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.     

基于拖曳锚轨迹方程的拖曳锚系泊优化分析

拖曳锚系泊作为目前海洋工程使用最为广泛的系泊方式，因其较低的制造和安装成本使其成为小型浮式平台系泊方式的首选。目前拖曳式系泊设计大多根据以往工程经验进行选择设计，由于经验设计中并没过多的泥面表面及以下锚链安装信息，因此在设计之初很难对系泊材料用量准确计算。不同重量的拖曳锚在同样的嵌深和同样的锚倾角条件下，所能提供的系泊力大不相同。同时，不同重量的拖曳锚的在相同土壤条件下拖曳嵌入轨迹不同，因而最大埋深不同。上述两方面原因导致不同重量的拖曳锚提供的系泊力不一样。因此，需要达到同样的系泊效果，泥线以下反悬链线和卧底链的长度在不同重量锚的系泊系统中是不同的。参照系泊成本各个分项在其生命周期各阶段的占比，材料制造成本与安装成本占据系泊成本的大部分，且安装成本往往正比于材料成本。因此降低系泊锚链的材料成本对整个系泊系统的成本降低起到了巨大的优化作用。通过结合拖曳锚的极限平衡法拖曳轨迹理论方程，以不同重量的拖曳锚在粘性土壤中的不同拖曳距离下提供额定系泊力时所需要的材料重量为系泊成本评价依据，从而得到浮式结构物系泊系统的一种设计优化方法。     

水下航行体通气超空泡减阻特性实验研究

为了研究超空泡的减阻效果,保证在较低流速下生成超空泡,在水洞中开展了水下航行体通气超空泡的实验研究.采用通气的方法在较低水速下生成人工通气超空泡,通过改变通气率和弗劳德数,获得了不同条件下通气空泡的长度,以及不同空泡长度下的模型阻力系数.研究表明,来流速度不变时,空泡长度随通气率的增加而增加,阻力系数随空泡长度的增加先递增后递减;空化器直径对阻力系数的影响较大,在大弗劳德数条件下,阻力系数会因空化器直径过大而出现随通气量的增加而变大的趋势.利用商用软件对超空泡形态及阻力系数作了数值仿真,并与实验结果作了对比,两者符合较好.     

深水定位系泊系统集成及控制系统研究

采用集中检测及控制技术将深水定位系泊系统集成为一个整体并加以控制,集成及控制系统模拟采样平台的运动坐标数据（采用仿真信号）,检测定位锚机锚索张力及长度,通过传感器检测风向、风速、海流和波浪,并实时显示和信息分析处理,建立适应复杂海洋环境下的多点系泊控制数学模型和算法演算,通过仿真实现深水定位系泊系统的自动定位控制功能。     

On the importance of mooring system parametrisation for accurate floating structure designs

This paper analyses the importance of mooring design parametrisation on the dynamic behaviour of mooring loads. An exhaustive sensitivity analysis is performed to evaluate the variability of mooring loads because of inaccuracies in the definition of model inputs, including physical and numerical parameters. Results show a relevant dependence on the length and significance in other parameters, such as the weight together with the hydrodynamic equivalent diameter and the drag forces. An inaccuracy below 1% in the mooring reference length can generate loads of up to twice the design, and an incorrect definition of the weight or the drag coefficient in the mooring design can lead to a design load variability of up to 30%. Stiffness plays a crucial role in snap events, reaching load differences of 19% depending on the stiffness selected.This research is based on a set of numerical models capable of predicting the mooring system response. A dynamic numerical model with two schemes of resolution is implemented and calibrated according to an experimental test campaign. Other sources of results provided by a quasi-static model and commercial software, Sesam (DNV-GL), are incorporated. In general, the dynamic numerical models show a good accuracy with an experimental database composed by a set of 2D prescribed movement tests at the fairlead of the mooring system.     

