Numerical simulation of rolling for 3-D ship with forward speed and nonlinear damping analysis

Strongly nonlinear characteristics of ship roll owing to viscous effect can be usually observed. To describe the nonlinear roll behavior, the CFD method has been frequently employed with obvious advantages compared with the traditional semi-empirical formula method in estimating the roll damping. Numerical simulations of free decay and forced rolling at various forward speeds and amplitudes for a 3-D ship hull are conducted in the present research to predict ship roll damping, in which a RANS solver is employed and a dynamic mesh technique is adopted and discussed in detail. Numerical results, including nonlinear flow characters around ships, rolling decay curves and damping coefficients, show that they are all in good agreement with available experimental data. The linear and nonlinear damping coefficients are estimated and analyzed by fitting with exponential functions for various rolling amplitudes, frequencies and speeds in the free decay simulations, and the damping coefficients are obtained by a polynomial fitting in the forced roll simulations. It is indicated that the damping coefficients increase with increasing rolling angle amplitude and velocity. It is also emphasized that the effect of forward speed is significant to roll damping and the nonlinear damping decreases with increasing velocity.     

三体船在斜浪规则波中运动响应预报方法研究

对于三体船在波浪中的水动力预报,航速效应和主体与片体间水动力干扰效应是水动力载荷合理预报的关键之处,需合理考虑.二维半势流理论可较为准确反映上述效应,已在三体船顶浪中垂向运动响应理论预报上获得了较为满意的结果.在此基础上,该文探讨了该方法在三体船斜浪中运动特别是横摇预报上的应用.为较准确获得三体船横摇运动时粘性阻尼贡献,采用三体船模静水中自由横摇衰减试验获得了自由横摇运动衰减曲线,并经能量法处理获得了船舶横摇阻尼系数.然后,结合二维半理论,获得了三体船运动数值解.数值解与斜浪规则波中运动响应模型试验结果符合较好,反映本文所采用的数值方法可较好地预测三体船斜浪中水动力载荷和运动,可用于三体船在波浪中耐波性评估.     

A study of hybrid prediction method for ship parametric rolling

The parametric rolling(PR) in the head or following waves has been considered as one of the main stability failure modes in the development of the 2nd generation Intact Stability criterion by the International Maritime Organization(IMO).According to previous studies,the estimation methods of the roll damping affect the prediction of the PR significantly,and most of them are based on experiment data or Ikeda’s empirical formula.The accuracy of the estimation method for the roll damping could be a key aspect for the validity of its prediction for the full scale ship.In this research,a hybrid prediction method is developed for the numerical prediction of the parametric rolling when experiment data are not available for the roll damping.Comparison study is also carried out between the hybrid method and a nonlinear dynamics method,where the roll damping is estimated by the simplified Ikeda’s method and the direct CFD prediction method in a direct non-linear simulation based on the 3-D CFD approach in the model scale.It is shown that the results of the hybrid method are in satisfactory agreements with the model experiment results,and the method can be used for analysis especially at the early design stage where experiment data are often not available.     

船体各种剖面的横摇阻尼与旋涡的形状

本文运用求解N－S方程的手法研究船体的横摇阻尼问题。利用作者原先提出的计算方法，数值模拟船体各种二维横剖面在横摇时的粘性流场。计算结果表明涡的位置与实验结果相当吻合，同时给出了横摇时各种横剖面的剪切应力与压力分布，比较各种不同剖面的旋涡形状可以发现，对于船舯附近的剖面，船体周围有两个涡，而对于船首或船尾附近的剖面，船体周围只有一个涡。利用这一数值模拟结果计算了包括粘性影响的横摇阻尼，计算结果表明横摇阻尼中压力成分比剪切应力成分要大。     

