Flush-mounted cavity hot-wire probes have been around since two decades, but have typically not been applied as often compared to the traditional wall hot-wires mounted several wire diameters above the surface. While the latter suffer from heat conduction from the hot wire to the substrate in particular when used in air flows, the former is belived to significantly enhance the frequency response of the sensor. The recent work using a cavity hotwire by Gubian et al. (2019) came to the surprising conclusion that the magnitute of the fluctuating wall-shear stress τ+w,rms reaches an asymptotic value of 0.44 beyond the friction Reynolds number Re τ ∼ 600. In an effort to explain this result, which is at odds with the majority of the literature, the present work combines direct numerical simulations (DNS) of a turbulent channel flow with a cavity modelled using the immersed boundary method, as well as an experimental replication of the study of Gubian et al. in a turbulent boundary layer to explain how the contradicting results could have been obtained. It is shown that the measurements of the mentioned study can be replicated qualitatively as a result of measurement problems. We will present why cavity hot-wire probes should neither be used for quantitative nor qualitative measurements of wall-bounded flows, and that several experimental short-comings can interact to sometimes falsely yield seemingly correct results.

Historical accidents and experimental investigations have made apparent that blast loaded concrete members are prone to fail in brittle shear rather than a ductile flexure mode. Air blasts from accidental detonations or explosives may cause severe damage to buildings and infrastructure and it is of great importance that load carrying members can withstand the impulse that arise to avoid progressive collapses. The aims of this thesis are, through explanation of blast loads as dynamic loads acting on structures and measuring of the effects of blast loads on reinforced concrete beams with shear reinforcement, to understand the mechanisms governing shear failure. Two hypotheses are therefore tested: That for a reinforced concrete beam with shear reinforcement, the mechanism governing dynamic shear failure is similar to that of static shear failure and that blast induced shear failure in reinforced concrete beams can be prevented through a sufficient amount of shear reinforcement. To meet the stated aims and test the hypotheses, a literature study was conducted together with numerical simulations using explicit non-linear finite element analysis software LS-Dyna.Previous experimental investigations on blast loaded reinforced concrete beams have displayed a possible shift in failure mode from a ductile flexural failure at static loading to a brittle shear failure at dynamic loading. The shifting may be a property of higher exciting frequencies of blast loads, inducing modes of vibration with larger portions of shear energy. The results obtained from the numerical analyses indicated that an increased ratio of shear reinforcement reduces the risks of a brittle shear failure as well as decrease beam deflections and concrete strains, while increasing strains in the tensile reinforcement.Analysis of the shear capacity and shear reinforcement design through methods given in Eurocode 2 and FKR 2011 were considered as supplementary to the FE analysis. FKR 2011 provided accurate estimations of the maximum dynamic support reactions. Eurocode 2 uses a more conservative approach resulting in lower values of the design shear strength.The conclusions are that for the given beam and blast load, brittle shear failures may be prevented through reduction of the spacing and increase of the bar diameter of the shear reinforcement. The increased plastic strain of the tensile reinforcement as well as measurements of shear crack widths, support reactions and strains in the concrete suggests that the beams with large ratio of shear reinforcement exhibit more ductile behaviour without reaching failure. / Historiska händelser och experimentella undersökningar har gjort gällande att betongelement utsatta för luftstötvåg är benägna att gå till spröda skjuvbrott snarare än sega böjbrott. Luftstötvågor från oavsiktliga detonationer eller explosivt gods kan orsaka svåra skador på byggnader och