38 research outputs found
Modélisation dynamique et commande optimale d'un système de réfrigération à base d'éjecteur
Recently, the ejector-based refrigeration system (ERS) has been widely used in the cooling
industry as an appropriate alternative to the compressor-based cooling systems. However,
the advantages of ERS such as the reliable operation and low operation and maintenance
costs are overshadowed by its low efficiency and design complexity. In this context, this
thesis presents the efforts to develop a control model enabling the ERS to operate in its
optimal operational conditions. The extensive experimental studies of ERS revealed that at
a fixed condenser inlet condition, there exists an optimal primary stream mass flow
rate (generating pressure) that simultaneously maximizes the compression ratio (Cr) and
exergy efficiency and minimizes the evaporating pressure. Then, the steady state
models of the heat exchangers were developed and used to investigate the influence of the
increase in generating pressure on the coefficient of performance (COP) of the system and
it showed that increasing the generating pressure reduces the COP, linearly. In order to
predict the choking regime of the ejector and explain the reasons of observed physical
phenomenon, the 1D model of a fixed geometry ejector installed within an R245fa ERS was
developed. The developed model demonstrated that the ejector operates in the subcritical
mode when the generating pressure is below the Cr optimum point, while it operates in
critical mode at or above the optimum generating pressure. Next, a dynamic model of the
ERS was built to evaluate the ERS transient response to an increase in the primary stream
mass flow rate. Since the ERS dynamics is mainly dominated by the thermal dynamics of
the heat exchangers, the dynamic models of the heat exchangers were developed using the
moving boundary approach and connected to the developed models of the ejector and steady
state models of the pump and expansion valve to build a single dynamic model of the system.
The built dynamic model of an ERS was used to estimate the time response of the system in
the absence of accurate experimental data of the system’s dynamics. Finally, a control model
was designed to drive an ERS towards its optimal operation condition. A self-optimizing, model-free control strategy known as Extremum seeking control (ESC) was adopted to
minimize evaporating pressure in a fixed condenser thermal fluid inlet condition. The innovative ESC model named batch phasor ESC (BPESC) was proposed based on estimating the gradient by
evaluating the phasor of the output, in batch time. The simulation results indicated that
the designed BPESC model can seek and find the optimum evaporating pressure with good performance in terms of predicting the steady state optimal values and the convergence rates.Récemment, le système de réfrigération à éjecteur (SRE) a été largement utilisé dans l'industrie du refroidissement en tant que solution de remplacement appropriée aux systèmes de refroidissement à compresseur. Cependant, les avantages du SRE, tels que le fonctionnement fiable et les faibles couts d'exploitation et de maintenance, sont éclipsés par son faible rendement et sa complexité de conception. Dans ce contexte, ce projet de recherche de doctorat a détaillé les efforts déployés pour développer une stratégie de commande permettant au système de fonctionner dans ses conditions opérationnelles optimales. Les études expérimentales approfondies du SRE ont révélé que, dans une condition d'entrée de condensateur constante, il existe un débit massique optimal du flux primaire (générant une pression) qui maximise simultanément le taux de compression
(Cr) et l'efficacité exergétique, et minimise la pression d’évaporation. Ensuite, les modèles à l’état d’équilibre des échangeurs de chaleur ont été développés et utilisés pour étudier l’influence de l’augmentation de la pression générée sur le coefficient de performance (COP) du système et il en ressort que l'augmentation de la pression génératrice réduit le COP de manière linéaire. Afin de prédire le régime d'étouffement de l'éjecteur et d'expliquer les raisons du phénomène physique observé, le modèle 1D d'un éjecteur à géométrie fixe installé dans un système SRE R245fa a été développé. Le modèle développé a démontré que l'éjecteur fonctionne en mode sous-critique lorsque la pression génératrice est inférieure au point optimal de Cr, alors qu'il fonctionne en mode critique à une pression égale ou supérieure à la pression génératrice optimale. Ensuite, un modèle dynamique du SRE a été développé pour étudier la