Date Published: February 8, 2018
Publisher: Public Library of Science
Author(s): Hao Tang, Liuxiong Xu, Fuxiang Hu, Alexander J. Werth.
Nylon (PA) netting is widely used in purse seines and other fishing gears due to its high strength and good sinking performance. However, hydrodynamic properties of nylon netting of different characteristics are poorly understood. This study investigated hydrodynamic characteristics of nylon netting of different knot types and solidity ratios under different attack angles and flow velocities. It was found that the hydrodynamic coefficient of netting panels was related to Reynolds number, solidity ratio, attack angle, knot type and twine construction. The solidity ratio was found to positively correlate with drag coefficient when the netting was normal to the flow (CD90), but not the case when the netting was parallel to the flow (CD0). For netting panels inclined to the flow, the inclined drag coefficient had a negative relationship with the solidity ratio for attack angles between 0° and 50°, but a positive relationship for attack angles between 50° and 90°. The lift coefficient increased with the attack angle, reaching the culminating point at an attack angle of 50°, before subsequent decline. We found that the drag generated by knot accounted for 15–25% of total drag, and the knotted netting with higher solidity ratio exhibited a greater CD0, but it was not the case for the knotless netting. Compared to knotless polyethylene (PE) netting, the drag coefficients of knotless PA netting were dominant at higher Reynolds number (Re>2200).
Meshes of traditional netting panels are composed of bars and knots. More recently, netting panels can be weaved without knots, and are called knotless netting. In knotted netting, single English knot and double English knot are commonly used knotted types for fishing nets, while in knotless netting, twisted Cross and Ultra Cross (named by Nichimo Co., Ltd, Shimonoseki, Japan) are the basic knotless type according to the way the mesh is wove. One parameter of a netting panel is the solidity ratio, which is defined as the ratio of projected area of bars and knots to the outline area of a netting panel. Different knot types influence the solidity ratio of netting, which may result in differential hydrodynamic forces. Usually the knotted netting has greater drag force than knotless netting of the same twine diameter and materials, and knotted netting panels with different knot types have different hydrodynamic characteristics .
In this study, the hydraulic mean depth (h) defined as the ratio of area to the perimeter of the mesh  was used as characteristic length for netting when it was set normal to the flow. We suggested that the flow through the plane netting could be considered as a flow in the pipe, the hydraulic mean depth can thus be used as the characteristic length to determine the range of Reynolds number in experiment. Considering the porosity of netting, the characteristic length of experimental netting was characterized by a combination of the hydraulic mean depth and porosity, h = 0.25 βl sin 2φ. A better fitting prediction result was presented in Fig 4. Our findings show that the drag coefficient decreased and approached a steady value distributed in a space between 1.2 and 1.4 when Rh ranged from 1.5×103 to 1.0×104, similar to that reported by Miyazaki and Takahashi .
Experiments were carried out to investigate the effects of solidity ratio, flow velocity and knot type on the hydrodynamic characteristics of plane netting in uniform flow at different attack angles. The following conclusions can be drawn from the experiments and subsequent analysis: