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Studies of the reaction mechanism and reinforcing effect of resorcinol and pre-condensates in polydienes
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Spectral and Spatial Investigation of Ocean Tide Signatures in GRACE(-FO) Post-Fit Residuals
No full textThe Gravity Recovery and Climate Experiment (GRACE, 2002–2017) was the first satellite mission to utilize low-low Satellite-to-Satellite Tracking (ll-SST), and the first mission able to monitor mass variations on Earth. Since 2018, the successor mission GRACE Follow-On (GRACE-FO) has been in orbit. The ll-SST concept involves two identical co-orbiting satellites separated by a distance of approximately 220 km. The distance between the satellites is precisely tracked by a K/Ka-band ranging system and serves as the main observable for the derivation of monthly gravity field solutions. These gravity field products are of major importance for studying mass variations within the Earth’s system. The process of computing the monthly gravity field solutions involves satellite orbit modeling and parameter estimation as central components. Since the precise distance measurements provided by the K/Ka-band ranging system contain information on all influences affecting satellite dynamics, various background models are employed during satellite orbit modeling. These models are used to separate the signals that should conventionally be part of the gravity field solutions from disturbances related, for example, to tides, gravitational attraction from celestial bodies, and rapid mass variations in the atmosphere and oceans. Imperfections of ocean tide models are considered among the primary factors limiting the quality of GRACE and GRACE-FO gravity field products. Ocean tide models are known to exhibit significant inaccuracies, especially in polar regions where precise satellite altimetry observations are lacking, and in shallow water regions where ocean tide dynamics are more complex than those in open ocean areas.
As part of this work, a spectral analysis was conducted for the first time to examine ocean tide signatures in the ll-SST post-fit residuals. Monthly gravity field solutions and the corresponding range-rate post-fit residuals for the period from April 2002 to September 2023 were computed. The obtained rangerate post-fit residuals were low-pass filtered, numerically differentiated, and assigned to a global 5°×5° grid. Lomb–Scargle periodograms were computed for the time series in each grid, and then analyzed for frequencies with significant amplitudes occurring on a global scale. In total, over 30 prominent tidal frequencies were identified, which correspond not only to the major gravitationally excited tidal constituents but also to minor degree-2 tides, degree-3 tides, non-linear tides, and radiational tides. With the exception of a few tidal constituents, the corresponding amplitude maps almost exclusively show increased amplitudes in polar regions, along coastlines and confined to some regions of the open ocean. The most complex region where a large number of tidal frequencies show increased amplitudes is the Weddell Sea. Although most of the identified tidal frequencies were considered during orbit modeling, meaning that the amplitudes in the periodograms represent residual signal relative to the ocean tide model used, several unmodeled frequencies were identified. These include degree-3 tides 3M1, 3L2, 3N2, 3M3, the compound tides 2SM2 and 2MK3/MO3, and the radiational triple S3, T3, R3, with the latter two sharing their frequencies with the compound tides SP3 and SK3, respectively. All of these unmodeled tides have been scarcely studied or not studied at all thus far. A global altimetry-constrained ocean tide model for 3M3 was just recently published. At the moment, for 3M1, 3L2, 3N2 altimetry-based solutions are only available for the latitudes from −66° to +66°, but not for the polar regions, where according to the performed spectral analysis large tidal variations exist. Data-constrained ocean tide solutions do not exist for the other frequencies. It was shown that purely hydrodynamic solutions can explain a large part of the degree-3 and radiational signal in the post-fit residuals. For the identified compound tides at the moment not even hydrodynamic models have been published yet. An analysis of altimetry data reveals a qualitative agreement with the 2SM2 and 2MK3/MO3 patterns observed in the post-fit residuals. The findings presented suggest that the analysis of ll-SST post-fit residuals offers significant potential for validating ocean tide models. Future research should also explore the potential for