URANS simulations of static and dynamic maneuvering for surface combatant: part 1. Verification and validation for forces, moment, and hydrodynamic derivatives

Part 1 of this two-part paper presents the verification and validation results of forces and moment coefficients, hydrodynamic derivatives, and reconstructions of forces and moment coefficients from resultant hydrodynamic derivatives for a surface combatant Model 5415 bare hull under static and dynamic planar motion mechanism simulations. Unsteady Reynolds averaged Navier–Stokes (URANS) computations are carried out by a general purpose URANS/detached eddy simulation research code CFDShip-Iowa Ver. 4. The objective of this research is to investigate the capability of the code in regards to the computational fluid dynamics based maneuvering prediction method. In the current study, the ship is subjected to static drift, steady turn, pure sway, pure yaw, and combined yaw and drift motions at Froude number 0.28. The results are analyzed in view of: (1) the verification for iterative, grid, and time-step convergence along with assessment of overall numerical uncertainty; and (2) validations for forces and moment coefficients, hydrodynamic derivatives, and reconstruction of forces and moment coefficients from resultant hydrodynamic derivatives together with the available experimental data. Part 2 provides the validation for flow features with the experimental data as well as investigations for flow physics, e.g., flow separation, three dimensional vortical structure, and reconstructed local flows.     

URANS prediction of roll damping for a ship hull section at shallow draft

It is analytically difficult to calculate roll damping of ships due to the effects of viscosity. Therefore, computational fluid dynamics (CFD) has become a powerful tool in predicting roll damping recently. The unsteady flow around a forced rolling hull section with bilge keels can be calculated using a commercial URANS code which includes the viscous effects. In this study, two-dimensional (2D) roll damping calculations for a S60 midsection with bilge keels including free surface effects are performed for shallow draft case. The first objective of the study is to show whether the URANS code can be used to predict roll damping coefficient correctly. The second one is to show why Ikeda’s estimation method is insufficient at shallow draft case. Sinusoidal forced roll motion calculation method of roll damping moment with the help of a sliding interface and a fixed roll axis is successfully applied to predict roll damping coefficient. The calculations are carried out for different roll motion periods and amplitudes to validate the accuracy of the URANS code for different cases. Numerical results are compared with experiments, which were carried out at the towing tank facility of Osaka Prefecture University (OPU), and Ikeda’s estimation method. The results show that the URANS code is capable of predicting roll damping coefficients in a good agreement with experimental results and can be used further to develop a better model for prediction of roll damping.     

基于LES方法圆柱绕流三维数值模拟(英文)

采用计算流体软件CFX5中large.eddy simulation(LES)模型计算了均匀流场中三维圆柱绕流的水动力特性.使用有限体积法对三维N-S方程进行求解.数值模拟着重研究了高雷诺数时展向各截面的压力、阻力、升力及涡管特性.数值计算结果表明:展向各截面柱体受力关于中截面对称且小于二维情况,柱体周围流场呈现明显的三维特性.     

基于CFD的内河船深浅水中操纵性预报

由于岸壁效应和浅水效应,内河船舶在限制水域作操纵运动时通常受到比在开阔水域中更大的水动力.这些水动力对船舶操纵性具有不利影响,有可能导致船舶碰撞或触底等海上事故.因此,为了在船舶设计阶段预报其操纵性能,考虑浅水效应和岸壁效应以准确计算内河船舶操纵运动水动力非常重要.本文基于CFD方法,通过对粘性绕流进行数值模拟,对长江中营运的三艘内河船舶的操纵运动水动力进行计算.首先,为了验证数值方法的可靠性,对标模KVLCC2纯横荡和纯首摇试验的水动力进行计算,并将计算结果与现有的试验数据进行对比.然后,对三艘内河船舶在不同水深下的静舵试验、纯横荡和纯首摇试验进行数值模拟,计算得到水动力及相应的线性水动力导数.最后,基于计算得到的水动力导数,获得Nomoto模型中的操纵性参数,对比分析三艘内河船舶在深浅水中的操纵性能.结果表明,本文方法可以揭示不同水深下三艘内河船舶的操纵性变化趋势.该方法可为船舶设计阶段内河船舶深浅水中的操纵性预报提供一种实用的工具.     

