水流作用下重力式网箱网衣张力分布

在水流作用下，网箱网衣要承受很大的水流作用力，在网箱实际使用中破网逃鱼现象时常发生。网衣属于柔性结构物，在外部荷载作用下会发生较大的变形和位移，因此其受力和变形特性较为特殊。为了获得水流作用下重力式网箱网衣的张力分布，从而为设计和生产提供理论参考依据，利用集中质量的方法建立了水流作用下网箱网衣受力和变形的数学计算模型。利用此数学模型对重力式网箱网衣在不同配重模式和网目形状下的变形和张力分布进行了数值模拟。通过数值模拟研究发现，网箱网衣张力分布形式和网目形状关系密切，配重形式对其影响较小。相同工况条件下，配置底图时网衣所受最大张力略大于单独沉子配重形式，同时菱形网目网衣所受最大张力略大于正方形网目的受力。     

圆形网衣在水流作用下的运动变形特性

研究网衣在水流作用下的运动变形是深入了解网箱耐流特性的关键。本研究基于集中质量点法,建立了圆形网衣在水流作用下的数值计算模型,并利用前人的试验结果对数值模型进行了验证。验证结果表明,无论是网衣的变形还是受力情况,模拟结果都与试验结果吻合良好。在此基础上,针对实际应用中的圆形网衣,在不同配重(GW1=400kg、GW2=800kg)和流速(U=0.3～0.9m/s)条件下,对网衣变形、网衣底端前后点位移、网衣底端倾角进行了数值模拟,探讨了流速、配重大小对网衣运动变形特性的影响。结果表明,水流速度的增大会加剧网衣的变形,在配重GW1、流速U=0.75m/s时,网衣容积损失率达到50%以上。网衣底端前点的水平位移大于后点的水平位移,前点的垂直位移则小于后点的垂直位移,随着流速的增大,网衣底端前后点的位移差异将更为明显。当配重增加时,网衣底端位移、倾角以及网衣容积损失率可相应减小,网衣变形得到一定改善。数值模拟证明,本研究所建立的数值计算模式具有较好的稳定性和解的收敛性,可为下一步波浪、流联合作用下的整体网箱模拟奠定良好的理论基础。     

平面网衣在水流作用下的受力和变形特性数值模拟研究

基于集中质量点法和牛顿第二定律,建立了网衣在水流作用下的受力和运动响应数学模型.采用计算机数值模拟方法,研究了水流作用下网衣的动态变形情况以及网衣受力平衡后的空间分布形状.在2种配重(GW1=4.12 N、GW2=16.38 N)和6种流速(U=0.17、0.22、0.28、0.33、0.39和0.44 m · s-1)条件下对网衣稳定后沿水流方向的总水流力、网衣底端的水平及垂直位移进行了数值计算.计算结果表明,配重及流速大小对网衣受力和变形特性具有较大的影响.随着水流速度的增大,网衣在水流作用下的运动变形加剧,且当配重增大时,网衣变形减小,网衣受力随流速的变化增幅明显加快.为了验证模型的正确性和有效性,文章还将数值模拟结果与前人的试验结果进行了比较,效果良好.     

HDPE圆柱形网箱与圆台形网箱受力变形特性的比较

介绍了一种用于模拟水流作用下网箱受力变形的数值计算模型,并利用海上实测数据对计算模型进行了验证。验证结果表明,网箱锚绳力的计算结果与实测数据十分接近,最大相对误差为7.0%。在此基础上,分别对圆柱形网箱和圆台形网箱在不同配重(GW₁=350 kg、GW₂=600 kg)和流速(U=0.3～0.9 m/s)条件下所受的水流力以及容积损失率进行了数值计算。结果表明,两种网箱无论在所受的水流力方面还是在变形后保持的网箱容积方面随流速和配重的变化趋势基本一致。圆柱形网箱的容积损失率远大于圆台形网箱的容积损失率,在较小流速(U=0.3 m/s)研究时,圆柱形网箱的容积损失率达到了28%左右,具有较明显的初始变形。圆台形网箱的初始变形很小,具有较好的耐流特性和抗变形能力。     

