A hybrid discontinuous in space and time Galerkin method for wave propagation problems

Medium‐frequency regime and multi‐scale wave propagation problems have been a subject of active research in computational acoustics recently. New techniques have attempted to overcome the limitations of existing discretization methods that tend to suffer from dispersion. One such technique, the discontinuous enrichment method, incorporates features of the governing partial differential equation in the approximation, in particular, the solutions of the homogeneous form of the equation. Here, based on this concept and by extension of a conventional space–time finite element method, a hybrid discontinuous Galerkin method (DGM) for the numerical solution of transient problems governed by the wave equation in two and three spatial dimensions is described. The discontinuous formulation in both space and time enables the use of solutions to the homogeneous wave equation in the approximation. In this contribution, within each finite element, the solutions in the form of polynomial waves are employed. The continuity of these polynomial waves is weakly enforced through suitably chosen Lagrange multipliers. Results for two‐dimensional and three‐dimensional problems, in both low‐frequency and medium‐frequency regimes, show that the proposed DGM outperforms the conventional space–time finite element method. Copyright © 2014 John Wiley & Sons, Ltd.     

局部约束结构振动的模态研究

制动盘/摩擦块系统的制动过程属于局部接触振动问题。摩擦块局部接触(约束)会对系统模态及固有频率造成影响,进而影响制动噪声的产生。将刹车盘简化为一维循环梁结构,并建立了在摩擦块作用下的运动方程。首先计算无接触时梁自由振动的模态(参考模态)。然后用线性弹簧代替局部接触,列写出连续条件并计算模态,得到所谓局部非连续基函数。将局部非连续基函数与参考模态进行正交化处理后,作为参考模态的补充,用于计算系统响应。与差分法结果比较表明,与传统模态方法相比,局部非连续基函数法更为准确。研究发现局部接触会抑制循环结构振动的对称性,导致正弦或余弦模态消失,以及刚度非线性和摩擦作用,会使振动是波动型的。该工作为基于局部非连续基函数法研究摩擦结构不稳定振动机理打下了基础。     

DDA中裂隙岩体渗流网络识别方法的改进及应用

裂隙网络是岩体渗流的主要通道,对其进行准确高效的识别是实现基于DDA的渗流应力耦合方法的重要步骤。通过建立环块体裂隙排列数组,以裂隙为搜索路径,提出更简单、效率更高的节点识别方法;同时提出外边界、边界节点及外边界角点的识别方法,构建出包括以上要素的完整裂隙网络,使外水压力荷载能有效施加在块体系统上,扩展DDA裂隙网络识别方法的适用性。将此方法应用于基于DDA的渗流应力耦合模型,并结合算例进行考虑渗流应力耦合的数值计算。结果表明,该方法可准确识别出复杂裂隙网络,可结合基于DDA的渗流应力耦合程序完成模拟计算。研究成果为裂隙岩体在渗流状态下的失稳判断提供了依据。     

谐振式微光学陀螺中非连续频率调制特性研究

谐振式微光学陀螺（RMOG）中,方波频率调制的三角波实现形式消除了双频率锯齿波中复位时间与复位误差引起的输出光强波动。理论分析了三角波实现方波频率调制的过程中波形转折点引起非连续频率调制对输出光强的影响。仿真分析了不同调制频率偏置下调制波形转折点以及不同激光频率与谐振频率偏差对输出的影响。利用时域梳状滤波器对输出信号进...     

An enhanced energy conserving time stepping algorithm for frictionless particle contacts

An Enhanced Energy Conserving Algorithm (EECA) formulation for time integration of frictionless contact–impact problems is presented. In it the energy, linear and angular momentum are conserved for every contact using an enhanced Penalty method. Previous formulations for these problems have shown that the total bodies' energy decreases for contact due to an artificial energy transfer between the penalty spring and the contacting bodies. Consequently, they are not able to reproduce a physical response after a single contact, introducing errors in trajectories and velocities. Through the conserving balance equations, EECA computes a physical response by inserting for every contact an additional amount of linear momentum and contact force. The structure of these equations defines the additional linear momentum to restore the energy and the enhanced Penalty method based on a spring and a dashpot. This method approximately enforces the first and second Kuhn–Tucker conditions. The new algorithm has been applied to several frictionless rigid problems using the Discrete Element Method. The first two problems consist of the simulation and analytical comparison of the Newton's Cradle and Carom problems (billiard pool problem). The last two are the hopper filling process and the breaking of a pool ball's triangular arrangement, both of which involve a medium number of contacts. Application of this formulation will be straightforward to elastic and general‐shaped bodies using the Finite Element Method. Copyright © 2010 John Wiley & Sons, Ltd.     

