变刚度楔形体板架落体砰击试验研究

通过对具有四种刚度的两个二维楔形体板架的自由落体试验,测量模型入水速度、砰击压力和结构响应,获得模型底部板架在砰击载荷作用下的响应特征;对结构模型在均布静压力作用下的应力响应进行结构力学分析;比较试验和计算得到的应力响应,获得作用在模型肋骨上的砰击压力的折减系数,从而使该结构在设计时仍可按常规的静力计算方法进行强度校核。通过试验,深入研究了折减系数与结构刚度的关系,并指出在船体局部结构设计中应充分注意刚度变化对折减系数的影响。     

船体结构局部强度设计中的砰击载荷确定方法

在分析船舶砰击载荷力学特性的基础上,首先对各种船体砰击载荷设计方法进行比较和开展了压力不均匀系数的研究。然后从工程实用的角度出发,引入和扩展了砰击压力"折减系数"的概念,将砰击压力转化为与结构应力响应等价的均布静压力,使结构仍可按静力强度计算方法来进行设计和校核。在此基础上,提出了船体结构局部强度设计中确定砰击载荷的思路,给出了在砰击载荷作用下各种局部结构强度计算的方法与步骤。通过实例应用和分析,验证了方法的实用性。该方法可应用于船体局部结构的设计实践。     

砰击载荷作用下船体局部结构优化设计

本文以某船船首结构为研究对象,基于Fluent与Sesam分别计算了船体砰击压力系数和砰击瞬时船体与波浪之间的相对速度,采用"两步走"的方法对船首底部及外飘处的砰击载荷进行研究。考虑砰击载荷在船首区域的时空分布,将该载荷分区施加在船首结构表面,计算了船首结构在砰击载荷作用下的结构响应。研究发现,在船体纵向方向,越靠近船首的位置,相对入水速度越大;在垂向方向,入水速度呈现先增大后减小的趋势。根据计算结果对船首局部结构进行了优化设计。     

船首外飘砰击强度评估方法

通过分析三维板架入水砰击的结构动力响应,引入等效静力系数,建立板架结构入水砰击的等效静力分析方法。系统地改变计算参数,分析了冲击速度、入水角度、网格尺寸、边界条件等因素对板架应力峰值和等效静力系数的影响,进而提出了一种评估船首外飘结构砰击强度的方法。该方法可以反映砰击载荷作用下局部板架的动态效应,又避免在单纯利用板架模型评估时,板架边界设置对应力响应峰值的影响。     

船艏自由落体砰击载荷模型试验研究

本文对船艏自由落体砰击载荷进行了模型试验研究,根据不同的落体高度与入水角进行了多次试验,研究了落体高度、入水角等因素对入水速度、砰击载荷及结构响应的影响。研究结果表明:船体入水速度、砰击压力、结构响应等随着落体高度的增大而增大;模型入水过程中,各测量点砰击压力峰值发生时刻存在一定的时间差;结构响应峰值发生时刻也存在一定的时间差;由于斜升角较小,球鼻艏底部的砰击压力峰值最大;外飘区域的砰击压力最大值仅为球鼻艏底部的30%～50%;同一水线面上,从船艏模型艏端向艉端砰击压力峰值逐渐减小;对同一横剖面,外飘下部区域的砰击压力峰值大于外飘上部区域的砰击压力峰值;由于砰击压力对外界影响因素非常敏感,砰击压力与结构响应具有一定的离散性。     

空气层对高速三体船连接桥砰击压力峰值影响二维仿真研究

利用LS-DYNA仿真软件研究了高速三体船连桥结构的砰击问题,建立了二维有限元模型,对高速三体船结构以不同的速度进行等速入水的情况进行了计算.研究发现,存在于高速三体船主船体和辅船体与水之间的空气层充当了缓冲垫,大大减小了连接桥的砰击压力峰值.通过对压力峰值与速度平方比值的无量纲系数的回归分析,发现该系数随着入水速度的增加成二次指数递减趋势;其次是假想不存在空气层进行仿真计算,与考虑空气层的计算进行比较分析,量化空气层对高速三体船连接桥砰击压力峰值的影响,并得出随着砰击速度的增加,空气层对压力峰值影响逐渐变小.     