réponse transitoire du SRE lors d’une augmentation du débit massique du flux primaire. Puisque la dynamique du SRE est principalement dominée par la dynamique thermique des échangeurs de chaleur, les modèles dynamiques des échangeurs de chaleur ont été développés à l'aide de l'approche des limites mobiles et connectés aux modèles développés de l'éjecteur et des modèles à l'état stationnaire de la pompe et de la vanne un seul modèle dynamique du système. En l’absence de données expérimentales précises sur la dynamique d’un système SRE, le modèle dynamique développé du SRE a été simulé numériquement pour étudier sa réponse temporelle. Enfin, une stratégie de commande extrêmale (ESC) a été élaboré pour régler automatiquement le SRE à ses conditions de fonctionnement optimales, c’est-à-dire pour trouver la vitesse de la pompe qui minimise la pression dans des conditions d'entrée de condenseur fixes. Afin de proposer une ESC implémentable en temps discret sur une installation réelle sujette à un bruit de mesure important et un traitement hors-ligne par trame, une nouvelle commande extrémale basée sur une approche par phaseur avec une procédure de traitement de signal par trame (BPESC) a été développée et simulée avec le modèle numérique. Les résultats de la simulation ont indiqué que le modèle BPESC peut trouver la vitesse optimale de la pompe avec de bonnes performances en termes de précision et de vitesse de convergence
Convective heat transfer control in turbulent boundary layers
Mención Internacional en el título de doctorThe sustainable development of our society opens up concerns in several fields of engineering,
including energy management, production, and the impact of our technology, being thermal
management a common issue to be addressed. The investigation reported in this manuscript
focuses on understanding, controlling and optimizing the physical processes involving
convective heat transfer in turbulent wall-bounded flows. The content is divided into two
main blocks, namely the investigation of classic open-loop active-control techniques to
control heat transfer, and the technological development of machine-learning strategies to
enhance the performance of flow control in the field of convective heat transfer.
The first block focuses on actuator technology, applying dielectric-barrier discharge
(DBD) plasma actuators and a pulsed slot jet in crossflow (JICF), respectively, to control
the convective heat transfer in a turbulent boundary layer (TBL) over a flat plate. In the
former, an array of DBD plasma actuators is employed to induce pairs of counter-rotating,
streamwise-aligned vortices embedded in the TBL to reduce heat transfer downstream of the
actuation. The whole three-dimensional mean flow field downstream of the plasma actuator
is reconstructed from stereoscopic particle image velocimetry (PIV). Infrared thermography
(IR) measurements coupled with a heated thin foil provide ensemble-averaged convective
heat transfer distributions downstream of the actuators. The combination of the flow
field and heat transfer measurements provides a complete picture of the fluid-dynamic
interaction of plasma-induced flow with local turbulent transport effects. The plasmainduced
streamwise vortices are stationary and confined across the spanwise direction due
to the action of the plasma discharge. The opposing plasma discharge causes a mass- and
momentum-flux deficit within the boundary layer, leading to a low-velocity region that
grows in the streamwise direction and which is characterised by an increase in displacement
and momentum thicknesses. This low-velocity ribbon travels downstream, promoting
streak-alike patterns of reduction in the convective heat transfer distribution. Near the wall,
the plasma-induced jets divert the main flow due to the DBD-actuator momentum injection
and the suction on the surrounding fluid by the emerging jets. The stationarity of the
plasma-induced vortices makes them persistent far downstream, reducing the convective
heat transfer.
Conversely, the target of the second paper in this first block is to enhance convective
heat transfer rather than reduce it. A fully modulated, pulsed, slot JICF is used to perturb
the TBL. The slot-jet actuator, flush-mounted and aligned in the spanwise direction, is
controlled based on two design parameters, namely the duty cycle (DC) and the pulsation
frequency (f). Heat transfer and flow-field measurements are performed to characterise the
control performance using IR thermography and planar PIV, respectively. A parametric
study on f and DC is carried out to assess their effect on the heat transfer distribution.
