Ph.D. - Ocean and Resources Engineering

Permanent URI for this collectionhttps://hdl.handle.net/10125/36918

Browse

Now showing 1 - 15 of 15

Designing wave-powered ocean observing: Experimental findings of an oscillating water column type wave energy converter with a submerged heave plate and V-shaped channels in irregular waves
(University of Hawai'i at Manoa, 2024) Ulm, Nicholas R.; Huang, Zhenhua; Ocean & Resources Engineering
The study of oscillating water column (OWC)-type wave energy converters (WEC) has primarily focused on applications in the nearshore environment with an end use in residential power grids. This dissertation examines the power performance of a new OWC geometry relative to blue economy objectives that focus on providing power in the intermediate-water-depth environment. This geometry, which is based on the H¯alona Blowhole, consists of a cylindrical OWC chamber affixed above a heave plate with V- shaped channels. This dissertation assesses a new method of experimentally investigating OWC-type WECs at model scale by comparing previously used methods of orifice plate parameterization with methods that rely on a single measurement of pressure. The study discusses the implications of this method relative to a new end use for autonomous underwater vehicle docking. The power performance is evaluated through experimental testing on a fixed and floating geometry OWC. In evaluating power performance, the impact of different representative power take-off (PTO) damping values and directional dependence is investigated in regular and irregular waves. The power performance and motion response of the floating variation is discussed relative to new objectives for a blue economy application of wave energy.
Coastal Defense in an Idealized Barrier Reef System Using Pile and OWC-Pile Breakwaters
(University of Hawai'i at Manoa, 2024) Huang, Shijie; Huang, Zhenhua; Ocean & Resources Engineering
Climate change has dramatically exacerbated coastal erosion, primarily through the effects of sea level rise, the intensification of tropical storms, and the increased frequency of high-wave events. These processes pose a severe threat to the tourism-based economy and the densely populated coastlines of Hawai‘i, where coastal erosion has become an urgent issue. Compounding the problem, traditional coastal defense structures—such as seawalls, breakwaters, and revetments—have proven ineffective for the unique environmental conditions of Hawaiian waters. Given the economic and environmental stakes, there is an immediate need for coastal engineers and researchers to propose and implement innovative, site-specific defense strategies to safeguard Hawai‘i’s valuable shorelines. The Hawaiian shoreline is generally well protected by extensive coral reefs in nearshore waters. These reefs serve as natural breakwaters, forming shallow water barriers that dissipate wave energy through wave breaking and by increasing friction against wave propagation. However, not all of Hawai‘i’s coastlines benefit from continuous reef protection. Gaps between reef patches, known as channels, leave certain beaches exposed to high-energy waves and offshore currents. The combination of high wave energy and offshore current can mobilize beach sediment and transport it offshore, accelerating beach erosion in vulnerable areas. Together with the lagoons frequently found in front of Hawaiian beaches, the reef, channel and the lagoon forms a typical topographic characteristic of barrier reef system. This dissertation aims to deepen the understanding of hydrodynamic processes within a barrier reef system and to elucidate the beach erosion mechanisms associated with these processes using Computational Fluid Dynamics (CFD) simulations. This detailed analysis will inform the development of tailored, innovative solutions to mitigate beach erosion in channel-fronted areas. Two specific solutions are explored and tested. The first one is a pile breakwater system, consisting of an array of cylindrical piles strategically placed to dissipate and reflect wave energy. This method offers a cost-effective alternative to conventional hard structures, particularly in the deep waters found within reef channels, where traditional approaches are both expensive and less effective. The second proposed solution is a dual-functional wave energy farm that provides both coastal protection and renewable energy generation. This system will utilize an array of Oscillating Water Columns (OWCs)--one of the most extensively studied wave energy harvesting devices---to form an OWC-pile breakwater. By combining shoreline protection with energy production, the OWC-pile breakwater system is well-suited for application in Hawai‘i, where there is an urgent need for effective coastal defense and a growing demand for sustainable energy solutions.
