柱间有相对运动坐底圆柱群的绕射辐射分析

圆柱群是深海海工建筑物的主要承力结构.目前对圆柱群进行的水动力分析大多考虑将圆柱群作为一个整体的情况,即柱群中各柱无相对运动;对于柱间有相对运动圆柱群的研究较少.为此,采用精确代数法研究柱间有相对运动坐底直立圆柱群的绕射-辐射问题.首先开展了柱群中各圆柱做给定单独运动的辐射分析;之后在有入射波浪的情况下,分析了各柱有相对运动圆柱群的绕射-辐射问题;进而求得各圆柱上的水动力、振动幅值等量.作为验证,先将退化情况的结果与已有文献进行了对比.进一步的计算结果表明,在某些波数下,分析中是否考虑柱间相对运动对结果影响较大.     

涡激振动驱动的柱群结构俘获海流能的稳定性分析

基于海流能发电涡激振动驱动俘获能量这一想法,对耦合连接的四圆柱结构在均匀海流流速下的自由涡激振动进行模拟发现:振动结构的响应幅值在较大和较小约化速度下,随组合间距比LH/D~2的变化相差较大。因此对U_r=5.71和U_r=14.29两种约化速度下结构位移幅值谱图、升力特性、相位差进行分析,结果表明:U_r=5.71时,不同间距比下各圆柱升力与位移间的相位角Φ不同,存在明显的主频且按较规则的正弦规律变化,四个圆柱对结构的振动都起激励作用,各圆柱俘获的水动能或转移到水流中的机械能相对稳定;而U_r=14.29时,各圆柱升力波动不规则,升力频率成分复杂,升力位移间的相位角Φ不明显,各圆柱俘获水动能不稳定,对结构振动起主要作用的是下游两圆柱。分析结果对柱群结构俘获海流能时其约化速度范围的确定有参考价值。     

波浪作用下上部带有透空结构的圆筒垂向水动力特性的解析研究

基于线性势流理论,应用特征函数展开方法和透空壁内流体速度与两壁间压力差成正比的线性模型,建立了上部带有透空结构的截断圆筒绕射问题和辐射问题的解析模型,进而导出浮体的垂荡运动响应幅值算子计算公式。通过与直立透空圆筒和非透空截断圆柱的相关结果进行对比,验证了该文推导的正确性。通过数值计算研究了透空系数、圆筒内外径比（内圆柱半径和圆筒半径之比）和吃水比（透空结构吃水和圆筒吃水之比）对整个圆筒的垂向水动力特性（运动响应幅值、附加质量、辐射阻尼和波浪激振力）的影响,并与非透空截断圆柱的计算结果进行对比分析。结果表明随着透空系数的增大,结构的附加质量和辐射阻尼呈增加趋势,垂荡运动响应幅值算子的峰值呈明显减小趋势,并且在低频区垂向波浪激振力相对较小,这有利于增加浮式结构的稳定性;内外径比和吃水比均对透空结构的垂向水动力特性影响较大。     

动水力简化计算方法对圆形桥墩传递函数的影响

刚性柱法、莫里森法和基频近似法是可供工程应用的不可压缩无粘性水体地震动水力的简化计算方法。为研究上述方法的差异性，将基于不可压缩无粘性水体辐射理论推导的考虑结构变形的精确解作为标准，从频域传递函数的角度，对各简化方法进行比较分析。以水中悬臂圆柱墩为例，选取2个尺寸参数并设计了84种不同尺寸的桥墩，输入脉冲激励得到墩顶节点的位移传递函数、墩底节点的剪力和弯矩传递函数，提取各传递函数的前两阶共振峰幅值和共振频率，基于标准解结果求得3种简化方法的误差。通过分析误差的范围及变化趋势，发现刚性柱法所得共振峰幅值更精确，基频近似法所得共振周期更精确，莫里森法误差受参数的影响显著。此外，基频近似法所得共振峰幅值大多偏小，而莫里森法所得幅值和周期大多偏大。     