Multi-fidelity optimization of a high-speed foil-assisted semi-planing catamaran for low wake

The wakes of high-speed passenger-only ferries that operated through Rich Passage, on the Seattle-Bremerton ferry route, caused beach erosion and damage to habitat. A task was initiated to design a low-wake high-speed vessel using multi-fidelity CFD based design optimization by using low-fidelity potential flow solvers for initial global design optimization and by using URANS solvers for high-fidelity tuning of the optimized design. This simulation based design process involved a close collaboration between ship designers, and hydrodynamics and CFD specialists, whose collective expertise guided the evolution of the design based on both hydrodynamic and structural aspects. The initial hull shape optimization using potential flow code was carried out by blending three different initial concepts provided by the designers. Subsequently, URANS was used to evaluate the potential flow optimized hull and to further optimize the hull configuration parameters, namely, the centre-of-gravity, demihull spacing, foil location, foil angle and slenderness ratio at different displacement conditions. The URANS based configuration optimization also took into account the far field wakes’ energy spectrum with an objective of reducing the energetic, low frequency far field wakes which are associated with beach flattening on the mixed sand and gravel beaches. Calculation of the far field wake using URANS would require an unfeasibly large domain size; therefore, a Havelock code with a source distribution matching the URANS calculated near field wave elevation was used to propagate the wakes into the far field. The end result of the optimization was a design with significantly reduced far field wake, which is currently being built for experimental testing.     

CFD validation studies for a high-speed foil-assisted semi-planing catamaran

This paper details the CFD validation studies carried out as a prerequisite for multi-fidelity CFD-based design optimization of high-speed passenger-only ferries aimed at reducing far-field wake energy that causes beach erosion. A potential flow program (WARP) and a URANS program (CFDSHIP) were validated using full-scale measurements of resistance, sinkage, trim, and far-field wake train obtained over a wide range of speeds for two high-speed semi-planing foil-assisted catamarans: Spirit (LOA-22 m) and 1060 (LOA-17 m). This study posed a unique combination of challenges for CFD modeling: the foil appended geometry required complicated surface overset grids, the effect of the waterjet and wind resistance had to be modeled, and a method had to be devised to extrapolate the calculated near-field elevation to get the far-field wake train using Havelock sources. A more concentrated effort was applied to the URANS verification and validation which forms the focus of this paper. The results show that URANS is able to accurately predict the resistance and motions for both vessels when coupled with models that account for the propulsors and air resistance. The overall accuracy of URANS for the performance analysis of the foil-assisted, semi-planing catamarans was adequate to warrant its use as a tool for subsequent design and optimization of a new vessel with significantly reduced wakes.     

肥大型船模操纵性水动力CFD预报的试验验证分析

肥大型船操纵性水动力的预报不仅是肥大型船操纵性设计多方案选型及优化的基础,而且是水面船操纵性水动力预报中的难点.文章探讨了CFD在肥大型船操纵性水动力预报方面的应用能力和预报精度,并进行了系列模型的CFD计算与验证分析,结果表明所建立的肥大型船操纵性水动力CFD数值预报方法具有良好的精度和普适性.     

Computational fluid dynamics (CFD) prediction of bank effects including verification and validation

Restricted waters impose significant effects on ship navigation. In particular, with the presence of a side bank in the vicinity of the hull, the flow is greatly complicated. Additional hydrodynamic forces and moments act on the hull, thus changing the ship's maneuverability. In this paper, computational fluid dynamics methods are utilized for investigating the bank effects on a tanker hull. The tanker moves straight ahead at a low speed in two canals, characterized by surface piercing and sloping banks. For varying water depth and ship-to-bank distance, the sinkage and trim, as well as the viscous hydrodynamic forces on the hull, are predicted by a steady state Reynolds averaged Navier–Stokes solver with the double model approximation to simulate the flat free surface. A potential flow method is also applied to evaluate the effect of waves and viscosity on the solutions. The focus is placed on verification and validation based on a grid convergence study and comparisons with experimental data. There is also an exploration of the modeling errors in the numerical method.     

无航速三体浮驳水动力特性频域分析

为了研究不同工作条件下无航速三体浮驳的水动力特性的变化规律,根据势流理论和波浪的辐射/衍射理论对无航速三体浮驳的水动力特性进行了分析,并以某三体浮驳为例应用水动力计算软件AQWA对其在不同工况下进行了水动力计算。分别求得了该三体浮驳在不同水深、不同吃水深度时的附加质量、附加阻尼、不同波向下的波浪激励力和幅值响应算子随波频的变化曲线,并对其进行了对比分析。结果表明,水深对三体浮驳附加质量、附加阻尼、波浪激励力和幅值响应算子均有较大的影响,特别是在低波频时影响较为显著;水深大于20倍吃水深度后三体浮驳的水动力系数可按照无限水深进行近似计算。吃水深度对三体浮驳纵荡和横荡幅值响应算子几乎无影响,对垂荡和纵摇幅值响应算子的影响较小,对横摇和艏摇幅值响应算子的影响较大。     

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。     

Numerical modelling of hydrodynamic responses of Ocean Farm 1 in waves and current and validation against model test measurements

