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

在大风浪中，船舶运动是强烈的非线性和不稳定的。本文研究了船舶非线性运动的特性，揭示了非线性粘性力对船舶非线性运动的影响，实验发现船舶在横浪中在宽波频范围内作大幅横摇运动，在一定波频下出现跳跃现象。横摇和垂荡固有频率相近的船舶，在高频高浪中会出现较大的不稳定横摇运动。研究表明，船舶快速漂移运动通过改变船舶的相遇频率、波浪诱导漂移力、粘性漂移阻力、横摇阻尼和波浪诱导横摇激励力矩，引起船舶大幅度的不稳定横摇运动。本文在课题组组长提出的横摇预报方法的基础上，推导了具有定常漂移运动的船舶横摇阻尼预报方法。结果表明，该方法的预测结果与实验结果吻合较好，并对大横摇运动船舶的漂移阻力特性进行了实验研究。实验表明，由于粘性的漂移阻力系数不是恒定的，而是取决于Kc数，这是通过考虑漂移运动的影响定义。对带有船底龙骨的垂荡体的粘性阻尼力特性进行了实验研究。结果表明，由于非对称的强涡脱落，水动力表现出很强的非线性

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