PA单丝网片水动力特性水槽试验研究

聚酰胺(PA)单丝网片具有较好的渔用材料性能,在渔具减阻降耗方面具有一定的开发利用潜力。通过水槽试验,设计研究了7种不同线面积系数(α=0.050～0.173) PA单丝双死结网片的水动力变化规律,并与PA复丝单死结网片、超高分子量聚乙烯(UHMWPE)复丝单死结网片,及PE绞捻网片的水动力特性进行对比分析。结果显示:PA单丝双死结网片垂直于水流时,阻力系数C_(D90)在1.043～1.411范围内与线面积系数成正相关关系;平行于水流时,阻力系数C_(D0)在0.223～0.359范围内与线面积系数成负相关关系,从而也证实了线面积系数对于网片阻力特性存在双重影响;当PA单丝双死结网片与水流方向呈倾斜角度时,阻力系数随冲角的增大而增大,而升力系数C_(Lθ)先增大后减小,在50°时达到最大值。对比3种不同材料网片的水槽试验,当网片垂直于水流时,PA单丝双死结网片的阻力系数比PA复丝单死结网片小9.8%,比UHMWPE复丝单死结网片大17.3%,比PE绞捻网片小14.7%;当网片平行于水流时,PA单丝双死结网片的阻力系数比PA复丝单死结网片大16.6%,比UHMWPE复丝单死结网片大35.3%,比PE绞捻网片小21.2%。从而说明在相同条件下,应用PA单丝网片可以降低水流阻力,为PA单丝网具的设计、制作与应用提供了理论依据。     

六角形目和无结菱形目经编网片阻力性能与运动变化的比较

通过水槽模型实验,对六角形目经编网片(RHN)和无结菱形目经编网片(KRNDM)不同流速(V,0.5、0.75、1.0、1.25、1.5、1.75、2.0 kn)、不同横向缩结系数(Et,0.60、0.65、0.707)下的网片阻力性能和运动变化进行了比较分析。结果如下:(1)网片固定时,六角形目网片阻力较大,无结菱形目网片阻力较小。在横向缩结系数范围(0.60~0.707)内,无结菱形目网片的阻力随着Et增大而逐渐减小,六角形目网片阻力则与Et变化无明显相关。(2)相同Et时,六角形目网片阻力始终较无结菱形目网片阻力大,且与无结菱形目网片之间的阻力差值随流速的增加逐渐增大。(3)Et对于水平位移和垂直位移的影响随网片的结构差异而不同。相同Et时,六角形目网片的水平位移均较大,无结菱形目网片较小。在设计选用网箱网片时(Et,0.60~0.707),首先应考虑网片网目结构和网线粗度,其次考虑横向缩结系数。     

渔具材料基本名词术语(五)

<正> 网片方向 netting direction网片尺度的方向。注:无结网片的方向,一般也与网线的总走向有关,但并不完全如此。因为,网线的总走向有时也不易判断。一般情况是,其网目最长轴方向与网线总走向相平行。如果网目的两个轴长相等,则网片的方向就无法确定。这时,网目的尺寸可按任—方向来确定。纵向(N)N-direction网片长度的方向。有结网片的纵向 N-direction of knottednetting与结网网线总走向相垂直的方向(见图8)。     

K型网目对渔用乙纶网片拉伸力学性能的影响

以网片断裂强力、网片断裂伸长率和网片断裂强力保持率为指标，研究了不同K型网目对渔用乙纶网片拉伸力学性能的影响。结果表明：K型网目下网目内径之比在1.01～1．20时，网片断裂强力随着网目内径之比的增大而下降，两者呈密函数相关。同规格线的网片，K型网目下且两网目内径之比相同时，小规格网目尺寸的网片断裂强力损失小，断裂伸长率高，而大规格网目尺寸的网片断裂强力损失大．断裂伸长率低。     

网具工艺名词术语(一)

<正> 网片(网衣)编织 braiding制作网片(网衣)时,纤维或纤维制品以一定规律交叉穿插成网目的工艺,简称织网.网片(网衣)编结 braiding制作网片(网衣)时,网线作结以构成网目的工艺,简称结网。注:本篇中的有关规定,主要适用于有结网片(网衣).网片(网农)纵向 N-direction网片(网衣)中,与结网网线总走向相垂直的方向(图1),代号 N。     

