection from a horizontal ice surface melting in pure water was conducted experimentally for the ambient water temperature from 2▿C to 10°C. Natural convection flow around upward-or downward-facing horizontal ice plate was divided into three regions according to the temperature variation of ambient water. The flow patterns of three regions were no flow, two-dimensional steady laminar flow and unsteady flow. Mean Nusselt number for the upward-facing surface had its maximum value at about 3°C of ambient water temperature. However, in the case of the downward-facing surface it increased as the ambient water temperature increased.     

Effects of Heat Treatment in a Wet Hydrogen Atmosphere on the Reliability of Sintered α-Silicon Carbide

Sintered α-SiC was exposed, for times up to 2 h, to a flowing wet H₂ atmosphere ( P _H2O= 1 × 10^-4 MPa) at temperatures of 1300°, 1400°, and 1500°C. The effect of such conditions on the reliability of the ceramic was estimated by comparing the Weibull modulus of the groups of specimens, tested in four-point flexure, before and after exposure. The Weibull modulus of as-polished specimens was 6.7, indicating a wide variation in room-temperature flexural strength. The Weibull modulus was increased to 14.2 by the heat treatment for 2 h in wet H₂ at 1400°C. The average strength was also improved from 347 to 446 MPa by such exposure. Heat treatment at 1300° and 1500°C also improved the reliability of the material, as indicated by increases in the Weibull modulus, but to less a degree than did exposure at 1400°C. The increases in reliability and average strength were attributed to the blunting of surface flaws by the formation of a thin SiO₂ layer on the sample surface.     

一种基于MVDR和CCBC的抗噪语音识别方法

提出了一种适用于抗噪声语音识别的方法，其特征提取过程基于最小方差无失真响应（Minimum variance distortionles sresponse，MVDR）谱估计方法，并对该特征进行频率弯折以提高其知觉分辨率，最后使用基于正则相关分析的谱变换补偿（Canonical correlation based on compensation，CCBC）法对该特征进行自适应处理，从而提高了系统的鲁棒性。在展览馆噪声、人群噪声和汽车噪声下，与基于传统Mel倒谱系数（MFCC）特征的系统进行了对比实验，结果表明使用本文方法的语音识别系统的识别率得到了显著的提高。     

基于PDA的通信装备故障诊断系统的设计与实现

基于PDA的通信装备故障诊断系统采用客户/服务器结构,通过TCP/IP网络进行身份认证和数据传输。文中针对网络信息系统的口令验证的安全问题提出了一种基于SHA的一次性口令的身份认证方法;给出了诊断系统总体流程,并设计了可配置的诊断流程;讨论使用SQLServerCE提供的远程数据访问RDA对象,实现PDA上的SQLServerCE数据库与远程PC服务器上的SQLServer数据库的数据传输。     

典型装药水下爆炸的殉爆规律研究

针对典型的壳装炸药殉爆更接近炸药实际使用状态,采用ANSYS／LSDYNA软件建立了典型装药水下殉爆的有限元仿真模型,通过计算得到了殉爆距离与安全距离,基于此加工了试验样弹,并进行了相应的水下殉爆试验。试验结果与数值仿真结果较为吻合,表明了本文的仿真模型能够有效描述带壳装药的水下殉爆情况,水下试验中南冲击波压力和气泡周期来判断被发装药是否殉爆是可行的。最后,在水下殉爆理论分析基础上,根据试验结果预计了大药量水中兵器战斗部的安全性。本文的研究可为水中兵器战场环境的安全性设计提供依据,并可为水中兵器战斗部殉爆毁伤能力评估提供参考。     

导弹武器系统贮存期间检测周期的确定方法

导弹装备具有长期贮存的特性,必须对贮存装备进行质量控制,通过定期监测和修复贮存失效的方法,使装备在贮存期间满足预期的可靠性要求,因此确定合理的检测周期十分必要.提出了一种确定检测周期的新方法,同时以实际数据为基础,综合运用可靠性与维修性理论、现代数理统计知识、定性与定量相结合等方法进行分析,应用Matlab软件进行仿真.     

Oxidation behaviour and strength of B4C-30 wt% SiC composite materials

The oxidation behaviour and the effect of oxidation on the room-temperature flexural strength of B₄C-30 wt% SiC composite material were investigated. The weight changes of the samples exposed to air at temperatures between 500 and 1000 °C were continuously monitored with a microbalance. At temperatures below 800 °C, the weight change of the specimen was negligible. As the temperature was increased to 800 °C, parabolic weight gain was observed. The rate of the weight gain increased with exposure temperature. The oxidation product formed on the surface was found to be a crystalline boric oxide (B₂O₃) by X-ray analyses. The oxide layer was severely cracked due to the thermal expansion mismatch between the oxide layer and the substrate. However, the room-temperature flexural strength was increased when the samples were exposed at temperature between 700 and 900 °C, apparently due to the blunting of strength-limiting defects at the surface. When the temperature was higher than 1000 °C, a severe reduction in strength was observed. The reliability of the composite material was also improved significantly by such exposures.     

Skin impedance from 1 Hz to 1 MHz

The impedance of skin coated with gel but otherwise unprepared was measured from 1 Hz to 1 MHz at ten sites on the thorax, leg, and forehead of ten subjects. For a 1-cm² area, the 1 Hz impedance varied from 10 kΩ to 1 MΩ, which suggests that the bipotential amplifier input impedance should be very high to avoid common-mode-to-differential-mode voltage conversion. The 1-MHz impedance was tightly clustered about 120 Ω. The 100-kHz impedance was about 220 Ω, which suggests that the variation in skin impedance can cause errors in two-electrode electrical impedance tomographs     

Thermal diagnostics front-end electronics for LISA Pathfinder

Precision temperature measurements are required in the LTP, the LISA technology package, for various diagnostics objectives. In this article, we describe in detail the front-end electronics design and the associated temperature sensors to achieve the LTP requirements: noise equivalent temperature of 10 microK Hz(-12) in the frequency range from 1 to 30 mHz at room temperature. We designed an ac Wheatstone bridge and a subsequent digital demodulation to minimize 1/f noise. We show experimental results where the required sensitivity in the measurement bandwidth is fulfilled.