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Characterization of low levels of turbulence generated by grids in the settling chamber of a laminar wind tunnel
Get PDFWind tunnel investigations of how Natural Laminar Flow (NLF) airfoils respond to atmospheric turbulence require the generation of turbulence, whose relevant characteristics resemble those in the atmosphere. The lower, convective part of the atmospheric boundary layer is characterized by low to medium levels of turbulence. The current study focuses on the small scales of this turbulence. Detailed hot-wire measurements have been performed to characterize the properties of the turbulence generated by grids mounted in the settling chamber of the Laminar Wind Tunnel (LWT). In the test section, the very low base turbulence level of Tuu ≅ 0.02% (10 ≤ f ≤ 5000 Hz) is incrementally increased by the grids up to Tuu ≅ 0.5%. The turbulence spectrum in the u-direction shows the typical suppression of larger scales due to the contraction between grids and test section. Still, the generated turbulence provides a good mapping of the spectrum measured in flight for most of the frequency range 500 ≤ f ≤ 3000 Hz, where Tollmien-Schlichting (TS)-amplification occurs for typical NLF airfoils. The spectra in v and w-direction exhibit distinct inertial subranges with slopes being less steep compared to the − 5/3 slope of the Kolmogorov spectrum. The normalized spectra in u-direction collapse together well for all grids, whereas in v- and w-directions the inertial- and dissipative subranges are more clearly distinguished for the coarser grids. It is demonstrated that the dissipation rate ε is a suitable parameter for comparing the wind tunnel turbulence with the atmospheric turbulence in the frequency range of interest. By employing the grids, turbulence in the range 4.4 × 10⁻⁷ ≤ ε ≤ 0.40 m²/s³ at free-stream velocity U∞ = 40 m/s can be generated in the LWT, which covers representative dissipation rates of free flight NLF applications. In the x-direction, the spectra of the v and w-components develop progressively more pronounced inertial- and dissipative subranges, and the energy below f ≈ 400 Hz decreases. In contrast, the spectral energy of the u-component increases across the whole frequency range, when moving downstream. This behavior can be explained by the combination of energy transport along the Kolmogorov cascade and the incipient return to an isotropic state
Nonlinear Flight Control Law Design for a Hybrid Quad-Plane Unmanned Aerial Vehicle
Get PDFHybrid quad-plane unmanned aerial vehicles (UAVs), also known as dual-system hybrid UAVs, combine the advantages of multicopters and fixed-wing UAVs and have recently been gaining increasing attention. However, quad-planes are characterized by complex structures, high nonlinearity, strong coupling, effector redundancy, and three flight regimes (hover, transition, and fixed-wing flight), posing significant challenges for researching their flight control laws. Classical linear control laws have been widely used for quad-planes, but their limited adaptability can lead to degraded flight performance.
To enhance the flight performance of a quad-plane named Scihunter, this thesis aims to design nonlinear flight control laws for it in the three flight regimes. These control laws should enable Scihunter to autonomously follow a predefined path and achieve stable and smooth transition flight. Nonlinear flight control laws, based on incremental nonlinear dynamic inversion (INDI) and linear parameter-varying (LPV) control methods, are proposed in this thesis because they are simple to design, easy to implement, and offer superior control performance. Moreover, due to the distinct flight characteristics in the three flight regimes, the variable structure control technique is employed in the control law design.
A brief analysis of Scihunter's mathematical model is first provided, followed by the proposed cascade control structures divided into inner and outer loops. Then, control laws based on INDI control are designed for each control loop in hover and fixed-wing flight. During the control law design for transition flight, an extension of INDI, incremental nonlinear control allocation, is adopted to address effector redundancy and achieve a smooth flight transition.
Subsequently, control laws based on LPV control are proposed for the three flight regimes. The loop shaping-based parameter-dependent Lyapunov function method is used to design the inner-loop control law. To reduce the controller order and improve computational efficiency, the proportional-integral-derivative (PID) control method is used to track outer commands. Given the over-actuated property of Scihunter in transition flight, the weighted least square control allocation strategy is utilized to allocate virtual control inputs to actual actuators to fulfill transition requirements.
