Kohn anomaly in graphene

Symmetry based analysis of the Kohn anomaly is performed. Kohn phonon frequencies and displacements are calculated by force constant method. It is shown that Kohn phonon vibrations cause electronic band gap opening.     

A nontrivial factor in determining current distribution in an ac HTS cable-proximity effect

A superconductor has zero resistance at the superconducting state. This unique property creates many exceptional phenomena, of which some are known and the others are not. Our experiments with multilayer high temperature superconductor (HTS) cable samples revealed a new phenomenon that alternating current had a tendency to flow in the inner and outer layers of the cables. We attribute the cause of this phenomenon to the electromagnetic interaction in an infinite electrical conductivity medium and term it su...     

A nonlinear analytical model for the squeeze-film dynamics of parallel plates subjected to axial flow

In this paper, we develop a theoretical model to predict the nonlinear fluid-structure interaction forces and the dynamics of parallel vibrating plates subjected to an axial gap flow. The gap is assumed small, when compared to the plate dimensions, the plate width being much larger than the length, so that the simplifying assumptions of 1D bulk-flow models are adequate. We thus develop a simplified theoretical squeeze-film formulation, which includes both the distributed and singular dissipative flow terms. This model is suitable for performing effective time-domain numerical simulations of vibrating systems which are coupled by the nonlinear unsteady flow forces, for instance the vibro-impact dynamics of plates with fluid gap interfaces. A linearized version of the flow model is also presented and discussed, which is appropriate for studying the complex modes and linear stability of flow/structure coupled systems as a function of the average axial gap velocity. Two applications of our formulation are presented: (1) first we study how an axial flow modifies the rigid-body motion of immersed plates falling under gravity; (2) then we compute the dynamical behavior of an immersed oscillating plate as a function of the axial gap flow velocity. Linear stability plots of oscillating plates are shown, as a function of the average fluid gap and of the axial flow velocity, for various scenarios of the loss terms. These results highlight the conditions leading to either the divergence or flutter instabilities. Numerical simulations of the nonlinear flow/structure dynamical responses are also presented, for both stable and unstable regimes. This work is of interest to a large body of real-life problems, for instance the dynamics of nuclear spent fuel racks immersed in a pool when subjected to seismic excitations, or the self-excited vibro-impact motions of valve-like components under axial flows.     

风暴潮过程中波浪对地形演变影响的数值研究

建立了一个波流耦合作用下泥沙输移的平面二维数值模型，以研究风暴潮过程中地形冲淤变化的规律及波浪的影响。水流和泥沙输运的模拟采用水沙耦合数值模型，以考虑强冲淤条件下地形变化与水流运动之间的相互作用。波浪场模拟采用SWAN模型，计算得到的波浪辐射应力输入水沙耦合数值模型实现波流耦合。采用建立的模型对风暴潮过程中近岸地形变化进行了分析，结果表明，波浪对地形演变的影响程度随着风暴潮潮位的增加而减小。当风暴潮潮位较小时，近潮汐通道处形成不对称的三角洲、入口处形成冲刷坑；当风暴潮潮位较高时，形成的三角洲不对称性较小。当风暴潮潮位升高，近岸水深与波长之比大于0.15时，波流耦合模型与不考虑波浪作用模型计算所得的地形变化的绝对误差为整体地形变化的5%，但前者计算时间增加了一倍，因此，波流耦合计算对于风暴潮潮位较低，近岸水深与波长之比小于0.15时具有重要意义。     