anläggningar och det är därför viktigt att bärande strukturer kan motstå impulsen som uppstår för att fortskridande ras ska undvikas. Målen med detta examensarbete är att, genom att förklara det dynamiska beteendet hos luftstötvågor som belastar konstruktioner och mätning av de effekter som uppstår av luftstötvågor på armerad betong med tvärkraftsarmering, förstå de mekanismer som ligger till grund för skjuvbrott. Som påföljd testas två hypoteser: Att mekanismerna som ligger till grund för dynamiska skjuvbrott hos armerade betongbalkar med tvärkraftsarmering är liknande som för de som styr statiska skjuvbrott, och att skjuvbrott till följd av luftstötvågor kan förhindras genom att förse balken med tillräcklig tvärkraftsarmeringsinnehåll. För att nå målen och testa hypoteserna så genomfördes en litteraturstudie tillsammans med numeriska simuleringar med explicit icke-linjär finita elementanalys i kommersiella programvaran LS-Dyna. Tidigare experimentella undersökningar av armerade betongbalkar utsatta för luftstötvåg har visar på en förflyttning i brottmod, från sega böjbrott vid statisk belastning till spröda skjuvbrott vid dynamisk belastning. Förflyttningen kan härstamma från luftstötvågens frekvensinnehåll som framkallar högre vibrationsmoder med större andel skjuvenergi.De erhållna resultaten från de numeriska analyserna indikerar att ett ökat tvärkraftsarmeringsinnehåll minskar risken för spröda skjuvbrott, minskar utböjning och töjningar I balken samtidigt som töjningar i dragarmeringen ökar.Analyser av tvärkraftskapaciteten och dimensionering av tvärkraftsarmering genom metoder givna i Eurocode 2 och FKR 2011 användes som komplement till finita elementanalysen. FKR 2011 gav träffsäkra uppskattningar av de dynamiska störreaktionerna grundade i dynamisk jämvikt. Emellertid var Eurocode 2 det mer konservativa tillvägagångssättet, vilket resulterade i lägre dimensionerande tvärkraftskapacitet. Slutsatserna som drogs var att, för den givna balken och luftstötvågen så kunde spröda skjuvbrott motverkas genom minskning av avstånden mellan tvärkraftsarmeringen och ökning av stångdiametern. De ökade plastiska töjningarna i dragarmeringen tillsammans med mätningar av skjuvsprickornas bredd, stödreaktionerna och töjningarna i betongen föreslår att balkarna med högt tvärkraftsarmeringsinnehåll visar på ett segare beteende utan att gå till brott.

<p>We use numerical simulations to examine the fragmentation of protostellar discs via gravitational instability (GI), a proposed formation mechanism for gas-giant planets and brown dwarfs. To accurately model heating and cooling, we have implemented radiative transfer (RT) in the TreeSPH code Gasoline, using the flux-limited diffusion approximation coupled to photosphere boundary cooling. We present 3D radiation hydrodynamics simulations of discs that are gravitationally unstable in the inner 40 AU; these discs do not fragment because the cooling times are too long. In prior work, one of these discs was found to fragment; however, we demonstrate that this resulted from an over-estimate of the photosphere cooling rate. Fragmentation via GI does not appear to be a viable formation mechanism in the inner 40 AU.</p> <p>We also present simulations of GI in the outer regions of discs, near 100 AU, where we find GI to be a viable formation mechanism. We give a detailed framework that explains the link between cooling and fragmentation: spiral arms grow on a scale determined by the linear gravitational instability, have a characteristic width determined by the balance of heating and cooling, and fragment if this width is less than twice their Hill radius. This framework is consistent with the fragmentation and initial fragment masses observed in our simulations. We apply the framework to discs modelled with the commonly-used beta-prescription cooling and calculate the critical cooling rate for the first time, with results that are consistent with previous estimates measured from numerical experiments.</p> <p>RT is fundamentally important in the star formation process. Non-ionizing radiation heats the gas and prevents small-scale fragmentation. Ionizing radiation from massive stars is an important feedback mechanism and may disrupt giant molecular clouds. We present methods and tests for our implementation of ionizing radiation, using the Optically-Thin Variable Eddington Tensor method.</p> / Doctor of Philosophy (PhD)

