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ABSTRACT A comprehensive ocean observation platform is under development, which can measure biological, chemical and physical properties in the upper ocean mixed layer and the data obtained can be sent to laboratories in real time. The displacement of the platform is about 500 tons and plans for it to be moored in the North Pacific at a depth of 5000m. The wind and wave conditions in that area of the sea are very severe and safety precautions on the platform including mooring lines are essential. Model experiments and numerical simulations on motions and mooring tensions of the ocean observation platform were made for the design of the mooring system and platform. In general, results of numerical simulations showed good agreement with those of experiments, though a few points needing improvement were discussed. INTRODUCTION Buoy systems have been very popular for ocean observations. The submerged buoy system to measure the current velocity, salinity and water temperature, the surface buoy system to monitor the salinity and water temperature in the upper ocean mixed layer, the wave rider buoy and super buoy systems for wave measurements are well known. Fortunately sensor systems for each of these purposes are small and require low power, and the scale of buoy systems is therefore usually small. For studies on the greenhouse effects the material circulation between the atmosphere and the ocean is important. Long-term measurements are required on not only the primary material of C02 and NH4 but also meteorological and ocean environmental conditions such as wind velocity, air temperature, wave height, current velocity, water temperature, salinity and plankton. Only a research vessel enable us to carry out such comprehensive observations, however it is difficult to conduct long-term observations by ship in the severe winter sea.
ABSTRACT A model for riser displacements generated by ocean currents is derived, for the case where the riser is cylindrical with uniform material properties in the axial direction. The current forces are assumed to be proportional to the square of the speed and account for changes due to variable riser diameters (buoyancy modules). Vortex induced vibrations are formulated through time varying drag- and lift coefficients. The model is sufficiently flexible to consider arbitrary currents that vary continuously with depth and time. Solutions of the model equations are developed in the time domain by expanding the riser displacement in terms of axially varying orthogonal functions. Data from a full scale drilling operation is used to validate the model code. It turns out that the oscillation frequency, displacements and mode shapes are fairly well reproduced by the model. INTRODUCTION Interactions between fluids and structures occur in many situations related to the offshore oil and gas industry. One of them, the interaction between risers and ocean currents, has received significant attention in recent years due to increasing activity in deeper water. Vortex induced vibration (VIV) is one aspect of the interactions and can take place in steady flows. Experimental studies of VIV have been carried out by several authors, among them Griffin (1980), Huse (1997), King (1977/1986) and Mo and Lie (1997). Field studies of VIV on deepwater drilling risers have also been performed under the "Norwegian Deepwater Programme" (NDP) (see e.g. Fumes et al. 1998).
Numerical Modelling of OWC-Shoreline Devices Including the Effect of Surrounding Coastline And Non-Flat Bottom
Brito-Melo, A. (Instituto Superior Técnico) | Hofmann, T. (Instituto Superior Técnico) | Sarmento, A.J.N.A. (Instituto Superior Técnico) | Clément, A.H. (Ecole Centrale de Nantes) | Delhommeau, G. (Ecole Centrale de Nantes)
ABSTRACT In the numerical modelling of wave energy plants of the Oscillating Water Column (OWC) type, it is usual to consider the structure isolated and standing on an horizontal bottom. However when dealing with inshore devices, located in a non flat bottom, the geometry of the surrounding shoreline may have a strong influence on the behaviour of the OWC system, in what concerns the resonance frequencies and the hydrodynamic coefficients. The purpose of this work is the application of a 3D radiation diffraction numerical code to the Azores European Pilot plant in the Island of Pico, taking into account the effect of the surrounding bathymetry and topography. The hydrodynamic problem is formulated in the frequency domain based on classical linear water wave theory. The present paper is an extension of a previous one (Brito-Melo et al., 1999). INTRODUCTION The European pilot plant in the island of Pico, in the Azores, is an OWC device consisting mainly of a concrete structure built upon the sea bottom and located in a natural convergent channel between the rocky coastline and a small islet. The structure forms a pneumatic chamber at its upper part (above water level) whose interior dimensions is 12x12m 2 at mean water level. An opening on the submerged part of the front wall enables the entrance of water within the chamber and by the effect of incident waves the water free surface inside the chamber is forced to oscillate (oscillating water column), inducing an up and down air displacement. The chamber is connected to the atmosphere by an air duct containing a reversible air turbine of the Wells type coupled to an electric generator. The alternating air flow exiting the interior chamber drives the turbine. A detailed description can be found in (Falcão et al., 1995).