波流入射方向对网格式锚碇网箱水动力特性的影响

运用集中质量方法和刚体运动学原理,建立了单体网格式网箱在波流联合作用下浮架、网衣和锚碇系统受力和运动的数学模型.利用物理模型试验对本数学模型的正确性进行了验证.在此基础上计算了2种波流作用方向、各工况下单体网格式网箱3类锚绳(锚碇锚绳、连接锚绳、网格锚绳)受力、浮架运动及网衣变形的结果,并分析了波流作用方向对网格式锚碇网箱水动力特性的影响.结果表明,当波流作用方向由正向变为斜向(45°)时,锚碇锚绳与连接锚绳受力有较大程度的增加,网格锚绳受力有较小程度的增加;浮架水平方向、竖直方向运动幅度有一定程度减小,浮架倾角及网衣变形变化不明显.     

基于OpenFOAM的全潜式网箱网衣系缚方式研究

本研究针对全潜式网箱的四周刚性柱–网衣系缚方式进行研究。通过模型实验测得柱–网结构模型在水流作用下, 不同流速、不同系缚方式下的网衣系缚点受力, 分析了不同系缚方式对网衣系缚点受力分布特性以及系缚点之间受力均匀性的影响。此外, 为进一步研究网衣系缚方式对周围流场的影响, 本研究基于混合体积法建立了网衣的流固耦合模型, 并在开源 CFD 软件 OpenFOAM 中开发得到了网衣流固耦合求解器, 通过数值模拟与模型实验相结合的方式研究了网衣的阻流效应以及网衣网线的张力分布特征和荷载传递方式。研究结果表明, 四周刚性柱–网结构模型中, 网衣系缚方式对水流作用下网衣的受力特性有着显著的影响, 网衣最大张力一般分布在与系缚点相连的网线上, 而且越靠近中间位置的网线和系缚点的受力也越大, 整体呈现出两端受力小中间受力大的对称现象; 在网衣背流侧, 靠近网衣沿水流方向投影的中心位置处流场受网衣阻流效应的影响会偏大, 流速衰减率也更高, 而且在同种工况下, 随着流速的变大, 网衣对流场流速的衰减效应也越明显。     

基于水动力性能的网箱网衣网目群化数值模拟方法研究

网衣是网箱的重要组成部分。为解决对网衣的运动变形以及受力进行数值研究存在的问题,基于Morison方程和集中质量法,提出了网目群化改进方法。改进方法保持了网目群化前后网衣的体积和质量不变,并通过调整水动力系数达到改进后的网衣所受水动力与未群化前相同。通过对圆形网衣进行数值模拟,并与相关研究进行对比,分析了圆形网衣的容积损失率和水平拖曳力。结果显示:圆形网衣在均匀流速的流场中,其水平拖曳力随着流速的增大,增加的速率也随之增大,随后达到稳态,数值基本不变;增加网衣底部配重可以有效减小网衣的变形,但是随着配重的增加,其变形抑制效果在逐渐降低。改进的网目群化方法准确有效、简化效果显著,对于提高网箱水动力学模拟效率和计算精度都具有较好的参考价值。     

船型桁架结构网箱单点系泊受力试验研究

安全高效的大型深海养殖网箱是引领深远海养殖发展的重要设施装备。该研究针对一种船型桁架结构的单点系泊深远海网箱开展了模型比尺为1∶40波浪流水池试验,比较分析了正常工况和失效工况2种不同单点系泊方式下网箱的系泊受力。在此基础上,分别围绕网衣、波浪和水流对网箱系泊受力的影响开展了试验研究。结果表明,相较于正常工况,失效工况的网箱系泊受力大幅增加,对网箱的安全性造成较大影响;在正常工况中,大浪条件网箱的受力随波高增加而增大,波高从12.5 cm增加到17.5 cm,网箱系泊受力增大3倍以上;在波流联合作用下,网箱系泊受力与流速变化呈正相关,与周期变化呈负相关;相较于无网衣,有网衣时网箱受力在纯波条件下增幅达1倍以上,波流联合作用下增幅为30%～60%。研究结果可为今后船型桁架结构网箱的系泊方案设计及海上安装提供理论依据和数据支撑。     

