多个同频摇荡剖面引起的波辐射

在内场中使用简单Ｇｒｅｅｎ函数的边界元方法与外场的速度势特征函数展开式相结合，用于求解多个同频摇荡剖面面引起的水面波辐射问题的频域解。方法适用于外场为定深的水域以及内场的复杂边界条件，各剖面的摇荡模态，幅值和相位可以互不相同。     

波浪中多船近距离并行航行的水动力干扰研究

采用三维势流理论及多体动力学理论建立了波浪中近距离并行航行多船波浪作用力及运动响应的计算模型.为考虑航速效应对船体间自由面的影响,采用三维移动脉动源Green(格林)函数来模拟船间的辐射波及绕射波.采用该数学模型求解了零航速及有航速情形下两船近距离并行时的流体动力项及运动响应,和模型试验结果比较验证了方法的可靠性.将该方法推广至三船并行航行的情形,重点分析了三船并行时水动力干扰和两船并行时水动力干扰的差异.     

有限区域求解流函数和速度势的迭代调整方法及其收敛性分析

全球区域求解流函数和速度势的解是唯一的,但有限区域内,由于受区域边界条件限制,流函数和速度势的计算以及风场的分解不是唯一的,减小或消除边界不确定对结果的影响是有限区域求解流函数和速度势以及分解风场最重要的问题．该文在Endlich迭代调整思想上,提出了能准确求解有限区域流函数和速度势且对边界条件要求较低的迭代调整方法．该方法也能准确地分解和重建风场,且风场重建的误差非常小．对该迭代方法的收敛性进行分析,发现其收敛性与不同方向网格的格距和调整系数有关．最后将该方法应用到Arakawa A-D网格和不规则区域,验证了该方法的可靠性．     

两个垂直圆柱在水波中的水动力相互作用

基于线性势流理论研究了两个垂直圆柱在水波中的水动力相互作用．两个圆柱中的一个固定在底部，另一个铰接在底部且可以在入射波方向以小振幅振动．本文研究了绕射波和辐射波，运用加法定理得到了每个圆柱表面速度势的简单的解析表达式，用级数形式显式表示了圆柱上的波浪激励力和力矩及振动圆柱的附加质量和辐射阻尼系数．级数的系数由代数方程组的解决定．给出了一些数值例子以说明诸如间距、圆柱的相对大小、入射角等各种参数对一阶力、定常二阶力、附加质量和辐射阻尼系数以及振动圆柱的响应等的影响．     

微通道液体流动双电层阻力效应

采用数值方法求解双电层的Poisson-Boltzmann方程和液体运动的Navier-Stokes方程,研究微通道双电层对压强梯度液体流动的阻力效应． 量纲分析表明,双电层阻力大小可以用一个无量纲的电阻力数表示．它与液体的介电系数、固体表面的zeta电位平方成正比,与液体的动力粘性系数、电导率以及微通道的宽度平方成反比．在计算流动诱导的流动电位势和电阻力时,提出电流密度平衡条件,可以消除传统电流平衡条件导致的固壁附近产生局部回流的不合理物理现象．还给出不同电阻力数的微通道流量、流量损失率、速度剖面的数值结果,合理解释了双电层对微通道液体流动的阻力效应．     

求解二维多区域声波散射问题的边界元方法

对于多散射区域的声波散射问题的外Neumann边值问题,用单层位势来逼近每个散射域上的散射波,再利用位势理论的跳跃关系将问题转换为第二类边界积分方程组的求解问题,然后用Nystrom方法进行了求解.对多个随机散射区域的声波散射问题,数值例子体现了该求解方法的可行性和准确性.     

通过槽孔耦合入导电腔内的瞬态电磁场

本文从等效定理着手,求解瞬态入射电磁波通过槽孔耦合入导电腔的问题。应用矩量法,并定义广义导纳矩阵后,推导出广义网络矩阵方程。最后用奇点展开法求得瞬态腔内场。文中讨论了求得自然频率、自然模矢量及耦合系数的方法。求得了与较重要的系统奇点对应的自然模分布,计算了相应的耦合系数作为入射角的函数。当入射电磁波为时间的阶梯脉冲时,具体计算了槽孔处磁流和腔内场的瞬态波形。     

