Effects of wall temperature on boundary layer stability over a blunt cone at Mach 7.99

Effects of wall temperature on stabilities of hypersonic boundary layer over a 7-degree half-cone-angle blunt cone are studied by using both direct numerical simulation (DNS) and linear stability theory (LST) analysis. Four isothermal wall cases with Tw/T0= 0.5, 0.7, 0.8 and 0.9, as well as an adiabatic wall case are considered. Results of both DNS and LST indicate that wall temperature has significant effects on the growth of disturbance waves. Cooling the surface accelerates unstable Mack II mode waves and decelerates the first mode (Tollmien–Schlichting mode) waves. LST results show that growth rate of the most unstable Mack II mode waves for the cases of cold wall Tw/T0=0.5 and 0.7 are about 45% and 25% larger than that for the adiabatic wall, respectively. Numerical results show that surface cooling modifies the profiles of rdut/dyn and temperature in the boundary layers, and thus changes the stability haracteristic of the boundary layers, and then effects on the growth of unstable waves. The results of DNS indicate that the disturbances with the frequency range from about 119.4 to 179.1 kHz, including the most unstable Mack modes, produce strong mode competition in the downstream region from about 11 to 100 nose radii. And adiabatic wall enhances the amplitudes of disturbance according to the results of DNS, although the LST indicates that the growth rate of the disturbance of cold wall is larger. That because the growth of the disturbance does not only depend on the development of the second unstable mode

长期保存起源信息封装技术的比较研究

文章详细介绍了四种国际主流起源信息的封装格式和应用方案，对其封装方法、元数据支持、工具支 持和易用性等进行了对比分析，进一步探讨了四种方法在实际应用中的问题及其应对策略，并建议在实际运用 中，应结合具体场景选择合适的封装格式和封装策略，以达到最佳效果。</p

构建以事件为核心的长期保存系统起源管理框架

［目的/意义］研究建立长期保存系统起源管理框架，通过有效管理起源信息，确保长期保存系统所 存档数据的真实可靠可用。［方法/过程］基于数字对象保存周期进行起源事件定义，基于 OAIS 保存流程进行 起源管理框架设计，以事件为核心进行起源管理功能模型和起源信息模型设计。［结果/结论］初步完成基于事 件的保存系统起源管理框架的设计，既遵循保存领域的相关标准，同时兼顾实践需求，对长期保存系统具有很 好的普适性和可行性，但其在有效性和实用性方面还有待进一步验证。</p

基于OAIS信息模型的起源信息封装策略研究与应用

［目的/意义］探索设计并实施长期保存系统起源信息封装策略。［方法/过程］首先分析 OAIS 信息 模型的详细组成，然后分析长期保存系统起源封装的特定服务需求，总结国际主流起源封装策略的基本特点， 提出基于 OAIS 信息模型的起源信息封装策略，并利用 Fedora 仓储系统的内容模型 FOXML 进行封装实现。 ［结果/结论］该策略遵循 OAIS 及 PＲEMIS 标准，全面记录起源信息，具有灵活可扩展的特点，但同时管理和使 用相对比较复杂。</p

构建以事件为核心的长期保存系统起源管理框架

［目的/意义］研究建立长期保存系统起源管理框架，通过有效管理起源信息，确保长期保存系统所 存档数据的真实可靠可用。［方法/过程］基于数字对象保存周期进行起源事件定义，基于 OAIS 保存流程进行 起源管理框架设计，以事件为核心进行起源管理功能模型和起源信息模型设计。［结果/结论］初步完成基于事 件的保存系统起源管理框架的设计，既遵循保存领域的相关标准，同时兼顾实践需求，对长期保存系统具有很 好的普适性和可行性，但其在有效性和实用性方面还有待进一步验证。</p

Direct numerical simulation of hypersonic boundary layer transition over a blunt cone with a small angle of attack

