트롤 전개판의 동적 거동 모델링 및 수치 시뮬레이션 연구

Abstract

It has been recognized that understanding the behavior and optimizing the performance of otter boards are required for improving the fishing efficiency in otter trawl fisheries. Because the otter boards perform the roles of keeping the towing depth of the fishing gear constant as well as spreading the net mouth for the secure swept area. The simple principle of the otter boards is that the lift force of the hydrodynamic resistance makes the otter boards to rise up toward outside to the towing direction. Several studies have been made on the methods in the circulating water channel for analyzing the critical factors which affect on the performance, such as the coefficient of the drag and lift as well as the varied pressure center. This paper is intended as a modeling and the numerical simulation of the motion of trawl otter boards in three dimensional spaces, as a part of the trawl system which consist of the ship and the fishing gear. The trawl system can be classified into two main groups: the otter boards as the rigid body and the net as the flexible body. This thesis describes the rigid body in six degrees of freedom, and the flexible body in mass-spring model refers to the literature. Those two types of bodies have the non-linear motion in the water as well as the coupled effect. In the model, based on the governing equations of the motion of the rigid body, the equations of external forces and moments were derived from hydrodynamic resistance, hydrostatics, added mass and the wave force. The simulations of the trawl system were performed through the integration of the combined non-linear equations numerically, Backward Euler of implicit method. In order to collect the data of the hydrodynamic coefficients such as drag, lift and moments, the model test were conducted and some literatures were referred. The model of the otter board was a simple cambered type made of stainless still with 13% of cambered ratio and 1.7 of aspect ratio. At the 3.5m/s of water flowing, the coefficients of drag, lift, moments and the center of pressure were achieved and the results were very similar to the other ones from the literatures. These data were used in the simulation to express the hydrodynamic characteristics of the otter board. In addition, two series of field experiments were carried out to measure the behavior of otter board in several fishing conditions. In the first experiment, the depth change of the otter boards were measured with two conditions: the change of towing speeds and the warp length. The warp length was 110m with ±20m and the towing speed was changed from 4.5kts to 3.5kts. The result from the first condition, the variation of the depth was about 18.95m deeper. The one from the second condition, it deepened 5.1m on the longer warp length(130m) and it returned to the original depth with the warp length shortened(110m). The simulation results showed the very close variation range to the each condition. In the second experiment, the angular displacement of otter boards and the tension of warps were observed with the condition of the varied towing speed from 4.4kts to 2.0kts at the interval of 0.5kts. The simulation results were compared to the results of experiment. The angular values between the experiments and the simulation were close and the change patterns were very similar. Finally, an exclusive simulation program for the otter board was developed to assess the behavior and the performance. The program calculates the otter boards behavior connected to the towing ship and the net, but it regards the towing force and the net resistance as the simple forces. In other words, it does not calculate the net modeled as mass-spring. The program ran stable and the calculation time were very fast compared to the full model simulation. The calculation results of the tension and the angular displacement was close to the one of the field experiments.