强迫运动柱体附加质量与阻尼系数的CFD计算

基于CFD的RANS方法,计入粘性和自由面的影响,模拟船舶剖面柱体的单自由度强迫摇荡运动,得到横荡、升沉、横摇及横荡对横摇的附加质量与阻尼系数,并与试验进行了比较,符合良好。两种不同网格进行试算比较表明:整体域移动网格的稳定性和计算效率要优于局部重划的非结构网格。最后,针对加装了舭龙骨的柱体进行强迫横摇运动模拟,求解大幅度横摇下的横摇阻尼,分析了舭龙骨尺度对横摇阻尼的影响。各种计算表明:无论是线性还是大幅度非线性运动,粘性CFD方法能有效地预报耐波性中各种运动模态的的水动力系数。     

浅窄航道船舶线性位置导数估算

本文采用简化的一元流分析方法，引入非均匀流的概念对浅水中的细长体理论作了修正，计算了船舶在浅窄航道中斜航时的线性位置导数，计算中对粘性影响作了适当修正。计算结果与试验值吻合较好，且本法简便，便于在船舶设计初始阶段对浅窄航道船舶线性位置导数的估算。     

Stability of a tumblehome hull under the dead ship condition

Some methods for direct stability assessment under the dead ship condition were currently developed by the international maritime organization(IMO)under the Second Generation Intact Stability Criteria.Model tests and simulations are carried out to validate the numerical methods used in assessing the stability under the dead ship condition.This is done in three stages.Firstly,the uncoupled roll mathematical model(1 DOF)is adopted to calculate the roll motion based on the irregular beam waves and the steady wind.Secondly,a drift free experiment is conducted to measure the roll motion under irregular beam waves with zero speed,and then two restrained experiments with counter weights and four springs are performed under the same condition.Finally,the effects of the drift and sway motions on stability under the dead ship condition are then verified by experimental results,and the results of the numerical methods are compared to the results of the model experiments.It is concluded that more accurate numerical methods could be developed for assessing the direct stability under the dead ship condition.     

NUMERICAL SIMULATIONS OF WAVE-INDUCED SHIP MOTIONS IN TIME DOMAIN BY A RANKINE PANEL METHOD

A time domain prediction of wave-induced ship motions by a Rankine panel method is investigated. Linear boundary conditions on free surface and mean wetted body surface are adopted, while the numerical damping method is used for the radiation conditions. The motions of two ships in regular head waves are computed by the present method. The related numerical results are compared with the experiment data and those from linear strip theory. The comparison shows satisfactory agreements for pitch and heave transfer functions.     

带自由液面的艇/桨干扰特性数值模拟与验证研究

带自由液面的艇/桨干扰特性研究是水动力学领域的重点与难点.此文采用数值模拟方法研究了潜艇近水面航行时的艇/桨干扰特性.首先介绍了计算方法:流场采用RANS方法结合RNGk -ε湍流模型求解;自由液面捕捉采用VOF方法;螺旋桨运转采用滑移网格方法;带螺旋桨潜艇表面与计算域内全部采用结构化网格进行离散.然后,对于SUBOFF潜艇模型在不同浸深下的阻力特性进行了数值模拟,分析了自由液面对于模型阻力的定量影响.最后,对于带螺旋桨的SUBOFF潜艇模型在深潜与近水面状态下的艇后推力、扭矩进行了数值模拟,分析了自航因子,研究了艇/桨干扰特性.计算结果与试验结果进行了详细的对比分析,验证了计算方法与计算结果的可靠性.本文的工作可为将来构建数值拖曳水池提供技术支撑.     

基于分位数回归的升船机变形监控模型构建方法

为深入分析升船机变形影响因素,提出了一种基于分位数回归的升船机变形监控模型构建方法。该方法根据升船机的结构特点,将温度、前期上游水位均值等因素引入候选影响因子集,采用自适应弹性网络分位数回归对影响因子进行筛选,建立各分位数下的回归模型,并根据拟合的良好性和检验的有效性原则选出最优的升船机变形监控模型。实例验证结果表明：相对于常规的逐步回归模型,本文方法构建的最优模型的预测精度波动性小,具有较强的稳定性,同时具有良好的长期预测能力。     

