考虑范德华力曲率效应的双壁碳纳米管外压屈曲

针对双壁碳纳米管外压屈曲问题,研究了层间范德华力的曲率效应对临界外压的影响。应用弹性双层圆柱壳模型,考虑层间范德华力不仅与层间距有关而且与挠度曲率的变化有关,导出了外压屈曲临界压力解析公式。计算得出在不同半径、不同长细比下,外压屈曲临界压力的数值结果,并与经典壳的结果和忽略范德华力曲率效应的结果做了比较。结果显示,对于小半径的双壁碳纳米管曲率效应对外压屈曲有效明显的影响。     

Rayleigh–Taylor Instability of Rotating Oldroydian Viscoelastic Fluids in Porous Medium in Presence of a Variable Magnetic Field

The instability of the plane interface between two oldroydian viscoelastic superposed fluids in the presence of uniform rotation and variable magnetic field in porous medium is considered. For potentially stable configuration, the system is found to be stable for disturbances of all wave numbers. The magnetic field succeeds in stabilizing certain wave-number range, which were unstable in the absence of magnetic field and rotation for the potentially unstable configuration. Sub cases of magnetic free and rotation free configurations are also considered, separately.     

基于粗粒化方法的类超级碳纳米管自由振动研究

超级碳纳米管是在碳纳米管结构基础上,将每一根碳-碳键替换为碳纳米管而形成的新型结构。类超级碳纳米管是超级碳纳米管对应的宏观结构,在保持外观结构的基础上将尺度放大到宏观尺度。本文建立了类超级碳纳米管的粗粒化模型。基于粗粒化方法,研究了类超级碳纳米管的自由振动。分析了内外管半径以及长度对类超级碳纳米管振动行为的影响。与原结构有限元进行对比,结果表明粗粒化方法能有效的计算类超级碳纳米管的振动行为。     

基于粗粒化方法的类超级碳纳米管自由振动研究

超级碳纳米管是在碳纳米管结构基础上,将每一根碳-碳键替换为碳纳米管而形成的新型结构。类超级碳纳米管是超级碳纳米管对应的宏观结构,在保持外观结构的基础上将尺度放大到宏观尺度。本文建立了类超级碳纳米管的粗粒化模型。基于粗粒化方法,研究了类超级碳纳米管的自由振动。分析了内外管半径以及长度对类超级碳纳米管振动行为的影响。与原结构有限元进行对比,结果表明粗粒化方法能有效的计算类超级碳纳米管的振动行为。     

Influence of a Longitudinal Magnetic Field on the Hall Effect in the Plasma Accelerator Channel

An analytic model of steady-state two-dimensional flows in coaxial plasma-accelerator channels in the presence of a longitudinal magnetic field is proposed. The solution of the problem is found in the smooth channel approximation for the MHD equations of an ideal two-component plasma. An example of the developing axisymmetric flows is given and the features of the plasma-dynamic processes are investigated. It is found that the Hall effect and the anode flow zone can be reduced using a longitudinal field and plasma rotation.     

一维流体饱和粘弹性多孔介质层的动力响应

本文研究了不可压流体饱和粘弹性多孔介质层的一维动力响应问题。基于粘弹性理论和多孔介质理论,在流相和固相微观不可压、固相骨架服从粘弹性积分型本构关系和小变形的假定下,建立了不可压流体饱和粘弹性多孔介质层一维动力响应的数学模型,利用Laplace变换,求得了原初边值问题在变换空间中的解析解,并利用Laplace逆变换的Crump数值反演方法,得到原动力响应问题的数值解。数值研究了饱和标准线性粘弹性多孔介质层的动力响应,分析了固相位移、渗流速度、孔隙压力及固相有效应力等的响应特征。结果表明,与不可压流体饱和弹性多孔介质相同,不可压流体饱和粘弹性多孔介质中亦只存在一个纵波,并且固相骨架的粘性对动力行为有显著的影响。     

黏弹性轴向运动梁非线性受迫振动稳态幅频响应

本文研究了黏弹性轴向运动梁横向受迫振动稳态幅频响应问题.在控制方程的推导中,对黏弹性本构关系采用物质导数.把多尺度法直接应用于梁横向振动的非线性控制方程,利用可解性条件消除长期项,得到系统稳态的幅频响应曲线.运用Lyapunov一次近似理论分析幅频响应曲线的稳定性.通过算例研究了黏性系数,外部激励幅值以及非线性项系数对稳态幅频响应曲线及其稳定性的影响.运用数值方法对两端固定边界下黏弹性轴向运动梁的控制方程直接数值解,分析梁横向非线性振动的稳态幅频响应,通过数值算例验证直接多尺度法的结论.     

