Occurrence Mechanisms of Isolated Giant Waves on the Ocean

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Recently the generation mechanisms and occurrence probability of Freak Waves (isolated giant wave) are being investigated by many researchers in the world. It is because these gigantic waves appear sporadically and become a threat against navigating ships and/or ocean structures.In this project, we kept the monthly meeting to deepen the consensus and to catch up with the current information in this field of research. During the period of this program, we organized the special session in SNAME and open symposiums concerning Freak Waves in collaboration with RIAM, Kyushu University. Many contributions by the members of the group were read in several International Conferences such as ISOPE, OMAE, RINA, IWHF, ICMS, INH, WNSC and Workshops on Rogue Waves. Among them an invited lecture was presented in KSOE International Workshop on Freak Waves held in Korea.In the case of long crested sea, we made use of Non-linear Schroedinger (NLS) equation which has been actively studied in the field of … More plasma physics and found pertinent solutions applicable to the swell of the ocean. One is the bound soliton solutions of the NLS equation which interact themselves to have the extremely large envelope within them. N-soliton solution is derived exactly from the specific but arbitrary initial value by use of the Inverse Scattering Method. The other is a sort of time dependent space periodic solution accompanying isolated extreme crests in its most developed stage which is known as the homoclinic orbit in non-linear dynamics context in general. We could confirm the validity of this solution in the field of hydrodynamics as well by means of extensive experiments in physical and numerical wave tank. It means the importance of the development of so called Benjamin-Feir instability of water wave train to appear as the isolated majestic crest in a sea way even in the surrounding random wave field. The wave making signal was fabricated for the ship model test in a wave flume. The result revealed that the bending moment load exceeded twice as large as those by ordinary heading waves. On the other hand, in the case of short crested waves, the exact theory does not exist in the present and numerical simulation in the full dimensions is not sufficient in either computation code or CPU power for the time being. We are now on going with the directional wave tank experiments and intend to have the directional wave spreading measurements in a wave tank.In order to confirm this idea in the actual ocean, a great deal of observations including buoy system available to have the wave direction and synthetic aperture radar measurements from artificial satellite in the outer ocean should be indispensable so that these tasks are to be scheduled in the next step of this project. Less