Numerical Analysis of the Motion of a Flexible Hose Net Used for Hauling a Box Chamber Net in Set Net Fishery

Abstract

A set net fishery is one of the most popular fishing methods and has a history of a few hundred years in Japan. The hauling operation of a box chamber net is one of the essential works for fishermen to harvest fishes, not losing and damaging them. However, the current net-hauling system requires many personnel powers with danger works in early morning. The present study proposes a flexible hose net for hauling a box chamber net in an automated way. Flexible hoses are arranged like a brick form and installed in the bottom of the box chamber net. Compressed air is injected into flexible hoses at one edge of the hose net. The water tank models of the set net and the hose net are made based on the similarity law of Tauchi, while the Reynolds dependency is not taken into account. The hose net rose like an S shape to drive fishes into the smaller domain gradually. Then a simple method of numerical analysis is represented to estimate the formation and the tension of the flexible hose net. For simplicity, the formation of a flexible hose net in the direction normal to air injection is uniform. The hose net is assumed to be flexible with and without air inside the hoses. Hence the hose net is approximated by a flexible cable. A maximal tension of the flexible hose net is the key parameter for its design. The tension is expected to be maximal at the middle point on the flexible hose net when the flexible hose net obtains the half of the total buoyancy. Therefore, the formation of the flexible hose net at this time is analyzed in comparison with the experimental result. The projected area, drag coefficient, current velocity, and the tension at the origin (pretension) are varied to investigate the sensitivity of these parameters to the formation of the flexible hose net. When the parameters related to the drag force on the fish cage were varied, little difference could be found among the analyzed results. The drag force on the flexible hose net exerts little effect on its formation, and is overwhelmed by the effects of gravity and buoyancy. The formation of the hose net is sensitive to the tension at the origin. As future studies, the tension will be measured at several points on the hose net in water tank testing by more precise tank model considering the stiffness of the hose and the Reynolds dependency. Then the numerical model will be expanded to three-dimension to take the time changes in the formation of the flexible hoses and drag force into account.