The vorticity fields are reconstructed from the Proper Orthogonal Decomposition (POD)
modes, retrieving phase information. The flow topology is considerably altered by the
jet pulsation, even compared to the case of a steady jet. The results show that both the jet penetration in the streamwise direction and the overall Nusselt number increase with
increasing DC. However, the frequency at which the Nusselt number is maximised is
independent of the duty cycle. A wall-attached jet rises from the slot accompanied by a
pair of counter-rotating vortices that promote flow entrainment and mixing. Eventually,
a simplified model is proposed which decouples the effect of f and DC in the overall
heat transfer enhancement, with a good agreement with experimental data. The cost of
actuation is also quantified in terms of the amount of injected fluid during the actuation,
leading to conclude that the lowest duty cycle is the most efficient for heat transfer
enhancement among the tested set.
The second block of the thesis splits into a comparative assessment of machine
learning (ML) methods for active feedback flow control and an application of linear genetic
algorithms to an experimental convective heat transfer enhancement problem. First,
the comparative study is carried out numerically based on a well-established benchmark
problem, the drag reduction of a two-dimensional Kármán vortex street past a circular
cylinder at a low Reynolds number (Re = 100). The flow is manipulated with two
blowing/suction actuators on the upper and lower side of a cylinder. The feedback employs
several velocity sensors. Two probe configurations are evaluated: 5 and 11 velocity probes
located at different points around the cylinder and in the wake. The control laws are
optimized with Deep Reinforcement Learning (DRL) and Linear Genetic Programming
Control (LGPC). Both methods successfully stabilize the vortex alley and effectively
reduce drag while using small mass flow rates for the actuation. DRL features higher
robustness with respect to variable initial conditions and noise contamination of the sensor
data; on the other hand, LGPC can identify compact and interpretable control laws, which
only use a subset of sensors, thus allowing reducing the system complexity with reasonably
good results.
The gained experience and knowledge of machine-learning methods motivated the
last study enclosed in this thesis, which utilises linear genetic algorithm control (LGAC)
to identify the best actuation parameters in an experimental application. The actuator
is a set of six slot jets in crossflow aligned with the freestream. An open-loop optimal
periodic forcing is defined by the carrier frequency (f), the duty cycle (DC) and the phase
between actuators (ϕ) as control parameters. The control laws are optimised with respect
to the unperturbed TBL and the steady-jet actuation. The cost function includes wall
convective heat transfer and the cost of the actuation, thus leading to a multi-objective
optimisation problem. Surprisingly, the LGAC algorithm converges to the same frequency
and duty cycle for all the actuators. This frequency is equivalent to the optimal frequency
reported in the second study of the first block of this thesis. The performance of the
controller is characterised by IR thermography and PIV measurements. The action of the
jets considerably alters the flow topology compared to the steady-jet actuation, yielding
a slightly asymmetric flow field. The phase difference between multiple jet actuation
has shown to be very relevant and the main driver of flow asymmetry. A POD analysis
concludes the shedding phenomena characterising the steady-jet actuation, while the
optimised controller exhibits an elongated large-scale structure just downstream of the
actuator.
The investigation carried out in this thesis sheds some light on the application of
different flow control strategies to the field of convective heat transfer. From the utilisation
of plasma actuators and a single jet in cross flow to the development of sophisticated
control logic, the results point to the exceptional potential of machine learning control in unravelling unexplored controllers within the actuation space. Ultimately, this work
demonstrates the viability of employing sophisticated measurement techniques together
with advanced algorithms in an experimental investigation, paving the way towards more
complex applications involving feedback information.El desarrollo sostenible de nuestra sociedad abre preocupaciones en varios campos de
la ingeniería, incluyendo la gestión de la energía, la producción y el impacto de nuestra
tecnología, siendo la gestión térmica un tema común a tratar. La investigación que se
presenta en este manuscrito se centra en la comprensión, el control y la optimización
de los procesos físicos que implican la transferencia de calor por convección en flujos
turbulentos de pared. El contenido se divide en dos bloques principales: la investigación
de las técnicas clásicas de control activo de lazo abierto para controlar la transferencia de
calor, y el desarrollo tecnológico de estrategias de aprendizaje automático para mejorar el
rendimiento del control del flujo en el campo de la transferencia de calor por convección.
