[pt] Coberturas de um disco quadriculado com buracos D são contados de acordo com volume (na variável formal q) e fluxo (em p1, p2, ..., pN). Consideramos propriedades algébricas dos resultados gerados pela função F (p1, p2, ..., pN, q). Para números fixos p2, ..., pN, q > 0 o polinômio f(p1) = F(p1, p2, ..., pN, q) tem todas as raízes reais (e negativas). / [en] Tilings of a quadriculated disk with holes D are counted according to vol- ume (in the formal variabel q) and flux (in p1; p2;... pN). We consider algebraic properties of the resulting generating function D (p1; p2; ...; pk; q). For p1; p2; ...; bpi; ...; pn; q > 0 the polynomial f(pi) = D (p1; p2; ... ; pi; ...; pn; q) has all roots real numbers (and negative).

Multiple coherent supernova explosions (SNe) in an OB association can produce a strong shock that moves through the interstellar medium (ISM). These shocks fronts carve out hot and tenuous regions in the ISM known as superbubbles. The density contour plot at three different times (0.5 Myr (left panel), 4 Myr (middle panel), 9.5 Myr (right panel)) showing different stages of superbubble evolution for n0 = 0.5 cm−3, z0 = 300 pc, and for NOB = 104. This density contour plot is produced using ZEUS-MP 2D hydrodynamic simulation with a resolution of 512 × 512 with a logarithmic grid extending from 2 pc to 2.5 kpc. For a detailed description of this figure, see Roy et. al., 2015. The evolution of a superbubble is marked by different phases, as it moves through the ISM. Consider an OB association at the center of a disk galaxy. Initially the distance of the shock front is much smaller than the disk scale height. The superbubble shell sweeps up the ISM material, and once the amount of swept up material becomes comparable to the ejected material during SNe, the superbubble enters a self-similar phase (analogous to the Sedov-Taylor phase of individual SNe). As the superbubble shell sweeps up material, its velocity decreases, and thus the corresponding post-shock temperature drops. At a temperature of ∼ 2 × 105 K (where the cooling function peaks), the superbubble shell becomes radiative and starts losing energy via radiative cooling. This radiative phase is shown in the left panel of Figure 1. The superbubble shell starts fragmenting into clumps and channels due to Rayleigh-Taylor instabilities (RTI) (which is seeded by the thermal instability; for details see Roy et. al., 2013) when the superbubble shell crosses a few times the scale height. This is represented in the middle panel of the same figure. At a much later epoch, RTI has a strong effect on the shell fragmentation and the top of the bubble is completely blown off (the right panel). In the first chapter of the thesis (reported in Sharma et. al., 2014), we show using ZEUS-MP hydrodynamic simulations that an isolated supernova loses almost all its mechanical energy within a Myr whereas superbubbles can retain up to ∼ 40% of the input energy over the lifetime of the starcluster (∼ few tens of Myr), consistent with the analytic estimate of the second chapter. We also compare different recipes (constant luminosity driven model (LD model), kinetic energy driven model (KE model) to implement SNe feedback in numerical simulations. We determine the constraints on the injection radius (within which the SNe input energy is injected) so that the supernova explosion energy realistically couples to the interstellar medium (ISM). We show that all models produce similar results if the SNe energy is injected within a very small volume ( typically 1–2 pc for typical disk parameters). The second chapter concentrates on the conditions for galactic disks to produce superbubbles which can give rise to galactic winds after breaking out of the disk. The Kompaneets formalism provides an analytic