College of Marine Sciences, Shanghai Ocean University, 999 Huchenghuan Road, Lingang New District, Shanghai, 201306, People's Republic of China.
National Engineering Research Center for Oceanic Fisheries, Shanghai, 201306, People's Republic of China.
Sci Rep. 2023 Apr 17;13(1):6249. doi: 10.1038/s41598-023-33230-y.
The interaction between fluid and the midwater trawl with stocked catches is extremely complex, but essential to improve the understanding of the drag force acting on the trawl, the behavior of the fishing structure during a trawling process, and to predict its selectivity process. The present study assesses the turbulent characteristics inside and around the midwater trawls with catch and without catch linked to its fluttering motion. The analysis is based on three-dimensional electromagnetic current velocity meter measurements performed in the multiple points inside and outside different parts of a 1/35 scaled midwater trawl model with the aim of access the main turbulent flow structure inside and around the gear. Time-averaged normalized flow velocity fields and turbulent flow parameters were analyzed from the measured flow data. Furthermore, Fourier analysis was conducted by watching the time-frequency Power spectrum content of instantaneous flow velocities fields, the fluttering trawl motions, turbulent kinetic energy, and momentum flux. Based on successive analyzes of mean flow characteristics and turbulent flow parameters, it has been demonstrated that the presence of catch inside the trawl net impacts the evolution of unsteady turbulent flow by creating large trawl fluttering motions that strongly affect the flow passage. The results showed that the time-averaged normalized streamwise and transverse flow velocities inside and around the trawl net with catch were 12.41% lower compared with that obtained inside and around the trawl without catch. The turbulent length scale and turbulent Reynolds number obtained in the different part of the trawl net with catch were about 33.05% greater than those obtained on the trawl net without catch, confirming that the unsteady turbulent flow developing inside and around the midwater trawl is influence by the catch and liner. It is observed that the motions of both the trawl without catch and the trawl with catch are mainly of a low-frequency activity and another component related to unsteady turbulent flow street. A complex fluid-structure interaction is then demonstrated where the fluttering motions of the trawl net affect the fluid flow inside and around trawl net, the fluid force, turbulent pattern, and simultaneously, the periodic unsteady turbulent flow influence the trawl motions.
水层流与带渔获物中层拖网之间的相互作用极其复杂，但对于提高对拖网拖曳力的认识、了解拖网过程中捕捞结构的行为以及预测其选择性过程至关重要。本研究评估了带渔获物和不带渔获物的中层拖网内部和周围的湍流动特性及其随拖网摆动的运动。分析基于在 1/35 比例中层拖网模型的多个点进行的三维电磁流速计测量，旨在获取渔具内部和周围的主要流结构。从测量的流数据中分析了时均归一化流速场和湍流动参数。此外，通过观察瞬时流速场、拖网摆动、湍流动能和动量通量的时频功率谱内容，进行了傅里叶分析。基于对平均流特性和湍流动参数的连续分析，表明渔获物在拖网内的存在通过产生强烈影响流道的大型拖网摆动来影响非定常湍流动的演化。结果表明，带渔获物的拖网内和周围的时均归一化流向和横向流速比不带渔获物的拖网内和周围的流速低 12.41%。带渔获物的拖网不同部分获得的湍流动长度尺度和湍流动雷诺数比不带渔获物的拖网大约 33.05%，这证实了非定常湍流动在中层拖网内和周围的发展受到渔获物和衬里的影响。观察到无渔获物和带渔获物的拖网的运动主要是低频活动，另一个与非定常湍流动街相关的组件。然后证明了一种复杂的流固相互作用，其中拖网的摆动运动影响拖网内和周围的流体流动、流体力、湍流动模式，同时周期性的非定常湍流动也影响拖网的运动。