Enhanced Fully Nonlinear Boussinesq-type Equations In Conserved Variable Form And Linear Analytical Properties With Compact Finite Difference Schemes
(University of Hawaii at Manoa, 2023) Heitmann, Troy; Cheung, Kwok Fai; Ocean & Resources Engineering
In coastal engineering applications, Boussinesq-type models are limited by orders of approximation originating in both the governing equations and numerical schemes employed. Dispersive model solutions reflect a composition of approximations dependent upon finite sampling intervals. This study aims to improve understanding of both theoretical and numerical facets, with the end goal of strengthening community awareness in model applicability. A modern approach to parameterize wave breaking in Boussinesq-type equations is to leverage the hyperbolic structure of the leading order nonlinear shallow water equations and approximate over-turning processes using shock capturing methods designed to conserve both mass and momentum. In this approach, it is well known that the governing partial differential equations (PDEs) must be expressed in conserved variable form to attain proper shock speeds. A new independent formulation covering a family of fully nonlinear, weakly dispersive Boussinesq-type equations is derived in conserved variable form by depth integrating Euler’s equations of motion under an irrotational flow assumption. A projected Taylor series expansion of the vertical velocity about an arbitrary material surface is utilized in the depth integration of the irrotational flow condition to give an expression for the horizontal velocity. Through a change of variables, the dependency of the horizontal velocity is expressed with reference to an arbitrary point of evaluation. A new weighted average of horizontal velocity expansions at the material surfaces defines the model velocity at a datum invariant reference attached to the flow depth. In comparison to existing theories, the approach introduces an additional term which enhances nonlinear dispersion. Imposing constraints on the orders of approximation, leading order theories are recovered, thus showing theoretical advancement. Transforming the governing PDEs into discrete approximations facilitates numerical simulation of nonlinear processes over a complex bathymetry, in which the approximations result in a system of modified PDEs (MPDEs) possessing unique solutions specific to the numerical methods employed. In practical application, practitioners are burdened with an unnecessary level of uncertainty during the selection of discretization parameters, despite their fundamental roles in the governing MPDEs. Beyond numerical experiments, there has been little effort to explicitly communicate numerical implications in Boussinesq-type models. For the Boussinesq-type equations derived herein, dispersion emerges through Taylor series expansions along the vertical axis, in which the methods of approximation mirror those used in finite difference methods. Therefore, a complementary finite difference framework is adopted in which the time integration is performed using linear multistep schemes. Difference operators, including those with compact support, are expressed in symbolic form for the purpose of generalization. Difference operators are expressed in symbolic form to promote generalization while seamlessly enabling the novel application of compact finite difference schemes. Applying Fourier-Laplace transforms, the symbolic operators are mapped into spectral space, where waveform resolution is evaluated as a function of time, ∆t, and space, ∆x, sampling intervals. The approach facilitates a complex propagation factor analysis of amplitude and phase modulations, both of which may be present in physical theory. To better accommodate operator interactions occurring in systems of equations, definitions of operator support and coefficients are adjourned to maintain complex degrees of freedom during the full system analysis. As a result, the solution to the MPDEs becomes an objective, as opposed to an outcome, when defining schemes. Developments on Boussinesq-type equations have largely focused on dispersion enhancements to the governing PDEs, in which family members having the same formal order of accuracy exhibit very different dispersive behaviors. The same level of research has not been carried out with regard to the respective MPDEs, where different schemes lead to unique dispersive solutions. The linearized MPDEs are cast into spectral space using Fourier-Laplace transforms. Substituting in the symbolic operators, the newly derived numerical dispersion relation for Boussinesq-type equations matches that of the PDEs provided the discrete operators are replaced by their continuous counterparts. The dispersion relation of the MPDEs is dependent upon not only on the wave number, k, and still water depth, h, but also ∆t and ∆x sampling intervals. The function domain of the celerity, or phase speed, is thus multidimensional, collapsing only to k and h in the limit of vanishing sampling intervals for stable consistent schemes. Several leading order Boussinesq-type equations are analyzed, in which the error associated with the MPDEs quantifies the bounds for application. Theories which exhibit increases in phase speed with relative depth are best suited to finite difference methods. This is due to a counter balancing decrease in phase speed imposed by finite difference methods. The transparency of error associated with the MPDEs gives further insights on the selection of sampling intervals and permits optimal mesh design for a given application.