不同热环境下的颤振问题初探

高超声速环境下,气动加热会严重影响飞行器结构的动力与颤振特性。文章研究了不同边界条件下气动热对结构动力学特性及颤振特性的影响。以全动平尾为对象,分析了不同热条件下其振动与颤振特性的变化。此外,采用解耦求解思路,进行了高温下弹性模量变化和热应力对平尾颤振特性影响的分析。结果表明,结构加热产生的热应力是导致其颤振速度下降的主要原因;平均温度升高会引起平尾颤振速度的明显下降,应引起重视。     

波流非线性相互作用对流场和载荷的影响

提出Stokes波与指数剖面流相互作用的五阶分析理论,所得质点速度与现有实验结果进行了比较,分析了线性叠加方法和近似理论引起的误差。结果表明,波流共存时的流场由纯波、纯流和相互作用项组成,流不仅直接改变了流场,而且通过改变波参数间接影响流场。指数剖面流能合理描述实际情况中的剪切流,由分析理论得到的波流场与实验值符合得很好。线性叠加方法能引起超过40％的结构物极限载荷误差。当取表观周期波、同时对流进行Wheeler伸展,近似理论误差最小。     

基于排队论的内场维修设备数量确定方法

在内场车间维修设备配置研究中,为了科学合理地确定维修设备数量需求,运用排队论的基本方法,结合飞机内场维修工作特点,提出了基于排队论的内场维修设备数量确定方法,构建了相应的数量确定模型,结合实例验证了该方法可行有效.结果表明方法可以提高飞机基层级内场车间维修设备数量确定的准确性.     

Torsional wave dispersion relations in a pre-stressed bi-material compounded cylinder with an imperfect interface

This paper studies the influence of the imperfectness of the contact condition on the torsional wave propagation in the initially stressed (stretched) bi-material compounded circular cylinder. The investigation is carried out within the scope of the piecewise homogeneous body model with the use of the Three-dimensional Linearized Theory of Elastic Waves in Initially Stresses Bodies. The mathematical formulation of the corresponding eigen-value problem is formulated and the solution method for that is developed. The two cases considered are the bi-material compounded cylinder consists of the solid inner and surrounding hollow outer cylinders (Case 1); the bi-material compounded cylinder consists of the hollow inner and surrounding hollow outer cylinders (Case 2). The mechanical relations of the cylinders’ materials are written through the Murnaghan potential. It is proven that the imperfectness of the contact condition does not influence the asymptotic-limit values of the wave propagation velocity. Moreover, the numerical results on the effects of the imperfectness of the boundary condition on the influence of the initial stresses on the wave propagation velocity are presented and discussed.     

On the hydrodynamic interaction of two circular cylinders oscillating in a viscous fluid

The present paper deals with the plane flow fields induced by two parallel circular cylinders with radiia andb oscillating in a direction which is i) parallel or ii) perpendicular to the plane containing their axes. The effect of the cylinders' hydrodynamic interaction on steady streaming has been studied analytically at high frequency by the method of matched asymptotic expansions.It is found that ifa=b the steady streaming is directed symmetrically to the cylinders while whenab (in the case i)) the secondary steady flow is directed towards the larger cylinder and one of the outer steady vortices disappears.It is shown in case i) that the drag force acting on each cylinder is smaller than the same force experienced on a single cylinder with the same radius which is placed in an unbounded oscillating flow. When the cylinder radii are equal, the drag is greater on the forward cylinder than on the rear one.In contrast, in case ii), wherea=b, it is shown that the drag on each of the two cylinders is greater than the drag acting on a single cylinder with the same radius placed in an unbounded oscillating stream and also each of the cylinders experiences a repulsive force in a direction perpendicular to the oscillating flow.     

Sound transmission into a thick hollow cylinder with the fixed-end boundary condition

In this paper sound transmission through the air filled finite thick cylinders exposed to the different incident acoustic wave is studied. The effect of end boundary conditions on the noise reduction of finite cylinders is evaluated. The uniform incident wave and the wave radiated from monopole and dipole sources are used in this study. Three positions are considered for the dipole source. Every position for the dipole source causes symmetric or antisymmetric pressure distributions on the external surface of the cylinder in tangential or axial direction. For the purpose of sound transmission analysis the linear three-dimensional theory of elasticity utilizing the technique of variables separation for the infinite circular cylinders is used to analyze the vibration of finite circular cylinder. In these analyzes the stress continuity condition on the inner and outer surfaces of the cylinder is satisfied using orthogonalization technique and velocity continuity condition is exactly satisfied on the interfacial surfaces. The sound transmission evaluation is carried out for cylinders with various half-length to outer-radius ratios. The results show that in the case of the fixed-end cylinder, the effect of boundary conditions on the noise reduction can be neglected for the half-length to outer-radius ratio of more than 10. Comparing between the obtained results from different acoustic sources shows that the obtained noise reductions from the uniform acoustic wave are less than those obtained from the monopole and dipole sources.     

