付加物が渦励振に与える影響の実験

Abstract

In Ocean development, many cylindrical shape structures are in use. These structures are subjected to many external forces, such as water current, waves and reaction force from the upper structure. Especially, the lift force induced by the shedding vortices cause the vibration of the structures. This phenomenon is called Vortex Induced Vibration, VIV. It is assumed that VIV causes fatigue damages on the structure because it contains high frequency component of vibration and it increases in drag force on the structure. One of the solutions to decrease VIV, the appendage such as splitter plates or fairings is used to be attached on the structures. However, it may happen that such appendages increase the amplitude of VIV in some situations. Meanwhile, for actual use, it must be considered not only the decreasing performance but also the ease of handling. If Computational Fluid Dynamics is used, it is easy to modify the shape of the appendage and simulate its performance in different circumstances. Before that, it is needed to reveal the influence of the motion and flow field around the structure with the appendages. In this study, a rigid circular cylinder with or without appendage was supported by coil springs in the circular tank. The motion and rotation of the cylinder were measured by tracking the markers attached on its cross section. In the same manner, those of the appendage were also measured. The flow field around the cylinder was visualized by PIV. Experimental results show that the appendage, such as splitter plate or fairing, sometimes can not decrease the amplitude of VIV. They also show that the appendage which can shorten the distance between the shedding vortices near the cylinder can decrease VIV.