<p>The formation of star clusters occurs in a complex environment and involve a large number of physical processes. One of the most important processes to consider is radiative feedback. The radiation released by forming stars heats the surrounding gas and suppresses the fragmentation of low mass objects. Ionizing radiation can also drive large scale outflows and disperse the surrounding gas. Owing to all this complexity, the use of numerical simulations to study cluster formation in molecular clouds has become commonplace. In order to study the effects of radiative feedback on cluster formation over larger spatial scales than previous studies, we present hydrodynamical simulations using the AMR code FLASH which make use of cluster particles. Unlike previous studies, these particles represent an entire star cluster rather than individual stars. We present a subgrid model for representing the radiative output of a star cluster which involves randomly sampling an IMF over time to populate the cluster. We show that our model is capable of reproducing the properties of observed clusters. The model was then incorporated into FLASH to examine the effects of radiative feedback on cluster formation in full hydrodynamical simulations. We find that the inclusion of radiative transfer can drive large scale outflows and decreases the overall star formation efficiency by a factor of 2. The inclusion of radiative feedback also increases the degree of subclustering. The use of cluster particles in hydrodynamical simulations represents a promising method for future studies of cluster formation and the large scale effects of radiative feedback.</p> / Master of Science (MSc)

Mergers are known to be essential in the formation of large scale structures and to have a significant role in the history of galaxy formation and evolution. Besides a morphological transformation, mergers induce important bursts of star formation. These starburst are characterised by high Star Formation Efficiencies (SFEs) and Specific Star Formation Rates, i.e., high Star Formation Rates (SFR) per unit of gas mass and high SFR per unit of stellar mass, respectively, compared to spiral galaxies. At all redshifts, starburst galaxies are outliers of the sequence of star-forming galaxies defined by spiral galaxies. We have investigated the origin of the starburst-mode of star formation, in three local interacting systems: Arp 245, Arp 105 and NGC7252. We combined high-resolution JVLA observations of the 21-cm line, tracing the Hi diffuse gas, with UV GALEX observations, tracing the young star-forming regions. We probe the local physical conditions of the Inter- Stellar Medium (ISM) for independent star-forming regions and explore the atomic-to-dense gas transformation in different environments. The SFR/H i ratio is found to be much higher in central regions, compared to outer regions, showing a higher dense gas fraction (or lower Hi gas fraction) in these regions. In the outer regions of the systems, i.e., the tidal tails, where the gas phase is mostly atomic, we find SFR/H i ratios higher than in standard Hi-dominated environments, i.e., outer discs of spiral galaxies and dwarf galaxies. Thus, our analysis reveals that the outer regions of mergers are characterised by high SFEs, compared to the standard mode of star formation. The observation of high dense gas fractions in interacting systems is consistent with the predictions of numerical simulations; it results from the increase of the gas turbulence during a merger. The merger is likely to affect the star-forming properties of the system at all spatial scales, from large scales, with a globally enhanced turbulence, to small scales, with possible modifications of the initial mass function. From a high-resolution numerical simulation of the major merger of two spiral galaxies, we analyse the effects of the galaxy interaction on the star forming properties of the ISM at the scale of star clusters. The increase of the gas turbulence is likely able to explain the formation of Super Star Clusters in the system. Our investigation of the SFR–H i relation in galaxy mergers will be complemented by highresolution Hi data for additional systems, and pushed to yet smaller spatial scales.

