False waves in wave records

A methodology to define discrete waves from free sea surface elevation time series is presented. The method allows an objective discrimination of false waves among small waves, avoiding the use of arbitrary criteria associated with zero-up-crossing, zero-down-crossing, or other definitions. The method uses the Hilbert Transform and the representation of the time series in the complex plane. A discrete wave corresponds to a 2π phase-advance in the complex plane. The waves between zero crossings which do not show 2π phase advance are considered false waves. Wave rider buoy records, measured off the west coast of Portugal, were employed in assessing some of the statistical implications of this methodology.     

Variability of hydrodynamic load predictions for a tension leg platform

A survey of diffraction/radiation analyses has been performed on behalf of the International Ship and Offshore Structures Congress, Committee I.2. This paper presents the results from the survey, including data from 17 organisations plotted in a common format. The variability of the computed hydrodynamic loads and responses is discussed, and it is concluded that there is a degree of uncertainty in results predicted on the basis of commonly used idealisations. Even in long period waves the variability between the computed results is considerable. To obtain reliable results at the shorter periods associated with heave, pitch and roll resonances of a deep water TLP, much finer meshes than those used by most of the participating organisations would be required.     

Model estimates for the mean gas exchange between the ocean and the atmosphere under the conditions of the present-day climate and its changes expected in the 21st century

Annual mean fluxes of CO₂ and oxygen across the sea surface are estimated with the use of numerical modeling for several regions located in the Gulf Stream and Kuroshio zones. The present-day climatic conditions and the climatic conditions expected in the middle and at the end of the 21st century are considered. Specific features of gas exchange under a strong wind that are associated with gas exchange by bubbles and with changes in the air-water difference of the gas concentrations were taken into account in the calculations. The estimates obtained differ substantially from the results based on the traditional approach, which disregards the above features. A considerable increase in the absorption of CO₂ by the ocean, which is mainly caused by the continuing increase in the CO₂ concentration in the air during its small changes in the ocean, is expected in the 21st century. At the same time, no trends are revealed in the annual mean fluxes of oxygen across the ocean surface. The conclusion is made that, in calculations of CO₂ absorption by the world ocean, it is necessary to take into account both specific features of gas transfer under a strong wind and an increase in the atmospheric concentration of CO₂.     

The buckling of plastic oblate hemi-ellipsoidal dome shells under external hydrostatic pressure

The paper presents a theoretical and an experimental investigation into the buckling of seven oblate hemi-ellipsoidal dome shells under external hydrostatic pressure. Four of the shells were made in glass reinforced plastic and three were made from a thermosetting plastic called solid urethane plastic. All the vessels were tested to destruction. The theoretical study was made with the aid of a non-linear finite element solution, where both geometrical and material non-linearity were allowed for. Good agreement was found between experiment and theory for all the vessels. The very oblate domes failed axisymmetrically. Theoretical convergence was good for the more oblate domes but it was not as good as for the less oblate domes. This may have been because the less oblate domes did not fail in a classical axisymmetric manner as was expected. This work is of much importance in ocean engineering.     

Water vapor in the atmosphere over the northern Tien Shan

Refined data of systematic measurements of total water vapor in the atmosphere from May 1980 to April 2005 are presented. The data were obtained at the Issyk Kul atmospheric-monitoring station by the method of solar molecular-absorption spectroscopy. Over 25 years, the annual mean water-vapor content in the atmosphere increased by 4.5% at a mean rate of increase of 0.18% per year. However, the water-vapor content decreased in the last five years. The results of statistical processing of experimental data (general statistical characteristics, correlation coefficients, composite oscillations) are described. A refined model is proposed for forecasts of temporal variations in the monthly mean and annual mean water-vapor contents for the coming years. The model includes a linear trend and the sum of oscillations with periods close to the periods of a number of well-known geophysical phenomena. Regression equations are proposed to relate the water-vapor content in the atmospheric column to the surface temperature and absolute humidity.     

The differences between a wing and a planing plate in two-dimensional flow

The normal force coefficient on a flat planing surface having arbitrary heave and pitch motion in two-dimensional flow is compared with the lift coefficient of a thin wing in an infinite fluid. Despite the totally different derivations, they are found to be identical (at large Froude numbers and low trim angles and allowing for the wing's interaction with twice as much fluid) at low reduced frequencies. For higher frequency motions, the wing's angle of attack induced lift and its pitch and heave damping are less than those of a planing surface, but the acceleration terms remain identical. The differences at the higher reduced frequencies are due to the fact that, in invisad irrotational flow, the planning plate cannot leave a vortex wake, whereas a wing does.It seems to follow that the “virtual mass” planing hull analysis can be applied to “quasi-static” problems involving wings and bodies in an infinite fluid without the slenderness restriction originally imposed by Jones (1946). Certainly, it is remarkable that the so called “quasi-steady” forces on a two-dimensional wing can be obtained in a few lines of elementary analysis. On the other hand, the method fails entirely when used to compute the pitching moment on a two-dimensional plate, even though it has been found to give good results for the three-dimensional case (Payne, 1981c).This work is offered as a very incomplete study of an intriguing relationship between two very different bodies of analysis. Much more work will need to be done before the relationship between the two approaches will be fully understood.