Fluid dynamics

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.

Boundary layer control on a circular cylindrical body through oscillating Lorentz
forcing is studied by means of numerical simulation of the vorticity-stream
function formulation of the Navier-Stokes equations. The model problem
considers axisymmetric seawater flow along an infinite cylinder controlled by an
idealized radially directed Lorentz force oscillating spatially and temporally.
Under optimum forcing parameters, it is shown that sustainable Lorentz induced
vortex rings can travel along the cylinder at a speed equivalent to the phase
speed of forcing . Wall stress is shown to locally change sign in the region
adjacent to the vortex, considerably decreasing net viscous drag . Adverse flow
behaviors are revealed as a result of studying the effects of the Reynolds
numbers, strength of the Lorentz force, and phase speed of forcing for boundary
layer control. Adverse flow behaviors inc I ude complex vortex configurations
found for suboptimal forcing resulting in a considerable increase in wall stress.

This thesis explores an approach for the measurement of the quality of power
generated by the Center of Ocean and Energy Technology's prototype ocean turbine. The
work includes the development of a system that measures the current and voltage
waveforms for all three phases of power created by the induction generator and quantifies
power variations and events that occur within the system. These so called "power quality
indices" are discussed in detail including the definition of each and how they are
calculated using LabYiew. The results of various tests demonstrate that this system is
accurate and may be implemented in the ocean turbine system to measure the quality of
power produced by the turbine. The work then explores a dynamic model of the ocean
turbine system that can be used to simulate the response of the turbine to varying
conditions.

Several modifications have been implemented to numerical simulation codes based on
blade element momentum theory (BEMT), for application to the design of ocean current
turbine (OCT) blades. The modifications were applied in terms of section modulus and
include adjustments due to core inclusion, buoyancy, and added mass. Hydrodynamic loads
and mode shapes were calculated using the modified BEMT based analysis tools. A 3D
model of the blade was developed using SolidWorks. The model was integrated with
ANSYS and several loading scenarios, calculated from the modified simulation tools, were
applied. A complete stress and failure analysis was then performed. Additionally, the
rainflow counting method was used on ocean current velocity data to determine the loading
histogram for fatigue analysis. A constant life diagram and cumulative fatigue damage
model were used to predict the OCT blade life. Due to a critical area of fatigue failure being
found in the blade adhesive joint, a statistical analysis was performed on experimental
adhesive joint data.

This thesis was about finding a recovery method for TiO2, using a TiO2 recovery technology, which was high enough to be economical ($10 - $15 per 1,000 gallons) to be adopted by wastewater treatment plants. When comparing recovery technologies, the top three which were investigated further through experimentation were a centrifuge, sedimentation tank, and microfilter membrane. Upon experimentation and research, the TiO2 recovery efficiencies of these technologies were 99.5%, 92.5%, and 96.3%, respectively. When doing economic analysis on these technologies comparing TiO2 efficiencies and capital and operational costs, the centrifuge was the most preferred economic option. Also, its cost did were in the economical range ($10 - $15/1,000 gallons) which makes even this technology economical. Besides that, important and valuable information about TiO2: settling behavior, particle size and zeta potential, interactions with COD, and filter operations (particle characterization) were discovered for future research and future testing on this issue.

Numerical simulations of waterjet inlets have been conducted in order to understand inlet performance during ship turning maneuvers. During turning maneuvers waterjet systems may experience low efficiency, cavitation, vibration, and noise. This study found that during turns less energy arrived at the waterjet pump relative to operating straight ahead, and that the flow field at the entrance of the waterjet pump exhibited a region of both low pressure and low axial velocity. The primary reason for the change in pump inflow uniformity is due to a streamwise vortex. In oblique inflow the hull boundary layer separates when entering the inlet and wraps up forming the streamwise vortex. These changes in pump inflow during turning maneuvers will result in increased unsteady loading of the pump rotor and early onset of pump rotor cavitation.

In the field of machine prognostics, vibration analysis is a proven method for
detecting and diagnosing bearing faults in rotating machines. One popular method
for interpreting vibration signals is envelope demodulation, which allows a technician
to clearly identify an impulsive fault source and its severity. However incipient faults -faults in early stages - are masked by in-band noise, which can make the associated impulses difficult to detect and interpret. In this thesis, Wavelet De-Noising (WDN) is implemented after envelope-demodulation to improve accuracy of bearing fault diagnostics. This contrasts the typical approach of de-noising as a preprocessing step.
When manually measuring time-domain impulse amplitudes, the algorithm
shows varying improvements in Signal-to-Noise Ratio (SNR) relative to background
vibrational noise. A frequency-domain measure of SNR agrees with this result.

The research presented in this thesis utilizes Blade Element Momentum (BEM) theory with a
dynamic wake model to customize the OrcaFlex numeric simulation platform in order to allow
modeling of moored Ocean Current Turbines (OCTs). This work merges the advanced cable modeling
tools available within OrcaFlex with well documented BEM rotor modeling approach creating a
combined tool that was not previously available for predicting the performance of moored ocean
current turbines. This tool allows ocean current turbine developers to predict and optimize the
performance of their devices and mooring systems before deploying these systems at sea. The BEM
rotor model was written in C++ to create a back-end tool that is fed continuously updated data on the
OCT’s orientation and velocities as the simulation is running. The custom designed code was written
specifically so that it could operate within the OrcaFlex environment. An approach for numerically
modeling the entire OCT system is presented, which accounts for the additional degree of freedom
(rotor rotational velocity) that is not accounted for in the OrcaFlex equations of motion. The properties
of the numerically modeled OCT were then set to match those of a previously numerically modeled
Southeast National Marine Renewable Energy Center (SNMREC) OCT system and comparisons were
made. Evaluated conditions include: uniform axial and off axis currents, as well as axial and off axis wave fields. For comparison purposes these conditions were applied to a geodetically fixed rotor, showing nearly identical results for the steady conditions but varied, in most cases still acceptable accuracy, for the wave environment. Finally, this entire moored OCT system was evaluated in a dynamic environment to help quantify the expected behavioral response of SNMREC’s turbine under uniform current.

The drag reduction by vortex fusion was investigated. A comparison of flow over a bundle of cylinders in uniform and in disturbed currents was performed in a water channel. The model was subjected to cross flow. A thin cylindrical wire located nearby upstream and leveled at half the height of the test model was used as a source of disturbance. A hydrogen bubble technique was utilized to observe the flow pattern. The accumulation of vortices at stagnating regions in front of a bundle of cylinders transformed into a counter-rotated curl at leading edges of each leading cylinder in the bundle. Measurements were carried out by a computerized data acquisition system. Drag coefficient measurements, digital spectral and fourier analyses were also performed. Results have shown that a drag reduction can be obtained by introducing a thin cylindrical wire in front of the stagnation.

A study of two-layer quasi-geostrophic vortex flow is performed to determine the effect of a current difference between the layers on a vortex initially extending through both the layers. In particular, the conditions under which the current difference can 'tear' the vortex are examined. In the first set of flows studied, the current difference is generated by a (stronger) third vortex in the upper layer located at a large distance from the (weaker) vortex under study. A set of flows are also considered in which an ambient geostrophic current difference is produced by a non-uniform background potential vorticity field. The results of the study will be useful in determining the conditions under which large geophysical vortex structures, such as cyclones and ocean rings, can extend to large depths even though the mean currents in the ambient flow change significantly along the vortex length.