CFD modelling of a small–scale fixed multi–chamber OWC device

Wave Energy Converters (WECs) have excellent potential as a source of renewable energy that is yet to be commercially realised. Recent attention has focused on the installation of Oscillating Water Column (OWC) devices as a part of harbor walls to provide advantages of cost–sharing structures and proximity of power generation facilities to existing infrastructure. In this paper, an incompressible three–dimensional CFD model is constructed to simulate a fixed Multi–Chamber OWC (MC–OWC) device. The CFD model is validated; the simulation results are found to be in good agreement with experimental results obtained from a scale physical model tested in a wave tank. The validated CFD model is then used for a benchmark study of 96 numerical tests. These investigate the effects of the PTO damping caused by the power take–off (PTO) system on device performance. The performance is assessed for a range of regular wave heights and periods. The results demonstrate that a PTO system with an intermediate damping can be used for all chambers in the MC–OWC device for most wave period ranges, except for the long wave periods. These require a higher PTO damping. An increased incident wave height reduces the device capture width ratio, but there is a noticeable improvement for long wave periods.     

Transient effects of tendon disconnection of a TLP by hull–tendon–riser coupled dynamic analysis

This paper deals with a numerical study of the transient effect of tendon disconnection on global performance of an extended tension leg platform (ETLP) during harsh environmental conditions of Gulf of Mexico (GoM). The ETLP has twelve tendons with twelve production top-tensioned risers (TTRs) and one drilling riser. The risers are attached by hydro-pneumatic tensioners. A time-domain nonlinear global-motion-analysis program for floating hulls coupled with risers/mooring lines is developed to model the transient effects associated with tendon disconnection at the top or the bottom. The sudden disconnection of one or more tendons causes the change of stiffness and natural periods, the imbalance of forces and moments of the total system, and possibly large transient overshoots in tension at the moment of disconnection. The breakage at the top and the unlatch at the bottom also make different impacts on the system. The transient responses and tensions are compared and discussed in the viewpoint of the robustness of the system. The survivability of a TLP with the loss of one or two tendons by accident during a lesser-than-extreme environment can also be checked by this kind of time-domain simulation technique.     

Effects of periodic saturation on stress–strain relationship of a sandstone mixture

While a crushed sandstone particle mixture, usually used as an engineering fill, is filled along bank of or in a large reservoir, it is subjected to periodic saturation induced by filling-drawdown cycles of reservoir water. The periodic saturation may induce post-construction settlement, and reduce stability of the filled body. In order to evaluate the effects of periodic saturation on stress–strain relationship of the mixture, several triaxial tests are carried out. According to experimental data, the periodic saturation may induce an increment of axial strain (Δε; <0.226%), a decrement of crest of deviator stress (Δ(σ₁?σ₃)_f; <0.192?MPa) and a decrement of angle of shearing resistance (Δφ; <3.70°). With increment of the number of periodic saturation cycles, the variations of the three parameters may be fitted by logarithmic curves. And with increase in the stress level for periodic saturation, the variations of the three parameters may be fitted by straight lines. Three fitting equations to predict the three parameters’ values, and an equation to calculate the settlement induced by the periodic saturation of a large-area filled foundation, are suggested.     

Transfer of the Substance of a Colored Drop in a Liquid Layer with Travelling Plane Gravity–Capillary Waves

Izvestiya, Atmospheric and Oceanic Physics - The evolution of the distribution pattern of an ink droplet freely falling on the wavy surface of a liquid is traced by video-recording methods. A...     

Deformation of a sandstone–mudstone particle mixture induced by periodic saturation

This study focuses on the evaluation on deformation induced by periodic saturation of a sandstone–mudstone particle mixture. Two types of triaxial tests, without and with periodic saturation, were performed. The strain–stress relationships from the two types of tests indicate that the periodic saturation may induce an increment of axial strain (Δε), and the Δε values are related to the ratio of confining to atmospheric pressure (σ₃/p_a), stress level for periodic saturation (L), and number of periodic saturation or cycles (N). The values of Δε are increasing along logarithmic curves with increment of N value from 1 to 20, and increase along straight lines with increasing L value from 0.18 to 0.82 or σ₃/p_a value from 1 to 4. Based on the analyses of experimental data, a logarithmic fitting equation, which is a function of N, L, and σ₃/p_a, is suggested to predict the Δε value. And based on the fitting equation and simple analyses on stress state, another equation, which may be used to estimate the settlement induced by periodic saturation of a large-area foundation filled using the sandstone–mudstone particle mixture, is also suggested.     