3种网箱用网片的阻力性能与运动变化比较

采用模型试验,3种不同网片分别为六角形目经编网片RHN、菱形目无结经编网片KRNDM、菱形目有结绞捻网片KTNDM,规格均为1.7mm(水平缩结长度)×1.7 mm(垂直缩结高度).在不同流速下,通过改变网片的水平缩结系数(Et分别为0.65、0.707、0.60),对网片的阻力性能和运动变化(水平位移和垂直位移)进行研究.结果如下:(1)网片固定时,菱形目网片的阻力在水平缩结范围(Et=0.60～0.707)内,随着缩结增大而逐渐减小.六角形目网片阻力在3者中最大,有结绞捻网片阻力最小,六角形目网片阻力与缩结关系不明显.3种网片之间的阻力差值随流速的增加而增大.(2)缩结系数对于水平位移和垂直位移的影响随网片的结构差异而不同.缩结系数相同时,六角形目网片的水平和垂直位移均为最大,有结绞捻网片均最小.(3)网片的阻力与水平垂直位移受迎流面积的影响.在缩结系数范围内(Et=0.60～0.707)设计网片时首先应考虑网片结构和网线粗度,其次考虑结节,最后考虑缩结系数.     

方形目网片手编和由菱形目网片改制的工艺研究

迄今为止，用于捕捞生产的网片，其网目都是由网线斜向交叉作结(或插、绞捻)而成。网线斜向交叉的结构决定了网目形状随缩结系数变化而保持相应状态的菱形，故称之为菱形目网片。网目的菱形结构能够从网具产生一直延用至今，其对捕捞生产具有很强的适应性是不容怀疑的。然而，生产实践表明，在水域封闭能力，网片局部形变，网目选择能力，     

网片的表面积和计算方法

<正> 捕捞作业过程中,流体对网具的阻力主要归结于对网片的阻力.历来在计算网片水中阻力时,都是以网片在运动方向的投影面积作为网衣阻力的计算依据,即网衣网线的粗度或直径是与阻力相关的主要参数,对于网片相对于流体作垂直运动时,这样的计算是正确的.但大部分的渔捞作业工况,如拖网拖曳时上下身网与水流的相对运动;围网放网时网衣迅速垂直下降与水流的相对运动以及流刺网的拖曳等等,其网衣与流体的相对运动夹角极小,几     

南麂列岛海域两种框型人工鱼礁水动力性能试验

为改良南麂列岛海域投放的人工鱼礁礁体存在的沉陷、位移现象,试验研究了人工鱼礁水动力性能,并进行对比验证。采用水槽试验方法,对两种框型人工鱼礁模型在5种水流速度(0.15、0.20、0.25、0.30、0.35 m/s)和4种迎流角(0°、15°、30°、45°)条件下的阻力进行测定,并计算阻力系数。以水槽试验所测得的阻力系数,结合波流动力学理论计算两种实物礁体的水阻力、抗滑移系数和抗倾覆系数。结果显示,两种礁体的阻力均与流速呈幂函数关系,在流速v=0.35 m/s时的阻力差比值最低;两种礁体在相同流速下的阻力差比值均随冲角增大而减小;在不同流速下,两种礁体的阻力系数均在冲角θ=15°时差异最小,在θ=45°时差异最大;随着冲角和迎流速度的增加,礁体的抗滑移、抗倾覆系数逐渐减小,两种礁体在海水流速u≥5节、冲角θ≥30°时抗滑移系数<1,但抗倾覆系数始终>1.2。研究表明,改良后的框型鱼礁在稳定性方面有一定的优势。     

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。     

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.     

Use of non-linear regression to evaluate drag force and volume coefficient of structure of square cage

ABSTRACT:   Cage aquaculture is increasing in importance in fisheries. However, in past studies the relationship between drag force and volume coefficient of the cage bag, and four key factors affecting its efficiency, i.e. flow velocity, ratio of depth to width of the cage, twine diameter to bar length of netting ( d/l ), and weight of the cage has been limited to partial regression and qualitative discussion. This paper establishes two non-linear regressions of the drag force and volume coefficient of a cage bag with seven flow velocities, five ratios of depth to width of the cage, four d/l , and five weights of the cage. The experimental data are processed using the forward selection method in the stepwise regression selection procedure in order to prioritize each factor, and then establish non-linear regressions of the drag force and volume coefficient using multiregression analysis. The results show that factors affecting the drag force are, in order of importance, flow velocity, ratio of depth to width of the cage, d/l and weight. With respect to the volume coefficient, factors in order of importance are flow velocity, weight, ratio of depth to width of the cage, and d/L. The two regressions can predict drag force and volume coefficient more comprehensively than those in previous studies. Finally, the regressions are applied to the full-scale structure of the cage by Tauti's modeling rules.     

