Les matériaux composites sont très utilisés dans diverses applications et sont parfoissoumis à des impacts hypervéloces (IHV), notamment dans le domaine spatial. La taille des impacteursétant proche de celle des torons de fibres, les simulations mésoscopiques ont tout leurintérêt mais nécessitent des modèles numériques aboutis pour chaque sous-Constituant. Le graphiteétant souvent utilisé comme matrice ou fibres, nous avons étudié son comportement dynamique.Ainsi, des expériences de pénétration et de cratérisation ont été menées sur un isographite poreux.L’analyse post-Mortem des cibles, associée à des calculs d’ordre de grandeur, apporte un éclairagenouveau sur la phénoménologie des impacts et fournit des renseignements utiles à la simulationnumérique. Un modèle pour matériaux poreux et fragiles, implémenté dans un code de dynamiquerapide, est utilisé. Basé en partie sur des propriétés statiques, il a été progressivement testé sur deschocs plans. Son utilisation pour la simulation des IHV donne de bons résultats. Toutefois, il convenaitde le valider en s’affranchissant du comportement du projectile. Pour cela, une campagne dechocs lasers a été menée. Des diagnostics in-Situ ont été utilisés et leur corrélation avec des analysespost-Mortem a permis l’identification de différents modes d’endommagement des cibles. Finalement,bien que l’équivalence entre IHV et chocs lasers ne soit pas démontrée, ces derniers se sont montréscomplémentaires en suggérant de futures évolutions du modèle numérique. / Composite materials are widely used in various applications and may be submittedto hypervelocity impacts (HVI), such as in the aerospace field. The size of the impactors beingclose to that of a strand of fibers, mesoscopic simulations are of great interest but they need reliablenumerical models for each meso-Constituent. Graphite often being used as fiber or matrix,we studied its dynamic behavior. Penetration and craterization experiments have been conductedonto porous isotropic graphite. Post-Mortem analysis of targets, associated to order-Of-Magnitudecalculations, sheds a new light on the phenomenology of impacts and brings useful informationfor numerical simulation. A model for porous and brittle materials, implemented into a hydrocode,is used. Partially based on static mechanical properties, it has been progressively tested on planeshocks. Its use for simulating HVI gives satisfying results. Nevertheless, it was necessary to validateit by disregarding the projectile behavior. In that purpose, a campaign of laser-Driven shocks hasbeen conducted. In-Situ diagnostics have been simultaneously used and their correlation with postmortemanalysis allowed the identification of different damaging regimes of the targets. Finally,although the equivalence between HVI and laser-Driven shocks is not proved, the latter turned outto be complementary, suggesting the future evolutions of the numerical model.

The next generation of European scientific satellites will carry extremely sensitive measurement devices that require platform stability orders of magnitude higher than current missions. It is considered that the meteoroid and space debris (M/SD) environment poses a risk to the success of these missions as disturbances induced by the impact of these particles at hypervelocity may degrade the platform stability below operational requirements. In this thesis, disturbances induced by the impact of M/SD particles at hypervelocity on a representative scientific satellite platform have been investigated. An extensive experimental impact test program has been performed, from which an empirical ballistic limit equation (BLE) which defines the conditions of structural perforation for composite sandwich panel structures with CFRP facesheets and aluminium honeycomb cores (CFRP/Al HC SP) has been defined. The BLE is used to predict impact conditions capable of inducing the different excitation modes relevant for a SP sandwich panel structure, enabling a significant reduction in the time and expense usually required for calibrating the protective capability of a new structural configuration. As experimental acceleration facilities are unable to cover the complete range of possible in-orbit impact conditions relevant for M/SD impact risk assessment, a Hydrocode model of the representative CFRP/Al HC SP has been constructed. A series of impact simulations have been performed during which the local impact-induced disturbance has been measured. The numerical disturbance signals have been validated via comparison with experimental disturbance measurements, and subsequently subject to a characterisation campaign to define the local elastic excitation of the SP structure equivalent to that induced by impact of a M/SD particle at hypervelocity. The disturbance characterisation is made such that it is applicable as an excitation force on a global satellite Finite Element (FE) model, allowing propagation of impact-induced disturbances throughout the complete satellite body to regions of critical stability (i.e. measurement devices). The disturbance induced upon measurement devices by M/SD impacts at both near- and far-body locations can then be made, allowing the threat to mission objectives to be assessed.

