Ph.D.  Ocean and Resources Engineering
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ItemPeriodicity and patterns of the global wind and wave climate([Honolulu] : [University of Hawaii at Manoa], [December 2013], 201312)Windgenerated waves propagate across the oceans transporting energy that shapes the shorelines, influences maritime commerce, and defines coastal landuse around the world. Understanding the role of the ocean wind and wave climate is imperative for ocean engineering practices with both societal impacts and scientific contributions. The focus of this dissertation is the description of the patterns and cycles of the wind and wave climate through the use of reanalysis datasets that cover 1979 to 2009. The dissertation consists of three major parts, which examine the validity of the reanalysis datasets for climate research, verify climate signals in the datasets with published indices, and explore the dominant modes of variability. Over thirty years of high quality data from the recent release of the ECMWF Reanalysis Interim (ERAI) and NCEP Climate Forecast System Reanalysis (CFSR) allows for studies of the global climate with unprecedented detail. Independent observations from altimeters and buoys to provide assessment of their consistency in time and space. Both have good spatial homogeneity with consistent levels of errors in the Northern and Southern Hemispheres representing a significant improvement over previous reanalyzes. ERAI is homogenous through time, while CFSR exhibits an abrupt decrease in the level of errors in the Southern Ocean beginning in 1994. Although ERAI proves to be a more consistent dataset, CFSR's increased resolution, enhanced small scale features, and ability to match the observed variability makes it an attractive option for climate research. The continuous 31 years of global wind and wave data from the CFSR datasets are assessed in terms of well established climate patterns and cycles. Quarterly averages and percentile plots of the wind speed and wave height illustrate the seasonal pattern and distributions of extreme events. Statistical analyses of the annual and interannual variability suggest relationships to established climate patterns. The data shows strong correlation with published indices of known atmospheric cycles of the Arctic Oscillation (AO), Antarctic Oscillation (AAO), and El Nino Southern Oscillation (ENSO) in both the wind and wave fields. The results are comparable with published climate studies confirming CFSR's use in the study of complex climate dynamics. A standard empirical orthogonal function method extracts dominant spatial structures from time series of CFSR. The results show strong zonal structures in the winds and saturation of swells across the ocean basins, but these dominant features obscure the periodicity of individual climate cycles. A combined method utilizing the Fourier transform and empirical orthogonal functions helps resolve cyclic features of the climate system. Each of the three major ocean basins is characterized by its dominant modes and periodicity. The analysis reveals that the Atlantic is saturated by signals from the Northern Hemisphere including a broad range of intraseasonal components similar to those of the AO. The Indian and Pacific are strongly influenced by interannual cycles from the ENSO and AAO. In addition, these two oceans have strong components with periods of 5090 days that have similar spatial structure to those with 25 years periods suggesting linkage between the two frequency components.

ItemNonlinear wave loads on decks of coastal structures([Honolulu] : [University of Hawaii at Manoa], [December 2013], 201312)This dissertation is concerned with the theoretical calculations of twodimensional nonlinear wave loads on a horizontal deck of the coastal structure located in water of finite depth. The deck may be fully submerged, partially inundated or fully elevated above the stillwater level. Two different approaches are used to calculate the waveinduced horizontal and vertical forces and overturning moment. One is based on the theory of directed fluid sheets, namely the GreenNaghdi (GN) theory of water waves, and the other is based on Euler's equations. The forces on the deck are calculated by integrating the timedependent pressure around the body. The Level I GN equations are used to obtain an unsteady solution of the problem of propagation of flow of an incompressible and inviscid fluid over a fully submerged thin horizontal plate, an idealized model of a horizontal deck. A theoretical formulation of the problem is provided, and the solution of the equations are approximated by finitedifference equations. Euler's equations are solved with a finitevolume formulation and an Euler scheme for time derivations to approximate the loads of the flow of an incompressible and inviscid fluid on the deck of a coastal structure, whether it is submerged or elevated. The free surface between the water and air is captured by an interface capturing approach, namely the Volume of Fluid method. The computations are performed by use of the InterFoam solver of the Computational Fluid Dynamic's program, OpenFOAM. The results section of this dissertation is mainly concerned with the loads due to nonlinear waves of solitary and cnoidal types. Results are compared with the available laboratory experiments, and with a linear solution of the problem. Comparisons of the results of the GN and Euler's equations show a close agreement between the two methods. The presence of girders, on a model of a bridge deck with girders, is studied by making a direct comparison with the flat plate, and by changing the number of girders on the model. It appears that the girders do not have any influence on the vertical force, and only a small influence on the horizontal force. The effect of formation of air pockets between the girders, in a model of an elevated bridge deck, is studied by adding air pressure relief openings to the deck of the structure. It is found that the entrapment of air pockets increases the vertical uplift force significantly. By use of the GN equations, a parametric study is performed to assess how the periodic wave loads on a submerged deck depend on the wave conditions (wave height, wave period and submergence depth) and deck geometry (deck width).

