Im Rahmen dieser Dissertation wird die Stärke der Multibunch-Instabilität (MBI), die von longitudinalen Dipol-, transversalen Dipol- und Quadrupolschwingungsmoden höherer Ordnung getrieben werden mithilfe von analytischen Rechnungen und Trackingsimulationen in dem Ausbauprojekt BESSY Variable Pulse Length Storage Ring (BESSY VSR) des Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB) untersucht und mit dem vorhanden aktiven Dämpfungssystem verglichen. Algorithmen für Trackingsimulationen werden hergeleitet und eine halbempirische Formel zur Abschätzung der transversalen quadrupolaren MBI wird präsentiert. MBI Studien bilden einen wesentlichen Teil der Beurteilung der BESSY VSR Hohlraumresonatoren und begleiteten und beeinflussten ihren Entwicklungsprozess. Mit Berechnungen auf Grundlage des neusten BESSY VSR Hohlraumresonatormodels kann Strahlstabilität als wahrscheinlich und unabhängig vom Füllmuster angesehen werden. Des weiteren wurden Messungen der MBI an BESSY II und der Metrology Light Source (MLS) der Physikalisch-Technischen Bundesanstalt durchgeführt, bei welcher die longitudinale langreichende Impedanz charakterisiert wurde. Transiente Strahllast wird in dieser Arbeit mit analytischen Formeln und neuen, experimentell überprüften Trackingsimulationen berechnet. Für das Standardfüllmuster von BESSY VSR wurde gezeigt, dass die besondere Konfiguration der Hohlraumresonatorfrequenzen zu einer relativ starken Beeinflussung der langen Elektronenpakete führt. Diese verkürzt das Elektronenpaket und vergrößert die Touschekverluste. / In the scope of this thesis, the strength of coupled bunch instabilities (CBIs) driven by longitudinal monopole higher order modes (HOMs) and transverse dipole and quadrupole HOMs is evaluated for the upgrade project BESSY Variable Pulse Length Storage Ring (BESSY VSR) at Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB), based on analytic calculations and tracking simulations, and compared to the performance of an active bunch-by-bunch feedback (BBFB). Algorithms for tracking codes are derived, and a semi-empirical formula for the estimation of transverse quadrupole CBIs is presented. CBI studies are an integral part of the benchmarking of the cavity models for BESSY VSR and have been accompanying and influencing their entire design process. Based on the BESSY VSR cavity model with highly advanced HOM damping, beam stability is likely to be reached with a BBFB system, independent of the bunch fill pattern. Additionally, measurements of CBIs have been performed at BESSY II and the Metrology Light Source of the Physikalisch-Technische Bundesanstalt (MLS), where the longitudinal long range impedance was characterized. Transient beam loading is evaluated by means of analytic formulas and new experimentally verified tracking codes. For the baseline bunch fill pattern of BESSY VSR, it is shown that the particular setup of cavity frequencies amplifies the transient effect on the long bunch, limiting its elongation and potentially resulting in increased Touschek losses.

L’objectif de ce sujet de thèse est de prédire l’apparition des instabilités plastiques (striction diffuse et striction localisée) dans les matériaux métalliques. Ces matériaux sont décrits par des modèles de comportement élasto-plastique couplés à l’endommagement. L'approche de Lemaitre, reliant l'endommagement à la déformation plastique équivalente et au taux de restitution de la densité d'énergie élastique, est adoptée. Parmi les différents critères et indicateurs qui sont considérés pour la prédiction des instabilités matériau, la théorie de bifurcation et les critères de type force maximum sont tout particulièrement analysés et comparés. Un objectif important de cette étude consiste à déterminer les mécanismes déstabilisants clés associés à cette modélisation du comportement, ainsi que l’impact des différents aspects physiques et des paramètres matériau sur l’apparition de la striction. Les développements résultants sont appliqués à une sélection représentative de matériaux métalliques afin prédire leurs limites de formabilités. Cette approche combinant des lois de comportement et critères de striction peut être utilisée comme outil théorique et numérique d’aide à la conception de nouveaux matériaux à ductilité améliorée / The aim of the present work is to predict the occurrence of plastic instabilities (diffuse and localized necking) in thin sheet metals. The prediction of these plastic instabilities is undertaken using an elastic–plastic model coupled with ductile damage, which is then combined with various plastic instability criteria theory. The