Of Rats and Men: Underwater Passive Acoustic Localization Investigations Using Relative Arrival Times and Blind Channel Estimation
(University of Hawaii at Manoa, 2022) Rideout, Brendan Pearce; Nosal, Eva-Marie; Ocean & Resources Engineering
Understanding the ecology of any organism requires an understanding of all its life stages. Underwateracoustics provides the ability to observe the submerged lives of marine mammals in ways not possible through visual means. The complexities of underwater acoustic propagation yield both challenges and opportunities to extract information from recorded data, of which the estimation of underwater location is one example. This dissertation presents two signal processing approaches related to passive underwater acoustic localization. Blind channel estimation is a computational approach to estimating a set of impulse responses basedon simultaneous recordings of the same unknown source by different receivers. The estimated impulse responses from this approach may simplify passive acoustic localization for some types of sound sources by making arrival times easier to identify. Differences among the received waveforms are interpreted as evidence of differences in the underlying channel impulse responses. Using a sparse assumption on these impulse responses, several different optimization approaches (OMP, CoSaMP, and NESTA) are applied to simulated ocean acoustic data. Estimated channels are a good fit to the true channel when the channels are static, but the introduction of a time-varying characteristic to the true channels negatively impacts channel recovery performance. Single hydrophone passive acoustic localization is the practice of estimating location characteristics of anunderwater sound source using acoustic recordings from a single receiver. This work develops an approach for estimating the horizontal range between a submerged source and receiver in a deep ocean environment without relying on modal dispersion. We develop a cost function and optimization approach that are robust to some significant sources of noise and environmental uncertainty. Results from simulations and ground-truthed measured data demonstrate the accuracy of this localization approach. Underwater acoustic data recorded by the ALOHA Cabled Observatory (ACO) are processed using thissingle hydrophone localization approach. Acoustic recordings at ACO made between February 2007 - September 2017 yield 41,481,171 fin whale 20 Hz call detections, of which 3,445,568 detections remain after pruning away suspected sei whale and duplicate call detections. Fin whale 20 Hz calls are concentrated in the November-April time period near ACO. Estimated fin whale call parameters, including inter-call interval, are more clearly estimated from the recorded data following pruning to reduce the false positive rate.
Study of wave interaction with vertical piles integrated with oscillating water columns
(University of Hawaii at Manoa, 2018-12) Xu, Conghao; Huang, Zhenhua; Ocean & Resources Engineering
Ocean wave energy is a source of abundant renewable and clean energy. However, a host of challenges including construction and maintenance costs and structural reliability have prevented the large-scale commercial application of ocean wave energy converters (WECs). Integrating WECs with shore-protection structures may significantly reduce the costs associated with wave energy utilization. One such integration is vertical piles integrated with oscillating water columns (OWCs), which can help achieve costs sharing and overcome the cost hurdles facing the wave energy industry. This study examines the performance of circular piles integrated with OWC devices (OWC-piles) in terms of wave energy extraction and wave scattering. Two configurations of OWC-piles, a loosely spaced configuration, and a closely spaced configuration, are investigated. For the loosely spaced configuration, the spacing is large enough so that the interference between adjacent OWC-piles can be ignored. So that the performance of the loosely spaced configuration can be studied by examining the performance of a standalone OWC-pile. In chapter 2, the performance of a standalone OWC-pile configuration is investigated theoretically, experimentally, and numerically. A quadratic power takeoff model is implemented in the study. The viscous loss associated with vortex shedding is discussed based on a comparison between the theoretical and experimental results. The possible effects of spatial non-uniformity including resonant sloshing are discussed. The performance of the loosely spaced configuration is discussed. In chapter 3, the study is extended to investigate experimentally the performance of a row of closely spaced OWC-piles in terms of wave energy extraction and wave scattering. A comparative evaluation of the performance of the proposed OWC-pile in both configurations are performed. In chapter 4, a computational fluid dynamics study is presented to understand the detailed hydrodynamics involved in the wave interaction with OWC-piles for both configurations. Chapter 5 reports an experimental study investigating the scour around a row of closely spaced piles without OWC device, which affects the safety of the pile structures, especially in extreme events such as tsunamis. The purpose of this study is to provide understanding of the scour induced by the unsteady jet flow created by the narrow gaps between piles. Future work includes a three-phase simulation of the sediment dynamics around OWC-pile structures, and numerical and experimental studies of the shore protection performance of the closely spaced OWC-piles. The three-phase flow model for these future research can be partially validated using data from chapter 5.
