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。     

扇形凹穴型微通道液体流动与传热特性的数值模拟

为得到扇形凹穴型微通道内单相液体流动与传热特性,以等截面矩形微通道为参照,利用FLUENT计算流体力学模拟与分析软件进行三维数值模拟,采用有限体积法离散模型和SIMPLEX算法进行层流计算,讨论扇形凹穴型微通道热沉在不同体积流量不同热流密度条件下流体流动特性和传热特性.模拟结果表明：较大Re条件下扇形凹穴型微通道具有很...     

山区桥墩抗滚石冲击的双腔椭圆偏心包裹结构研究

山区桥梁墩柱因刚性大而抗冲击效果差,常被滚石冲击损毁,但目前还缺乏有效防治措施。为此,在科学解析滚石对桥墩的弹塑性冲击破坏机理的前提下,基于耗能减震理论提出了一种充填泡沫铝的双腔椭圆偏心桥墩包裹抗冲击结构。继而综合MTS试验数据,采用动力有限元技术对滚石冲击下桥墩防护新结构的动力响应进行计算和分析,并优化结构设计。结果表明：定向椭圆偏心包裹泡沫铝层可显著延长冲击历时,降低冲击荷载,其结构形式经济高效;包裹层厚度适当增加有利于控制墩柱对冲击的动力响应,但过厚也会因结构整体性增大而弱化甚至降低防护效果;双腔充填的\     

SH波对浅埋相邻多个圆孔作用的动力分析

建立了求解浅埋相邻多个圆形孔洞对稳态SH波的散射及浅埋结构动应力分析的解析方法。利用SH波散射的对称性、复变函数与多极坐标法，构造了一个可以满足水平地面上应力自由边界条件的浅埋相邻多圆孔对SH波散射的波函数；利用这一波函数，则可将这一个半空间的问题转化成对一个全空间中的相邻多圆孔的散射问题的求解。该问题的解答，最终又可归结为对一组无穷代数方程组的求解问题，并可利用截断有限项的方法对其进行计算，作为例题对抗爆问题进行分析。最后给出了当稳态SH波垂直于水平面入射时，有关浅埋相邻2个圆孔附近的动应力集中问题的算例和数值结果，并讨论了波数与浅埋2个圆孔中心至自由表面距离和2个圆孔中心距离变化对动应力集中的影响。     

圆截面坯料拔长时空腔形成机理

本文分析了圆截面坯料在平砧下拔长时的变形特点及空腔形成机理,提出了为保证锻件质量和提高拔长效率应采取的措施.     

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.