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.     

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。     

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。     

流速和放网模式对金枪鱼围网网具形态的影响

网具形态是金枪鱼围网作业性能的一个重要指标,受网线材料、沉子配重、海流等诸多因素影响。为探究放网模式和流速对金枪鱼围网网具形态的影响,本研究依照上海开创远洋渔业有限公司金枪鱼原型围网,根据田内准则制作模型网,在日本下关西日本日网公司专业动水槽进行模型实验。实验设置3种放网模式（顺流放网、侧流放网、背流放网）,3种流速（0 cm/s、5 cm/s、8 cm/s）,比较各放网模式下,流速对金枪鱼围网网具沉子纲沉降深度、纵向网衣轮廓、浮沉子纲水平包围面积和浮沉子纲水平包围形状的影响。结果显示：（1）0 cm/s流速下放网,网具沉子纲沉降深度最大（1.21 m）,浮沉子纲水平包围面积最大（8.44 m²,9.89 m²）,纵向网衣轮廓最佳;（2）顺流5 cm/s条件下放网,网具浮沉子纲水平包围形状最佳;（3）顺流、背流放网,网具最大沉降深度随着流速增加逐渐减小[1.21 m（0 cm/s） > 1.17 m（5 cm/s） > 1.13 m（8 cm/s）,1.21 m（0 cm/s） > 1.16 m（5 cm/s） > 1.10 m（8 cm/s）],网具最大沉降深度与流速显著相关（>0.05）;（5）3种放网模式下,流速与网具浮沉子纲水平初始包围面积不相关（<0.05）。综上建议,有流时为确保网具形态最佳及网具沉降深度最大,应首先采用侧流放网。     

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.     

南极磷虾桁杆拖网网型结构与作业性能之间的关系

桁杆拖网是南极磷虾连续泵吸桁杆拖网捕捞系统重要组成部分之一,为深入了解该系统中桁杆拖网的作业性能特征,促进网具作业性能优化,本研究通过模型试验开展了网型结构和沉子重量对网具作业性能影响的探索,网型结构包括6种(3种腹网宽度,每种宽度采用2种缝合方式),分别以A、B、C、AA、BB和CC表示,沉子重量3种。综合比较表明,不同试验网型作业性能优劣排序为BB>CC>C>A>B>AA,但差异不显著。缩小腹网宽度和斜目缝合均能有效提高网口垂直扩张、降低能耗系数,提高网具作业性能,尤其适用于较高拖速的作业场景。沉子重量与网具阻力、网口垂直扩张均呈显著正相关,但与能耗系数相关性不明显;为促进网口垂直扩张,合适的沉子重量配备随拖速增加而递增,拖速1.5 m/s的条件下,各试验网型其沉子配重应不低于3.05 t。试验条件下,各网型的网具阻力、能耗系数变化趋势较平稳,递变斜率变化较小,网具阻力与拖速呈幂函数关系,指数介于1.49～1.64。以上结论可为南极磷虾桁杆拖网属具配备以及优化设计提供参考。     

帆张网帆布面积对网具作业性能的影响

帆布面积增加在促进网口扩张的同时,也使网具整体阻力增加。为了明晰帆布面积对网具整体作业性能的影响,以网口面积与网具阻力的比值(K)作为帆张网网具性能比较参数,通过模型实验分析帆布面积对网具作业性能的影响,结果显示,流速为0.24 m/s时,K值随帆布面积增加而递增,但增速逐渐趋缓,帆布面积趋于0.288 m~2时,K呈现接近峰值的特征;流速≥0.4 m/s时,K值保持平稳,与帆布面积相关性不明显;因此,帆布面积趋于0.288 m~2时,此时帆布面积与网具匹配系数(P)趋于0.072,网具可在各种流速条件下均保持较好的作业性能,根据实验观察,此时背腹网网衣受力均匀,网型较好;网具整体阻力(F)与帆布面积(A)和流速(V)呈幂函数关系,其表达方式为F=334.32×A~(0.5)×V~(1.61);帆布展弦比对K值变化趋势影响较小;以上结论可为帆张网帆布、锚与网具的合理配置提供科学依据。     

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.     