The present study deals with the development of a single-stage implosion-driven hypervelocity launcher. A thin-walled tube filled with helium surrounded by explosives acts as a driver for the launcher. Implosion of the tube drives a strong shock that reflects back and forth between the projectile and the implosion pinch, generating very high temperatures and pressures. Simple analytic models were used to approximate the performance of the pump tube and its use as a driver for a launcher. Experiments to evaluate the implosion dynamics and performance of the pump tube were carried out, and implosion-driven launcher experiments demonstrated muzzle velocities above 4 km/s with 5-mm-diameter aluminum projectiles. Projectile integrity was verified by high-speed photography. Disagreement of experimental data with the analytical models of performance is mostly due to failure to seal the chamber of the launcher, resulting in loss of driver gas, and pump tube expansion, which weakens the precursor shock.

No description available.

Numerical simulations of galaxy formation and observational Galactic Astronomy are two fields of research that study the same objects from different perspectives. Simulations try to understand galaxies like our Milky Way from an evolutionary point of view while observers try to disentangle the current structure and the building blocks of our Galaxy. Due to great advances in computational power as well as in massive stellar surveys we are now able to compare resolved stellar populations in simulations and in observations. In this thesis we use a number of approaches to relate the results of the two fields to each other. The major observational data set we refer to for this work comes from the Radial Velocity Experiment (RAVE), a massive spectroscopic stellar survey that observed almost half a million stars in the Galaxy. In a first study we use three different models of the Galaxy to generate synthetic stellar surveys that can be directly compared to the RAVE data. To do this we evaluate the RAVE selection function to great detail. Among the Galaxy models is the widely used Besancon model that performs well when individual parameter distribution are considered, but fails when we study chemodynamic correlations. The other two models are based on distributions of mass particles instead of analytical distribution functions. This is the first time that such models are converted to the space of observables and are compared to a stellar survey. We show that these models can be competitive and in some aspects superior to analytic models, because of their self-consistent dynamic history. In the case of a full cosmological simulation of disk galaxy formation we can recover features in the synthetic survey that relate to the known issues of the model and hence proof that our technique is sensitive to the global structure of the model. We argue that the next generation of cosmological galaxy formation simulations will deliver valuable models for our Galaxy. Testing these models with our approach will provide a direct connection between stellar Galactic astronomy and physical cosmology. In the second part of the thesis we use a sample of high-velocity halo stars from the RAVE data to estimate the Galactic escape speed and the virial mass of the Milky Way. In the course of this study cosmological simulations of galaxy formation also play a crucial role. Here we use them to calibrate and extensively test our analysis technique. We find the local Galactic escape speed to be 533 (+54/-41) km/s (90% confidence). With this result in combination with a simple mass model of the Galaxy we then construct an estimate of the virial mass of the Galaxy. For the mass profile of the dark matter halo we use two extreme models, a pure Navarro, Frenk & White (NFW) profile and an adiabatically contracted NFW profile. When we use statistics on the concentration parameter of these profile taken from large dissipationless cosmological simulations we obtain an estimate of the virial mass that is almost independent of the choice of the halo profile. For the mass M_340 enclosed within R_340 = 180 kpc we find 1.3 (+0.4/-0.3) x 10^12 M_sun. This value is in very good agreement with a number of other mass estimates in the literature that are based