12 傾斜を有する水中線状構造物のVIV応答

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Vortex induced vibration (VIV) responses of underwater line structures inclined against the normal of free-stream direction had been measured. Two 1440mm and 9500mm in length circular section line structures with 12.7mm diameter had been used. The VIV frequency of an inclined underwater line structure depends on the normal component of free-stream velocity because VIV is a vibration balancing the hydrodynamic force due to the vortex shedding and the restoring force and the hydrodynamic force by vortex shedding depends on the normal component of free-stream velocity. The tangential component of free-stream velocity has an effect on the VIV, too. When an underwater structure is set with an angle of inclination, an axial direction friction force comes out by the tangential velocity component. The structure inclination changes the natural frequency of the structure because the natural frequency of a line structure depends on the tension. The VIV response of an inclined underwater structure strongly depends on the normal velocity component. The VIV responses measured by accelerometers depend on the deduced velocity consisted of the normal component of flow velocity, 1st-order natural frequency of the structure and the structure diameter. The VIV responses of the underwater line structures were almost same as VIV responses of rigid circular cylinders excepting at the region occurring 2nd-mode resonance of the vortex shedding with the 2nd-order natural frequency. The natural frequency of a spring mounted mass system linearly depends on the order number. However, the natural frequency of a tensioned line structure is non-linear in the order number. The frequency of the In-Line VIV was about 2 times of the Cross-Flow VIV frequency, but the mode number of In-Line shape was not only 2 times of the Cross-Flow shape mode number. The shape mode depends on the relation between the VIV frequency and the natural frequency. The displacement of Cross-Flow VIV was about 4 times of the In-Line VIV displacement. However, bending stresses in the In-Line direction were almost same as the Cross-Flow direction bending stresses, because the mode number of In-Line shape was about 2 times of the Cross-Flow shape mode number.
社団法人日本船舶海洋工学会の論文

12 傾斜を有する水中線状構造物のVIV応答

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