Problems and prospects of creating a global land–ocean seismic network

The paper discussed the advantages and limitations of seismic signal detection on the ocean bottom. The need to create long-term seismic monitoring systems in areas of industrial development on the shelf and continental slope, as well as in areas with high seismic and tsunami hazards, is justified. The results of employing broadband bottom seismographs during expeditions of the Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) are described. Autonomous broadband bottom seismographs with operational communication via satellite or radio channels are proposed for creating a global marine seismic network.     

Simulation of Typhoon Muifa using a mesoscale coupled atmosphere–ocean model

A mesoscale coupled atmosphere–ocean model has been developed based on the GRAPES(Global and Regional Assimilation and Prediction System) regional typhoon model(GRAPES_TYM) and ECOM-si(estuary, coast and ocean model(semi-implicit)). Coupling between the typhoon and ocean models was conducted by exchanging wind stress, heat, moisture fluxes, and sea surface temperatures(SSTs) using the coupler OASIS3.0. Numerical prediction experiments were run with and without coupling for the case of Typhoon Muifa in the western North Pacific. To investigate the impact of using more accurate SST information on the simulation of the track and the intensity of Typhoon Muifa, experiments were also conducted using increased SST resolution in the initial condition field of the control test. The results indicate that increasing SST resolution in the initial condition field somewhat improved the intensity forecast, and use of the coupled model improved the intensity forecast significantly, with mean absolute errors in maximum wind speed within 48 and 72 h reduced by 32% and 20%, respectively. Use of the coupled model also resulted in less pronounced over-prediction of the intensity of Typhoon Muifa by the GRAPES_TYM. Moreover, the effects of using the coupled model on the intensity varied throughout the different stages of the development of Muifa owing to changes in the oceanic mixed layer depth. The coupled model had pronounced effects during the later stage of Muifa but had no obvious effects during the earlier stage. The SSTs predicted by the coupled model decreased by about 5–6°C at most after the typhoon passed, in agreement with satellite data. Furthermore, based on analysis on the sea surface heat flux, wet static energy of the boundary layer, atmospheric temperature, and precipitation forecasted by the coupled model and the control test, the simulation results of this coupled atmosphere–ocean model can be considered to reasonably reflect the primary mechanisms underlying the interactions between tropical cyclones and oceans.     

Tuning a physically-based model of the air–sea gas transfer velocity

Air–sea gas transfer velocities are estimated for one year using a 1-D upper-ocean model (GOTM) and a modified version of the NOAA–COARE transfer velocity parameterization. Tuning parameters are evaluated with the aim of bringing the physically based NOAA–COARE parameterization in line with current estimates, based on simple wind-speed dependent models derived from bomb-radiocarbon inventories and deliberate tracer release experiments. We suggest that A = 1.3 and B = 1.0, for the sub-layer scaling parameter and the bubble mediated exchange, respectively, are consistent with the global average CO₂ transfer velocity k. Using these parameters and a simple 2nd order polynomial approximation, with respect to wind speed, we estimate a global annual average k for CO₂ of 16.4 ± 5.6 cm h^?1 when using global mean winds of 6.89 m s^?1 from the NCEP/NCAR Reanalysis 1 1954–2000. The tuned model can be used to predict the transfer velocity of any gas, with appropriate treatment of the dependence on molecular properties including the strong solubility dependence of bubble-mediated transfer. For example, an initial estimate of the global average transfer velocity of DMS (a relatively soluble gas) is only 11.9 cm h^?1 whilst for less soluble methane the estimate is 18.0 cm h^?1.     

