영각, 레이놀즈수 및 공극률에 따른 평면 망지의 유체역학적 계수

Abstract

Abstract

It is feasible to decide the proper size of the fishing gears by measuring the hydrodynamic forces acting on the fishing gears. It would be also useful to analyze the deformation and the behavior of fishing gears and operate fishing gears. In a fish cage system, the stability of the structure is variable in accordance with the hydrodynamic forces to the structure. In particular, the netting is most vulnerable in fishing gears and also has high fatigue from hydrodynamic forces. It is most important to clearly investigate the hydrodynamic forces acting on the netting in order to prevent various possible accidents in fishing and aquaculture operations. Fishing gears are mostly composed of the netting that is highly flexible and deformable from external forces. Furthermore, the hydrodynamic forces acting on the netting are changed according to current, attack angle, material and others. It is possible to figure out hydrodynamic forces from calculating hydrodynamic coefficients varied according to specific parameters. Hydrodynamic coefficient is an important parameter in determining the shape and tension acting on the fishing gear and, also depend on attack angle, Reynolds number and net solidity. Started with the simple model testing, computer-based methods with numerical analysis has been recently used to numerically analyze the submerged behavior and deformation of flexible structures. Therefore, it would be required to establish more accurate hydrodynamic coefficients as a fundamental data used in the numerical model for analyzing the response of fishing gears. In this study, the flume tank tests were performed to measure the hydrodynamic forces for the 8 kinds of nettings which have different mesh sizes, but the same total projected area with the same twine thickness. In order to establish the standardized hydrodynamic coefficients of the nettings, the coefficients were derived according to attack angle, Reynolds number and solidity ratio. Furthermore, changes of the derived coefficients were compared and analyzed with attack angle, Reynolds number and solidity ratio.

The results of the nettings tests were as follows;

1. Drag coefficient was influenced by attack angle, solidity ratio and Reynolds number in order.

2. Lift coefficient had the dependence of attack angles, however, not dependence of solidity ratios and Reynolds numbers.

3. The drag coefficients of the nettings were tended to increase with increased solidity ratios.

4. The drag coefficient was tended to be mostly constant with increased Reynolds number when the low attack angles applied, but decreased when the higher attack angles applied with increased Reynolds number.

5. The drag coefficient was tended to decrease with increased Reynolds number in the range of 263 to 527, even though it is constant with that of values higher than 527.