基于船波相对运动的船艏砰击仿真方法

在分析船波相对运动表达式的基础上计算船艏典型剖面的船波相对运动,探讨船艏入水过程中的砰击问题,对比船体某剖面3种入水仿真模型计算所得的砰击载荷,讨论三维外形和航行速度对船艏剖面砰击外载荷的影响。在该分析中,船体在规则波浪中的运动用基于三维势流理论的水动力软件AQWA计算获得,船波相对运动通过理论推导计算获得,用对船艏结构施加强迫运动的方式模拟船波相对运动的真实过程。采用An sys/Ls-Dyna软件的流固耦合分析进行入水仿真,流体划分为ALE体积网格,船艏视为刚体,划分为Lagrange有限元网格。对比结果表明：在三维模型中,相邻剖面引起剖面最大压力点处的液面变化对该点的砰击压力有增大效果,航速有增大剖面砰击压力的作用,减小船艏底部纵向斜升角有利于降低砰击压力。     

基于船波相对运动的船艏砰击仿真方法

在分析船波相对运动表达式的基础上计算船艏典型剖面的船波相对运动,探讨船艏入水过程中的砰击问题,对比船体某剖面3种入水仿真模型计算所得的砰击载荷,讨论三维外形和航行速度对船艏剖面砰击外载荷的影响。在该分析中,船体在规则波浪中的运动用基于三维势流理论的水动力软件AQWA计算获得,船波相对运动通过理论推导计算获得,用对船艏结构施加强迫运动的方式模拟船波相对运动的真实过程。采用An—sys／Ls—Dyna软件的流固耦合分析进行入水仿真,流体划分为ALE体积网格,船艏视为刚体,划分为Lagrange有限元网格。对比结果表明：在三维模型中,相邻剖面引起剖面最大压力点处的液面变化对该点的砰击压力有增大效果,航速有增大剖面砰击压力的作用,减小船艏底部纵向斜升角有利于降低砰击压力。     

船首外飘砰击设计载荷直接计算

文章结合三维线性势流理论和砰击速度的长期分析方法,求解出船体外飘位置的设计砰击速度;以首垂线和静水面交点处的设计砰击速度为目标值,给出了用于确定船首外飘砰击设计载荷的等效设计波,进而得到了设计状态下船体外飘剖面与波面相对运动关系;将船体剖面与波面间的相对运动关系等效转化为船体剖面与静水面的相对运动,利用显式有限元方法实现了外飘剖面砰击设计载荷的预报。针对直接计算方法中涉及的设计砰击速度、砰击压力和砰击压力系数,对比分析了文中结果和相应的规范值或试验值,论证了文中船舶外飘砰击压力设计载荷直接计算方法的合理性。     

波浪作用下三维楔形体入水砰击数值模拟

当船舶航行于恶劣海况时,船舶会发生砰击现象。砰击现象是指船体发生剧烈的摇荡运动导致出水并再次入水,由于船舶入水砰击是瞬态过程,所以会在短时间内产生巨大的砰击压力,造成船体的变形甚至失效,因此准确预报入水砰击压力对保证船舶安全航行和作业具有重要意义。本文建立三维楔形体模型来模拟船首部位,结合有限体积法与动网格技术,引入VOF模型,数值模拟了波浪作用下不同刚度三维楔形体垂直入水的过程。研究发现不同刚度的三维楔形体分别入水的过程中,弹性结构入水砰击压力的峰值要小于刚性结构,弹性效应会一定程度减缓砰击的发生,为今后工程实践提供有价值的参考。     

Benchmark study on slamming response of flat-stiffened plates considering fluid-structure interaction