基于严格范德华力作用的多壁碳纳米管磁弹性振动

给出了一种求解在任意两管之间严格范德华力相互作用下多壁碳纳米管磁弹性振动频率的解析方法.研究结果表明,在轴向磁场的作用下,严格范德华力相互作用对多壁碳纳米管最高磁弹性振动频率的影响大于对最低振动频率的影响;严格范德华力作用下多壁碳纳米管的最高磁弹性振动频率要高于经典范德华力作用下多壁碳纳米管的最高磁弹性振动频率;严格范德华力对磁弹性振动频率的影响依赖于碳纳米管层间距的变化和管的层数,且随着多壁碳纳米管层数的增加而趋于一个稳定值.本文的研究结果对于碳纳米管作为基本元件在纳米电子元件中的实际应用具有一定的参考价值.     

偏置磁场对磁致伸缩纵向导波换能器效能的影响研究

为提高磁致伸缩导波换能器的激励效能,本文研究了磁场结构参数对偏置磁场空间分布的影响。基于COMSOL有限元仿真平台,对磁致伸缩换能器磁场分布特性进行了数值计算,研究了磁路结构形式、磁路和永磁铁数量等对偏置磁场分布的影响,最后优化出适合纵向导波激励的磁场结构参数。检测实验结果表明,随着偏置磁场磁路数量的增加,偏置磁场强度增大,磁场径向均匀性更好,磁致伸缩换能器的效能也相应提高;在相同磁路数量的条件下,永磁体数量的改变对换能器效能影响较小,四磁路偏置磁场最优;轭铁中部增加永磁铁后的磁路结构的偏置磁场的轴向均匀性更高,其激励效能更好。     

Convection in a Porous Medium with Variable Gravity Field and Magnetic Field Effects

The problem of convection in a variable gravity field with magnetic field effect is studied using methods of linear instability theory and non-linear energy theory. Then, the accuracies of both the linear instability and global non-linear energy stability thresholds are tested using a three-dimensional simulation. The strong stabilizing effect of gravity field and magnetic field is shown. Moreover, the results support the assertion that the linear theory is very accurate in predicting the onset of convective motion, and thus regions of stability.     

碳含量对碳素钢磁场摩擦磨损性能的影响与作用机制研究

基于自制的HY-100销-盘式磁场摩擦磨损试验机,对比研究了有无外加磁场条件下,低碳钢、中碳钢和高碳钢的滑动干摩擦行为和其磨损特性,并使用扫描电子显微镜、金相显微镜等分析了摩擦微观表面与次表面,探讨了碳含量对碳素钢磁场摩擦的影响规律和作用机理. 试验结果显示:根据碳含量的不同,有无磁场条件下碳素钢摩擦系数的变化也不同,在外加磁场条件下,低碳钢摩擦系数降低,中碳、高碳钢摩擦系数升高;随着碳含量升高,磁场对其摩擦系数的影响越小. 外加磁场可以有效改善碳素钢的磨损性能,随着碳含量升高,磁场对其磨损率的改善程度越大. 分析其原因可能为铁碳竞争氧化,这既保证了铁氧化物的减磨效应,也减缓了铁氧化物积累后的剥落. EDS能谱分析结果表明高碳钢的氧铁比最低,和铁碳氧化竞争模型推论相一致.      

The Effect of Magnetic Field Dependent Viscosity on Thermosolutal Convection in a Ferromagnetic Fluid Saturating a Porous Medium

The effect of magnetic field dependent viscosity on thermosolutal convection in a ferromagnetic fluid saturating a porous medium is considered for a fluid layer heated and soluted from below in the presence of uniform magnetic field. Using linearized stability theory and normal mode analysis, an exact solution is obtained for the case of two free boundaries. For case of stationary convection, medium permeability has a destabilizing effect, whereas a stable solute gradient and magnetic field dependent viscosity have a stabilizing effect on the system. In the absence of magnetic field dependent viscosity, the destabilizing effect of non-buoyancy magnetization is depicted but in the presence of magnetic field dependent viscosity non-buoyancy magnetization may have a destabilizing or stabilizing effect on the onset of instability. The critical wave number and the critical magnetic thermal Rayleigh number for the onset of instability are also determined numerically for sufficiently large values of buoyancy magnetization parameter M₁ and the results are depicted graphically. The principle of exchange of stabilities is found to hold true for the ferromagnetic fluid saturating a porous medium heated from below in the absence of stable solute gradient. The oscillatory modes are introduced due to the presence of the stable solute gradient, which were non-existent in its absence. A sufficient condition for the non-existence of overstability is also obtained. The paper also reaffirms the qualitative findings of earlier investigations which are, in fact, limiting cases of the present study.     