assimilating ll-SST measurements into hydrodynamic models.Das Gravity Recovery And Climate Experiment (GRACE, 2002–2017) war die erste Satellitenmission, die das Low-Low-Satellite-To-Satellite-Tracking (LL-SST) einsetzte, und die erste Mission, die in der Lage war, Massenvariationen auf der Erde zu verfolgen. Seit 2018 befindet sich die Nachfolgemission GRACE Follow-On (GRACE-FO) im Orbit. Das LL-SST-Konzept besteht aus zwei identischen Satelliten, die die Erde in einem gemeinsamen Orbit hintereinander umkreisen und durch einen Abstand von etwa 220 km getrennt sind. Dabei wird der Abstand zwischen den Satelliten präzise von einem K/Ka-Band-Ranging-System erfasst und dient als Hauptbeobachtung für die Berechnug monatlicher Schwerefeldlösungen. Diese Schwerefeldprodukte sind von zentraler Bedeutung für die Untersuchung von Massenvariationen im Erdsystem. Der Prozess der Berechnung monatlicher Schwerefeldlösungen umfasst die Satellitenorbitmodellierung und die Parameterschätzung als zentrale Komponenten. Da die präzisen Distanzmessungen des K/Ka-Band-Ranging-Systems Informationen über alle auf die Satellitendynamik wirkenden Einflüsse enthalten, werden während der Satellitenorbitmodellierung verschiedene Hintergrundmodelle eingesetzt, um die Signale, die konventionell Teil der Schwerefeldlösungen sein sollten, von Störbeiträgen zu trennen, die beispielsweise mit Gezeiten, gravitativer Anziehung durch Himmelskörper und schnellen Massenvariationen in der Atmosphäre und den Ozeanen zusammenhängen. Ungenauigkeiten der Ozeangezeitenmodelle gelten als wesentliche Faktoren, welche die Qualität der GRACE- und GRACE-FO-Schwerefeldprodukte beeinträchtigen. Ozeangezeitenmodelle weisen insbesondere in Polargebieten erhebliche Ungenauigkeiten auf, da dort präzise Satellitenaltimetrie-Beobachtungen fehlen, sowie in Flachwasserbereichen, wo die Dynamik der Ozeangezeiten komplexer ist als in offenen Ozeangebieten.
Im Rahmen dieser Arbeit wurde erstmals eine Spektralanalyse durchgeführt, um Ozeangezeiten-Signaturen in den LL-SST-Post-Fit-Residuen zu untersuchen. Monatliche Schwerefeldlösungen und die dazugehörigen K-Band-Range-Rate-Post-Fit-Residuen für den Zeitraum von April 2002 bis September 2023 wurden berechnet. Die erhaltenen Range-Rate-Post-Fit-Residuen wurden einer Tiefpassfilterung unterzogen, numerisch differenziert und einem globalen 5°×5°-Gitter zugewiesen. Lomb–Scargle-Periodogramme wurden für die Zeitreihen in jedem Raster berechnet und anschließend auf Frequenzen mit signifikanten Amplituden auf globaler Ebene analysiert. Insgesamt wurden über 30 auffällige Gezeitenfrequenzen identifiziert, die nicht nur den wichtigsten gravitationsbedingt angeregten Gezeitenkonstituenten entsprechen, sondern auch kleineren Grad-2-Gezeiten, Grad-3-Gezeiten, nichtlinearen Gezeiten und Strahlungsgezeiten. Mit Ausnahme einiger weniger Gezeitenkomponenten zeigen die entsprechenden Amplitudenkarten fast ausschließlich erhöhte Amplituden in Polarregionen, entlang von Küstenlinien und in einigen begrenzten Regionen des offenen Ozeans. Die komplexeste Region, in der eine große Anzahl von Gezeitenfrequenzen erhöhte Amplituden aufweist, ist das Weddellmeer. Obwohl die meisten der identifizierten Gezeitenfrequenzen während der Orbitmodellierung berücksichtigt wurden, was bedeutet, dass die Amplituden in den Periodogrammen ein Residualsignal gegenüber dem verwendeten Ozeangezeitenmodell darstellen, wurden mehrere unmodellierte Frequenzen identifiziert. Dazu gehören die Grad-3-Gezeiten 3M1, 3L2, 3N2, 3M3, die zusammengesetzten Gezeiten 2SM2 und 2MK3/MO3, sowie die Strahlungsgezeiten S3, T3, R3, wobei die beiden letzteren ihre Frequenzen jeweils mit den zusammengesetzten Gezeiten SP3 und SK3 teilen. Alle diese unmodellierten Gezeiten sind bisher kaum oder gar nicht untersucht worden. Ein globales Ozeangezeitenmodell für 3M3, das Altimetriedaten verwendet, wurde erst kürzlich veröffentlicht. Zurzeit sind für 3M1, 3L2, 3N2 altimetriebasierte Lösungen nur für die Breitengrade von −66° bis +66° verfügbar, nicht jedoch für die Polarregionen, wo gemäß der durchgeführten Spektralanalyse große Gezeitenvariationen existieren. Messdatenbasierte Ozeangezeitenlösungen sind für die anderen Frequenzen nicht vorhanden. Es wurde gezeigt, dass rein hydrodynamische Lösungen einen großen Teil des Grad-3- und des Strahlungssignals in den Post-Fit-Residuen erklären können. Für die identifizierten zusammengesetzten Gezeiten gibt es derzeit noch keine veröffentlichten hydrodynamischen Modelle. Eine Analyse von Altimetriedaten zeigt eine qualitative Übereinstimmung mit den 2SM2- und 2MK3/MO3-Mustern in den Post-Fit-Residuen. Die vorgestellten Ergebnisse deuten darauf hin, dass die Analyse von LL-SST-Post-Fit-Residuen, ein erhebliches Potenzial zur Validierung von Ozeangezeitenmodellen bietet. Zukünftige Untersuchungen sollten auch das Potenzial der Assimilation von LL-SST-Messungen in hydrodynamische Modelle untersuchen
