PREDICTION OF SHIP VISCOUS WAKE IN WAVES

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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.