El primer bloque se centra en la tecnología de los actuadores, aplicando actuadores de
plasma de descarga de barrera dieléctrica (dielectric barrier dicharge, DBD) y un chorro
con forma de ranura pulsado en flujo cruzado (jet in crossflow, JICF), respectivamente,
para controlar la transferencia de calor por convección en una capa límite turbulenta
(turbulent boundary layer, TBL) sobre una placa plana. En el primero, se emplea un
conjunto de actuadores de plasma DBD para inducir pares de vórtices contra-rotativos,
alineados con la corriente e incrustados en la TBL para reducir la transferencia de calor
aguas abajo de la actuación. El campo de flujo medio tridimensional completo aguas abajo
del actuador de plasma se reconstruye a partir de la velocimetría de imagen de partículas
estereoscópica (particle image velocimetry, PIV). Las mediciones de termografía infrarroja
(IR) junto a una fina lámina calentada proporcionan distribuciones de transferencia de calor
convectiva promediadas aguas abajo de los actuadores. La combinación de las mediciones
del campo de flujo y de la transferencia de calor proporciona una imagen completa de
la interacción fluido-dinámica del flujo inducido por el plasma con los efectos locales de
transporte turbulento. Los vórtices en el sentido de la corriente inducidos por plasma
son estacionarios y están confinados transversalmente debido a la acción de la descarga
de plasma. La descarga de plasma en oposición causa un déficit de flujo de masa y de
momento dentro de la capa límite, lo que conduce a una región de baja velocidad que
crece en la dirección de la corriente y que se caracteriza por un aumento de los espesores
de desplazamiento y de momento. Esta zona de baja velocidad se desplaza corriente abajo,
promoviendo patrones de reducción similares a rayas en los que se reduce la transferencia
de calor por convección. Cerca de la pared, los chorros inducidos por el plasma desvían el
flujo principal debido a la inyección de momento del actuador DBD y a la succión sobre el
fluido circundante por parte de los chorros emergentes. La estacionariedad de los vórtices
inducidos por el plasma los hace persistentes aguas abajo, reduciendo la transferencia de
calor por convección.
Por el contrario, el objetivo del segundo trabajo de este primer bloque es mejorar la
transferencia de calor por convección en lugar de reducirla. Se utiliza un JICF, pulsado
y con forma de ranura, totalmente modulado para perturbar la TBL. El actuador de chorro, montado a ras y alineado en la dirección transversal, se controla en base a dos
parámetros de diseño, a saber, el ciclo de trabajo (DC) y la frecuencia de pulsación (f).
Se realizan mediciones de la transferencia de calor y del campo de flujo para caracterizar
el rendimiento del control mediante termografía IR y PIV planar, respectivamente. Se
lleva a cabo un estudio paramétrico de f y DC para evaluar su efecto en la distribución de
la transferencia de calor. Los campos de vorticidad se reconstruyen a partir de los modos
de descomposición ortogonal adecuada (POD), recuperando la información de fase. La
topología del flujo se ve considerablemente alterada por la pulsación del chorro, incluso
en comparación con el caso de un chorro estacionario. Los resultados muestran que tanto
la penetración del chorro en la dirección de la corriente como el número Nusselt global
aumentan con el incremento de DC. Sin embargo, la frecuencia a la que se maximiza el
número Nusselt es independiente del ciclo de trabajo. Un chorro adherido a la pared sale
de la ranura acompañado de un par de vórtices contrarrotantes que promueven el arrastre
y la mezcla del flujo. Finalmente, se propone un modelo simplificado que desacopla el
efecto de f y DC en la mejora global de la transferencia de calor, con un buen acuerdo
con los datos experimentales. También se cuantifica el coste de la actuación en términos
de la cantidad de fluido inyectado durante la actuación, llegando a la conclusión de que el
ciclo de trabajo más bajo es el más eficiente para la mejora de la transferencia de calor
entre el conjunto probado.
