Motion Analysis of Flexible Hose Based on Water Tank Experiment

Abstract

Set net fishery faces the problems of decrease in harvest, with lack of labor and aging of fishermen. These problems make it necessary to develop a new automatic harvesting way of set net fishery. An automatic net-hauling system is one of the promising ways, while the system using rubber tubes and polyethylene pipes are not prevailing because of the high maintenance cost and the difficulty in the operation. In the present study, the flexible hose net is proposed to harvest fish in the box chamber net of set net fishery. The flexible hose net is installed on the water bottom below the box chamber net. Compressed air is injected from one edge of the hose net to haul the box chamber net gradually, resulting in cornering fish in the other edge. In the preset study, the feasibility of the net-hauling system for set net fishery was examined by water tank experiment. The variation in the formation of the hose net and the time for sinking and floating were examined, changing the parameters such as air pressure and buoyancy balance. As a result, the hose net sank automatically if the weight attached to the hose net was 39% of the total buoyancy of the hose net with full of air. The time spent for sinking operation was about 160 s, which corresponds to about 10 min for actual hose net according to the similarity law. However, it should be noted that the similarity of water pressure could not be reproduced in the water tank experiment. To reduce the sinking time, the initial inner pressure of the hose net must be the atmospheric pressure before the beginning of the sinking operation. There is an inflection point between the flexible hoses with and without air. It is worried that air flow in the flexible hoses may be impeded. So the inside structure of the flexible hoses may have to be improved to secure the air flow in any condition. In relation to the difficulty in reproducing the water pressure condition in the water tank, the numerical analysis will be required to reproduce the motion of the hose net and its sinking time both for scaled and full-scale models. In the future, three-dimensional coupled model of fluid, structure, and air flow will be developed and validated by the experimental data, beginning with the simple two-dimensional modeling of the motion of the hose net.