Digital access to mobility: socio-spatial inequalities in public transport app use
No full textPublic transport apps have become one of the most popular digital interfaces for accessing travel information and managing personal mobility. Decision-makers in the transport sector rely heavily on them when introducing new digital services, which range from seat reservations to subsidised digital tickets. At the same time, not all passengers are ready to adopt these apps. The risk of certain social groups being overlooked determines the importance of public transport app use being critically analysed.
This dissertation examines the factors that influence user adoption and engagement with public transport apps, paying particular attention to their socio-spatial aspects. The study draws on regression modelling and machine-learning methods to analyse online and offline survey data collected in the Hanover Region in Germany. The results show that socio-demographic factors such as older age, disability, and limited language proficiency are related to a lower probability of using public transport apps. Older age, lower income, disability, and a migration background are also related to lower user engagement with app features. These findings highlight digital inequality among passengers, including for those who have the apps installed. Furthermore, app adoption varies spatially: residents in less accessible areas are more likely to use public transport apps, possibly due to a higher demand for travel information. Simultaneously experiencing app-related digital inequality and mobility inequality, for example due to residing in an area with a poorer public transport supply, may exacerbate both issues and lead to digital mobility inequality. Beyond that, app engagement is determined by the perceived usefulness of travel information, ticketing and advanced app features, adopted in relation to the frequency with which users take public transport, use their smartphones and engage in online activities. Thus, public transport app use reflects complex digital and mobility behaviour patterns. Understanding them informs inclusive app development and digitalisation policies in public transport sector
Bicarbonate-Rich Mineral Water and Human Health: Biological Mechanisms and Clinical Evidence
No full textBackground and aim: Nutrition plays a crucial role in both subjective well-being and maintaining optimal health. A substantial body of research has demonstrated that the contemporary Western diet, characterized by a high proportion of animal protein, highly processed foods, and a low proportion of fruit and vegetables, is associated with an increased risk of lifestyle diseases. This diet induces an excess of acid, which must be counterbalanced to maintain the normal functioning of the organism. Bicarbonate plays a key role in these processes as a buffer base. Therefore, the objective of this thesis was to provide a comprehensive overview of the current state of research on the effects of bicarbonate-rich mineral water and to critically evaluate potential areas of application concerning the prevention and treatment of diseases and symptoms. To obtain more detailed insights into the effects on the acid-base balance, a human study was also conducted, taking into account both short-term and long-term effects. Given the tendency of mineral water with high bicarbonate levels to exhibit elevated sodium concentrations, particular emphasis was placed on investigating its effects on blood pressure.
Methods: Reviews: A comprehensive literature search was conducted for intervention studies that evaluated the effects of bicarbonate-rich mineral water on human health. Due to the substantial amount of data obtained, the results of the literature search were critically summarized in two distinct narrative reviews. Intervention study: The monocentric, placebo-controlled, randomized clinical study included healthy men and women aged 30-65 years who were not taking any antihypertensive medication and adhered to an omnivorous diet. The participants consumed either 1.5-2 liters/day of mineral water with low mineral content or mineral water with high bicarbonate and sodium levels. The effects of the intervention were assessed by evaluating various blood and urine parameters, both after three days of water consumption and at the end of the four-week intervention period. Furthermore, blood pressure measurements were performed at the study center.
