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The hydrodynamic characteristics of polyethylene (PE) netting and chain link wire netting with diŠerent types of twine diameter and mesh size for aquaculture net cages were examined by experiments in a ‰ume tank.The drag coe‹cient C D0 of plane netting set parallel to the water ‰ow increases in the order of PE netting with diamond mesh, PE netting with square mesh and chain link wire netting. The drag coe‹cient C D90 of the chain link wire netting set normal to the water ‰ow was 0.8 0.9 times that of the PE netting. Moreover, the drag coe‹cient C Du and lift coe‹cient C Lu of PE netting and chain link wire netting set at various angles of attack to the water ‰ow were estimated by empirical equations as functions of C D0 , C D90 , mesh factor and angle of attack, which provide a relevant basis for analyzing and predicting the behavior characteristics of cages in the sea.

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Dynamic responses of tuna purse seine to currents were numerically studied with regard to the vertical subsidence and global load distribution, and determinant attributes were identified. We rebuilt the submerged geometry of a purse seine net using the lumped mass method with hydrodynamic coefficients obtained from measurements of the prototype material, as well as a mesh grouping method, which corrects the twine diameter and netting material density for the equivalent net by introducing the compensation coefficient. Uneven tension distribution showed that it was vulnerable to high loads in the bunt area at shooting and along the lead line at pursing. High loads were present at the convex sections of net circles in the direction of the current. Higher shooting speeds resulted in well-balanced sinking, while a reduced hanging ratio of netting panels was beneficial to faster sinking. While large mesh sizes reduce water resistance and increase sinking velocity, extensive use should be cautioned in terms of the sensibility to stress. Comparing two options of different proportions of large-mesh panels, mesh sizes doubled for 15 strips versus 5 strips, suggesting that the 15 strip option would pose a higher risk of strand vulnerability, while the 5 strip option may be a more balanced alternative with a lower mesh density and a lower mesh stress.