on independent data sets and analysis techniques. In the last part of this thesis we investigate a new possible channel to generate a population of Hypervelocity stars (HVSs) that is observed in the stellar halo. Commonly, it is assumed that the velocities of these stars originate from an interaction with the super-massive black hole in the Galactic center. It was suggested recently that stars stripped-off a disrupted satellite galaxy could reach similar velocities and leave the Galaxy. Here we study in detail the kinematics of tidal debris stars to investigate the probability that the observed sample of HVSs could partly originate from such a galaxy collision. We use a suite of $N$-body simulations following the encounter of a satellite galaxy with its Milky Way-type host galaxy. We quantify the typical pattern in angular and phase space formed by the debris stars and develop a simple model that predicts the kinematics of stripped-off stars. We show that the distribution of orbital energies in the tidal debris has a typical form that can be described quite accurately by a simple function. The main parameters determining the maximum energy kick a tidal debris star can get is the initial mass of the satellite and only to a lower extent its orbit. Main contributors to an unbound stellar population created in this way are massive satellites (M_sat > 10^9 M_sun). The probability that the observed HVS population is significantly contaminated by tidal debris stars appears small in the light of our results. / Ein häufig verfolgter Ansatz Galaxien wie unsere Milchstraße besser zu verstehen, sind numerische Simulationen, d.h. das Nachvollziehen ihrer Entstehung und Entwicklung mit Hilfe von Computern. Dieses Vorgehen erlaubt das Betrachten solcher Objekte von einem evolutionären Standpunkt aus. Eine andere Herangehensweise verfolgt die Galaktische Astronomie, welche über Sternbeobachtungen den aktuellen Zustand der Milchstraße untersucht. Hier wird versucht, die konstitutiven Bestandteile unserer Galaxie zu erkennen, um dadurch ein besseres Verständnis ihrer Struktur zu erlangen. Die enorme Rechenleistung moderner Supercomputer und die Entwicklungssprünge im Bereich der digitalen Himmelsdurchmusterungen haben dazu geführt, dass inzwischen mit beiden Ansätzen vergleichbare Populationen von einzeln beobachtbaren Sternen studiert werden können. In der vorliegenden Arbeit werden verschiedene Möglichkeiten untersucht, die Ergebnisse dieser beiden astrophysikalischen Disziplinen, welche bislang weitgehend getrennt von einander betrieben wurden, sinnvoll zu kombinieren. Der überwiegende Teil der Beobachtungsdaten, die dabei verwendet werden, wurde im Zuge des Radial Velocity Experiments (RAVE) gesammelt, einer spektroskopischen Durchmusterung der Sterne fast des gesamten Südhimmels. Um die Daten des RAVE-Projekts statistisch auswerten zu können, musste zuerst die detaillierte Auswahlfunktion der Durchmusterung rekonstruiert werden, d.h. die Wahrscheinlichkeit, dass ein Stern von RAVE beobachtet wurde, musste, in Abhängigkeit von den Eigenschaften des Sterns, bestimmt werden. Der Hauptteil der Dissertation gliedert sich in drei weitgehend unabhängige Studien. Im ersten Teil wird die oben erwähnte Auswahlfunktion benutzt, um voraus zu sagen, was das RAVE Projekt beobachtet hätte, falls bestimmte theoretische Modelle unserer Milchstraße zu träfen. Auf diese Art und Weise umgehe ich das problematische Unterfangen, die Beobachtungsdaten zu einem physikalischen Modell zu verallgemeinern. Die Problematik hierbei liegt darin, dass astronomische Beobachtungen nicht direkt physikalisch relevante Größen, wie Massen oder Alter der Sterne, liefern, sondern scheinbare Helligkeiten oder Winkelpositionen. In dieser Studie wird der umgekehrte Weg beschritten und synthetische Beobachtungen aus den Modellen generiert. Untersucht wurden dabei sowohl klassische analytische Modelle als auch Modelle, die aus numerischen Simulationen resultieren. Letztere wurden zu ersten Mal überhaupt auf diese Art und Weise getestet und es zeigt sich, dass solche Modelle den klassischen in bestimmten Aspekten, die mit der Entwicklungsgeschichte der Milchstraße verknüpft sind, überlegen sind. Im zweiten Teil der Arbeit werden die RAVE-Daten benutzt um die Masse der Milchstraße, bzw. die