Springing Vibrationの起振力に関する一考察

抄録

This paper discusses some problems on the exiciting force of springing vibration. The results obtained from this study are as follows. (1) Experiments by ship models were carried out to confirm phenomena of springing vibration. From the tests, ship hull vibration caused by waves is found to occur when the natural frequency of the model coincides with the harmonics of the encountered wave frequency. (2) An equation of motion of the simplified model for springing vibration is derived in order to explain the mechanism of ship hull vibration due to waves. Two forces, buoyant force and virtual mass force, are derived theoretically as the exiciting force of the springing vibration. But it is found that the virtual mass force reduces to a half of that derived by Watanabe, and the motion exhibits non-linear characteristics because of the existence of time dependent term in the governing equation of motion. (3) Forces of the springing vibration are calculated taking into account the sub-surface effect for rectangular, triangular and circular cross-sections. Results of calculation show that this effect is remarkable for the higher harmonics of the bouyant force. (4) A numerical calculation method of the exiciting force for arbitrary ship sections and wave forms is presented. From the results of calculation by this method, it is shown that not only shape of ship's cross-section but also wave form are closely related to the generation of several components of the exiciting force with the higher harmonics of the encounter wave frequency.