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。     

淹没丁坝附近三维水流运动大涡数值模拟

通过大涡数值模拟研究了丁坝附近水流特性。在数值模拟过程中基本方程采用大涡模拟方程,由Smagorinsky模型得到小尺度涡粘性系数,在此基础上给出了大涡模拟数值计算方法;并利用试验观测数据,验证了数值模拟结果,同时比较了上挑、正挑、下挑3种淹没的不透水丁坝附近水流特性     

规则波中参数横摇直接稳性评估方法研究

国际海事组织(IMO)正在制定第二代完整稳性衡准,其中参数横摇是非常重要的稳性失效模式。为提供一种计算可靠且便于工程应用的参数横摇直接稳性评估方法,该文采用一种实用的三维时域方法进行规则波中船舶参数横摇运动研究。在该方法中,应用组合Rankine源和时域格林函数的混合源法求解辐射势和绕射势,对船体平均湿表面进行压力积分求解辐射力和绕射力,而Froud-Krylov力和静水力是通过对船体瞬时湿表面压力积分获得的,同时考虑垂荡-横摇-纵摇三自由度之间的耦合作用,以及非线性横摇阻尼的影响。以国际标模C11集装箱船为研究对象,数值分析不同航速和波陡条件下的参数横摇运动规律,将计算结果与已有的试验结果进行了对比,并依据IMO提案中关于参数横摇的定量验证标准进行准确性评估,评估结果为IMO参数横摇直接稳性评估方法的制定提供了相关技术支撑。     

基于神经网络的船体剖面水动力机器学习与验证

该文以船体剖面水动力为研究对象,梳理了二维剖面几何特征和频率等与水动力关联关系。基于FFD方法生成系列船体剖面,并通过格林函数法计算出水动力系数形成相应的数据库。建立两种不同参数描述的几何剖面和水动力学习模型,采用神经网络建模训练获得了两种剖面水动力预报模型,对船体剖面横荡、垂荡、横摇和横荡-横摇四个模态的附加质量与阻尼系数进行预报,并将模型预测值与真实值、以及通过切片理论获得的全船垂荡水动力系数进行比较验证。研究表明,该模型能高效预报船体水动力,精度较高泛化能力强,为不同参数模型预报船舶水动力,为船舶设计的不同阶段提供了手段。     

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.     

水下航行体壁面脉动压力的大涡模拟研究

湍流壁面脉动压力是重要的水动力噪声源,开展相应的计算与试验研究十分必要。作者在大涡模拟的理论框架下,结合精细网格生成技术,对于水下航行体的壁面脉动压力进行了数值预报。首先,详细描述了所使用的大涡模拟方程与动态Smagorinsky亚格子模型,介绍了离散求解的数值方法。其次,利用大涡模拟计算了SUBOFF模型主体与附体上的表面压力分布,并利用试验结果进行了验证,分析了大涡模拟方法计算定常流动的可靠性。再次,计算了平板的湍流壁面脉动压力,并与CSSRC的消音风洞试验结果进行了对比,分析了大涡模拟方法计算非定常流动的可靠性。最后,对于水下航行体模型主体上三个位置与围壳上四个位置处的壁面脉动压力进行了大涡模拟,得到了脉动压力1/3OCT频谱,分析了频谱高频与低频特性以及衰减特性,并用试验结果进行了验证。研究表明,本文建立的水下航行体壁面脉动压力的数值预报方法是可靠的。     

均匀流绕旋转圆柱分离流动的数值模拟

本文利用离散涡模型与边界层理论相结合的方法，研究了高雷诺数下，均匀流绕旋转圆柱的分离流动以及有关动力学特性，并验证了Magnus效应，数值模拟过程中，边界层方程用Keller盒式法求解，离散涡采用涡量均匀分布的圆形涡团模型并以涡核半径随时间增长来模拟涡旋的粘性扩散效应。在计算绕旋转圆柱的流动中，无须人为地引进非对称扰动，到一定时候就能自动形成交替脱落的涡街，旋转圆柱周向速度与来流速度比值α的变化范围为0.05-0.3。计算所得升阻系数、分离点的位置以及压力分布与理论和实验结果相符。     

具有垂荡板的Spar平台流体力学特性的数值模拟研究

该文采用大涡模拟的方法对具有垂荡板的Spar平台振荡运动进行了模拟与计算。验证了数值模拟方法的可靠性,并分析研究了垂荡板的厚径比与运动振幅对其水动力系数的影响。结果表明,阻尼系数与垂荡板Keulegan–Carpenter(KC)数呈正相关关系。对垂荡板周围的涡结构进行研究,验证了垂荡板周围的涡结构可分为交互式、独立式与互逆式三类。三种不同的涡结构取决于垂荡板的厚径比与数。在此基础上提出三种涡结构的分类界限,总结出不同涡结构情况下,Spar平台无因次阻尼系数的经验公式。     