Effects of Mudstone Particle Content on Shear Strength of a Crushed Sandstone–Mudstone Particle Mixture

The present study focuses on the shear strength, deformation, and particle crushing of sand which is mixed by crushed sandstone and mudstone particles. Two triaxial tests—one for unsaturated specimens and the other for saturated specimens—were performed, along with sieve analysis tests. Results obtained from the triaxial tests indicate that, with the increment of the mudstone particle content by weight, the angle of internal friction decreases, the cohesion increases and then decreases, and both the initial angle of shearing resistance at the atmospheric pressure and decremental angle increase and then decrease. Compared to the unsaturated specimens, the deviator stress or normalized deviator stress of the saturated specimens at the same axial stress may decrease due to the wetting action. Thus, the shear strength of the tested materials may be reduced by the wetting action. Results of sieve analysis tests indicate that the mixing of some mudstone particles into the sandstone particle mixture may reduce the amount of particle crushing, but the wetting action may increase the amount. The amount of the crushed particle may arrive at a minimum of 40% of mudstone particle content by weight.     

Numerical Modeling of Tropical Cyclone-Induced Storm Surge in the Gulf of Oman Using a Storm Surge–Wave–Tide Coupled Model

Ocean Science Journal - A storm surge is a complex phenomenon in which current, tide, and waves interact with each other. Even if the wind is the main force of driving the surge, waves and tide are...     

Damping of gravity–capillary waves on water surface covered with a visco-elastic film of finite thickness

Results of laboratory studies of the damping of gravity–capillary waves on a water surface covered with a film of petroleum product (diesel fuel) in a wide range of change in a film thickness are presented. A nonmonotonic dependence (with a local maximum) of a damping coefficient on film thickness is discovered. Numerical calculations of the dispersion equation for gravity–capillary waves in the presence of a viscoelastic film of arbitrary thickness, which confirmed the existence of the maximum of the damping coefficient as a function of film thickness, are performed. Based on a comparison of the calculation results and the data of laboratory measurements of wave characteristics, the values of parameters in the diesel fuel films are estimated in a wide range of a change in their thickness.     

The sources of Antarctic bottom water in a global ice–ocean model

Two mechanisms contribute to the formation of Antarctic bottom water (AABW). The first, and probably the most important, is initiated by the brine released on the Antarctic continental shelf during ice formation which is responsible for an increase in salinity. After mixing with ambient water at the shelf break, this salty and dense water sinks along the shelf slope and invades the deepest part of the global ocean. For the second one, the increase of surface water density is due to strong cooling at the ocean–atmosphere interface, together with a contribution from brine release. This induces deep convection and the renewal of deep waters. The relative importance of these two mechanisms is investigated in a global coupled ice–ocean model. Chlorofluorocarbon (CFC) concentrations simulated by the model compare favourably with observations, suggesting a reasonable deep water ventilation in the Southern Ocean, except close to Antarctica where concentrations are too high. Two artificial passive tracers released at surface on the Antarctic continental shelf and in the open-ocean allow to show clearly that the two mechanisms contribute significantly to the renewal of AABW in the model. This indicates that open-ocean convection is overestimated in our simulation. Additional experiments show that the amount of AABW production due to the export of dense shelf waters is quite sensitive to the parameterisation of the effect of downsloping and meso-scale eddies. Nevertheless, shelf waters always contribute significantly to deep water renewal. Besides, increasing the P.R. Gent, J.C. McWilliams [Journal of Physical Oceanography 20 (1990) 150–155] thickness diffusion can nearly suppress the AABW formation by open-ocean convection.     

Transport and retention of a conservative tracer in an isolated creek–marsh system

A study of tracer transport and retention in a small tidal creek and marsh system located in the southeastern US was conducted using a three-dimensional hydrodynamic model and data from a dye tracer release. The model simulated tidally driven flow, including inundation and drying out of the marshes and the dispersal of the dye tracer. Flow measurements in the tidal creek showed that the simulations appeared to generally duplicate the tidally driven flow into and out of the tidal creeks and marshes. The dye tracer experiment was conducted to test the hydrodynamic model's ability to simulate dispersal of a point release of pollutants into the creek during an incoming tide. The simulations reproduced much of the experimental measurements, but bathymetric errors and lack of resolution of the smallest arms of the tidal creeks affected the ability to faithfully reproduce the initial peak in measured dye tracer concentrations at a sampling location far from the boundary. The lack of the smallest tidal creeks led to some trapping of water and dye in the marshes. Two independent estimates of flushing time yielded values between 1.6 and 2.4 days. An uncertainty analysis indicates that model simulations are sensitive to variations in parameters such as water depth and marsh grass density. On the other hand, omission of the smallest tidal creeks in simulations may be partially offset by decreasing marsh grass density. Further improvements must rely on more accurate and detailed bathymetric data that resolves the smaller arms of the tidal creeks and includes quantitative information about the density and distribution of marsh grass.     