Results: Reviews: The initial review provides a summary of the effects on the buffer capacity of the organism. The findings indicate that the consumption of bicarbonate-rich mineral water results in a decrease in renal net acid excretion (NAE), accompanied by an increase in bicarbonate (HCO3-) levels and a rise in base excess (BE) in the blood. Furthermore, the beneficial effects regarding the prevention of kidney stones are outlined, and evidence of reduced bone resorption is demonstrated. The second review focuses on metabolic changes. The findings indicate that cardiovascular risk factors, including parameters of glucose and lipid metabolism, are positively impacted, and no adverse effects on blood pressure are anticipated. The manuscript also describes positive effects regarding complaints in the upper gastrointestinal tract. Potential benefits for liver health are also highlighted. Intervention study: In the context of acid-base balance regulation, the consumption of bicarbonate-rich mineral water has been demonstrated to be more efficacious than mineral water with a low mineral content. The intervention study demonstrated an increase in urine pH and a decrease in NAE in urine. Furthermore, alterations in blood were detected, suggesting an enhancement in buffer capacity. The effects were already apparent after a short period of consumption (three days) and persisted until the end of the intervention period.
Conclusion: The available data suggest that mineral water with high bicarbonate content has the potential to enhance the body's buffering capacity, thereby contributing to the stabilization of the acid-base balance. This has a favorable impact on metabolism and a wide range of health aspects. In addition, the consumption of mineral water with a high bicarbonate and sodium content does not appear to pose a risk of increasing blood pressure
Population Models with Size and Spatial Structure - Well-posedness & Applications
No full textThis thesis is concerned with the investigation of size-and-spatially structured population mod-
els and related applications. The first part contains the theoretic fundamentals for this thesis,
upon which we rely for subsequent properties and applications. This involves proving well-
posedness of the model in terms of deriving generalized solutions via the method of character-
istics and utilizing the explicit form of solutions to prove that they admit the structure of a
strongly continuous semigroup, called the size-diffusion semigroup. Next, favorable properties
of the parabolic structure in the spatial variable are used to derive regularizing effects of the
size-diffusion semigroup, and its generator is characterized. The theoretic part is rounded off
with several consequences of the explicit form of the size-diffusion semigroup including the
proof that the generator has compact resolvent, a perturbation result based on the theory of
Miyadera and Voigt, and an ansatz for investigating quasilinear problems.
The second part of this thesis contains two applications for size-and-spatially structured popu-
lation equations. The first one models growth and dispersion of tumor cells inside healthy tissue
and consists of several coupled equations. The individual growth of the proliferating tumor
cells in combination with their spatial movement being composed of size-dependent diffusion
and haptotactic behavior present the main issues in the investigation of local existence and
positivity of mild solutions. The chapter is closed off with global existence of mild solutions
under stronger assumptions on the spatial diffusion of tumor cells.
The second application consists of a perturbed model for cell population evolution with division.
The inclusion of a divisional process as a linear perturbation yields a particular asymptotic
behavior for the resulting semigroup called asynchronous exponential growth. This entails that
the long-term behavior of the solutions can be described by exponential growth or decay in
time while stabilizing to a finite-dimensional space with respect to the size-variable. A crucial
prerequisite for this result is eventual compactness of the first generation of the perturbed
semigroup, which is then combined with spectral properties of the perturbed generator, as
well as a result by Krein and Rutman on the point spectrum of compact and strongly positive
operators
Dresdener Kreis 2009 : 10. Dresdener Kreis Elektroenergieversorgung : 02. bis 03. April 2009 in Duisburg
No full textDer Sammelband "Dresdener Kreis 2009: 10. Dresdener Kreis Elektroenergieversorgung vom 02. bis 03. April 2009 in Duisburg" beinhaltet neben einem kurzen Bericht zum 10. Treffen des Dresdener Kreises 2009 wissenschaftliche Beiträge von Doktoranden der beteiligten Hochschulinstitute zum Thema Elektroenergieversorgung. Der Dresdener Kreis setzt sich aus der Professur für Elektroenergieversorgung der Technischen Universität Dresden, dem Fachgebiet Elektrische Energiesysteme der Universität Duisburg-Essen, dem Fachgebiet Elektrische Energieversorgung der Leibniz Universität Hannover und dem Lehrstuhl Elektrische Netze und Erneuerbare Energie der Otto-von-Guericke Universität Magdeburg zusammen und trifft sich einmal im Jahr zum fachlichen Austausch an einer der beteiligten Universitäten
Data-driven patient-specific fluid resuscitation
No full textAutomated fluid resuscitation systems use control algorithms to determine fluid infusion rates for patients recovering from, e.g., hemorrhage or dehydration. Instead of relying on models that require patient-specific parameter identification, we present a data-driven patient-specific alternative that bypasses modeling and identification. Specifically, we implement a predictive controller that only uses blood-pressure data, and manages to regulate the patient’s blood-pressure to a specified setpoint. We demonstrate the results in simulation on 100 virtual patients created using a well-established model of cardiovascular hemodynamics
Hybrid quantum-classical computation – From infrastructure to algorithms
No full textQuantum computers hold the potential of solving problems that are intractable using classical means. Yet, to date no quantum computer has actually solved a problem of general interest that could not have been solved otherwise. Bridging this gap from promised theoretical usefulness to practical relevance is an interdisciplinary challenge, requiring experimental progress, solutions of classical software challenges as well as advances in quantum algorithm design. It is these final two points in which this thesis aims to contribute.