With the continuous growing of the aquaculture industry and increasingly limited fish farming sites at close to shore areas both in Norway and worldwide, there is a need to develop fish farms suitable for aquaculture production in typical offshore environments. For this purpose, SALMAR has developed and deployed the Ocean Farm 1 facility for offshore fish farming.The main purpose of this paper is to develop a reliable numerical model and investigate the motion responses of the Ocean Farm 1 structure in waves and current. The established numerical model consists of the Ocean Farm 1's frame structure (with rigidly-connected circular column components), the net and the mooring system. The hydrodynamic external loads and coefficients of the frame structure are obtained by using potential flow theory. The quadratic drag load on the individual circular columns of the frame structure is formulated by a given drag coefficient. The loads on the net are formulated by using the screen model, where the Reynold number dependent lift and drag forces are formulated as a function of the solidity ratio Sn of the net, relative inflow angle and velocity. The hydrodynamic loads on the mooring lines are formulated using the Morison's equation and the structural responses of the mooring lines are obtained using a nonlinear FE model.With the developed numerical model, time domain simulations are performed. The simulation results are firstly validated against measured data from the decay tests, current tests, and regular wave tests. After the validation, numerical simulations are performed in different irregular wave and current combined weather conditions and the obtained motion response of Ocean Farm 1 are discussed and compared with available measurement data.     

自由面和岸壁对限制航道中船舶操纵性水动力的影响

本文在二阶抛物面离散后的船体表面单元上布置强度呈线性分布的Rankine源，在离散后的岸壁单元上和用贴体网格法离散船体附近自由面的单元上布置均匀Rankine源，在船体中纵面上布置强度在水深方向呈阶梯变化的附着涡线．计算了考虑自由而影响下船舶平行于岸壁航行时的水动力，还着重讨论了自由面和岸壁对限制航道中船舶水动力的影响．     

A stability analysis of tandem offloading systems at sea

In this article, we analyze the linear stability of tandem offloading systems in wind, current, and waves. The wind and current forces are evaluated with the help of published experimental data, while the hydrodynamic coefficients and wave drift forces are rigorously estimated by using a three-dimensional singularity distribution method based on potential theory. The bow hawser and mooring lines are described quasistatically by elastic catenary equations. In order to examine the linear static and dynamic stability of the system, the equations for surge, sway, and yaw are linearized. The effect of design parameters such as turret position, mooring stiffness, and hawser length and stiffness on stability is investigated based on linearized model. The stability analysis clarifies the mechanism of the limit cycle for tandem offloading systems, which is known as fishtailing motion. The theoretical results of the shape and amplitude of the limit cycle are found to be in good agreement with those of simulations and experiments.     

海上浮式结构物的水动力仿真研究

基于粘性计算流体力学的方法建立三维数值波浪水池,模拟有限振幅波的传播,并计算三维球体在规则波环境下所受的波浪力。采用两相不可压的RANS方程求解非定常不可压缩粘性流体,并采用流体体积函数(VOF)法对自由面进行动态模拟。通过编写用户自定义函数(UDF)设置边界入口速度和波高,实现在波浪水池尾部1~2倍波长区域消波,最终求出有限振幅波的模拟结果以及规则波中三维球体所受的波浪力,该结果与势流理论边界元法得到的结果在趋势上吻合良好。该研究方法为模拟分析其他海洋结构物在波浪中的水动力奠定基础,丰富与扩展了数值波浪水池的应用。     

三维竖轴潮流水轮机水动力性能研究

竖轴水轮机作为潮流能转换为电能的核心装置,其水动力性能的优劣将会直接影响到整体发电系统的效率。为了研究大型竖轴水轮机叶片安装角对水轮机水动力性能的影响,基于多参考系模型(MRF),采用Fluent软件对流场中的模型进行3D数值模拟。在转速和来流速度保持不变,改变安装角时,分析同种翼型5个不同安装角叶片对潮流能水轮机的水动力性能的影响。同时分析在同一安装角和旋转速度条件下,不同来流速度对水轮机水动力性能的影响。结果表明,叶片安装角对竖轴潮流水轮机的能量利用率影响较大,来流速度对水轮机叶片表面的静压力和输出功率具有一定的影响。研究结果对今后竖轴水轮机的设计和生产具有借鉴意义。     

The effects of free surface and end cell on flow around a finite circular cylinder with low aspect ratio

Vortex-induced motion is an oscillatory phenomenon which occurs to a floating body with low aspect ratio. The basic phenomenological study about the effects of free surface and end cell on flow around a finite fixed circular cylinder was investigated in this study using particle image velocimetry and hydrodynamic force measurement. It was found from the former experiment that the wake of the cylinder is influenced by the both end cell and free surface. Blowup and back flow are generated from the end cell, and their effects are suspended by free surface. The result of hydrodynamic force measurement showed the effect of Reynolds number, Froude number, and the aspect ratio of the floating body on the hydrodynamic force. Fluctuating components of hydrodynamic coefficients decrease for increasing Reynolds number, Froude number, and the aspect ratio. On the other hand, the mean drag coefficient increases as Froude number increases and decreases as the aspect ratio increases. The interpretation to these results was discussed in comparison with flow structures observed in the experiment. In addition, it was found that the effect of Reynolds number on the mean drag coefficient changes at different aspect ratios. A possible interpretation to this phenomenon was proposed.     

