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51.
This paper is concerned with modeling composite beams with spanwise heterogeneity. We first formulate the original three-dimensional problem in an intrinsic form which admits a geometrically exact formulation. Taking advantage of slenderness of beam structure and smallness of heterogeneity, we use the variational asymptotic method to systematically obtain an effective beam model through simultaneous homogenization and dimensional reduction. This approach is implemented in the commercial code VABS using the finite element technique for the purpose of dealing with composite beams with spanwise heterogeneity in real applications. A few examples are used to demonstrate the capability of this new model. 相似文献
52.
Chang-Yong Choi 《Journal of Mechanical Science and Technology》2006,20(12):2230-2241
This paper presents a sole application of boundary element method to the conjugate heat transfer problem of thermally developing
laminar flow in a thick walled pipe when the fluid velocities are fully developed. Due to the coupled mechanism of heat conduction
in the solid region and heat convection in the fluid region, two separate solutions in the solid and fluid regions are sought
to match the solid-fluid interface continuity condition. In this method, the dual reciprocity boundary element method (DRBEM)
with the axial direction marching scheme is used to solve the heat convection problem and the conventional boundary element
method (BEM) of axisymmetric model is applied to solve the heat conduction problem. An iterative and numerically stable BEM
solution algorithm is presented, which uses the coupled interface conditions explicitly instead of uncoupled conditions. Both
the local convective heat transfer coefficient at solid-fluid interface and the local mean fluid temperature are initially
guessed and updated as the unknown interface thermal conditions in the iterative solution procedure. Two examples imposing
uniform temperature and heat flux boundary conditions are tested in thermally developing region and compared with analytic
solutions where available. The benchmark test results are shown to be in good agreement with the analytic solutions for both
examples with different boundary conditions. 相似文献
53.
Zi-Wei Zheng Zhi-Xing Yang Chang-Yong Pan Yi-Sheng Zhu 《Broadcasting, IEEE Transactions on》2005,51(2):237-243
As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency-division multiplexing (OFDM) is extensively used in modern digital television terrestrial broadcasting (DTTB) systems, such as the cyclic prefix OFDM (CP-OFDM) based DVB-T DTTB systems and the time domain synchronous OFDM (TDS-OFDM) based DMB-T DTTB systems, to support high performance bandwidth-efficient multimedia services. OFDM-based DTTB systems are very sensitive to the nonlinear distortion at the transmitter and the imperfect parameters estimation at the receiver. In this paper, the combined effects of nonlinear distortion and imperfect parameters estimation on the performance of the OFDM-based DTTB systems are investigated, a closed form of symbol error probability of the OFDM-based DTTB systems with nonlinear distortion and imperfect parameters estimation is derived. Theoretical results show closely matching with those obtained by simulations for both the CP-OFDM based DVB-T DTTB systems and the TDS-OFDM based DMB-T DTTB systems. 相似文献
54.
Jung-Goo Cho Ju-Won Baek Chang-Yong Jeong Geun-Hie Rim 《Industry Applications, IEEE Transactions on》1999,35(1):15-20
A novel zero-voltage and zero-current-switching (ZVZCS) full-bridge pulsewidth modulation converter is presented to simplify the circuits of the previously presented ZVSCS converters. A simple auxiliary circuit, which consists of one small capacitor and two small diodes, is added in the secondary to provide ZVZCS conditions to primary switches, as well as to clamp secondary rectifier voltage. The additional clamp circuit for the secondary rectifier is not necessary. The auxiliary circuit includes neither lossy components nor additional active switches, which makes the proposed converter efficient and cost effective. The principle of operation, features, and design considerations are illustrated and verified on a 2.5 kW 100 kHz insulated-gate-bipolar-transistor-based experimental circuit 相似文献
55.
