Ph.D. - Ocean and Resources Engineering

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https://hdl.handle.net/10125/36918

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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
(2024) Ulm, Nicholas R.; Huang, Zhenhua; Ocean & Resources Engineering
Coastal Defense in an Idealized Barrier Reef System Using Pile and OWC-Pile Breakwaters
(2024) Huang, Shijie; Huang, Zhenhua; Ocean & Resources Engineering
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.

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