El segundo bloque de la tesis se divide en una evaluación comparativa de los métodos
de aprendizaje automático (machine learning, ML) para el control activo del flujo
por retroalimentación y una aplicación de algoritmos genéticos lineales a un problema
experimental de mejora de la transferencia de calor por convección. En primer lugar, el
estudio comparativo se realiza numéricamente a partir de un problema de referencia bien
establecido: la reducción de la resistencia aerodinámica de una calle de vórtices de Kármán
bidimensional tras un cilindro circular a un número de Reynolds bajo (Re = 100). El
flujo se manipula con dos actuadores de soplado/succión en la parte superior e inferior
de un cilindro. La retroalimentación emplea varios sensores de velocidad. Se evalúan
dos configuraciones de sondas: 5 y 11 sondas de velocidad situadas en diferentes puntos
alrededor del cilindro y en la estela. Las leyes de control se optimizan con el aprendizaje
profundo por refuerzo (Deep Reinforcement Learning, DRL) y el control por programación
genética lineal (Linear Genetic Programming Control, LGPC). Ambos métodos estabilizan
con éxito la calle de vórtices y reducen de manera efectiva la resistencia al tiempo que
usan caudales másicos pequeños para la actuación. El DRL se caracteriza por una mayor
robustez con respecto a la variación de la condición inicial y a la contaminación por ruido
de los datos de los sensores; por otro lado, el LGPC puede identificar leyes de control
compactas e interpretables, que sólo utilizan un subconjunto de sensores, lo que permite
reducir la complejidad del sistema con resultados razonablemente buenos.
La experiencia adquirida y el conocimiento de los métodos de aprendizaje automático
motivaron el último estudio incluido en esta tesis, que utiliza el control por algoritmo
genético lineal (Linear Genetic Algorithm Control, LGAC) para identificar los mejores
parámetros de actuación en una aplicación experimental. El actuador es un conjunto de
seis chorros con forma de ranura en flujo cruzado y alineados con la corriente principal.
Se define una ley de forzado periódica en lazo abierto mediante la frecuencia portadora
(f), el ciclo de trabajo (DC) y la fase entre actuadores (ϕ) como parámetros de control.
Las leyes de control se optimizan con respecto a la TBL no perturbada y la actuación
de chorro constante. La función de coste incluye la transferencia de calor por convección de la pared y el coste de la actuación, lo que da lugar a un problema de optimización
multiobjetivo. Sorprendentemente, el algoritmo LGAC converge a la misma frecuencia y
ciclo de trabajo para todos los actuadores. Esta frecuencia es equivalente a la frecuencia
óptima reportada en el segundo estudio del primer bloque de esta tesis. El rendimiento
del controlador se caracteriza mediante termografía IR y mediciones PIV. La acción de los
chorros altera considerablemente la topología del flujo en comparación con la actuación
de los chorros constantes, dando lugar a un campo de flujo ligeramente asimétrico. La
diferencia de fase entre la actuación de múltiples chorros ha demostrado ser muy relevante
y el principal impulsor de la asimetría del flujo. Un análisis POD concluye los fenómenos
de desprendimiento de vórtices que caracterizan la actuación de chorro constante, mientras
que el controlador optimizado muestra una estructura alargada a gran escala justo aguas
abajo del actuador.
La investigación llevada a cabo en esta tesis arroja algo de luz sobre la aplicación
de diferentes estrategias de control de flujo en el campo de la transferencia de calor por
convección. Desde la utilización de actuadores de plasma y un único chorro en flujo cruzado
hasta el desarrollo de una sofisticada lógica de control, los resultados apuntan al excepcional
potencial del control por aprendizaje automático para desentrañar controladores
inexplorados dentro del espacio de actuación. En última instancia, este trabajo demuestra
la viabilidad de emplear sofisticadas técnicas de medición junto con algoritmos avanzados
en una investigación experimental, allanando el camino hacia aplicaciones más complejas
que implican información de retroalimentación.The work enclosed in this thesis has been partially supported by the Universidad Carlos
III de Madrid through a PIPF scholarship awarded on a competitive basis, and by the
following research projects: ARTURO (Active contRol of Turbulence for sUstainable aiRcraft
propulsiOn), ref. PID2019-109717RB-I00/AEI/10.13039/501100011033, funded by
the Spanish State Research Agency (SRA); the 2020 Leonardo Grant for Researchers and
Cultural Creators AEROMATIC (Active flow control of aerodynamic flows with machine
learning), funded by the BBVA Foundation with grant number IN[20]_ING_ING_0163;
and GloWing Starting Grant, funded by the European Research Council (ERC), under
grant agreement ERC-2018.StG-803082.Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i VirgiliPresidente: Octavio Armas Vergel.- Secretario: Manuel García-Villalba Navaridas.- Vocal: Gioacchino Cafier
Optimization Methods Applied to Power Systems Ⅱ
Electrical power systems are complex networks that include a set of electrical components that allow distributing the electricity generated in the conventional and renewable power plants to distribution systems so it can be received by final consumers (businesses and homes). In practice, power system management requires solving different design, operation, and control problems. Bearing in mind that computers are used to solve these complex optimization problems, this book includes some recent contributions to this field that cover a large variety of problems. More specifically, the book includes contributions about topics such as controllers for the frequency response of microgrids, post-contingency overflow analysis, line overloads after line and generation contingences, power quality disturbances, earthing system touch voltages, security-constrained optimal power flow, voltage regulation planning, intermittent generation in power systems, location of partial discharge source in gas-insulated switchgear, electric vehicle charging stations, optimal power flow with photovoltaic generation, hydroelectric plant location selection, cold-thermal-electric integrated energy systems, high-efficiency resonant devices for microwave power generation, security-constrained unit commitment, and economic dispatch problems
