细长体出水过程中阻尼系数变化分析

Variation of the damp coefficients of a slender body during water exit

针对细长体出水过程中黏性阻尼系数和兴波阻尼系数的变化,采用弹性体振动理论和流体势流理论相结合的方式对其进行研究.对于黏性阻尼项,建立简化的涡激振动模型,将涡激振动的阻力和升力项考虑到结构振动方程中,获得考虑涡激振动后的阻尼系数增量.对于兴波阻尼项,采用时域格林函数法,首先计算规则球体振动问题,并将计算结果与相关文献对比,发现二者吻合良好.在验证算法的基础上,采用细长体模型,分别计算不同振动频率和不同出水高度对于兴波阻尼系数的影响.计算结果表明:涡激振动引起的黏性阻尼增量随着洋流增大、波浪增高或者空泡长度减短而增加;兴波阻尼和细长体的出水高度、固有振型和振动频率密切相关,所对应的细长体在头部出水、振动频率0.5 Hz左右诱发的兴波阻尼最大.一阶和二阶弹性振动诱发的兴波阻尼系数较小,工程中可忽略.

To study the variation of the damp coefficients of a slender body during water exit, the vibration theory of an elastic body and the potential flow theory of fluid are adopted. For viscous damp coefficient, vortex induced vibration （ VIV ） model is introduced and the lift and drag forces are included in the structure vibration equation. The increment of damp coefficient by VIV is therefore obtained. For wave-making damp coefficient, time-domain Green Function is adopted. The problem of a sphere oscillating on the free surface is calculated first and compared with the published paper, and the results agree well with each other, which validates the numerical model and procedure in this paper. Based on this, the wave-making damp coefficient of a slender body under different vibration frequencies and different lengths during water exit are calculated and analyzed. The results show that the increment of damp coefficient by VIV rises with the increase of current, wave and the reduction of cavitation, while the wave-making damp coefficient is closely related to the length of water exit, the natural vibration mode and the vibration frequency of the slender body. For the case studied in this paper, the wave-making damp coefficient peaks when the head of the body comes out of the water and the vibration frequency is around 0.5 Hz, and the wave-making damp coefficient induced by the first and second order elastic vibration respectively is quite small, which can be neglected in the engineering application.