Dates: 04/03/09 (Part I) and 04/17/09 (Part II)
Speaker: W. Xiao
Title: Predicting Rogue Waves from Directional Spectra
Abstract:
Rogue waves are defined as waves whose wave height is multiple times larger than the
significant wave height. Those waves are suspected to be the reason for many platform
damage and ship loss. There are two essential research topics in this area. The first
one is to understand the generation mechanisms of these giants and the second is to
develop a reliable statistical model to predict the rogue wave occurrence. In this
talk, we describe an investigation of the occurrence, statistics, and generation
mechanisms of rogue waves in the open sea using large- scale three-dimensional
phase-resolved wavefield simulations. A large number of phase-resolved nonlinear
wave-fields, initially specified by directional wave spectra, is obtained by applying
direct simulations. The typical domain size of such numerical nonlinear wave-fields is
O(10^3 km^2) over evolution time of O(hr). From the simulated nonlinear wave-fields,
rogue wave events are identified and their occurrence statistics are studied. It is
shown that the classic linear and second-order narrow-banded theories significantly
underestimate the rogue wave occurrence in wave-fields with moderately large wave
steepness, relatively narrow spectra bandwidth and narrow directional spreading. The
influence of key wave spectrum parameters (such as typical steepness, spectra bandwidth
and directional spreading) on the rogue wave occurrence is analyzed. It shows, unlike
the case found in long-crested wavefield, the occurrence of rogue wave does not only
depend on the so-called Benjamin-Feir index but also the crest length and group length
of the 3D wavefield. The first part of the talk also serves as a short up-to-date review
on statistical models developed for large waves.