Ph.D. - Ocean and Resources Engineering
Permanent URI for this collection
Browse
Recent Submissions
1 - 10 of 12
-
Item
-
Item
-
Item
-
Item
-
ItemPeriodicity and patterns of the global wind and wave climate([Honolulu] : [University of Hawaii at Manoa], [December 2013], 2013-12)Wind-generated waves propagate across the oceans transporting energy that shapes the shorelines, influences maritime commerce, and defines coastal land-use 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 (ERA-I) 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. ERA-I is homogenous through time, while CFSR exhibits an abrupt decrease in the level of errors in the Southern Ocean beginning in 1994. Although ERA-I 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 inter-annual 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 intra-seasonal components similar to those of the AO. The Indian and Pacific are strongly influenced by inter-annual cycles from the ENSO and AAO. In addition, these two oceans have strong components with periods of 50-90 days that have similar spatial structure to those with 2-5 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], 2013-12)This dissertation is concerned with the theoretical calculations of two-dimensional 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 still-water 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 Green-Naghdi (GN) theory of water waves, and the other is based on Euler's equations. The forces on the deck are calculated by integrating the time-dependent 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 finite-difference equations. Euler's equations are solved with a finite-volume 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 wave-induced flow through a submerged surface([Honolulu] : [University of Hawaii at Manoa], [December 2013], 2013-12)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 free-standing disk model, i.e., placed away from any flume walls. The wave-induced 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 capacitive-type 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 wave-induced velocity.
-
ItemBoussinesq-type model for nearshore wave processes in fringing reef environment([Honolulu] : [University of Hawaii at Manoa], [December 2010], 2010-12)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 Boussinesq-type equations, which include a continuity and a momentum equation with conserved variables, contain the conservative form of the nonlinear shallow-water equations for shock capturing. The finite volume method with a Godunov-type scheme provides a conservative numerical procedure compatible to the present governing equations. A fifth-order TVD (Total Variation Diminishing) reconstruction procedure evaluates the inter-cell variables, while a directional splitting scheme with a Riemann solver supplies the inter-cell flux and bathymetry source terms in the two-dimensional 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], 2011-08)Hydroelasticity is an important issue in the design of modern-day 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 rigid-body 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.
-
ItemDepth-integrated free-surface flow with non-hydrostatic formulation([Honolulu] : [University of Hawaii at Manoa], [May 2012], 2012-05)This dissertation presents the formulation of depth-integrated wave propagation and runup models from a system of governing equations for two-layer non-hydrostatic flows. The conventional two-layer non-hydrostatic formulation is re-derived from the continu-ity and Euler equations in non-dimensional form to quantify contributions from nonlin-earity and dispersion and transformed into an equivalent integrated system, which sepa-rately describes the flux and dispersion-dominated processes. The formulation includes interfacial advection to facilitate mass and momentum exchange over the water col-umn. This equation structure allows direct implementation of a momentum conserving scheme and a moving waterline technique to model wave breaking and runup without in-terference from the dispersion processes. The non-hydrostatic pressure, however, must be solved at the layer interface and the bottom simultaneously from the pressure Poisson equation, which involves a non-symmetric 9-band sparse matrix for a two-dimensional vertical plane problem. A parameterized non-hydrostatic pressure distribution is intro-duced to reduce the computational costs and maintain essential dispersion properties for modeling of coastal processes. The non-hydrostatic 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 two-layer formulation to a hybrid system with unknown non-hydrostatic pressure at the bottom only and a tridiagonal matrix in the pressure Poisson equation. The hybrid system reduces to a one-layer model for a linear distribution of the non-hydrostatic pressure. Fourier analysis of the governing equations of the two-layer, hybrid, and one-layer systems yield analytical expressions of the linear dispersion and shoaling gradient as well as the super and sub-harmonics transfer functions. The two-layer 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 one-layer 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 second-order wave theory, the two-layer system shows overall underestimation of the nonlinearity, while the hybrid system reasonably describes the super and sub-harmonics for kd ranging from 0 to 3. The two-layer, hybrid, and one-layer 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 non-hydrostatic terms in the vertical direction. A semi-implicit scheme integrates the depth-integrated flow in time with the non-hydrostatic pressure determined from a Poisson-type 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 two-layer 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 one-layer model exhibits stable and robust performance even when the wave characteristics are beyond its applicable range.