bifurcation-based criteria and the maximum force criterion used in this work are formulated within a general three-dimensional modeling framework, and then applied for the particular case of plane-stress conditions for sheet metals. Some theoretical relationships or links between the different investigated necking criteria are established, which allows a hierarchical classification in terms of their conservative character in predicting critical necking strains. The resulting numerical tool is implemented into the finite element code ABAQUS/Standard to predict forming limit diagrams, in both situations of a fully three-dimensional formulation and a plane-stress framework. This approach, that combines constitutive equations to necking criteria, serves as a useful tool in the design of new materials with improved ductility

Dans le cadre de la fusion par confinement inertiel, nous présentons une analyse des effets du champ magnétique sur le développement linéaire des instabilités de Richtmyer-Meshkov, en magnétohydrodynamique idéale d’une part, et de Rayleigh-Taylor au front d’ablation, dans les phases d’accélération et de décélération d’autre part.A l’aide d’un code linéaire de perturbation, nos simulations mono mode nous permettent de confirmer, pour l’instabilité de Richtmyer-Meshkov, la stabilisation apportée par la composante du champ magnétique parallèle au vecteur d’onde des perturbations de l’interface, dont l’amplitude oscille au cours du temps. Nous montrons que la prise en compte de la compressibilité n’apporte pas de changements significatifs par rapport au modèle impulsionnel incompressible existant dans la littérature. Dans nos simulations numériques bidimensionnelles, en géométrie plane, de l’instabilité de Rayleigh-Taylor dans la phase d’accélération, nous prenons en compte le phénomène d’auto-génération de champ magnétique induite par cette instabilité. Nous montrons qu’il est possible d’atteindre des valeurs de champ de l’ordre de quelques teslas et que la croissance de l’amplitude des perturbations transite plus rapidement vers un régime de croissance non-linéaire avec, notamment, un développement accru de la troisième harmonique. Nous proposons également une adaptation d’un modèle existant, étudiant l’effet d’anisotropie de conductivité thermique sur le taux de croissance de l’instabilité de Rayleigh-Taylor au front d’ablation, pour tenter de prendre en compte les effets des champs magnétiques auto-générés sur le taux de croissance de l’instabilité de Rayleigh-Taylor. Enfin, dans une étude numérique à deux dimensions, en géométrie cylindrique, nous analysons les effets des champs magnétiques auto-générés par l’instabilité de Rayleigh-Taylor dans la phase de décélération. Cette dernière étude révèle l’apparition de champs magnétiques pouvant atteindre plusieurs milliers de teslas sans pour autant affecter le comportement de l’instabilité de Rayleigh-Taylor. / In the context of inertial confinement fusion we investigate effects of magnetic fields on the development in the linear regime of two hydrodynamic instabilities: Richtmyer-Meshkov instability using ideal magnetohydrodynamics and ablative Rayleigh-Taylor instability in both acceleration and deceleration stages.Direct numerical simulations with a linear perturbation code enable us to confirm the stabilizing effect of the component of the magnetic field along the perturbations wave vector. The amplitude doesn’t grow linearly in time but experiences oscillations instead. The compressibility taken into account in the code does not affect predictions given by an already existing impulsive and incompressible model.As far as Rayleigh-Taylor instability is concerned we study the effects of self-generated magnetic fields that arise from the development of the instability itself. In the acceleration stage we perform two dimensional simulations in planar geometry. We show that magnetic fields of about 1T can be generated and that the instability growth transits more rapidly into nonlinear growth with the enhancement of the development of the third harmonic. We also propose an adaptation of an existing model that aims at studying thermal conductivity anisotropy effects, to take into account the effects of the self-generated magnetic fields on the Rayleigh-Taylor instability growth rate.Finally, in the deceleration stage, we perform two dimensional simulations in cylindrical geometry that take into account self-generation of magnetic fields due to the instability development. It reveals magnetic fields of about several thousands of teslas that are not strong enough though to affect the instability behavior.