This paper presents a benchmark study on the slamming responses of offshore structures’ flat-stiffened plates. The objective was to compare the fluid-structure interaction (FSI) simulation methodologies, modeling techniques, and established researchers' experiences in predicting slamming pressure. Three research groups employing the most common commercial software packages for numerical FSI simulations (i.e. LS-Dyna ALE, LS-Dyna ICFD, ANSYS CFX, and Star-CCM+/ABAQUS) participated in this study. Wet drop test data on flat-stiffened aluminum plates of light-ship-like bottom structures available in the open literature was utilized for validation of the FSI modeling. A summary of the experimental conditions including the geometry model and material properties, was distributed to the participants prior to their simulations. A parametric study on flat-stiffened steel plates having actual scantlings used in marine installations was performed to investigate the effect of impact velocity and plate rigidity on slamming response. The FE simulation results for the total vertical forces acting on the stiffened plates and their structural responses to those forces, as obtained from the participants, were analyzed and compared. The reliable and accurate predictions of slamming loads using the aforementioned commercial FSI software packages were evaluated. Additionally, equivalent static slamming pressures resulting in the same permanent deflections, as observed from the FSI simulations, were reported and compared with analytical models proposed by the Classification Standards DNV and existing experimental data for calculation of the slamming pressure. The study results showed that the equivalent load model depends on the water impact velocity and plate rigidity; that is, the equivalent static pressure coefficient decreases with an increase in impact velocity, and increases when impacting structures become stiffer.     

Prediction of slamming pressure considering fluid-structure interaction. Part II: Derivation of empirical formulations

This is Part II in a series of papers. Part I [1] investigated the slamming responses of flexible flat stiffened steel and aluminum plates using the nonlinear explicit finite element code LS-Dyna with the Multi-Material Arbitrary Lagrangian-Eulerian (MMALE) solver. Subsequently, a simplified finite element FSI model of water hitting structures that is realistically close to the slamming phenomenon occurring on the bottom part of offshore structures was proposed. The proposed FSI methodology presented in Part I was verified by comparison with the relevant test data. It was evident that the use of the proposed numerical method presented in Part I was very effective for a benchmarking investigation of slamming load considering the hydroelastic effect. However, the method required much effort in terms of computation time and power analysis resources. The present study, Part II, aimed, as an alternative to the FSI analysis approach, to develop empirical formulae for prediction of slamming loads acting on deformable flat stiffened plates used in marine applications. This paper begins by describing the limitations of the existing approaches based on theoretical, experimental and even numerical studies conducted in the past for estimation of slamming loads. Next, it presents, based on the simulation methodology developed in Part I, rigorous parametric studies that had been performed on actual scantlings of marine-seagoing structures. The effects of structural geometry and water impact velocity on slamming pressure are then investigated in detail. Subsequently, the parametric results are analyzed and utilized to derive empirical formulations for the prediction of slamming loads acting on flat stiffened plates of marine structures. The accuracy and reliability of the proposed formulations are established by comparison with the results of the test and other existing formulations. The proposed formulations are expected to be used for the purposes of the design without any time-consuming FSI analysis of advanced and optimal structures that are robust to slamming.     

Uncertainty due to discretization on the ALE algorithm for predicting water slamming loads