ABSTRACT We introduced daily crop development and growth functions into theBiosphere-Atmosphere Transfer Scheme (BATS) coupled to the National Center for Atmospheric Research Regional Climate Model (NCAR RegCM2). Coupled RegCM/BATS simulations were performed over the conterminous United States for a dry (1988) and a favorably moist (1991) growing seasons at a spatial resolution of 90 x 90 km. We found that differences in air and ground temperature, mixing ratio, and precipitation between these two seasons were significantly larger in the interactive cases compared to the control ones. The differences in interannual variability were due to the fact that the interactive version of the model was able to capture extensive plant wilt that resulted from drought conditions of summer 1988, the control version was not. Inhibited plant growth resulted in significant decreases in the surface moisture flux to the atmosphere, therefore increasing the sensible heat flux. This acted as a positive feedback to the climate model's suggested dry conditions of 1988. The interactive version of RegCM/BATS that we developed agreed better with observations for the maximum daily air temperature over the central Great Plains for each of the two years in question compared to the control version. Interannual variability ranges simulated with the interactive version of the model also agreed better with observations compared to control ones. INTRODUCTION The principal objective of this paper is to investigate the effect, over a predominantly agricultural central U.S. domain, of the interactively-simulated crop development and growth on the interannual variability in the warm-season surface and air temperature, mixing ratio, and precipitation. Heat, moisture, and momentum fluxes over land strongly depend on the condition of surface vegetation. Experimental data indicate that some biophysical parameters (Leaf Area Index, bulk canopy resistance, roughness length) vary significantly throughout the growing season.
ABSTRACT By first deriving the appropriate Green's function, a model is developed that allows the interaction of ice-coupled waves with a crack to be studied analytically. Simple formulae for the reflection and transmission coefficients emerge that have not been reported before. The coefficients are found to be strongly dependent on wave period; as period is increased the reflection coefficient drops rapidly to zero, where transmission is perfect, before rising to a low maximum and then decreasing asymptotically to zero again as period is increased further. The mathematical technique employed is amenable to other problems of this type. INTRODUCTION Vast areas of the Arctic Ocean are covered with a continuous veneer of sea ice, broken only by cracks, leads and pressure ridges. Such features, which may stretch for tens of kilometres, form when changes in the wind especially cause divergent or convergent stresses to develop in the ice sheet. The ice will crack as it is pulled apart and, if divergent stresses persist, a lead will form that may freeze over. A wind change will then produce a pressure ridge or a shear ridge. Because waves generated in the open sea are known to penetrate far into the Arctic ice cover, there has been some interest in the possibility of using waves as a tool to "remotely sense" average ice thickness. Such an idea seems plausible now because, over the last twenty years or so, sustained effort has been put into understanding how waves and sea ice in its many forms interact. (See Squire et al, 1995 for a recent review.) However, the bulk of the theories that have been developed to model waves propagating in sea ice assume that the ice behaves as a uniform plate without flaws.
INTRODUCTION Sea ice is characterized by strength anisotropy as a function of load application mode which may be normal (┴) or parallel (||) to the freezing plane. This problem was addressed in a number of papers (Frederking and Timco, 1984). Anisotropy coefficient, kR introduced by the Moscow Construction Engineering Institute (MISI) is obtained as: where R= is the mean value of ultimate uniaxial compressive strength of ice when load direction is normal to the freezing plane and R': is that when load direction is parallel to the freezing plane (MISI Report, 1992). Note that the introduction of anisotropy coefficient involves a number of issues opened to argument. It is known from the law of probability that the ratio of mean values is not the mean value in itself (as opposed to the production of mean values), i.e. kR may be a measure of the mean value but the error of the measure will be unknown. Furthermore, the mean value depends substantially on the number of tests, which is not taken into account in the expression (I). However, anisotropy coefficient was used by MISI and estimated based on a number of data used by some authors involved in anisotropy studies. It varied from 1.04–2.0 (MISI Report, 1992). Anisotropy coefficient was estimated as kR =1.39 during the expedition on the icebreaker "Yermak" in the sea of Okhotsk in 1982 (Report on Expedition... 1982). Institute "SakhalinNIPImorneft" performed ice studies in the Chayvo Bay also using anisotropy coefficient (Polomoshnov, 1990; Polomoshnov et al, 1992). However, no reliable results were obtained: anisotropy coefficient was either over or below 1. Note that the majority of ice strength tests were performed on samples cut out normal to the freezing plane, therefore, knowledge of ice strength anisotropy is required adequate evaluation of the results obtained and substantiation
- North America > United States (0.50)
- Europe > Russia > Central Federal District > Moscow Oblast > Moscow (0.26)
- Asia > Russia > Far Eastern Federal District (0.15)
ABSTRACT A novel wave energy device (the 'Wave Rotor') is driven by the action of the Magnus effect on two parallel counter-rotating cylinders. This paper presents the results of experimental investigations of a model wave rotor both driven in still water and absorbing energy from incident waves, together with data on spinning cylinders in uniform steady flow and in waves. An estimate is made of the productivity of a full-scale device. INTRODUCTION The wave rotor (Retzler 1991) comprises cylinders that span between arms that turn about a central axis parallel with the wave crests (figure 1). Circulation in opposite senses around the two cylinders generates lift forces producing a moment about the central axis. In orbital flow the device rotates continuously at the wave frequency and power is extracted by providing damping to the rotation. Magnus lift forces on spinning cylinders in waves can be several times larger than the inertial forces depending on spin magnitude and wave size (Budal & Lillebekken 1985, Retzler 1987), offering the potential of a wave energy device of higher power for a given displacement. An earlier lift-driven wave energy device has been described by Hermans, van Sabben and Pinkster (1990). It used a foil, mounted on a shaft immersed in waves, which also performed full rotations synchronously with the waves. The device described here uses two cylinders for lift to give a couple around the centre of mass, hence requiring no counterbalance. Furthermore a spinning cylinder, unlike a foil, has no angle of attack and hence does not stall. EXPERIMENTS The wave flume The experiments were carried out in a wave flume 12.8m long, 0.425m wide, with a water depth of 0.7m. An electricallydriven absorbing wavemaker with force feedback generates waves over a frequency range of 0.5 – 2.0Hz.
