A study on non-linear viscous farces acting on a ship with large amplitude of motions

在大浪中，船舶运动是强烈的非线性和不稳定的。本文研究了非线性船舶运动的特性，揭示了非线性粘性力对非线性船舶运动的作用。实验发现，在大波束海域，船舶在较宽的波频范围内发生较大的横摇运动，在某一波频范围内出现跳变现象。横摇和升沉固有频率相近的船舶，在高频的高浪中会出现较大的不稳定横摇运动。研究表明，船舶的快速漂移运动通过改变船舶的遇浪频率、浪致漂移力、粘性漂移阻力、横摇阻尼和浪致横摇激励力矩等因素引起了船舶的大而不稳定的横摇运动。这一事实表明，在大波束海中，船舶的横摇运动应视为船舶波频运动和稳定漂移运动的耦合运动。在本课题首席研究员的横摇预测方法的基础上，推导出稳定漂移运动船舶横摇阻尼的预测方法。结果表明，该方法的预测结果与实验结果吻合较好。通过实验研究了大横摇船舶的漂移阻力特性。实验表明，粘滞阻力系数不是恒定的，而是与Kc数有关，Kc数是考虑了漂移运动的影响而确定的。推导了该系数的预测方法。实验研究了作用于舱底龙骨的船体的粘性阻尼力的特性。研究发现，由于不对称的大振幅强涡脱落，水动力力表现出较强的非线性

In heavy waves, ship motions are strongly non-linear and unstable. In the present study, the characteristics of the non-linear ship motions are investigated and the roles of non-linear viscous forces on them are revealed.It is experimentally found that in heavy beam seas a ship is experienced large amplitude roll motion in wide range of wave frequencies and a jump phenomena appears at a certain wave frequency. The large and unstable roll motions appear for a ship with similar roll and heave natural frequencies and in high wave with high frequency. it is revealed that fast drift motion of the ship cause the large and unstable roll motion by changing encounter frequency, wave-induced drift force, viscous drift resistance, roll damping and wave-induced roll exciting moment. This fact suggests that roll motion of a ship in heavy beam seas should be treated as an coupled motion of wave-frequency ship motions and steady drift motion.A prediction method of roll damping of a ship with steady drift motion is deduced on the basis of a roll prediction method by the head investigator of the present research project. It is confirmed that the predicted results by the method are in fairly good agreement with experimental results.The drift resistance acting on a ship with large roll motion is experimentally investigated to find the characteristics. The experiments revealed that the drift resistance coefficient due to viscosity is not constant but depends on Kc number which is defined by taking the effect of drift motion into account. A prediction method of the coefficient is deduced.The characteristics of viscous damping forces acting a heaving body with bilge keels are experimentally investigated. It is found that strong non-linearity appears in the hydrodynamic forces because of unsymmetrical strong vortex shedding in large amplitude