近海风力发电机基础桩群潮汐水动力脉动特性试验研究

近海风电机组基础较多采用圆柱高桩混凝土承台结构,潮汐水动力作用可能激励风机-塔架-基础桩高耸结构的振动而影响安全,该文通过水力学模型试验研究8圆柱基础桩群潮汐水动力脉动特性。结果表明:脉动压力可视作平稳的随机过程,时域幅值特征表现出与单圆柱差异较大的特点,频域能量特征揭示了水流的脉动主频和旋涡脱落频率。     

近水面状态有限长圆柱壳受迫振动的输入功率流和声辐射特性

基于声固耦合模型,采用波传播方法分析近水面状态有限长圆柱壳受迫振动的输入功率流和声辐射特性,并考虑了自由液面的影响。通过与有限元仿真结果进行对比分析,验证了该方法的正确性。当圆柱壳接近自由液面时,自由液面的存在致使水下圆柱壳输入功率流和声辐射特性的峰值频率向高频偏移,且响应幅值也略有增大。随着潜深的增大,自由液面对圆柱壳输入功率流特性的影响逐渐减小。当潜深大于5倍半径时,自由液面对耦合系统输入功率流的影响可以忽略,但其对耦合系统声辐射特性的影响不可忽略。     

圆柱碰撞冲击噪声理论分析与数值仿真

基于接触动力学相关理论和弹性体线接触理论模型,建立了圆柱碰撞冲击理论数学模型,结合声学理论对圆柱碰撞冲击辐射噪声进行了理论预估。采用有限元法与瞬态边界元法相结合的瞬态噪声数值仿真方法,对圆柱碰撞冲击噪声进行数值仿真,实现了瞬态冲击噪声辐射声波的可视化。分析结果表明圆柱碰撞冲击噪声主要由加速度辐射噪声产生,圆柱碰撞冲击噪声辐射声场具有明显指向性。对比理论分析结果、数值仿真结果以及Hertz点接触模型计算结果可知,理论分析结果与数值仿真结果吻合较好,而Hertz点接触模型将圆柱体线接触模型简化为点接触模型,导致接触时间延长,接触力和冲击噪声幅值降低。基于数值仿真方法及理论分析,研究了冲击速度及材料弹性模量对圆柱碰撞冲击噪声的影响。     

附加整流装置的圆柱体涡激振动数值研究

对附加不同整流装置的圆柱体横向单自由度涡激振动特性进行了系统研究。重点分析了约化速度3.0≤U_r≤12.0范围内,附加不同整流装置的圆柱振动幅值,受力系数,振动频率和泻涡模式。研究结果表明:附加不同整流装置的圆柱体其水动力特性较附加整流装置的静止圆柱体复杂很多,采用附加整流装置的静止圆柱体绕流结果评价整流装置性能好坏欠妥。     

仿海蛇柔性桨状蛇尾推进水动力学研究

海蛇躯体不同的推进段对推力的贡献不同,侧面压扁成桨状尾部是海蛇自适应进化最典型的特征,显著增强了其运动速度、机动性和效率。本文以柔性蛇尾为研究对象,建立蛇尾摆动流固耦合水动力分析模型,采用求解Reynolds-averaged Navier-Stokes(RANS)方程方法,研究蛇尾尺度参数-运动学参数-推进力之间的动力学关系。以NACA006翼型为基础,分析了不同蛇尾运动频率、幅值以及几何参数对推进力的影响。结果表明,摆动频率和摆动幅值对推进力的影响非独立的,本文中模型在摆动频率大于0.7 Hz、摆动幅值小于45°时,推进力随摆动频率、摆动幅值增大而增大;在改变蛇尾外形参数优化时,推进力随蛇尾侧面积增加而增加,通过改变弦长增大侧面积的方式有利于增加推进力。     

气泡群振荡及噪声数值模拟研究

针对水中气泡群振荡及辐射噪声问题,基于均质混合流模型,考虑气泡动力学作用,建立数学模型;并采用拉格朗日有限体积法,对气泡群振荡过程及辐射噪声进行数值模拟。重点研究了初始气泡半径、含气率及气泡群尺度等对振荡辐射噪声的影响。结果表明,随着初始含气率、气泡群尺度增大,气泡辐射噪声频率逐渐减小、声压幅值逐渐增大即声强增大;初始气泡半径对辐射噪声频率影响较小,但声压幅值随其增大而减小。     