基于AQWA的双浮箱-双水平板浮式防波堤水动力分析

运用AQWA软件,针对双浮箱-双水平板浮式防波堤,建立不规则波作用下浮体锚链的耦合模型,研究时域情况下,改变水平板与浮箱的垂直间距,分析浮体运动响应以及锚链受力情况;并深入探讨了不同波浪周期和波高情况下水平板长度对浮体运动响应以及锚链受力的影响.结果表明,增加水平板与浮箱的垂直间距和水平板长度,可以有效减小浮体运动响应和锚链受力.波高和波浪周期是浮体重要的水动力因素,波高越大,波浪周期越长,浮体运动响应越剧烈;当浮体水平板长度达到21米左右时,整个防波堤达到最佳锚泊状态.     

Effect of bed vicinity on vortex shedding and force coefficients of fluid flow on an offshore pipeline

The effect of rigid bed proximity on flow parameters and hydrodynamic loads in offshore pipelines exposed to turbulent flow is investigated numerically. The Galerkin finite volume method is employed to solve the unsteady incompressible 2D Navier–Stokes equations. The large eddy simulation turbulence model is solved using the artificial compressibility method and dual time-stepping approach. The proposed algorithm is developed for a wide range of turbulent flows with Reynolds numbers of 9500 to 1.5×10⁴. Evaluation of the developed numerical model shows that the proposed technique is capable of properly predicting hydrodynamic forces and simulating the flow pattern. The obtained results show that the lift and drag coefficients are strongly affected by the gap ratio. The mean drag coefficient slightly increases as the gap ratio increases, although the mean lift coefficient rapidly decreases. The vortex shedding suppression happen at the gap ratio of less than 0.2.     

Effect of injection angle on artificial cavitation using the design of experiment method

Using the supercavitation phenomenon is necessary to reach high velocities underwater. Supercavitation can be achieved in two ways: natural and artificial. In this article, the simulation of flows around a torpedo was studied naturally and artificially. The validity of simulation using theoretical and practical data in the natural and artificial phases was evaluated. Results showed that the simulations were consistent with the laboratory results. The results in different injection coefficient rates, injection angles, and cavitation numbers were studied. The obtained results showed the importance of cavitation number, injection rate coefficient, and injection angle in cavity shape. At the final level, determining the performance conditions using the Design of Experiment (DOE) method was emphasized, and the performance of cavitation number, injection rate coefficient, and injection angle in drag and lift coefficient was studied. The increase in injection angle in the low injection rate coefficient resulted in a diminished drag coefficient and that in the high injection rate coefficient resulted in an enhanced drag coefficient.     

旋转圆柱绕流的流场特性

在亚临界区Re=1.4×105条件下,基于大涡模拟（LES）方法对均匀来流作用下的旋转圆柱绕流进行了数值模拟。通过与非旋转时的实验和计算结果比较,验证了文中计算的准确性。在此基础上,对不同转速条件下（0≤α≤2,α为圆周线速度与自由来流速度的比值）的圆柱绕流进行了数值研究。结果表明,圆柱旋转可以有效地抑制其旋涡脱落,随着转速的增加,可大幅提高其升阻比,主要表现为阻力系数减小而升力系数线性增大。当α=2时,阻力系数和升力系数在经过短暂过渡期后达到稳定。     

通气空泡流中洞壁效应对空化数和压力场影响的研究

文章在均质平衡多相流模型的基础上耦合输运方程型空化模型,通过求解混合介质的RANS方程、RNG k-ε湍流输运方程以及各相的质量输运方程,采用通用CFD软件—FLUENT数值模拟了水洞中带圆盘空化器航行体模型的定常通气空泡流动,研究了圆形截面闭式空泡水洞中洞壁效应对通气空化数和压力场的影响。得到的阻塞空化数线性正比于圆盘水洞直径比,且与三维圆盘自然空泡流的势流近似解基本一致,分析了洞壁效应作用下空化流场内的压力分布特点,并根据计算结果拟合了一定适用条件下通气空泡长度、最大直径和模型阻力系数的近似公式。     

Optimization of submerged floating tunnel cross section based on parametric Bézier curves and hybrid backpropagation - genetic algorithm

