ABSTRACT

Characteristics of the two-crossing wave field in finite water depth have been discussed, based on both laboratory experiments and a 3rd-order wave theory proposed by the present authors. The result shows that the maximum wave height of the crossing waves Is largest around the crossing angle of 0° and is smallest around the crossing angle of 90°. Spatial variations in the maximum wave heights of crossing waves Increase with decreasing difference between the two primary wave periods. The direction of the horizontal maximum water particle velocity inclines toward the direction of the primary wave with the larger velocity at the phase of wave crest, whereas toward the direction of the other wave with the smaller velocity at the phase of wave trough. As the crossing angle and the difference between two primary wave periods become large, the horizontal orbit of the water particle motion trends to close to a circle from an oval with approaching the water surface from the bottom. In addition, the fundamental frequencies of two primary waves are modulated by the wave propagation direction.

INTRODUCTION

Real sea waves are called short-crested waves or multidirectional random waves which are composed of many component waves with different wave heights, periods and propagation directions. Considering recent multipurpose utilization of coastal and ocean regions, the most concerned waves are almost strongly nonlinear. Then, it may be general that the nonlinear interaction among component waves should be taken into account for construction of offshore structures and preservation of sea environment in multidirectional random wave field. Researches to clarify the primary characteristics of the nonlinear interaction among component waves have been actively carried out in the fields of unidirectional and multidirectional waves since 1950s. Tick (1963), Pierson (1993) and so on have studied the former wave field. We will consider the nonlinear interactions among component waves with directionality.

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