Numerical simulation of waves generated by landslides using a multiple-fluid Navier–Stokes model

This work reports on the application and experimental validation, for idealized geometries, of a multiple-fluid Navier–Stokes model of waves generated by rigid and deforming slides, with the purpose of improving predictive simulations of landslide tsunamis. In such simulations, the computational domain is divided into water, air, and slide regions, all treated as Newtonian fluids. For rigid slides, a penalty method allows for parts of the fluid domain to behave as a solid. With the latter method, the coupling between a rigid slide and water is implicitly computed (rather than specifying a known slide kinematics). Two different Volume of Fluid algorithms are tested for tracking interfaces between actual fluid regions. The simulated kinematics of a semi-elliptical block, moving down a water covered plane slope, is first compared to an earlier analytical solution. Results for the vertical fall of a rectangular block in water are then compared to earlier experimental results. Finally, more realistic simulations of two- and three-dimensional wedges sliding down an incline are compared to earlier experiments. Overall, in all cases, solid block velocities and free surface deformations are accurately reproduced in the model, provided that a sufficiently resolved discretization is used. The potential of the model is then illustrated on more complex scenarios involving waves caused by multi-block or deformable slides.     

Historic developments in macrozoobenthos of the Rhine–Meuse estuary: From a tidal inlet to a freshwater lake

Water works during the 1960s and 1970s changed the northern part of the Rhine–Meuse estuary in the south-west of the Netherlands into a freshwater lake, from west to east divided into three basins called the Haringvliet, the Hollands Diep and the Biesbosch. Concurrently water quality parameters (e.g. nutrients and pollutants) changed drastically during the last 50 years. This study combines macrozoobenthic monitoring data from the region from 1960 to 2001 with trends in abiotic parameters to evaluate historic developments of the communities, including densities, species numbers and diversity, and assess future developments as a first step to a rehabilitation of the estuary as planned for January 01, 2008. During the 1960s, the macrozoobenthic densities of Oligochaeta and/or Polychaeta dominated communities increased with a gradual decrease of saltwater intrusion and salinity variability. The first years after the basins became stagnant, the species numbers per sample and the Shannon diversity were high due to the coexistence of salt and freshwater species. An increase in nutrient and pollutant loads led to a decrease in the macrozoobenthos densities. As water and sediment quality gradually improved, nowadays the former estuary contains high diversity and high density macrozoobenthic communities, whereas Oligochaeta and/or Polychaeta were dominant in the 1960s, and Bivalvia and Gastropoda were more abundant during the 1970s. Macrozoobenthic communities moved from the east to west with a time-lag, which may primarily be attributed to changing salinities, salinity variances and oxygen levels. Therefore, the current communities of the Haringvliet show similarities with the communities that occurred already during the 1960s in the Biesbosch. This study shows the value of macrozoobenthos monitoring data over longer periods. The possible impact of a new saltwater inlet in the west of the Haringvliet, allowing in the near future saltwater to enter 11.5 km eastward, yet alternated by frequent flushing with freshwater to ascertain that the salt intrusion does not reach further, on the development of the macrozoobentic communities is discussed.     

The processes of semi-enclosed basin–ocean water exchange across a tidal mixing zone

To assess each of the two possible mechanisms responsible for the fortnightly modulation of semi-enclosed basin–ocean water exchange (‘density tides’), a set of numerical experiments is carried out using a vertically two-dimensional numerical model with realistic situations in Puget Sound in mind. It is found that, although the localized vertical viscosity (or the localized vertical diffusivity) enhanced in the entrance sill region primarily controls the bottom-water transport (or the bottom-water density) during spring tides, it does not lead to any appreciable variations of bottom-water density (or bottom-water transport). This indicates that the fortnightly modulation of vertical viscosity and that of vertical diffusivity both play important roles in creating density tides. In the real ocean, the vertical viscosity and diffusivity are enhanced simultaneously during spring tides, so that it is difficult to discriminate between both effects on density tides. This causes the widespread misunderstanding that density tides are mainly caused by the decreased advection of dense bottom-water from outside due to the enhanced vertical viscosity during spring tides.     