Extratropical Storm-Generated Swells Induced Vulnerability Effects on the Tropical Islands of Hawaii
(University of Hawaii at Manoa, 2018-08) Onat, Yaprak; Ocean & Resources Engineering
The poleward shift of strong extratropical storms due to global warming’s effect on baroclinicity raises the question of how the storm intensification affects the susceptibility of distant remote islands under high wave energy environments. This study aims to identify the effective linkages between the intensification of extratropical storms and the corresponding swells in order to reduce the uncertainty in prioritizing vulnerable coastal systems in Hawai‘i. The minimum mean sea level pressure and geopotential height, and maximum vorticity are used as a criteria to define strong cyclonic activity from an atmospheric reanalysis dataset to hindcast swell states of the North Pacific from 2007-2017. The de-seasonalized trend of the northwest swells and the spatial distribution of the wave exposure are visualized in an index-based coastal vulnerability GIS model to classify coastal exposure. The correlation between strong extratropical cyclones and swells show an increase in the frequency of swells, which accounted for a quarter of the total swells reaching the Hawaiian Islands over the record period. The significant wave height and peak period of the associated swells at the northwest of O‘ahu displays a significant upward trend of up to 0.51 m and 1.72 s in open ocean respectively, while keeping a rather stable direction range of 325-330º during the record period. These swells contribute to the already alarming 34% of the medium to high vulnerability of the coastlines of the Hawaiian Islands. Understanding the dominant factors affecting shoreline vulnerability and the impact of strong extratropical storm-generated swells related to their susceptibility allows the formulation of better strategies to more effectively mitigate the potential risk for Pacific Island communities. The value of this work lies in both identifying the swell trends and customizing the proposed framework to determine crucial elements that increase the susceptibility of critically exposed shoreline segments. This work provides a guide for policymakers to promote public awareness and support deliberation, planning, and design of adaptation strategies.