Cette recherche se focalise sur les problèmes d’instabilités de combustion hautes fréquences dans les moteurs fusées. Ces instabilités sont connues pour avoir des effets néfastes et peuvent, dans certains cas, causer la destruction du système propulsif. Pour éviter l’apparition de ces instabilités, il est important de connaître les mécanismes qui entretiennent ces phénomènes dynamiques et de comprendre le couplage complexe entre l’injection, la combustion et la résonnance acoustique du système. Ce travail comprend trois parties.La première partie traite de la simulation numérique de jets non-réactifs et réactifs soumis à différentes conditions de modulation afin de comprendre les interactions entre les jets, les flammes et leur environnement. Les calculs numériques de jets ronds non-réactifs ainsi que des flammes plus complexes formées par des injecteurs coaxiaux dans des conditions transcritiques ont été effectuées avec des simulations aux grandes échelles (SGE), adaptées aux conditions gaz réels à l’aide du solveur AVBP-RG. Les jets ronds ont été soumis à des fluctuations de vitesse transverse. Il a été trouvé que pour toutes les amplitudes et fréquences de modulation, le jet est déformé et oscille dans la direction transverse. Ce comportement peut être représenté par un modèle. Les flammes coaxiales ont été soumises à une modulation de débit et de pression. La modulation induit des variations du dégagement de chaleur global. Un modèle mathématique reliant les paramètres modulés au dégagement de chaleur est proposé.La seconde partie contient les travaux expérimentaux. Dans ce cadre, un nouveau banc expérimental a été développé pour l’étude de cavités couplées pressurisées (NPCC). Le couplage entre le plénum (ou dôme) et la chambre a été étudié. Un modèle reliant les fluctuations de pression et de vitesse en sortie des injecteurs a été développé et comparés aux données d’essais. Le banc NPCC a aussi été utilisé pour acquérir plus de connaissances sur le niveau d’amortissement. Les coefficients d’amortissement ont été déterminés.La dernière partie de ce document traite du développement d’un modèle ordre réduit qui représente des mécanismes qui entretiennent et amortissent les instabilités de combustion hautes fréquences. Cette description dynamique a été incorporée dans un code de stabilité haute fréquence (STAHF). Ce code a été utilisé pour étudier un moteur à ergols liquides d’une puissance de 87 MW (le banc BKD du DLR en Allemagne) qui présente des instabilités hautes fréquences. Après le recalage de certains paramètres de contrôle, STAHF a été capable de retrouver des résultats obtenus d’essais au DLR. / This research concerns some of the issues raised by high frequency combustion instabilities in rocket engines. These instabilities are known to have detrimental effects leading, in some cases, to the destruction of the propulsion system. To avoid the appearance of such instabilities it is important to gain an understanding of the processes driving such dynamical phenomena. One has to consider the complex coupling between injection, combustion and the acoustic resonances of the system. The present work contributes to this objective by developing three items.The first deals with numerical simulations of non-reactive and reactive jets submitted to different modulation conditions to understand the interaction between jets, flames and their environment. Numerical simulations of non-reactive round jets as well as more complex flames formed by coaxial injectors operating under transcritical conditions were carried out using large eddy simulation (LES) adapted to real gas situations by making use of the AVBP-RG flow solver. Round jets were submitted to transverse velocity fluctuations. It has been found that for all amplitudes and frequencies of modulation, the modulated jet is deformed and oscillates. This behavior can be represented by a model. The coaxial flames were submitted to mass flow rate and pressure modulation. For these cases it has been found that the modulation induces variations of the global heat release rate. A mathematical relationship between the modulated parameters and the heat release rate has been proposed.The second item includes experimental investigations. For this purpose a New Pressurized Coupled Cavities (NPCC) laboratory test rig has been developed. The possible coupling between the plenum and the thrust chamber was studied. A model, linking pressure and velocity fluctuations between the plenum and the thrust chamber, has been developed. The laboratory test rig was also used to gather some knowledge on the levels of damping and the damping coefficients could be determined.The last item of this document deals with the development of a reduced order dynamical model which includes some of the driving and damping mechanisms of high frequency combustion instabilities. This dynamical description was implemented in a high frequency stability code (STAHF). This code was used to examine a 87 MW liquid rocket engine (BKD operated at DLR, Germany) exhibiting high frequency oscillations. After the adjustment of some control parameters, STAHF was able to retrieve some the features observed in experiments carried out at DLR.

