Cette thèse porte sur la résolution numérique du modèle multi-dimensionnel d'écoulement cisaillé en eau peu profonde. Dans le cas d'un mouvement unidimensionnel, ces équations coïncident avec les équations de la dynamique de gaz pour un choix particulier de l'équation d'état. Dans le cas multi-dimensionnel, le système est complètement différent du modèle de la dynamique de gaz. Il s'agit d'un système EDP hyperbolique 2D non-conservatif qui rappelle un modèle de turbulence barotrope. Le modèle comporte trois types d'ondes correspondant à la propagation des ondes de surface, des ondes de cisaillement et à celle de la discontinuité de contact. Nous présentons dans le cas 2D un schéma numérique basé sur une nouvelle approche de ``splitting" pour les systèmes d'équations non-conservatives. Chaque sous-système ne contient qu'une seule famille d'ondes: ondes de surface ou ondes de cisaillement, et discontinuité de contact. La précision d'une telle approche est testée sur des solutions exactes 2D décrivant l'écoulement lorsque la vitesse est linéaire par rapport aux variables spatiales, ainsi que sur des solutions décrivant des trains de rouleaux 1D. Finalement, nous modélisons un ressaut hydraulique circulaire formé dans un écoulement convergent radial d'eau. Les résultats numériques obtenus sont clairement similaires à ceux obtenus expérimentalement: oscillations du ressaut et son rotation avec formation du point singulier. L'ensemble des validations proposées dans ce manuscrit démontre les aptitudes du modèle et de la méthode numérique pour la résolution des problèmes complexes d'écoulements cisaillés en eau peu profonde multidimensionnels. / This thesis is devoted to the numerical modelling of multi-dimensional shear shallow water flows. In 1D case, the corresponding equations coincide with the equations describing non--isentropic gas flows with a special equation of state. However, in the multi-D case, the system differs significantly from the gas dynamics model. This is a 2D hyperbolic non-conservative system of equations which is reminiscent of a generic Reynolds averaged model of barotropic turbulent flows. The model has three families of characteristics corresponding to the propagation of surface waves, shear waves and average flow (contact characteristics). First, we show the ability of the one-dimensional conservative shear shallow water model to predict the formation of roll-waves from unstable initial data. The stability of roll waves is also studied.Second, we present in 2D case a new numerical scheme based on a splitting approach for non-conservative systems of equations. Each split subsystem contains only one family of waves (either surface or shear waves) and contact characteristics. The accuracy of such an approach is tested on exact 2D solutions describing the flow where the velocity is linear with respect to the space variables, and on the solutions describing 1D roll waves. Finally, we model a circular hydraulic jump formed in a convergent radial flow of water. Obtained numerical results are qualitatively similar to those observed experimentally: oscillation of the hydraulic jump and its rotation with formation of a singular point. These validations demonstrate the capability of the model and numerical method to solve challenging multi--dimensional problems of shear shallow water flows.

La compréhension et l’amélioration de l’environnement vibratoire des charges utiles demande la mise au point de démarches prédictives maîtrisées qui permettent de comprendre les phénomènes de transmission des ondes de chocs d’origine pyrotechnique dans le lanceur Ariane5. Plus particulièrement, la maîtrise du comportement transitoire des coques sandwichs en nid d’abeilles, principaux constituants de l’Adaptateur de Charges Utiles – structure porteuse des satellites, est nécessaire pour prédire les vibrations au pied des équipements électroniques des satellites et des lanceurs. Cette problématique présente un caractère multi-échelle tant d’un point de vue temporel (charge mobile supersonique, temps d’analyse) que spatial (dimensions des structures du lanceur, taille des cellules en nid d’abeilles, longueurs d’ondes liées aux hautes fréquences). Celui-ci a été traité dans cette thèse en s’appuyant d’une part, sur une qualification à la fois analytique et numérique des modèles classiques homogénéisés des plaques sandwichs en nid d’abeilles pour la gamme de fréquence mise en jeu et d’autre part, sur une application des stratégies de remaillage adaptatif pour la propagation des ondes développées dans le cadre de la méthode de Galerkin espace-temps discontinue en temps. Deux catégories de modèles de plaques épaisses ont été ainsi construites dans le but d’enrichir la cinématique classique de plaques épaisses de Mindlin-Reissner qui s’est avérée être insuffisante pour correctement représenter le comportement dynamique hors-plan des plaques sandwich en nid d’abeilles. Ainsi ont été analysés les modèles dits monocouches basés sur un enrichissement de la cinématique par ajout de degrés de liberté dans l’épaisseur, et les modèles multicouches composés d’une superposition de trois plaques avec une homogénéisation séparée des matériaux. Il a été montré que ces deux sortes de modèles améliorent la description des phénomènes de hautes fréquences, notamment ceux de flexion et de cisaillement transverse qui sont plus délicats à retranscrire. Toutes les études numériques ont été effectuées avec un code