流体动压超光滑加工关键工艺参数优化

流体动压超光滑加工材料去除主要受工件表面流体动压和剪切分布的影响,根据材料去除的理论模型分析了影响材料去除的关键工艺参数。基于流体动力学仿真和具体实验对抛光轮浸没深度、抛光轮转速和抛光轮间隙对流体动压超光滑加工的材料去除速率的影响规律进行了研究。分析结果表明:抛光轮的浸没深度对材料去除速率影响不大;材料去除速率随着抛光轮转速的减小、抛光间隙的增大而减小;考虑实际使用条件,最优抛光轮转速为300 r/min、抛光间隙为25μm、抛光轮浸没深度为(2/3)R。同时对抛光头温度稳定性进行了具体实验测试,其在装置启动后4 h基本达到热平衡,通过试运行预热的方式可有效避免温升变化对抛光间隙的影响。     

凹槽构型对平板气层减阻影响试验分析

通过平板凹槽喷气减阻模型试验,研究了航速和气流量等因素对平板气层减阻规律的影响,并利用水下摄像系统对不同凹槽构型内的气层形态进行了观测。试验结果表明:不喷气时凹槽的存在会使平板的阻力增加;喷气可以有效减小平板阻力,存在饱和气流量;饱和气流量随着槽深的增加而减小,饱和气流量下平板的绝对减阻率随航速的增加而降低;设置凹槽构型是保持平板下表面气层稳定性的有效措施,合适的凹槽构型可以明显改善平板底部喷气流场,提高平板喷气减阻效果;在Fr为0.119时,饱和气流量下平板的绝对减阻率可以达到40%以上。     

光纤通道调度算法研究

针对光纤通道协议中没有规定数据调度算法的问题,在满足系统实时性的情况下,经过对FC-AE-ASM协议的深入分析,提出一种基于令牌桶的优先级和加权轮转相结合的调度算法,同时给出了权值分配和轮转周期的确定方法。在以消息的延迟率作为算法性能的评价指标的基础上,经过仿真,对调度算法的性能进行了验证,结果显示出该算法可以很好地满足航空电子系统实时性要求。     

基于概率分布的硬件木马检测特征选择方法

针对基于旁路信号分析的硬件木马检测中存在的高维以及信号冗余度高等问题,寻求一种特征选择方法在降维、降低冗余度方面的可行性,通过考虑样本概率的分布情况,提出了一种以散度和Bhattacharyya距离作为可分性判据的特征选择方法。首先分析了旁路信号的特征选择问题,然后阐述了基于散度和Bhattacharyya距离的可分性判据,最后在FPGA中植入硬件木马,采用K-L方法进行实验,通过对样本特征选择前后的检测效果发现,这种特征选择方法不仅有助于分辨"金片"与含木马的待测芯片间旁路信号的统计差异,而且与分类错误率建立联系,更好地实现了硬件木马的检测。     

线粘弹性材料中三维裂纹问题的加料有限元法

将加料有限元法扩展应用于线粘弹性材料三维断裂问题.为了反映裂纹尖端的奇异性,在裂尖附近的奇异区采用若干八节点六面体加料单元和过渡单元,非奇异区采用常规八节点六面体单元.三种单元分区混合使用形成求解域网格划分.基于Boltzmann叠加原理,推导了粘弹性材料的增量型本构关系,进而获得了增量加料有限元列式,并通过附加自由度计算粘弹性介质中裂纹应变能量释放率.数值算例验证了方法的正确性和有效性.     

军用飞机航材配送中心选址方法与模型

航材保障是装备保障工作的重要组成部分,直接影响部队作战和训练任务的完成。深入开展军用飞机航材配送中心选址方法研究,探索新的航材配送中心选址方法与手段,对于配送中心的科学选址,提高航材保障工作效益具有重要意义。针对军用飞机航材保障特点,综合考虑任务成功率、经济性和战时道路损毁等因素,提出了军用飞机航材保障配送中心选址方法,建立了配送中心的选址模型。最后,给出了具体算例,验证所建模型的科学性和有效性。     

压力喷嘴常温下雾化特性实验研究

为了满足工程中对喷嘴雾化特性测量的高精度、低成本要求,建立了喷嘴雾化特性实验系统,以自来水为雾化工质,对蒸发冷却压力喷嘴在常温、常压下的雾化特性进行了实验研究。用容积法测量了喷嘴的流量,用最小二乘法对测量数据进行回归分析,得到了喷嘴流量、雾化压力和喷孔直径之间的关联式与流量系数;用机器视觉方法测量了喷嘴的雾化锥角与雾滴尺寸,得到了雾化锥角与雾滴尺寸随雾化压力变化的规律。结果表明:随着雾化压力的增大,喷嘴雾化锥角不断增加,雾滴尺寸不断减小;随轴向距离的增大,D0.632、D0.5、D32有不同幅度的增大,而D0.9则基本不变。     

Bounding the optimal burn‐in time for a system with two types of failure

Burn‐in is a widely used method to improve the quality of products or systems after they have been produced. In this paper, we consider the problem of determining bounds to the optimal burn‐in time and optimal replacement policy maximizing the steady state availability of a repairable system. It is assumed that two types of system failures may occur: One is Type I failure (minor failure), which can be removed by a minimal repair, and the other is Type II failure (catastrophic failure), which can be removed only by a complete repair. Assuming that the underlying lifetime distribution of the system has a bathtub‐shaped failure rate function, upper and lower bounds for the optimal burn‐in time are provided. Furthermore, some other applications of optimal burn‐in are also considered. © 2004 Wiley Periodicals, Inc. Naval Research Logistics, 2004     

Research on the influences of confining pressure and strain rate on NEPE propellant: Experimental assessment and constitutive model

In order to study the influences of confining pressure and strain rate on the mechanical properties of the Nitrate Ester Plasticized Polyether (NEPE) propellant, uniaxial tensile tests were conducted using the self-made confining pressure system and material testing machine. The stress-strain responses of the NEPE propellant under different confining pressure conditions and strain rates were obtained and analyzed. The results show that confining pressure and strain rate have a remarkably influence on the mechanical responses of the NEPE propellant. As confining pressure increases (from 0 to 5.4 MPa), the maximum tensile stress and ultimate strain increase gradually. With the coupled effects of confining pressure and strain rate, the value of the maximum tensile stress and ultimate strain at 5.4 MPa and 0.0667 s⁻¹ is 2.03 times and 2.19 times of their values under 0 MPa and 0.00333 s⁻¹, respectively. Afterwards, the influence mechanism of confining pressure on the NEPE propellant was analyzed. Finally, based on the viscoelastic theory and continuous damage theory, a nonlinear constitutive model considering confining pressure and strain rate was developed. The damage was considered to be rate-dependent and pressure-dependent. The constitutive model was validated by comparing experimental data with predictions of the constitutive model. The whole maximum stress errors of the model predictions are lower than 4% and the corresponding strain errors are lower than 7%. The results show that confining pressure can suppress the damage initiation and evolution of the NEPE propellant and the nonlinear constitutive model can describe the mechanical responses of the NEPE propellant under various confining pressure conditions and strain rates. This research can lay a theoretical foundation for analyzing the structural integrity of propellant grain accurately under working pressure loading.     

在柴油机放热曲线计算中两个关键问题的解决方法

对影响柴油机放热曲线精度计算的因素进行了讨论 ,并对影响放热率计算的主要问题如动态上止点修正、示功图光顺处理两个主要环节进行了分析研究 ,提出了解决方法 .