Evolution of skewness and kurtosis of weakly nonlinear unidirectional waves over a sloping bottom

We consider the effect of slowly varying depth on the values of skewness and kurtosis of weakly nonlinear irregular waves propagating from deeper to shallower water. It is known that the equilibrium value of kurtosis decreases with decreasing depth for waves propagating on constant depth. Waves propagating over a sloping bottom must continually adjust toward a new equilibrium state. We demonstrate that weakly nonlinear waves may need a considerable horizontal propagation distance in order to adjust to a new shallower environment, therefore the kurtosis can be notably different from the equilibrium value for each corresponding depth both on top of and beyond a bottom slope. A change of depth can provoke a wake-like spatially non-uniform distribution of kurtosis on the lee side of the slope. As an application, we anticipate that the probability of freak waves on or near the edge of the continental shelf may exhibit a rather complicated spatial structure for wave fields entering from deep sea

The influence of currents, long waves and wind on gravity-capilary

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1995.Includes bibliographical references (p. 211-216).by Karsten Trulsen.Ph.D

Laboratory evidence of freak waves provoked by non-uniform bathymetry

We show experimental evidence that as relatively long unidirectional waves propagate over a sloping bottom, from a deeper to a shallower domain, there can be a local maximum of kurtosis and skewness close to the shallower side of the slope. We also show evidence that the probability of large wave envelope has a local maximum near the shallower side of the slope. We therefore anticipate that the probability of freak waves can have a local maximum near the shallower side of a slope for relatively long unidirectional waves. Copyright 2012 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Phys. Fluids 24, 097101 (2012

Numerical simulations of the charging of dust particles by contact with hot plasmas

International audienceCharging of individual dust particles in contact with hot plasmas is studied by numerical methods. The dust particle is treated as a rigid solid body, composed by either perfectly insulating or conducting material. The collisionless plasma, consisting of electrons and singly charged ions, is simulated by Particle-in-Cell methods in two spatial dimensions. It is demonstrated that the surface conditions, i.e. roughness, of the dust particles are significant for the charging. In a streaming plasma, a dust grain develops an electric dipole moment which varies systematically with the relative plasma flow. The strength and direction of this dipole moment depends critically on the material. We observe also Langmuir oscillations excited in the vicinity of the particles, and analyze the spatial variation of their spectral distribution

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 $\alpha$ and the enhancement coefficient $\gamma$. We discuss how freak waves in a random sea state are more likely to occur for large values of $\alpha$ and $\gamma$. 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

Offshore-borebransjen og verdsettelse av Ocean Rig ASA

Offshore-borebransjen er i denne utredningen definert som all virksomhet med mobile offshoreenheter, plattformboring og andre relaterte tjenester som brønnoperasjoner og engineering i tilknytning til boring. Utredningen går først i dybden på hele offshore-borebransjen. Jeg ser blant annet på konkurranseforhold i bransjen samtidig som jeg blant annet analysere andre makroøkonomiske faktorer som påvirker offshore-borebransjen. Etter at offshore-borebransjen er grundig analysert ser jeg nærmere på Ocean Rig ASA. Dette er et lite og ungt selskap med en god posisjon innenfor segmentet for boreoperasjoner på dypt vann. Ocean Rig eier og opererer to halvt nedsenkbare borerigger. I tillegg til at jeg ser på selskapets historiske prestasjoner blir Ocean Rigs strategi og risikoforhold grundig drøftet før jeg går i gang med selve verdsettelsen. Verdsettelsen er i første rekke basert på Discounted Cash Flow, men verifiseres også ved hjelp av multiplikatorer. Sensitivitetsanalyser blir også gjennomført, disse viser at det er vanskelig å sette en bestemt verdi på et selskap. Derfor angir jeg et verdiintervall som jeg mener er fornuftig. Basert på de analysene jeg har gjennomført anser jeg selskapet som undervurdert ved Oslo Børs. Derfor vil jeg også gi en kjøpsanbefaling på aksjen