Hydrodynamic forces acting on a circular cylinder oscillating in waves and a small current

A complete theory to obtain semianalytical solutions of the wave drift damping for a circular cylinder freely oscillating in waves is developed. The wave drift damping can be significantly increased by heave and pitch motions. Effects of the draft of the cylinder and effects of the water depth are shown. The effective evaluation of the free-surface integral and the corner problem are also discussed, and the computed results of the far and near field formulations are compared. These semianalytical solutions may prove to be important in providing validation of results obtained by numerical techniques such as the higher-order boundary element method.     

波浪对潮流影响的数值模拟及其应用

文章将水动力因素分为潮流特征周期与波浪特征周期两部分,建立了考虑波浪影响的潮流方程。通过考虑波浪引起的辐射应力、表面和底部边界层、Stokes漂流以及Coriolis波浪应力等因素,利用数值方法分析了波浪对潮流场的影响,并将其应用到渤海区域中,分析渤海内波浪对于潮流的影响。     

基于多次透射公式和无奇异边界元法模拟全非线性数值波浪水池

文章基于势流理论对全非线性的三维数值水池进行了模拟,其控制方程由无奇异边界积分方程法(Desingularized Boundary Integral Equation Method,DBIEM)进行离散求解,在求解全非线性的自由面微分方程时,文中采用混合欧拉—拉格朗日法(Mixed Eulerian-Lagrangian,MEL)和四阶Adams-Bashforth-Moulton(ABM4)预报—修正方法,为了避免结果发散即增强数值稳定性,文中采用B样条法来光顺波面.同时,在远方辐射控制面上采用多次透射公式方法(Multi-transmitting Formula,MTF)来进行消波,文中得到的结果与理论解进行了比较,结果表明该方法可用来有效模拟全非线性的数值波浪水池.     

湍流边界层的两层模型积分法

湍流边界层的两层模型(TLM)微分法在过去的十几年里得到了迅速发展.然而,目前除了对简单的模型外,还未能得到完全可靠的速度剖面模型.本文针对平板湍流边界层讨论一种新的两层模型积分法,该方法的内层采用壁面率,而外层使用了剪应力呈抛物线分布的Clauser湍流粘性公式.计算结果与1/7幂次律单层模型进行了比较,吻合较好.如果考虑压力梯度的影响,还可以将该方法推广到轴对称和三维湍流边界层中去.     

采用THOBEM方法研究波流与结构物相互作用的数值模拟(英文)

To study wave-current actions on 3-D bodies a time-domain numerical model was established using a higher-order boundary element method (HOBEM). By assuming small flow velocities, the velocity potential could be expressed for linear and higher order components by perturbation expansion. A 4th-order Runge-Kutta method was applied for time marching. An artificial damping layer was adopted at the outer zone of the free surface mesh to dissipate scattering waves. Validation of the numerical method was carried out on run-up, wave exciting forces, and mean drift forces for wave-currents acting on a bottom-mounted vertical cylinder. The results were in close agreement with the results of a frequency-domain method and a published time-domain method. The model was then applied to compute wave-current forces and run-up on a Seastar mini tension-leg platform.     

开口结构声辐射计算的虚拟边界法

计算薄壁开口结构声辐射问题时,一般采用以结构表面压力差作为变量的间接边界元法.当结构两表面的法向量不一致或者两表而振动速度不相等时,间接边界元法无法应用,只能采用直接边界元对结构的内外边界均进行离散求解.对于厚度较小的结构,内外表面均离散的直接边界元法会导致数值积分奇异的困难.针对上述不足,文章提出一种计算开口结构声辐射问题的虚拟边界法:在开口结构上人为地加上边界,使其成为一个或多个相互独立的封闭结构,对这样的封闭结构分内外声场分别建立方程,并利用虚拟边界上的压力和速度连续条件联立求解,从而避免了间接边界元和直接边界元法计算开口结构声辐射问题时存在的缺陷;同时采用文中的方法还可以同时得到声场中非物面边界上质点的振动速度和压力,这是常规边界元法无法做到的.数值算例表明文中的方法是正确可行的.     

船舶在远场水下爆炸载荷作用下动态响应的数值计算方法

提出了一个利用MSC／DYTRAN数值模拟水面船舶在远距离水下爆炸载荷作用下动力响应的方法。用FORTRAN语言编译用户子程序，在近场水域边界处加上冲击波载荷以模拟远场爆炸效应，进而利用DYTRAN中强大的流固耦合计算功能，计算船体在水下冲击波作用下的动态响应。同时研究了边界定义和单元划分对冲击波传播的影响。该方法弥补了DYTRAN计算远场水下爆炸的某些不足，计算所得到的船体附近的自由场压力与经验公式的结果基本一致，船体的冲击响应与相关实验结果比较表明本文计算结果可信。     

The hydrodynamic forces acting on a cylinder array oscillating in waves and current

The problem of the interaction of multiple cylinders oscillating in waves and slow current is considered. The interaction is represented by waves emitted from adjacent cylinders towards the cylinder under consideration. Wave drift forces and moment in the horizontal plane are calculated by the far-field method based on the conservation of momentum or angular momentum. A semianalytical formula for the calculation of the wave drift damping is then deduced. The conservation of the integrals in these formulae is proved. Special treatments to improve the accuracy of results are discussed. Comparisons between calculated results and experimental measurements are made, showing satisfactory agreement. Effects of various combinations of current direction and incident wave angle on the wave drift damping and damping moment are also examined.     