The coronal mass ejections (CMEs) are phenomena that evidence the complex solar activity. During the CME evolution in the solar wind (SW) the shock and sheath (Sh) are established. With these, the transfer of energy and shock thermalization have origin through several processes like instabilities and particle acceleration. Here, we present two studies related to CMEs. In the first study, we analyze the existence of the KelvinHelmholtz instability (KHI) at the interfaces CMESh and ShSW. For this purpose, we assumed two CMEs that propagate independently in the slow and fast SW. We model velocities, densities and magnetic field strengths of sheaths, and SW in the CMEs flanks, in order to solve the Chandrasekhar condition for the magnetic KHI existence. Our results reveal that KHI formation is more probably in the CME that propagate in the slow SW than in CME propagating in the fast SW. It is due to large shear flow between the CME and the slow SW. Besides we find that the interface ShSW is more susceptible to the instability. In the second study, we examine the distributions of particle acceleration and turbulence regions around CME-driven shocks with wave-like features. We consider these corrugated shock as the result of disturbances from the bimodal SW, CME deflection, irregular CME expansion, and the ubiquitous fluctuations in the solar corona. We model smooth CME-driven shocks using polar Gaussian profiles. With the addition of wave-like functions, we obtain the corrugated shocks. For both shock types are calculated the shock normal angles between the shock normal and the radial upstream coronal magnetic field in order to classify the quasi-parallel and quasi-perpendicular regions linked to the particle acceleration and turbulence regions, respectively. Our calculations show the predisposition of the shock to the particle acceleration and indicate that the irregular CME expansion is the relevant factor in the particle acceleration process. We consider that these wave-like features in shocks may be essential in the study of current problems as injection particle, instabilities, downstream-jets, and shock thermalization. / As ejeções de massa coronal (do inglês coronal mass ejections, CMEs) são consideradas traçadores da atividade solar. Durante a evolução das CMEs no vento solar (do inglês solar wind, SW), o choque e o envoltório (do inglês sheath, Sh) são estabelecidos. Nesta fase, a transferência da energia e a termalização do choque podem ter origem através de vários processos, entre eles instabilidades e aceleração de partculas. Aqui nós apresentamos dois estudos relacionados às CMEs. No primeiro estudo, analisamos a existência da instabilidade KelvinHelmholtz (KHI) nas interfaces CMESh e ShSW. Para isto, supomos duas CMEs que se propagam independentemente no SW lento e rápido. Modelamos as velocidades, densidades e a intensidade do campo magnético dos envoltórios e SW nos flancos das CMEs, a fim de resolver a condição de Chandrasekhar para a existência da KHI magnética. Nossos resultados revelam que a formação da KHI pode ser mais provável na CME que se propaga no SW lento do que na CME que se propaga no SW rápido. Isto é devido a um maior cisalhamento entre a CME e o SW lento. Além disso, encontramos que a interface ShSW é ser mais suscetvel à instabilidade. No segundo estudo, examinamos as distribuições das regiões de aceleração de partculas e turbulência em choques ondulados com caractersticas semelhantes a ondas. Assumimos choques ondulados como resultado de perturbações do SW bimodal, deflexão da CME, expansão irregular da CME, e flutuações onipresentes na coroa solar. Construmos choques sem ondulações usando perfis Gaussianos. Com adição de funções semelhantes a ondas, obtemos os choques ondulados. Para ambos tipos de choques, calculamos os ângulos entre o vector normal ao choque e o campo magnético coronal radial, assim classificamos as regiões como quase-paralelas e quase-perpendiculares que são ligadas às regiões de aceleração de partculas e turbulência, respectivamente. Nossos cálculos mostram a predisposição do choque para o fenômeno de acceleração de partculas, e indicam que a expansão irregular da CME é o fator de maior relevância neste processo. Consideramos que assumir ondulações nos choques pode ser essencial nos estudos de problemas atuais como injeção de partculas, instabilidades, jatos e termalização dos choques.

