Variations in hydrodynamic characteristics of netting panels with various twine materials, knot types, and weave patterns at small attack angles

Hao Tang,Xuefang Wang,Shuchuang Dong,Liuxiong Xu,Fuxiang Hu,Cheng Zhou

doi:10.1038/s41598-018-35907-1

Abstract

It is essential to conduct hydrodynamic experiments for fishing gear at small attack angles along the flow direction to better understand the hydrodynamic characteristics of netting and application of gear. The hydrodynamic characteristics of netting panels made of different materials at small attack angles were investigated by a self-designed setup; this is essential for the effective use of netting on different types of gears. As confirmed by experiments, the measured drag of designed frame without netting accounted for less than 20% of the total setup drag including experimental netting and remained in a steady state under various current speeds and small attack angles, indicating that the self-designed frame setup is suitable for such trials. The drag coefficient was determined by varying the attack angle, solidity ratio, Reynolds number, knot types, weave pattern, and twine materials at small attack angles. The results indicate that the drag coefficient increased as the attack angle increased, but decreased as the solidity ratio and Reynolds number increased. The drag generated by knot accounted for 21% of the total drag of nylon (PA) netting. For braided knotless netting, the drag coefficient of PA netting was about 8.4% lower than that of polythene netting (PE) and 7% lower than that of polyester netting (PES). Compared with twined netting, the braided netting exhibited a higher resistance to flow, corresponding to higher values of drag coefficient.

Highlights

A good understanding of hydrodynamic characteristics of fishing gear is essential for the development and operation of efficient gears with a low impact on environment and nontarget species
Small differences in drag coefficient can lead to large differences in force, and while evaluating the forces acting on a net, the effect of attack angle cannot be ignored, especially small attack angles
Because the underwater shape is affected by current, fishing gear structures, and fishing operations, the movement of fishing gear is shown in an approximate angle state along the current direction

Summary

Introduction

A good understanding of hydrodynamic characteristics of fishing gear is essential for the development and operation of efficient gears with a low impact on environment and nontarget species. Extensive studies have been conducted on fishing gear consisting of fabric nets focusing on the hydrodynamic characteristics of fabric itself[4,5,6,7,8] Aarsnes et al.[9] derived a formula for drag coefficient as a function of solidity ratio (the ratio of twine diameter to the bar length) and angle of incidence (angle between the current direction and normal to a net plane) based on tow tank data; the effect of various attack angles was not considered. Hosseini et al.[7] obtained the drag and lift coefficients of nylon (PA) knotted netting from flume tank experiments to simulate the sinking behavior of purse seine; the hydrodynamic coefficients depend on Reynolds number and attack angle. The progress in studies of hydrodynamic characteristics of a netting at small attack angles is slow because of the limitations of experimental setup including the frame, installation methods, and precision instrument

Objectives

Methods

Results

Conclusion