URANS simulations of ship motion responses in long-crest irregular waves

In this paper, numerical prediction of ship motion responses in long-crest irregular waves by the URANS-VOF method is presented. A white noise spectrum is applied to generate the incoming waves to evaluate the motion responses. The procedure can replace a decade of simulations in regular wave with one single run to obtain a complete curve of linear motion response, considerably reducing computation time. A correction procedure is employed to adjust the wave generation signal based on the wave spectrum and achieves fairly better results in the wave tank. Three ship models with five wave conditions are introduced to validate the method. The computations in this paper are completed by using the solver naoe-FOAM-SJTU, a solver developed for ship and ocean engineering based on the open source code OpenFOAM. The computational motion responses by the irregular wave procedure are compared with the results by regular wave, experiments and strip theory. Transfer functions by irregular wave closely agree with the data obtained in the regular waves, showing negligible difference. The comparison between computational results and experiments also show good agreements. The results better predicted by CFD method than strip theories indicate that this method can compensate for the inaccuracy of the strip theories. The results confirm that the irregular wave procedure is a promising method for the accurate prediction of motion responses with less accuracy loss and higher efficiency compared with the regular wave procedure.     

用时域法求解二维二阶非线性水波的绕射问题

本文用二阶理论在时期围内研究非线性水波的绕射问题。一、二阶问题分别满足各自的自由表面条件和物面条件。用一阶Ａｄａｍｓ格式并配合时步处理来计算自由表面上的波高和速度势，而流场内的速度势通过求解有限元方法得到。采用人工阻尼区来吸收反射波，对水平半圆柱在水面的绕射问题进行了计算，得到了自由表面波高时间历程和圆柱所受到的波浪力，计算结果和有关文献进行了比较，其符合程度是令人满意的。     

An iterative Rankine boundary element method for wave diffraction of a ship with forward speed

A 3-D time-domain seakeeping analysis tool has been newly developed by using a higher-order boundary element method with the Rankine source as the kernel function. An iterative time-marching scheme for updating both kinematic and dynamic free-surface boundary conditions is adopted for achieving numerical accuracy and stability. A rectangular computational domain moving with the mean speed of ship is introduced. A damping beach at the outer portion of the truncated free surface is installed for satisfying the radiation condition. After numerical convergence checked, the diffraction unsteady problem of a Wigley hull traveling with a constant forward speed in waves is studied. Extensive results including wave exciting forces, wave patterns and pressure distributions on the hull are presented to validate the efficiency and accuracy of the proposed 3-D time-domain iterative Rankine BEM approach. Computed results are compared to be in good agreement with the corresponding experimental data and other published numerical solutions.     

浮码头非线性撞击问题的样条加残配点法

本文在分析浮码头撞击的线性振动基础上，针对广泛采用的防冲设备－橡胶护舷，提出非线性动力响应的分析方法。对复合体系非线性动力方程应用样条加残配点法进行直接积分。考虑非线性阻尼，采用切线刚度及拉格朗日插值函数，给出撞击力及撑杆动内力的最大值，为工程应用提供有效方法。     

船体砰击问题的非线性边界元分析

本文运用边界元法分析外飘和Ｕ型船体载面的砰击问题，在分析过程中自由液面采用完全非线性边界条件以及计算自由液面的大变形，而边界单元则采用线性单元假设。数值计算表明，针对二维楔形体常速入水的结果与已有的结果吻合得很好，具有外飘和Ｕ型特性的船体截面在入水砰击过程中其压力分布及其演变具有完全不同的特性。     

