Hin-Fatt Cheong

Reflection and transmission characteristics of regular and random waves from a submerged horizontal plate

Abstract Reflection and transmission characteristics of regular and random waves from a horizontal thin plate which is fixed at a certain submergence below the water surface, have been investigated. The experimental results are compared with the theoretical solution obtained numerically by the finite-element method using the linear-wave diffraction theory. The optimum conditions of the submerged plate to achieve minimum transmission of the waves, can be deduced from the results. Optimum plate width was found to be about 0.5–0.7 of the wave length above the plate with plate submergence kept at around 0.05–0.15 to induce wave breaking which is the main cause of energy dissipation.

A size-based ecosystem model for pelagic waters

Size-based models offer a holistic approach to understanding the structure and function of pelagic ecosystems, compared to simple models in which the entire system is represented by a single phytoplankton and zooplankton species. In this study, a one-dimensional size spectral model is constructed to simulate the temporal and vertical variability in the microbial community, nutrients and primary productivity. The approach taken is based on published allometric relationships governing physiological and ecological processes for phytoplankton and zooplankton. Since these allometric relationships are objective estimators of ecological parameters and are not dependent on knowledge of particular species, these models can be used to describe general features of the ecosystem. The model is driven by light, temperature and nutrient availability and is applied to an oligotrophic system in the Sargasso Sea-the Bermuda Atlantic Time Series (BATS) station, where field data on size spectra has recently been acquired. The model is able to reproduce the general overall features of this oceanic ecosystem fairly well: It depicts a late winter/early spring bloom and low production during the summer and fall months. In addition, a deep subsurface phytoplankton biomass maximum is produced in the summer, consistent with field measurements. The model prediction for size spectra is reasonably successful, although further refinement is necessary. As a whole, the model is fairly robust and can also be applied to ecosystems of different trophic state.

Laboratory study of plunging wave impacts on vertical cylinders

The characteristics of pressures associated with plunging wave impacts on a vertical cylinder are presented. Despite a high variability in the peak pressures, spatial distributions of the impact pressure time histories, both in the vertical direction and around the seaward front of the cylindrical surface, are found to vary systematically for a range of cylinder locations in the wave plunging region. Through correlations between the incident wave kinematics and the impact pressures, the mechanics of the impact process have been identified and elucidated. These results serve as a useful reference for future numerical and theoretical modelling of extreme wave loads on surface-piercing vertical cylinders.

Three-dimensional finite difference model for transport of conservative pollutants

Abstract A three-dimensional finite difference transport model appropriate for the coastal environment is developed for the solution of the three-dimensional convection–diffusion equation. A higher order upwind scheme is used for the convective terms of the convection–diffusion equation, to minimise the numerical diffusion. The validity of the numerical model is verified through five test problems, whose exact solutions are known.

Multilevel finite-difference model for three-dimensional hydrodynamic circulation

A three-dimensional finite-difference multilevel hydrodynamic model is developed using an explicit scheme on a staggered grid. The model has been tested against four cases, namely (i) wind-induced circulation (ii) density-driven circulation (iii) seiche oscillation in a closed basin and (iv) tide-induced circulation in a open channel. The results obtained in the present study compare well with those obtained from the corresponding analytical solutions under idealised conditions for the above four cases. The model was also tested against the case of circulation induced by wind and Coriolis force and the results obtained are compared with the results of Davies and Owen (1979).

Boundary fitted grid models for tidal motions in Singapore coastal waters

This paper describes the application of a 2-D nested model in rectangular coordinates and an orthogonal boundary fitted grid (BFG) model to tidal motions in Singapore coastal waters. The basic mathematical model for shallow water waves in the BFG model is derived from the transformation of corresponding depthaveraged equations in rectangular coordinates. The curvilinear coordinate system is obtained from the solution of a grid generator based on the elliptic generation technique. The BFG model is applied to the Singapore coastal region and its surrounding waters with a complicated domain and internal island groups. The tidal flow behaviour including current circulation, time history of sea levels and currents as well as patterns of residual currents are investigated. The numerical results are successfully compared with the field measurements.

Prediction of extreme 3-sec. gusts accounting for seasonal effects

Abstract In conventional statistical analysis of extreme 3-sec. gusts, extremal distributions are often used to fit the annual maximum value obtained from available records, invoking the assumption of stationarity and statistical independence. This paper presents an alternative approach in the form of an exponential distribution for the magnitude of the gust above a pre-determined base level compounded with a nonhomogenous Poisson model for the occurrence of the exceedances. The base level of 13.5 m/s is found to be appropriate. The prediction of the extreme gusts using this method agrees well with a modified conventional extreme value analysis which accounts for the seasonal variations within a year through the monthly extremal distribution parameters. The compound distribution approach is preferred over the conventional method since it does not rely solely upon the maximum value but rather utilizes directly more of the available information.

Reflection and transmission of random waves by a horizontal double-plate breakwater

The reflection and transmission coefficients of a horizontal double plate system were derived in terms of the reflection and transmission coefficients of a single plate case. A spectral decomposition technique was used to evaluate the spectral parameters for the incident, reflected and transmitted wave spectra. Experiments were conducted for the case of a seaward surface plate and a submerged leeward plate and it was established that the optimum degree of submergence for the leeward plate would be about 0.10 to 0.20 for minimum wave transmission.

Forces on a smooth submarine pipeline in random waves — A comparative study

Abstract Forces on a smooth submarine pipeline, fixed horizontally near a plane boundary, have been investigated under random wave conditions. The submarine pipeline was subjected to Pierson-Moskowitz spectrum (P-M spectrum) at various energy levels. The water particle kinematics were computed based on the linear random wave model and the Morison equation was chosen as the wave force predictor model. The inline hydrodynamic coefficients of drag and inertia were evaluated using two different methods, one in the frequency domain and the other in the time domain. Five mathematical formulations were considered for the analysis of transverse wave forces and these were compared in terms of the correlation coefficient. The transverse force was also analyzed in terms of the transverse root mean square (rms) coefficient. The inline hydrodynamic coefficients of drag and inertia and the transverse rms coefficient were correlated with the Keulegan-Carpenter number or period parameter, the relative clearance of the pipeline from the bed and the depth parameter. Finally, the results of the random wave tests were compared with those of regular waves under similar pipeline conditions.

Wave induced pressures and forces on a fixed submerged inclined plate

Abstract The diffraction of water waves by a submerged fixed inclined plate is modelled using the finite element method (FEM). Wave induced pressures and forces on the submerged fixed inclined plate due to incident monochromatic waves are analysed using the finite element model. The fluid domain is discretised using 8 node isoparametric elements with 3 node quadratic line elements on the free surface, bed and radiation boundaries. The accuracy of the numerical technique is demonstrated by comparing the FEM results with the analytical solution obtained by long wave solution for a thin submerged fixed horizontal plate in shallow water. The horizontal and vertical force coefficients are computed for various wave periods and inclinations of the plate. The reflection and transmission coefficients are also computed and presented for various submergence depths. The possible use of the submerged inclined plate as a breakwater and its advantages over the horizontal plate are discussed.