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.     

固定方式对水流作用下桩柱式围网网片力学特性的影响

采用集中质量点法和网目群化法,结合数值模拟技术探讨了桩柱式围网网片单元在水流作用下的力学特性,着重分析了不同网片宽度和固定方式下网片单元的网线张力分布、节点偏移和网片系缚点受力特性。结果显示,水流作用下,桩柱式围网网片的最大张力主要分布于网片顶部和底部力纲的两侧,最大张力值与网片宽度成正比;网片的最大偏移部位主要分布于网片的上下端,最大偏移量与网片宽度成正比;系缚点最大受力出现在网片的顶端和底部,且其受力在数值上均远远大于中间系缚点;研究发现,随着系缚点数的增加,各系缚点的受力不断下降,网线张力和偏移量也迅速减小;当系缚点超过一定数量时,网线张力和偏移量将不再显著下降,表明存在一个临界系缚点数。本研究可为桩柱式围网工程的设计与安装提供参考。     

Circulation and larval fish transport within a tidally dominated estuary

In March 1996 two surveys of larval fish abundance and water flow were carried out within the estuarine region near Beaufort Inlet, NC. Each survey extended over two full semidiurnal tidal cycles and included measurements of larvae concentration and velocity distribution at several locations. There was a large across-channel variation in the subtidal flow passing through Beaufort Inlet, with net inflow over the eastern and central portions of the inlet and net outflow on the western side of the inlet. This pattern was consistent with moored current meter measurements of a previous study, and was reproduced by a numerical model circu-lation forced only by the M₂ tide. A net ingress of larvae from the open ocean into the estuary was observ-ed during both surveys. Most larvae entered the estuary over the eastern and central portions of the inlet, where the subtidal flow was up-estuary. However, the mean circulation played a minor role in the net movement of larvae into the estuary. Rather, net up-estuary transport of larvae was principally due to variation of larval abundance with tidal flow; with abundance during flood tide usually far exceeding ebb tide abundance. This mode of transport was likely driven by a behavioural response to tidal flow in which larvae tended to descend to the bottom on falling tides and reside throughout the water column on rising tides.     

Mesh selectivity of encircling gill net for gizzard shad Konosirus punctatus in the coastal sea of Korea

ABSTRACT:   Fishing experiments were carried out in Yeongil Bay, Korea using an encircling gill net with four different mesh sizes and two different hanging ratios to measure the mesh selectivity for gizzard shad Konosirus punctatus . Twenty-six trials were conducted giving a total catch of 485 gizzard shad. The 'share each length's catch total' (SELECT) analysis with maximum likelihood method was used to fit the different functional models, the normal, lognormal, and bi-normal model, for a selectivity curve to the catch data. In addition, two cases in which the relative fishing intensity was either estimated or fixed were compared. The bi-normal model with the fixed relative fishing intensity was found to fit the data best. For the selectivity curve on the bi-normal model, the relative length (the ratio of fish length to mesh size) with the maximum efficiency was obtained as 3.70. From this, the focal fish lengths in the commercial encircling gill net, with mesh size ranging from 5.0 to 6.0 cm, were inferred to be 18.5–22.2 cm.     

Experimental study of the drag on fine-mesh netting

The hydrodynamic forces on netting have been investigated by many researchers. They proposed equations to estimate the drag and lift coefficients of the netting, taking the effects of the Reynolds number and solidity ratio into account. However, there are few studies on the hydrodynamic forces on netting with a fine mesh, which is sometimes used in Japan for farming small fish, such as young sardines. The size of the fine mesh is a few millimeters, whereas the solidity ratio of the netting is similar to that of standard netting. The drag on fine-mesh netting may increase because of the effects of blocking. In the present study, the drag on fine-mesh netting was examined by means of a towing experiment in a water tank. The experiment included an investigation of the drag on a planar net, a half-circular net, a circular net, and a circular net with a bottom, to understand the effect of the attack angle of the water current and wake. As a result, the drag coefficient of the plane net was in the range of those proposed in the previous studies. The effect of the attack angle of the water current was approximately reproduced by the cosine function. The reduction factors for the water current velocity through the circular net also agreed with those proposed in the previous studies, whereas it was approximately at the maximum among the previous estimations. Consequently, the drag on fine-mesh netting could be well explained by the estimates from the previous studies. However, the drag at the bottom of the net, parallel to the water current, was overestimated if hydrodynamic forces act on each transverse twine normal to the flow direction. The effects of the interactions among the twines, which are aligned parallel to the flow direction, should be specifically examined as a future study.     