expression for the adiabatic evolution of a superbubble. In our calculation, we include radiative cooling, and implement the supernova explosion energy in terms of constant luminosity through out the life-time of the OB stars in an exponentially stratified medium (Roy et. al., 2013). We use hydrodynamic simulations (ZEUS-MP) to determine the evolution of the superbubble shell. The main result of our calculation is a clear demarcation between the energy scales of sources causing two different astrophysical phenomenon: (i) An energy injection rate of ∼ 10−4 erg cm−2 s−1 (corresponding Mach number ∼ 2–3, produced by large OB associations) is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (ii) A larger energy injection scale ∼ 10−3 erg cm−2 s−1, or equivalently a surface density of star formation rate ∼ 0.1 M⊙ yr−1 kpc−2 corresponding to superbubbles with high Mach number (∼ 5–10) produces galactic-scale superwinds (requires superstar clusters to evolve coherently in space and time). The stronger energy injection case also satisfies the requirements to create and maintain a multiphase halo (matches with observations). Roy et. al., 2013 also points out that Rayleigh-Taylor instability (RTI) plays an important role in the fragmentation of superbubble shell when the shell reaches a distance approximately 2–3 times the scale-height; and before the initiation of RTI, thermal instability helps to corrugate the shell and seed the RTI. Another important finding of this chapter is the analytic estimation of the energetics of superbubble shell. The shell retains almost ∼ 30% of the thermal energy after the radiative losses at the end of the lifetime of OB associations. The third chapter considers the escape of hydrogen ionizing (Lyc) photons arising from the central OB-association that depends on the superbubble shell dynamics. The escape fraction of Lyc photons is expected to decrease at an initial stage (when the superbubble is buried in the disk) as the dense shell absorbs most of the ionizing photons, whereas the subsequently formed channels (created by RTI and thermal instabilities) in the shell creates optically thin pathways at a later time (∼ 2–3 dynamical times) which help the ionizing photons to escape. We determine an escape fraction (fesc) of Lyc photons of ∼ 10 ± 5% from typical disk galaxies (within 0 ≤ z (redshift) ≤ 2) with a weak variation with disk masses (reported in Roy et. al., 2015). This is consistent with observations of local galaxies as well as constraints from the epoch of reionization. Our work connects the fesc with the fundamental disk parameters (mid-plane density (n0), scale-height (z0)) via a relation that fescαn20z03 (with a ≈ 2.2) is a constant. In the fourth chapter, we have considered a simple model of molecule formation in the superbubble shells produced in starburst nuclei. We determine the threshold conditions on the disk parameters (gas density and scale height) for the formation of molecules in superbubble shells breaking out of disk galaxies. This threshold condition implies a gas surface density of ≥ 2000 M⊙ pc−2, which translates to a SFR of ≥ 5 M⊙ yr−1 within the nuclear region of radius ∼ 100 pc, consistent with the observed SFR of galaxies hosting molecular outflows. Consideration of molecule formation in these expanding superbubble shells predicts molecular outflows with velocities ∼ 30–40 km s−1 at distances ∼ 100–200 pc with a molecular mass ∼ 106–107 M⊙, which tally with the recent ALMA observations of NGC 253. We also consider different combinations of disk parameters and predict velocities of molecule bearing shells in the range of ∼ 30–100 km s−1 with length scales of ≥ 100 pc, in rough agreement with the observations of molecules in NGC 3628 and M82 (Roy et. al., 2016, submitted to MNRAS).

