On the aerodynamic forces of a plunging airfoil

The generation of aerodynamic forces by a plunging NACA0012 airfoil at a Reynolds number of 20,000 was studied for a range of non-dimensional plunge frequenciesk and amplitudesh using a 2D unsteady compressible Navier-Stokes solver, an unsteady panel method (UPM) and Garrick’s analysis. Calculations using these two methods indicate that the forces collapse reasonably well withkh (or equivalents the Strouhal number), but are only weak functions ofk. In contrast results from the NS code indicate that the forces are dependent on bothk andkh independently, with large variations at low frequencies. The frequency dependence was found to be a result of vortex shedding from the leading edge of the airfoil, and appears to result from two factors. Firstly at high plunge frequenciesk, the leading edge vortex does not have sufficient time to grow, whereas at lowk the vortex can become a sizeable fraction of the airfoil chord before separating. Secondly once the vortex separates, it is convected downstream over the surface of the airfoil. Due to the low pressure in the vortex core, thrust is maintained while the vortex is upstream of the airfoil maximum thickness point (where the airfoil surface is tilted upstream and the vortex low pressure creates an upstream suction force). Once past this point, the airfoil surface is tilted downstream and the vortex contributes to drag rather than thrust. At high plunge frequencies the vortex cannot be convected far downstream before the motion cycle creates another leading edge vortex on the opposite side of the airfoil, so the impact is lessened. At lowk however the vortex travels far downstream over the airfoil surface, causing drag for a larger portion of the flapping cycle, and therefore lower propulsive efficiency. These results have strong implications on how the thrust and the propulsive efficiency can be maximised by controlling the relative amplitudes and phases of combined pitching and plunging motions of an airfoil.     

A systematic method of adaptive joint design considering different assembly sequence in sheet metal product

In sheet metal assembly, not only the component variations and tool errors, but also the component structure (joint type) and assembly process (assembly sequence) affect the final dimensional quality. In this paper, a systematic method for adaptive joint design considering different assembly sequence is proposed to meet the in-process dimensional adjustability of KCs (key characteristics). First, the adaptive characteristic of the sheet metal joint is depicted. Then, the mathematical model in order for concurrently optimizing both joint type and different assembly sequence is presented. How to evaluate the combination of joint type and assembly sequence is carried out according to two conditions: (1) for single KC, and (2) for multiple KCs. The KC confliction is considered to ensure the important KCs. Genetic algorithm is used to resolve the optimization of joint design. An example is chosen to demonstrate our method finally, and various joint designs are acquired according to different assembly sequences by this means. The proposed methods make it possible for us to improve the dimensional quality of product in the design stage.     

Preparation and Characterization of Superconducting B1-Type Mo1-xNbxN Thin Films

Solid-solution thin films, Mo_1−xNb_xN, with B1 (NaCl) structure have been deposited on substrates at 450°C by reactive sputtering of composite targets with Mo and Nb metals in an equimolar Ar and N₂ gas mixture. The lattice parameter of the solid solution increased linearly with an increase of Nb content, x . The superconducting transition temperature, T _c, of the Mo-rich films ( x ≦ 0.12) was around 6 K, which was lower than that expected from theoretical prediction. The residual resistivity ratio, r =ρ(300 K)/ρ(20 K), of the films varied with x , and a minimum value of r was observed in the vicinity of x = 0.5.     

稀土改性镍催化剂对CO2甲烷化反应的影响

以稀土为助剂 ,金属镍为活性组份 ,研究了不同稀土对镍催化剂的改性作用 ,同时用活性比表面、H2 -TPD、CO2 - TPD及 XPS等方法对催化剂进行了表征。结果表明 ,稀土对镍催化剂的 CO2 甲烷化活性有明显的促进作用 ,其中以稀土 Sm的改善效果最好 ;海泡石和 Sm既能增大镍催化剂的活性比表面积 ,又能提高 H2 和 CO2 的吸附量 ,同时降低反应的活化能 ,增大镍上的电子云密度。     

USB信息认证器的设计

给出了一种PC使用的USB信息认证外围设备。它使用USB接口,可为电子商务、电子政务提供高强度信息安全保障它基于抗篡改的硬件设计。通过USB接口与计算机相连接,可提供数据完整性、数字签名和数字认证等安全功能。     

Formulation and characterization of anisotropic conductive adhesive paste for microelectronics packaging applications

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. In this paper, several anisotropic conductive adhesive (ACA) pastes were formulated, which consist of diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A as polymer matrix, imidazoles as curing agents, and different sizes of silver (Ag) powders or gold (Au)-coated polymer spheres as conductive particles. The effects of ACA resin and different curing agents, as well as different conductive particles, on flexible substrate of the flip-chip joint were studied. The results show that the size and type of different conductive particles have very limited influence on an ACA flip-chip joint. The ACA resin as well as the curing agent can affect the reliability of the joint. The same results can be applied for the failure analysis of ACA flip-chip technology.     

On the kinetics of carbon nanotube growth by thermal CVD method

The role of ammonia (NH₃) on obtaining good quality vertically aligned multi-walled carbon nanotubes (CNTs) in thermal chemical vapor deposition (CVD) method has been widely studied. It was generally agreed that NH₃ helps to maintain catalyst metal surface active by reacting with amorphous carbon. In this article, a systematic study in varying the temperature and mixing ratio of gases was conducted in order to clarify the role of NH₃ and revealed a criterion for optimized condition window in the growth processes. In addition, this study has also carried out a statistical analysis through intensive TEM observations on the tube diameters, bamboo spacing, and the formation rate of each diaphragm under various temperatures and carbon source/NH₃ ratios. While the formation of the separation diaphragms were indeed a result of bulk diffusion of carbon atoms from bottom of the Ni nanoparticle following thermal dehydrogenization to the top of the Ni nanoparticle, there were other carbon atoms diffusing presumably via surface diffusion to the CNT-metal interface and contributed to the growth of tube wall; in other words, the CNTs growth is simultaneous renucleation and growth processes, instead of a continuous renucleation and growth process. This kinetics-based mechanism in combination with the proposed role of NH₃ could not only successfully explain the effects of the process parameters including temperature and the mixing gas ratio, but also could be used for pursuing the goal of lower growth temperature for thermal CVD method which is very important for many applications of CNTs.     

Numerical analysis of projection welding on auto-body sheet metal using a coupled finite element method

A comprehensive finite element method employing a subroutine to link up submodules of commercial code ANSYS is proposed to perform analysis of projection welding in quantitative detail. In order to solve the complexity due to dynamic changes in heat and electrical current flow paths, as well as temperature-dependent material properties, information about contact interfaces and the geometry of the projection areas have been taken into account. By updating parameter information among these submodules in an incremental manner, a truly thermal-electrical-mechanical coupled numerical analysis is realized for projection welding simulation. A case study of an automotive door assembly welding process is carried out and a series of experiments is conducted to confirm the validity of the newly developed method. The agreement between the experimental and numerical results is satisfactory, indicating that the incrementally coupled finite element method may be suitable for projection welding research. Finally, future work to extend this method in optimizing projection welding process design is also presented.