MECHANISM OF SOUND GENERATION BY FLOW OVER TWO CAVITIES IN TWO DIMENSIONS

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有限元法应用于微波和光波器件的问题和发展

本文就有限元法应用于微波和光波器件的原理、问题和最新进展作了综合评述。文中讨论了适用于不同问题的变分公式的意义；着重讨论了避免和消除虚模的方法和处理开放边界的技术；还对代表该领域发展趋势的三个方面，即边棱元法。非线性结构及有限元程序的前后处理问题进行了评论和展望。     

Microwave surface resistance of epitaxial YBa2Cu3O7 thin films at 18.7 GHz measured by a dielectric resonator technique

We used a dielectric resonator technique for highly sensitive measurements of the temperature dependence of the microwave surface resistanceR _s of 1×1 cm² superconducting films at 18.7 GHz. It consists of a sapphire disc positioned on the film under investigation within a copper cavity which is acting as a radiation shield. In the TE₀₁ oscillation mode the highly reproducible quality factor of about 10⁵ results in a sensitivity of ±50 forR _s measurements. The temperature dependence ofR _s can be measured up to values as high as 1 . We have investigated several YBa₂Cu₃O₇ thin films prepared by high oxygen pressure d.c. sputtering on LaAlO₃ and NdGaO₃. Our best films exhibit a pronounced nonlinear behavior of the d.c. resistivity(T) with(300K)/(100K) values of about 3.7. Those films show, besides the initial fall-off just belowT _c , a further strong decrease ofR _s at low temperatures. This was observed both at 18.7 GHz and 87 GHz, as measured by a conventional cavity end plate replacement technique. ForTT_c/2 these films exhibit an exp (–T _c/T) dependence ofR _s with-values around 0.4. These observations may be explained by a superconducting energy gap with 2/kT _c0.8 for charge carriers localized in the CuO chains for YBa₂Cu₃O₇.     

非对称级联型全光纤Fabry-Perot腔谱特性研究

为了进一步优化光纤光栅法布里-珀罗窄带滤波器的滤波性能,提出了基于光纤布拉格光栅的全光纤型非对称级联法布里-珀罗腔（FBG-FP）结构。通过传输矩阵方法,建立了数学仿真分析模型,推导出FBG-FP结构产生单峰谐振需满足的条件。对其传输特性进行了详尽的理论分析和模拟仿真,数值分析结果表明：通过对光纤布拉格光栅折射率调制深度以及谐振腔长等结构参量的优化组合,可以改善非对称双腔结构谐振光谱的通带平坦性和过渡带滚降特性;双腔FP器件可以得到更高的波长选择度,因此可用作信道化滤波与可调单频光纤激光器。     

基于CFD的摆线泵进出油腔对容积效率影响的分析

为提高摆线泵的容积效率,优化泵内部的流场分布,减少回流和空化现象.利用CFD方法对摆线泵内部流场进行数值模拟,定量分析月牙状进出油腔对泵容积效率的影响.仿真结果表明,适当减小G0并增大F0可以改善泵的内部流场分布,提高容积效率.仿真结果可以为摆线泵的结构设计提供参考.     

中间主应力对球形孔扩张的影响

基于Mohr—Coulomb理论推导的球形孔扩张问题的弹塑性解没有考虑中间主应力的影响。因而与实际结果有误差。为此，本文利用双剪强度理论建立了球形孔扩张问题时的弹塑性解。通过算例表明：基于双剪强度理论所求的球形孔扩张的塑性区半径小于M—C准则。据此可充分发挥材料自身的承载能力，减少支护，对实际工程具有重要的理论和实际意义。     

Stress singularity analysis in the restorations of premolar class ii cavities

Abstract

This paper presents the singular stress analysis near the apex of a structure formed during dental restoration of a premolar class II cavity. Based on the elasticity theory, the stresses may go to infinity at the junctions of different materials (e.g. dentine, enamel, restoration materials). Tensions will cause material separation and then material fracture. In order to reduce the failure probability, the degree of stress concentration has to be reduced. The stress singularity order and the stress intensity factor are two parameters, which are often used in fracture analysis. The objective of this paper is to find conditions such that non‐singular stress fields are possible. Three critical positions in the restoration structure are discussed.

They are the tips of interface between (1) enamel and restoration; (2) dentine and restoration; and (3) enamel, dentine and restoration. In the last two cases, the restoration may be bonded or debonded to enamel or dentine. After employing Kolosov‐Muskhelishvili complex functions together with the eigenfunction expansion method, the singularity orders are computed theoretically. Weak stress singularity conditions can be sought by properly selecting cutting angles or restoration materials.     

Changes in Coupled Vibration Frequencies and Modes of Wall-Cavity Systems Induced by Stiffness Variation in the Structure

Problems of fluid structure interactions are governed by a set of fundamental parameters. This work aims at showing through simple examples the changes in natural vibration frequencies and mode shapes for wall-cavity systems when the structural rigidity is modified. Numerical results are constructed using ANSYS software with triangular finite elements for both the fluid （2D acoustic elements） and the solid （plane stress） domains. These former results are compared to proposed analytical expressions, showing an alternative benchmark tool for the analyst. Very rigid wall structures imply in frequencies and mode shapes almost identical to those achieved for an acoustic cavity with Neumann boundary condition at the interface. In this case, the wall behaves as rigid and fluid-structure system mode shapes are similar to those achieved for the uncoupled reservoir case.     

监控量测在浅埋地下大断面平顶框架结构施工中的应用分析

城市浅埋大断面地下结构因其结构和建设的复杂性往往成为工程建设的重点和难点。施工中如何利用监控量测技术实时监控围岩、支护结构、中间构造结构等之间的受力转换往往成为施工关键。结合北京城市快速轨道交通东直门车站地下方厅工程实例,探讨和分析了应用浅埋平顶群洞暗挖法施工过程中监控量测技术和方法,并对监测结果进行了比较分析,验证了施工方法的合理有效性。     

微波脉冲与带缝嵌套腔体耦合的研究

为了解带缝腔体内耦合场分布特性,利用时域有限差分方法对微波脉冲与带缝嵌套矩形腔体系统(系统1)和带缝嵌套圆柱形腔体系统(系统2)的耦合过程进行了数值计算。计算表明两系统的外、内孔缝附近均存在场增强现象;两系统耦合过程存在相同的外孔缝耦合主脉冲、外孔缝场增强因子和脉宽展开现象,其内部腔体(Q2)中心点耦合场强峰值接近相等。相比各自的外部腔体(Q1)以及相同入射脉冲下的孔缝、同体积单层腔体内的耦合场强,两嵌套腔体的Q2内耦合场要弱。对两个系统的共振峰、共振频率点以及最大耦合函数曲线的研究结果表明,系统2在耦合过程中的腔体调制现象更明显,且在两种系统中,内孔缝使Q2内耦合波基于Q1内耦合波平均衰减约10 dB。