Des trous noirs supermassifs (TNs) de plusieurs millions de masses solaires occupent le centre de la plupart des galaxies proches. La découverte du TN Sagittarius A* au centre de notre galaxie, La Voie lactée, l'a confirmé. Pour autant, certaines galaxies semblent dépourvues de TNs (par exemple NGC205, M33), ou alors ne posséder un TN que de quelques milliers de masses solaires. D'autre part, des TNs dans leur forme la plus lumineuse, appelés quasars, dont la luminosité est plus importante que des centaines de fois celle d'une galaxie toute entière, ont été observés à très grand décalage spectral, lorsque l'Univers n'était alors âgé que d'un milliard d'années. Les modèles de formation des TNs doivent expliquer à la fois l'existence des TNs de faibles masses observés aujourd'hui dans les galaxies de faibles masses, mais aussi leur prodigieux homologues quasars dans l'Univers jeune. La formation des TNs pose encore de nos jours de nombreuses questions: comment se forment les TNs au début de l'histoire de l'Univers? Quelle est leur masse initiale? Quelle est la masse minimale d'une galaxie pour posséder un TN? Pour répondre à ces questions et pour étudier la formation des TNs dans le contexte de l'évolution des galaxies, nous avons utilisé des simulations hydrodynamiques cosmologiques, qui offrent l'avantage de suivre l'évolution temporelle de nombreux processus comme la formation stellaire, l'enrichissement en métaux, les mécanismes de rétroactions des TNs et des supernovae. J'ai particulièrement dirigé mes recherches sur les trois principaux modèles de formation des TNs à partir du reliquat des premières étoiles, d'amas d'étoiles, ou encore par effondrement direct. / Supermassive black holes (BHs) harboured in the center of galaxies have been confirmed with the discovery of Sagittarius A* in the center of our galaxy, the Milky Way. Recent surveys indicate that BHs of millions of solar masses are common in most local galaxies, but also that some local galaxies could be lacking BHs (e.g. NGC205, M33), or at least hosting low-mass BHs of few thousands solar masses. Conversely, massive BHs under their most luminous form are called quasars, and their luminosity can be up to hundred times the luminosity of an entire galaxy. We observe these quasars in the very early Universe, less than a billion years after the Big Bang. BH formation models therefore need to explain both the low-mass BHs that are observed in low-mass galaxies today, but also the prodigious quasars we see in the early Universe.BH formation is still puzzling today, and many questions need to be addressed: How are BHs created in the early Universe? What is their initial mass? How many BHs grow efficiently? What is the occurrence of BH formation in high redshift galaxies? What is the minimum galaxy mass to host a BH? We have used cosmological hydrodynamical simulations to capture BH formation in the context of galaxy formation and evolution. Simulations offer the advantage of following in time the evolution of galaxies, and the processes related to them, such as star formation, metal enrichment, feedback of supernovae and BHs. We have particularly focused our studies on the three main BH formation models: Pop III remnant, stellar cluster, and direct collapse models.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Quasinormal modes of plane-symmetric anti-de Sitter (AdS) black holes in d spacetime dimensions are investigated. Following the gauge invariant prescription developed by Kodama, Ishibashi and Seto (2000), fundamental equations for gravitational perturbation in such a background are constructed. Within such a prescription, metric perturbations naturally split into three disjoint classes. Namely, tensor, vector and scalar perturbations. However, different gauge invariant quantities are chosen in the present work, because they are more suited to the particular boundary conditions usually imposed to find quasinormal modes in AdS spacetimes than those used by Kodama, Ishibashi and Seto. In particular, the quantities used here present also the so called hydrodynamic modes, i. e., shear modes for vector perturbations and sound wave modes for the scalar ones, what is not found using the former quantities. It is also shown that there is just one shear mode, which does not depend upon the number of spacetime dimensions (d). Moreover, it is also found a general expression for the sound wave modes in terms of the number of the parameter d for scalar perturbations, and that there is no such a hydrodynamic