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.     

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.     

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。     

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。     

Numerical study of roll motion of a 2-D floating structure in viscous flow

In the present study,an open source CFD tool,Open FOAM has been extended and applied to investigate roll motion of a 2-D rectangular barge induced by nonlinear regular waves in viscous flow.Comparisons of the present Open FOAM results with published potential-flow solutions and experimental data have indicated that the newly extended Open FOAM model is very capable of accurate modelling of wave interaction with freely rolling structures.The wave-induced roll motions,hydrodynamic forces on the barge,velocities and vorticity fields in the vicinity of the structure in the presence of waves have been investigated to reveal the real physics involved in the wave induced roll motion of a 2-D floating structure.Parametric analysis has been carried out to examine the effect of structure dimension and body draft on the roll motion.     

A new methodology for the CFD uncertainty analysis

With respect to the measurement uncertainty, this paper discusses the definition, the sources, the classification and the expressions of the CFD uncertainty. Based on the orthogonal design and the statistics inference theory, a new verification and validation method and the related procedures in the CFD simulation are developed. With the method, two examples of the CFD verification and validation are studied for the drag coefficient and the nominal wake fraction, and the calculation factors and their interactions which would significantly affect the simulation results are obtained. Moreover, the sizes of all uncertainty components resulting from the controlled and un-controlled calculation factors are determined, and the optimal combination of the calculation factors is obtained by an effect estimation in the orthogonal experiment design. It is shown that the new method can be used for the verification in the CFD uncertainty analysis, and can reasonably and definitely judge the credibility of the simulative result. As for CFD simulation of the drag coefficient and the nominal wake fraction, the results predicted can be validated. Although there is still some difference between the simulation results and the experiment results, its approximate level and credibility can be accepted.     

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.     

ARIMA与ANN组合预测模型在中长期径流预报中的应用

基于时间序列预测模型及BP神经网络,提出了新的组合预测方法.该方法采用三层结构的BP神经网络来构造组合预测模型,运用时间序列模型预测方法得出的预测结果,采用历史滚动法将前5年的预测结果数据作为BP网络的输入,以当前年份的预测结果为网络期望输入,建立了ARIMA-ANN组合预报模型.利用Matlab7神经网络工具箱对塔里木河上游源流卡群水文站的年径流量进行了预报及验证.结果表明:组合模型的预报结果精度高,容错能力强,是中长期径流预报的有效方法.     

Efficient data assimilation method based on chaos theory and Kalman filter with an application in Singapore Regional Model

Data assimilation is a methodology that utilizes information from observations, and assimilates it into numerical models, with the intention of improving the quality and accuracy of the model outputs. This paper introduces a hybrid data assimilation scheme, which combines a temporal error prediction algorithm based on local model (LM) inspired by chaos theory and a spatial error distribution scheme through the propagation of error covariances derived from the Kalman filter (KF). Local model is only capable of predicting the model errors at the stations where the observations are available, while the effect of traditional Kalman filter is limited to a time horizon where the improved initial conditions are washed out. The hybrid scheme outlined in this paper is performed in two steps: (i) predicting the model errors at the measurement stations using the local model approach and (ii) distributing the predicted errors over the computational domain using the Kalman filter. Incorporating error distribution with error prediction assimilates limited information from the observed data into non-measurement stations. Therefore all stations of interest are able to be benefited. The proposed hybrid scheme has been implemented in the Singapore Regional Model (SRM) constructed by Delft3D modelling system, with the improvements in the assimilated characteristics discussed in detail.     

数值波浪水池中航行船舶辐射问题的数值模拟

基于CFD方法,建立了数值波浪水池,就Wigley-Ⅲ船模在不同航速下摇荡运动辐射问题进行了数值模拟,给出了一种求解航行船舶辐射运动的水动力系数的方法.计算分析了船体相关模态运动的附加质量与阻尼,与势流理论结果、DUT(Delft University of Technology)的试验数据进行了比较,吻合良好.对比和研究表明,本文的方法比实验更易实现和控制,能细致描述船舶周围的流场,在船舶水动力性能的分析预报等方面具有广阔的应用前景.     