Numerical Dispersion in Non-Hydrostatic Modeling of Long-Wave Propagation
(University of Hawaii at Manoa, 2018-08) Li, Linyan; Ocean & Resources Engineering
Numerical discretization with a finite-difference scheme is known to introduce truncation errors in the form of frequency dispersion in depth-integrated models commonly used in tsunami research and hazard mapping. While prior studies on numerical dispersion have focused on the shallow-water equations, we include the depth-integrated non-hydrostatic pressure and vertical velocity through a Keller box scheme and investigate the properties of the resulting system. Fourier analysis of the discretized governing equations gives rise to a dispersion relation in terms of the time step, grid size, and wave direction. The interworking of the dispersion relation is elucidated by its lead-order approximation, one and two-dimensional numerical experiments, and a case study of the tsunami generated by the 2010 Mentawai Mw 7.8 earthquake. The dispersion relation, aided by its lead-order approximation from the Taylor series expansion, shows that coupling between the spatial discretization and non-hydrostatic terms results in significant reduction of numerical dispersion outside the shallow-water range. The time step, which counteracts numerical dispersion from spatial discretization, only has secondary effects within the applicable range of Courant numbers. Numerical dispersion also decreases for wave propagation oblique to the principal axes of the grid due to effective increase in spatial resolution. A numerical flume experiment of standing waves indicates minor contributions from the implicit solution scheme of the non-hydrostatic terms. A second numerical experiment verifies the properties deduced from the analytical results and demonstrates the effectiveness of discretization in altering progressive waves over a two-dimensional grid. The computational results also demonstrate generation of spurious, short-period trailing waves from hydrostatic model with insufficient numerical dispersion. Since the governing equations for the non-hydrostatic system trend to underestimate dispersion in shoaling water, the numerical effects are complementary in producing a solution closer to Airy wave theory. A case study of the 2010 Mentawai Mw 7.8 earthquake and tsunami event, which has a compact source adjacent to a deep trench, demonstrates the role of dispersion in wave propagation and the implications for the commonly-used source inversion techniques. Non-dispersive models are often used with an initial static sea-surface pulse derived from seafloor deformation in computation of tsunami Green's functions. We compare this conventional approach with more advanced techniques, which use Green's functions computed by a dispersive model with an initial static sea-surface pulse and with the surface waves generated from kinematic seafloor deformation. The fine subfaults needed to resolve the compact rupture results in dispersive waves that require a non-hydrostatic model. The Green's functions from the hydrostatic model are overwhelmed by spurious, grid-dependent short-period oscillations, which are filtered prior to their application. These three sets of tsunami Green's functions are implemented in finite-fault inversions with and without seismic and geodetic data. Seafloor excitation and wave dispersion produce more spread-out waveforms in the Green's functions leading to larger slip with more compact distribution through the inversions. If the hydrostatic Green's functions are not filtered, the resulting slip spreads over a large area to eliminate the numerical artifacts from the lack of dispersion. The fit to the recorded tsunami and the deduced seismic moment, which reflects the displaced water volume, is relatively insensitive to the approach used for computing Green’s functions.
Periodicity and patterns of the global wind and wave climate
(University of Hawaii at Manoa, 2013-12) Stopa, Justin Edward
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.
Nonlinear wave loads on decks of coastal structures
(University of Hawaii at Manoa, 2013-12) Hayatdavoodi, Masoud
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).
Wave energy capture: The focusing of wave-induced flow through a submerged surface
(University of Hawaii at Manoa, 2013-12) Carter, Richard William
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.
Boussinesq-type model for nearshore wave processes in fringing reef environment
(University of Hawaii at Manoa, 2010-12) Roeber, Volker
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.
Hydroelasticity of marine vessels advancing in a seaway
(University of Hawaii at Manoa, 2011-08) Das, Suvabrata
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.
Depth-integrated free-surface flow with non-hydrostatic formulation
(University of Hawaii at Manoa, 2012-05) Bai, Yefei
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.