The coronal mass ejections (CMEs) are phenomena that evidence the complex solar activity. During the CME evolution in the solar wind (SW) the shock and sheath (Sh) are established. With these, the transfer of energy and shock thermalization have origin through several processes like instabilities and particle acceleration. Here, we present two studies related to CMEs. In the first study, we analyze the existence of the KelvinHelmholtz instability (KHI) at the interfaces CMESh and ShSW. For this purpose, we assumed two CMEs that propagate independently in the slow and fast SW. We model velocities, densities and magnetic field strengths of sheaths, and SW in the CMEs flanks, in order to solve the Chandrasekhar condition for the magnetic KHI existence. Our results reveal that KHI formation is more probably in the CME that propagate in the slow SW than in CME propagating in the fast SW. It is due to large shear flow between the CME and the slow SW. Besides we find that the interface ShSW is more susceptible to the instability. In the second study, we examine the distributions of particle acceleration and turbulence regions around CME-driven shocks with wave-like features. We consider these corrugated shock as the result of disturbances from the bimodal SW, CME deflection, irregular CME expansion, and the ubiquitous fluctuations in the solar corona. We model smooth CME-driven shocks using polar Gaussian profiles. With the addition of wave-like functions, we obtain the corrugated shocks. For both shock types are calculated the shock normal angles between the shock normal and the radial upstream coronal magnetic field in order to classify the quasi-parallel and quasi-perpendicular regions linked to the particle acceleration and turbulence regions, respectively. Our calculations show the predisposition of the shock to the particle acceleration and indicate that the irregular CME expansion is the relevant factor in the particle acceleration process. We consider that these wave-like features in shocks may be essential in the study of current problems as injection particle, instabilities, downstream-jets, and shock thermalization. / As ejeções de massa coronal (do inglês coronal mass ejections, CMEs) são consideradas traçadores da atividade solar. Durante a evolução das CMEs no vento solar (do inglês solar wind, SW), o choque e o envoltório (do inglês sheath, Sh) são estabelecidos. Nesta fase, a transferência da energia e a termalização do choque podem ter origem através de vários processos, entre eles instabilidades e aceleração de partculas. Aqui nós apresentamos dois estudos relacionados às CMEs. No primeiro estudo, analisamos a existência da instabilidade KelvinHelmholtz (KHI) nas interfaces CMESh e ShSW. Para isto, supomos duas CMEs que se propagam independentemente no SW lento e rápido. Modelamos as velocidades, densidades e a intensidade do campo magnético dos envoltórios e SW nos flancos das CMEs, a fim de resolver a condição de Chandrasekhar para a existência da KHI magnética. Nossos resultados revelam que a formação da KHI pode ser mais provável na CME que se propaga no SW lento do que na CME que se propaga no SW rápido. Isto é devido a um maior cisalhamento entre a CME e o SW lento. Além disso, encontramos que a interface ShSW é ser mais suscetvel à instabilidade. No segundo estudo, examinamos as distribuições das regiões de aceleração de partculas e turbulência em choques ondulados com caractersticas semelhantes a ondas. Assumimos choques ondulados como resultado de perturbações do SW bimodal, deflexão da CME, expansão irregular da CME, e flutuações onipresentes na coroa solar. Construmos choques sem ondulações usando perfis Gaussianos. Com adição de funções semelhantes a ondas, obtemos os choques ondulados. Para ambos tipos de choques, calculamos os ângulos entre o vector normal ao choque e o campo magnético coronal radial, assim classificamos as regiões como quase-paralelas e quase-perpendiculares que são ligadas às regiões de aceleração de partculas e turbulência, respectivamente. Nossos cálculos mostram a predisposição do choque para o fenômeno de acceleração de partculas, e indicam que a expansão irregular da CME é o fator de maior relevância neste processo. Consideramos que assumir ondulações nos choques pode ser essencial nos estudos de problemas atuais como injeção de partculas, instabilidades, jatos e termalização dos choques.

