On the validity of planar, thick curved beam models derived with respect to centroidal and neutral axes

Most dynamic analyses of planar curved beams found in the literature are carried out based on a curved beam model which assumes that the neutral axis coincides with the centroidal axis of the curved beam. This assumption leads to governing equations of motion which are relatively simple with analysis results that have acceptable accuracy for shallow curved beams. However, when a curved beam is not shallow and/or its cross section is not doubly symmetric, the offset distance between the neutral and centroidal axes may be large enough to influence the in-plane dynamics of the curved beam even for small motion. In this paper, the validity of this underlying assumption for modeling a linear curved beam is examined. To this end, two sets of equations of motion governing the in-plane dynamics of a planar curved beam are derived, in a consistent manner for comparison, based on the linear strain-displacement relations and Hamilton’s principle. The first set of equations is derived from the displacement components measured with reference to the neutral axis of the curved beam while the second set is derived with respect to the centroidal axis of the cross section. The curved beam is considered extensional and the effects of rotary inertia and radial shear deformation are included. In addition to the curvature parameter that characterizes the wave motion for both curved beam models, an eccentricity parameter is introduced in the first model to account for the offset between the neutral and centroidal axes. The dynamic behavior predicted by each curved beam model is compared in terms of the dispersion relations, frequency spectra, cutoff frequencies, natural frequencies and modeshapes, and frequency responses. In order to ensure that the comparison is accurate, the wave propagation technique is applied to obtain exact wave solutions. It is shown that, when the curvature parameter is not small, the underlying assumption has a substantial impact on the accuracy of the linear dynamic analysis of a curved beam.     

频率约束板结构拓扑优化

本文利用ICM（独立、连续、映射）方法建立了频率约束下平板结构重量最轻的拓扑优化模型。采用指数函数作为单元重量、单元质量及单元刚度的过滤函数。通过瑞利商对刚度过滤函数倒变量的泰勒一阶展式,将频率约束近似显式化。利用对偶理论将含有大量设计变量的约束优化模型转化为易于求解的少设计变量拟无约束优化模型,通过序列二次规划将转化模型进行求解,提高了求解的效率。本文选择MSC.Patran&Nastran软件及PCL二次开发语言构架了平板结构频率约束拓扑优化问题的软件。数值算例表明：本文的方法具有迭代稳定性和收敛高效性。     

深水环境下隔水管的横向自由振动分析

隔水管是海洋钻井工程中的必要设备。在深水环境下,隔水管的疲劳失效问题普遍存在,隔水管的强烈振动是导致其失效的主要原因之一。本文采用微元法,得到了隔水管在各种载荷作用下的横向振动规律及其横向振动的固有频率的分布规律,进一步分析了各种因素对隔水管横向振动固有频率的影响。结果表明:隔水管的长度增加时,固有频率减小;随着顶部张力比的增加,隔水管的横向振动的固有频率也增加;随着壁厚的增加,隔水管横向振动的固有频率逐渐降低;而随着钻井液密度的增加,隔水管横向振动的固有频率增加;海水的存在使隔水管的横向振动固有频率降低。该研究有利于避免隔水管在使用过程中出现强烈的共振现象,为减小隔水管的失效提供了理论依据。     

一种谐振式微加速度计表芯非线性动力学分析及实验

在研制基于静电刚度谐振式微加速度计过程中,发现增大激励电压可以提高输出信噪比,但响应的振动幅度将不稳定,同时谐振频率也将会出现漂移。针对上述问题,建立了静电驱动微机械谐振系统等效行为模型,非线性动力学理论分析结果与实验现象一致,总结出需要从加速度计结构参数优化和减小激励电压两个方面来减小频率漂移和提高分辨率。将结构优化准则应用到制造的微加速度计上,实验结果表明:在5 V敏感电压下,闭环条件下单梁加速度计灵敏度为58 Hz/g,分辨率为3.5 mg。     

The effect of frequency and sample shape on fatigue behaviors of DZ125 superalloy

The effect of frequency and sample shape on fatigue behaviors of DZ125 superalloy are systematically studied. The results show that fatigue fracture still occurs above the cycle of 10⁸ for tests carried out at the frequency of f = 20 kHz and stress ratio R = ?1, so the traditional fatigue limit at cycle of 10⁷ is not appropriate for fatigue design. Fatigue fracture surfaces are perpendicular to stress axis for cylindrical and plate specimens, and the fatigue cracks originate from the extra surface of the specimens. Fatigue crack is apt to propagate from cutting direction to forward direction, which occurs mainly in the second propagation stage at higher stress amplitude. There is an obvious frequency effect for DZ125 superalloy. The higher the test frequency is, the more serious the effect of frequency on fatigue behaviors of the alloy. After the frequency correction, the ultra-high cycle fatigue S-N curve well coincide with the traditional fatigue S-N curve.     