Trapping and Near-Trapping by Arrays of Cylinders in Waves

In this paper, some recent developments and new results concerning the trapping of waves by arrays of vertical circular cylinders is presented. In particular, the cases are examined when there is a circular arrangement of cylinders and both finite and infinite periodic linear arrays of identical cylinders. Only for the infinite array is there pure trapping of waves – known as Rayleigh–Bloch or edge waves – which, for particular dominant wavenumbers, reduce to the well-known trapped-mode solutions for a cylinder between two parallel walls having either Neumann or Dirichlet conditions upon them. This latter case is considered separately and some new results are presented. In the circular array and finite linear array the concept of near-trapping is introduced where large resonant motions are found to occur at certain frequencies of the incident wave field. In the case of the finite linear array, these near-trapping frequencies are related to the Rayleigh–Bloch trapped-wave frequencies for the infinite array. Finally, the case when there are two or more lines of cylinders in the linear array is examined.     

Control of the diffracted response of wire arrays with double periods

The possibility of controlling the diffracted response of a periodic structure is investigated by using dual-period arrays, i.e., periodic arrays with a compound unit cell. We consider wire gratings in which each period comprises several cylinders with circular cross sections and all the cylinder axes are contained in the same plane. It is shown that this kind of structure permits one to control the diffracted response, regardless of the cylinder material and the incident polarization. Our numerical results suggest that the effect produced by wire gratings with dual-period characteristics is basically a geometric effect, and it can be present for other shapes of individual scatterers within each subarray.     

沿水平向运动的水中截断圆柱体辐射波浪问题的解析解答

根据线性势波理论,分析了水中截断圆柱体作水平简谐运动时结构周围的辐射波浪。利用分离变量法,分别得到了含有未知常数的三个流体子域速度势的简谐表达形式,并采用一个较为简单的匹配方法使其在流体子域的共同边界上满足压力和法向速度的连续条件。于是求解得到了速度势,进而可得到由等效附加质量和附加阻尼表示的柱体侧面上的动水力。不仅能考虑自由表面波对动水压力的影响,也适用于位于任意水深处的截断圆柱体。实例分析表明,该方法具有较高的计算精度;同时对于截断圆柱体,采用Morison方程中的动水附加惯性力项会高估柱体侧面上的动水力。     

Flows of inelastic non-Newtonian fluids through arrays of aligned cylinders. Part 1. Creeping flows

Numerical simulations are presented for flows of inelastic non-Newtonian fluids through periodic arrays of aligned cylinders, for cases in which fluid inertia can be neglected. The truncated power-law fluid model is used to define the relationship between the viscous stress and the rate-of-strain tensor. The flow is shown to be dominated by shear effects, not extension. Numerical simulation results are presented for the drag coefficient of the cylinders and the velocity variance components, and are compared against asymptotically valid analytical results. Square and hexagonal arrays are considered, both for crossflow in the plane perpendicular to the alignment vector of the cylinders (flows along the axes of the array as well as off-axis flows), and for flow along the cylinders. It is shown that the observed strong dependence of the drag coefficient on the power-law index (through which the stress tensor is related to the rate-of-strain tensor) can be described at all solid area fractions by scaling the drag on a cylinder with appropriate velocity and length scales. The velocity variance components show only a weak dependence on the power-law index. The results for steady-state drag and velocity variances are used in an approximate theory for flows accelerated from rest. The numerical simulation data for unsteady flows agree very well with the approximate theory.     

Diffraction of hydroelastic waves by multiple vertical circular cylinders

The diffraction problem of hydroelastic waves beneath an ice sheet by multiple bottom-mounted circular cylinders is considered. The elastic thin-plate theory is adopted to model the ice sheet, while the linearized velocity potential theory adopted for the fluid flow. The velocity potential corresponding to each cylinder is expanded into a series of eigenfunctions, and the total potential is expressed as a summation of these expansions over the entire NC number of cylinders. For each cylinder, the Green’s second identity is used outside its domain to obtain a set of linear equations. For each different cylinder, the domain used is different. NC cylinders give NC sets of coupled linear equations. Investigations are made for different arrangements of cylinders, piercing through ice sheets. Results for the wave forces on the cylinders with clamped and free conditions of the ice edge are obtained. Physical phenomena corresponding to cylinders arranged in square, in an array, in a double-array and in a staggered double array are discussed.     