À ce jour, les différentes méthodes de reconstruction des mouvements du plasma à la surface du Soleil qui ont été proposées présupposent une MHD idéale (Welsch et al., 2007). Cependant, Chae & Sakurai (2008) ont montré l’existence d’une diffusivité magnétique turbulente à la photosphère. Nous introduisons une généralisation de la méthode du Minimum Energy Fit (MEF ; Longcope, 2004) pour les plasmas résistifs. Le Resistive Minimum Energy Fit (MEF-R ; Tremblay & Vincent, 2014) reconstruit les champs de vitesse du plasma et la diffusivité magnétique turbulente qui satisfont à l’équation d’induction magnétique résistive et qui minimisent une fonctionnelle analogue à l’énergie cinétique totale. Une séquence de magnétogrammes et de Dopplergrammes sur les régions actives AR 9077 et AR 12158 ayant chacune produit une éruption de classe X a été utilisée dans MEF-R pour reconstruire les mouvements du plasma à la surface du Soleil. Les séquences temporelles des vitesses et des diffusivités magnétiques turbulentes calculées par MEF-R sont comparées au flux en rayons X mous enregistré par le satellite GOES-15 avant, pendant et après l’éruption. Pour AR 12158, nous observons une corrélation entre les valeurs significatives de la diffusivité magnétique turbulente et de la vitesse microturbulente pour les champs magnétiques faibles. / To this day, the various methods proposed for the reconstruction of plasma motions at the Sun’s surface are all based on ideal MHD (Welsch et al., 2007). However, Chae & Sakurai (2008) have shown the existence of an eddy magnetic diffusivity at the photosphere. We introduce a generalization of the Minimum Energy Fit (MEF; Longcope, 2004) for resistive plasmas. The Resistive Minimum Energy Fit (MEF-R; Tremblay & Vincent, 2014) infers velocity fields and an eddy magnetic diffusivity which solve the resistive magnetic induction equation and minimize an energy-like functional. A sequence of magnetograms and Dopplergrams documenting the active regions AR 9077 and AR 12158 are used as input in MEF-R to reconstruct plasma motions at the Sun’s surface. Time series of the inferred velocities and eddy magnetic diffusivities are compared to the soft X-ray flux observed by GOES-15. We find a positive correlation between significant eddy magnetic diffusivities and microturbulent velocities for weak magnetic fields in AR 12158.

Ce manuscrit se concentre sur l’étude de la dynamique de drainage de gouttes d’eau dans une phase continue d’huile de paraffine. Les gouttes sont de taille millimétrique, déformables et évoluent dans un domaine de simulation carré de 1 cm de coté. La simulation du comportement de tels systèmes pose le problème général de la description numérique des écoulements multiphasiques non stationnaires. Un modèle simplifié dans une géométrie à deux dimensions est proposé et simulé en volumes finis. Il inclut les propriétés physico-chimiques des interfaces et notamment les phénomènes de coalescence et l’évolution d’un tensioactif soluble dans les gouttes. L’effet des conditions aux limites sur le drainage d’une unique goutte est étudié. Le rôle de la coalescence sur ce drainage est également décrit pour un modèle de deux gouttes. Quelques simulations sont enfin proposées avec des systèmes dispersés plus complexes. / This manuscript focuses on the description of the settling dynamics of water droplets in a continuous phase of paraffin oil. Droplets are of millimetre size, deformable and evolve in a square simulation domain of 1 cm side. The simulations of the behaviour of such systems raise the general problem of the numerical description of the flows occurring in multiphase unsteady systems. A simplified model in a two dimensional geometry is used and integrated with a finite volume numerical technique. It includes the interfacial mechanical and chemical properties and in particular the coalescence phenomena and the evolution of a water soluble surfactant. The effect of the boundary conditions on the drainage of a unique droplet is studied. The role of drop-drop coalescence on this drainage is also described for a model with two droplets. Some simulations are finally proposed with more complex dispersed systems.