Hydrodynamic responses of longline aquaculture facility with lantern nets in waves

To prevent occurrences of structure failure, it is essential to study the mooring line tension and motions of the longline aquaculture facilities with lantern nets. This study describes a physical model experiment that attempts to determine an optimal structure design. The experiment investigated effects of three facility design factors on the mooring line tension and movement of the lantern net and mainline: incident wave angles, lantern net layout depths, and lantern net layout forms (horizontal arrangement/staggered arrangement). The experimental results indicate that the mooring line tension decreases with increasing wave periods. The amplitude and variation of windward mooring line tension are much larger and more complex than that of leeward. The vertical movement amplitude and variation range of the lantern net and mainline are much larger than that of horizontal movement amplitude. Decreasing the incident wave angle, deepening the arrangement depth of lantern nets or adopting staggered arrangement can reduce the mooring line tension in the case of longer wave period. The analysis indicates that increasing the layout depths of the lantern nets and applying the staggered layout form are beneficial in reducing the vertical motion amplitude and elliptical excursion of the mainline. The incident wave angle has a relatively weak effect on the motion of the main and lantern net. Increasing the layout depths of lantern nets will reduce the horizontal and vertical movement amplitudes and elliptical excursion of the lantern nets. Changing the layout forms of the lantern nets from horizontal to staggered will increase the horizontal motion amplitude and decrease the vertical motion amplitude and elliptical excursion of the lantern nets.     

罩网自由沉降过程的数值模拟研究

通过集中质量法建立罩网网具的数学模型,对罩网自由沉降过程进行数值模拟研究。模拟结果通过罩网4种配重沉降试验实测数据的验证,并展示了罩网自由沉降过程的空间形状变化。研究发现:1)随沉降进行罩网网口形状变为近似圆形,迎流面较圆钝,背流面存在沿海流方向的突出部;2)4种配重间网口质量点拉力、水动力、沉降速度均存在显著性差异且随配重的增加而增大;3)网口质量点的拉力先骤然增大,之后呈现先变小后增大的波动,最后缓慢增大;网口质量点的水动力、沉降速度先骤然变小,之后呈现先增大后变小的波动,最后缓慢减小;4)研究认为在罩网作业所允许的配重范围内,沉降速度随配重的不断增加而持续增大。     

Analysis of juvenile tuna movements as correlated random walk

We explore how a stochastic model provides the most promising avenue towards predicting fish movement. To construct a stochastic model describing fish movement, trajectories of ten juveniles in a water tank were analyzed from a stochastic point of view. The heading angle was defined as a random variable. Our analysis found that the most probable forward heading angle was between 0° and 22.5° (probability ~78%), followed by angles between 22.5° and 45° (probability ~10%). We also found that the choice of future heading angle depends on the current heading angle. Therefore, we treated heading angle state as a first-order Markov process and constructed a correlated random walk model describing juvenile movement in a water tank. Our stochastic model simulated a trajectory similar to observed trajectories. We used the model as a tool for estimating the probability distribution of potential fish path outcomes. We derived the distribution of potential outcomes from a large number of simulations (N = 1000) and investigated these trajectories. We collected a set of juvenile trajectories that collided with the tank and estimated the probability of juvenile collisions with the tank.     