JEONG-HYUN PARK KYUNG-HEE KIM JEONG-MAN CHO SUNG-KI LIM 《Journal of Materials Science》1998,33(23):5695-5702
A new forming method, pressureless powder packing (PLPP), was studied to fabricate the - and -Al2O3 tubes. Alkali sources were infiltrated into the pores of -Al2O3 tube preforms that had been prepared by the PLPP forming method. The composition for the synthesis of -Al2O3 phase was Na2O · 0. 138Li2 · 4.4Al2O3. The -Al2O3 fraction of calcined and sintered bodies was increased with the increase of calcination temperature, and phase formation was largely affected by the type of starting -Al2O3. Large particle size and narrow size distribution of fused -Al2O3 resulted in uniform green microstructure that enhanced the homogeneity of alkali salts after infiltration, which was very important for the -Al2O3 formation. Sintered microstructure was uniform in all specimens but further development was required for density improvement. 相似文献
56.
SUNG-TAE LEE DAE-HAN LEE SANG-MIN LEE SANG-SOO HAN SANG-HYUNG LEE SUNG-KI LIM 《Bulletin of Materials Science》2016,39(3):729-735
Ca-doped Na+- β/β′′-alumina was synthesized using a solid-state reaction. The changes in the properties of Na+- β/β′′-alumina resulting from the presence of Ca impurity were studied. Ca (0–5 wt%) was added to the respective samples, which were then sintered. The specimens were characterized using X-ray diffraction, scanning electron microscopy, densimetry and impedance analysis. In the sintered specimens, the β′′-alumina phase fraction decreased as Ca content increased, whereas the relative sintered density increased. The surface morphology of Ca-doped Na+- β/β′′-alumina specimens showed a Ca-rich layer, which was the main cause of increase in the specific resistance. 相似文献
57.
C.‐S. LIM F. J. PARRA‐VELANDIA N. CHEN P. ZHANG S. L.‐M. TEO 《Journal of microscopy》2014,255(3):150-157
The advent of scanning electron microscopy has facilitated our understanding of the biology in relation to surface microstructure of many invertebrates. In recent years, interest in biomimetics and bio‐inspired materials has further propelled the search for novel microstructures from natural surfaces. As this search widens in diversity to nurture deeper understanding of form and function, the need often arises to examine rare specimens. Unfortunately, most methods for characterization of the microtopography of natural surfaces are sacrificial, and as such, place limiting constraints on research progress in situations where only a few rare specimens are known, such as the rich resources lodged in natural history museum collections. In this paper, we introduce the use of optical coherence tomography (OCT) as a noninvasive tool for bioimaging surface microtopography of crab shells. The technique enables the capture of microstructures down to micron level using low coherence near‐infrared light source. OCT has allowed surface microtopography imaging on crab shells to be carried out rapidly and in a nondestructive manner, compared to the scanning electron microscope technique. The microtopography of four preserved crab specimens from Acanthodromia margarita, Ranina ranina, Conchoecetes intermedius and Dromia dormia imaged using OCT were similar to images obtained from scanning electron microscope, showing that OCT imaging retains the overall morphological form during the scanning process. By comparing the physical lengths of the spinal structures from images obtained from OCT and scanning electron microscope, the results showed that dimensional integrity of the images captured from OCT was also maintained. 相似文献
58.
并行多任务分配是多agent系统中极具挑战性的课题, 主要面向资源分配、灾害应急管理等应用需求, 研究如何把一组待求解任务分配给相应的agent联盟去执行. 本文提出了一种基于自组织、自学习agent的分布式并行多任务分配算法, 该算法引入P学习设计了单agent寻找任务的学习模型, 并给出了agent之间通信和协商策略. 对比实验说明该算法不仅能快速寻找到每个任务的求解联盟, 而且能明确给出联盟中各agent成员的实际资源承担量, 从而可以为实际的控制和决策任务提供有价值的参考依据. 相似文献
59.
The paper describes the structure of an interactive manual data input programming system which was developed for drilling and which uses a ‘shop floor oriented’ language. This programming system incorporates ‘cutting technology’, to provide an unskilled machine operator with computer assistance in achieving optimum drilling conditions for a wide range of engineering materials, and has been designed to be used in conjunction with a microprocessor-based NC drilling system, the structure of which is also briefly described. 相似文献
60.
Diameter-dependent electromechanical properties of GaN nanowires 总被引:1,自引:0,他引:1
The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires. 相似文献