Flow of a Newtonian Fluid between Eccentric Rotating Cylinders and Related Problems

An incompressible Newtonian fluid is contained in the annular region between two infinite cylinders, one or both of which rotate with constant angular velocities about their respective axes. The first-order inertial correction to the forces exerted by the fluid on the cylinders is obtained in explicit algebraic form. The results are applied to the related problem in which the inner cylinder executes a planetary motion about the axis of the outer cylinder. They are also applied to the problem of the transverse sedimentation of a long cylinder in a half space of fluid bounded by a rigid wall. Certain anomalies which arise in this case are noted.     

Optimal control of a cylinder rotating in a viscous fluid

Under consideration is the axisymmetric problem of optimal boundary control of a mechanical system consisting of two coaxial cylinders and an incompressible viscous fluid filling the region between them. The control parameter is the angular velocity of the outer cylinder. The goal is to stop the interior cylinder at a prescribed time with minimal energy expense. We prove that the problem is uniquely solvable and obtain the optimality system.     

The structure of fully nonlinear Taylor vortices

The structure of nonlinear short-wavelength Taylor vorticesin the flow between rotating concentric cylinders is considered.In the short-wavelength limit, the nonlinear vortex motion isgoverned by a mean-flow-first-harmonic interaction problem.The initial structure of the nonlinear vortex state is shownto be governed by a multilayer structure in which the vortexis constrained to lie between the inner cylinder and a positioninternal to the flow regime. This position is dependent uponthe Taylor number and it is found that there is a critical valueof the Taylor number at which the vortex first impinges on theouter boundary. The vortex field then develops a double boundarylayer structure at both the inner and outer boundary as theTaylor number is increased past this critical Taylor number.     

Thermal analysis of moving induction heating of a hollow cylinder with subsequent spray cooling: Effect of velocity,initial position of coil,and geometry

In this paper, temperature analysis of the complete process of moving induction heat treatment is performed using numerical methods. A non-linear and transient magneto-thermal coupled problem with a moving coil which is considered as moving heat source, is investigated by an efficient finite-element procedure. A vertical hollow circular cylinder is heated by the moving coil at a given velocity along it, and the heated parts then quenched by a moving water–air spray. The effects of natural convection with air on the both inner and outer surfaces of cylinder, and also radiation of outer surface of cylinder with ambient are taken into account. For quenching of work-piece, a specific kind of atomized spray cooling which utilizes a mixture of water and air with different mass fractions is used. This procedure includes moving boundary conditions, temperature-dependent properties, and change in magnetic permeability of specified alloy at the Curie temperature. Obtained numerical results have been verified by comparison with analytical solutions using Green’s function methods. Also, the effect of velocity, initial position of inductor and inner to outer radius ratio on temperature distribution are investigated.     

Diffraction of water waves by a slotted dual cylindrical caisson breakwater

The linearized theory of water waves is used to examine the diffraction of incident regular waves by a slotted dual cylindrical caisson breakwater. The breakwater consists of a vertically slotted outer cylinder circumscribing an impermeable inner cylinder. Under the assumption that the wavelength is much greater than its thickness, each panel is replaced by a thin structure and the permeability is modeled by suitable boundary conditions applied on its surface. The mixed boundary-value problem for the outer cylinder is transformed to dual series relations and the least-square method is applied to get the forces on the structure and associated diffraction field. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analytical solution of the quasi-static thermoelasticity problem in a pressurized thick-walled cylinder subjected to transient thermal loading

Thermoelasticity problem in a thick-walled cylinder is solved analytically using the finite Hankel transform. Time-dependent thermal boundary conditions are assumed to act on the inner surface of the cylinder. For the mechanical boundary conditions two different cases are assumed: Traction–displacement problem (traction is prescribed on the inner surface and the fixed displacement boundary condition on the outer one) and Traction–Traction problem (tractions are prescribed on both the inner and outer surfaces of the hollow cylinder). The quasi-static solution of the thermoelasticity problem is derived analytically, i.e., the transient thermal response of the cylinder is derived and then, quasi-static structural problem is solved and closed form relations are extracted for the thermal stresses in the two problems. The results show to be in accordance with that cited in the literature in the special cases.