In this thesis we investigated two major issues in astrophysical flows: the transport of magnetic fields in highly conducting fluids in the presence of turbulence, and the turbulence evolution and turbulent dynamo amplification of magnetic fields in collisionless plasmas. The first topic was explored in the context of star-formation, where two intriguing problems are highly debated: the requirement of magnetic flux diffusion during the gravitational collapse of molecular clouds in order to explain the observed magnetic field intensities in protostars (the so called \"magnetic flux problem\") and the formation of rotationally sustained protostellar discs in the presence of the magnetic fields which tend to remove all the angular momentum (the so called \"magnetic braking catastrophe\"). Both problems challenge the ideal MHD description, usually expected to be a good approximation in these environments. The ambipolar diffusion, which is the mechanism commonly invoked to solve these problems, has been lately questioned both by observations and numerical simulation results. We have here investigated a new paradigm, an alternative diffusive mechanism based on fast magnetic reconnection induced by turbulence, termed turbulent reconnection diffusion (TRD). We tested the TRD through fully 3D MHD numerical simulations, injecting turbulence into molecular clouds with initial cylindrical geometry, uniform longitudinal magnetic field and periodic boundary conditions. We have demonstrated the efficiency of the TRD in decorrelating the magnetic flux from the gas, allowing the infall of gas into the gravitational well while the field lines migrate to the outer regions of the cloud. This mechanism works for clouds starting either in magnetohydrostatic equilibrium or initially out-of-equilibrium in free-fall. We estimated the rates at which the TRD operate and found that they are faster when the central gravitational potential is higher. Also we found that the larger the initial value of the thermal to magnetic pressure ratio (beta) the larger the diffusion process. Besides, we have found that these rates are consistent with the predictions of the theory, particularly when turbulence is trans- or super-Alfvénic. We have also explored by means of 3D MHD simulations the role of the TRD in protostellar disks formation. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low mass stars. Previous studies showed that an enhanced microscopic diffusivity of about three orders of magnitude larger than the Ohmic diffusivity would be necessary to enable the formation of a rotationally supported disk. However, the nature of this enhanced diffusivity was not explained. Our numerical simulations of disk formation in the presence of turbulence demonstrated the efficiency of the TRD in providing the diffusion of the magnetic flux to the envelope of the protostar during the gravitational collapse, thus enabling the formation of rotationally supported disks of radius ~ 100 AU, in agreement with the observations. The second topic of this thesis has been investigated in the framework of the plasmas of the intracluster medium (ICM). The amplification and maintenance of the observed magnetic fields in the ICM are usually attributed to the turbulent dynamo action which is known to amplify the magnetic energy until close equipartition with the kinetic energy. This is generally derived employing a collisional MHD model. However, this is poorly justified a priori since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless-MHD description. We have studied here the turbulence statistics and the turbulent dynamo amplification of seed magnetic fields in the ICM using a single-fluid collisionless-MHD model. This introduces an anisotropic thermal pressure with respect to the direction of the local magnetic field and this anisotropy modifies the MHD linear waves and creates kinetic instabilities. Our collisionless-MHD model includes a relaxation term of the pressure anisotropy due to the feedback of the mirror and firehose instabilities. We performed 3D numerical simulations of forced transonic turbulence in a periodic box mimicking the turbulent ICM, assuming different initial values of the magnetic field intensity and different relaxation rates of the pressure anisotropy. We showed that in the high beta plasma regime of the ICM where these kinetic instabilities are stronger, a fast anisotropy relaxation rate gives results which are similar to the collisional-MHD model in the description of the statistical properties of the turbulence. Also, the amplification of the magnetic energy due to the turbulent