10,007 research outputs found
Freak Waves in Random Oceanic Sea States
Freak waves are very large, rare events in a random ocean wave train. Here we
study the numerical generation of freak waves in a random sea state
characterized by the JONSWAP power spectrum. We assume, to cubic order in
nonlinearity, that the wave dynamics are governed by the nonlinear Schroedinger
(NLS) equation. We identify two parameters in the power spectrum that control
the nonlinear dynamics: the Phillips parameter and the enhancement
coefficient . We discuss how freak waves in a random sea state are more
likely to occur for large values of and . Our results are
supported by extensive numerical simulations of the NLS equation with random
initial conditions. Comparison with linear simulations are also reported.Comment: 7 pages, 6 figures, to be published in Phys. Rev. Let
Predicting rogue waves in random oceanic sea states
Using the inverse spectral theory of the nonlinear Schrodinger (NLS) equation
we correlate the development of rogue waves in oceanic sea states characterized
by the JONSWAP spectrum with the proximity to homoclinic solutions of the NLS
equation. We find in numerical simulations of the NLS equation that rogue waves
develop for JONSWAP initial data that is ``near'' NLS homoclinic data, while
rogue waves do not occur for JONSWAP data that is ``far'' from NLS homoclinic
data. We show the nonlinear spectral decomposition provides a simple criterium
for predicting the occurrence and strength of rogue waves (PACS: 92.10.Hm,
47.20.Ky, 47.35+i).Comment: 7 pages, 6 figures submitted to Physics of Fluids, October 25, 2004
Revised version submitted to Physics of Fluids, December 12, 200
Wave Kinematics at High Sea States
Measurements of currents close to the ocean surface and within the crests of large, steep waves have been acquired with an incoherent bistatic sonar mounted on the seafloor. The sonar uses a single narrow-beam transmitter/receiver and three fan-beam receivers set in a triangular configuration around the source. Acoustic pulses transmitted from the seafloor are scattered by bubble clouds and the sea surface to the four receivers and may be transformed into velocity components as a function of elevation. Individual estimates of the currents at, and close to, the surface are made with sufficient temporal resolution to identify kinematics in the crests of large waves. Observations acquired in the Danish sector of the North Sea are examined to evaluate both the potential merits and limitations of the measurement approach. At lower wind speeds, sidelobe scatter from the surface reaches the receiver simultaneously with the volume scattered signal arriving from a few meters beneath, contaminating the velocity measurement at this depth. At higher wind speeds, bubble clouds and increased roughness of the surface combine to suppress this effect, permitting reliable near-surface measurement. A numerical simulation has been implemented to explore some aspects of sonar performance including turbulent velocity fluctuations and bubble density gradients. Additional analysis is carried out to examine bubble suppression of sidelobe scatter. The observations lead to some conclusions regarding wave kinematics during a storm in which the wind speed reached ∼17 m s−1. At the ocean surface, the downwind velocity in the crests of large waves substantially exceeds that predicted by the second-order Stokes model, but in the wave troughs the current is close to the nonlinear prediction
The Gamow-Teller Resonance in Finite Nuclei in the Relativistic Random Phase Approximation
Gamow-Teller(GT) resonances in finite nuclei are studied in a fully
consistent relativistic random phase approximation (RPA) framework. A
relativistic form of the Landau-Migdal contact interaction in the spin-isospin
channel is adopted. This choice ensures that the GT excitation energy in
nuclear matter is correctly reproduced in the non-relativistic limit. The GT
response functions of doubly magic nuclei Ca, Zr and Pb
are calculated using the parameter set NL3 and =0.6 . It is found that
effects related to Dirac sea states account for a reduction of 6-7 % in the GT
sum rule.Comment: 9 pages, 1 figur
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