波流作用下深水网箱受力及运动变形的数值模拟

基于已建立的浮架和网衣数学模型,对不同波况和流速共同作用条件下HDPE深水网箱所受的锚绳力、波流力、容积损失率以及浮架倾角进行数值计算,设计的波流要素值为:波高H=4～6 m,周期T=6.0～8.6 s,流速U=0.3～0.9 m/s.结果表明,网箱锚绳受力、波流力和容积损失率均与波高和流速成正比,与周期的关系不明显,且网箱系统所受的波流力约为网箱迎浪侧两根锚绳受力的合力.在波高H=4～6 m、流速U=0.75 m/s时,网箱容积损失率达到47%～56%,网箱变形较为严重,为此建议网箱养殖区域应选择流速小于0.75 m/s的海区较为适宜.周期对网箱容积损失率的影响很小,对浮架倾角的影响较为明显,波高和流速不变时,随着周期的增大,浮架倾角会有所减小.本研究旨在探讨波浪流对深水网箱受力及运动变形的影响,为高海况网箱养殖的风险评估提供参考依据.     

深水养殖网箱试验中网衣相似准则的应用

通过分析与试验相结合的方法对重力相似准则在网衣试验中的适用性进行研究。试验以在国内应用较广的田内相似准则作为参照,分析讨论了两种相似准则的异同,推导得出了重力相似准则结合大小尺度比在网衣模型设计中的质量校正公式及刚度校正公式。并据此按照两种相似准则,设计了包括网衣、底杆和沉子系统的两套模型,模型上端固定于设置了受力传感器的刚性直杆上。试验在纯流条件下进行,分别设置5组流速,试验水深0．8m。各组流速重复试验3次,受力由固定于刚性直杆上的传感器测量获得,网衣的变形则通过系缚在网衣上的二极管的坐标确定。在进行受力分析时,由于两相似准则的受力比尺不同,因此必须将模型受力按照对应比尺统一转换为原型值,同时利用受力与流速的回归关系式将模型受力校正为同一流速下的数值然后再进行比较。试验结果表明,相同流速条件下,两种相似准则的模型受力及变形总体上吻合较好,说明重力相似准则在网衣试验中采用质量校正及刚度修正后同样能够得到令人满意的结果,而且试验中模型的大尺度比达到1：20,这无疑为涉及波浪问题的深水网箱模型试验的设计提供了保障,从而避免了田内相似准则在深水网箱试验研究中应用的局限性。     

HDPE圆形重力式网箱受力变形特性的数值模拟

该研究旨在综合探讨不同网箱周长、浮管管径、网衣高度及网目大小对整体网箱受力变形的影响,为网箱的科学合理选型提供数据参考。设定的网箱周长40—80m,浮管管径250～630mm,网衣高度6～20m,网目大小45～115mm。通过数值模拟方法对4种规格高密度聚乙烯圆形网箱在不同组合条件下网箱锚绳受力、波流力以及容积损失率进行了数值计算。结果表明,大规格网箱的锚绳受力、波流力更大,容积损失率更小,锚绳数量的增加可以大大降低锚绳受力。相比浮管管径,网衣高度和网目大小对网箱受力变形的影响更显著。整体网箱的受力变形随着网衣高度的增加而增大,随网目的增大而减小。     

Typical hydrodynamic models for aquaculture nets: A comparative study under pure current conditions