dynamo action when considering an initial seed magnetic field is similar to the collisional-MHD model, particularly when considering an instantaneous anisotropy relaxation. The models without any pressure anisotropy relaxation deviate significantly from the collisional-MHD results, showing more power in small-scale fluctuations of the density and velocity field, in agreement with a significant presence of the kinetic instabilities; however, the fluctuations in the magnetic field are mostly suppressed. In this case, the turbulent dynamo fails in amplifying seed magnetic fields and the magnetic energy saturates at values several orders of magnitude below the kinetic energy. It was suggested by previous studies of the collisionless plasma of the solar wind that the pressure anisotropy relaxation rate is of the order of a few percent of the ion gyrofrequency. The present study has shown that if this is also the case for the ICM, then the models which best represent the ICM are those with instantaneous anisotropy relaxation rate, i.e., the models which revealed a behavior very similar to the collisional-MHD description. / Nesta tese, investigamos dois problemas chave relacionados a fluidos astrofísicos: o transporte de campos magnéticos em plasmas altamente condutores na presença de turbulência, e a evolução da turbulência e amplificação de campos magnéticos pelo dínamo turbulento em plasmas não-colisionais. O primeiro tópico foi explorado no contexto de formação estelar, onde duas questões intrigantes são intensamente debatidas na literatura: a necessidade da difusão de fluxo magnético durante o colapso gravitacional de nuvens moleculares, a fim de explicar as intensidades dos campos magnéticos observadas em proto-estrelas (o denominado \"problema do fluxo magnético\"), e a formação de discos proto-estelares sustentados pela rotação em presença de campos magnéticos, os quais tendem a remover o seu momento angular (a chamada \"catástrofe do freamento magnético\"). Estes dois problemas desafiam a descrição MHD ideal, normalmente empregada para descrever esses sistemas. A difusão ambipolar, o mecanismo normalmente invocado para resolver estes problemas, vem sendo questionada ultimamente tanto por observações quanto por resultados de simulações numéricas. Investigamos aqui um novo paradigma, um mecanismo de difusão alternativo baseado em reconexão magnética rápida induzida pela turbulência, que denominamos reconexão turbulenta (TRD, do inglês turbulent reconnection diffusion). Nós testamos a TRD através de simulações numéricas tridimensionais MHD, injetando turbulência em nuvens moleculares com geometria inicialmente cilíndrica, permeadas por um campo magnético longitudinal e fronteiras periódicas. Demonstramos a eficiência da TRD em desacoplar o fluxo magnético do gás, permitindo a queda do gás no poço de potencial gravitacional, enquanto as linhas de campo migram para as regiões externas da nuvem. Este mecanismo funciona tanto para nuvens inicialmente em equilíbrio magneto-hidrostático, quanto para aquelas inicialmente fora de equilíbrio, em queda livre. Nós estimamos as taxas em que a TRD opera e descobrimos que são mais rápidas quando o potencial gravitacional é maior. Também verificamos que quanto maior o valor inicial da razão entre a pressão térmica e magnética (beta), mais eficiente é o processo de difusão. Além disto, também verificamos que estas taxas são consistentes com as previsões da teoria, particularmente quando a turbulência é trans- ou super-Alfvénica. Também exploramos por meio de simulações MHD 3D a influência da TRD na formação de discos proto-estelares. Sob condições MHD ideais, a remoção do momento angular do disco progenitor pelo campo magnético da nuvem pode evitar a formação de discos sustentados por rotação durante a fase principal de acreção proto-estelar de estrelas de baixa massa. Estudos anteriores mostraram que uma super difusividade microscópica aproximadamente três ordens de magnitude maior do que a difusividade ôhmica seria necessária para levar à formação de um disco sustentado pela rotação. No entanto, a natureza desta super difusividade não foi explicada. Nossas simulações numéricas da formação do disco em presença de turbulência demonstraram a eficiência da TRD em prover a diffusão do fluxo magnético para o envelope da proto-estrela durante o colapso gravitacional, permitindo assim a formação de discos sutentados pela rotação com raios ~ 100 UA, em concordância com as observações. O segundo tópico desta tese foi abordado no contexto dos plasmas do meio intra-aglomerado de galáxias (MIA). A amplificação e manutenção dos