Tsunami and storm wave impacts on coastal bridges
(University of Hawaii at Manoa, 2014-12) Seiffert, Betsy Rose
Wave loads on coastal bridges due to tsunami and storm waves are studied through a set of laboratory experiments and numerical calculations. Effects of wave nonlinearity and entrapped air on wave loading under conditions where the bridge may be partially or fully inundated are of particular interest. In addition, effects of compressibility and scaling are investigated through numerical calculations. With the destruction of bridges during recent events such as the 2011 Tohoku tsunami and hurricanes Katrina in 2005 and Ivan in 2004, this highlights the importance of this research in understanding the mechanisms of failure during such events to prevent future coastal bridge failures. Destruction of these bridges is not only financially costly, but can prevent emergency services from reaching coastal communities, thus potentially contributing to loss of life. Along with the bridges, this research is applicable to other coastal and offshore structures, such as piers, submerged breakwaters and offshore platforms, in which wave loading or entrapped air is of concern. To investigate this problem, an extensive set of experiments is conducted on a flat plate, bridge model with girders, and a bridge model with varying percentages of trapped air that serves as a valuable benchmark for understanding wave loads on coastal structures, and bridges in particular. The wave cases tested include an extensive set of water depths, wave amplitudes and wave periods (for periodic waves) to cover a wide range of solitary and shallow-water to intermediate-water depth cnoidal waves. A range of model elevations was also tested to cover a range where the entire model is fully submerged below the water surface, to where the model is fully elevated above the water surface, and in the case of the model with girders, the girders are fully elevated as well. Experiments were conducted in a two-dimensional wave flume located in the University of Hawaii at Manoa's Hydraulics Lab in the Civil and Environmental Engineering Department. The models used were representative of a 1:35 scale model of a two-lane coastal bridge, typical in an island community. To study the effects of entrapped air, a series of experiments is conducted where side panels were added to each side of the model to trap air between the girders. Then different percentages of air-relief openings are added to the panels to capture a range of cases where no air can escape between the girders, to where all the air can escape and the wave can freely interact with the bottom of the bridge deck. Data from these experiments show the largest vertical uplift forces and forces in the direction of wave propagation on a flat plate and a bridge model occur when the structure is near the still-water level (SWL). For the cases where air is trapped, the addition of air relief openings significantly reduces uplift forces. Many current empirical relations estimating wave loads on coastal bridges only take hydrostatic effects into account. When compared with empirical estimations, data from these experiments show both hydrostatic and hydrodynamic forces must be taken into consideration. Comparison is also made with numerical calculations solving incompressible Euler's equations by use of the CFD software OpenFOAM, discussed in Hayatdavoodi (2013), Seiffert, Hayatdavoodi & Ertekin (2014), and Hayatdavoodi, Seiffert & Ertekin (2014b) with excellent agreement. Effects of compressibility and scaling are tested numerically by solving compressible and incompressible Euler's equations. Numerical calculations show that the effects of compressibility on the long duration forces are small. Calculations also show that when Froude scaling is applied to forces on the model scale, the forces agree well with force calculations at the prototype scale. These results have important design implications for bridge engineers.
Characterization of underwater acoustic sources recorded in reverberant environments with application to scuba signatures
(University of Hawaii at Manoa, 2014-12) Gemba, Kay Leonard
The ability to accurately characterize an underwater sound source is an important prerequisite for many applications including detection, classification, monitoring and mitigation. Unfortunately, anechoic underwater recording environments, required to make ideal recordings, are generally not available. Current methods adjust source recordings with spatially averaged estimates of reverberant levels. However, adjustments can introduce significant errors due to a high degree of energy variability in reverberant enclosures and solutions are inherently limited to incoherent approximations. This dissertation introduces an approach towards a practical, improved procedure to obtain an anechoic estimate of an unknown source recorded in a reverberant environment. Corresponding research is presented in three self-contained chapters. An anechoic estimate of the source is obtained by equalizing the recording with the inverse of the channel's impulse response (IR). The IR is deconvolved using a broadband logarithmic excitation signal. The length of the IR is estimated using methods borrowed from room acoustics and inversion of non-minimum phase IR is accomplished in the least-squares sense. The proposed procedure is validated by several experiments conducted in a reverberant pool environment. Results indicate that the energy of control sources can be recovered coherently and incoherently with root-mean-square error (RMSE) of-70 dB (10-70 kHz band). The proposed method is subsequently applied to four recorded SCUBA configurations. Results indicate that reverberation added as much as 6.8 dB of energy. Mean unadjusted sound pressure levels (0.3-80 kHz band) were 130 5.9 dB re 1 Pa at 1 m. While the dereverberation method is applied here to SCUBA signals, it is generally applicable to other sources if the impulse response of the recording channel can be obtained separately. This dissertation also presents an approach to separate all coloration from the deconvolved IR. This method can be used to estimate the channel's IR or the magnitude spectrum of the combined electrical equipment. The procedure is validated using synthetic results of an imagesource model and the channel's IR is recovered over the full band with a RMSE of-31 dB.

Browse

Recent Submissions