The net is regarded as the most critical component in marine aquaculture facilities as it is the only barrier which protects the environment from fish escapes. Accurate predictions of the net cage deformation and drag force on the nets are needed, both for ensuring fish welfare and for dimensioning of the mooring system. Thus, an appropriate hydrodynamic model is essential. In practice, two types of hydrodynamic force models, i.e., the Morison type and the Screen type, are commonly used to calculate the hydrodynamic forces on nets. Application of the models depends on the underlying structural model and the availability of data. A systematic review of hydrodynamic models is therefore undertaken to compare the models and various parameterisations, in aid of model selection during the design. In this study, eleven commonly used hydrodynamic models, i.e., five Morison models and six Screen models, are reviewed comprehensively, and implemented into a general finite element (FE) solver for dynamic simulations. Sensitivity studies on different current velocities, inflow angles and solidities of the nets are carried out. Moreover, different wake effects are also considered in numerical simulations. The numerical results from different models are compared against existing experimental data under pure current conditions. Suggestions for selection of suitable hydrodynamic models are provided, based on the model comparison.     

Numerical simulation of the effects of structure size ratio and mesh type on three-dimensional deformation of the fishing-net gravity cage in current

In our previous research, the hydrodynamic behavior of a two-dimensional (2D) plane fishing net in current was simulated. Based on the research, a model of a three-dimensional (3D) net is established by using the lumped mass method. To verify the validity of the numerical model, model test results by other authors are cited and compared with the numerical results. The simulated results are in good agreement with experimental ones. In this paper, the 3D net model is applied to investigate the effects of structure size ratio (RDH) and mesh type on the 3D net deformation of the gravity cage in current. The numerical results indicate that the decrease of RDH is practically feasible in improving the cage net deformation. With a sinker system the net deformation with diamond mesh is greater than that with square mesh. When the bottom-collar sinker system is applied, with an increase in current velocity, the net deformation with diamond mesh is less than with square mesh. The results of this study provide a better understanding of the hydrodynamic behavior of the gravity cage.     

Accuracy improvement of numerical simulation with the determination of drag coefficients of floating collars

To install aquaculture facilities in the open sea, which is significantly influenced by tidal currents and waves, technology must be developed to withstand severe natural conditions. Many studies have been performed on cage structures that can operate in such an environment. To analyze the performance of the cage structure, it is necessary to derive the drag coefficient of the floating collar according to the degree of submergence of the floating collar in the sea; however, studies on this have not been published. Therefore, in the present study, we determined the drag coefficients according to the extent of submersion of high-density polyethylene (HDPE) pipes, which are the most commonly used pipes for cage floating collars. Using three types of HDPE pipes with different diameters, we derived drag coefficients for 1/4, 2/4, 3/4, and complete submersion of the pipe’s cross section at different attack angles and current speeds. The drag coefficient of the HDPE pipe derived from a flume-tank experiment was employed for modeling the floating collar. Additionally, a numerical simulation was performed. The deformation of the sides of the cage according to the current speeds agreed well with the model test and numerical simulation results, and the overall calculation error for the tension of the mooring line was within ±5%. Furthermore, the accuracy of the simulation was improved by about 7% through the numerical calculation method of the floating collar applied in this study. According to the results of the model test and numerical simulation, cage structures with various properties can be modeled consistently using a mass–spring model, enabling relatively accurate numerical calculations and behavior simulation of the cage.     

Field measurements of cage deformation using acoustic sensors

As aquaculture continues to supply an increasing share of the worldwide seafood demand, it will become critical for farmers to maximize their efficiency. Presently, the majority of marine finfish are produced in gravity type net pens which can deform when they are subjected to currents. The water velocity loading affects the overall net shape which results in net cage volume loss and consequently, increases fish stress and decreases growth rates.In this study, an acoustic method is utilized to monitor the deformation of a small-scale fish cage deployed in currents. Twelve acoustic sources and four hydrophones were deployed on and around a small scale net pen for 60 days to monitor the net cage movement and volume. Local current velocities were recorded using two current meters, one inside and one outside the net pen. Three volume approximation techniques were examined, using the positions of the acoustic sources to predict net chamber volume as it responded to the currents. A numerical model of the system was then configured, set with loads under similar water velocities and results between field measurements and the model were compared.The use of acoustic sources and hydrophones to monitor cage deformation was shown to accurately monitor net deformation. Field measurements compared well to numerical model predictions, with errors ranging from −3.8% to 32%, depending upon the number of acoustic sources employed in the volume calculations. At low water velocities, six acoustic sources were found to accurate predict the net pen volume. In higher currents, a minimum of nine acoustic sources was recommended.     