éléments finis qui emploie des solveurs adaptatifs dynamiques basés sur la méthode de Galerkin espace-temps discontinue en temps. Cette méthode d’intégration en temps introduit un amortissement numérique dépendant du pas de temps et qui peut interférer avec un amortissement physique susceptible d’être introduit dans un modèle numérique et conduire au final à un amortissement total différent de celui qui est attendu. Cette interaction a été analysée et mise en évidence dans ce travail à travers l’introduction de l’amortissement de Rayleigh dans les modèles de propagation de chocs. Les outils et les modèles de propagation ainsi développés ont été validés sur plusieurs structures académiques et industrielles. Des comparaisons avec des données expérimentales sur des structures industrielles de grande taille, plus particulièrement sur un Adaptateur de Charges Utiles d’Ariane5, sont effectuées et soulignent la cohérence de notre approche ainsi que la fiabilité et l’efficacité des modèles de propagation proposés. / Reliable and efficient numerical models for the pyrotechnic shock wave propagation in structures of the Ariane5 launcher are necessary for a good understanding and a predictive analysis of the payload vibration environment. More precisely, the correct modeling of the dynamic behaviour of the honeycomb sandwich shells, the main material composing the payload adaptor, is essential to control the vibration environment of the payload and the embarked electronic equipments and so to prevent them from damages caused by the shock wave propagation. The topic is obviously a multi-scale problem from both temporal and spatial points of view : short time intervals imposed by supersonic moving loads vs. large total time interval that the slowest waves need to travel throughout the adaptor ; very short wavelengths of high frequency waves, and very small size of the honeycomb cells vs. large structure dimensions. To take into account all involved space-time scales in a reliable and efficient way, the herein study is based both on the analytical and numerical qualification of the classical homogenized models of honeycomb sandwich shells for the frequency range introduced by the pyrotechnic shock wave, and on a dynamic solver based on the well-known space-time discontinuous Galerkin method, allowing the use of adaptive remeshes for the wave propagation. The classical Mindlin-Reissner’s kinematics of thick plates being inefficient to correctly represent the dynamic out-of-plane behaviour of the honeycomb sandwich plates, two kinds of its enrichment are considered : One-layered models based on an enrichment of the kinematics by adding degrees of freedom in the thickness, and multi-layered models composed of a superposition of three plates with separated material homogenisations. It has been shown theoretically and numerically that, both types of enrichment allow more precise descriptions of flexure and transverse shear modes in the high frequency range. However, the multi-layered models give much more promising results, as the important role played by the honeycomb core for the transverse shear behaviour of the whole sandwich is not “smeared” in a one-layered homogenized model. All the numerical studies were conducted with a finite element code which uses a dynamic solverbased on the time discontinuous space-time Galerkin method. The built-in numerical damping of this solver can interfere with a physical damping potentially introduced by the numerical model and results in a global damping totally unexpected. This interaction has been analysed and underlined in this work thanks to the introduction of the Rayleigh damping in the shock wave propagation models. Theoretical and numerical tools and propagating models thus developed have been validated on several academic and industrial structures. Comparison with experimental data on large size industrial structures, especially a real size payload adaptor, is performed and emphasizes the coherence of our approach and the reliability and the efficiency of the proposed propagating models.

O efeito da terapia por ondas de choque (TOC) na consolidação óssea, após osteotomias bilaterais dos fêmures e osteossínteses com hastes bloqueadas, foi estudado em 8 cães. Os fêmures direitos constituíram o grupo controle e os esquerdos, o grupo tratado, que recebeu 2000 pulsos de ondas de choque de 18 kV, energia de 5mJ (-6dB), na linha da fratura. Radiografias após 4, 8 e 12 semanas, revelaram maior proliferação periosteal no grupo tratado. Os resultados dos exames cintilográficos (razão tratado/controle), realizados na 2a, 4a, 6a, 8a, 10a e 12a semanas, foram estatisticamente superiores no grupo tratado. A TOC provocou aumento da atividade osteogênica na consolidação de fraturas agudas / The effect of extracorporeal shock wave therapy (ESWT) on bone healing, after bilateral femoral osteotomies and osteosynthesis with interlocking nails, was studied in 8 dogs. The right femurs composed the control group, and the left femurs, the treated group, which received 2000 pulses of shock wave, with 18 kV, 5 mJ (-6dB) energy on the fracture line. Radiographs after 4, 8 and 12 weeks, revealed increased periosteal proliferation in the treated group. The results (treated/control ratio) of the scintigraphic exams, performed on weeks 2, 4, 6, 8, 10 and 12, were statistically higher for the treated group. The ESWT led to increased osteogenic activity on acute fracture's bone healing.