Drag force acting on biofouled net panels

Measurements were made to assess the increase in drag on aquaculture cage netting due to biofouling. Drag force was obtained by towing net panels, perpendicular to the incident flow, in experiments conducted in a tow tank and in the field. The net panels were fabricated from netting stretched within a 1 m² pipe frame. They were towed at various speeds, and drag force was measured using a bridle-pulley arrangement terminating in a load cell. The frame without netting was also drag tested so that net-only results could be obtained by subtracting out the frame contribution. Measurements of drag force and velocity were processed to yield drag coefficients.

Clean nets were drag tested in the University of New Hampshire (UNH) 36.5 m long tow tank. Nets were then exposed to biofouling during the summer of 2004 at the UNH open ocean aquaculture demonstration site 1.6 km south of the Isles of Shoals, New Hampshire, U.S.A. Nine net panels were recovered on 6 October 2004 and immediately drag tested at sea to minimize disturbing the fouling communities. The majority of the growth was skeleton shrimp (Caprella sp.) with some colonial hydroids (Tubularia sp.), blue mussels (Mytilus edulus) and rock borer clams (Hiatella actica). Since the deployment depth was 15 m (commensurate with submerged cages at the site), no algae were present. The net panels had been subject to several different antifouling treatments, so the extent of growth varied amongst the panels. Drag force measurements were made using a bridle-pulley-load cell configuration similar to that employed in the tow tank. Fixtures and instruments were mounted on an unpowered catamaran that was towed alongside a workboat. Thus, the catamaran served as the “carriage” for field measurements.

Increases in net-only drag coefficient varied from 6 to 240% of the clean net values. The maximum biofouled net drag coefficient was 0.599 based on net outline area. Biofouled drag coefficients generally increased with solidity (projected area of blockage divided by outline area) and volume of growth. There was, however, considerable scatter attributed in part to different mixes of species present.     

Drag force acting on biofouled net panels

Measurements were made to assess the increase in drag on aquaculture cage netting due to biofouling. Drag force was obtained by towing net panels, perpendicular to the incident flow, in experiments conducted in a tow tank and in the field. The net panels were fabricated from netting stretched within a 1 m² pipe frame. They were towed at various speeds, and drag force was measured using a bridle-pulley arrangement terminating in a load cell. The frame without netting was also drag tested so that net-only results could be obtained by subtracting out the frame contribution. Measurements of drag force and velocity were processed to yield drag coefficients.Clean nets were drag tested in the University of New Hampshire (UNH) 36.5 m long tow tank. Nets were then exposed to biofouling during the summer of 2004 at the UNH open ocean aquaculture demonstration site 1.6 km south of the Isles of Shoals, New Hampshire, U.S.A. Nine net panels were recovered on 6 October 2004 and immediately drag tested at sea to minimize disturbing the fouling communities. The majority of the growth was skeleton shrimp (Caprella sp.) with some colonial hydroids (Tubularia sp.), blue mussels (Mytilus edulus) and rock borer clams (Hiatella actica). Since the deployment depth was 15 m (commensurate with submerged cages at the site), no algae were present. The net panels had been subject to several different antifouling treatments, so the extent of growth varied amongst the panels. Drag force measurements were made using a bridle-pulley-load cell configuration similar to that employed in the tow tank. Fixtures and instruments were mounted on an unpowered catamaran that was towed alongside a workboat. Thus, the catamaran served as the “carriage” for field measurements.Increases in net-only drag coefficient varied from 6 to 240% of the clean net values. The maximum biofouled net drag coefficient was 0.599 based on net outline area. Biofouled drag coefficients generally increased with solidity (projected area of blockage divided by outline area) and volume of growth. There was, however, considerable scatter attributed in part to different mixes of species present.     

