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.     

水流作用网衣过程的数值模拟

网衣是深水网箱的主要组成部分，也是整个网箱系统中受力最为复杂的部件之一。文章介绍了水动力作用下一种基于集中质量法的网衣数学模型，并引用前人的试验结果对数学模型进行了验证。在此基础上，利用此数学模型在不同配重（CW1=400kg和CW2=800kg）和流速（U=0．3—0．6m·s^-1）条件下分别对网衣所受的水流力、网衣形状和网衣运动位移三者随时间的变化过程进行了数值模拟，给出了网衣达到稳定状态前、后网衣受力变形的计算结果，揭示了网衣在水流作用过程中的动态变化规律，并进一步分析了配重及流速大小对网衣受力、变形及运动特性的影响。     

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。     

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。     

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.     

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。     

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。     

水流作用下自升降式张力腿网箱水动力学特性试验研究

自升降式张力腿网箱因其在水流作用下能够下潜到水中,具有在恶劣海况条件下,避免海水表面的风浪冲击,降低网箱系统被破坏风险的优点,具有良好的市场前景。根据田内准则,运用水槽模型试验方法,研究了周长60 m,网高16 m张力腿网箱在水流作用下的缆绳张力、网箱下潜深度、容积损失率与流速的关系。结果显示:张力腿网箱缆绳张力随流速增加而增加,且主要受力集中在迎流面的缆绳上;张力腿网箱可以在水流作用下自动下潜,流速0.8 m/s时,下潜深度可达11.91～17.79 m,从而能够实现网箱抗风浪的目的,另外可根据需要调整浮架浮力使网箱沉降深度达到合理水层区间;在低流速下各配置张力腿网箱容积损失率较小,在流速0.8 m/s时,最大容积损失率32.5%,小于类似配置的重力式圆形网箱。研究结果可为张力腿网箱的设计与应用提供参考。     

网衣缩结对圆柱形网箱性能影响的初步研究

采用网箱模型试验，对周长50m、深8m的HDPE双管圆形重力式网箱，通过改变网衣装配方法，在不同流速条件下对网箱性能进行了研究。结果表明，菱形目装配缩结系数0．707时网箱的阻力比0．65时小，但在静态时，缩结系数0．707比0．65时容积损失率大;力纲装配对网箱在水流作用下的形态产生影响。     

波浪作用下圆形网箱浮架系统的运动特性分析

作为深水养殖网箱的重要组成部分，浮架系统在网箱抗风浪性能中扮演着极为重要的角色，研究浮架系统在波浪作用下的运动特性是深入了解网箱抗风浪性能的关键。基于集中质量法将圆形浮架离散成众多微元，并运用线性波浪理论和刚体运动学原理，建立了波浪作用下网箱浮架系统的运动响应数学模型。针对圆形浮架的实际尺寸参数，在不同波高（H=4.2～7.0 m）和周期（T=7.2, 8.6 s）条件下，采用计算机数值模拟方法，对浮架前后点运动轨迹、浮架质心水平和垂直位移、浮架倾角进行了数值计算。通过对计算结果的比较及分析表明：（1）浮架垂直位移幅度大于水平位移幅度，波浪周期对浮架垂直位移的影响要小于对水平位移的影响；（2）浮架水平位移、垂直位移以及倾角都和波高具有显著的正比关系。在波高H=4.2～7.0 m范围内，周期T=7.2 s时，水平位移、垂直位移和倾角的增量分别为1.84 m、1.0 m和7.18°；周期T=8.6 s时，三者的增量分别为1.66 m、1.05 m和5.43°；（3）周期增大，浮架垂直位移增加，水平位移和倾角减小。     

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.     

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.     

Numerical simulation of the flow field inside and around gravity cages

A three-dimensional (3D) numerical model is established to simulate the flow field inside and around the gravity cages in a current. The realizable k–ɛ turbulence model was chosen to describe the flow, and the governing equations are solved by using the finite volume method. In the numerical model the cylindrical cage is divided into 16 plane nets around the circumference and a bottom net, and the net is modeled using the porous media model. The unknown porous coefficients are determined from the hydrodynamic force on the net under different flow velocities and attack angles using the least squares method. In order to validate the numerical model, the numerical results of the plane nets were compared with the data obtained from two physical model tests. The comparisons show that the numerical results are in good agreement with the experimental data. Using the present model, this paper presents the flow field inside and around the gravity cages with different spacing distances and cage numbers. This study provides information about the flow field inside and around the fishing net cages.     

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.     

波浪作用下一种鲆鲽类方形网箱水动力特性数值模拟研究

针对鲆鲽类方形重力式网箱在纯波浪条件下的水动力特性,采用数值模拟的方法对鲆鲽类方形深水重力式网箱的主要部件——浮架系统、配重系统、锚碇系统进行了模拟。将模拟结果与实验结果进行分析比较表明,数模与物模各量值吻合良好,平均相对误差均不超过9%,表明数值模拟的方法能较好地模拟鲆鲽类方形深水重力式网箱的水动力特性。在此基础上,用数值模拟的方法,进一步研究了底框质量和网衣高度的改变对网箱各参数的影响,模拟结果显示,底框质量改变主要影响锚绳受力和底框倾角,底框质量从80 g升至100 g再升至140 g的过程中,锚绳受力平均增幅分别为13%和19%,同时增大底框质量有助于减小底框运动倾角,其平均降幅分别为8.8%和9.3%;网衣高度由20 cm增至30 cm后,锚绳受力平均增加15%,同时网衣的增高导致对浮架和底框的牵制作用加大,使得浮架和底框的运动幅度有所减小,降幅不超过10%。在以上研究结果的基础上,为鲆鲽类网箱的设计与优化提供了参考建议。     

Numerical study on wave attenuation inside and around a square array of biofouled net cages

In this study, waves propagating through a square array of 16 net cages with different levels of biofouling are numerically studied using a three-dimensional computational fluid dynamics (CFD) model. A porous-media fluid model is adopted to simulate both clean and biofouled netting of a cage array in waves. A numerical wave tank is built, and the oscillating-boundary method is adopted to generate waves. The flow motion is solved by the Navier-Stokes equations, and the free water surface is captured using the volume of fluid (VOF) method. The numerical model is validated by comparing the numerical data with corresponding experimental measurements of a net-cage model with clean netting. To analyze wave attenuation, a numerical analysis of wave elevation both inside and around the cage arrays is presented, which considers the effect of biofouling. Based on the results of the present study, the effect of biofouling on wave elevation is noticeable; the damping effect of the cage array increases with increasing level of biofouling. Furthermore, the incident angle of waves has a noticeable effect on the wave field inside and around the cage array.     

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.