京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19474号 / 理博第4134号 / 新制||理||1595(附属図書館) / 32510 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 荒木 武昭, 教授 佐々 真一, 教授 山本 潤 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

With the growing threat of orbital debris impacts to space structures, the development of space shielding concepts has been a critical research topic. In this study, numerical simulations of the hypervelocity impact response of stacked aluminum 6061-T6 sheets were performed to assess the effects of layering on penetration resistance. This work was initially motivated by set of experimental tests where a stack of four aluminum sheets of equal thickness was observed to have a higher hypervelocity ballistic resistance than a monolithic aluminum sheet with the same total thickness. A set of smoothed particle hydrodynamic simulations predicted a 40% increase in the ballistic limit for a 6-layer target compared to a monolithic sheet. In addition, the effect of variable sheet thickness and sheet ordering on the impact resistance was investigated, while still maintaining a constant overall thickness. A set of thin layers in front of a thick layer generally lead to a higher predicted ballistic limit than the inverse configuration. This work demonstrates an increase in the performance of advanced space shielding structures associated with multi-layering. This suggests that it may be possible to dramatically improve the performance of such structures by tailoring the material properties, interfaces, and layering concepts.

No description available.

No description available.

No description available.

We employ the AdS/CFT correspondence and hydrodynamics to analyze the transport properties of \(2+1\) dimensional electron fluids. In this way, we use theoretical methods from both condensed matter and high-energy physics to derive tangible predictions that are directly verifiable in experiment. The first research topic we consider is strongly-coupled electron fluids. Motivated by early results by Gurzhi on the transport properties of weakly coupled fluids, we consider whether similar properties are manifest in strongly coupled fluids. More specifically, we focus on the hydrodynamic tail of the Gurzhi effect: A decrease in fluid resistance with increasing temperature due to the formation of a Poiseuille flow of electrons in the sample. We show that the hydrodynamic tail of the Gurzhi effect is also realized in strongly coupled and fully relativistic fluids, but with modified quantitative features. Namely, strongly-coupled fluids always exhibit a smaller resistance than weakly coupled ones and are, thus, far more efficient conductors. We also suggest that the coupling dependence of the resistance can be used to measure the coupling strength of the fluid. In view of these measurements, we provide analytical results for the resistance as a function of the shear viscosity over entropy density \(\eta/s\) of the fluid. \(\eta/s\) is itself a known function of the coupling strength in the weak and infinite coupling limits. In further analysis for strongly-coupled fluids, we propose a novel strongly coupled Dirac material based on a kagome lattice, Scandium-substituted Herbertsmithite (ScHb). The large coupling strength of this material, as well as its Dirac nature, provides us with theoretical and experimental access to non-perturbative relativistic and quantum critical physics. A highly suitable method for analyzing such a material's transport properties is the AdS/CFT correspondence. Concretely, using AdS/CFT we derive an estimate for ScHb's \(\eta/s\) and show that it takes a value much smaller than that observed in weakly coupled materials. In turn, the smallness of \(\eta/s\) implies that ScHb's Reynolds number, \(Re\), is large. In fact, \(Re\) is large enough for turbulence, the most prevalent feature of fluids in nature, to make its appearance for the first time in electronic fluids. Switching gears, we proceed to the second research topic considered in this thesis: Weakly coupled parity-breaking electron fluids. More precisely, we analyze the quantitative and qualitative changes to the classical Hall effect, for electrons propagating hydrodynamically in a lead. Apart from the Lorentz force, a parity-breaking fluid's motion is also impacted by the Hall-viscous force; the shear-stress force induced by the Hall-viscosity. We show that the interplay of these two forces leads to a hydrodynamic Hall voltage with non-linear dependence on the magnetic field. More importantly, the Lorentz and Hall-viscous forces become equal at a non-vanishing magnetic field, leading to a trivial hydrodynamic Hall voltage. Moreover, for small magnetic fields we provide analytic results for the dependence of the hydrodynamic Hall voltage on all experimentally-tuned parameters of our simulations, such as temperature and density. These dependences, along with the zero of the hydrodynamic Hall voltage, are distinct features of hydrodynamic transport and can be used to verify our predictions in experiments. Last but not least, we consider how a distinctly electronic property, spin, can be included into the hydrodynamic framework. In particular, we construct an effective action for non-dissipative spin hydrodynamics up to first order in a suitably defined derivative expansion. We also show that interesting spin-transport