ItemWave energy capture : The focusing of waveinduced flow through a submerged surface([Honolulu] : [University of Hawaii at Manoa], [December 2013], 201312)A submerged impervious horizontal disk is positioned near the free surface. Piercing this body is a tubular section, having an opening flush with the top surface and extending completely through the body. Waves passing over this surface will induce an oscillating fluid flow within this tubular section. The magnitude of the oscillation is dependent upon the structure's dimensions relative to environmental conditions such as the wave period, the wave height and submergence depth, as well as the extent to which surface waves are focused within this region. Both the numerical and experimental results of this phenomenon, which are pertinent to the development of a new wave energy converter, are described. The flow within the opening of the submerged surface is modeled by use of the Green function method within the confines of linear potential theory. The numerical predictions are compared with the experimental data. Monochromatic waves propagate over the submerged surface of a freestanding disk model, i.e., placed away from any flume walls. The waveinduced flow through the submerged surface is measured by two different sensors: an electromagnetic flow sensor and a particle image velocimetry laser. Wave elevation is recorded using capacitivetype wave gauges. Phasing of wave elevation to the vertical velocity through the tubular section is also discussed. Of the parameters that were varied decreasing the submergence depth of the disk resulted in the most significant increase in vertical waveinduced velocity.

ItemBoussinesqtype model for nearshore wave processes in fringing reef environment([Honolulu] : [University of Hawaii at Manoa], [December 2010], 201012)The extended lagoons and steep flanks of most fringing reefs produce unique coastal processes that are challenging to numerical wave models developed for continental shelf conditions. This dissertation describes the formulation and validation of a coastal wave model applicable to fringing reef environment. The governing Boussinesqtype equations, which include a continuity and a momentum equation with conserved variables, contain the conservative form of the nonlinear shallowwater equations for shock capturing. The finite volume method with a Godunovtype scheme provides a conservative numerical procedure compatible to the present governing equations. A fifthorder TVD (Total Variation Diminishing) reconstruction procedure evaluates the intercell variables, while a directional splitting scheme with a Riemann solver supplies the intercell flux and bathymetry source terms in the twodimensional horizontal plane. Time integration of the governing equations provides the conserved variables, which in turn provide the flow velocities through a linear system of equations derived from the dispersive terms in the momentum equations. The model handles wave breaking through momentum conservation based on the Riemann solver without the use of predefined empirical coefficients for energy dissipation. A series of numerical experiments verify the dispersion characteristics of the model. The computed results show very good agreement with laboratory data for wave propagation over a submerged bar, wave breaking and runup on plane beaches as well as wave transformation over fringing reefs. The model accurately describes transition between supercritical and subcritical flows as well as development of dispersive waves in the processes.

