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Prof. Lev Shemer - "Steep and Breaking Waves (A personal perspective)" | The Racah Institute of Physics

Prof. Lev Shemer - "Steep and Breaking Waves (A personal perspective)"

Date: 
Wed, 16/10/201312:00-13:30
Location: 
Danciger B building, Seminar room
One can attribute the considerable attention given to very steep water-waves in basic science in part to the fact that they represent a commonly observed manifestation of an essentially nonlinear phenomenon. The appearance of such waves is extremely important in marine and off-shore engineering, as well as in geosciences where they strongly affect the exchange of momentum, mass and energy between ocean and atmosphere. In this century, it became fashionable to talk about the so called rogue, or freak, waves which appear locally and spontaneously in the sea. Waves with heights exceeding twice the significant wave height are commonly considered to belong to this category. Circumstances leading to the appearance of these waves remain not completely clarified. Moreover, extremely steep waves eventually break. Breaking is the major factor causing wave energy damping, nevertheless, in spite of extensive studies in recent decades, the mechanisms leading to breaking are not yet fully understood. In the study of extreme waves, distinction can be made between deterministic and random waves. To gain a better understanding of the complex wave field in the sea, it is imperative to investigate much simpler nonlinear gravity wave fields. To this end, it is instructive to decouple between the stochastic nature of gravity wave fields in the ocean, their nonlinearity and the directional spreading. The talk addresses several aspects of a combined experimental and numerical investigation of steep nonlinear non-breaking and breaking unidirectional waves carried out in our laboratory:
• Methods of generating a single very high wave at a prescribed location in a tank. Two methods were compared: Peregrine soliton as a solution of the nonlinear Schrödinger (NLS) equation, and focusing of numerous wave components at a prescribed instant by constructive interference of various harmonics. While being basically linear, the latter process is strongly affected by nonlinearity.
• Lagrangian dynamics and kinematics of a wave on a verge of breaking and investigation of validity of different breaking criteria.
• Investigation of rogue wave appearance in random wave fields (time permitting).
Experiments were conducted in two experimental facilities: the Large Wave Channel (GWK) in Hanover, Germany, which is 300 m long, and a smaller 18 m long wave tank at Tel-Aviv University. The NLS equation, the modified NLS (Dysthe) equation and the spatial version of the Zakharov equation served as theoretical models.