06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
Browse
404 results
Search Results
Item Open Access Measurement of soot precursor particles under atmospheric and low pressure conditions by means of time-of-flight mass spectrometry(2009) González Baquet, Tania; Aigner, Manfred (Prof. Dr.-Ing.)During the last decades a great progress has been achieved in the understanding of the combustion of hydrocarbons. The gas phase reactions governing the first steps in the combustion process are well understood. The existing models describing the growth of soot particles and the formation of soot aggregates are widely accepted, as well. However, the so-called inception, i.e. the mechanism leading to the formation of the first solid particles from gas phase molecules, is still a controversial issue. This is mainly due to the lack of adequate experimental techniques capable of detecting particles in the low nanometer range like those created in the nucleation process in flames. At present most of the combustion models stress the importance of PAH formation and growth in the soot formation process. Other models, however, propose a soot formation mechanism based on the formation of large three dimensional structures without crystallinity. In the present work, the detection and characterization of soot precursor particles, as transition species between gas phase molecules and solid soot particles in the combustion process, is attempted by means of mass spectrometry. To this end a ”custom-built” reflectron time-of-flight mass spectrometer of high sensitivity and with a large mass range is used. Measurements are carried out in different premixed ethylene laboratory flames at different pressures and in a wide range of stoichiometries. Additionally, the exhaust of a gasoline and a diesel engine is investigated. These measurements require the development of a sampling technique capable of transporting the sample from atmospheric conditions to the high vacuum of the mass spectrometer. The resulting fast pulsed sampling system minimizes undesirable sampling line effects while it enables the generation of an optimized molecular beam. Photo ionization of the sample is provided by an excimer laser. The main findings of this work can be summarized as follows: 1. Different types of soot precursor particles can coexist in the flame. For the first time two different types of soot precursor particles with diameters ranging from approximately 1 to 5 nm have been simultaneously detected. The different soot precursor particle modes, in the following referred to as mode A and mode B, show different features. Thus, the existence of at least two different types of soot precursor particles is postulated. Mode A particles are found in a wide range of flame stoichiometries. They are characterized by an ionization order close to two and show a fragmentation threshold of around 0.12 MW/cm2. These particles are considered amorphous, more characteristic of low temperature flames and associated to the soot precursor particles described by D’Alessio et al.. Mode B particles are only observed in a limited stoichiometric range associated with rather high flame temperatures. Mode B particles show an ionization order close to one and a relatively high fragmentation threshold close to 2.24 MW/cm2. These particles are considered to be similar to the ones described by Dobbins et al., i.e. stacks of planar PAHs. 2. Soot precursor particles, although considered to be very reactive, can survive the flame and be emitted. 3. Soot precursor particles are found in significant amounts only at flame stoichiometries above the soot threshold. The lower stoichiometric limit for particle generation is still an issue discussed in the combustion community. All results of this study indicate that the onset of particle formation takes place at flame stoichiometries close to the soot threshold. Consequently, the emission of soot precursor particles seems not to be an outstanding problem in the case of gasoline combustion engines, since the latter work under fairly stoichiometric burning conditions and are characterized by a homogeneous fuel-mixture. This is confirmed by the measurements carried out in the exhaust gas of a gasoline generator. Conventional diesel engines work under globally lean burning conditions but are characterized by a heterogeneous fuel-mixture. Consequently, high particle emissions are expected. The measurements carried out in the exhaust gas of a diesel generator, however, show negligible soot precursor particle emissions. In this case soot precursor particles are oxidized due to the excess of oxygen in the exhaust gas. Soot precursor particle losses due to coagulation with soot particles are also expected. This work demonstrates the utility of time-of-flight mass spectrometry for the detection and study of soot precursor particles. The experimental data presented in this thesis provide new information about the transition region between gas phase molecules and soot particles in the combustion process. This improves the understanding of the soot formation process and stimulates the revision of current combustion models.Item Open Access Faserverbundwerkstoffe im Automobilbau: methodischer Ansatz zur Analyse von Schäden(2012) Schmidt, Alexandra Priska; Drechsler, Klaus (Prof. Dr.