Dhanak, Manhar

The goal of the work described in this thesis is to design a flow augmentation device to increase the power capture and efficiency of a small-scale floating Under-Shot Water Wheel (USWW) currently being developed by Florida Atlantic University research funded by the U.S Department of Energy. The flow concentrator subsystem is intended to maximize the kinetic energy extracted by the marine hydrokinetic (MHK) energy collection device through modification of the local flow field across the capture plane. The primary objective is to increase the velocity and/or rate of mass inflow through the turbine through inserting a streamlined body in the region of interest. By utilizing the resulting flow field to increase hydraulic forcing on the waterwheel blades, the torque and/or RPM of the USWW can be increased. Based on experimental testing in the FAU wave tank at 1:5 prototype scale (280 mm wheel diameter) the flow concentrator was shown to produce an increase in device power coefficient of 17-55% measured over a velocity range of 0.16-0.45 m/s.

In this research an attempt is made at explaining the physical processes behind energy dissipation during wave breaking, through spectral analysis of the resulting sound. The size of an air bubble can be directly linked to the frequency of the sound that is heard using the simple harmonic solution to the Rayleigh–Plesset equation. It indicates the inverse relationship between frequency and bubble size. And this relationship has been used to identify wave breaking in general [MANASSEH 2006]. Now this research goes a step farther and looks at how the frequency spectrum of the sound changes with time, in an effort to understand the general pattern and from that to deduce an empirical equation that describes the breaking down of turbulence during a wave breaking event.
Two main processes have been identified, with the second process having three main indicators that are necessary to evidence wave breaking. The first process is a near instantaneous shattering of the initial air bubble into much smaller metastable bubbles of a size that appears to be common for all waves independent of wave height. Then in the second process, the bubbles continue to break down following a recognisable pattern.

Automatic target recognition of unexploded ordnances in side scan sonar imagery has been a struggling task, due to the lack of publicly available side-scan sonar data. Real time image detection and classification algorithms have been implemented to combat this task, however, machine learning algorithms require a substantial amount of training data to properly detect specific targets. Transfer learning methods are used to replace the need of large datasets, by using a pre trained network on the side-scan sonar images. In the present study the implementation of a generative adversarial network is used to generate meaningful sonar imagery from a small dataset. The generated images are then added to the existing dataset to train an image detection and classification algorithm. The study looks to demonstrate that generative images can be used to aid in detecting objects of interest in side-scan sonar imagery.

Tidal currents are a renewable energy resource and the work presented is in the field of harnessing tidal currents for electrical power generation. The main objective of the research is to provide information on rotational flow effects, caused by the spinning of the earth, around obstacles on the sea floor, in support of developing robust design of an underwater turbine array. This research looks at a gravity based linear array, a single turbine deep, with its largest dimension several kilometers long. The primary goal is to model a Taylor column above a linear array. The Taylor column has closed streamlines or stagnant flows inside of it and the flows around the column accelerate asymmetrically. The layout design of the array is intended to minimize the effect of the stagnant flows by predicting the location where closed streamlines could develop. The design is for the array and not for a turbine. Also, the locations where the energetic flows through the array have the longest periods are identified.
Numerical modeling with ANSYS Fluent failed repeatedly to accurately model rotational effects around an obstacle with a minimal relative current so as to form a Taylor column. Instead, Johnson’s (1982) analytical solutions for quasigeostrophic flows over elongated topography are used to study how the blocking parameter influences streamlines with changes in velocity typical of a tidal change. The streamlines illustrate the location over an array where the flows are accelerated and also where closed streamlines form.

This study analyzes the hydrodynamic performance of an advanced catamaran vehicle using computational fluid dynamics (CFD) simulations and experimental testing data in support of system identification and development of a physics-based control system for unmanned surface vehicle (USV) operations in coastal waters. A series of steps based on increasing complexity are considered sequentially in this study. First the steady flow past the static vehicle, then the vehicle with a fixed orientation advancing in calm water, and finally the vehicle moving with two degrees of freedom (DOF) in calm water as well as head seas.
The main objective of the study is to assess the role of general multiphase unsteady Reynolds Averaged Navier Stokes (RANS) as a predictive tool for the hydrodynamic performance of an USV. A parametric analysis of the vehicle performance at different Froude number and wave steepness in shallow waters is conducted. The characteristics of the wave resistance, heaving and pitching motion, wave-hull interactions, and free surface flow patterns are investigated. The study will aid in the design of a robust physics-based control system for the vehicle and provide a tool for prediction of its performance.

Geometric modification as the most effective passive flow control method has recently received wide attention due to its enormous potential in enhancing performance characteristics of airfoils or hydrofoils without expensive manufacturing and maintenance cost. Two primary passive flow control modifications, known as leading-edge tubercles and internal slots and their applications in airfoils/hydrofoils have been investigated in this dissertation. For the hydrofoil, since free surface effects cannot be neglected, the interaction between the hydrofoil-motion induced waves on the free surface and the hydrofoil has been studied as well. In the theoretical approach aspect, an empirically-based model based on an iteration scheme has been proposed for predicting the lift coefficients of twisted airfoils with leading-edge tubercles by using experimental data for untwisted airfoils. With both numerical and experimental investigations, this dissertation has discussed the application of a custom optimized-design internal slot on a NACA 634-021 airfoil blade to allow ventilation of flow through the slot from the pressure side to the suction side of the blade, in support of delaying flow separation, and stall. The combined effect of an internal slot in an airfoil and transverse leading-edge tubercles on its performance has been further studied both numerically and experimentally. Moreover, performance of a NACA 634-021 hydrofoil in motion under and in close proximity of a free surface for a large range of AoAs has been studied. Lift and drag coefficients of the hydrofoil at different submergence depths are investigated both numerically and experimentally. The results of the numerical study are in good agreement with the experimental results. The agreement confirms the new finding that for a submerged hydrofoil operating at high AoAs close to a free surface, the interaction between the hydrofoil-motion induced waves on the free surface and the hydrofoil results in mitigation of the flow separation characteristics on the suction side of the foil and delay in stall, and improvement in hydrofoil performance. A similarly submerged hydrofoil with a custom-designed internal slot has further been studied. The performance characteristics of the slotted hydrofoil in the presence of the free surface are investigated both numerically and experimentally.

The observation of turbulence in the Florida Current is presented with the use of velocity measurements collected with an Acoustic Doppler Current Profiler (ADCP). The research is conducted through application of the theories of Taylor and Kolmogorov and related derivations, and processing tools of MATLAB software to this Eulerian observation of flow [1]. The velocity profile of the Florida Current is deduced in terms of its turbulent character with shear, acceleration, gradient, Reynolds Number, Reynolds Stress, Welch power spectrum density of current velocity, wavenumbers of Taylor’s hypothesis and Kolmogorov, wavenumber spectrum, eddy diameters, diapycnal diffusivity, and the Richardson Number. Processing methods are validated with results of other research conducted in the Florida Current with the use of a Multi-Scale Profiler, and an Advanced Microstructure Profiler for determination of shear, dissipation, diffusivity, and estimates of turbulent eddy diameters based on Taylor’s Hypothesis [1][4]. A spectral analysis is developed and is compared with Kolmogorov’s -5/3-Law. The process and the results of the analysis are described.