Study of a Compressed Air Vessel for Controlling the Pressure Surge in Water Networks: CFD and Experimental Analysis

Air chambers show good ability in controlling the pressure surge from a water hammer (WH) phenomenon. To simulate an air chamber and study the behavior of air inside it, a compressed air vessel (CAV) is considered in a pressurized system. The current work consists of experimental tests and one-dimensional (1D) and two-dimensional (2D) computational fluid dynamics (CFD) simulations for an air pocket within a CAV in the case of rapid pressurization and the occurrence of WH in a pressurized system. The pressure variations create vorticity and turbulence with oscillating behaviors, but the available 1D models are unable to simulate those phenomena adequately. Therefore, by using the measured data, proper CFD analysis is conducted considering the effect of the wall, y ⁺ , mesh size, turbulence, and the wall treatment method to better understand the behavior of the system. Results of the CFD simulation show that realizable k-ε turbulence model, when coupled with the enhanced wall treatment (EWT) method, works adequately for modeling the pressure oscillation. The volume of fluid (VOF) model and the piecewise linear interface calculation (PLIC) method have presented good ability in the prediction of the air-water interface.     

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.     

随浪中船舶复原力计算和试验研究

目前国际海事组织第二代完整稳性衡准正在制定中,纯稳性丧失被列入五种稳性失效模式之一,随浪中复原力变化是纯稳性丧失薄弱性衡准和稳性直接评估的关键因素。该文以C11集装箱船为对象,对随浪不同波浪条件下的横摇复原力进行了试验测量,得到了不同横摇角度时的复原力变化,分析了不同波浪条件下复原力的变化规律;并开发波浪中复原力计算方法和程序,对随浪中船舶横摇复原力进行了计算分析,最后和试验结果进行了对比,有效验证了计算方法的准确性和有效性,为纯稳性丧失直接评估方法的实现奠定了基础,为船舶第二代完整稳性衡准的建立提供了技术支撑。     

Numerical estimation of bank-propeller-hull interaction effect on ship manoeuvring using CFD method

This paper presents a numerical investigation of ship manoeuvring under the combined effect of bank and propeller. The incompressible turbulent flow with free surface around the self-propelled hull form is simulated using a commercial CFD software(ANSYS-FLUENT). In order to estimate the influence of the bank-propeller effect on the hydrodynamic forces acting on the ship, volume forces representing the propeller are added to Navier-Stokes equations. The numerical simulations are carried out using the equivalent of experiment conditions. The validation of the CFD model is performed by comparing the numerical results to the available experimental data. For this investigation, the impact of Ship-Bank distance and ship speed on the bank effect are tested with and without propeller. An additional parameter concerning the advance ratio of the propeller is also tested.     

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.     

横流中有限宽窄缝射流的旋涡结构

本文应用PIV流动显示技术详细测量了有限宽度窄缝射流从底部射入横流的流场结构。在此基础上采用湍流数学模型及相应的混合有限分析方法，得到横流中有限宽窄缝射流的时均及瞬态流场、涡量场，以此来分析流场、涡量场特性，揭示各种具有拟序结构性质的复杂涡系的产生、发展及演化过程。实验观察到射流迎、背流剪切涡层卷起形成类开尔文涡列。本文还探讨了横流绕流分离旋涡的生成机理及其转向。另外，观测到射流喷口上游的马蹄涡系以及射流逆流区下游尾迹区旋涡的层状结构形式。通过拓扑分析的方法，给出了横流中有限宽窄缝射流旋涡近区流场的拓扑性质。     

智能碾压监测系统在平寨水库堆石坝的应用

在平寨水库大坝填筑施工中,采用了智能碾压监测系统。本文详细论述了该系统工作机理,并对监测的铺料厚度、碾压遍数、碾压搭接、行走速度等碾压参数与现场挖坑试验的干密度值相关性和趋势性进行了分析,为平寨水库大坝填筑质量过程控制提供可靠依据。     

船舶横摇的参数激励及对安全性的影响

本文研究了横浪情况下的横摇参数激励问题，在时域域范围内求解了横摇参数激励方程，分析了参数激励对横摇运动响应和船舶安全性的影响。