campos magnéticos observados no MIA são normalmente atribuidas à ação do dínamo turbulento, que é conhecidamente capaz de amplificar a energia magnética até valores próximos da equipartição com a energia cinética. Este resultado é geralmente derivado empregando-se um modelo MHD colisional. No entanto, isto é pobremente justificado a priori, pois no MIA o caminho livre médio de colisões íon-íon é da ordem das escalas dinâmicas, requerendo então uma descrição MHD não-colisional. Estudamos aqui a estatística da turbulência e a amplificação por dínamo turbulento de campos magnéticos sementes no MIA, usando um modelo MHD não-colisional de um único fluido. Isto indroduz uma pressão térmica anisotrópica com respeito à direção do campo magnético local. Esta anisotropia modifica as ondas MHD lineares e cria instabilidades cinéticas. Nosso modelo MHD não-colisional inclui um termo de relaxação da anisotropia devido aos efeitos das instabilidades mirror e firehose. Realizamos simulações numéricas 3D de turbulência trans-sônica forçada em um domínio periódico, mimetizando o MIA turbulento e considerando diferentes valores iniciais para a intensidade do campo magnético, bem como diferentes taxas de relaxação da anisotropia na pressão. Mostramos que no regime de plasma com altos valores de beta no MIA, onde estas instabilidades cinéticas são mais fortes, uma rápida taxa de relaxação da anisotropia produz resultados similares ao modelo MHD colisional na descrição das propriedades estatísticas da turbulência. Além disso, a amplificação da energia mangética pela ação do dínamo turbulento quando consideramos um campo magnético semente, é similar ao modelo MHD colisional, particularmente quando consideramos uma relaxação instantânea da anisotropia. Os modelos sem qualquer relaxação da anisotropia de pressão mostraram resultados que se desviam significativamente daqueles do MHD colisional, mostrando mais potências nas flutuações de pequena escala da densidade e velocidade, em concordância com a presença significativa das instabilidades cinéticas nessas escalas; no entanto, as flutuações do campo magnético são, em geral, suprimidas. Neste caso, o dínamo turbulento também falha em amplificar campos magnéticos sementes e a energia magnética satura em valores bem abaixo da energia cinética. Estudos anteriores do plasma não-colisional do vento solar sugeriram que a taxa de relaxação da anisotropia na pressão é da ordem de uma pequena porcentagem da giro-frequência dos íons. O presente estudo mostrou que, se este também é o caso para o MIA, então os modelos que melhor representam o MIA são aqueles com taxas de relaxação instantâneas, ou seja, os modelos que revelaram um comportamento muito similar à descrição MHD colisional.

In this thesis, we aim to further elucidate the phenomenon of galaxy evolution in the environment of galaxy clusters using the methodology of numerical simulations. For that, we have developed hydrodynamic models in which idealized gas-rich galaxies move within the ICM of idealized galaxy clusters, allowing us to probe in a detailed and controlled manner their evolution in this extreme environment. The main code used in our simulations is RAMSES, and our results concern the changes in gas composition, star formation rate, luminosity and color of infalling galaxies. Additionally to processes taking place inside the galaxies themselves, we have also described the dynamics of the gas that is stripped from those galaxies with unprecedented resolution for simulations of this nature (122 pc in a box including an entire 1e14 Msun cluster), finding that clumps of molecular gas are formed within the tails of ram pressure stripped galaxies, which proceed to live in isolation within the ICM of a galaxy cluster for up to 300 Myr. Those molecular clumps possibly represent a new class of objects; similar objects have been observed in both galaxy clusters and groups, but no comprehensive description of them has been given until now. We additionally create a hydrodynamic model for the A901/2 multi-cluster system, and correlate the gas conditions in this model to the locations of a sample of candidate jellyfish galaxies in the system; this has allowed us to infer a possible mechanism for the generation of jellyfish morphologies in galaxy cluster collisions in general. / Nesta tese, nós visamos a contribuir para o entendimento do fenômeno da evolução de galáxias no ambiente de aglomerados de galáxias usando a metodologia de simulações numéricas. Para isso, desenvolvemos modelos hidrodinâmicos nos quais galáxias idealizadas ricas em gás movem-se em meio ao gás difuso de aglomerados