Model Predictive Control of Impedance Source Inverter for Photovoltaic Applications
A model predictive controlled power electronics interface (PEI) based on impedance source inverter for photovoltaic (PV) applications is proposed in this disssertation. The proposed system has the capability of operation in both grid-connected and islanded mode. Firstly, a model predictive based maximum power point tracking (MPPT) method is proposed for PV applications based on single stage grid-connected Z-source inverter (ZSI). This technique predicts the future behavior of the PV side voltage and current using a digital observer that estimates the parameters of the PV module. Therefore, by predicting a priori the behavior of the PV module and its corresponding effects on the system, it improves the control efficacy. The proposed method adaptively updates the perturbation size in the PV voltage using the predicted model of the system to reduce oscillations and increase convergence speed. The experimental results demonstrate fast dynamic response to changes in solar irradiance level, small oscillations around maximum power point at steady-state, and high MPPT effectiveness from low to high solar irradiance level. The second part of this work focuses on the dual-mode operation of the proposed PEI based on ZSI with capability to operate in islanded and grid-connected mode. The transition from islanded to grid-connected mode and vice versa can cause significant deviation in voltage and current due to mismatch in phase, frequency, and amplitude of voltages. The proposed controller using MPC offers seamless transition between the two modes of operations. The main predictive controller objectives are decoupled power control in grid-connected mode and load voltage regulation in islanded mode. The proposed direct decoupled active and reactive power control in grid connected mode enables the dual-mode ZSI to behave as a power conditioning unit for ancillary services such as reactive power compensation. The proposed controller features simplicity, seamless transition between modes of operations, fast dynamic response, and small tracking error in steady state condition of controller objectives. The operation of the proposed system is verified experimentally. The final part of this dissertation focuses on the low voltage ride through (LVRT) capability of the proposed PV systems during grid faults such as voltage sag. In normal grid condition mode, the maximum available power from the PV panels is injected into the grid. In this mode, the system can provide reactive power compensation as a power conditioning unit for ancillary services from DG systems to main ac grid. In case of grid faults, the proposed system changes the behavior of reactive power injection into the grid for LVRT operation according to the grid requirements. Thus, the proposed controller for ZSI is taking into account both the power quality issues and reactive power injection under abnormal grid conditions
Design and Implementation of Internal Model Based Controllers for DC/ AC Power Converters
The aim of this thesis is to design and implement an advanced control system for a working three-phase DC to AC power converter. Compared to' the traditional PI controller used widely in industry, the new voltage controller can track the reference voltage with improved accuracy and efficiency in the presence of different kind of local loads, and also works well in the single phase voltage control. This voltage controller is combined with a power controller to yield a complete controller. An important aspect of this work is the hardware implementation of the whole system. Main parts ofthis thesis are: ???????? 1. Review ofH-infinity and repetitive control techniques and their applications in power converters. 2. Design of a new voltage controller to eliminate the DC component in the output voltages, and taking into account the practical issues such as the processing delay due to the digital signal processor (DSP) implementation. 3. Modelling and simulation of the converter system incorporating different control techniques and with different kinds of loads. 4. Hardware implementation and the two-processor controller. The parallel communication between the DSPs. 5. The main problems encountered in???????????????????? hardware implementation and programming. The software used to initialize DSPs, implement the discretetime voltage controller and other functions such ~ generations of space vector pulse width modulation (SVPWM) signals, circuit protections, analog to digital (AD) cOl)versions, data transmission, etc. 6. Experimental results the under circumstances of no load connected to the converter, pure three-phase resistive loads, three-phase unbalanced resistive' loads and the series resistor-inductor loads. /Imperial Users onl
Nonlinear hydrodynamic modelling of wave energy converters under controlled conditions
One of the major challenges facing modern industrialized countries is the provision of energy:
traditional sources, mainly based on fossil fuels, are not only growing scarcer and
more expensive, but are also irremediably damaging the environment. Renewable and
sustainable energy sources are attractive alternatives that can substantially diversify the
energy mix, cut down pollution, and reduce the human footprint on the environment.