The direct numerical simulation of boundary layer transition over a 5&deg; half-cone-angle blunt cone is performed. The free-stream Mach number is 6 and the angle of attack is 1&deg;. Random wall blow-and-suction perturbations are used to trigger the transition. Different from the authors&rsquo; previous work [Li et al., AIAA J. 46, 2899(2008)], the whole boundary layer flow over the cone is simulated (while in the author&rsquo;s previous work, only two 45&deg; regions around the leeward and the windward sections are simulated). The transition location on the cone surface is determined through the rapid increase in skin fraction coefficient (Cf). The transition line on the cone surface shows a nonmonotonic curve and the transition is delayed in the range of 0&deg; &le; &theta; &le; 30&deg; (&theta; = 0&deg; is the leeward section). The mechanism of the delayed transition is studied by using joint frequency spectrum analysis and linear stability theory (LST). It is shown that the growth rates of unstable waves of the second mode are suppressed in the range of 20&deg; &le; &theta; &le; 30&deg;, which leads to the delayed transition location. Very low frequency waves VLFWs� are found in the time series recorded just before the transition location, and the periodic times of VLFWs are about one order larger than those of ordinary Mack second mode waves. Band-pass filter is used to analyze the low frequency waves, and they are deemed as the effect of large scale nonlinear perturbations triggered by LST waves when they are strong enough.The direct numerical simulation of boundary layer transition over a 5&deg; half-cone-angle blunt cone is performed. The free-stream Mach number is 6 and the angle of attack is 1&deg;. Random wall blow-and-suction perturbations are used to trigger the transition. Different from the authors&rsquo; previous work [ Li et al., AIAA J. 46, 2899 (2008) ], the whole boundary layer flow over the cone is simulated (while in the author&rsquo;s previous work, only two 45&deg; regions around the leeward and the windward sections are simulated). The transition location on the cone surface is determined through the rapid increase in skin fraction coefficient (Cf). The transition line on the cone surface shows a nonmonotonic curve and the transition is delayed in the range of 20&deg; &le; &theta; &le; 30&deg; (&theta; = 0&deg; is the leeward section). The mechanism of the delayed transition is studied by using joint frequency spectrum analysis and linear stability theory (LST). It is shown that the growth rates of unstable waves of the second mode are suppressed in the range of 20&deg; &le; &theta; &le; 30&deg;, which leads to the delayed transition location. Very low frequency waves (VLFWs) are found in the time series recorded just before the transition location, and the periodic times of VLFWs are about one order larger than those of ordinary Mack second mode waves. Band-pass filter is used to analyze the low frequency waves, and they are deemed as the effect of large scale nonlinear perturbations triggered by LST waves when they are strong enough

Wide-Range Tunable Dynamic Property of Carbon Nanotube-Based Fibers

Carbon nanotube (CNT) fiber is formed by assembling millions of individual tubes. The assembly feature provides the fiber with rich interface structures and thus various ways of energy dissipation, as reflected by the non-zero loss tangent (>0.028--0.045) at low vibration frequencies. A fiber containing entangled CNTs possesses higher loss tangents than a fiber spun from aligned CNTs. Liquid densification and polymer infiltration, the two common ways to increase the interfacial friction and thus the fiber's tensile strength and modulus, are found to efficiently reduce the damping coefficient. This is because the sliding tendency between CNT bundles can also be well suppressed by the high packing density and the formation of covalent polymer cross-links within the fiber. The CNT/bismaleimide composite fiber exhibited the smallest loss tangent, nearly as the same as that of carbon fibers. At a higher level of the assembly structure, namely a multi-ply CNT yarn, the inter-fiber friction and sliding tendency obviously influence the yarn's damping performance, and the loss tangent can be tuned within a wide range, as similar to carbon fibers, nylon yarns, or cotton yarns. The wide-range tunable dynamic properties allow new applications ranging from high quality factor materials to dissipative systems

Bio-Inspired Aggregation Control of Carbon Nanotubes for Ultra-Strong Composites

High performance nanocomposites require well dispersion and high alignment of the nanometer-sized components, at a high mass or volume fraction as well. However, the road towards such composite structure is severely hindered due to the easy aggregation of these nanometer-sized components. Here we demonstrate a big step to approach the ideal composite structure for carbon nanotube (CNT) where all the CNTs were highly packed, aligned, and unaggregated, with the impregnated polymers acting as interfacial adhesions and mortars to build up the composite structure. The strategy was based on a bio-inspired aggregation control to limit the CNT aggregation to be sub 20--50 nm, a dimension determined by the CNT growth. After being stretched with full structural relaxation in a multi-step way, the CNT/polymer (bismaleimide) composite yielded super-high tensile strengths up to 6.27--6.94 GPa, more than 100% higher than those of carbon fiber/epoxy composites, and toughnesses up to 117--192 MPa. We anticipate that the present study can be generalized for developing multifunctional and smart nanocomposites where all the surfaces of nanometer-sized components can take part in shear transfer of mechanical, thermal, and electrical signals

塔里木沙漠公路防护林土壤微生物生物量与土壤环境因子的关系

为探讨极端干旱区风沙土土壤微生物与土壤环境因子的作用规律,采用相关分析法研究了塔里木沙漠公路防护林地土壤微生物生物量与理化因子和酶活性的关系.结果表明:土壤容重和粒径减小(R0.85)时,防护林地中土壤微生物数量和生物量有增大趋势,由容重与微生物量的相关性主导;土壤养分含量与土壤微生物数量和生物量呈正相关,主要由速效养分和放线菌、微生物生物量C、P的相关性所致;土壤酶活性与土壤微生物数量和生物量的相关性差异较大,R在0.51～0.91,主要取决于蔗糖酶、磷酸酶与放线菌、微生物量C的相关;土壤盐分增加不利于土壤微生物生物量的积累(R0.63).实践中应为干旱区林地土壤微生物营造良好的土体,促进土壤物质循环