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Simulating the Hydrodynamics of Offshore Floating Wind Turbine Platforms in a Finite Volume Framework
There is great potential for the growth of wind energy in offshore locations where the structures are exposed to a variety of loading from waves, current and wind. A variety of computer-aided engineering (CAE) tools, based largely on engineering models employing potential-flow theory and/or Morison\u27s equation, are currently being used to evaluate hydrodynamic loading on floating offshore wind turbine platforms. While these models are computationally inexpensive, they include many assumptions and approximations. Alternatively, high-fidelity computational fluid dynamics models contain almost no assumptions, but at the cost of high computational expense. In this work, CFD simulations provide detailed insight into the complex fluid flow that has not been captured experimentally, nor can be attained with reduced-order models.
This work includes a thorough validation of the various CFD toolboxes necessary for simulating offshore floating wind turbine platforms in the ocean environment, from numerical wave propagation to fluid-structure interactions. The fundamental physics of flow around complex structures is examined through various studies to better understand the effects of a fluid interface, truncated ends, structure size, multi-member arrangements and environmental conditions. These factors are explored in terms of drag, lift and frequency of the loads. Additionally, motion of structures in free decay tests and waves are investigated. The work provides insight into the complex fluid flow around floating offshore structures of small draft in a variety of environmental conditions. CFD simulations are used to assess assumptions and approximations of reduced-order engineering models, and explain why, and in which conditions, these models perform inaccurately. Finally, the work provides suggestions for improvements to engineering tools often used for hydrodynamics modeling of floating offshore wind turbines
Asymptotic theory for a Leidenfrost drop on a liquid pool
Droplets can be levitated by their own vapour when placed onto a superheated
plate (the Leidenfrost effect). It is less known that the Leidenfrost effect
can likewise be observed over a liquid pool (superheated with respect to the
drop), which is the study case here. Emphasis is placed on an asymptotic
analysis in the limit of small evaporation numbers, which proves to be a
realistic one indeed for not so small drops. The global shapes are found to
resemble "superhydrophobic drops" that follow from the equilibrium between
capillarity and gravity. However, the morphology of the thin vapour layer
between the drop and the pool is very different from that of classical
Leidenfrost drops over a flat rigid substrate, and exhibits different scaling
laws. We determine analytical expressions for the vapour thickness as a
function of temperature and material properties, which are confirmed by
numerical solutions. Surprisingly, we show that deformability of the pool
suppresses the chimney instability of Leidenfrost drops
Digital Microfluidics as a Reconfiguration Mechanism for Antennas
This dissertation work concentrates on novel reconfiguration technologies, including design, microfabrication, and characterization aspects with an emphasis on their applications to multifunctional reconfigurable antennas. In the literature, reconfigurable antennas have made use of various reconfiguration techniques. The most common techniques utilized revolved around switching mechanisms. Other techniques such as the incorporation of variable capacitors, varactors, and physical structure manipulation surfaced recently to overcome many problems faced in using switches and their biasing. Usage of fluids (micro-fluidic or otherwise) in antennas provides a conceptually easy reconfiguration mechanism in the aspect of physical alteration. However, a requirement of pumps, valves, etc. for liquid transportation makes the antenna implementations rather impractical for the real-life scenarios. This work reports on design and experiments conducted to evaluate the electrowetting on dielectric (EWOD) driven digital microfluidics as a reconfiguration mechanism for antennas
Design of robust slow-speed ships for sustainable operation
Phd ThesisMulti-objective optimisation that considers the energy efficiency and economic success is an
important aspect of ship design and operation. Both the hydrodynamic and economic
performance characteristics need to be addressed in the early stages of the design, and secured
during the life span of a ship. Because of the conflicting nature of these two objectives, there
are various trade-offs at stake in the goal for making ships more efficient and greener to comply
with IMO regulations while reducing the building and operating costs and increasing the
profitability at the same time for all stakeholders especially owners and operators.
In attempt to reduce the amount of greenhouse gas emissions from ships, and hence to achieve
a lower EEDI value, this research approaches the problem of improving the energy efficiency
of ships. That is achieved by optimising the hull design over a speed range through parametric
modification to reduce resistance and required power, and also through adopting slow steaming
concept.
Moreover, the research aims to determine the best practice to reduce the annual cost of running
a ship and to increase the annual revenue as well as to make the ship a more profitable
investment over her life span. The profit per tonne.mile and the net present value NPV are
estimated in the economic analysis to be used as indicators to compare alternative designs for
different routes and market conditions scenarios. To achieve this aim, the main operational and
economic aspects such as the fluctuations in the fright rates and fuel prices in the shipping
market are covered in the economic analysis. In addition, the acquiring price and salvage value
are included in order to obtain solid comparisons.