Shock wave-boundary layer interactions (SWBLIs) have been studied ex-tensively due to their practical importance in the design of high speed ve-hicles. These interactions, especially the ones leading to shock induced separation are typically unsteady in nature and can lead to large fluctuating pressure and thermal loads on the structure. The resulting shock oscil-lations are generally composed of high-frequency small-scale oscillations and low-frequency large-scale oscillations, the source of the later being a subject of intense recent debate. Motivated by these debates, we study in the present work, the SWBLI at a compression ramp and on a forward-facing step (FFS) at a Mach number of 2.5. In the case of compression ramps, a few ramp angles are studied ranging from small (10 degree) ramp angle to relatively large values of up to 28 degrees. The FFS configuration, which consists of a 90 degree step of height h, may be thought of as an extreme case of the compression ramp geometry, with the main geometri-cal parameter here being (h/δ), where δis the thickness of the oncoming boundary layer. This configuration is less studied and has some inherent advantages for experimentally studying SWBLI as the size of the separa-tion bubble is large. In the present experimental study, we use high-speed schlieren, unsteady wall pressure measurements, surface oil flow visualiza-tion, and detailed particle image velocimetry (PIV) measurements in two orthogonal planes to help understand the features of SWBLI in the com-pression ramp geometry and the forward-facing step case. The SWBLI at a compression ramp has been more widely studied, and our measurements show the general features that have been seen in earlier studies. The upstream boundary layer is found to separate close to the ramp corner forming a separation bubble. The streamwise length of the separa-tion bubble is found to increase with the ramp angle, with a consequent shift of the shock foot further upstream. At very small ramp angles up to 10 degrees, there is no evidence of separation, while at large ramp angles of 28 degrees, the separation bubble extends upstream to about 3.5δ(δ=boundary layer thickness). In all cases, the separation bubble is however very small in the wall normal direction, typically known to be about 0.1δ, and hence is difficult to directly measure in experiments using PIV. Shock foot measurements using PIV show that the shock has a spanwise ripple, which seems directly related to the high-and low-speed streaks in the in-coming boundary layer as recently shown by Ganapathisubramani et al. (2007). The forward-facing step configuration may be thought of as an extreme case of the compression ramp geometry, with a ramp angle of 90 degrees. This configuration has not been extensively studied, and is experimentally convenient due to the large separation bubbles formed ahead of the step. In the present work, extensive measurements of the mean and unsteady flow around this configuration have been done, especially for the case of h/δ=2, where his the step height. Pressure measurements in this case, show clear low-frequency motions of the shock at non-dimensional frequencies of about fh/U∞≈ 0.02. In this case, PIV measurements show the pres-ence of a large mean separation bubble extending to about 4hupstream and about 1hvertically. Instantaneous PIV measurements have been done in both cross-stream (streamwise and wall-normal plane) and in the span-wise (streamwise-spanwise) plane. Instantaneous cross-stream PIV mea-surements show significant variations of the shock location and angle, be-sides large variations in the recirculation region (or separation bubble), this being determined as the area having streamwise velocities less than zero. From a large set of individual PIV instantaneous fields, we can estimate the correlation of the measured shock location to both downstream effects like the area of the recirculation region, and upstream effects like the presence of high-/low-speed streaks in the oncoming boundary layer. We find that the shock location measured from data outside the boundary layer is more highly correlated to downstream effects as measured through the recircu-lation area compared to upstream effects in the boundary layer. However, we find that the shock foot within the boundary layer has ripples in the spanwise direction which are well correlated to the presence of high-/low-speed streaks in the incoming boundary layer. These spanwise ripples are however found to be small (less than one h) compared to the highly three-dimensional shape of the recirculation region with spanwise variation of the order of 3 step heights. In summary, the study shows that the separated region ahead of the step is highly three-dimensional. The shock foot within the boundary layer is found to have ripples that are well correlated to fluctuations in the in-coming boundary layer. However, we find that the large-scale nearly two-dimensional shock motions outside the boundary layer are not well cor-related to the fluctuations in the boundary layer, but are instead well cor-related with the spanwise-averaged separation bubble extent. Hence, the present results suggest that for the forward-facing step configuration, it is the downstream effect caused by the separation bubble that leads to the observed low-frequency shock motions.

