PREDICTION OF SHIP VISCOUS WAKE IN WAVES

舰船在波浪中的推进性能预测是舰船整体性能评估的重要内容之一。众所周知，尾迹对推进性能的影响在推进因素中占主导地位。然而，由于尾迹的粘性影响，其预测一直是一个难题。本研究的目的是阐明船舶在波浪中粘性尾流的物理机制，并评价合理的预测方法。在本研究的三年期间，第一年致力于尾流振荡速度的测量和分析。结果表明，在振荡幅度较大的区域，平均尾迹减小。第二年，基于内部尾迹区域的非定常边界层假设和外部的势流进行计算，预测波浪运动和船舶运动对尾迹值的影响。除尾迹变化较大的区域外，与实验结果吻合较好。最后一年进行了基于二维Navier-Stokes方程的振荡椭圆圆柱正航速计算，模拟船舶在波浪中的粘性尾流。计算结果与第一年的实测尾流值有相似的趋势。综上所述，本研究阐明了船尾尾迹振荡场在波浪运动中的物理机制，并尝试对尾迹振荡值进行预测。对预测方法的改进还有待进一步研究。

The prediction of ship propulsive performance in wave is one of the most important items in the assessment of the entire ship performances. The effect of the wake on the propulsive performance has been known to be predominant among the propusive factors. However, its prediction has been a difficult problem due to the viscous fluid effect on the wake. The objective of the present research is to elucidate the physical mechanism of the ship viscous wake in waves and to evaluate the reasonable prediction methods.In the three-year period of the present research, the first year was devoted to the measurement of the oscillating stern flow velocities and its analysis. It was found that the mean wake decreases in the region where the amplitude of the oscillation becomes large.In the second year, the computations based on the unsteady boundary layer assumption in the internal wake region and the potential flow outside were made to predict the change of the wake values due to the wave motion and the ship motion. The agreement with the experiment was quite well except for the region of large wake change.The final year has been devoted to carry out the computations based on 2D Navier-Stokes equation for an oscillating elliptic cylinder at forward speed, which simulates the ship viscous wake flow in waves. The result of the computations shows a similar tendency with the measured wake values which was done in the first year.It can be summarized that the present research has clarified the physical mechanism of the oscillating wake field at ship stern in its motions in waves, and attempted to predict the oscillating wake values. Research for the improvement of the prediction method still remains.