Dans le contexte de la fusion par confinement inertiel (FCI), le lissage optique est une technique utilisée pour obtenir une irradiation laser aussi homogène que possible, en modifiant les propriétés de cohérence temporelle et spatiale des faisceaux laser. L'utilisation du lissage optique est une nécessité sur les lasers de puissance comme le Laser MégaJoule (LMJ) pour limiter le développement des instabilités paramétriques issues de l'intéraction laser-plasma, et parmi elles, la rétrodiffusion Brillouin stimulée (RBS). Ces instabilités entraînent des défauts d'irradiation sur cible et peuvent aussi être une source d'endommagement dans la chaîne optique. Cependant ces techniques peuvent entraîner d'autres problèmes au niveau de la chaîne laser, tels que la conversion de modulation de phase en modulation d'amplitude (FM-AM), néfastes au bon déroulement des expériences et pouvant également endommager les chaînes laser.On comprend donc qu'il est nécessaire de trouver un compromis autour du lissage optique. L’évolution du compromis du lissage est cependant compliquée car la quantification des gains et des pertes est très difficile à établir. Ainsi, tant que la quantification n’est pas faite, le compromis n’évolue pas : le lasériste souhaite toujours moins de lissage et « l’expérimentateur » toujours plus de lissage mais aucun des deux ne peut apporter suffisamment d’éléments quantitatifs pour faire pencher la balance. Cette thèse propose donc de poser les premières briques permettant d'arriver à ce compromis pour le LMJ, à l'aide d'études théoriques et numériques.Nous comparons soigneusement le lissage longitudinal (LSSD) et transversal (TSSD) par dispersion spectrale dans une configuration de lissage idéale pour chaque cas. Avec des codes 3D, nous avons simulé la RBS dans un plasma d'or, typique des expériences de FCI et favorable au développement de la RBS. Nous montrons que, contrairement aux idées reçues, l'évolution temporelle de la RBS présente certaines différences entre les deux systèmes de lissage. Premièrement, les valeurs asymptotiques des niveaux de saturation ne sont pas tout à fait les mêmes. Avec une simple description des rayons et le calcul du gain RBS pour chaque rayon, nous avons pu expliquer cette différence. En outre, la dynamique de la RBS est également quelque peu différente. Nous avons montré que la dynamique RBS est déterminée par l'évolution temporelle des propriétés des surintensités et en particulier par la longueur d'interaction effective entre la lumière rétrodiffusée Brillouin et les points chauds. Cette longueur d'interaction effective dépend à la fois de la vitesse longitudinale et de la longueur des points chauds. En effet, la synchronisation des longueurs d'interaction effectives des deux schémas de lissage synchronise également la croissance des courbes de rétrodiffusion avant saturation.Nous montrons, également qu'il est possible de faire évoluer les paramètres de lissage du LMJ en illustrant une nouvelle façon de réduire la conversion FM-AM inévitablement présente dans les lasers de forte puissance. En répartissant le spectre total habituellement utilisé par un quadruplet (regroupement de 4 faisceaux), en deux parties de spectres identiques plus petits sur les faisceaux de gauche et de droite, la conversion FM en AM est considérablement réduite de 30% à 5% tout en maintenant la performance de lissage pour la RBS. Nous avons également montré que le temps de cohérence qui en résulte n'a aucun effet sur le niveau maximal de RBS atteint. De la même façon, il faudra étudier l'impact de ces évolutions sur d'autres instabilités telles que le diffusion Raman stimulée ou le transfert d'énergie par croisement de faisceaux. / In the context of inertial confinement fusion (ICF), optical smoothing is a technique used to obtain the most homogeneous laser irradiation possible, by modifying the temporal and spatial coherence properties of the laser beams. The use of optical smoothing is a necessity on high-power lasers such as the Laser Mégajoule (LMJ) to limit the development of parametric instabilities resulting from laser-plasma interaction, and among them, stimulated Brillouin backscattering (SBS). These instabilities lead to target irradiation defects and can also be a source of damage in the optical lines. However, these techniques can lead to other problems in the laser lines, such as the conversion of phase modulation to amplitude modulation (FM-to-AM), which is harmful to the proper conduct of the experiments and can also damage the laser optics.It is therefore a necessity to find a compromise around optical smoothing. The evolution of the smoothing compromise is however complicated because the quantification of gains and losses is very difficult to establish. Thus, as long as quantification is not done, the compromise does not evolve: the laserist always wants less smoothing and the experimentalist always more smoothing, but neither of them can bring enough quantitative elements to tip the balance. This thesis therefore proposes to lay the first groundwork for reaching this compromise for the LMJ, using theoretical and numerical studies.We carefully compare longitudinal (LSSD) and transverse (TSSD) smoothing by spectral dispersion in an ideal smoothing configuration for each case. With 3D codes, we simulated SBS in a gold plasma, typical of ICF experiments and favourable to the development of SBS. We show that, contrary to popular belief, the temporal evolution of SBS shows some differences between the two smoothing schemes. First, the asymptotic values of saturation levels are not quite the same. With a simple description using light rays and the calculation of the SBS gain for each ray, we were able to explain this difference. In addition, the dynamics of SBS are also somewhat different. We have shown that the SBS dynamics is determined by the temporal evolution of the properties of the hot-spots and in particular by the effective interaction length between the Brillouin backscattered light and the hot-spots. This effective interaction length depends on both the longitudinal velocity and the length of the hot-spots. Indeed, the synchronization of the effective interaction lengths of the two smoothing schemes also synchronizes the growth of the backscatter curves before saturation.We also show that it is possible to change the smoothing parameters of the LMJ by illustrating a new way to reduce the FM-to-AM conversion inevitably present in high-power lasers. By splitting the total spectrum usually used by a quadruplet (grouping of 4 beams) into two parts of smaller identical spectra on the left and right beams, the FM-to-AM conversion is significantly reduced from 30% to 5% while maintaining the smoothing performance for SBS. We have also shown that the resulting coherence time of the laser has no effect on the maximum level of SBS achieved. Similarly, the impact of these developments on other instabilities such as stimulated Raman scattering or crossed beam energy transfer will also need to be investigated.