Masse der in ihr enthaltenen dunklen Materie, ab zu schätzen. Zur Eichung der Analysemethode wird dabei wieder auf Ergebnisse von Simulationen zurück gegriffen, die die Entwicklung von ähnlichen Galaxien wie der Milchstraße verfolgt haben. Zuerst wird die lokale Entweichgeschwindigkeit, d.h. die Mindestgeschwindigkeit, die ein Körper benötigt, um unsere Galaxie zu verlassen, bestimmt. Die beste Abschätzung beträgt 533 (+54/-41) km/s. Anhand dieser Schätzung kann, in Kombination mit vereinfachten analytischen Modellen der Materieverteilung in unserer Galaxie, die Masse der Milchstraße innerhalb von 180 kpc auf 1,3 (+0,4/-0,3) x 10^12 Sonnenmassen bestimmt werden. Dieses Ergebnis bestätigt frühere unabhängige Massenschätzungen, die auf anderen Beobachtungsdaten und anderen Analysestrategien basieren. Im letzten Teil der Arbeit wird eine spezielle Population von Sternen im Außenbereich unserer Galaxie untersucht, sogenannte Hyperschnellläufersterne (HSS). Diese wurde in einer weiteren Himmelsdurchmusterung, dem Sloan Digital Sky Survey (SDSS), gefunden. Die Besonderheit dieser Sterne besteht in ihren extrem hohen Geschwindigkeiten oberhalb der Entweichgeschwindigkeit. Allgemein wird angenommen, dass die Sterne ihre Geschwindigkeiten im Zuge der Spaltung eines Doppelsternsystems durch Gezeitenkräfte nahe des supermassereichen Schwarzen Lochs im Zentrum der Milchstraße erreichen. Vor Kurzem wurde jedoch ein alternatives Szenario vorgeschlagen. Nach diesem können solche Sterne auch während des Einfalls einer Satellitengalaxie auf die Milchstraße entstehen. Diese Hypothese wird anhand von numerischen Simulationen, die diese Situation nachbilden, getestet. Es zeigt sich, dass HSS auf diese Weise entstehen können, aber dass die beobachtete Population höchstwahrscheinlich einen anderen Ursprung hat.

Understanding the failure behavior of polymers subjected to an ultrahigh strain rate (UHSR) impact is crucial for their applications in any protective shielding. But little is known about how polymers respond to UHSR events at the macroscale, or what effect their chemical makeups and morphology contribute. This dissertation aims to answer these questions by characterizing the responses of polymers subjected to UHSRs, investigating how the polymer molecular architecture and morphologies alter the macroscopic response during UHSRs via hypervelocity impact (HVI), linking the behaviors of UHSR events between the macro- and nano-length scales, and determining the consequences of UHSR impacts on polymer chains. Macroscale UHSR impacts are conducted using a two-stage light gas gun (2SLGG) to induce an HVI. Different molecular weights and thicknesses of polycarbonate were considered. The HVI behavior of polycarbonate is characterized using both real-time and postmortem techniques. The response depends on target thickness and impact velocity (vi). However, negligible difference is observed between the HVI results for the two differing entanglement densities. These contrasts previous conclusions drawn on the nanoscale during UHSR impacts which capture an increase in the energy arrested from the projectile with increasing entanglement density. To link the UHSR phenomena from nano to macroscale, laser-induced projectile impact testing (LIPIT) is conducted on polymethyl methacrylate (PMMA) thin films on the nanoscale in addition to ballistic and 2SLGG impacts at macroscale. Applying Buckingham-Π theorem, scaling relationships for the minimum perforation velocity and the residual velocity across these length scales were developed. It is shown that the ratios between target thickness to projectile radius, between projectile and target density, and the velocity of the compressive stress wave traveling through the target are the governing parameters for the UHSR responses of polymers across theses length scales. The effect UHSRs have on the polymer is investigated via ex-situ analysis by capturing polymer debris using a custom-built debris catcher. Different material-vi combinations are examined. X-ray diffraction and differential scanning calorimetry are used to characterize the HVI debris. Evidence of char was found within the debris. This dissertation advances the knowledge regarding the failure behavior of polymer materials subjected to UHSRs.

