報告書番号

U02012

タイトル（和文）

流れ方向に振動する円柱に作用する非定常抗力の特性

タイトル（英文）

Characteristics of Dynamic Drag Force Acting on a Circular Cylinder Oscillating in the Streamwise Direction

概要 （図表や脚注は「報告書全文」に掲載しております）

円柱状のプラント構造物の抗力方向流力振動の解明を目的として，流れ方向に強制加振される円柱に作用する非定常抗力を，換算流速が1～4の範囲で，振幅が直径の2.5～10％と比較的小さい条件において測定した．その結果，測定結果から求めた付加減衰力の特性が既往の自由振動実験結果と定量的にも整合すること，定常流中の付加質量係数が静止流体中の値から大きく変化し，振動の発生限界が換算減衰率のみでなく質量比にも若干依存することがわかった．また，抗力方向付加減衰力が円柱と流体の相対速度変化に起因した流体減衰と渦放出に起因した励振力の合力であると考え，測定結果に基づき，両者の影響の定量的評価を行った．この結果，ある限界流速以上では，流体減衰の効果を渦放出に起因した励振力が上回り，両者の合力が減衰力から励振力へ切り替わること，円柱振幅が大きくなると，相対的に流体減衰の効果が上回り，振動が抑制されることがわかった．

概要 （英文）

Precise measurement of dynamic drag force acting on a circular cylinder oscillating in the streamwise direction was carried out in a water tunnel under small forcing amplitude conditions. In the previous report, the measurement method of dynamic drag force and the estimation method of added damping force were described. This report describes the experimental results with setting forcing amplitude at 2.5%, 5.0% and 10%. The measured dynamic drag force was reduced to added mass coefficient and exciting parameter, which denotes dimensionless exciting fluid force. As a result, it was found that the exciting parameter is strongly dependent on cylinder amplitude and its characteristics agree well quantitatively with the in-line oscillation behavior of an elastically supported cylinder in a cross flow obtained by previous free oscillation tests. The added mass coefficient varies depending on reduced velocity, and the stability criteria of reduced damping is slightly affected by the mass ratio. Furthermore, the effects of fluid damping, which is caused by the change of flow velocity relative to cylinder motion, and excitation force induced by vortex shedding were qualitatively estimated on the basis of the experimental results in detail. It seems reasonable to conclude that the relative balance between the fluid damping and the vortex excitation force controls whether dynamic drag force excites cylinder oscillation in in-line direction or damps it. As the cylinder amplitude increases, the fluid damping effect overcomes the vortex excitation force, which leads to suppression of cylinder vibration.

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