The sinking performance of the tuna purse seine gear with large-meshed panels using numerical method

This paper reports an improved version of the numerical method used in a previous study on the dynamic simulation of purse seine gear in three dimensions. The improvement is achieved by refining the mass–spring model to take into account both the drag coefficient as a function of the attack angle and Reynolds number as applied to the setting operation of the purse seine gear. The validity of the numerical simulation is assessed by comparing the measured and calculated values for the sinking depth of the net. The numerical simulation is used to examine the sinking performance of the different designs in which large meshed-panels and netting materials are used together in the main body section of the netting. The results indicate that, compared to the prototype net, nets bearing larger mesh panels require more sinking depth with much more pronounced operational depth at corresponding times of the fishing operation when heavier netting material is used. Moreover, in the new net designs, lower tensile forces are exerted on both ends of the pure wire during pursing. The new net constructions with regard to the operational depth represent alternatives that may reduce the potential problem of frequent failed setting of the tuna purse seine gear.     

Aquaculture Net Drag Force and Added Mass

The hydrodynamic loads on plane net samples of differing mesh geometry are measured in steady and oscillating flows. The steady loads on plane nets are also numerically simulated. The net is modeled as an inter-connected system of lumped masses and springs. The loads are computed for each twine segment and applied to the lumped masses at the segment ends. The equations of motion are formulated for the coupled dynamics of the masses and solved numerically. Drag data from the experiments is compared with analytical and numerical models and existing empirical formulae. Results for steady flows indicate that drag coefficients for nets and cylinders, as a function of the Reynolds number, have identical trends with consistent offsets. It is concluded that the drag coefficient for nets is equivalent to the drag coefficient for cylinders (and spheres for knotted nets) modified by a function of net solidity. For unsteady flows, the drag and added mass are extracted from the total wave force by applying a vector approach. It is shown that drag and added mass coefficients are not well quantified by conventional non-dimensional parameters (i.e. Keulegan–Carpenter and Reynolds numbers). The unsteady drag coefficient is presented as a function of wave particle velocity, wave period and net porosity. It is proposed that the added mass coefficient be expressed by an assumption of an effective thickness—conceptually the width of water affected by the net, which is a function of wave frequency and net solidity.     

Fundamental studies on the hydrodynamic resistance of small pot traps

ABSTRACT:   The hydrodynamic resistance of small pot traps has been conducted in order to establish some basic information. The specific objectives of the study was to measure the hydrodynamic force and estimate the critical setting condition for traps. Five types of traps with different materials were used in the experiment: a netted semi-cylinder shape, a wire semi-cylinder shape, a heart shape, a box shape, and a cylinder shape. The hydrodynamic force of each trap was measured in a flume tank. Flow speeds in the flume tank were 0.1, 0.2, 0.3, 0.4, and 0.5 m/s. Attack angles for this study were 0, 15, 30, 45, 60, 75, and 90 degrees. At an attack angle of 0 degrees the main axis of the trap was parallel to the water flow and at 90 degrees it was vertical. The values of the hydrodynamic drag coefficient varied with traps: netted semi-cylinder shape, 2.75–5.96; wire semi-cylinder shape, 2.81–4.49; heart shape, 2.77–3.66; box shape, 2.39–2.97; and for cylinder shape, 3.57–3.67. The flow speed (0.5 m/s) was effective to set the netted semi-cylinder, wire semi-cylinder, box, and cylinder shaped traps. The same flow speed applied to the heart-shaped trap was only effective to a maximum of 30 degrees attack angles and below.     

立式V型曲面网板的水动力性能

采用正交优选法来考察网板板面折角、展弦比以及后退角对立式V型曲面网板水动力性能的影响。试验结果证明影响网板水动力性能的最重要的因素是网板板面折角,其次是展弦比和后退角。当网板的板面曲率为14%、板面折角为12°、展弦比为1.60、后退角为10°时,网板具有较高的水动力性能。当冲角为25°和28°时,网板的升力系数均为1.68。另外,通过对优选网板添加模拟海底的试验证明,网板在底层作业时,其临界冲角从28°减小为25°;在常用工作冲角范围内,网板在底层时的扩张性能要高于中层,同时,网板的升阻比也略有上升,并能在较宽的冲角范围内持续保持较高的扩张性能。