The cross-section geometry of a submerged floating tunnel (SFT) has a large effect on hydrodynamic characteristics, structural behavior and service level, making the tunnel cross section the primary factor in optimizing efficiency. Minimizing the mean drag and the dynamic variability in the lift of the SFT cross section under bi-directional (i.e., tidal) flow has a dramatic impact on the reduction of structural displacements and mooring loads. Based on a parametric Bézier curve dynamically comprising the leading-edge radius, tunnel height and width to define the SFT geometry, a sensitivity analysis of the Bézier curve parameters for a fixed aspect ratio with prototype dimensions under uniform flow conditions was conducted by applying Computational Fluid Dynamics (CFD), and the pressure distribution around the SFT cross-section surface was analyzed. A theoretical method comprising the Kármán vortex street parameters was employed to verify the CFD simulation results. In order to determine the SFT cross section with optimal hydrodynamic properties, the mean drag and Root Mean Square (RMS) lift coefficients were selected as optimization objectives, and four Bézier curve parameters were the input variables, in a neural network and genetic algorithm optimization process (a hybrid BP-GA structure), which is less likely to become trapped in local minima. The results show the optimal tunnel cross section has a mean drag and a RMS lift coefficient reduced by 0.9% and 6.3%, respectively, compared to the original CFD dataset.     

深海系泊缆的动刚度特性

针对纤维缆在交变载荷作用下存在的动刚度问题,基于集中质量法的思想建立了系缆的动张力计算模型,编制了能够有效分析系泊系统动力响应的源程序.将张力程序计算结果与商业软件计算结果进行对比,验证了算法程序的准确性,并在此基础上分析了纤维缆复杂的应力应变行为,采用统计的原理计算平均张力和张力幅值,将动刚度引入模型中探究其对系泊张力的影响.此外,分析系统在不同运动形式及环境载荷下的张力结果,研究考虑动刚度后系统参数对系泊张力的影响规律.     

基于实验设计方法的人工空泡喷射角度影响（英文）

Using the supercavitation phenomenon is necessary to reach high velocities underwater. Supercavitation can be achieved in two ways: natural and artificial. In this article, the simulation of flows around a torpedo was studied naturally and artificially. The validity of simulation using theoretical and practical data in the natural and artificial phases was evaluated. Results showed that the simulations were consistent with the laboratory results. The results in different injection coefficient rates, injection angles, andcavitation numbers were studied. The obtained results showed the importance of cavitation number, injection rate coefficient, and injection angle in cavity shape. At the final level, determining the performance conditions using the Design of Experiment(DOE) method was emphasized, and the performance of cavitation number, injection rate coefficient, and injection angle in drag and lift coefficient was studied. The increase in injection angle in the low injection rate coefficient resulted in a diminished drag coefficient and that in the high injection rate coefficient resulted in an enhanced drag coefficient.     

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.     

超空泡航行体楔形舵片流体动力学特性数值模拟

舵片是保证超空泡航行体运动稳定性和控制航行弹道的重要部分。文章基于均质平衡流模型和SST(Shear Stress Transport)湍流模型,计算了单独舵片的流体动力特性,并与试验数据进行了对比,结果符合较好,验证了计算模型的有效性。基于此方法,计算了单独舵片发生空化后在不同操舵状态下的非定常流体动力变化。结果表明,在攻角相同时,操舵状态下舵片的非定常升力系数和定常结果差别不大,而非定常阻力系数大于定常结果,并且操舵速度越快,阻力系数越大。另外计算了舵片发生空化后的流体动力系数,结果显示在攻角相同时,舵片的阻力系数和升力系数均小于其在全湿状态下的结果;在空化状态下,舵片升力系数的斜率小于全湿状态,并且舵片升力系数的斜率是变化的,存在某临界攻角,攻角大于此临界值时,升力系数的斜率减小,而此临界攻角恰好为舵片的吸力面刚刚出现空化时的攻角;操舵状态下舵片的阻力系数和升力系数的变化规律与定常结果一致,但是数值偏小。     

基于滑移网格技术计算螺旋桨水动力性能研究

基于RANS方程的CFD软件数值模拟螺旋桨定常和非定常的水动力性能.定常计算采用多重参考系MRF模型,分别采用标准k-ε的湍流模型,RNG k-ε湍流模型和Reliable k-ε湍流模型模拟在不同进速系数时的推力系数和转矩系数.将模拟的数值结果与试验值相比较,计算结果表明,采用Reliable k-ε湍流模型计算出的推力系数与转矩系数与试验值基本吻合,并以该结果为初始场,通过滑移网格技术,采用单机并行计算螺旋桨非定常水动力性能.相较于定常计算结果更加接近试验值,说明滑移网格技术具有更高的精准度,更加适用于计算螺旋桨的水动力性能.     

