Study On Oscillation And Stability Of Mid-trawl Codend

Trawling is one of the most productive fishing methods in the world.With the continuous exploitation and utilization of fishery resources,more attention is paid to environmental protection and energy saving in fishing gear fishing method,which leads to the change of the concept of fishing gear design.With the optimization of fishing gear performance,scholars pay more attention to the research of environmental friendly fishing gear.As an important part of the trawl,the codend plays the role of collecting and storing the catch.Previous studies assumed that the trawl remained stable in the towing process and and optimized the trawl parameters on this basis to improve the selectivity and fishing efficiency of the trawl.Considering the fact that the net is unstable in the towing process,this study explores the effect of different factors on the oscillating characteristics(amplitude and frequency)of the codend,and proposes a scheme to improve the stability of the codend.This study focuses on the issues such as "the oscillating pattern and effect factors of the codend in the middle trawl" and "the response of the codend stability characteristics to the codend structure and mesh characteristic parameters".Firstly,based on the measured data at sea,Morlet wavelet transform was used to process the oscillating data of the codend,and the effects of operating parameters(towing speed,warp length,warp tension and catch size)on the oscillating characteristics of the codend were analyzed.Secondly,the effect of mesh size on codend oscillation was taken as the research object,and the applicability of Morlet wavelet transform and FFT methods was compared by model test.Thirdly,through the appropriate data processing method obtained above,the different effect parameters(mesh size,twine diameter,codend length,knot direction,material,catch,etc.)on the stability of codend was explored.Fourthly,the distribution of the flow field of the codend were analyzed by model test and numerical simulation.Finally,based on the above research,the scheme that to improve the stability of the Antarctic Krill trawl codend was proposed,and the performance of the improved codend was evaluated through the flume model test.The main results of this study are as follows:(1)The measured results at sea show that the positional oscillation period of the codend is between 50 and 90 s,and it increases with increasing warp tension but decreases during the towing stage.The positional amplitude of the codend was between 0 and 4 m during the net shooting and hauling stages,and between 0.2 and 0.6 during the towing stage.The codend oscillation amplitude increased with the increase of the warp tension,towing speed,and catch size,while decreased as the warp length increased.In addition,the periodicity of the codend oscillations increased as the towing speed increases during the net shooting and hauling stages,while decreased at the towing stage.This basic results from the codend fluttering motions could be useful to improve the understanding of fish behavior and gear shape that modify the hydrodynamic force acting on the codend instantaneously.(2)Morlet wavelet transform and FFT model tests were used to explore the influence of mesh size on the stability of codend,and the applicability of the two data processing methods was evaluated.The results showed that: The time evolutions were oscillatory quasi-periodic and related to the low-frequency activity.The codend drag oscillation frequency was mainly 0.30–0.45 Hz and decreased with the mesh size,and the amplitude and wavelet coefficient of drag force increased as the mesh size decreased.The codend position oscillates frequency was 0.30–0.45 Hz in X and z-directions.The drag oscillation frequency was mainly 0.30–0.45 Hz and synchronized with the position motion.The dominant frequencies of these codend motions and the associated drag showed low-frequency activities,and the wavelet spectrum was transferred from small-scale to large-scale motions.In addition,the codend with a greater mesh size showed more stable behavior with a lower frequency and oscillatory motions compared to the codend with a small mesh size led to a decrease in drag on this codend.In addition,we recommend that the FFT can be used to obtain the amplitude and frequency directly for codend steady oscillation.(3)The model test analyzed the influence of various factors(mesh size,twine diameter,codend length,knot direction,material,catch,etc.)on the oscillating characteristics of the codend.The results showed that drag of the codend increases with decreasing mesh size,increasing bar diameter,and increasing codend length.The T90 codend has greater drag force than the T0 codend.Under the condition of empty codend,the frequency of drag oscillation was 0.35-0.40 Hz,while the condition of codend with catch,the frequency of drag and position oscillation was 0.29-0.49 Hz.Under the condition of empty codend,the drag amplitude of the codend decreases with increasing mesh size,decreasing bar diameter,and increasing codend length.The drag amplitude of the T90 codend was smaller than that of the T0 codend.When there was catch in the codend,the drag amplitude of the codend increased with the increase of flow velocity,and the decrease of mesh size,the decrease of bar diameter,and the increase of codend length.The study used the T90 for the codend and found that it increased the amplitude of the codend drag.Nylon codend showed large drag amplitude compared to polyethylene codend under the same flow velocity.It was found that the codend with liner was smaller than that of codend without liner below 0.8 m/s but larger at flow velocity above 0.8 m/s.The knotless of the linner showed smaller drag amplitude compared to the linner with knots.The amplitude of the codend position was decreased with increase in the bar diameter and the length of the codend.The T90 codend showed smaller amplitude compared to T0 codend.The amplitude was smaller compared to codend with liner,and the amplitude of codend with knot liner was similar to the knotless liner.(4)Statistical model analysis shows that the weight of each factor affecting the drag amplitude of empty codend is: flow velocity,twine material,knot direction,mesh size,bar diameter,and codend length.For the drag amplitude of codend with catch,the impact weight of each factor in decreasing order was: flow velocity,mesh size,codend length,bar diameter,twine material,and knot direction.For the codend position amplitude in the x direction,the impact weight of each factor in decreasing order was: codend length,mesh size,bar diameter,twine material,flow velocity,and knot direction.For the codend position amplitude in the z direction,the impact weight of each factor in decreasing order was: mesh size,twine material,bar diameter,flow velocity,codend length,and knot direction.(5)With the increase in the catch size,the drag force,codend motion,Reynolds number,and codend volume increase,while the drag coefficient decreases.The oscillation amplitudes of the drag force and codend motion increase as the catch size increases and mainly involve a lowfrequency activity.The mean flow velocity inside and around the unlined codend without catch was found to be 5.81%,14.39%,and 27.01% greater than those obtained inside and around the unlined codend with catch,the codend with liner and without catch,and the codend with both liner and catch,respectively.In addition,the POD results demonstrate that an unsteady turbulent flow develops inside and around the codend,This flow involves the eddy flow caused by the blocking of the flow passage due to the presence of the liner,the vortex shedding caused by the intense codend motion and the presence of the catch,and the turbulence due to the codend wake.Finally,it was found that the Strouhal numbers related to the vortex low frequency of the codend with both catch and liner are greater than the vortex flow frequency of the unlined codend without catch.(6)The trawl-J was worse than the original trawl,with an average drag force of 2.4%-6.0%larger than that of original trawl,but the drag and position amplitude of the trawl-J were smaller than those of the original trawl.The codend cross-sectional area of trawl-K was more expanded than the original trawl,effectively improving the opening of meshes,the average drag force of the trawl-K was increased by 5.1%-7.6% compared to the original trawl.The trawl position and drag amplitude of the trawl-K was more stable than those of the original trawl and trawl-J.Therefore,it is recommended to use a codend made of a larger mesh size with 50% and T90,while keeping the other parameters the same as the original trawl,to improve the stability of the fishing trawl and the selectivity of the codend.