一种大面积比喷管的分段式设计与数值分析

为了满足大推力上面级发动机大面积比喷管的设计需求,采用了排放冷却前段和单壁辐射冷却尾段的分段式设计方案。在排放冷却前段传热计算的基础上,通过对内流场进行数值模拟,重点研究了单壁尾段在引入上游排放冷却气氢情况下的冷却特性和喷管效率。结果表明:对于大面积比喷管,采用带二次流的单壁金属喷管延伸段是现实可行的,有望达到较好的冷却保护效果并提高喷管效率。     

Grinding performance evaluation of porous composite-bonded CBN wheels for Inconel 718

For high-efficiency grinding of difficult-to-cut materials such as titanium and nickel alloys, a high porosity is expected and also a sufficient mechanical strength to satisfy the function.However, the porosity increase is a disadvantage to the mechanical strength. As a promising pore forming agent, alumina bubbles are firstly induced into the abrasive layer to fabricate porous cubic boron nitride（CBN） wheels. When the wheel porosity reaches 45%, the bending strength is still high up to 50 MPa with modified orderly pore distribution. A porous CBN wheel was fabricated with a total porosity around 30%. The grinding performance of the porous composite-bonded CBN wheel was evaluated in terms of specific force, specific grinding energy, and grinding temperature, which were better than those of the vitrified one under the same grinding conditions. Compared to the vitrified CBN wheel, clear straight cutting grooves and less chip adhesion are observed on the ground surface and there is also no extensive loading on the wheel surface after grinding.     

激波聚焦诱导气液两相爆震燃烧的数值模拟

对以激波聚焦和增加障碍物方式诱导煤油-空气气液两相爆震燃烧的过程进行了数值模拟.采用欧拉-拉格朗日方法建立了脉冲爆震发动机(PDE)中气液两相流的喷射、雾化、掺混过程.研究发现环形爆震波在爆震管凹腔内经过反射、汇聚后能够引燃可燃混合物.而在障碍物处,激波的反射和再反射聚焦能够形成高温高压点(2700K,25MPa),产生局部爆炸,有助于形成稳定的脉冲爆震燃烧(波面速度为1900m/s,温度为2 950K),有效地缩短由缓燃向爆震转变(DDT)距离至0.45m.     

含噪信号经验模式分解虚假分量识别方法

针对含噪信号Hilbert-Huang变换存在虚假分量,提出改进的奇异值分解(SVD)方法进行降噪,改进包含两个部分:一是利用重构相空间代替传统矩阵如Hankel矩阵,以去掉信号冗余,再者提出奇异值能量熵分量差分法,更易于定出重构奇异值阶次;二是提出了频谱比值法对虚假分量进行辨识,更有效辨识出虚假分量.首先利用经验模式分解(EMD)得到本征模式分量(IMF),识别并剔除趋势项,重构信号,然后进行SVD,重构降噪后的信号,消除虚假分量,最后进行时频分析.联合方法应用于含噪仿真信号,信噪比(signal noise ratio,SNR)提高了5.5%,虚假分量辨识率提高至100%,用于双跨转子故障振动信号,得到正确的时频结果,表明了所提方法识别含噪信号虚假分量的有效性.     

肋角度对气膜冷却特性的影响

在光滑二次流通道的基础上,分析对比了两种带肋通道(135°肋和45°肋)对气膜冷却特性的影响.采用瞬态液晶测试技术获得了气膜孔下游表面传热系数比与气膜冷却效率分布.使用Fluent软件RANS数值方法对相应结构进行了数值模拟,并使用了realizable k-ε湍流模型.光滑二次流通道模型中,气膜孔内流线呈螺旋状分布,导致较大的孔内速度分离与流动损失.冷气射流分成两部分,其中一股形成一对偏斜的对转涡.135°肋结构中,二次流通道上部分的旋转涡为顺时针方向,使得气流易于流入气膜孔,气膜孔内流线呈直线分布.45°肋结构中,二次流通道上部分旋转涡为逆时针方向,增强了气膜孔内旋转涡.45°肋结构中冷气流入气膜孔之后的流动结构与光滑二次流通道结构相似.135°肋结构气膜冷却效率最大而表面传热系数比最低.     

飞机下滑波束导引系统设计及仿真

下滑波束导引系统是飞机在自动着陆时所采用的一种重要无线电波束导引系统。首先,简要阐述了下滑波束导引系统的基本工作原理;然后,对导引系统的运动学环节及下滑耦合器进行分析,对飞机下滑波束导引系统结构进行设计;最后,在MATLAB平台下,对所设计的下滑波束导引系统进行大量仿真研究。仿真结果显示。所设计的下滑波束导引系统结构是合理的,其控制系统是稳定的,且其稳态性能也得到改善。     

降低PLC系统峰均比的低复杂度改进SLM算法

针对信道慢时变特性及噪声复杂的特点,提出了适用于低压电力线通信的降低峰均比的改进算法.改进算法对原始输入序列中的部分数据进行循环移位来获得一系列不同的新数据序列;在每个新数据序列尾部插入相应的边带信息组成一个输入候选序列;对这一系列不同的输入候选序列分别实施傅里叶反变换,得到不同的输出序列;从中选择峰均比最小的用于传输,达到降低系统峰均比的目的.结果表明:改进算法能够有效地降低系统的峰均比,并且能够获得更好的误码率性能,同时具有比传统选择性映射方法更低的计算复杂度.     

Enhancing buckling capacity of a rectangular plate under uniaxial compression by utilizing an auxetic material

Auxetic materials have previously been shown to enhance various performances due to its unusual property of becoming fatter when uniaxially stretched and thinner when uniaxially com-pressed (i.e., the materials exhibit a negative Poisson’s ratio). The current study focuses on assessing the potential of an auxetic material to enhance the buckling capacity of a rectangular plate under uniaxial compression. The in-plane translational restraint along the unloaded edges that was often neglected in open literature is taken into consideration in our buckling model proposed in this study. The closed-form expressions for the critical buckling coefficient of the rectangle are provided and the predicted results agree well with those determined by the finite element method. Further-more, the results indicate that the buckling performance of a rectangular plate under uniaxial com-pression can be significantly improved by replacing the traditional material that has a positive Poisson’s ratio with an auxetic material when there is in-plane translation restraint along the unloaded edges.     

Evolution of the orbital elements for objects with high area-to-mass ratios in geostationary transfer orbits

A new population of uncatalogued objects in geosynchronous Earth orbits (GEO), with a mean motion of about 1 rev/day and eccentricities up to 0.6, has been identified recently. The first observations of this new type of objects were acquired in the framework of the European Space Agency’s (ESA) search for space debris in GEO and the geostationary transfer orbit (GTO) using the ESA 1-m telescope on Tenerife. Earlier studies have postulated that the perturbations due to the solar radiation pressure can lead to such large eccentricities for GEO objects with a high area-to-mass ratio (A/M). The simulations showed that the eccentricities of GEO objects with large A/M exhibit periodic variations with periods of about one year and amplitudes depending on the value of A/M. The findings of these studies could be confirmed by observations from the ESA 1-m telescope on Tenerife.     

缩短工时创造就业岗位的现实选择

缩短工时能够创造就业机会，提供有效的就业岗位。本文分析比较了国内外就业理论和创造就业实际操作过程的政策选择，认为：应该抓紧对实行35小时／周工作制的研究，并尽快出台相关法案。