Finally, simulations are conducted in four cases: a nominal case, a measurement noise case, a wind disturbance case, and a model parameter uncertainty case, to evaluate the designed control laws. To provide a benchmark for comparison, the conventional flight control law for quad-planes, designed using the PID control method, is chosen as a comparison group. Simulation results corroborate the performance improvements achieved by the proposed control laws
IDIOME IM DAF-UNTERRICHT. Ergebnisse einer Frequenzuntersuchung für das Projekt PhraseoLab
Get PDFKorpusgesteuerte und korpusbasierte Untersuchungen führen überzeugend vor Augen, dass die Sprache zu einem viel stärkeren Grad aus konventionalisierten Mehrworteinheiten besteht als früher angenommen wurde. Daraus ergibt sich das Postulat, ihnen einen angemessenen Stellenwert im Fremdsprachenunterricht einzuräumen. Dieser Aufgabe versucht das von der EU kofinanzierte Projekt PhraseoLab gerecht zu werden, dessen Ziel es ist, eine digitale Lernressource zum Erlernen deutscher Phraseologie im Rahmen eines plurilingualen Ansatzes zu erarbeiten. Im Projekt werden drei Gruppen fester Mehrworteinheiten didaktisch aufbereitet: Kollokationen, Routineformeln und Idiome, der Schwerpunkt des vorliegenden Beitrags liegt auf Idiomen. Da der Status unterschiedlicher Gruppen von Mehrworteinheiten in der Phraseodidaktik umstritten ist, wird zuerst die Rolle der Idiome im DaF-Unterricht diskutiert. Anschließend werden die Ergebnisse einer Frequenzuntersuchung von Idiomen in der geschriebenen (DWDS) und gesprochenen Sprache (DWDS, DGD-Korpora) dargestellt, deren Ziel darin besteht, eine empirisch fundierte Selektion erwerbsrelevanter Idiome vorzulegen
(Meta-)Narrationen – Beschreibend-reflexive Zugänge zu lebensweltlicher Mehrsprachigkeit in der Lehramtsausbildung
Get PDFObwohl sprachliche Vielfalt in der Schweiz gesetzlich geregelt ist und gefördert wird, funktioniert ein Grossteil der Kantone einsprachig. Auch im schulischen Kontext dominieren nach wie vor monolingual geprägte Normvorstellungen. In diesem Zusammenhang interessiert uns, wie Menschen mit anderen Erstsprachen als Deutsch in biografisch-narrativen Interviews mit Lehramtsstudierenden über mehrsprachiges Aufwachsen in der Deutschschweiz berichten, und wie dieses ko-konstruierte Wissen von den Studierenden in Metatexten reflektiert wird. Mit Bezug auf narrativ-biographische Zugänge, die wir in der soziolinguistischen und diskursorientierten Erzählforschung verorten, ergründen wir zudem, inwiefern narrativ-sprachbiographische Zugänge in der Hochschullehre helfen, monolingual geprägte Normvorstellungen zu hinterfra-gen und Mehrsprachenkompetenz zu normalisieren
The interplay of corporate finance and sustainability: Evidence from sustainability-linked lending, transparency initiatives and corporate litigation
Get PDFThis dissertation explores the interaction between corporate sustainability and corporate finance across three thematic areas. The first thematic area examines how the publication of the Carbon Disclosure Project’s (CDP) list, comprising companies that have refused to respond to the questionnaire on environmental data, affects these companies’ firm value (Chapter 2). Thereby, this chapter provides insights into whether this non-disclosure campaign conducted by the CDP is effective in exerting capital market pressure on companies that refuse to disclose environmental data. The analysis identifies a significantly negative valuation effect for smaller and less profitable companies on the CDP’s list.
The second thematic area focuses on sustainability-linked lending (Chapters 3 to 6). Chapters 3 and 4 provide an overview of the design of the sustainability-related attributes which define sustainability-linked bonds (SLBs) and sustainability-linked loans (SLLs). More specifically, these chapters discuss the sustainability targets and the financial incentivization mechanisms that connect these targets to the financing terms of these debt products. Subsequently, Chapters 5 and 6 address empirical questions in sustainability-linked lending. Chapter 5 initially examines the pricing of SLLs in the debt capital market and finds that SLLs are priced with initial spreads that are, on average, 9.5 basis points lower than the initial spreads of conventional loans from the same issuer with similar characteristics. This pricing advantage for the borrower is greater for borrowers characterized by a strong environmental profile and for loans originated with a lending syndicate characterized by strong environmental characteristics. Chapter 6 investigates the perception of SLB issuances in the equity market by analyzing abnormal stock returns around the announcement of SLB issues. The study identifies a positive announcement effect, which however only remains significant for certain issuers with specific characteristics in the two subsequent trading weeks. Over this period, issuers with a strong environmental profile, as well as those that have not issued use-of-proceeds bonds before the SLB issue, experience significantly more positive stock market reactions than the corresponding issuers that have weaker environmental attributes or those that have issued use-of-proceeds bonds previously.