One of the most promising quantum computing architectures is based on trapping ions in surface-electrode ion-traps. In order to productively operate such trapped-ion quantum computers, we develop and benchmark a compiler that optimizes ion shuttling sequences in order to efficiently map abstract quantum circuits onto a physical trapped-ion chip. This is a necessary component of the infrastructure of a quantum computing setup, however it is alone not sufficient to execute arbitrary quantum algorithms.
In particular, a very promising class of quantum algorithms are so-called hybrid quantum-classical algorithms which leverage the computational resources of both quantum and classical computers. To effectively implement such algorithms, a suitable software setup is necessary to allow for tight interplay between the two types of computers without wasting precious coherence time. To solve this challenge, we propose, implement and benchmark our setup for Interpreted Quantum-Classical Computations (IQCCs), demonstrating its feasibility for trapped-ion quantum computers.
For the second part of this thesis we focus on the development of quantum algorithms that make use of this setup. As a first application, we investigate quantum simulations of gauge theories. Such theories are prevalent throughout modern physics and are characterized by local constraints that the dynamics are subject to. Errors in noisy simulations of such theories results in these constraints being violated and the simulations becoming unphysical. We show that this problem can be remedied by measuring gauge operators, decoding possible errors on a classical computer and actively correcting them, which reduces simulation errors. Additionally we interpret the scheme as a sympathetic cooling setup, which allows thermal control of the simulation.
We finally turn to the task of preparing ground states of frustration-free Hamiltonians in quantum computers. Studying ground states of various systems is of paramount importance for solving optimization problems and understanding phenomena such as magnetism, superconductivity and superfluidity. We frame the ground-state preparation problem as an error correction problem, wherein energy is measured locally, measurements are decoded and corrections are applied to reduce the energy. In physically relevant examples we show up to exponentially improved runtimes over previous, similar algorithms and give a general run-time bound in terms of the Hamiltonian gap.Quantencomputer haben das Potential Probleme zu lösen, die mit klassischen Methoden unzugänglich sind. Dennoch hat bis heute kein Quantencomputer je ein Problem von allgemeiner Relevanz gelöst, das nicht auch klassisch lösbar gewesen wäre. Das Überwinden dieser Lücke zwischen der versprochenen Überlegenheit und tatsächlicher praktischer Anwendung ist eine Herausforderung, die interdisziplinäre Zusammenarbeit erfordert. Neben experimentellen Fortschritten ist auch die Entwicklung von klassischer Software-Infrastruktur zum erfolgreichen Betrieb eines Quantencomputers notwendig, sowie Fortschritte bezüglich effizienterer Quantenalgorithmen. Diese letzten beiden Punkte sind es, bei denen diese Arbeit anstrebt einen Beitrag zu leisten.
Eine der vielversprechendsten Quantencomputerplattformen basiert auf Ionen, die in Ionenfallenchips gefangen werden. Für deren Betrieb wird ein Compiler benötigt, der logische Quantenschaltkreise in Instruktionen zur Manipulation dieser Ionen übersetzt. Solch ein Compiler wird in dieser Arbeit entwickelt und analysiert.
