Shan-Hwei Ou

Simulating typhoon waves by SWAN wave model in coastal waters of Taiwan

The SWAN wave model is typically designed for wave simulations in the near-shore region and thus is selected for evaluating its applicability on typhoon waves in the coastal waters around Taiwan Island. Numerical calculations on processes of wave heights and periods during the passages of four representative typhoons are compared with measured data from field wave stations on both east and west coasts. The results have shown that waves due to typhoons of paths 2, 3 and 4 can be reasonably simulated on east coastal waters. However, discrepancies increase for the simulated results on west coastal waters because the islands central mountains partly damage the cyclonic structures of the passing-over typhoons. It is also found that the included nested grid scheme in SWAN could improve the accuracy of simulations in coastal waters to facilitate further engineering practices.

On the prediction of beach changes by a new 2-D empirical eigenfunction model

Abstract A new two-dimensional (2-D) empirical eigenfunction model is developed for the prediction of beach changes due to cross-shore and longshore sediment transports. Beach profile data, measured every two months along six segmented detached breakwaters, were analyzed to generate spatial and temporal orthogonal eigenfunctions. The temporal eigenfunctions are predicted through the use of the Markov process and a linear regression method as a time series to forecast changes in the sea bottom. This model simplifies the complex representation of eigenfunctions presented by Hsu et al. (1986), and takes advantage of this simplication to reduce artificial errors and save computer time for further applications. The predictability of the proposed model is examined through field observations as well as predictions of existing empirical eigenfunction models. The result shows that this 2-D empirical eigenfunction model is efficient and advisable for the prediction of the variability of beach changes around coastal structures.

A complementary mild-slope equation derived using higher-order depth function for waves obliquely propagating on sloping bottom

In this paper a complementary mild-slope equation (CMSE) is derived in order to investigate the transformation of progressive waves obliquely propagating over the sloping bottom more realistically. We introduce a new depth function which includes the wave refraction and the influence of the bottom slope α, perturbed to the second-order in the integral equation. A new depth-integrated mild-slope equation is derived, by using the above mentioned depth function, to model a time-harmonic motion of small amplitude waves in varying water depth. The simulated results reveal that the proposed model provides a significant improvement in the calculation of the wavenumber and the group velocity at different bottom slopes. With the increasing bottom slope, the discrepancies in the reflection coefficient of Bragg scattering between the analytical solution and the one calculated from the conventional mild-slope equation (MSE) and the modified MSE (MMSE) are seen to steadily increase. The group velocity of the waves, wh...

On the mass transport of water waves in a turbulent boundary layer

Abstract An analytical theory describing the mass transport velocity induced by water waves in a turbulent boundary layer is presented. A one-layer time-invariant eddy viscosity model, which is more realistic under natural conditions, is specified to calculate steady streaming within boundary layer. A simplified analysis is developed based on the wave friction factor relating the bottom shear stress and free stream velocity. A predicted reversal of the mass transport is obtained for the case of long waves. The results are similar to those based on time-dependent eddy viscosity models. The present theory is much simpler mathematically than existing theories. The numerical results are compared favorably with available data.

TWO-DIMENSIONAL EMPIRICAL EIGENFUNCTION MODEL FOR THE ANALYSIS AND PREDICTION OF BEACH PROFILE CHANGES

More than 150 tests have been analyzed in order to describe the dynamically stable profiles of rock slopes and gravel beaches under wave attack. Relationships between profile parameters and boundary conditions have been established. These relationships have been used to develop a computer program. This program is able to predict the profiles of slopes with an arbitrary shape under varying wave conditions, such as those found in storm surges and during the tidal period.This paper investigates the utility of winds obtainable from a numerical weather prediction model for driving a spectral ocean-wave model in an operational mode. Wind inputs for two operational spectral wave models were analyzed with respect to observed winds at three locations in the Canadian east coast offshore. Also, significant wave heights obtainable from the two spectral models were evaluated against measured wave data at these locations. Based on this analysis, the importance of appropriate wind specification for operational wave analysis and forecasting is demonstrated.

Growth of kidney and antikidney vortices over a square jet in crossflow

The present paper reports on triple-decked evolution of the kidney vortices and the growth of unsteady antikidney-type vortices over a square jet in crossflow. Incompressible direct numerical simulations are performed with Reynolds number 2000 and jet to crossflow velocity ratio 2.5, to extract such flow details. Out of the three different decks, the lower deck kidney vortices maintain their steady appearance over the jet hole; however, vortices in the middle and the upper decks were unsteady in nature. Frequent shedding of the frontal hovering vortices and their convex wrapping over the front part of the jet hole are found to be responsible for the multidecked growth of the kidney vortices. On the other hand, unsteady antikidney-type vortices are noted to grow over the central part of the hole, and their growth is verified to be attributed due to concave windward folding of the frontal jet shear layer. Notably, the present findings remained consistent with existing experimentally observed features of suc...

Wave boundary layers in rough turbulent flow

A one-layer time-invariant eddy viscosity model is specified to develop a mathematical model for describing the essential features of the turbulent wave boundary layer over a rough bed. The functional form of the eddy viscosity is evaluated based on a modified one-equation turbulence model in which the eddy viscosity varies in time and space. The present eddy viscosity model simplifies much of the mathematical complexity in many existing models. Predictions from the present model have been compared favorably with a wide range of experimental data. It is found that the eddy viscosity model adopted in the present study is physically reasonable.

Three-dimensional evolution of vortical structures and associated flow bifurcations in the wake of two side-by-side square cylinders

In this paper, we focus on the three-dimensional growth of the vortical structures behind two square cylinders when placed in a side-by-side arrangement and examine different possible modes of flow bifurcation along their spanwise extended corelines. For this, unsteady three-dimensional flow simulations are conducted with five different values of gap/diameter ratios (g*), namely, 2.1, 1.7, 0.7, 0.5, and 0.2, that cover important known phases, i.e., coshedding (g*=2.1 and 1.7), asymmetric (g*=0.7 and 0.5), and single-body type (g*=0.2) of wake evolution. In order to exploit flow physics within the transition range, the Reynolds number of the flow is taken as 100. Notably, for all the investigated cases, parallel vortex shedding has been observed behind the two cylinders. However, with the decrease of gap ratio g*, the downstream flow gradually lost stability and the corelines of the shedded vortices appeared in a wavy fashion. While for g*=2.1 and 1.7, the simulated streamwise flows exhibit antiphase and i...

WAVE FIELD BEHIND THE PERMEABLE DETACHED BREAKWATER

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.

WWM Extended to Account for Wave Diffraction on a Current Over a Rapidly Varying Topography

The WWM (wind wave model) is extended to account for wave refraction-diffraction for wind waves propagating over a rapidly varying seabed in the presence of current. The wave diffraction effect is introduced into the wave action balance equation through the correction of wavenumber and propagation velocities using a diffraction corrected parameter. The approximation is based on the mild-slope equation for wave refraction-diffraction with current effect on a rapidly varying sea bottom. The relative importance of additional terms that influence the corrected diffraction parameter in the presence of currents was first introduced. The comparison of numerical results with other numerical models and experiments show that the validity of the model for describing wave propagating over a rapidly varying bottom with current effect is satisfactory. The implementation of this phase-decoupled refraction-diffraction approximation in WWM shows capability of the present model can be used in most practical engineering situations.© 2008 ASME