ItemHydroelasticity of marine vessels advancing in a seaway([Honolulu] : [University of Hawaii at Manoa], [August 2011], 201108)Hydroelasticity is an important issue in the design of modernday marine vessels, because of their flexibility associated with lighter construction materials and higher design speeds. The present study extends the hydroelasticity method by including effects of vessel forward speed and utilizes a direct solution approach instead of the modal superposition method, which requires structural details not available in early design stages. The model has two components, describing respectively, the elastic deformation of the vessel and the motion of the fluid. Small amplitude assumptions of the surface waves and vessel deformation lead to linearization of the problem, which is solved in the frequency domain. The formulation adopts a translating coordinate system with the vessel speed. The linear free surface boundary conditions account for the modification of the steady flow around the vessel. The radiation condition for the scattered waves takes into account the Doppler effect due to forward speed. A boundary element model describes the potential flow associated with the current and waves around the vessel. A finite element model relates the structural deformation to the fluid pressure through the kinematic and dynamic boundary conditions on the wetted body surface. This direct coupling of the structural and hydrodynamic systems leads to a system of equations in terms of the body surface oscillation, which includes elastic and rigidbody motions. The model is verified and validated in part with laboratory data on a rigid hull advancing in head seas and with published numerical results from the modal superposition method without vessel forward speed. A parametric analysis of a Wigley hull shows the forward speed introduces new resonance modes that amplify the response and stress of the vessel. The model provides a useful design tool to investigate the effects of vessel elastic deformation and forward speeds on structural performance and seakeeping.

ItemDepthintegrated freesurface flow with nonhydrostatic formulation([Honolulu] : [University of Hawaii at Manoa], [May 2012], 201205)This dissertation presents the formulation of depthintegrated wave propagation and runup models from a system of governing equations for twolayer nonhydrostatic flows. The conventional twolayer nonhydrostatic formulation is rederived from the continuity and Euler equations in nondimensional form to quantify contributions from nonlinearity and dispersion and transformed into an equivalent integrated system, which separately describes the flux and dispersiondominated processes. The formulation includes interfacial advection to facilitate mass and momentum exchange over the water column. This equation structure allows direct implementation of a momentum conserving scheme and a moving waterline technique to model wave breaking and runup without interference from the dispersion processes. The nonhydrostatic pressure, however, must be solved at the layer interface and the bottom simultaneously from the pressure Poisson equation, which involves a nonsymmetric 9band sparse matrix for a twodimensional vertical plane problem. A parameterized nonhydrostatic pressure distribution is introduced to reduce the computational costs and maintain essential dispersion properties for modeling of coastal processes. The nonhydrostatic pressure at mid flow depth is expressed in terms of the bottom pressure with a free parameter, which is optimized to match the exact linear dispersion relation for the water depth parameter up to kd = 3. This reduces the integrated twolayer formulation to a hybrid system with unknown nonhydrostatic pressure at the bottom only and a tridiagonal matrix in the pressure Poisson equation. The hybrid system reduces to a onelayer model for a linear distribution of the nonhydrostatic pressure. Fourier analysis of the governing equations of the twolayer, hybrid, and onelayer systems yield analytical expressions of the linear dispersion and shoaling gradient as well as the super and subharmonics transfer functions. The twolayer system reproduces the linear dispersion relation within a 5% error for water depth parameter up to kd = 11. The hybrid system with an optimized free parameter yields the same dispersion relation as the extended Boussinesq equations. The onelayer system shows a major improvement of the dispersion properties in comparison to the classical Boussinesq equations, but is not sufficient to model coastal wave transformation. The linear shoaling gradient serves as analytical tool to measure wave transformation over a plane slope although it is secondary compared to the linear dispersion relation. In comparison to secondorder wave theory, the twolayer system shows overall underestimation of the nonlinearity, while the hybrid system reasonably describes the super and subharmonics for kd ranging from 0 to 3. The twolayer, hybrid, and onelayer systems share common numerical procedures. A staggered finite difference scheme discretizes the governing equations in the horizontal dimension and the Keller box scheme reconstructs the nonhydrostatic terms in the vertical direction. A semiimplicit scheme integrates the depthintegrated flow in time with the nonhydrostatic pressure determined from a Poissontype equation. Numerical results are verified and validated through a series of numerical and laboratory experiments selected to measure model capabilities in wave dispersion, shoaling, breaking, runup, drawdown, and overtopping. The twolayer model shows good performance in handling these processes through its integrated structure, but slightly underestimates the wave height in shoaling. The hybrid model provides comparable results with the twolayer system in general and slightly improved performance in shoaling calculations due to better approximation of nonlinearity. The onelayer model exhibits stable and robust performance even when the wave characteristics are beyond its applicable range.