求解外域辐射和散射声场的分形有限元方法

应用二级分形有限元方法计算了外域声场.用一人工边界把外域声场分为两部分,人工边界以内使用常规有限元方法,人工边界以外的无限大区域使用分形有限元方法.使用分形有限元方法,一方面形成几何自相似网格使得相邻层之间的单元刚度矩阵和质量矩阵具有非常简单的关系,另一方面引用自动满足无限远辐射条件的全域插值函数把节点自由度变换为一组广义坐标,因而计算量可以大大减少.数值计算结果表明:该方法对于计算无限大外域声场是有效的.     

基于势流理论的船舶操纵水动力分析

以船舶操纵水动力预报为研究背景,以无界流场势流理论为基础,采用边界元法,对目标Wigly船在斜航运动状态下的流场速度和转首力矩进行计算。分析了斜航时船舶周围流场的分布规律,研究了转首力矩随船舶漂角的变化规律。结果表明,该研究对预报斜航时船舶的运动规律具有较好的应用前景。     

Steady streaming of viscous surface layer in waves

An investigation of the drift velocity induced by water waves of a contminated surface layer is carried out. The theoretical analysis is based on a thin boundary layer on the free surface. The results of the analysis reveal that the drift velocity of a viscous layer on the water surface is 7/4 times the Stokes prediction ofc(ak) ² wherec is the wave celerity andak the wave slope. The present experimental investigation confirms the validity of the theoretical prediction for the drift velocity for a lightly contaminated surface layer; however, for a heavily contaminated surface layer, the experimental results exceed the theoretical prediction. An investigation for a heavily contaminated layer is carried out assuming an inextensible surface layer. Thus, in the experiment, vinyl sheets are used to substitute the contaminated layers. By balancing the wave-induced mean thrust force with the mean drag force, the drift velocity is obtained and compared with the experimental results. Based on the theoretical and experimental analyses, formulae for predicting the drift velocities for laminar and turbulent flow conditions are proposed.     

非结构化网格浸入边界法的流固耦合数值模拟(英文)

This paper presents an improved unstructured grid immersed boundary method. The advantages of both immersed boundary method and body fitted grids which are generated by unstructured grid technology are used to enhance the computation efficiency of fluid structure interaction in complex domain. The Navier-Stokes equation was discretized spacially with collocated finite volume method and Euler implicit method in time domain. The rigid body motion was simulated by immersed boundary method in which the fluid and rigid body interface interaction was dealt with VOS (volume of solid) method. A new VOS calculation method based on graph was presented in which both immersed boundary points and cross points were collected in arbitrary order to form a graph. The method is verified with flow past oscillating cylinder.     

惯性导航系统中陀螺斜坡漂移的测量及补偿

当惯性陀螺工作在方位姿态时,有一定的斜坡漂移,它将引起惯性导航系统中的严重的航向和位置误差,所以有必要估计和补偿它.本文描述了斜坡漂移产生的原因和特性,详细讨论了一种在BBA校准后利用惯导系统的输出来估计和补偿斜坡漂移和常值漂移的方法,以及提出了几个在实际操作中需注意的问题.软件仿真和实验都证明了这种方法的有效性.     

Determining the hydrodynamic forces on a planing hull in steady motion

A combination of methods was developed that can determine hydrodynamic forces on a planing hull in steady motion. Firstly, a potential-based boundary-element method was used to calculate the hydrodynamic pressure, induced resistance and lift. Then the frictional resistance component was determined by the viscous boundary layer theory. Finally, a particular empirical technique was applied, to determine the region of upwash geometry and determine spray resistance. Case studies involving four models of Series 62 planing craft were run. These showed that the suggested method is efficient and capable, with results that are in good agreement with experimental measurements over a wide range of volumetric Froude numbers.     