Steady-State MHD Flows in Nozzles with an External Longitudinal Magnetic Field

Steady-state MHD flows in channels of the nozzle type in the presence of an external longitudinal magnetic field can be divided into two significantly different classes. Subcritical flows, in which the Alfvén velocity calculated from the longitudinal magnetic field is less than the plasma velocity, have mainly the same properties as flows in a transverse magnetic self-field and their quantitative characteristics depend only slightly on the longitudinal magnetic field strength. Supercritical flows with the opposite inequality for the velocities correspond to strong longitudinal magnetic field. The main difference is the transitions between different forms of energy (kinetic, thermal, and electromagnetic). The present study contains a classification of possible flows, namely, sub- and supercritical and sub-, super-, and transonic flows with respect to the fast and slow magnetosonic and Alfvén velocities. Examples of these flows are given. The effect of the problem parameters on the flow properties is investigated.     

磁场对不同温度场中输流悬臂碳纳米管动态特性的影响

本文在采用经典欧拉-伯努利梁模型的基础上,引入考虑小尺度效应的非局部弹性理论,着重研究不同温度场中输流悬臂单层碳纳米管系统（SWCNT）在外加纵向磁场作用下的颤振失稳问题。基于哈密顿原理获得了该流固耦合系统的振动控制方程及相应的边界条件,应用微分变换法（DTM法）求解此高阶偏微分方程,通过数值计算研究了不同温度场中施加纵向磁场对系统动力学特性的影响。结果表明：施加纵向磁场在不同温度场中都将增强输流悬臂碳纳米管的动态稳定性。然而,这种增强程度却与温度场的变化量有关,在不同温度变化量下,磁场对系统稳定性的增强程度有一个峰值,这意味着,实际应用中,为了提高这类流固耦合系统的动态稳定性,一味提高纵向磁场强度并不可取。     

温度场作用下悬臂输流碳纳米管的颤振失稳分析

以非局部弹性理论为基础,采用欧拉-伯努利梁模型,考虑碳纳米管的小尺度效应,应用哈密顿原理获得了温度场作用下的悬臂输流单层碳纳米管(SWCNT)的振动控制方程以及边界条件,依靠微分变换法(DTM法)对此高阶偏微分方程进行求解,通过数值计算研究了温度场中悬臂单层输流碳纳米管的振动与颤振失稳问题.结果表明:管内流体流速、温度场中温度变化情况与小尺度参数都会对系统振动频率以及颤振失稳临界流速产生影响.其中,小尺度效应将会降低悬臂输流系统的稳定性,使系统更为柔软;而高温场与低温场对系统动态失稳的影响不同,低温场中随温度变化值的增加,系统的稳定性提高;高温场这一作用效果恰好与之相反.     

Effect of Thermal Modulation on the Onset of Convection in Walters B Viscoelastic Fluid-Saturated Porous Medium

The linear stability of Walters B viscoelastic fluid-saturated horizontal porous layer is examined theoretically when the walls of the porous layer are subjected to time-periodic temperature modulation. Three types of boundary temperature modulations are considered namely, symmetric, asymmetric, and only the lower wall temperature is modulated while the upper wall is held at constant temperature. A regular perturbation method based on small amplitude of applied temperature field is used to compute the critical values of Rayleigh number and the corresponding wave number. The shift in critical Rayleigh number is calculated as a function of modulation frequency, viscoelastic parameter, and Prandtl number. The effect of all three types of modulations is found to be destabilizing as compared to the unmodulated system. This result is in contrast to the system with other types of fluids. Besides, the influence of physical parameters on the control of convective instability of the system is discussed.     

旋转运动导电圆板的磁弹性参数振动分析

针对磁场中旋转运动圆板,在动能、应变能表达式基础上,根据哈密顿原理导出圆板的磁弹性振动方程．应用伽辽金积分法,得到横向磁场中旋转变速运动圆板的轴对称参数振动微分方程．通过坐标变换得到包含两个变系数项的马蒂厄振动方程．应用弗洛凯理论和平均法对系统的参数振动问题进行求解．通过数值计算得到周期稳定图、对应的振动响应特性图和相轨迹图．结果表明：在稳定区域内,系统的幅频曲线呈现为周期或概周期变化形式;在不稳定区内,系统的幅频响应曲线呈现为发散变化形式．     

横向磁场中软铁磁薄板的物理非线性混沌振动

对处于横向均匀磁场中四边简支的软铁磁矩形薄板,在横向均布载荷作用下,考虑物理非线性和磁弹性耦合作用,由伽辽金法推导出磁弹性振动微分方程,求得了系统的异宿轨道参数方程,并根据Melnikov函数方法,推导并求解了振动系统异宿轨道的MelnikOV函数,给出了判断该系统发生Smale马蹄变换意义下混沌振动的条件和混沌判据.     

Double-Diffusive Convection in Bidispersive Porous Medium with Chemical Reaction and Magnetic Field Effects

Transport in Porous Media - This study features a model for double-diffusive convection in a bidisperse porous medium where a vertical magnetic field chemical reaction’s effects are present....