Numerical uncertainty due to discretization on the Arbitrary Lagrangian-Eulerian (ALE) Finite Element method is investigated in the study. The paper quantifies uncertainty using two ITTC recommended methods, and also applies a constant Courant-Friedrichs-Lewy (CFL) number based discretization approach, instead of performing the independent grid and time-based discretization recommended by ITTC. As a case study, water entry of a flat bottom rigid and flexible plate is simulated considering various entry velocities. The total slamming loads and structural responses on both the rigid and elastic bottom plates are predicted and validated against available experimental data. Results indicate that numerical errors due to discretization differ in the various parameters and from case to case. They do affect the analysis of slamming loads and associated structural responses, and the hydroelasticity analysis as well. The hydroelasticity effects on the slamming force generally increase as the entry velocity increases, however, the quantitative results differ much for models with different grids. For example, when the hydroelasticity effect is estimated using the finer model, the deviation of the total slamming force on the elastic plate relative to the one on the rigid body are 56%, 57%, and 63% respectively for the three constant entry velocities, whereas the estimations are −27%, −4% and 3% with the coarser model. The study concludes that the uncertainty due to discretization in ALE is not just case-specific, but also parameter specific. The uncertainty quantification procedures with a constant CFL number based refinement are recommended to investigate the uncertainty comparing to the individual grid and time step study, in particular for the ALE solution where the time step is adjusted automatically as the grid changes. Thus, consideration should be given to updating the ITTC guidelines to incorporate the constant CFL based discretization approach.     

基于混合两步法的船舶砰击载荷预报研究

文章针对船舶结构设计时重点关注的艏部砰击载荷问题,综合考虑计算效率及精度,提出了基于势流理论和计算流体力学方法的混合两步法。第一步,采用三维势流理论预报波浪中有航速船舶的运动响应,分析船波相对运动;第二步,根据预报砰击载荷所在位置的船体横剖面,建立等截面的三维立体模型,采用基于有限体积法与动网格技术的计算流体力学方法,基于第一步得到的相对运动结果模拟落体入水过程,计算砰击压力。使用该两步法预报了某超大型油轮在压载工况顶浪航行时候的艏部砰击压力,并讨论了相对运动和砰击压力的时域历程规律。文中数值预报结果得到了水池模型实验的验证,表明该方法的可行性和预报结果的合理性。     

满足油船共同结构规范的船底砰击加强分析

共同结构规范(CSR)对油船船底砰击的结构评估提出了明确而严格的要求,通过对规范条款的分析,归纳总结了CSR对船底砰击加强的具体要求,阐述了有效控制砰击、加强结构重量的手段。并以某Aframax型油船为例,从结构重量和施工工艺两方面,对抵御砰击压力的船底结构进行不同加强方案的比较分析。     

舰船主体结构流体冲击响应主动控制方法和分析

本文以探索性的方式提出了一种有效降低高速舰船主体结构流体冲击瞬态响应的主动最优控制方法,讨论了最优控制原理、观测器和作动器的设计,建立了一套舰船主体结构流体冲击瞬态响应控制仿真系统。通过对一艘实船的仿真分析给出了一些有理论和实际意义的结论。     

结构模糊可靠性分析

结构破坏准则实际上是带有模糊性的，要合理判断结构强度必须依靠模糊概率方法。本文采用基于模糊判决的限界搜索法，给出了计算结构模糊可靠性清晰解的方法。以受集中力的两端刚固梁和船首底板砰击强度为例进行了数值计算，并讨论了容差变化对模糊可靠度指标的影响。     

基于有限元方法的船首底部结构砰击强度分析

船艏底部作为整船中的重要受力区域,船舶航行时,其与波浪会发生相对碰撞,即产生砰击现象,从而对船体的结构安全造成隐患。本文以某型半潜运输船为例,结合中国船级社(CCS)规范,根据Ochi-Mottor理论进行砰击压力极值计算,并采用有限元方法对该型船舶艏部区域的结构强度进行分析。     

极端波浪与海洋结构物的强非线性作用研究综述

鉴于极端波浪的极大破坏力,其与海洋结构物的强非线性作用研究正日益受到重视。为了评估极端波浪可能带来的严重破坏,有必要对极端波浪作用下海洋结构物的波浪爬升与抨击、强非线性波浪力、结构载荷与运动响应等问题开展深入研究。国内外许多学者采用数值计算、模型实验及小波分析等手段对这些问题开展了探索研究,获得了一些有益的研究结论。该文对极端波浪与海洋结构物相互作用的研究现状和现有结论作了综述,可为进一步开展深入研究提供有益参考。