方尾船在浅水中兴波的数值研究

本文从含移动压力的浅水波方程出发，计算方尾船在浅水中的兴波。船的运动看成对自由水面的压力扰动，直接加到浅水方程组。采用运动网格的有限元法求解浅水波动方程。在船体周围的开边界上使用无反射的边界条件。本文给出三维自由水面波高图、深度平均的水平流速以及方尾船兴波阻力系数、船体沉浮量、纵倾角的数值结果。从物理意义的定性分析来看，数值结果是合理的，计算方法是有效的。计算结果表明，兴波阻力系数、船体沉浮量、纵     

基于MPS方法数值模拟三维LNG液舱的晃荡问题

该文基于自主开发的无网格粒子方法求解器MLParticle-SJTU,将MPS方法(moving particle semi-implicit)应用到三维LNG液舱的晃荡问题中,分别研究了LNG液舱在单自由度纵摇和横摇激励下,装载高度和激励频率对液舱晃荡作用的影响。首先,对装载高度为70%H的LNG液舱在单自由度纵摇和横摇激励下的晃荡问题分别进行了数值模拟,并将计算结果与实验结果进行比较,验证了MPS方法的可靠性。其次,对比不同装载高度,不同激励频率下液舱晃荡的拍击压力和流场分布情况,分析了装载高度和激励频率对晃荡作用的影响,并分析了一阶固有频率和共振频率随装载高度的变化规律。数值结果表明:激励频率对晃荡幅度影响较大;当激励频率在一阶固有频率附近时,会发生剧烈的晃荡现象;52.5%H为危险装载高度,在共振频率下,产生的拍击压力最大。     

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.     

不可压缩旋转涡对的流噪声计算研究

相隔一定距离的两个同向旋涡相互诱导而形成绕中点旋转的涡对,进而诱发流噪声。该文基于不可压缩流场提供的非稳态流动信息,采用黏声分离法研究水流场中旋转涡对的声学特性。计算结果同理论值进行比较,并探讨了计算条件对计算稳定性和准确性的影响。计算结果表明:在选取合适的计算参数(人工阻尼、网格尺寸和时间步长)的情况下,黏声分离法方法适用于预测水流噪声。对不可压缩旋转涡对的声场特性进行分析,得到旋转涡对声压图案类似四极子,在远场接近圆柱波;随着旋转马赫数的增加,声压级增大。     

Direct calculation method of roll damping based on three-dimensional CFD approach

The effect of roll damping on the prediction of a ship's motion is significant. A method based on the 3-D CFD approach for calculating the roll damping of a ship is proposed in this paper. The free decay experiments of 4 different types of ships at zero velocity are carried out. The roll damping coefficients are calculated and compared with the experiment results, with a good agreement. It is shown that the method adopted in this paper could be used to simulate the free rolling of a ship in the calm water at zero velocity very well and with good, stable accuracy for the prediction of the roll damping, and also with good applicability. This method may be further developed for the prediction of a full scale ship in the future.     

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.     

A numerical study on dispersion of particles from the surface of a circular cylinder placed in a gas flow using discrete vortex method

The dispersion of particles emitted from the surface of a circular cylinder placed in a gas flow at the Reynolds number of 200 000 is numerically investigated using the discrete vortex method coupled with a Lagrangian approach for solid particle tracking. The wake vortex patterns, the temporal-spatial distributions and trajectories as well as the dispersion functions for particles with various Stokes numbers（St） ranging from 0.001 to 1.0 are obtained. The numerical results reveal that：（1） Solid particles on the cylinder surface are picked up and then transported away from the cylinder by the wake vortex flow.（2） Solid particles emitted from the cylinder surface always follow the vortices in the cylinder wake, and the response of particles to wake vortices is directly related to their Stokes numbers（particles with St= 0.001, 0.0038, 0.01 can distribute both in the vortex core and around the vortex periphery, whereas those with St= 0.1, 1.0 can not enter the vortex core and congregate mainly around the vortex periphery）.（3） The particles move in rolling state in the wake region, and the dispersion intensity of particles in the lateral direction decreases remarkably as the Stokes number of particles is increased from 0.001 to 1.0.     

FORCE REDUCTION OF FLOW AROUND A SINUSOIDAL WAVY CYLINDER

A large eddy simulation of cross-flow around a sinusoidal wavy cylinder at Re=3000 was performed and the load cell measurement was introduced for the validation test. The mean flow field and the near wake flow structures were presented and compared with those for a circular cylinder at the same Reynolds number. The mean drag coefficient for the wavy cylinder is smaller than that for a corresponding circular cylinder due to the formation of a longer wake vortex generated by the wavy cylinder. The fluctuating lift coefficient of the wavy cylinder is also greatly reduced. This kind of wavy surface leads to the formation of 3-D free shear layers which are more stable than purely 2-D free shear layers. Such free shear layers only roll up into mature vortices at further downstream position and significantly modify the near wake structures and the pressure distributions around the wavy cylinder. Moreover, the simulations in laminar flow condition were also performed to investigate the effect of Reynolds number on force reduction control.     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

MODELLING AND COMPUTATION OF UNSTEADY TURBULENT CAVITATION FLOWS

1. INTRODUCTION Cavitatating flows are important phenomena happened in industry setups and many under-water vehicles. It leads to performance losses and modifications of blade load when happened in machines, especially turbo-machinery. Nevertheless, cavit…