ABSTRACT A Wells turbine has inherent disadvantages: lower efficiency and poorer starting characteristics. In this case, the guide vanes in front of and after rotor may be one of the most effective equipment for the improvement of the turbine performance. Several papers demonstrated the usefulness of 2D guide vanes so far. In order to achieve the further improvement of the performance of the Wells turbine, the effect of 3D guide vanes has been investigated experimentally by a model testing under steady flow conditions. And then, the running and starting characteristics under sinusoidally oscillating flow conditions have been obtained by a computer simulation using quasi-steady analysis. As a result, it is found that the running and starting characteristics of the Wells turbine with 3D guide vanes are superior to those with 2D guide vanes. INTRODUCTION Several of the wave energy devices currently studied in the United Kingdom, Japan, Portugal, India and other countries make use of the principle of the oscillating water-air column for converting wave energy to low-pneumatic energy which in turn can be converted into mechanical energy. In this case, the development of a bi-directional air turbine has come up as an important problem. So far, a number of self-rectifying air turbines with different configurations have been proposed, including the Wells turbine (Gato et al., 1988; Inoue et al., 1986a, 1986b; Kaneko et al., 1986; Raghunathan et al., 1987, 1994; Raghunathan, 1995; Setoguchi et al., 1986; Suzuki et al., 1985; White, 1995), a turbine using self-pitch-controlled blades (Raghunathan et al., 1997; Sannento et al., 1987, Takao et al., 1997), an impulse turbine with self-pitch-controlled guide vanes (Setoguchi et al., 1996), an impulse turbine with fixed guide vanes (Setoguchi et al., 1999) and so on (Kaneko et al., 1992).
ABSTRACT This paper presents an interaction problem of a half cylinder membrane structure and the viscous flow around it. The flow fields are numerically solved by the Navier-Stokes (NS) equation method. The unsteady incompressible 2D-NS and continuity equations are solved by a time marching numerical scheme. Finite element method (FEM) considering large deflection effect with membrane finite element is used to estimate the elastic characteristics of the membrane. In this paper, the vortexinduced vibration is reproduced in the numerical computation and significant influences of the unsteady aeroelastic behavior are confirmed. INTRODUCTION Membrane structure is characterized by its structural simplicity and movability offered by its lightness. These are advantages related to the fact that the wide space can easily be secured with low construction cost. For example, a sports dome and a multipurpose hall are classified as membrane structures which have become possible at the advent of synthetic fiber and glass fiber. In addition, the advantage remains undiminished in other membrane manufactures such as sails, balloons, airships, parachute and even umbrellas. However, these membrane structures or manufactures have some supports to keep the forms. Actually, the weakness of the membrane structure to the external force is a disadvantage comparing with rigid structures such as concrete buildings. Therefore, important factors in designing the membrane structures are accurate prediction and controls of the influences of the external force such as wind, snow and the water pressure. Furthermore, research of the interaction problem of the membrane structure and the external force is required in many engineering fields.
Abstract Through the use of a combination of captive tests and some heuristic procedures, Sphaier, Fernandes, Pontes and Correa (1998, 1999, 2000) developed a quadratic model, that have been inherited from the ship maneuvering field, to account for the hydrodynamic lateral force and turn moment. The present paper incorporates some improvements to the model including a heuristic expression to determine the longitudinal force. It is derived from towing tests for different heading angles and takes into account the scale effects through the Reynolds number and the form effect, the viscous pressure resistance, through the Prohaska coefficient k. The process of determining the coefficients depends not only on the tests but also on the knowledge of the added masses. Results from the potential theory (WAMIT, 1995) and from experiments (Clarke, Gedling, and Hine, 1982) are used. Using the maneuvering model a dynamic stability analysis is carried out comparing the behavior of the ship and the model. INTRODUCTION In the last forty years an extensive effort has been developed to define mathematical expressions to represent the forces and moments acting on ships traveling on the sea. Beginning with Abkowitz (1964) and Eda and Crane (1965) and followed by Norrbin (1970) and others the so-called maneuvering models for ships in normal speeds have been developed. Although a mathematical model has been used to establish the fundamentals of the theory, it is dependent on experimental tests. With the use of ships as stationary storage units some of these ideas have been imported and, in some case used directly. Using similar tests Wichers (1986), Molin and Bureau (1980), Obokata (1984) and others have developed different models based on the cross-flow model and also used on the maneuvering theory of ship.