Viele moderne Quantenalgorithmen beruhen auf dem Kombinieren von Quanten- und klassischen Rechenresourcen. Um bei solchen hybriden Algorithmen ein enges Zusammenspiel aller Rechnerkomponenten zu erlauben ohne wertvolle Kohärenzzeit zu verschwenden, ist weitere Softwareinfrastruktur notwendig, welche ebenfalls in dieser Arbeit entworfen wird. Es wird ein Aufbau für interpretierte quanten-klassische Berechnungen vorgestellt und dessen Machbarkeit durch die Laufzeitanalyse relevanter hybrider Algorithmen gezeigt.
Im zweiten Teil dieser Arbeit werden Anwendungen entworfen, die sich solche hybride quanten-klassische Berechnungen zu Nutze machen.
Als erstes wird hierzu die Simulation von Gittereichtheorien untersucht. Solchen Theorien sind die Grundlage vieler Zweige moderner Physik und sind charakterisiert durch lokale Zwangsbedingungen, wie z.B. das Gaußsche Gesetz.
Fehler in Simulationen können dazu führen, dass diese Zwangsbedingungen verletzt werden und Simulationen unphysikalisch werden. In dieser Arbeit wird eine Methode vorgestellt, dieses Problem zu lösen durch Messungen lokaler Eichgrößen, klassischer Dekodierung der Messergebnisse und anschließender aktiver Korrektur. Damit wird eine Verbesserung von Simulationsergebnissen und thermische Kontrolle des Systems demonstriert.
Als letztes widmet sich diese Arbeit dem Problem der Grundzustandspräparation frustrationsfreier Hamiltonians auf Quantencomputern. Dies ist von fundamentaler Bedeutung z.B. für das Verständnis von Magnetismus und Supraleitung.
Wir formulieren das Problem der Grundzustandspräparation als eine Fehlerkorrekturaufgabe, wobei lokale Energiemessungen klassisch dekodiert und anschließend korrigiert werden, um die Energie zu reduzieren. In physikalisch interessanten Beispielen demonstrieren wir eine bis zu exponentiell bessere Laufzeit als bekannte, ähnliche Algorithmen und bestimmen eine allgemeine Laufzeitschranke in Abhängigkeit der Energielücke des Hamiltonians
Das Leitbild einer nachhaltigen Kernberuflichkeit : Leitbild für die Berufsbildung
No full textEin Berufsbildungssystem ist auf ein Leitbild angewiesen. Das hier zur Diskussion gestellte Leitbild einer nachhaltigen Kernberuflichkeit, welches für ein zukunftsfähiges Berufsbildungssystem von grundlegender Bedeutung ist, soll diese Lücke schließen. Es zielt darauf ab, die Merkmale und Prinzipien einer nachhaltigen und somit zukunftsorientierten Berufsbildung zu skizzieren. Betont wird, dass die Berufsbildung eng mit dem Beruf verknüpft ist und relevante Grundsätze, die zu einem Beruf und dessen Weiterentwicklung führen, weiter Bestand haben werden. Hierbei ist eine Balance zwischen Beständigkeit und Veränderung wesentlich, um den Zusammenhang zwischen Berufsbildungssystem und Arbeitsmarkt aufrechtzuerhalten und gleichzeitig neue Anforderungen einer digitalisierten und vernetzten Arbeits- und Lebenswelt zu integrieren.
Das vorliegende Leitbild zeigt auf, welche Grundsätze für eine zum Beruf führende Berufsbildung gelten sollen. Sie sollen den Ausgangspunkt für alle Maßnahmen, Programme und Ordnungsanliegen bilden und in der Umsetzung detaillierter ausgestaltet werden.A vocational education and training (VET) system relies on a guiding principle, which the German system currently lacks. The presented guiding principle of sustainable core vocational training, which is fundamental to a future-proof VET system, aims to fill this gap. It seeks to outline the characteristics and principles of sustainable and thus future-oriented VET. It emphasizes that VET is closely linked to occupations and that relevant principles leading to occupations and their further development will continue to hold true. A balance between continuity and change is essential to maintain the connection between the VET system and the labor market while simultaneously integrating the new demands of a digitized and networked working and living environment. This guiding principle demonstrates which principles should apply to VET leading to occupations. These principles should form the basis for all measures, programs, and regulatory concerns and be developed in the implementation of VET
Computational Modeling, Inverse Design, and Characterization of Electro-Mechanical Willis Metamaterials
No full textArtificially engineered materials, known as metamaterials, have been developed to generate extraordinary effective properties useful for modulating dynamic material responses. A specific class of metamaterials, referred to as Willis materials, is characterized by an effective description termed Willis coupling, which is absent in their microstructures and enables coupling between momentum and strain, as well as stress and velocity. Recently, the understanding of Willis materials has evolved, leading to the development of composites in which mechanical and non-mechanical fields are highly coupled. Particularly, when applying the Willis homogenization approach to piezoelectric composites by Salomon and Shmuel [1], the momentum is also macroscopically coupled to the electrical field, resulting in a new form of electro-momentum coupling. The emergence of this Willis-type cross-coupling offers an additional approach to controlling elastic wave responses through non-mechanical stimuli and supports an alternative way to excite wave propagation via electrical means.