远程多光束激光相干场成像拍频误差校正研究

多光束激光相干成像技术是地基观测空间目标的重要方式,各光束的稳定性和性能一致性直接决定着系统成像质量,目前针对系统频率稳定性对成像质量的影响以及激光源频漂的补偿已有一定研究,然而针对多光束发射系统,由驱动放大噪声及声光移频噪声引起的各光束间独立的频漂还需进一步抑制.基于此,本文提出了动态解调和置信区间解调两种抑制方法,理论仿真了动态解调对缓慢频漂抑制的可行性,同时实验证明了置信区间解调法对成像效果的提升,并在200 m和1.2 km的湍流环境中对该解调方法进行了验证.研究表明,置信区间解调法对于各拍频间独立漂移有较好的实时补偿效果,能够有效抑制发射阵列中由声光调制及驱动放大引入的频率噪声,对水平距离1.2 km外的25 mm目标成像角分辨率达到4μrad.本研究为未来远程大功率发射阵列成像中的频率噪声抑制提供了较好的技术方案.     

Parameters of LC molecules’ movement measured by dielectric spectroscopy in wide temperature range

Dielectric properties of a nematic liquid crystal (NLC) mixture ZhK-1282 were investigated in the frequency range of 10²–10⁶Hz and a temperature range of ?20 to 80?°С. On the basis of the Debye’s relaxation polarization model dielectric spectra of temperature dependence of the orientational relaxation time τ and the dielectric strength δe were numerically approximated at the parallel orientation of a molecular director relative to alternating electric field. Influence of ester components in the mixture plays crucial role in relaxation processes at low temperature and external field frequency. The activation energy of the relaxation process of a rotation of molecules around their short axis was measured in a temperature interval of ?20 to ?+15?°С in which the dispersion of a longitudinal component of the dielectric constant takes place. The energy of potential barrier for polar molecules rotation in the mesophase was calculated. The value of the transition entropy from the nematic to isotropic phase was obtained from this calculation. The values of the coefficient of molecular friction and rotational diffusion were obtained by different methods. The experimental data obtained are in a satisfactory agreement with the existing theoretical models.     

Numerical investigation of the effect of dual frequency sonication on stable bubble dynamics

A computational study aiming to simulate an oxygen single acoustic bubble oscillation under a dual-frequency sonication was presented in this paper. The non-linear response of the bubble to the superposition of two fields of ultrasonic waves was investigated through dynamics parameters, collapse ratios and average velocities. The main goal of this analyze is to link the properties of the wave resulting from the dual-frequency excitation to the dynamics behavior of the bubble. The obtained results prove that, in contrast with the mono-frequency, coupling a wave to lower frequencies enhances the collapse duration and raises the compression ratio in the case of 35 kHz, while associating any of the studied waves to a higher frequency elevates the number of bubble oscillations during a time interval as compared to mono-frequency. The total sonochemical production has been investigated in accordance with the dynamics results, as well as the proportions of the three predominant free radicals, that show a dependency on the value of the basic frequency.     

Clock frequency estimation under spontaneous emission

We investigate the quantum dynamics of a driven two-level system under spontaneous emission and its application in clock frequency estimation. By using the Lindblad equation to describe the system, we analytically obtain its exact solutions, which show three different regimes: Rabi oscillation, damped oscillation, and overdamped decay. From the analytical solutions, we explore how the spontaneous emission affects the clock frequency estimation. We find that under a moderate spontaneous emission rate, the transition frequency can still be inferred from the Rabi oscillation. Our results enable potential practical applications in frequency measurement and quantum control under decoherence.     

高重频CO_2激光干扰技术研究

本文针对高重频CO_2激光干扰技术展开分析和研究。首先,概述了激光对抗武器的概念、分类和特点,以及激光对抗武器的干扰与破坏的主要目标对象。其次,分析了红外探测器激光干扰损伤效果的检测方法。再次,分析了光学元件激光变形损伤效果的检测方法以及光学薄膜的激光损伤检测方法。按照上述检测方法进行实验,分别对红外单元探测器、光学元件、光学薄膜和其他材料的激光损伤阈值实验数据进行分析与讨论。最后,总结了全文并且展望了激光干扰技术的发展。