Shock-shock interactions are among the most basic gas-dynamic problem, and are almost unavoidable in any high speed light, where shock waves generating from different sources crosses each other paths. These interactions when present very close to the solid surface lead to very high pressure and thermal loads on the surface. The related practical problem is that experienced at the cowl lip of a scramjet engine, where the interfering shock waves leads to high heat transfer rates which may also lead to the damage of the material. The classification by Edney (1968) on the shock-shock interaction patterns based on the visualization has since then served the basis for such studies. Though the problem of high heating on the surface in the vicinity of the shock-shock interactions has been studied at length at supersonic Mach numbers, the study on the topic at the hypersonic Mach numbers is little sparse. Even in the studies at hypersonic Mach numbers, the high speeds are not simulated, which is the measure of the kinetic energy of the ow. Very few experimental studies have addressed this problem by simulating the energy content of the ow. Also, some of the numerical studies on the shock-shock interactions suggest the presence of unsteadiness in the shock-shock interaction patterns as observed by Edney (1968), though this observation is not made very clearly in the experimental studies undertaken so far. In the present study, experiments are carried out in a conventional shock tunnel at Mach number of 5.62 (total enthalpy of 1.07 MJ/kg; freestream velocity of 1361 m/s), with the objective of mapping the surface pressure distribution and surface convective heat transfer rate distribution on the hemispherical body in the presence of the shock-shock interactions. A shock generator which is basically a wedge of angle = 25 , is placed at some dis-dance in front of the hemispherical body such that the planar oblique shock wave from the shock generator hits the bow shock wave in front of the hemi-spherical body. The relative distance between the wedge tip and the nose of the hemispherical body is allowed to change in di erent experiments to capture the whole realm of shock-shock interaction by making the planar oblique shock wave interact with the bow shock wave at different locations along its trajectory. The study results in a bulk of data for the surface pressure and heat transfer rates which were obtained by placing 5 kulites pressure transducers, 1 PCB pressure transducer and 21 platinum thin lm gauges along the surface of the hemispherical body in a plane normal to the freestream velocity direction. Along with the measurement of the surface pressure and the surface heat transfer rates, the schlieren visualization is carried out to capture the shock waves, expansion fans, slip lines, present in a certain shock-shock interaction pattern and the measured values were correlated with the captured schlieren images to evaluate the ow build up and steady and useful test time thereby helping in understanding the ow physics in the presence of the shock-shock interactions. From the present study it has been observed that in the presence of Edney Type-I and Edney Type-II interaction, the heat transfer rates on the hemi-spherical body are symmetrical about the centerline of the body, with the peak heating at the centerline which drops towards the shoulder. For Edney Type-III, Edney Type-IV, Edney Type-V and Edney Type-VI interaction pattern, the distribution in not symmetrical and shifts in peak heat transfer rates being on the side of the hemispherical from which planar oblique shock wave is incident. Also, it is observed that for the interactions which appear within the sonic circle, Edney Type-III and Edney Type-IV, the heat transfer rates observe an unsteadiness, such that the gauges located close to the interaction region experiencing varying heat transfer rates during the useful test time of the shock tunnel. Few experiments were conducted at Mach 8.36 (total enthalpy of 1.29 MJ/kg; freestream velocity of 1555.25 m/s) and Mach 10.14 (total enthalpy of 2.67 MJ/kg; freestream velocity of 2258.51 m/s) for the con gurations representing Edney Type-III interaction pattern to further evaluate the unsteady nature observed at Mach 5.62 ows. The unsteadiness was evident in both the cases. It is realized that the short test times in the shock tunnels pose a constraint in the study of unsteady flow fields, and the use of tailored mode operation of shock tunnel can alleviate this constraint. Also, limited number of experiments in the present study, which are carried out in a Free Piston Shock Tunnel, helps to understand the need to conduct such study in high enthalpy test conditions.

