漂流ブイの上下揺れに及ぼす粘性流体力の影響

Abstract

In recent years, a system to measure sea conditions in real time for carrying in a buoy is developed. In this system, wave conditions, those are wave height, period and direction, are obtained from calculating the buoy's motion equation with its accurate response operator functions using measured buoy's motion. Therefore, it is necessary to take the functions in design stage, in order to design for an optimum form of buoy for installed sea area. First, the accuracy of an existing motion calculation program based on potential theory is checked. It is found that the calculation overestimates the harmonic heaving amplitude for some buoy.Then, in order to understand the reasons, the characteristics of the hydrodynamic forces acting on buoy are investigated with a forced heaving motion test. From the flow observation around a buoy using visualization technique, large vortexes shedding under the body in the moment when buoy has maximum upward velocity are observed, and it is confirmed that viscous drag acts on buoy. Using a vertical cylinder model, the viscous drag coefficient acting on aft-body in the moment when the model has maximum upward velocity is measured by the forced heaving motion test for several KC numbers and 2D CFD calculation based on CIP method is also carried out, and the characteristics of the viscous drag are investigated and an empirical estimation formula is presented. Finally, a motion calculation including the viscous drag is carried out and the results are compared with measured results. It is confirmed that the calculated results are improved and shows good agreement with measured results.