I present a comprehensive investigation of fast stars in the Milky Way, from brisk disc stars to stars escaping the Galaxy. My thesis is that fast stars are the smoking guns of extreme stellar collisions and explosions, and so can act as an intermediary to studying these theoretically-unconquered astrophysical processes. In Chapter 1 I give a history of fast stars, address what it means for a star to be fast, and describe the processes that accelerate stars. I concisely summarise the Gaia mission, whose recent data releases heavily influenced this thesis. Supernovae in binary systems can fling away the companion; if a runaway companion can be associated with a supernova remnant, then together they reveal the evolution that led to the supernova. However, these associations are difficult to establish. In Ch. 2, I develop a sophisticated Bayesian methodology to search the nearest ten remnants for a companion, by combining data from Gaia DR1 with a 3D dust-map and binary population synthesis. With Gaia DR2, I will identify companions of tens of supernova remnants and thus open a new window to studying late-stage stellar evolution. It is unknown why 17% of B stars are spinning near break-up; these stars are termed Be stars because of emission lines from their ejected material. Their rapid spin could be due to mass transfer, but in Ch. 3 I show this would create runaway Be stars. I demonstrate using a hierarchical Bayesian model that these exist in sufficient numbers, and thus that all Be stars may arise from mass transfer. The stars escaping the Milky Way are termed hypervelocity stars. In Ch. 4, I overturn the consensus that the hypervelocity stars originated in the Galactic centre by showing that a Large Magellanic Cloud (LMC) origin better explains their distribution on the sky. In Ch. 5 I present three ground-breaking hypervelocity results with Gaia DR2: 1) only 41 of the 524 hypervelocity star candidates are truly escaping, 2) at least one of the hypervelocity stars originates in the LMC, and 3) the discovery of three hypervelocity white dwarf runaways from thermonuclear supernovae.

This dissertation is concerned with the understanding of physico-chemical properties of energetic materials (EMs). Recently, a substantial amount of work has been directed towards calculations of equations of state and structural changes upon compression of existing EMs, as well as elucidating the underlying chemistry of initiation in detonating EMs. This work contributes to this effort by 1) predicting equations of state and thermo-physical properties of EMs, 2) predicting new phases of novel EMs, and 3) examining the initial stages of chemistry that result in detonation in EMs. The motivation for the first thrust, is to provide thermodynamic properties as input parameters for mesoscale modeling. Such properties are urgently sought for a wide range of temperatures and pressures, and are often difficult or even impossible to obtain from experiment. However, thermo-physical properties are obtained by calculating structural properties and vibration spectra using density function theory and employing the quasi-harmonic approximation. The second thrust is directed towards the prediction and investigation of novel polymorphs of known azide compounds to identify precursor materials for synthesis of polymeric nitrogen EMs. Structural searches are used to identify new polymorphs, while theoretical Raman spectra for these polymorphs are calculated to aid experimentalists in identifying the appearance of these azide compounds under high pressure. The final thrust is concerned with elucidating the initial chemical events that lead to detonation through hypervelocity collision simulations using first-principles molecular dynamics. The chemical mechanisms of initiation are determined from the atomic trajectory data, while heats of reaction are calculated to quantify energy trends of chemical transformations.