L’étude du comportement des composantes des intérieurs planétaires dans des conditions extrêmes de pression (megabar) et température (milliers de Kelvin) est essentielle afin de construire des modèles fiables décrivant l’évolution et la structure des planètes. Dans ce travail, nous avons étudié plusieurs composantes par compression par choc laser sur les installations LULI2000 (France) et GEKKO XII (Japon).Nous avons employé des chocs décroissants pour étudier des conditions de haute-pression / haute-température. Afin d’accéder à des conditions de température modérée, nous avons utilisé des techniques de pre-compression statique (couplage compression par choc — cellules à enclumes de diamant) et dynamique (génération de doubles chocs).Nous avons étudié l’équation d’état des mélanges eau-ethanol-ammoniac et de l’eau et ammoniac purs, d’intérêt pour la description des intérieurs des planètes géantes de glace. L’étude de l’ammoniac a été particulièrement délicate en raison de sa forte réactivité et donc de la complexité du design des cibles ; nous présentons les premières données obtenues par choc laser, dans un domaine de pression jamais exploré. Les données des mélanges confirment des calculs ab initio récents basés sur une approximation de mélange linéaire.Nous avons également mesuré la réflectivité des mélanges liquides et de la silice, une composante-clé des intérieurs des planètes terrestres. Nous avons ensuite estimé la conductivité électrique — un paramètre crucial pour modéliser la génération des champs magnétiques planétaires dans les intérieurs via un mécanisme dynamo — de ces composantes.Eau, ammoniac et mélanges eau-ethanol-ammoniac affichent des réflectivités différentes, ce qui suggère que l’eau pure ne peut pas être considérée comme représentative des mélanges planétaires dans les modèles dynamo.Par ailleurs, nous avons apporté une confirmation expérimentale de calculs ab initio récents selon lesquels la conductivité de la silice n’est pas monotone le long d’une ligne isotherme pour des températures modérées.Nos données supportent des calculs qui prédisent qu’une dynamo peut avoir lieu dans les océans de magma dans des super-Terres ainsi que dans la jeune Terre. / Characterising the behaviour of planetary interiors’ components at extreme conditions (megabar pressures, temperatures of a few thousand Kelvin) is essential to build reliable models describing the evolution and structure of planets. In this thesis, we investigated various components on a wide set of conditions using laser-driven shock compression techniques at the LULI2000 (France) and GEKKO XII (Japan) facilities.Single decaying shocks were employed to study high-pressure / high-temperature states. To reach moderate-temperature conditions, closer to planetary interior profiles, we employed static and dynamic pre-compression techniques coupling Diamond Anvil Cells to shock compression and generating double shocks, respectively.We studied the equation of state of water-ethanol-ammonia mixtures and of pure liquid water and ammonia, of interest for icy giant structure models. Pure ammonia measurements have been particularly challenging due to cell design complexity in reason of its reactivity; we provide the first data obtained with laser shocks, in a pressure domain up to now unexplored. Mixtures data are in agreement with recent ab initio calculations based on the linear mixing approximation.We measured the optical reflectivity of liquid mixtures and silica, a key component of rocky planets’ interiors. From reflectivity data we estimated the electrical conductivity of such components — a crucial parameter for modelling the generation of planetary magnetic fields in the interiors via a dynamo mechanism.Water, ammonia, and water-ethanol-ammonia mixtures exhibit different reflectivity (hence conductivity) behaviours as a function of pressure and temperature. This suggests that pure water should not be used in dynamo models as representative of the icy mixtures.Moreover, we provide the first experimental confirmation of recent ab initio studies showing that the conductivity of silica along isothermal lines is not monotonic at moderate temperatures. Our data provide experimental support for the calculations predicting a dynamo action to occur in super-Earths’ and early Earth’s magma oceans.

Die vorliegende Arbeit beschreibt die Ergebnisse der Synthese und Charakterisierung der Hochdruckphase von Aluminiumnitrid mit Kochsalzstruktur (rs-AlN). Die Versuche wurden mittels Schockwellensynthese unter Verwendung der „flyer-plate-Methode“ mit anschließender Probenrückgewinnung durchgeführt. Für verschiedene Aluminiumnitridpulver mit einer Ausgangsporosität k = rho_solid/rho_porous von 1,5 bis 2,5 wurde bei einem Druck von 15 bis 43 GPa die Phasenumwandlung von der Wurtzitstruktur (w-AlN) in die Kochsalzstruktur (rs-AlN) bewirkt. Es ist damit erstmals gelungen, rs-AlN mit dynamischen HP-HT-Methoden herzustellen und damit Probenmengen im Milligramm- bis Grammbereich zu erhalten. Dadurch ist es möglich Untersuchungen durchzuführen, die zur weiteren Erforschung und Charakterisierung des Materials beitragen sollen. Im Fokus liegen dabei insbesondere die Untersuchung der mechanischen, thermischen und chemischen Stabilität, um die Eignung des Materials zur Herstellung ultraharter Komposite zu evaluieren. Die geschockten Pulver bestehen aus einem Phasengemisch aus dem Ausgangsmaterial (w-AlN), der Hochdruckphase (rs-AlN), Aluminiumoxid