考虑流固耦合的双弯头压裂管汇的振动特性

高压压裂过程中,压裂车与交流管汇相连接的弯头极易因管汇的剧烈振动发生疲劳损坏,降低了弯头的使用寿命。为此,基于流固耦合理论,推导了高压弯管的运动方程,对弯头在波动流作用下的流体力学模型和固体运动模型引入摩擦耦合因素,并运用ANSYS Workbench软件进行仿真计算,分析了双弯头的连接角度、弯管的壁厚、曲率半径等参数对弯管固有频率的影响,通过现场实测数据对仿真计算结果进行了验证。结果表明:(1)双弯头连接角度对固有频率影响较大;(2)双弯头固有频率随着弯头内径的增大近似线性增大,随着弯头曲率半径的增大而减小。结论认为,(1)双弯管连接角度介于75°~105°更有利于减小管汇的振动强度,弯管的振动强度最大可减小约30%;(2)在流固耦合的作用下,双弯头弯管的固有频率随弯管内径的增长呈近似线性增长的关系,压裂现场压力为60 MPa左右时应选取内径为?101.4 mm(4 in)的压裂管线;(3)在压裂现场激振频率不高的情况下,弯管内径为70 mm时,弯管的曲率半径应该尽量控制在160 mm左右。     

Interference management for spectral coexistence in a heterogeneous satellite network

With the advent of the fifth generation of mobile radio communication by 2020, there will be many challenges such as increasing service demand with low delay in providing billions of end users called the satellite mobile users. It is expected that terrestrial communication systems will be faced with a dense network having many small cells anywhere and anytime. Therefore, there are some remote regions in the world where terrestrial systems cannot provide any services to end users. Furthermore, because of lack of spectral resources, it is very important that the spectrum is shared between satellite systems and terrestrial equipment by a suitable solution to interference management. In this paper, a heterogeneous satellite network that includes low earth orbit (LEO) satellite constellation and terrestrial equipment is proposed to provide low delay services. In this type of structure, interference management based on transmission power control between LEO satellite systems and mobile users is very important for obtaining high throughput. Moreover, in order to mitigate interference, transmission power control is shown based on noncooperative Stackelberg game under many subgames through pricing‐based algorithm and convex optimization method. Finally, the simulation results show that the performance of this study's system model will be improved through the proposed algorithm.     

Shared human–robot path following control of an unmanned ground vehicle

This paper considers the shared path following control of an unmanned ground vehicle by a single person. A passive measure of human intent is used to blend the human and machine inputs in a mixed initiative approach. The blending law is combined with saturated super-twisting sliding mode speed and heading controllers, so that exogenous disturbances can be counteracted via equivalent control. It is proven that when the proposed blending law is used, the combined control signals from both the human and automatic controller respect the actuator magnitude constraints of the machine. To demonstrate the approach, shared control experiments are performed using an unmanned ground vehicle, which follows a lawn mower pattern shaped path.     

基于强化学习的合作频谱分配算法

为了解决认知无线电网络中的频谱分配问题,提出了一种基于用户体验质量的合作强化学习频谱分配算法,将认知网络中的次用户模拟为强化学习中的智能体,并在次用户间引入合作机制,新加入用户可以吸收借鉴其他用户的强化学习经验,能够以更快的速度获得最佳的频谱分配方案;并且在频谱分配过程中引入了主用户和次用户之间的价格博弈因素,允许主用...     

智能轮椅中人机界面的分类及评估

本文针对目前智能轮椅中已出现的诸如语音交互、头部运动等种类繁多的人机界面进行了归纳整理，从人的意识行为和能动性角度提出了一种新的分类方法，进而对各类人机界面的适用人群、硬件构成、成本估算以及产业化前景等方面进行了分析。从而有利于人机界面的行业标准的制定，也使得开发人员能够迅速定位自己开发产品所需的人机界面。     

三维反常声电光互作用最佳工作模式的确定

通过对三方、四方、六方和立方晶系的纵向电光效应和三维反常声光效应的研究,选定了立方晶系的GaP晶体作为三维声电光器件的最佳晶体,并确定了其最佳工作模式为:电场和光传播方向沿[111]方向;3个超声纯切变波的传播和偏振方向都在与[111]垂直的平面内,互成60°角。研究结果为三维声电光器件的设计与制作打下了良好基础。