The hydrodynamic characteristics of minnow netting composed of high-strength polyethylene (Dyneema) with diŠerent types of twine diameter and mesh size were evaluated through experiments conducted in a ‰ume tank. The drag coe‹cient C D90 of Dyneema minnow netting set normal to the water ‰ow was 8 and 25 smaller than that of minnow netting made of the conventional materials of polyamide and polyvinyl alcohol, respectively.The drag coe‹cient C D0 of minnow netting set parallel to the water ‰ow showed a slight diŠerence between the materials. Additionally, C D90 and C D0 were expressed as functions of Reynolds number and mesh factor a. Furthermore, the drag and lift coe‹cients of minnow netting set at various angles to the water ‰ow were estimated by empirical equations as functions of C D90 , C D0 and attack angle u. Table 1 に示す。
For model testing of trawl gear utilizing an opening device such as an otter board, the ratio in area of the model opening device to the full-scale device was proposed based on modiˆed Tauti's law. A sea trial with a fullscale midwater trawl net of 27.44 m in total length and model experiment with 1/10 scale model were conducted.Two otter board models of diŠerent scale were used for the model experiment: otter board model I of 1/60 area scale based on the modiˆed Tauti's law, and model II of 1/100 area scale based on the original Tauti's law. As a result, a distinct diŠerence both in the distance of the two otter boards and in net-mouth height was found between the converted values from the model test with model II based on Tauti's law and the observed values in the fullscale sea trial. In contrast, no such diŠerence was observed in the results of model I. In addition, it was veriˆed that the speed ratio based on the modiˆed Tauti's law was useful for predicting the drag of the test trawl gear with opening device.
A fish-cage flotation/submersion system using flexible hoses is proposed to achieve horizontally stable floating and sinking motions. Waterproof flexible hoses are inserted into polyethylene pipes installed at the top of the frame of the fish cage. These hoses flatten when they are devoid of air and water as the fish cage is submerged. The injection of high-pressure air regenerates buoyancy and enables the fish cage to rise in the water. The advantage of this system is the uniform, circumferential generation of buoyancy at the top of the frame, which suppresses inclination of the fish cage and concomitant deformation of the flexible chemical fiber nets. Tank model testing was carried out to examine the inclination and the floating velocity of the fish cage. Tauchi’s similarity law was applied to make a 1/30 model of the full-scale fish cage. The tank model of the fish cage was installed in the ocean engineering basin at the University of Tokyo, and it was made to float and sink in water with and without currents. The inclination and position of the fish cage were measured using video camera images. As a result, the proposed fish cage was observed to float stably in still water in contrast to systems based on the existing method. When subjected to water currents, the new fish cage inclined by a maximum of 18° just after leaving the bottom; the inclination was reduced with further ascension. The ratio of buoyancy to gravity, the rate of air injection, and the arrangement of the flexible hoses should be optimized to achieve a more stable motion. The floating velocity for the rising fish cage in still water was then analyzed. The drag coefficient of the fish cage, as calculated from experimental data, corresponded to that estimated from a structural analysis of the fish cage. Analysis projected accelerated motion for 0.02 s after the fish cage rose from the bottom, while acceleration lasted a few seconds in the tank model test. This is because uniformly accelerated motion was assumed in the analysis, while the acceleration actually varies from zero to a constant acceleration, because of the difference between gravity and the varying buoyancy of the flexible hoses.
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