und -oxynitriden, sowie amorphen Aluminiumhydroxiden. Die höchste Ausbeute an rs-AlN (~41 Ma% bei 2 mm Probenhöhe) kann bei Drücken von 24 GPa und einer Ausgangsporosität k von 2,1 erhalten werden. Anhand dem Auftreten verschiedener Al-O-N Phasen kann die Schocktemperatur für die einzelnen Versuche abgeschätzt werden (<1700 °C bis <2000 °C). Die Phasenumwandlung wird durch die Temperaturerhöhung aufgrund der Schockkompaktion der Pulver aktiviert. Als entgegenwirkender Prozess wurde die thermisch aktivierte Rückwandlung in die Niederdruckphase w-AlN aufgrund einer zu hohen Post-Schocktemperatur und einem zu langsamen Abkühlen der Probe postuliert. Daraus ergibt sich eine optimale Temperatur für den Versuchsaufbau von 1700 bis 1900 °C, bei der die höchsten rs-AlN Anteile beobachtet wurden. Eine Verringerung der Probenhöhe erhöht den Einfluss von Mehrfachreflektionen in der Probe und trägt damit zur Verbesserung der Umsetzung bei. Für drei Nanopulver (Kristallitgröße <25 nm) wurde die teilweise Umwandlung in die Kochsalzstruktur beobachtet, wohingegen für ein gröberes Nanopulver und ein Submikropulver (Kristallitgröße >45 nm) kein rs-AlN in den geschockten Proben nachgewiesen werden konnte. Es wird ein Stabilisierungsmechanismus der Kochsalzstruktur durch Kristallitgrößeneffekte vorhergesagt. Die Verringerung der Kristallitgröße führt zur Herabsetzung des Umwandlungsdrucks w-AlN -> rs-AlN. Es lässt sich daher schlussfolgern, dass für kleinere Partikel die Hochdruckmodifikation aufgrund der geringeren Entfernung vom chemischen Gleichgewicht bei Normalbedingungen stabilisiert werden kann, wohingegen für größere Kristallite die Rückwandlung in die Ausgangsphase geschieht. Weitere Stabilisierungsmechanismen wurden diskutiert. Mithilfe einer Rietveld-Verfeinerung der Röntgendiffraktogramme wurde die Gitterkonstante des rs-AlN mit a = 4,044 ± 0,001 Å und die Kristallitgröße mit 15,3 ± 0,2nm bestimmt. Die mittels hoch-aufgelöster Transmissionselektronenmikroskopie (TEM) bestimmte Kristallitgröße (10 bis 20 nm) ist in guter Übereinstimmung mit den Ergebnissen der Rietveld-Verfeinerung. Mit 27Al Kernspinresonanzspektroskopie (NMR) wurde die oktaedrische Al–N-Umgebung (AlN6) mit einer korrigierten chemischen Verschiebung von 2 ppm nachgewiesen. Anhand der IR-Spektren wird eine Al–N-Schwingungsbande des rs-AlN bei ca. 490 cm−1 vermutet. Dynamisch-thermische Untersuchungen zeigen, dass nanokristallines rs-AlN bei 600 °C beginnt zu Aluminiumoxid zu oxidieren und damit keine größere Beständigkeit im Vergleich zum w-AlN zeigt. Die thermisch aktivierte Rückwandlung des rs-AlN in die Niederdruckphase wurde ab 1200 °C (in Argon) bzw. 1100 °C (im Vakuum) bei einer Heizrate von 10 K/min beobachtet. Eine gute chemische Beständigkeit des Aluminiumnitrid mit Kochsalzstruktur gegenüber Wasser, Natronlauge und Säuren (HCl, H2SO4, H3PO4, HNO3 und Königswasser) wurde in Langzeit-Löslichkeitsversuchen nachgewiesen.:1. Einleitung 2. Grundlagen 3. Methoden und experimentelle Details 4. Ergebnisse 5. Diskussion 6. Schlussfolgerungen und Ausblick / In the present work the results of the synthesis and characterisation of the high-pressure phase of aluminium nitride with rocksalt structure (rs-AlN) are presented. The experiments were carried out with the flyer-plate-method with subsequently sample recovery. For different aluminium nitride powders with starting porosities k = rho_solid/rho_porous of 1.5 to 2.5 the phase transition from wurtzite structure (w-AlN) to the rocksalt structure (rs-AlN) was induced at a pressure of 15 to 43 GPa. This is to our knowledge the first succesful synthesis of rs-AlN with dynamic HP-HT methods. With this advance, samples in the milligram or gram range can be produced. Therefore further investigations to characterise the material are possible, especially the study of the mechanical, thermal and chemical stability to validate the potential for the production of ultrahard composites. The shocked samples consist of a phase mixture from the starting material (w-AlN), the high-pressure phase (rs-AlN), aluminium oxide and oxynitrides, as well as amorphous aluminium hydroxides. The highest yield of rs-AlN (~41 wt% at 2 mm sample height) can be obtained at a pressure of 24 GPa and a starting porosity k of 2.1. The shock temperature can be estimated by the formation of different Al-O-N phases (<1700 °C to <2000 °C). The phase transition is activated by the raise of temperature due to shock compression. A thermal activated reconversion to the low-pressure phase w-AlN caused by a high post-shock temperature and a slow cooling of the sample is postulated as a contrary process. This results in an optimum temperature of 1700 to 1900 °C for this set-up. A decrease of the sample height increases the influence of multiple reflections and therefore causes a better transformation. A partial conversion to rs-AlN was observed for three nanopowders (crystallite size <25 nm), whereas for a more coarse nanopowder and an submicronpowder (crystallite size >45 nm) no rs-AlN could be found in the shocked samples. A stabilisation mechanism of the rocksalt phase by crystallite size effects is predicted. The reduction of the crystallite size causes a decrease of the transition pressure for w-AlN -> rs-AlN. It can be concluded, that for smaller particles the high-pressure phase can be stabilised at ambient conditions on the basis of the smaller distance from equilibrium, whereas for larger particles the reconversion to the