de galáxias idealizados, permitindo um estudo detalhado e controlado da evolução destas galáxias neste ambiente extremo. O principal código usado em nossas simulações é o RAMSES, e nossos resultados tratam das mudanças em composição do gás, taxa de formação estelar, luminosidade e cor de galáxias caindo em aglomerados. Adicionalmente a processos acontecendo dentro das próprias galáxias, nós também descrevemos a dinâmica do gás que é varrido dessas galáxias com resolução sem precedentes para simulações dessa natureza (122 pc em uma caixa incluindo um aglomerado de 1e14 Msun inteiro), encontrando que aglomerados de gás molecular são formados nas caudas de galáxias que passaram por varrimento de gás por pressão de arraste, aglomerados estes que procedem a viver em isolamento em meio ao gás difuso de um aglomerado de galáxias por até 300 Myr. Esses aglomerados moleculares possivelmente representam uma nova classe de objetos; objetos similares foram previamente observados tanto em aglomerados quanto em grupos de galáxias, mas um tratamento compreensivo deles não foi apresentado até agora. Nós adicionalmente criamos um modelo hidrodinâmico para o sistema multi-aglomerado A901/2, e correlacionamos as condições do gás nesse modelo com a localização de uma amostra de galáxias jellyfish nesse sistema; isso nos permitiu inferir um possível mecanismo para a geração de morfologias jellyfish em colisões de aglomerados de galáxias em geral.

Dans cette thèse, nous étudions la formation de domaines ferroélectriques dans des nanostructures, à partir d'une modélisation faisant intervenir les équations de Ginzburg-Landau et d’Électrostatique, ainsi que des conditions aux limites d'application potentielle. Dans la première partie de la thèse, les nanostructures sont constituées d'une couche ferroélectrique entièrement enclavée dans un environnement paraélectrique. Nous introduisons un modèle depuis un couplage de ces équations et élaborons, pour son investigation, un schéma numérique faisant usage d’Éléments Finis. Des simulations numériques montrent l'efficacité de ce schéma, qui permet d'établir, par exemple, l'existence de cycles d'hystérésis sous l'influence de paramètres aussi bien physiques que géométriques. Dans la seconde partie, les nanostructures sont constituées d'une couche ferroélectrique partiellement enclavée qui s'intercale entre deux couches paraélectriques. Deux modèles sont proposés à partir d'une variante du couplage réalisé dans la première partie, et se distinguent dans la prescription des conditions aux limites. Des conditions de type Neumann interviennent dans le premier modèle, pour lequel un schéma numérique aussi basé sur des approximations par Eléments Finis est introduit. Dans le second modèle, des conditions périodiques sont prises en considération ; un schéma numérique s'appuyant ici sur une hybridation des méthodes de Différences Finies et d'Eléments Finis est présenté. Les simulations numériques basées sur ces deux schémas permettent de renseigner sur les permittivités dites effectives, des nanostructures, ou encore sur la constitution des parois de domaines ferroélectriques / In this thesis, we study the formation of ferroelectric domains in nanostructures by modeling based on the Ginzburg-Landau and Electrostatics equations, together with boundary conditions that are suitable for real applications. In the first part of the thesis, the nanostructures are made up of a ferroelectric layer, fully enclosed in a paraelectric environment. We introduce a model based on the coupled system of equations and then develop, for its investigation, a numerical scheme using Finite Elements. Numerical simulations show the efficiency of this scheme, which allows us to establish, for instance, the existence of hysteresis cycles under the influence of physical or geometric parameters. In the second part, the nanostructures are made up of a partially enclosed ferroelectric layer that lies between two paraelectric layers. Two models are introduced from a variant of the coupling performed in the first part, and differ in the prescription of the boundary conditions. Neumann type conditions are prescribed in the first model, for which a numerical scheme also based on Finite Element approximations is developed. In the second model, periodic conditions are taken into account; a numerical scheme based on a combination of Finite Difference and Finite Element methods is presented. Numerical simulations from these schemes allow us, for instance, to investigate the so-called effective permittivities, of the nanostructures, or the formation of ferroelectric domain walls