mode for the tensor sector. Horowitz-Hubeny power series method is used in numerical analysis to find the dispersion relations for the first few quasinormal modes, and also for the hydrodynamic modes. This analysis is performed for five and six spacetime dimensions in the case of tensor perturbations, and for four, five and six dimensions in the cases of vector and scalar perturbations. The dispersion relations of regular modes present the same general behavior for all kinds of perturbations, Re(w) → q and Im(w) → 0 in the limit q → ∞, where w and q are the normalized frequency and the normalized wave number, respectively. / Investiga-se os modos quase-normais gravitacionais de buracos negros plano-simétricos anti-de Sitter em d dimensões, cuja geometria das seções espaciais é plana e cuja topologia pode ser plana, cilíndrica ou toroidal. Deduz-se equações fundamentais de perturbação gravitacional para este background, seguindo o formalismo invariante de gauge desenvolvido por Kodama, Ishibashi e Seto (2000), segundo o qual as perturbações métricas são naturalmente separadas em três setores ortogonais: tensorial, vetorial e escalar. Entretanto, são escolhidas diferentes quantidades invariantes de gauge tais que sob condições de contorno apropriadas fornecem os modos quase-normais hidrodinâmicos do buraco negro em questão. Particularmente, no limite hidrodinâmico, os modos de cisalhamento nas perturbações gravitacionais vetoriais e modos de onda sonora nas perturbações escalares são encontrados explicitamente. Mostra-se que o modo de cisalhamento é único e independe do número de dimensões, apresenta-se uma expressão para o modo de onda sonora válida para qualquer dimensão e verifica-se que as perturbações gravitacionais tensoriais não apresentam modos hidrodinâmicos. Utiliza-se o método de Horowitz-Hubeny para calcular numericamente os primeiros modos quase-normais comuns para cada setor de perturbação e apresentam-se as respectivas relações de dispersão Re(w) × q e Im(w)×q, onde w são as freqüências quase-normais e q é o número de onda normalizados. Também obtêm-se numericamente os modos hidrodinâmicos e suas relações de dispersão. Os modos quase-normais das perturbações tensoriais são calculados para buracos negros plano-simétricos anti-de Sitter em cinco e seis dimensões, e os modos quase-normais das perturbações vetoriais e escalares são calculados para buracos negros em quatro, cinco e seis dimensões. Observa-se que as relações de dispersão apresentam um comportamento geral onde Re(w) → q e Im(w) → 0 conforme q → ∞ independentemente do tipo de perturbação, número de dimensões e do modo quase-normal analisado.

The correspondence of stationary, axisymmetric, asymptotically flat space-times and bundles over a reduced twistor space has been established in four dimensions. The main impediment for an application of this correspondence to examples in higher dimensions is the lack of a higher-dimensional equivalent of the Ernst poten- tial. This thesis will propose such a generalized Ernst potential, point out where the rod structure of the space-time can be found in the twistor picture and thereby provide a procedure for generating solutions to the Einstein field equations in higher dimensions from the rod structure, other asymptotic data, and the requirement of a regular axis. Examples in five dimensions are studied and necessary tools are developed, in particular rules for the transition between different adaptations of the patching matrix and rules for the elimination of conical singularities.

The black hole information paradox arises when quantum mechanical effects are considered in the vicinity of the event horizon of a black hole. In this report we describe the fundamental properties of quantum mechanical systems and black holes that lead to the information paradox, with focus on quantum entanglement. While first presented in 1976, the information paradox is as of yet an unsolved problem. Two of the proposed solutions, black hole complementarity and firewalls, are discussed. / Svarta hålets informationsparadox uppkommer när man tar hänsyn till kvantmekaniska effekter i närheten av händelsehorisonten av ett svart hål. I denna rapport beskrivs de grundläggande egenskaper hos kvantmekaniska system och svarta hål som leder till informationsparadoxen, med fokus på kvantintrassling. Paradoxen, som presenterades 1976 men än idag är ett olöst problem, förklaras sedan. Två av de förslagna lösningarna till paradoxen, svarta hål-komplementaritet och firewalls, diskuteras.