effects appear at second order in the derivative expansion. Namely, we show that the fluid's rotation polarizes its spin. This is the hydrodynamic manifestation of the Barnett effect and provides us with an example of hydrodynamic spintronics. To conclude this thesis, we discuss several possible extensions of our research, as well as proposals for research in related directions. / Wir verwenden die AdS/CFT-Korrespondenz und die Theorie der Hydrodynamik, um die Transporteigenschaften von \(2+1\)-dimensionalen Elektronisches Flüssigkeiten zu untersuchen. Somit nutzen wir sowohl theoretische Methoden der Fest\-körper\-physik als auch der Hochenergiephysik, um konkrete Vorhersagen zu treffen, die unmittelbar in Experimenten verifiziert werden können. Zunächst betrachten wir das Forschungsfeld der stark gekoppelten Elektronischen Flüssigkeiten. Motiviert durch die frühen Ergebnisse für die Transporteigenschaften schwach gekoppelter Flüssigkeiten von Gurzhi untersuchen wir, ob sich ähnliche Eigenschaften auch in stark gekoppelten Flüssigkei\-ten manifestieren. Dabei konzentrieren wir uns insbesondere auf den hydrodynamischen Teil des Gurzhi-Effekts, in welchem der Widerstand der Flüssigkeit mit steigender Temperatur sinkt, weil sich im untersuchten Material ein Poiseuillefluss von Elektronen bildet. Wir zeigen, dass dieser hydrodynamische Teil des Gurzhi-Effekts auch in stark gekoppelten und vollständig relativistischen Flüssigkeiten realisiert ist, einige Eigenschaften sich hierbei aber quantitativ unterscheiden. Insbesondere zeigen stark gekoppelte Flüssigkeiten immer kleinere Widerstände als schwach gekoppelte, und sind damit wesentlich effektivere Leiter. Wir schlagen darüber hinaus vor, die Abhängigkeit des Widerstands von der Kopplung zu nutzen, um die Kopplungsstärke der Flüssigkeit zu messen. Für diese Messungen stellen wir analytische Ergebnisse bereit, welche den Widerstand als Funktion des Quotienten aus Scherviskosität und Entropiedichte \(\eta/s\) der Flüssigkeit ausdrücken. Dabei ist \(\eta/s\) selbst eine bekannte Funktion der Kopplungsstärke in den Grenzfällen schwacher und unendlich starker Kopplung. In einer weiteren Untersuchung stark gekoppelter Flüssigkeiten schlagen wir Scandium-substituiertes Herbertsmithit (ScHb) als neuartiges, stark gekoppeltes Diracmaterial vor, welches auf dem Kagome-Gitter basiert. Die hohe Kopplungsstärke und die Dirac-Eigenschaften dieses Materials vermitteln uns theoretischen und experimentellen Zugang zu nicht perturbativer relativistischer und quantenkritischer Physik. Um die Transporteigenschaften eines solchen Materials zu untersuchen, stellt die AdS/CFT-Korrespondenz eine hervorragend geeignete Methode dar. Konkret nutzen wir AdS/CFT, um eine Abschätzung von \(\eta/s\) in ScHb herzuleiten. Der so ermittelte Wert ist wesentlich kleiner als der entsprechende Messwert für schwach gekoppelte Materialien. Der kleine Wert von \(\eta/s\) wiederum impliziert, dass die Reynolds-Zahl \(Re\) in ScHb groß ist. Tatsächlich ist \(Re\) hinreichend groß, um erstmals Turbulenz in Elektronisches Flüssigkeiten beobachten zu können, ein Effekt, der auch in viele anderen Flüssigkeiten in der Natur vorkommt. Wir gehen zum zweiten Forschungsthema über, welches in der vorliegenden Arbeit besprochen wird: schwach gekoppelte, paritätsbrechende Elektronisches Flüssigkeiten. Wir betracthen die hydrodynamische Bewegung von Elektronen in einen zwei dimensionalen Kanal, und untersuchen die sich ergebenden quantitativen und qualitativen Änderungen gegenüber dem klassischen Hall-Effekt. Außer der Lorentzkraft ist die Bewegung einer paritätsbrechenden Flüss auch den Einflüssen der Hallviskositätskraft ausgesetzt, welche die von der Hall Viskosität induzierte Scherspannungskraft ist. Wir zeigen, dass das Wechselspiel dieser beiden Kräfte zu einer hydrodynamischen Hall-Spannung führt, die nicht linear vom magnetischen Feld abhängt. Noch wichtiger ist, dass Lorentz- sowie hallviskose Kraft für ein nicht verschwindendes Magnetfeld gleich werden und damit zu einer trivialen hydrodynamischen Hall-Spannung führen. Darüber hinaus geben wir für kleine Magnetfeldstärken analytische Ergebnisse an, die die Abhängigkeit der hydrodynamischen Hall-Spannung von allen experimentell festgelegten Parametern unserer Simulation, wie z.B. Temperatur und Dichte, beschreiben. Diese Abhängigkeiten sind zusammen mit der verschwindenden hydrodynamischen Hall-Spannung charakteristische Eigenschaften hydrodynamischen Transports und können daher verwendet werden, um unsere Vorhersagen experimentell zu verifizieren. Zu guter Letzt untersuchen wir, wie eine charakteristische Eigenschaft von Elektronen, der Spin, in die hydrodynamische Theorie einbezogen werden kann. Dazu konstruieren wir eine effektive Wirkung, die nicht dissipative Spin-Hydrodynamik bis zur ersten Ordnung in einer geeigneten Ableitungsentwicklung beschreibt. Wir zeigen darüber hinaus, dass in zweiter Ordnung dieser Entwicklung interessante Spin-Transporteffekte auftreten. Dabei stellt sich heraus, dass die Rotation der Flüssigkeit seinen Spin polarisiert. Dies ist die hydrodynamische Manifestation des Barnett-Effekts, die als Beispiel für hydrodynamische Spintronics dient. Zum Abschluss der vorliegenden Arbeit diskutieren wir mehrere mögliche Erweiterungen unserer Untersuchungen und unterbreiten Vorschläge für weitergehende Forschung in verschiedene Richtungen.