Building on the closed-form expressions derived by Shmuel et al. [1], this dissertation develops a numerical framework for ensemble averaging that facilitates the calculation of effective dynamic properties of two-dimensional periodic piezoelectric composites and supports materials design applications. The subsequent task is to design a heterogeneous structure, assisted by topology optimization, to maximize electro-momentum (EM) coupling resulting from the effective electro-mechanical constitutive parameters. Further analysis is performed on the optimized unit cells to demonstrate asymmetric wave propagation due to EM coupling induced by mechanical and electrical loads. Based on the form of effective properties of such piezoelectric metamaterials, the scattering experiment approach is developed to homogenize a heterogeneous medium, where the local (imaginary) parts of Willis coupling as well as EM coupling cause asymmetric responses of the reflection waves. When accounting for the asymmetry of reflection amplitudes, a topology optimization approach is employed to design a one-way zero reflection system. The analysis is conducted on layered piezoelectric media with introduction of material loss as an additional factor to control the asymmetric reflection amplitudes and to effectively embrace the non-Hermitian physics. In addition to the framework of dynamic homogenization methods, the micromechanical technique applied in elastodynamics [2] and electromagnetism [3] is extended to periodic heterogeneous piezoelectric media. This method is derived based on the spatial transformations of field variables and Eshelby tensors into Fourier series, and the conversion of integrals into a system of linear equations. Since the Bloch form is specialized to mechanical and electrical fields, the resultant macroscopic dispersion relations are exactly reproduced in the microscopic counterparts. The propagation of free waves is accordingly examined by comparing their microscopic responses with effective ones to verify the homogenization work in the case of long wavelengths.
The concept of Willis materials is notably extended to flexoelectric composites [4], whose non-mechanical fields are additionally induced by strain gradients. Their homogenized constituent shows that the momentum is also coupled to the strain gradient term through an effective description termed gradient elasto-momentum (GEM) coupling. These heterogeneities are homogenized using ensemble averaging, which is incorporated into the dynamic homogenization approach of Willis to deliver a metamaterial system that possesses this coupling term. The existence of strain gradients, which can break the inversion symmetry of a unit cell, is thus significant for generating the imaginary parts of all cross-coupling terms without requiring asymmetry in microstructures. The study also finds an interesting feature that EM coupling not only arises from intrinsic piezoelectric parameters but is also generated by intrinsic flexoelectric couplings. Successive to the introduction of such flexoelectric Willis materials, physical restrictions on their properties are derived from assumptions about reciprocity, passivity, and causality. In particular, reciprocity imposes symmetric constraints on high-order elasticity, flexoelectric tensor, and GEM coupling, in addition to classical terms in piezoelectricity. For passivity, restrictions are placed on the real part of the power flow, ensuring that it is non-negative definite. Inequality relations between the Hermitian and skew-Hermitian parts of the effective material parameters are then defined, followed by the derivation of bounds for the GEM response. Causality, associated with the concept of passivity, imposes restrictions on the flexoelectric Willis constitutive relations, which are modeled as history functions to represent cause-effect relationships. This leads to the definition of complex-valued material parameters in which the real and imaginary parts are interdependent. Since the physical principles underlying the new Willis medium are theoretically understood, it is of interest to explore its applications in wave manipulation. Specifically, the scattering behavior in layered flexoelectric Willis structures is analyzed to show the role of GEM coupling in inducing asymmetry in reflection phases. Moreover, by adjusting external circuit conditions, the intrinsic non-centrosymmetric feature of dielectric materials causes non-symmetries in the effective properties between a given flexoelectric composition and its inversion, which can be harnessed to further modulate dynamic material responses and asymmetric wave phenomena