Ocean energy, including energy generated from the motion of wave, is a tremendous untapped
energy resource that could make a decisive contribution to the future supply of
clean energy. However, numerous obstacles must be overcome for ocean energy to reach
economic viability and compete with other energy sources. Energy can be generated from
ocean waves by wave energy converters (WECs). The amount of energy extracted from
ocean waves, and therefore the profitability of the extraction, can be increased by optimizing
the geometry and the control strategy of the wave energy converter, both of which
require mathematical hydrodynamic models that are able to correctly describe the WEC-
uid interaction. On the one hand, the accuracy and representativeness of such models
have a major in
uence on the effectiveness of the WEC design. On the other hand, the
computational time required by a model limits its applicability, since many iterations or
real-time calculations may be required. Critically, computational time and accuracy are
often mutually contrasting features of a mathematical model, so an appropriate compromise
should be defined in accordance with the purpose of the model, the device type, and
the operational conditions. Linear models, often chosen due to their computational convenience,
are likely to be imprecise when a control strategy is implemented in a WEC: under
controlled conditions, the motion of the device is exaggerated in order to maximize power
absorption, which invalidates the assumption of linearity. The inclusion of nonlinearities
in a model is likely to improve the model's accuracy, but increases the computational
burden. Therefore, the objective is to define a parsimonious model, in which only relevant
nonlinearities are modelled in order to obtain an appropriate compromise between accuracy
and computational time. In addition to presenting a wider discussion of nonlinear
hydrodynamic modelling for WECs, this thesis contributes the development of a computationally
efficient nonlinear hydrodynamic model for axisymmetric WEC devices, from
one to six degrees of freedom, based on a novel approach to the nonlinear computation of
static and dynamic Froude-Krylov forces
Symmetry in Electromagnetism
Electromagnetism plays a crucial role in basic and applied physics research. The discovery of electromagnetism as the unifying theory for electricity and magnetism represents a cornerstone in modern physics. Symmetry was crucial to the concept of unification: electromagnetism was soon formulated as a gauge theory in which local phase symmetry explained its mathematical formulation. This early connection between symmetry and electromagnetism shows that a symmetry-based approach to many electromagnetic phenomena is recurrent, even today. Moreover, many recent technological advances are based on the control of electromagnetic radiation in nearly all its spectra and scales, the manipulation of matter–radiation interactions with unprecedented levels of sophistication, or new generations of electromagnetic materials. This is a fertile field for applications and for basic understanding in which symmetry, as in the past, bridges apparently unrelated phenomena―from condensed matter to high-energy physics. In this book, we present modern contributions in which symmetry proves its value as a key tool. From dual-symmetry electrodynamics to applications to sustainable smart buildings, or magnetocardiography, we can find a plentiful crop, full of exciting examples of modern approaches to electromagnetism. In all cases, symmetry sheds light on the theoretical and applied works presented in this book
Advances in Computer Science and Engineering
The book Advances in Computer Science and Engineering constitutes the revised selection of 23 chapters written by scientists and researchers from all over the world. The chapters cover topics in the scientific fields of Applied Computing Techniques, Innovations in Mechanical Engineering, Electrical Engineering and Applications and Advances in Applied Modeling