low-pressure phase occurs. By a Rietveld refinement of the X-ray diffractograms, the lattice constant of rs-AlN and the crystallite size was determined to be a = 4.044 ± 0.001 Å respectively 15.3 ± 0.2 nm. The crystallite size of rs-AlN (10 to 20 nm) determined with high-resolution transmission electron microscopy (TEM) is in good agreement with the results of the Rietveld refinement. The octahedral Al–Npolyhedral (AlN6) was demonstrated by 27Al nuclear magnetic resonance spectroscopy (NMR) with a corrected chemical shift of 2 ppm. Based on infrared spectroscopy (FTIR) an AlN vibration band at about 490 cm−1 is assumed. Dynamic thermal analysis show, that the rs-AlN starts to oxidise to alumina at 600 °C and thus have no greater resistance in comparison with w-AlN. The thermal activated reconversion of rs-AlN to the low-pressure phase starts at 1200 °C (in argon) respectively 1100 °C (under vacuum) at a heating rate of 10 K/min. The aluminium nitride with rocksalt structure shows a good chemical resistance against water, caustic soda and acids (HCl, H2SO4, H3PO4, HNO3 and aqua regia).:1. Einleitung 2. Grundlagen 3. Methoden und experimentelle Details 4. Ergebnisse 5. Diskussion 6. Schlussfolgerungen und Ausblick

<p>Nanosecond spark plasma actuators provide an opportunity to reduce pollutants by promoting efficient combustion in engines or provide targeted, tunable, flow control over vehicles, due to their ability to influence flow and combustion through multiple mechanisms. The plasma actuators can be physically unobtrusive, can be turned on and off and their low duty cycle, large bandwidth, and light weight make them more appealing than other control approaches. One method by which these plasma actuators interact with the environment is by inducing a complex local flow field and in order, to design scalable, high frequency actuators effectively, it is necessary to first understand the flow induced by a single spark discharge. Most experimental analysis on the flow induced by spark discharges has been restricted to qualitative descriptions of the flow field, primarily due to the difficulties associated with measuring such a transient and highly complex flow with sufficient spatiotemporal resolution. Quantitative, experimental characterization of the flow induced by a spark discharge remains lacking. </p><p> </p><p>A spark discharge produces a shock wave and a hot gas kernel with a complex flow field following the shock. In this work, combined experimental and theoretical characterization of the spark induced flow is performed through a series of high spatiotemporal resolution measurements of the density and velocity fields and reduced-order modeling. The work investigates the mechanisms driving the cooling and vorticity generation in spark induced flow and the 3D nature of the flow field. Planar (2D-3C) and volumetric (3D-3C) velocity measurements are taken using stereoscopic particle image velocimetry (SPIV) and tomographic PIV, respectively. Density measurements are taken using background oriented schlieren (BOS) and high speed schlieren imaging is used to capture the shock wave induced by the spark.</p><p> </p><p>The work shows that spark plasma discharges induce vortex rings whose vorticity is likely generated due to baroclinic torque arising from the non-uniform strength of the induced shock wave. The hot gas kernel cools in two stages: an initially fast cooling regime, followed by a slower cooling process. Reduced order analytical models are developed to describe the cooling observed in the fast regime and the role of the vortex rings in the entrainment of cold ambient gas and the cooling of the hot gas kernel. The results show that the vortex rings entrain ambient gas and drive cooling in the fast, convective regime, cooling approximately 50% of the hot gas within the first millisecond of the induced flow. An increase in the electrical energy deposited in the spark gap increases the shock strength and curvature and increases the vortex ring strength, thereby increasing the cooling rate and expansion of the hot gas kernel. The volumetric velocity measurements capture one of the two induced vortex rings and provide a framework for the improvements needed in future tomographic PIV experiments of the spark induced flow field, necessary in assessing the 3D nature of the induced vortex rings.</p><p> </p><p> The results of this work provide the first set of quantitative, experimental data on flow induced by nanosecond spark discharges that can be used for validation of computational fluid dynamics (CFD) simulations. The results demonstrate that spark plasmas induce vortex ring-driven mixing flows and the results on mixing and cooling of the hot gas kernel can be extended to any passive scalars present in the flow field as well as inform pulsation frequencies and actuator designs for flow and combustion control. The results from the reduced order modeling can inform future studies and applications of nanosecond spark discharges and can be extended to a variety of other types of plasma discharges like laser sparks, long duration sparks and surface discharges with similar induced flow fields.<br></p>