Experimental investigations on hydrodynamic interactions between the cylinder and nets of a typical offshore aquacultural structure in steady current

A series of physical experiments were performed in steady current to investigate the hydrodynamic interactions between the cylinder and nets which constitute a main part of offshore aquacultural platforms. The hydrodynamic characteristics of only the cylinder, only nets and combined cylinder-net structures are measured and analyzed systematically under different current velocities, inflow angles and solidity ratios of nets. Based on experimental data, fitted formulas for hydrodynamic coefficients of a single resin net are proposed and compared to previously published empirical formulas. It is observed that the existence of the cylinder brings an increment up to 9.2% to drag coefficient of net panels whose solidity ratios are higher than 0.347, whereas this effect is negligible for nets with lower solidity ratios and perpendicular to the inflow. For nets inclined with 45°, the increment of the drag coefficient of net panels due to the existence of the cylinder is more significant (up to 22.9%) than that for perpendicularly placed nets. Furthermore, the existence of nets also leads to a noticeable increase in the drag coefficient of the cylinder, up to 40.18% for a relatively large net solidity ratio of 0.458, representative of biofouling condition. The increment increases with the rise of the solidity ratio of nets and it is larger for nets placed perpendicularly to the inflow than inclined. Effects of the cylinder and nets on lift coefficients of each other are a bit complicated, leading to an increase or reduction of lift coefficients depending on the inflow velocity, inflow angle and net solidity ratios. Finally, it is worth noting that the hydrodynamic interaction between the cylinder and nets deserves to be considered in current practice of hybrid methods by combining potential flow theory, Morison and screen models for aquacultural structures, especially for biofouling conditions.     

Large-eddy simulation of cross-flow around ship sections

The present work is motivated by phenomena occurring in the flow field around ship-like bodies with an incoming lateral flow (cross-flow, 90 ° drift angle). Three-dimensional unsteady flows around different ship sections are investigated by using computational fluid dynamic (CFD) tools with large-eddy simulation (LES) subgrid-scale turbulence model. The simulation results are compared to measurements at several Reynolds numbers in the 90–200,000 range. Focus in our investigation is on the characterization of the motion of vortex structures generated by the separated flow. Another target in the study is to obtain better knowledge of the hydrodynamic forces acting on the sections. Computed pressure and drag coefficients are compared with experimental measurements. The comparison between simulations and measurements shows that an LES model can predict the flow field around ship sections in detail and the hydrodynamic forces acting on the sections.     

大型船舶系泊模式设计及优化研究

通过理论设计、仿真计算及模型试验的方法,开展设有岛式上建和外飘舷台的大型特殊船舶的系泊模式设计过程及优化研究,基于对缆绳张力、船体运动等试验结果分析,最终形成风浪流环境下某特殊船型的码头系泊模式方案,大幅度降低了因依靠传统舾装数设计无法考虑波浪影响。而在实际系泊作业中带来的风险,为船舶码头系泊作业提供科学可靠的技术支撑。     

局部系泊失效下半潜式平台的动力响应特性

运用ANSYS/AQWA软件建立半潜式平台和锚泊系统的三维分析模型,研究平台主体的运动响应特性和系泊链张力响应特征,并对系泊链断裂前后2种状态下的平台动力响应特性进行对比分析。结果显示:当局部系泊链断裂后,系泊链最大张力最高增加18.07%,系泊链的可靠性最高降低了146.16倍;而平台的横摇角最大值增加了15.99%,横荡最远距离增大100%;系泊链3断裂时,平台的纵荡最远距离增大116%。因此,需要避免环境载荷迎面系泊链断裂的情况,一旦发生断裂,需要立即采取措施,以避免系泊系统发生连锁失效反应,同时采取适当措施降低平台横荡、纵荡和横摇的增加,从而保证平台和人员安全。