One of the most persistent problem in solar physics is the identification of the mechanism that heats the solar corona and accelerates the fast solar wind. Magneto-hydrodynamic (MHD)waves play a crucial role in heating of the solar corona and acceleration of the solar wind. Different types of oscillations have been now observed by various instruments. These are interpreted as due to ubiquitous presence of MHD waves. The magnetic field plays a fundamental role in the propagation and properties of these MHD waves. The topology (structure)of the magnetic fields are different in different regions of the solar atmosphere viz., active regions (high-lying closed magnetic fields), quiet Sun (low-lying closed magnetic fields) and coronal holes (open magnetic fields). The purpose of this dissertation is to study the nature of these propagating MHD waves in different regions of the solar atmosphere. It is believed that polar coronal holes which connects the inner corona and the solar wind, are the source regions of the fast solar wind. The on-disk part of a polar coronal hole can be divided into network and internetwork regions. Long time series(sit-and-stare)data have been obtained from the SUMER/SoHO spectrometer in N iv 765Å and Ne viii 770Å spectral lines to search for the presence of waves in these two different regions from a statistical approach. The network bright regions indicate the presence of compressional waves with a dominant period of ≈ 25 min in both the lines. Moreover, we found that there is a difference in the nature of the wave propagation in the bright (‘network’), as opposed to the dark (‘internetwork’) regions, with the latter sometimes showing evidence of downwardly propagating waves that are not seen in the former. This is consistent with the magnetic topology, as open field lines are rooted in network regions whereas internetwork region has low lying closed field lines. From a measurement of propagation speeds, we found all waves are subsonic, indicating that the majority of them are slow magneto-acoustic in nature. The off-limb part of coronal holes can be divided into plume and inter-plume regions. The simultaneous observations were performed with EIS/Hinode and SUMER/SoHO spectrometer in Fe xii 195Å and Ne viii 770Å spectral lines respectively. We detected the presence of accelerating waves in a polar inter-plume region with a period of 15 min to 20 min in both the spectral lines and a propagation speed increasing from 130 ± 14 km s−1 just above the limb, to 330 ± 140 kms s−1 around 160” above the limb. These waves can be traced to originate from a bright region of the on-disk part of the coronal hole which can be visualized as the base of the coronal funnels. The adjacent plume region also shows the presence of propagating disturbance with the same range of periodicity but with propagation speeds in the range of 135 ± 18 kms s−1 to 165 ± 43 kms s−1 only. We found that the waves within the plumes are not observable (may be getting dissipated) far off-limb whereas this is not the case in the inter-plume region. We suggested that the waves are likely either Alfv´enic or fast magneto-acoustic in the inter-plume regions and slow magneto-acoustic in the plume regions. These results support the view that the inter-plume regions area preferred channel for the acceleration of the fast solar wind. The quiet Sun can be further divided into bright magnetic (network), bright non-magnetic and dark non-magnetic (internetwork) regions. Simultaneous observations were performed in Ca ii filtergram from SOT/Hinode, TRACE 1550Åpassband and with SUMER/SoHO spectrometer in N iv 765ÅandNe viii 770Åspectral lines to study the oscillations in these different regions. We detected the presence of long period oscillations with periods between 15 min to 30 min in bright magnetic regions. The oscillations were detected from chromospheric height to low coronal heights. Power maps showed that low period powers are mainly concentrated in dark regions whereas long period powers are concentrated in bright magnetic regions. We proposed that these 15 min and above periods can propagate up to the coronal heights through ‘magneto¬acoustic portals’. However in this case only with the spectral imaging data, it was not possible to identify the mode of wave propagation. To detect the presence of waves in active regions, we have analysed the imaging and spec¬troscopic data acquired during the total solar eclipse of 2006 and 2009 respectively. We found the oscillations of periods 27 s and 20 s in imaging data obtained in green (Fe xiv 5303Å) and red (Fe x 6374Å) coronal emission lines respectively. Significant oscillations with high proba¬bility estimates were detected at boundary of active region and in the neighbourhood, rather than within the loops itself. We also reported the detection of oscillations in intensity, velocity and line width having periods in the range of 25 s to 50 s with spectroscopic data again obtained in green and red coronal emission lines. These high frequency oscillations were interpreted in terms of presence of fast magneto-acoustic waves or torsional Alfv´en waves. These detected propagating MHD waves may carry sufficient energy to heat the corona and provide enough momenta to accelerate the fast solar wind. In addition, these waves may also provide input for the measurement of coronal magnetic field using the technique of ‘coronal seismology’.