One of the current goals of research in hypersonic, airbreathing propulsion is access to higher Mach numbers. A strong driver of this goal is the desire to integrate a scramjet engine into a transatmospheric vehicle airframe in order to improve performance to low Earth orbit (LEO) or the performance of a semi-global transport. An engine concept designed to access hypervelocity speeds in excess of Mach 10 is the shock-induced combustion ramjet (i.e. shcramjet). This dissertation presents numerical studies simulating the physics of a shcramjet vehicle traveling at hypervelocity speeds with the goal of understanding the physics of fuel injection, wall autoignition mitigation, and combustion instability in this flow regime. This research presents several unique contributions to the literature. First, different classes of injection are compared at the same flow conditions to evaluate their suitability for forebody injection. A novel comparison methodology is presented that allows for a technically defensible means of identifying outperforming concepts. Second, potential wall cooling schemes are identified and simulated in a parametric manner in order to identify promising autoignition mitigation methods. Finally, the presence of instabilities in the shock-induced combustion zone of the flowpath are assessed and the analysis of fundamental physics of blunt-body premixed, shock-induced combustion is accelerated through the reformulation of the Navier Stokes equations into a rapid analysis framework. The usefulness of such a framework for conducting parametric studies is demonstrated.

<p>While hypervelocity impacts are ubiquitous throughout the solar system and have received decades  of  research,  the dynamic  fragmentation that  occurs  during an impact has received relatively little attention. This is made more troublesome by the fact that, by volume, more material in the target is altered by the tensile stresses of the rarefaction wave that relieves the pressure of the shock wave, compared to the amount excavated by the impact itself. This tensionally affected material  can  include  Grady-Kippfragments,  fragments  of  material  that  were  broken  apart according to a dynamic fragmentation model developed by Grady and Kipp in 1980. By using their model and inserting it into the Eulerian hydrocode iSALE, we have been able to examine the role tensile stressesand dynamic fragmentation play in hypervelocity impacts. We started by finding the limits on Grady-Kipp fragmentation on an already well studied surface, the Moon. We found that fragment sizes are weakly dependent on impactor size and impact velocity. For impactors 1 km in diameter or smaller, a hemispherical zone centered on the point of impact contains meter‐scale fragments. For an impactor 1 km in diameter this zone extends to depths of 20 km. At larger impactor  sizes,  overburden  pressure  inhibits  fragmentation  and  only  a  near‐surface  zone  is fragmented. For a 10‐km‐diameter impactor, this surface zone extends to a depthof ~20 km and lateral distances ~300 km from the point of impact. This suggests that impactors from 1 to 10 km in diameter can efficiently fragment the entire lunar crust to depths of ~20 km, implying that much of the modern day megaregolith can be created by single impacts rather than by multiple large impact events.</p> <p>With the extent of in-situ fragmentation examined we turned ourattention to getting our dynamic fragmentation code to run smoothly with iSALE’s PorTens. PorTens is a change made to iSALE to allow for pore space creation in material undergoing tensile stresses and pressures in order to keep thermodynamic consistency. Importantly, wefound that when the two routines are combined, porosity increases substantially, and that the large basins currently observed on the Moon’s surface are likely most responsible for the high porosity detected by the Gravity Recovery and Interior Laboratory (GRAIL) mission. Additionally, we discovered that deep lying porosity seems to be additive, suggesting that even without the influence of the largest impactors it is possible for porosity to increase over time. The final, and possibly most consequential conclusion from this work is the ability of tensile stresses and pressures can create potential sitesof refugia for early life that may have existed on early Earth or possibly Mars.</p> <p>Our final dive into hypervelocity impacts focuses on modeling fragments of ejecta. To study this, we have restructured the original fragmentation code substantially. Because most of the damage occurring in the ejecta is done in shear, our previously used Grady-Kipp implementation is not able to provide any useful data, without first making some necessary changes. Much of shear stresses occurring during the passage of a shockwave is accommodated by ductile deformation. Thus, we allow tensile damage to accumulate independently of any calculated shear damage. This simple assumption allows us to track fragment size within ejecta curtains.We then present the results of fragment size vs velocity for different sized impactors.</p> <p><br></p>