Water-tank experiment and static numerical analysis of the mooring system of a controllable depth cage

Recent variations in marine environments have increased the risk of aquaculture accidents at sea. This risk can be reduced by installing fish cages at the desired depth, based on environmental conditions such as wave height, the vertical profiles of water temperature, algal concentration, and dissolved oxygen concentration. Most submergible fish cages can be located at only two depths: the sea surface and a submerged depth. In the present study, a fish cage installed at various depths, i.e., a controllable depth cage (CDC), is proposed for avoiding undesirable environments. A water tank experiment is conducted to measure the drag of the cage and the static deformation of the mooring system using a scale model of the actual cage. Then, a simple numerical model based on the balance of forces on each component is developed to analyze the position and the attitude of the cage and the mooring tension of the system. The numerical model is verified by comparing the experimental and numerical results. The outcome showed that the cage and floats moved downstream at an increasing velocity. The results of the numerical simulation supported those of the water tank experiment. However, the simulated vertical positions of a cage and floats were higher compared with experimental results. Additionally, the inclination of angle increased alongside increasing velocity in the numerical simulation, whereas a complex variation was observed in the experiment. This happened because of underestimating the drag on the mooring rope in lower water current velocities; additionally, cage lift was not considered in the numerical model. Despite these discrepancies, the tension of each mooring rope was well predicted because of the dominant tension of the horizontal component. In future studies, the balance of forces on the rope should be predicted more precisely, and variations in cage drag and inclination angle should be included in the numerical model. Additionally, the effect of waves should be considered alongside water currents to ensure the safety of the CDC.     

Dynamical analysis of net cage structures for marine aquaculture: Numerical simulation and model testing

A numerical model for analyzing dynamic properties of a net-cage system exposed in the open sea is proposed. The model is based on a lumped-mass method. In this model, the mooring lines are divided into linear elements and the net cage divided into several plane surface elements. The interconnected points or corners are called nodes or lumped-mass centers. The external force is calculated on each element and then equally distributed to its nodes. By adding the contributed forces from the neighboring elements, a system of motion equations for nodes is formed. The volume reduction coefficient of a net cage is estimated by the ratio of minimum volume of net cage during fluid and structure interaction to the original volume. In general, the numerical results are in good agreements with the experimental data. However, the results also show that if the Reynolds number is lower than the suggested range of 1400–1800, the numerical model may underestimate the environmental forces on a net-cage system.     

Dynamical analysis of net cage structures for marine aquaculture: Numerical simulation and model testing

A numerical model for analyzing dynamic properties of a net-cage system exposed in the open sea is proposed. The model is based on a lumped-mass method. In this model, the mooring lines are divided into linear elements and the net cage divided into several plane surface elements. The interconnected points or corners are called nodes or lumped-mass centers. The external force is calculated on each element and then equally distributed to its nodes. By adding the contributed forces from the neighboring elements, a system of motion equations for nodes is formed. The volume reduction coefficient of a net cage is estimated by the ratio of minimum volume of net cage during fluid and structure interaction to the original volume. In general, the numerical results are in good agreements with the experimental data. However, the results also show that if the Reynolds number is lower than the suggested range of 1400–1800, the numerical model may underestimate the environmental forces on a net-cage system.     

Dynamic properties of a flexible net sheet in waves and current—A numerical approach

The dynamic properties of a flexible net sheet exposed to waves and current were investigated by using a numerical model. The net was modelled by dividing it into super elements, and the structural and hydrodynamic forces were calculated for each element. The movement of the net was found from time integrating the equation of motion at each node. The model was used to study the influence of different parameters on the behaviour of the net. Through six different cases the impact of the following five parameters were investigated: (i) floater movement, (ii) wave period/height, (iii) current velocity, (iv) net solidity and (v) bottom weight.