It is well-known that, increasing the flow rate in polymer extrusion, the flow becomes unstable and the smooth extrudate surface becomes wavy and disordered to an increasing degree. In order to investigate the mechanisms responsible for these instabilities we perform a linear stability analysis of the steady extrusion of a viscoelastic fluid flowing through a planar die under creeping flow conditions. We consider the Phan-Thien-Tanner (PTT) model to account for the viscoelasticity of the material. We employ the mixed finite element method combined with an elliptic grid generator to account for the deformable shape of the interface. The generalized eigenvalue problem is solved using Arnoldi’s algorithm. We perform a thorough parametric study in order to determine the effects of all material properties and rheological parameters. We investigate in detail the effect of interfacial tension and the presence of a deformable interface. It is found that the presence of a finite surface tension destabilizes the flow as compared to the case of the stick-slip flow. We recognize two modes which are found to become unstable beyond a critical value of the Weissenberg number and perform an energy analysis to examine the mechanisms responsible for the destabilization of the flow and compare against the mechanisms that have been suggested in the literature. / --

Current lean-premixed fuel injector designs have shown great potential in terms of reducing emissions of pollutants, but such designs are susceptible to combustion instabilities in which aerodynamic instability plays a major role and also has an effect on mixing of air and fuel. In comparison to prototype testing with combustors running in operating conditions, computational approaches such as Large Eddy Simulations (LES) offer a much more cost-effective alternative in the design stage. However, computational models employed by LES require validation by experimental data. This is one of the main motivations behind the present experimental study. Combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) instrumentation allowed simultaneous measurements of velocity vector and a conserved scalar introduced into the fuel stream. The results show that the inner swirl shear layer features two pairs of vortices, which draw high concentration fuel mixture from the central jet into the swirl stream and causes it to rotate in their wakes. Such periodic entrainment also occurs with the characteristic frequencies of the vortices. This has clear implications for temporal variations in fuel/air ratio in a combusting flow; these bursts of mixing, and hence heat release, could be a possible cause of mixing-induced pressure oscillation in combusting tests. For the first time in such a flow, all 3 components of the turbulent scalar flux were available for validation of LES-based predictions. A careful assessment of experimental errors, particularly the error associated with spatial filtering, was carried out. Comparison of LES predictions with experimental data showed very good agreement for both 1st and 2nd moment statistics, as well as spectra and scalar pdfs. It is particularly noteworthy that comparison between LES computed and measured scalar fluxes was very good; this represents successful validation of the simple (constant Schmidt number) SGS model used for this complex and practically important fuel injector flow. In addition to providing benchmark data for the validation of LES predictions, a new experimental technique has been developed that is capable of providing spatially resolved residence time data. Residence times of combustors have commonly been used to help understand NOx emissions and can also contribute to combustion instabilities. Both the time mean velocity and turbulence fields are important to the residence time, but determining the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three dimensional nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using a time-resolved PLIF technique, but a second camera for PIV was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures. Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after injection is stopped: this is equivalent to the sum of the convection time and the half-life values. The technique was applied to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core, where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NOx production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales.

No description available.