Study Of Tuna Purse Seine Performance Based On Numerical Simulation

Modern tuna purse seine net consists of various netting panels with different mesh sizes, forming an extremely large strip-typed structure for effective capture of pelagic species. During the process of fishing manipulation, a circumferential casting motion attempt to surround the shoals in the space, and then hauling the purse line to close the bottom of the net. Once tightened of the purseline, the net just likes a bowl shape, and there is no any probability for fish inside it to escape. An effectual catch is immediately dependent on the spatial volume and shape of net, rapidity of sinking, and the bottom closure of the net during pursing. The performance of tuna purse seine vary with the structure of fishing gear, environmental conditions on fishing grounds and fishing operations.Study approaches in regard to the design and modification of purse seine used to be applied mainly based on sea trial, model experiment and dynamical simulation. As such, this study simulated the submerged dynamical behavior of purse seine gear based on a mass-spring model to realize the visualization of the shooting and pursing process of purse seine gear. We tested the he hydrodynamic characteristics of knotless nylon netting with various mesh geometries in the flume tank, which provided the empirical coefficient formula of drag and lift for the establishment of dynamical model. Some fundamental studies in terms of net motion, transformation and tension force by model experiment and sea trial were conducted to provide the underlying parameters as the reference for dynamical model and were used for the accuracy verification. The results were as follows:(1) To realize the precise visualization of purse seine simulation, it is need to obtain the hydrodynamic coefficients of real netting. This study estimated the semi-empirical formulae of the knotless twisted nettings and Ultra Cross nettings in the context of drag coefficient with the net panel in normal free stream. The normal drag coefficient of UC netting was then determined in terms of the Reynolds number and solidity ratio. The differences in the material roughness or structural pattern also influenced the drag coefficient. As the net panel is inclined in the free stream, drag and lift coefficients are considered related to attack angle. When the attack angle is less than 50°(close to parallel state), drag coefficient increases as solidity ratio shrinks. For angles exceeding 50°(close to normal state), drag coefficient rises with the increased solidity. The dual effects of solidity on inclined drag coefficient are considered dependent on the drag coefficients normal to and parallel to free stream simultaneously. The dependence of lift coefficient on solidity with the increasing attack angle is directly opposite to inclined drag coefficient. In this case, Reynolds number is negligible and the empirical formula was derived; shadow effect may occur in the case of fluid transiting from upstream strands to downstream strands, causing a reduction of stream velocity and exerted fluid force. The drag coefficient formula is expressed as cooperative dependence of both Reynolds number and solidity ratio.(2) To obtain the input parameters in relation to the motion and applied force of seine gear in the simulation, this study employed model experiment to measure the parameters in terms of the deformation, movement and acting force of the seine net, and quantize the geometric transformation of leadline and floatline, opening area, maximum sinking depth, the moving and drifting of seiner under different shooting patterns for the purpose of the validation of numerical simultion. The results show a water-drop shape by leadline shrinking continually during pursing in static water. For other cases, there is a bending deformation appeared in cross set and elliptical shape in the front set. The shape in back set turns to be more elongated and flatter. Shooting with current(front set) produced the greatest encircled volume by minimizing the area contraction. The different types of sinking curves for the middle part of leadline among shooting patterns were observed: flat, curving inward and curving outward, respectively. Shooting against current(back set) is considered as the optimal strategy of setting the net judged from the sinking behavior. Under the effect of current, the horizontal movement of the net and the drifting of the seiner into the circle due to pursing are recorded, in which the moving distance in back set is maximal, followed by that in cross set and front set. The drifting distance in front set is much greater than that in the other two shootings patterns. At the beginning of pursing, purse line at the bunt is primarily subjected to tension force and the tension at both sides increase with pursing process continuously. At the conclusion of pursing, the tension at both side reached the peak, 300 and 200 k N for bunt and wing end respectively in full scale values. Moreover, the tension exerting at bunt is more than that at wing end.(3) The dynamical behavior of purse seine gear based on a mass-lumped method was simulated. In the structure of physical model, the knots of mesh were assumed as mass points with all of the external and internal forces concentrated on it, while the mesh bars connecting with mass points were regarded as springs providing tensile forces. The motion equations were solved by implicit predictor-corrector method to ensure the precision and stability. The twine diameter and compensation for material density of the equivalent net were determined based on the criteria of hydrodynamic and quality conservation. The visualization of the shooting and pursing process of purse seine gear was realized. Loading distribution showed the bunt section was subject to more tension before pursing, also by the lower part of the leadline. During the conclusion of pursing, due to the effect of purseline tension, loads concentrated on the lower section of leadline. As exerted by current of 0.4 Kn in the model, the dimensional shape of netting deformed considerably and the maximum sinking depth achieved at 250 m.(4) To verify the validity of numerical simulation results, the standardized sinking depth in sea trial was compared with the simulation result and the dimensional configuration in the simulation was compared with that in the model experiment. It proved to be of precision for the simulation results, but remained underestimation and over-estimation in some cases. The virtual design or improvement of gear was accomplished by adjusting the modeling parameters as to test the properties under different conditions. The results showed that, there is difference with respect to the sinking behavior of leadline in the shooting with different current profiles; shooting speed had the positive effect to the sinking; the reduction of hanging ratio can increase the sinking depth; the enlargement of mesh size can benefit to the sinking of net.