The final thematic area of this dissertation examines the influence of security class action lawsuits (SCAs) on the corporate governance structures of the defendant companies’ competitors (Chapter 7). Initially, the study presents evidence for two benefits resulting from robust governance structures in the context of corporate litigation. Specifically, the results suggest that robust corporate governance significantly mitigates the risk of being sued in an SCA and, in the event of litigation, also protects the value of the defendant company and to a lower extent that of its competitors. The study argues that these two benefits of robust governance structures should incentivize the competitors of SCA defendants to revise their own governance mechanisms following an SCA in the same industry. Consequently, the study analyzes the development of the SCA defendants’ industry competitors’ governance mechanisms after the SCA using a generalized difference-in-difference approach. The results provide evidence on significantly enhanced governance structures among the competitors of SCA defendants in the years following the SCA. A deeper analysis of the individual governance components reveals that the competitors of SCA defendants particularly enhance the independence of their boards by increasing the proportion of outside directors and the expertise of their boards by increasing the proportion of directors with financial or industry-specific backgrounds
Exploring magneto- and multicaloric materials for room and cryogenic temperature applications
Get PDFA future sustainable society requires energy-efficient and environmentally friendly technologies to successfully tackle the challenges of climate change, population growth, and a rising standard of living. The significance of cooling for society emphasizes the need to find efficient and environmentally friendly refrigeration alternatives. Solid-state caloric cooling represents such an alternative technology. In this thesis, Ni(-Co)-Mn-Ti all-d Heusler alloys, Fe₂AlB₂-type MAB phases, La(Fe,Si)₁₃-type compounds, and the novel Co₄(OH)₆(SO₄)₂[enH₂] organic-inorganic hybrid material are explored for magneto- and multicaloric cooling applications at room and cryogenic temperatures. Concerning room temperature applications, Ni(-Co)-Mn-Ti all-d Heusler alloys are optimized and comprehensively investigated. As a result, very high isothermal entropy changes are achieved in moderate magnetic fields and record-breaking adiabatic temperature changes are reported in high magnetic fields. However, hysteresis is identified as the main obstacle, limiting the caloric performance under cyclic conditions. Fe₂AlB₂-type MAB phases do not exhibit hysteresis and are therefore investigated as low-criticality and low-cost alternatives, showing small to moderate magnetocaloric effects around room temperature. At cryogenic temperatures, a multicaloric cooling approach with isotropic pressure and magnetic field as external stimuli is investigated, using a low-criticality and low-cost La₀.₇Ce₀.₃Fe₁₁.₆Si₁.₄ compound. This approach, though challenging to technically implement, achieved high isothermal entropy changes at natural gas, oxygen, and nitrogen liquefaction temperatures. In combination with adiabatic temperature change measurements of Ni-Co-Mn-Ti all-d Heusler alloys, universal limitations of hysteretic first-order phase transition materials are revealed for hydrogen liquefaction applications, directing the search for suitable candidates to second-order phase transition materials. Following this guideline, the novel compound Co₄(OH)₆(SO₄)₂[enH₂] shows the highest isothermal entropy changes of any rare-earth-free material for magnetocaloric hydrogen liquefaction so far. In general, it is demonstrated that caloric cooling technology, particularly at cryogenic temperatures, has great potential to contribute to carbon-neutrality and energy-efficiency in pursuit of a sustainable future
Potential Analysis and Feasibility Study on the Hydrothermal Utilization of Rivers — Using Marburg on the Lahn River as Case Study
Get PDFDue to their high specific heat capacity and constant thermal regeneration through flow, rivers are a promising renewable environmental heat source. The suitability of a river for hydrothermal use depends on river section-specific and site-specific factors. This paper presents a methodological proposal for a river section-specific potential analysis and site-specific feasibility study. The presentation of measured values in the form of hydrographs and duration curves as well as the use of suitable mean values and confidence intervals are central steps in the evaluation. The application of the potential analysis showed that the discharge in particular influences the size of the heat potential, while the water temperature determines whether and how the hydrothermal use is possible. The focus of this work was on the analysis of water temperature. Its significance for hydro-thermal use is multi-layered, and an assessment should be carried out at different levels. Using two specific sites on the Lahn river as a case study, the feasibility study method is demonstrated in this paper
Context in synthetic biology: Memory effects of environments with mono-molecular reactions
Get PDFSynthetic biology aims at designing modular genetic circuits that can be assembled according to the desired function. When embedded in a cell, a circuit module becomes a small subnetwork within a larger environmental network, and its dynamics is therefore affected by potentially unknown interactions with the environment. It is well-known that the presence of the environment not only causes extrinsic noise but also memory effects, which means that the dynamics of the subnetwork is affected by its past states via a memory function that is characteristic of the environment. We study several generic scenarios for the coupling between a small module and a larger environment, with the environment consisting of a chain of mono-molecular reactions. By mapping the dynamics of this coupled system onto random walks, we are able to give exact analytical expressions for the arising memory functions. Hence, our results give insights into the possible types of memory functions and thereby help to better predict subnetwork dynamics
Thick ceramic barrier coatings derived from transition metal modified polyorganosilazanes for high temperature applications
Get PDFAs part of the DFG-funded research training group MatCom-ComMat, this project focuses on developing novel ceramic coatings for use as environmental and thermal barriers on eutectic Mo-20Si-52.8Ti or similar substrates. This new material system aims to enable the operation of combustion engines, such as aerospace and power plant gas turbines, at temperatures exceeding 1200 °C to increase their efficiency.