<p>Raman spectroscopy is a molecular spectroscopy technique that uses monochromatic light to provide a fingerprint to identify structural components and chemical composition. Depending on the changes in the unit-cell parameters and volume under the application of stress and temperature, the Raman spectrum undergoes changes in the wavenumber of Raman-active modes that allow identification of sample characteristics. Due to the various advantage of mechanical Raman spectroscopy (MRS), the use of this technique in the characterization and modeling of chemical changes under stress and temperature have gained popularity. </p> <p> Quantitative information regarding the local behavior of interfaces in an inhomogeneous material during shock loading is limited due to challenges associated with time and spatial resolution. Recently, we have extended the use of MRS to high-strain rate experiments to capture the local thermomechanical response of mock energetic material and obtain material properties during shock wave propagation. This was achieved by developing a novel method for <i>in‑situ</i> measurement of the thermo‑mechanical response from mock energetic materials in a time‑resolved manner with 5 ns resolution providing an estimation on local pressure, temperature, strain rate, and local shock viscosity. The results show the solid to liquid phase transition of sucrose under shock compression. The viscous behavior of the binder was also characterized through measurement of shock viscosity at strain rates higher than 10⁶/s using microsphere impact experiments.</p> <p> This technique was further extended to perform Raman spectral imaging over a microscale domain of the sample with a nano-second resolution. This was achieved by developing a laser-array Raman spectral imaging technique where simultaneous deconvolution of Raman spectra over the sample domain was achieved and Raman spectral image was reconstructed on post-processing. We developed a Raman spectral imaging system using a laser array and analysis was performed over the interface of sucrose crystals bonded using an epoxy binder. This study provides the Raman spectra over the microstructure domain which enabled the detection of localized melting under shock compression. The distribution of shock pressure and temperature over the microstructure was obtained using mechanical Raman analysis. The study shows the effects of an actual interface on the propagation of shock waves where a higher dissipation of shock energy was observed compared to an ideal interface. This increase in shock dissipation is accompanied by a decrease in both the maximum temperature, as well as the maximum pressure within the microstructure during shock wave propagation.</p>

O presente trabalho consiste no uso do Método dos Elementos Finitos (MEF) para a análise de interação fluido-estruturas de barras com foco em problemas transientes envolvendo explosões ou outras ações com propagação de ondas de choque. Para isso é necessário o estudo de três diferentes aspectos: a dinâmica das estruturas computacional, a dinâmica dos fluidos computacional e o problema do acoplamento. No caso da dinâmica das estruturas computacional deve-se identificar em função da cinemática de deformações, quais são os requisitos para que um elemento seja adequado para analisar tais problemas, tendo em vista que a formulação deve admitir grandes deslocamentos. Para evitar problemas relacionados com aproximações de rotações finitas, opta-se por empregar uma formulação descrita em termos de posições e que leva em consideração os efeitos de empenamento da seção transversal. No caso da dinâmica dos fluidos computacional, busca-se uma formulação para escoamentos compressíveis que seja estável e ao mesmo tempo sensível ao movimento da estrutura, sendo empregado um algoritmo de integração temporal explícito baseado em características com as equações governantes descritas na forma Lagrangeana-Euleriana Arbitrária (ALE). No que se refere ao acoplamento, busca-se modularidade e versatilidade, empregando-se um modelo particionado fraco (explícito) de acoplamento e técnicas de transferência das condições de contorno (Dirichlet-Neummann), sendo estudados os efeitos de utilizar transferência bidirecional ou unidirecional dessas condições de contorno. / This work consists in the use of the Finite Element Method (FEM) for numerical analysis of fluid-bar structures, focusing on transient problems involving explosions or other actions with shock waves propagation. For this purpose, one needs to study three different aspects: the computational structural dynamics, the computational fluid dynamics and the coupling problem. Regarding computational structural dynamics, one need firstly to identify the requirements for an element to be adequate to analyze such problems, taking into account the fact that such element should admit large displacements. In order to avoid problems related to finite rotation approximations and to give a realist representation of a 3D bar structure, we chose a formulation defined in terms of positions and that considers the cross-section warping effects. Regarding computational fluid dynamics, we seek for a stable formulation for compressible flows, and at same time, sensitive to the movement of the structure, leading to an explicit time integration algorithm based on characteristics with governing equations described in the Arbitrary Lagrangian-Eulerian (ALE) form. Regarding to coupling, we chose to use a weak (explicit) partitioning coupling model in order to ensure modularity and versatility. The developed coupling scheme is bases on boundary conditions transfer techniques (Dirichlet-Neummann), and we study the effects of using bidirectional or unidirectional boundary conditions transfers.