地铁盾构隧道管片受力分析

盾构管片接头刚度系数一直是影响盾构管片设计计算结果正确性的直接影响因素。通过建立盾构管片及弯曲螺栓的三维非连续介质模型,以及设置合理的管片接头接触模型,消除了接头刚度系数对计算结果的影响。对盾构管片在实际荷载作用下的受力与变形情况进行研究。研究表明:管片拱顶与拱底接头向内侧张开,两侧拱腰接头向外侧张开。非连续介质模型中,盾构管片的分块拼装形式以及管片间连接螺栓单元的设置改变了管片环的整体刚度,管片接头附近出现了不均匀的波浪形云图。相对于惯用法模型,三维非连续介质模型的轴力值在接头处有明显突变,并且其最大正弯矩值小于惯用法模型计算结果。两种计算方法得出的最大负弯矩值相差不大,本次研究取得的成果对盾构管片设计工作有一定的参考价值。     

双排刚性桩屏障对平面SH波的隔离性状研究

基于Twersky的理论,引入圆柱体对声波和电磁波的多重散射解析解,对用排桩隔离环境振动的问题,提出了一种新的求解任意排列、任意直径刚性桩对平面SH波多重散射的理论方法,解决了传统的假设单重散射的计算方法中不考虑桩列作为整体屏障从而忽略桩间相互干涉关系的不足.随后,在数值计算分析中讨论了散射重数,排间距,桩间距等因素对双排刚性桩隔振效果的影响,对实际工程中利用排桩进行振动污染治理和屏障隔振的设计提出了有意义的结论.     

变截面压电悬臂梁系统的建模与实验验证

为对变截面压电悬臂梁系统的性能进行预测、分析和优化,该文对部分覆盖压电材料的悬臂梁数学进行了建模和分析。首先根据系统的动能、悬臂梁和压电片的热力学焓等能量关系的描述,应用拉格朗日方程得到系统的机电运动方程;再采用假设模态法并依据压电悬臂梁的边界条件和相容条件,给出系统的模态振型和自然频率的计算方法。实验结果表明,测量得到的一、二阶模态频率与理论模型计算的误差小于1%,测得的频率响应和理论预测吻合,提高了压电悬臂梁模型预测的精度。     

基于图像边缘特征的深度上采样算法

针对低分辨率深度图像上采样容易导致边缘模糊问 题,提出了一种基 于图像边缘特征的深度上采样算法。一方面,利用相同低分辨率深度和彩色图像的相关性系 数,自适应调 节深度图像边缘上采样过程中深度和彩色的权重;另一方面,结合上采样值和低分辨率深度 图像中邻近像 素值,对低分辨率深度图像的不连续区域进行求精操作以进一步减少边缘模糊现象。实验结 果表明,本文算法 的性能优于近年文献中提出的算法。本算法上采样深度图像的平均坏点率(B PR)为2.07%,均方根误差(RMSE )为3.46,峰值信噪比(PSNR)为38.58dB,绘制 虚拟视点的平均PSNR为39.58dB。     

高效率低电磁干扰的升压型DC–DC转换器

A synchronous boost DC-DC converter with an adaptive dead time control （DTC） circuit and antiringing circuit is presented. The DTC circuit is used to provide adjustable dead time and zero inductor current detection for power transistors and therefore, a high efficiency is achieved by minimizing power losses, such as the shoot-through current loss, the body diode conduction loss, the charge-sharing loss and the reverse inductor current loss. Simultaneously, a novel anti-ringing circuit controlled by the switching sequence of power transistors is developed to suppress the ringing when the converter enters the discontinuous conduction mode （DCM） for low electromagnetic interference （EMI） and additional power savings. The proposed converter has been fabricated in a 0.6 #m CDMOS technology. Simulation and experimental results show that the power efficiency of the boost converter is above 81% under different load currents from 10 to 250 mA and a peak efficiency of 90% is achieved at about 100 mA. Moreover, the ringing is easily suppressed by the anti-ringing circuit and therefore the EMI noise is attenuated.