To achieve this, the preceramic polymer Durazane 1800 was chemically modified with varying amounts of the transition metals hafnium and tantalum as well as boron because these tailored precursors yield Si_x(Hf_aTa_{1-a})(B_b)C_yN_z ceramics with desirable properties, including low thermal conductivity, high oxidation resistance and relatively high coefficients of thermal expansion for polymer-derived ceramics. The precursors demonstrated excellent processability for coating applications, allowing for the preparation of X-ray amorphous ceramic coatings on various substrates via spin coating followed by thermal treatment. The resulting coatings exhibited good adhesion, homogeneity, evenness, and thermal shock resistance up to a critical ceramic layer thickness above 500 nm. To increase the total coating thickness, a multilayer approach was followed, which, however, has been time and energy intensive. To overcome these drawbacks, a new method for the general fabrication of wet-chemically prepared multilayer coatings was developed and registered as a patent. Using this method, filler- and crack-free polymer-derived ceramic coatings with a thickness of more than 10 μm were prepared for the first time.
At 1200 °C, these ceramic multilayer coatings were able to grant excellent protection against oxidation for both silicon and Mo-20Si-52.8Ti substrates up to at least 50 h. However, for all substrate materials except silicon, severe damage to the coatings occurred due to interdiffusion, an increasing mismatch in thermal expansion coefficients or oxidation of the substrate leading to its expansion or volatilization. Thus, for application as a turbine material as described above, i) a change in the composition of the ceramic coating, ii) the implementation of an interjacent diffusion barrier, or iii) an adjustment in the composition of the substrate is considered to be required in order to transfer the good oxidation properties of the individual materials to the materials compound. Since the developed fast process is not limited to a single precursor, it is expected that a ceramic coating with a changed composition can be similarly prepared. Thereby, the described failure mechanisms may be evaded or inhibited. Furthermore, the prepared ceramic coatings are attractive candidates for environmental barrier coatings on various substrates.
The implementation of fillers was explored as well, explicitly targeting the use case of a thermal barrier coating. Therein, mostly crack-free and adherent Si_x(Hf_1)(B_{0.5})C_yN_z coatings up to thicknesses above 100 μm on a bond coat on silicon substrate could be obtained. These coatings showed strong oxidation when exposed to air at 1200 °C for 100 hours. Still, they remained adherent and mostly crack-free. Exploring this system with other passive and active fillers, such as Hf₆Ta₂O₁₇, holds promise for advancing these findings
Steffen Pappert, Melanie Schröter & Ulla Fix (Hg.). 2008. Verschlüsseln, Verbergen, Verdecken in öffentlicher und institutioneller Kommunikation
Get PDFDer weitaus größte Teil der Forschungsliteratur, die sich um die kommunikativen Kontextbedingungen sprachlicher Bedeutung bemüht, basiert in Nachfolge der Konzeptionen von Jürgen Habermas (1981) und Paul Grice (1975) auf der Annahme, dass sich die Kommunikationspartner um möglichst vollständige, schnelle und direkte Offenlegung ihrer Äußerungsabsichten bemühen. Dabei wird von einer irgendwie zu beschreibenden Korrelation zwischen einem Sprachdatum und einer positiv auf den Kommunikationspartner gerichteten Handlungsintention ausgegangen. Nicht gelingende Kommunikation, z.B. Missverstehen, wird dementsprechend in aller Regel als bilaterales Scheitern gedeutet, zum Beispiel in der Kontextualisierungstheorie Gumperz’ (1982) und in den Missverstehenslinguistiken Wolfgang Falkners (1997), Deborah Tannens (1994) oder Margret Seltings (1987). Ausnahmen von der Prämisse der verständigungswilligen Kommunikationspartner gibt es wenige, darunter Gabriel Falkenbergs Linguistik der Lüge (1982). In dem auf der Interaktionalen Soziolinguistik aufbauenden Modell Volker Hinnenkamps zu Missverständnissen in der sprachlichen Interaktion (1998) gibt es einige Passagen, die in diese Richtung weisen, so diskutiert Hinnenkamp bewusstes Missverstehen aus kommunikationsstrategischen Gründen (1998: 271-301)