VanZwieten, James H.

This research focused on maximizing the power generated by an array of ocean
current turbines. To achieve this objective, the produced shaft power of an ocean current
turbine (OCT) has been quantified using CFD without adding a duct, as well as over a
range of duct geometries. For an upstream duct, having a diameter 1.6 times the rotor
diameter, the power increased by 8.35% for a duct that extends 1 diameter upstream.
This research also focused on turbine array optimization, providing a
mathematical basis for calculating the water velocity within an array of OCTs. After
developing this wake model, it was validated using experimental data. As the
downstream distance behind the turbine increases, the analytic results become closer to
the experimental results, with a difference of 3% for TI = 3% and difference of 4% for TI
= 15%, both at a downstream distance of 4 rotor diameters.

This work details the development of tools and controllers for station keeping
control of twin screw vessels. A fundamental analysis is conducted of the dynamics of
twin screw displacement hull vessels and their actuator systems, where the response
characteristics and maneuverability are quantified through a series of full scale trials
conducted in different environmental conditions while recording the environmental
conditions, actuator states, and geodetic and inertial measurements. The data from these
maneuvers were repeatable from run to run and thus provide valuable benchmarks for
several maneuvers and the measured actuator response provides valuable set points of
performance characteristics/limitations for control development. A comprehensive
general simulation of small twin screw displacement hull boats is developed as a tool to
estimate ship and actuator responses in support of developing and tuning of control
systems. The model and computer simulation is capable of modeling a wide range of the
surface vessels, including their actuators and environmental conditions. This model
proved to be accurate, when compared to the sea trial data, and model estimates have rms velocity errors for the various steady maneuvers of 1.2-4.6% for surge, 12.6-17.9% for
sway, and 7.6-20.2% for yaw.
A path following station keeping controller is developed that uses Lyapunov
stability analysis to determine the path the vessel should follow to effectively eliminate
position error. This controller showed good performance for several different
environmental conditions. Encouraged by these finding, three additional station keeping
control methodologies are developed for twin screw surface ships. All four of these
controllers are examined for their robustness to environmental conditions, as well as their
sensitivity to sensor precision, sensor update rates, and actuator limitations. All
controllers are evaluated in sea state 4 yielding rms position errors from 3.3 to 16.2 m,
the rms surge and sway accelerations are under 0.62 m/s , and the engine shifting
frequencies are between 0.011 and 0.145 Hz. These four controllers are then tested over a
wide range of environmental conditions, sensor precisions and update rates, and actuator
response rates. The results from these tests give quantitative data that will aid in selecting
the appropriate controller for a specific application, and will assist in selecting
appropriate sensors.

The ocean currents off Florida are a renewable and
energy dense resource capable of providing Florida
with about 25% of its electricity needs. This current
is strongest at the sea surface and decreases
in strength with depth such that the individual rotor
blades on ocean current turbines (OCT) deployed to
harness this resource will operate in stronger currents
when positioned vertically upwards than when vertically
downwards. This current shear will induce cyclic
loadings on the rotor blades unless active control is
used to reduce these load variations. A direct adaptive
individual blade pitch controller is implemented
into a numerical model simulating an OCT operating
in the Gulf Stream. The adaptive controller is analyzed
with the OCT simulated in both stationary and
moored configurations. The results concluded that
the IBP controller reduced the amplitude of the loads
in the stationary and moored simulations by 91.18%
and 92.3%, respectively.

An Ocean Current Turbine (OCT) numerical simulation for creating, testing and
tuning flight and power takeoff controllers, as well as for farm layout optimization is
presented. This simulation utilizes a novel approach for analytically describing oceanic
turbulence. This approach has been integrated into a previously developed turbine
simulation that uses unsteady Blade Element Momentum theory. Using this, the
dynamical response and power production of a single OCT operating in ambient
turbulence is quantified.
An approach for integrating wake effects into this single device numerical
simulation is presented for predicting OCT performance within a farm. To accomplish
this, far wake characteristics behind a turbine are numerically described using analytic
expressions derived from wind turbine wake models. These expressions are tuned to
match OCT wake characteristics calculated from CFD analyses and experimental data. Turbine wake is characterized in terms of increased turbulence intensities and decreased
mean wake velocities. These parameters are calculated based on the performance of the
upstream OCT and integrated into the environmental models used by downstream OCT.
Simulation results are presented that quantify the effects of wakes on downstream turbine
performance over a wide range of relative downstream and cross stream locations for
both moored and bottom mounted turbine systems. This is done to enable the
development and testing of flight and power takeoff controllers designed for maximizing
energy production and reduce turbine loadings.

A motion compensated ADV system was evaluated to determine its ability to
make measurements necessary for characterizing the variability of the ambient current in
the Gulf Stream. The impact of IMU error relative to predicted turbulence spectra was
quantified, as well as and the ability of the motion compensation approach to remove
sensor motion from the ADV measurements. The presented data processing techniques
are shown to allow the evaluated ADV to be effectively utilized for quantifying ambient
current fluctuations from 0.02 to 1 Hz (50 to 1 seconds) for dissipation rates as low as
3x10-7. This measurement range is limited on the low frequency end by IMU error,
primarily by the calculated transformation matrix, and on the high end by Doppler noise.
Inshore testing has revealed a 0.37 Hz oscillation inherent in the towfish designed and
manufactured as part of this project, which can nearly be removed using the IMU.

Diversifying US energy production to include renewables has been a popular topic of discussion in recent years. In-stream hydrokinetic energy, electricity production from moving currents without the use of dams, has potential for significant power production with technically feasible US electricity production estimated at 14 GW from rivers, 50 GW from tides, and 19 GW from ocean currents; which is equivalent to approximately 17% of 2011 US power production. This work focuses on improving the power production from in-stream hydrokinetic turbines using adaptive torque control, and quantifies increased energy production by comparisons with standard fixed-gain torque control. This research uses numerical modeling to acquire power production estimates under simulated conditions. With these results we can quantify potential energy gains for three representative in-stream hydrokinetic rotor designs.

The "C-Plane" is a submerged ocean current turbine that uses sustained ocean currents to produce electricity. This turbine is moored to the sea floor and is capable of changing depth, as the current profile changes, to optimize energy production. A 1/30th scale physical prototype of the C-Plane is being developed and the analysis and control of this prototype is the focus of this work. A mathematical model and dynamic simulation of the 1/30th scale C-Plane prototype is created to analyze this vehicle's performance, and aid in the creation of control systems. The control systems that are created for this prototype each use three modes of operation and are the Mixed PID/Bang Bang, Mixed LQR/PID/Bang Bang, and Mixed LQG/PID/Bang Bang control systems. Each of these controllers is tested using the dynamic simulation and Mixed PID/Bang Bang controller proves to be the most efficient and robust controller during these tests.