Previous experimental studies have reported wave-like transport of heat in a small number of material systems, such as superfluids like helium II and crystal solids like bismuth. This phenomenon was henceforth referred to as 'second sound'. These rare observations of second sound are partly due to the challenge of obtaining accurate theoretical predictions. In this work, we use an ab-initio framework to study phonon hydrodynamics in 3D crystal fluorides and alkali hydrides, including sodium fluoride (NaF), lithium fluoride (LiF), lithium hydride (LiH), and sodium hydride (NaH). Moreover, we predict the existence of phonon hydrodynamics regime in bilayer graphene systems, including AA-bilayer graphene and AB-bilayer graphene. First, we obtain the second and third-order interatomic force constants using first-principles calculations, which are based on density functional theory (DFT). Secondly, we calculate the lattice thermal conductivity and phonon scattering rates by solving the Boltzmann transport equation (BTE). Thirdly, we apply Guyer's condition to show the phonon hydrodynamics regime based on the average Normal, Umklapp, and Boundary scattering rates. Finally, we examine the effect of different pseudopotentials on the thermal, electronic, and mechanical properties as well as the phonon hydrodynamics regime. In addition, we report the effect of isotopes on the lattice thermal conductivity and phonon hydrodynamics regime. Our calculations predict the existence of the second sound in NaF at 15 K and 8.3 mm characteristic length, consistent with previous experimental work. Based on Guyer's condition, the hydrodynamic window was determined in terms of characteristic lengths (~10² - ~10⁸ nm) and temperatures (up to ~80 K) for fluorides and alkali hydrides. On the other hand, second sound in 2D materials has been predicted to exist at much higher temperatures relative to 3D materials. We report the existence of a second sound for AA-bilayer graphene and AB-bilayer graphene above room temperature at a characteristic length of ~100 nm.

No description available.

No description available.

The accurate prediction of forces and motions on autonomous underwater vehicles (AUVs) operating close to the wavy free surface is imperative to their usefulness as oceanic research and warfare craft. Maneuvering models for underwater vessels are typically constrained to deep water motions where surface effects are negligible; however, a number of modeling assumptions that are applicable for deep water motions become invalid when the vessel is in proximity to the air-water interface. This dissertation investigates several aspects for the inclusion of free surface effects in maneuvering predictions of a shallowly submerged underwater vehicle. A lumped parameter maneuvering model for deeply submerged motion is improved to accommodate depth dependent effects by updating hydrodynamic derivatives using strip theory and boundary element method analysis. This new model can predict near-surface maneuvering motions of an AUV operating in calm or wavy waters. Alternative free surface affected motion predictions are offered by the Lagrangian Nonlinear Maneuvering and Seakeeping (LNMS) model, which provides motion predictions of a vehicle under waves using calculations from first principle energy considerations. While both models provide their own approach to shallowly submerged vehicle motion predictions, each model suffers from its own limiting hydrodynamic modeling assumptions such as linearized free surface boundary conditions, potential flow assumptions, and slowly varying motions. An investigation into the errors from these simplifying assumptions, including under prediction of the steady-state wave making forces and neglect of viscous effects, led to the creation of an innovative impulse motion model for the calculation of hydrodynamic parameters reducing the need for simplifying assumptions. The significant, novel contributions to near-surface AUV maneuvering research provided in this dissertation are listed below: 1. Creation of a free-surface affected lumped parameter maneuvering and seakeeping model using depth corrected hydrodynamic parameters from strip theory and boundary element method analysis 2. Investigation into the errors associated with linearized free surface boundary conditions and potential flow assumptions during the prediction of near-surface steady-state motions 3. Development of an impulse motion simulation procedure using 3D Unsteady Reynolds- Averaged Navier-Stokes Equation (URANSE) solvers to calculate the infinite frequency hydrodynamic added mass of a shallowly submerged underwater vehicle from rest and constant forward speed / Doctor of Philosophy / Autonomous underwater vehicles (AUVS) are an increasingly used tool in the exploration, defense, and study of our oceans and seaways. An essential aspect for the creation of various AUV systems is the accurate prediction of forces and motions while operating in a variety of different conditions, including near the wavy water surface. Maneuvering models that predict the motions of underwater vehicles often opt for deep water simplifying assumptions where the free surface has no effect; however, these assumptions aren't always valid. This dissertation looks to better understand the effects that a free surface has on AUV motion predictions and how these effects can be captured, understood, and incorporated within different maneuvering models. This goal is achieved by updating a previously constructed deep water maneuvering model to account for proximity to the free surface as well as exploring new methods that calculate the hydrodynamic parameters of a vehicle operating at these depths. With these findings, AUVs will be better informed to move as intended while operating in important combat and research zones of the ocean.