Power-law fluids flow and heat transfer over two tandem square cylinders: effects of Reynolds number and power-law index

The low-Reynolds numbers free-stream flow of power-law fluids and forced convection heat transfer around a square cylinder and two square cylinders in a tandem arrangement are numerically investigated. In the single cylinder case, the power-law index and Reynolds numbers range from n = 0.7 ? 1.4 and Re = 60 ? 160 at Pr = 0.7. In the tandem case, the spacing between the cylinders is four widths of each cylinder side and the power-law index ranges from 0.7 to 1.6 at Re = 40, 100, 160 and Pr = 0.7. All simulations are performed with a finite volume code based on the SIMPLEC algorithm and a non-staggered grid. The effect of spatial resolution on the results is also studied for a single cylinder and tandem cylinders. The mean and instantaneous streamlines, vorticity and temperature contours, the global quantities such as pressure and friction coefficients, the rms lift and drag coefficients, Strouhal and Nusselt numbers are determined and discussed for various power-law indexes at different Reynolds numbers. A comparison between the results of the single cylinder case and the two cylinders in tandem arrangement shows that there are relatively similar results for the single cylinder and the upstream cylinder of the tandem case.     

Free-surface flow of a fluid body with an inner circular cylinder in impulsive motion

An analytical theory and numerical computations are developed for the two-dimensional free-surface flow of an initially circular layer of inviscid fluid surrounding a rigid circular cylinder. The two cylinders are initially concentric. The fluid packet is released from rest and the flow suddenly starts forced by gravity and by the simultaneous impulsive motion of the inner body. A small-time expansion of the fully nonlinear free-surface problem is developed and a closed-form solution is found up to third order for an arbitrary radius of the rigid cylinder. For the gravitational flow around the body at rest, the solution is extended up to fourth order. Free-surface profiles and hydrodynamic forces on the cylinder are calculated and discussed against numerical solutions of the exact unsteady nonlinear problem. Some basic features, such as the formation of an almost uniform layer surrounding the upstream side of the body, are captured by the theory quite well and only later on in time significant quantitative differences appear. Similarly, the behaviour of hydrodynamic loads is rather well predicted during initial stages preceding larger fluctuations observed on a longer time-scale.     

Vibration of infinite piezoelectric cylinders with anisotropic properties using cylindrical finite elements

Natural frequencies for free vibration of infinite piezoelectric cylinders are computed using finite elements that are formulated in cylindrical coordinates. The finite-element method is used to model the cross-section of the cylinder in r, theta coordinates using circular sectors. Material constants that are functions of theta are allowed to vary in each circular sector and are computed using standard tensor transformations. The accuracy of the finite-element formulation is verified using previous results for isotropic cylinders and axisymmetric piezoelectric cylinders. New results are tabulated for frequencies of free vibration of solid and hollow piezoelectric cylinders of LiNbO(3) of crystal class 3m. Displacements for typical mode shapes are illustrated graphically.     

串列双圆柱静止绕流的二维数值仿真分析

利用CFX软件建立二维流场模型,采用有限体积法针对串列双圆柱的静止绕流现象进行了数值模拟计算。首先计算了雷诺数Re=200,不同间距时上下游圆柱的斯托罗哈数,并与参考文献的计算结果进行了对比,证明了该文计算的可靠性。然后分析了不同间距时上下游圆柱的升力系数和阻力系数的变化特点,得出了Re=200时双圆柱绕流的临界间距在3.375D~3.5D之间。最后通过对不同间距下流场变化的研究得出:上下游圆柱的间距小于临界间距时,上游圆柱不存在旋涡脱落;超过临界间距时,上游圆柱出现旋涡脱落;下游圆柱始终存在旋涡脱落现象。研究成果能够为计算流体力学和空气动力学技术的发展提供理论基础。