Effect of towing speed and net mouth size on catch efficiency in framed midwater trawls

ABSTRACT:   To investigate fish density using rigid-mouth, towed net gear, such as a framed midwater trawl (FMT), it is necessary to know the catch efficiency. The catch efficiency with an FMT is described as the rate of fish entering the net (entering rate) versus the rate of fish retention, i.e. the mesh selectivity. This study investigated the change in the catch efficiency under several towing conditions using three types of FMT of different net mouth areas (4.0, 12.3, and 16.0 m²) with the same mesh size. The catch efficiency of the FMT was evaluated by comparing catch per unit effort obtained from each towing condition. The catch efficiency of FMT varied with towing speed and net mouth area. Particularly, the effect of net mouth area on the catch efficiency was greater than that of towing speed. Additionally, under the same towing condition, the catch efficiency for large length fish was lower than that for small length fish.     

Size selectivity of a trammel net for oval squid Sepioteuthis lessoniana

ABSTRACT:   The size selectivity of a trammel net for herded oval squid Sepioteuthis lessoniana in Tateyama Bay, Chiba Prefecture, was estimated by comparison between the mantle length frequency distributions of oval squid caught by a trammel net and by a set net. The measured mesh sizes of the inner net of the trammel net and of the final section of the set net were 85.3 and 11.3 mm, respectively. In the trammel net fishery where oval squid are herded into the net, most of the oval squid are caught in the bag-shaped inner net. Hence, the logistic function was employed for the size selectivity curve of the trammel net. The 'share each length's catch total' (SELECT) model was implemented for the estimation of the selectivity curve. The size selectivity r ( l ) of the trammel net for the oval squid was expressed as a logistic function of the mantle length l : r ( l ) = exp(−18.57 + 0.88  l )/[1 + exp(−18.57 + 0.88  l )]. From these logistic parameter estimates, the 50% selection mantle length and selection range ( L ₇₅– L ₂₅) were calculated as 21.07 and 2.49 cm, respectively. The selection probability of oval squid whose mantle girth was equivalent to the mesh perimeter of the inner net was 0.09. Accordingly, oval squid of a girth smaller than the mesh perimeter were likely to pass through the mesh to escape from the net.     

Hydrodynamic performance investigation on an upper and lower water exchange device

Water exchange is an important way to guarantee its good quality between surface and bottom seawater, and it plays a significant role in water environment and ecology as well as fish farming systems. The conceptual experiment (Yin et al., 2018) was extended to present an Upper and Lower Water Exchange Device for strengthening the vertical water exchange performance, and a 3-D CFD mathematical model combining with Reynolds-Averaged Navier-Stokes equations, standard k-ε equations and VOF technique was developed to investigate its hydrodynamic performance and the vertical water exchange capacity for regular waves. The extrapolated relative error and the grid convergence index were used to estimate the discretization error. A series of experiments were conducted to measure the wave surface and water velocity histories, the experimental data were used to validate the mathematical results, and they agreed well with each other. It was found that the inside water motion is determined mainly by the wave overtopping and water entrainment. Generally, the net flow rate decreases at first and then increases with increasing wave steepness. In addition, the ratio of crest freeboard height to incident wave height plays a significant role in water exchange performance. A small ratio leads to an upward movement of the inside water, whereas a high value contributes to a downward net flow to some extent. Finally, a simple formula on the net flow rate was deduced with the dimensional analysis method and the least squares method, and it was validated well with the related data.     

方形箱网结构减流效果试验

根据降低流速对深水网箱养殖的影响这一需要,依据目前深水网箱的尺寸和渔具模型试验方法,设计并制作一个浮式箱网结构模型,同时进行水槽实验。实验在箱网的尾部设定一系列的断面和测量点,通过三维流速仪测量各点在不同实验流速下的流速,通过分析相关数据,探讨了该箱网减流结构尾部的流速分布和减流效果。实验流速采用了32.23 cm/s、50.19 cm/s、67.79 cm/s,分别相当于实际流速45.58 cm/s、70.79 cm/s、95.67 cm/s。实验测量了减流装置尾部减流区域50 cm到125 cm处的流速分布,相当于实际海域中距离减流装置尾部15 m到37.5 m处的流速分布。实验结果显示,箱网尾部的流速减小区域是在网箱网袋与来流垂直的投影面上,减流百分比在20%～45%左右,而箱网两侧的流速略有增加。从实验结果来看,这一结构能有效减小流速,通过合理的设计和在海域中布设,能够满足深水网箱养殖的减流需要。