台风强度变化最快时涡度的解析解

利用旋转坐标系中的基本方程推导得到动能方程,通过动能的局地变化率定义刻画台风强度变化率的能量泛函,对泛函作变分得到Euler-Lagrange方程.分析方程可知,当台风强度变化率达到最大时,摩擦力、气压梯度力、重力和动能梯度满足四力平衡关系.因此,这四个力确定的向量可以作为台风强度的预报因子,更准确地确定台风系统强度变化率达到最大的时间点.进一步通过风场变分分解提取到有旋场中的最大涡旋,得到台风强度变化最快时涡度和流场的解析解,对研究台风发展过程中速度的变化趋势和台风的空间结构具有实用价值,为台风路径和强度预报提供了一定的理论指引.     

Failure of type 316 stainless steel cladding during thermal transients

To obtain a basic understanding of the deformation and failure behaviors of 20% cold-worked type 316 stainless steel cladding during reactor over-power and loss-of-coolant transients, the mechanical response of the cladding was investigated, using a true 2 to 1 stress-state transient tube-burst method at a heating rate of 5.6°C/s (10°F/s).The uniform diametral strain to failure of the tubular cladding specimens was uniquely defined, based on strain profiles of the failure specimens and high-speed motion pictures taken during the tests. The effects of initial hoop stress, specimen length and metallurgical condition on the transient tube-burst response were studied. The heat-to-heat variation of the test results were examined.The uniform diametral strains to failure for the three heats of cladding tested in the present study were found to be consistently higher than the strains reported by Hunter et al.     

Microstructural evolution,mechanical and thermal properties of TiC-ZrC-Cr3C2 composites

Dense TiC-ZrC-Cr₃C₂ composites with various TiC content from 19.6 mol% to 78.4 mol% have been fabricated by hot-pressing sintering at 1950 °C using 2.0 mol% Cr₃C₂ as sintering aid. The effect of TiC content on the microstructure, mechanical and thermal properties of TiC-ZrC-Cr₃C₂ composites are investigated systematically. The single (Zr, Ti, Cr)C solid solution is obtained when TiC content is 19.6 mol%, while with increasing TiC content, the composites begin to consist of Zr-rich (Zr, Ti)C solid solution and Ti-rich (Ti, Zr, Cr)C solid solution phase. SEM and EDS analysis confirm that Cr element is not favorable to diffuse into ZrC lattice to form (Zr, Cr)C solid solution. Flexural strength and Vickers hardness increase gradually with increasing TiC content, but fracture toughness does not improve significantly. Fracture toughness are in the range of 3.34–4.01 MPa∙m^1/2 for all composites, and the optimum value reaches 4.01 MPa·m^1/2 with 49.0 mol% TiC. Experimental results of the thermal expansion coefficient reveal that the addition of TiC raises the thermal expansivity of TiC-ZrC-Cr₃C₂ composites. Noticeably, the thermal conductivities of TiC-ZrC-Cr₃C₂ composites show a decrement trend with increasing TiC content, not as theoretical predicting by the rule of mixtures. For instance, the thermal conductivity at 25 °C ranges from 18.0 W/m∙K for 8Z2T2C composite down to 10.6 W/m∙K for 2Z8T2C composite.     

商务英语函电中的语言特点研究

商务英语函电是国际贸易中贸易双方交流沟通的主要途径,经常使用缩略词、专业术语、正式用语等词汇,大量采用固定句、被动句和长句,具有措辞礼貌体谅、用语清晰简洁的特点。通过从词汇、句法、文体三个角度对商务英语函电语言特点的研究,并进行掌握和吸收,有助于更好地进行函电写作和有效地进行商务沟通,促进国际贸易的健康发展。     

塑料齿轮注塑模设计及成型工艺分析与优化

翘曲变形是影响塑料齿轮传动精度的最大因素。为获得最小变形量,结合注塑成型数值模拟和正交试验设计、极差分析,得到最优工艺参数方案,满足塑件使用要求;用三维软件完成了齿轮注塑模具的总装结构设计,阐述了三板模自动脱模的模具工作过程。     

Photoluminescent and tough hydrogels from silk fibroin and polyacrylic acid complexed with europium

Biocompatible, tough, and photoluminescent hydrogels are highly desirable for biomedical applications in vivo. Herein, hybrid hydrogels prepared from silk fibroin (SF) and polyacrylic acid (PAA) and complexed with europium, named as SF-PAA-Eu³⁺ hydrogels, exhibit good comprehensive properties. Owing to the intensive molecular interactions among SF, PAA, and Eu³⁺, SF-PAA-Eu³⁺ hydrogels show a greatest tensile strength of 0.58 MPa, elongation of 443%, and work of fracture of 1.65 MJ/m. In vivo imaging experiment in a mouse subcutaneous implantation model revealed excellent and sustained photoluminescence of the SF-PAA-Eu³⁺ hydrogels for 24 h. The work provides a strategy for designing functional SF-based hydrogels for imaging applications in vivo.     

地方高等职业教育专业设置与区域经济的适应性分析——以云南省10所高职院校为例

地方高等职业教育与区域经济发展是否适应,和其专业建设密不可分。选取了云南省10所具有代表性的高职院校作为研究对象,深入考察它们的专业设置情况,样本选择兼顾了层次和公私立之差异,从专业设置类别和布点数等来分析其与云南省三大产业的匹配情况。并提出了云南高职教育专业设置与调整建议,及需要遵循的原则。     

Artificial intelligence based ensemble approach for intrusion detection systems

Internet attacks pose a severe threat to most of the online resources and are a prime concern of security administrators these days. In spite of many efforts, the security techniques are unable to detect the intrusions accurately. Most of the methods suffer from the limitations of a high false positive rate, low detection rate and provide one solution which lacks the classification trade-offs. In this work, an effective two-stage method is proposed to produce a pool of non-dominating solutions or Pareto optimal solutions as base models and their ensembles for detecting the intrusions accurately. It generates Pareto optimal solutions to a chromosome structure in stage 1 formulating Pareto front. Whereas, another approximation to the Pareto front of optimal solutions is made to obtain non-dominating ensembles in the second stage. The final prediction ensemble solutions are computed from individual predictions using majority voting approach. Applicability of the suggested method is validated using benchmark dataset NSL-KDD dataset. The experimental results show that the recommended method provides better results than conventional ensemble techniques. The recommended method is also adequate to generate Pareto optimal solutions that address the issue of improving detection accuracy for minority as well as majority attack classes along with handling classification tradeoff problem. The proposed method resulted detection accuracy of 97% with FPR of 2% for KDD dataset respectively. The most attractive feature of the proposed method is that both generation of base classifier and their ensemble thereof are multi-objective in nature addressing the issue of low detection accuracy and classification tradeoffs.     

The effects of PVP surfactant in the direct and indirect hydrothermal synthesis processes of ceria nanostructures

CeO₂ nanostructures with completely different morphologies were successfully prepared using the same cerium source and mineralizer through the direct and indirect hydrothermal methods with different introducing strategies of PVP surfactant. The CeO₂ nanostructures tend to form the morphologies of nano-flowers and nano-cubes through the indirect and direct hydrothermal methods, respectively. X-ray diffraction (XRD) analyses revealed that both as-prepared nanostructures are composed of CeO₂ with a standard fluorite structure. The different synthesis mechanisms and corresponding chemical evolutions of the as-prepared CeO₂ nanostructures are discussed based on the different introducing strategies of PVP surfactant in the direct and indirect hydrothermal processes. Investigation of the UV-shielding ability of both CeO₂ nanostructures suggested that the UV absorbance of the nano-flowers is much higher than that of the nano-cubes.