三角形网格的自动生成及其局部加密技术研究

将目前比较流行的三角形网格生成方法前沿生成法和delaunay方法结合起来,形成一种新的三角形网格生成方法。利用节点的间距函数来控制计算区域内网格尺寸的变化和疏密过渡,优先处理边界尺寸最长边,逐渐在计算域内过渡生成网格尺寸减小的三角形,最终实现计算区域的三角化。同时利用源项加密和边界加密的方法将工程区域网格细化。     

浅水中低速斜航船舶水动力预报及验证与确认分析

浅水中的斜航船舶受到浅水阻塞效应和不对称流的综合影响。为预报该运动中的船舶水动力,文章采用基于定常雷诺平均纳维—斯托克斯方程的计算流体动力学方法,对浅水中做斜航运动的船舶粘性绕流场进行数值模拟。考虑低航速运动的特点,忽略航速影响下的自由面兴波,由数值计算得到水动力系数在漂角影响下的变化规律。针对计算精度问题,在数值模拟中从验证和确认角度分析和评估计算结果：通过网格收敛性分析分析数值误差与不确定度;结合试验数据考察计算模型的误差。此外,从计算区域尺度、湍流模型、边界条件、船体下沉和纵倾作用方面对模型误差的影响因素进行探讨,可为改进计算模型、提高数值模拟精度提供参考依据。     

Time-domain simulation for water wave radiation by floating structures （Part A）

Direct time-domain simulation of floating structures has advantages： it can calculate wave pressure fields and forces directly; and it is useful for coupled analysis of floating structures with a mooring system. A time-domain boundary integral equation method is presented to simulate three-dimensional water wave radiation problems. A stable form of the integration free-surface boundary condition （IFBC） is used to update velocity potentials on the free surface. A multi-transmitting formula （MTF） method with an artificial speed is introduced to the artificial radiation boundary （ARB）. The method was applied to simulate a semi-spherical liquefied natural gas （LNG） carrier and a semi-submersible undergoing specified harmonic motion. Numerical parameters such as the form of the ARB, and the time and space discretization related to this method are discussed. It was found that a good agreement can be obtained when artificial speed is between 0.6 and 1.6 times the phase velocity of water waves in the MTF method. A simulation can be done for a long period of time by this method without problems of instability, and the method is also accurate and computationally efficient.     

桁架式Spar平台与系泊/立管系统的全时域非线性耦合动态分析

开发了对浮式平台系统进行耦合动态分析的全时域程序。采用二阶时域方法计算水动力荷载,在此方法中,对物面边界条件和自由水面边界条件进行泰勒级数展开,利用Stokes摄动展开分别建立相应的一阶、二阶边值问题,而且此边值问题的计算域不随时间变化。采用高阶边界元方法计算每一时刻流场中的速度势,利用四阶预报校正法对二阶自由水面边界条件进行数值积分。在自由表面加入一个人工阻尼层来避免波浪的反射。对于系泊缆索/立管/张力腿的动力分析,在一个总体坐标系中对控制方程进行描述,采用基于细长杆理论的有限元方法进行求解。在耦合动态分析中,采用Newmark方法对平台和系泊缆索/立管/张力腿的运动方程同时进行求解。利用开发的耦合分析程序对一个桁架式Spar平台的运动响应进行了数值模拟,给出了平台的位移和系泊缆索/立管上端点的张力,并得到了一些重要结论。     

Risk zone of wrack hitting marine structure simulated by 2D hydraulic model

The wrack or the ship out of control will drift with flow.One of the most important factors that drive the ship is flow current which moves circularly in tidal area.The wrack from same place always drifts in different ways if the start time is different.So,during the ship drifting period,the drift trace is also determined by both wave and wind forces.The drift direction is limited by water depth which must be deeper than ship draft. These marine structures that can not afford the hit of wrack or will destroy the wrack must be well considered when they are placed near harbor and waterway or other water area with ship running.The risk zone should be consulted according to tide and weather conditions to protect structures and ships in necessary.A method is presented here to simulate the risk zone by 2D numerical hydraulic model with tidal current,wave,wind and water depth considered.This model can be used to built early-warning and protect system for special marine structure.     

Automatic two-dimensional layout using a rule-based heuristic algorithm

 Efficient layout strategies are introduced for automatic nesting using a rule-based heuristic approach to improve the layout efficiency. Since a large portion of the complexity of the parts layout problem results from the overlapping computation, geometric redesign techniques are also suggested to reduce the complexity of the problem for a fast and reliable means of performing overlapping computation. A new heuristic sliding technique is developed to find a near-optimum layout location from all the feasible arrangements in such a manner that two or more arbitrary parts do not overlap or intersect. An identification method with a pivoting point is suggested to calculate the boundary of the overlap region in a fast computation time-frame. Resource plate clipping using virtual memory, based on a polygon clipping algorithm, is also proposed as a technique to reduce the geometric conditions of the resource plate and the overlap computation time by updating a new stock boundary of the resource plate for the layout space of the next part after each part is placed. The aim of this article is to develop a rule-based heuristic nesting system to achieve a new automatic layout on the AutoCAD system. For this implementation, some nesting examples are demonstrated. A rule-based heuristic approach can be desirable in terms of the layout efficiency and considering the computational time for nesting problems, and is proposed as a real-time layout simulation method in the industrial field. Received: October 7, 2002 / Accepted: March 27, 2003 RID="*"