Manoeuvring prediction based on CFD generated derivatives

This paper presents numerical predictions of ship manoeuvring motions with the help of computational fluid dynamics(CFD) techniques. A program applying the modular concept proposed by the Japanese ship manoeuvring mathematical modelling group(MMG) to simulate the standard manoeuvring motions of ships has been initially developed for 3 degrees of freedom manoeuvring motions in deep water with regression formulae to derive the hydrodynamic derivatives of the vessels. For higher accuracy, several CFD generated derivatives had been substituted to replace the empirical ones. This allows for the prediction of the maneuverability of a vessel in a variety of scenarios such as shallow water with expected good results in practice, which may be significantly more time-consuming if performed using a fully CFD approach. The MOERI KVLCC2 tanker vessel was selected as the sample ship for prediction. Model scale aligned and oblique resistance and Planar Motion Mechanism(PMM) simulations were carried out using the commercial CFD software Star CCM+. The PMM simulations included pure sway and pure yaw to obtain the linear manoeuvring derivatives required by the computational model of the program. Simulations of the standard free running manoeuvers were carried out on the vessel in deep water and compared with published results available for validation. Finally, simulations in shallow water were also presented based on the CFD results from existing publications and compared with model test results. The challenges of using a coupled CFD approach in this manner are outlined and discussed.     

波浪周期对系泊船舶运动影响试验研究

系泊船舶泊稳是港口设计和营运过程中的重要问题,系泊船舶泊稳标准从实质上讲就是限制系泊船舶的运动量。较长周期波浪作用下的系泊船舶泊稳问题愈来愈受到各方的关注,选取了一艘10万吨级散货船进行横向不规则波作用下的系泊模型试验,在同一波高下变化入射波浪周期,通过系列试验,分析波浪周期对系泊船舶横移、横摇和升沉运动的影响。研究结果表明,系泊船舶横移运动量总体上均随着波浪周期的增大而增大,横移运动对波浪周期的响应敏感程度是变化的;系泊船舶横摇和升沉运动量均随着波浪谱峰周期的增大先增大然后减小,当波谱谱峰周期与相应的船舶固有周期接近时,横摇和升沉运动量均出现了一次峰值。     

The dynamics of the floodwater and the damaged ship in waves

The interaction between the dynamics of the floodwater and the damaged ship in waves is investigated by applying an integrated method, which couples a seakeeping solver and a Navier-Stokes solver. To reveal the effects of the water flooding and the sloshing on the damaged ship behaviour, the motion of a Ro-Ro ferry in regular beam seas is simulated, including the ship under the intact condition with and without internal water and the ship under a damaged condition. It is found that the shift of the natural roll frequency of a damaged ship and the decrease of its roll response are mainly due to the water sloshing inside the compartment. The effect of the resonant sloshing leads to the presence of a ship's second peak response at higher frequencies and it is significantly reduced by the water flooding through the damaged opening. The influence of the flooding and the sloshing on the ship behaviour is small with a further increase of the wave frequency.     

AN ASYMPTOTIC SOLUTION OF VELOCITY FIELD IN SHIP WAVES

1. INTRODUCTION It is well known that behind the ships, V-shaped steady-state ship waves are created while ships move on the water. The classical Kelvin ship wave pattern, which is obtained when a ship is taken as a point impulse moving at a constant velo…     

Ship waves in rivers: Environmental criteria and analysis methods for measurements

A literature review shows that the most important physical quantities determining environmental impact of ship waves in a waterway are the fluid velocities, maximum and minimum water levels, and size of drawdown events. Fluid velocity can vary strongly in the vertical so that the usual measurements at a single point are not enough unless made near where the effects are most important, often the bed. Customary use of wave height as a measure of impact has been misleading, because the all-important fluid velocity is of a scale given by wave height divided by wave period. A good and simple estimate of the surface velocity as a disturbance scale is shown to be given by the time derivative of the free surface height. The most important role of linear wave theory is to explain and understand the physics and measurement procedures, such as done here in several places. Its use for obtaining numerical results is criticised. Instead, three integral measures of impact are proposed, all of which use surface elevation measurements and which require no essential mathematical approximations or wave-by-wave analysis. The methods are applied to a study of ship waves on the Danube River. A number of results are presented, including examination of the effects of measurement frequency. After a ship passage, due to repeated shoreline reflections of the wake waves, the river is brought into a long-lasting state of short-crested disturbances, with finite fluid velocities. The environmental consequences of this might be important. After the primary and secondary ship waves it could be called the tertiary wave system.