Forecasting space–time variability of wave heights in the Bay of Bengal: a genetic algorithm approach

The present work employs a genetic algorithm to carry out wave height forecasting in the Bay of Bengal. The use of empirical orthogonal function analysis allows the spatial extending of the forecast to the entire basin. The chaotic nature of the process limits the horizon of usable forecasts to 48 h in advance. Statistical evaluation of the quality of forecast leads to encouraging results. A major advantage of this method is that, once the forecast equations are derived, they can be used directly without the necessity of having a numerical wave model as an intermediate step.     

Eddies and a mesoscale deflection of the slope current in the Faroe–Shetland Channel

The mesoscale dynamics of the Scottish side of the Faroe–Shetland Channel have been investigated using synoptic in situ and remote sensing observations. A cold core cyclonic eddy, identified from an AVHRR image, had a diameter of about 50 km and surface current speeds of up to 50 cm s^-1; it appeared to be attached to the 800 m isobath as it moved north-eastward along the edge of the channel at about 8 cm s^-1. Speeds in the slope current were about 50 cm s^-1 but increased to 70 cm s^-1 where the current was compressed by the eddy. Offshore, over the 1000 m isobath in the cooler water, speeds in the current were slower (ca. 20 cm s^-1). North-west of the Shetlands the offshore edge of the slope current was deflected across the channel for a distance of about 70 km from the shelf edge. The speed of drifters in the slope current increased to over 60 cm s^-1 as they moved anti-cyclonically around this deflection. CTD profiles suggest that the movement of the surface waters was mirrored in the deep water of the channel. The deflection carried a very large quantity of North Atlantic Water into the central part of the channel; its cause and ultimate fate are not known, although it is likely to have had a significant impact on the dynamics of the channel.     

Determination of wave–shoreline dynamics on a macrotidal gravel beach using Canonical Correlation Analysis

This paper describes the application of Canonical Correlation Analysis (CCA) to derive forcing–response relations between the wave climate and shoreline position on a macrotidal gravel barrier located in the southwest of the U.K., and to develop a tool to determine shoreline positions from wave records. The data sequences comprise wave climate recorded by a nearshore directional wave buoy and video-derived shorelines over a time span of one year and a half. The hydrodynamic conditions are used to determine the probability density function of wave heights and alongshore energy fluxes. These are then related to shoreline change through a CCA analysis. The CCA analysis identifies patterns of behaviour of the wave conditions and the shoreline position, and the relation between both patterns is found to provide useful information about the beach response to wave action. The analysis shows that the movement of sediment is greater at the southern end of the study area and that there is an immediate shoreline response to the wave action. In the case of coastal management it is more often the case that wave forecasts are available on a routine basis. The ability of the CCA to provide useful estimates of shorelines from wave conditions was tested by using measured waves to calculate the corresponding shoreline position from additional data at the end of the sequences. Shoreline positions determined with the CCA agreed well with the measured ones. Thus, the CCA is found to be a useful tool to determine unknown shoreline positions and support effective coastal management if good quality hydrodynamic and morphological data are available to input into the initial set-up of the technique.     

Hydrodynamic analysis of three–unit arrays of floating annular oscillating–water–column wave energy converters

This paper deals with the hydrodynamic analysis of an array of Oscillating Water Column (OWC) devices, made up of coaxial cylinders, which are floating either independently or as a unit forming a floating platform. The platform is considered either free – floating or as TLP configuration connected to the sea bottom. Numerical results concerning the three boundary value problems, namely, the diffraction, the motion – and the pressure – dependent radiation ones are given. They have been obtained through an analytical solution method using matched axisymmetric eigenfunction expansion formulations. In all cases the interaction phenomena with neighbouring bodies have been taken properly into account using the physical idea of multiple scattering. Numerical results for the first – and the mean second – order wave forces, the hydrodynamic interaction coefficients along with pressure hydrodynamic parameters, inner air pressure and free–surface oscillation amplitude inside and outside of each device are parametrically evaluated and supplemented by experimental data.