An optimisation framework using a VBA macro code has been developed based on the concept
of Pareto optimality to assess decision making, and to determine robust designs as well as
operational profiles based on results from the hydrodynamic model, environmental impact
model, and the economic model. The optimisation process is carried out for a Panamax tanker
case study using 5 parameters and a set of constraints for the hull parameters and speed.
The outcome from the optimisation framework is a set of Pareto optimal solutions where weight
factors are appointed to give the flexibility when addressing the importance of each individual
function. The solutions are presented graphically to form what is known as Pareto front which
determines the design space and the trade-offs between the different competing objective
ii
functions. This optimisation framework could assist decision making where it is possible to
choose a robust design or designs that offer a near-optimum performance regardless any
fluctuations in the market and or the operation profile, and eliminate any significant sub-optimal
design
The broaching of ships in following seas
The two aims of this work were: (1) to develop a theoretical
technique for determining the conditions where a broach would occur,
and (2) to identify the principal factors affecting the liability of
a ship to broach.
The first step was to develop a mathematical model based on the
conventional manoeuvring equations with coefficients which were
functions of the ship's longitudinal position in the wave, but independent
of encounter frequency. Next, a theoretical method for calculating
the values of some of the coefficients as functions of wave
position was developed using a strip theory approach and the results
compared with those obtained experimentally. The experimental technique
involved using a planar motion mechanism to oscillate a constrained
model balanced on a wave created by a wave dozer in a circulating
water channel. Although the agreement was poor and experimental
scatter high for some of the coefficients, the more important ones
were predicted quite well using the theory. Constrained model experiments were also --carried out in calm
water in order to determine the approximate value of the roll coupling
terms and it was found that, since they were small, the roll equation
could be ignored as a first approximation.
It was then possible to study the stability of the lateral and
longitudinal motions separately for various wavelengths and to determine
that the principal factor causing a broach was the large wave induced
yaw moment combined with the small restoring moment available
from the rudder operating with reduced effectiveness. The lateral and
longitudinal equations were then combined using a digital/analogue
hybrid simulation permitting the conditions which caused a broach to
be determined. When the results from the simulation-were compared
with results which had already been carried out by the
Admiralty Marine Technology Establishment at Haslar there was fairly
good agreement, implying that this method could be used to determine
whether a proposed design would meet an acceptable standard. Finally, possible improvements to the simulation were suggested
and guidelines for reducing the liability to broach were given both
for the operator and the designer
Dynamics and distribution of immunoglobolin E receptors : a dialog between experiment and theory
This dissertation explores the dynamics and distribution of immunoglobulin E receptors (FceRI) on mast cells by drawing on the techniques of experimental and theoretical physics. The motivation for these investigations is provided by a considerable interest in the transmembrane signaling mechanisms of immunoreceptors, especially when triggered with membrane-bound ligands. Experimental investigations quantify the spatiotemporal dynamics of the redistribution of FceRI due to membrane-bound monovalent ligands, using total internal reflection fluorescence microscopy and single-particle tracking. When mast cells contact such substrates, receptor clusters form at cell-substrate contact points. The initial rate of accumulation of receptors into these contact points or cell protrusions is consistent with diffusion-limited trapping. Over longer timescales (\u3e10 s), individual clusters move with both diffusive and directed motion components and eventually coalesce to form a large central receptor patch surrounded by a receptor cluster depletion zone. Detailed analysis of single-particle trajectories show that receptors maintain their diffusivity when confined within receptor clusters, and increase their diffusivity (above that of monomeric unliganded FceRI) in central patches. To study the kinetics of central patch formation, a new coalescence theory described by a melding process, which is not instantaneous, was developed. In these theoretical investigations, the difficult problem of moving boundaries is encountered. To handle the complexity, which stems from boundary growth due to particle melding, the study is divided into three parts. The first is about stationary trapping problems investigated by the standard defect technique, and the second is about a validity study of an adiabatic approximation for moving boundaries. In the last part of this dissertation, a new coalescence theory is developed, which is based on a completely self-consistent approach. Here, the time dependence of the moving boundary is not prescribed but obtained through feedback. Comparison of experiment and theory shows that observed biological cluster coalescence is delayed at early times and occurs at a faster rate at later times than predicted by a simple theory. The incompatibility at early times is addressed by a generalization of the theory to incorporate a time-dependent melding process by a memory concept, which quantitatively explains the observed delay
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