Orientadores: Alberto Vazquez Saa, Orlando Luis Goulart Peres / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-24T13:09:57Z (GMT). No. of bitstreams: 1 Oliveira_ClaitonPimentelde_D.pdf: 6246469 bytes, checksum: 74e9bd5e915848a1681572fd38bcf297 (MD5) Previous issue date: 2014 / Resumo: Matéria, ao se acumular ao redor de um objeto compacto (e.g., um buraco negro), se configura naturalmente na forma de um disco grosso (toro) em rotação. A matéria do disco pode ser considerada como um fluido, e suas estruturas de equilíbrio hidrodinâmico podem ser obtidas a partir das equações básicas da hidrodinâmica. Nesse trabalho apresento uma extensa revisão da teoria básica de discos grossos de acreção, no âmbito das teorias clássica e relativística, incluindo uma análise da chamada órbita circular marginalmente estável. Formulo o problema incluindo a autointeração gravitacional do toro, caso em que o problema das estruturas de equilíbrio se torna um problema de fronteira livre, o que dificulta a obtenção das soluções. Reviso os métodos e técnicas numéricas já utilizadas ao se atacar esse problema e desenvolvo um código numérico próprio, chamado BLATOS, que gera soluções autogravitantes de toros ao redor de buracos negros. Desenvolvo ainda uma metodologia para se aplicar o método nodal dos elementos espectrais a domínios não limitados. O desenvolvimento desse novo tipo de elemento, os chamados elementos infinitos, gera uma extensão natural a elementos não limitados com bordas curvas assintóticas. Aplico as soluções numéricas obtidas no estudo da instabilidade runaway, mostrando como a identificação da situação de instabilidade pode ser feita a partir dessas soluções. A partir do código numérico é possível alterar o perfil de rotação e a razão das massas toro/buraco negro, de forma a se realizar um estudo do espaço de soluções / Abstract: Matter, accumulating around a compact object (e.g., a black hole), appears naturally in the form of a thick disk (torus) in rotation. The material of the disk can be considered as a fluid, and its hydrodynamic equilibrium structures can be obtained from the basic equations of hydrodynamics. In this work I present an extensive review of the basic theory of thick accretion disks, in the framework of the classical and relativistic theories, including an analysis of the so called marginally stable circular orbit. I formulate the problem including the torus self gravitational interaction, in which case the equilibrium structures problem becomes a free boundary problem, making it difficult getting the solutions. I revise the methods and numerical techniques used to attack this problem and I develop a numeric code, named BLATOS, that generates autogravitating tori solutions around black holes. Further, I develop a methodology for applying the nodal spectral element method to unbounded domains. The development of this new type of element, the so called infinite element, generates a natural extension to unbounded elements with asymptotic curved edges. I apply the resulting numerical solutions in the study of runaway instability, showing how the identification of the instability can be done from these solutions. The rotation law and the torus/black hole mass ratio can be changed from the numerical code in order to conduct a study of the solution space / Doutorado / Física / Doutor em Ciências

Neste trabalho estudamos a dinâmica de discos torcidos finos e espessos para compreender melhor a propagação da deformação nestes discos. No caso dos discos finos, estudamos a física do efeito Bardeen-Petterson e aplicamos este modelo para explicar o jato em escalas de parsec e kilo-parsec da galáxia NGC 1275. Encotramos que o efeito Bardeen-Petterson reproduziu muito bem a forma do jato e com isto derivamos os parâmetros do disco como raio, valores das viscosidades azimutal e vertical, lei de potência da densidade superficial e spin do buraco negro. Para uma melhor compreensão da física destes discos, realizamos simulações GRMHD de discos moderadamente finos tanto planos como inclinados para estudar a evolução do ângulo de inclinação entre os momentos angular do buraco negro e do disco de acresção assim como o ângulo de torção que está associado com a precessão do disco. Encontramos que quando o disco de acresção e o buraco negro rotacionam no mesmo sentido, o ângulo de inclinação entre os momentos angular apresentou um comportamento oscilatório na parte interna do disco e permaneceu constante na parte externa em acordo com as previsões teóricas. Já quando o buraco negro rotacina no sentido oposto ao disco de acresção, encontramos pela primeira vez numa simulação GRMHD evidências de alinhamento, ocorrendo um alinhamento de 10\\% do angulo entre os momentos angulares do disco e buraco negro. Além disso, comprovamos pela primeira vez numa simulação GRMHD a não isotropia do stress. Utilizando um modelo semi-analítico, comparamos os resultados de nossas simulações com este modelo, utilizando os dados