Analysis of unsteady flow over Offshore Wind Turbine in combination with different types of foundations

Environmental effects have an important influence on Offshore Wind Turbine (OWT) power generation efficiency and the structural stability of such turbines. In this study, we use an in-house Boundary Element (BEM)—panMARE code—to simulate the unsteady flow behavior of a full OWT with various combinations of aerodynamic and hydrodynamic loads in the time domain. This code is implemented to simulate potential flows for different applications and is based on a three-dimensional first-order panel method. Three different OWT configurations consisting of a generic 5 MW NREL rotor with three different types of foundations (Monopile, Tripod, and Jacket) are investigated. These three configurations are analyzed using the RANSE solver which is carried out using ANSYS CFX for validating the corresponding results. The simulations are performed under the same environmental atmospheric wind shear and rotor angular velocity, and the wave properties are wave height of 4 m and wave period of 7.16 s. In the present work, wave environmental effects were investigated firstly for the two solvers, and good agreement is achieved. Moreover, pressure distribution in each OWT case is presented, including detailed information about local flow fields. The time history of the forces at inflow direction and its moments around the mudline at each OWT part are presented in a dimensionless form with respect to the mean value of the last three loads and the moment amplitudes obtained from the BEM code, where the contribution of rotor force is lower in the tripod case and higher in the jacket case and the calculated hydrodynamic load that effect on jacket foundation type is lower than other two cases.     

用源分布方法求解浅水中二维任意剖面在自由表面上垂荡的附加质量和阻尼系数

用源分布方法求解了二维任意剖面在浅水中垂向振荡问题，计算了系列60方型系数0.7船型剖面在不同水深吃水比情况下的垂荡水动力系数，讨论了水深对水动力系数的影响，并与Frank无限水深的结果作了比较。研究结果表明，水深对二维船型剖面水动力系数具有较大的影响。这种影响在水深吃水比小于5.0时已相当明显，当水深吃水比小于2.0时将变得显著。另外，水深对水动力系数的影响与振荡频率有关，并且在低频处更为明显。     

通过水下储能系统的蓄电池单元的流:三个接触的似花结构的球状物(英文)

An LES simulation of flow over an accumulator unit of an underwater compressed air energy storage facility was conducted. The accumulator unit consists of three touching underwater balloons arranged in a floral configuration. The structure of the flow was examined via three dimensional iso surfaces of the Q criterion. Vortical cores were observed on the leeward surface of the balloons. The swirling tube flows generated by these vortical cores were depicted through three dimensional path lines. The flow dynamics were visualized via time series snapshots of two dimensional vorticity contours perpendicular to the flow direction; revealing the turbulent swinging motions of the aforementioned shedding-swirling tube flows. The time history of the hydrodynamic loading was presented in terms of lift and drag coefficients. Drag coefficient of each individual balloon in the floral configuration was smaller than that of a single balloon. It was found that the total drag coefficient of the floral unit of three touching balloons, i.e. summation of the drag coefficients of the balloons, is not too much larger than that of a single balloon whereas it provides three times the storage capacity. In addition to its practical significance in designing appropriate foundation and supports, the instantaneous hydrodynamic loading was used to determine the frequency of the turbulent swirling-swinging motions of the shedding vortex tubes; the Strouhal number was found to be larger than that of a single sphere at the same Reynolds number.     

Experimental investigation of TARMAX model for modeling of hydrodynamic forces on cylinder-like structures

A new approach that models lift and drag hydrodynamic force signals operating over cylindrical structures was developed and validated. This approach is based on stochastic auto regressive moving average with exogenous (ARMAX) input and its time-varying form, TARMAX. Model structure selection and parameter estimation were discussed while considering the validation stage. In this paper, the cylindrical structure was considered as a dynamic system with an incoming water wave and resulting forces as the input and outputs, respectively. The experimental data, used in this study, were collected from a full-scale rough vertical cylinder at the Delft Hydraulics Laboratory. The practicality of the proposed method and also its efficiency in structural modeling were demonstrated through applying two hydrodynamic force components. For this purpose, an ARMAX model is first used to capture the dynamics of the process, relating in-line forces provided by water waves;secondly, the TARMAX model was applied to modeling and analysis of the lift forces on the cylinder. The evaluation of the lift force by the TARMAX model shows the model is successful in modeling the force from the surface elevation.