基于网具内外流场特性的南极磷虾拖网结构优化研究

Antarctic krill fishery is an important new growth point of China's distant-water fishery, and the development and utilization of this resources has become a major scientific and technological innovation project in China's space exploration and strategic resources development. Krill fishing has traditionally been conducted using large pelagic trawls because of the weak swimming ability and aggregation characteristic of Antarctic krill. In previous studies, physical experiments were used to reduce fishing gear resistance and improve mouth spread by changing operating parameters and mesh parameters, However, the basic research on the effect of the related factors on the distribution of flow fields inside and around the gear has been neglected, and the optimization of trawl gear performance has not been solved from the mechanism, resulting in the uncertainty of the research results. The flow fields inside and around the trawl net are measured using the particle image velocimetry (PIV) technique and the acoustic Doppler velocimeter (ADV) to explain the mechanism of the distribution of flow fields inside and around the trawl system based on physical standpoint, build the flow-velocity reduction index model, and illuminate the mechanism and influencing factors of flow field distribution in different parts of trawl. The changes of movement pattern of krill and hydrodynamic force action on different parts of trawl are obtained from this study, and a scheme for improving the trawl performance is constructed based on flow field distribution of the gear. The results will help optimal design of Antarctic krill trawl and implement ecological efficiency and low energy consumption fishing.

南极磷虾渔业是我国远洋渔业极为重要的新增长点，该资源的规模化开发与利用已成为我国空间拓展与战略资源开发领域中一项重大科技创新工程。南极磷虾游泳能力较弱，具有集群特性，中层拖网是捕获南极磷虾最为合适的高效渔具。针对拖网渔具性能优化，以往的研究多利用物理实验通过改变有关参数，减少渔具阻力，提高网具扩张，但却忽略了因素的改变对网具内外流场分布的影响这一基础性研究，并未从机理上解决问题，从而导致研究结果的不确定性。本研究拟结合粒子图像测速(PIV)技术和声学多普勒流速仪(ADV)对南极磷虾拖网模型网内外流场的流速矢量进行测定，从物理学角度探究拖网网具内外流场空间分布的变化规律，构建流速衰减指标模型，阐释拖网不同部位流场分布的发生机理和影响因素，揭示入网渔获的运动模式及其与拖网不同部位水动力作用机理的关系，建立基于流场分布特征的网具性能优化体系，为改进南极磷虾拖网设计，实现绿色高效捕捞提供科学支撑。

本研究分三个步骤实现南极磷虾拖网渔具的改进设计思路和方向的确立：首先，采用CFD数值模拟和物理模型试验探究了不同规格网片水动力及流场特性变动规律，构建了流速衰减模型；其次，利用物理试验方法获取了网囊和磷虾拖网周围流场分布规律；最后，结合海上局部附网法选择性试验，获取了磷虾入网后在拖网不同部位的逃逸情况，并基于流场分布规律和虾类行为特性的考虑，提出了南极磷虾拖网的优化方案，量化出内衬网等关键参数对拖网工程性能的影响效果。研究结果发现：①有、无结节的十字型和网目结构下游均具有流速衰减现象，对于网目结构，流速衰减率随冲角的减小而增大，且不同结构尾部的流速衰减率在10%左右。②对比动水槽试验结果，验证了构建的CFD数值模型可准确地对不同因子作用下网片受力和变形进行模拟。③局部附网法海上实测结果显示，磷虾逃逸行为主要发生在拖网第一段网身大网目处，而第二段网身近乎无逃逸行为；昼、夜对于磷虾穿过网目的逃逸结果有显著性差异。④拖网网具内外存在非定常湍流，且拖网尾流会引起紊流和旋涡脱落，造成拖网位置及周围流场呈周期性振荡。⑤网囊末端结构位置和振荡的变化造成流速下降，越接近网囊末端，流速越低；有渔获的网囊垂直方向的速度比变化较大，装配内衬网的网囊在中部速度最低。⑥随着渔获量的增加，网囊阻力和振荡效果增大；内衬网、结节类型和结构片式对网囊水动力和振荡特性影响显著。⑦内衬网比例减少50%，拖网阻力减小约20%，网口面积增加约5%；拖网水阻力随着浮沉比减小，网板扩张增大而增大。上述研究结果以渔具基础研究中流场分布特性为抓手，拓展了渔具振荡特性的研究内容，深入地剖析出网具水动力作用机制，为生态高效低能耗拖网渔具研发奠定了科学基础。