da simulações de disco plano como entrada e obitivemos os mesmos comportamentos das simulações tanto no caso prógrado quanto no caso retrógrado mostrando que o alinhamento é devido ao regime onda. / In this work we studied the dynamics of twisted thin and thick disks to better understand how the warp propagates in these discs. In the case of thin discs, we studied the physics of the Bardeen-Petterson effect and we applied this model to explain the shape of the jet in both parsec and kilo-parsec scales of the galaxy NGC 1275. We found that the Bardeen-Petterson effect could explain very well the shape of the jet and with that we derived the disc parameters such as its radius, the values of the kinematic azimutal and vertical viscosities, the power-law of the surface density and the spin of the black hole. To better understand the physics of such discs, we have performed GRMHD simulations of moderatelly thin tilted disks to study the evolution of the tilt angle between the angular momentum of the accretion disk and black hole and also the twist angle which is associated with the precession of the disc. We found that when the accretion disc and the black hole are rotating in the same direction, the tilt angle showed an oscillatory behavior in the inner parts of the disk while in the outer parts it remained constant in agreement with the theorical modelos. However, when both rotate in the opposite direction, we found for the very first time in a GRMHD simulation, evidences of alignment of 10\\% of the tilt angle. Besides that, we prove for the first time in a GRMHD simulation that the stress is far from being isotropic. Using a semi-analitic model, we compared the results of our simulations with this model, using the datas of the untilted simulations as inputs and we found the same behaviors found in the simulations even in prograde case as in the retrograde case showing that the alignment is due to bending waves.

Philosophiae Doctor - PhD / Wireless sensor networks (WSNs) have become a popular research area that is widely gaining the attraction from both the research and the practitioner communities due to their wide area of applications. These applications include real-time sensing for audio delivery, imaging, video streaming, and remote monitoring with positive impact in many fields such as precision agriculture, ubiquitous healthcare, environment protection, smart cities and many other fields. While WSNs are aimed to constantly handle more intricate functions such as intelligent computation, automatic transmissions, and in-network processing, such capabilities are constrained by their limited processing capability and memory footprint as well as the need for the sensor batteries to be cautiously consumed in order to extend their lifetime. This thesis revisits the issue of the energy efficiency in sensor networks by proposing a novel clustering approach for routing the sensor readings in wireless sensor networks. The main contribution of this dissertation is to 1) propose corrective measures to the traditional energy model adopted in current sensor networks simulations that erroneously discount both the role played by each node, the sensor node capability and fabric and 2) apply these measures to a novel hierarchical routing architecture aiming at maximizing sensor networks lifetime. We propose three energy models for sensor network: a) a service-aware model that account for the specific role played by each node in a sensor network b) a sensor-aware model and c) load-balancing energy model that accounts for the sensor node fabric and its energy footprint. These three models are complemented by a load-balancing model structured to balance energy consumption on the network of cluster heads that forms the backbone for any cluster-based hierarchical sensor network. We present two novel approaches for clustering the nodes of a hierarchical sensor network: a) a distance-aware clustering where nodes are clustered based on their distance and the residual energy and b) a service-aware clustering where the nodes of a sensor network are clustered according to their service offered to the network and their residual energy. These approaches are implemented into a family of routing protocols referred to as EOCIT (Energy Optimization using Clustering Techniques) which combines sensor node energy location and service awareness to achieve good network performance. Finally, building upon the Ant Colony Optimization System (ACS), Multipath Routing protocol based on Ant Colony Optimization approach for Wireless Sensor Networks (MRACO) is proposed as a novel multipath routing protocol that finds energy efficient routing paths for sensor readings dissemination from the cluster heads to the sink/base station of a hierarchical sensor network. Our simulation results reveal the relative efficiency of the newly proposed approaches compared to selected related routing protocols in terms of sensor network lifetime maximization.