-Ing.)Schadensanalyse beinhaltet die Beurteilung geschädigter Bauteile von der Schadenserkennung bis zur Entscheidung über Tolerierbarkeit, Reparatur oder Austausch. Es gibt derzeit keine allgemeine Bewertungsgrundlage, um die zulässige Schadensgröße für Bauteile aus kohlenstofffaserverstärkten Kunststoffen (CFK) an Fahrzeugen im praktischen Einsatz zu ermitteln. Die vorliegende Arbeit liefert einen methodischen Ansatz zur Entscheidungsfindung. Die Arbeit befasst sich mit Schäden, die nachträglich in die Fahrzeugstruktur eingebracht werden. Dies kann etwa durch Fehlgebrauch, Unfälle oder Steinschläge geschehen. Fertigungsfehler werden nicht berücksichtigt. In einem Versuchsprogramm wurde die Restfestigkeit von vorgeschädigten CFK Rohren mit unterschiedlichem Lagenaufbau unter Druckbelastung ermittelt. Realitätsgetreue Impacts werden zunächst durch einfache Bohrungen sowie Längs- und Quernuten auf akademische Weise angenähert und die Ergebnisse mit ungeschädigten Proben verglichen. Der Einfluss der Faserorientierung sowohl auf die Schadensausbreitung bei Impactschäden als auch auf die Restfestigkeit wird betrachtet. Deutliche Zusammenhänge zwischen Schadensgröße und Bruchlast konnten ermittelt und analytisch beschrieben werden. Dadurch ist eine Vorhersage der Restfestigkeit bei weiteren Schadensgrößen möglich. Zur potenziellen Reduktion der Versuchsumfänge wird die Möglichkeit der Computersimulation geprüft. Es wird deutlich, dass Simulationsmodelle die umfangreiche empirische Datenbasis derzeit nicht vollständig ersetzen können. Anhand des seitlichen Dachrahmens eines Automobils wird gezeigt, nach welchen Kriterien die Entscheidung über kritische oder unkritische Schäden erfolgt. Dazu wird der Standardlastfall des Dacheindrücktests verwendet. Die Ergebnisse der Versuche dienen als Bewertungsgrundlage. Ein 7-Punkte-Plan fasst die entwickelte Methodik zusammen und dient als Grundlage für die Analyse. Das exemplarisch durchgeführte Vorgehen kann auf beliebige Geometrien und weitere Lastfälle übertragen werden. Die Erkenntnisse dieser Arbeit können bereits in der Entwicklungsphase eines Automobils als Entscheidungsgrundlage dienen. Unter Berücksichtigung eines geeigneten zerstörungsfreien Prüfverfahrens zur Detektion der Schädigungen eignet sich die vorgestellte Methodik auch zum Einsatz im automobilen Kundenservice. Es wurden verschiedene Methoden miteinander verglichen. Neben Ultraschall und Computertomographie bietet sich die Optische Lock-In Thermographie für den praktischen Einsatz an.Item Open Access Aeroacoustic simulation of turbulent boundary layer induced automotive gap noise(2021) Erbig, Lars; Munz, Claus-Dieter (Prof. Dr. rer. nat.)Item Open Access Modeling, testing and application of tuned liquid multi-column dampers for floating offshore wind turbines(2024) Yu, Wei; Cheng, Po Wen (Prof. Dr.)Item Open Access Modell-basierte Systemsimulation eines Kleinsatelliten mit einem FPGA-basierten On-board-Computer(2009) Falke, Albert; Röser, Hans-Peter (Prof. Dr.-Ing.)Die vorliegende Dissertation beschäftigt sich schwerpunktmäßig mit der Systemsimulation des Satelliten Flying Laptop unter Weltraumbedingungen, mit dem Ziel die Funktionalitäten des Satellitensystems als Ganzes zu verifizieren. Simulation bezeichnet allgemein das Nachahmen des Verhaltens eines Systems oder Prozesses zum Zwecke der Analyse von Systemen, die für die theoretische oder formelmäßige Behandlung zu kompliziert sind. Auch im Kontext einer Kleinsatellitenmission ist das Satellitensystem bereits so komplex, dass man ohne die systemweite Simulation keine detaillierte Analyse unter Berücksichtigung der vielen miteinander interagierenden Komponenten mehr durchführen kann. Bereits seit einigen Jahren verwenden renommierte Satellitenhersteller Simulationstechnologien, um bereits während der Satellitenentwicklung und Fertigung den größtmöglichen Missionserfolg zu gewährleisten. Eine dieser Technologien ist die Modell-basierte Entwicklungs- und Verifikationsumgebung der Firma EADS Astrium GmbH aus Friedrichshafen. Sie bietet ein systematisches und standardisiertes Entwicklungs- und Verifikationsrahmenwerk im Sinne eines systemweiten Satellitensimulators an, um die Entwicklung von Satelliten, die Verifikation der On-board Software und den Gesamtfunktionsnachweis des Satelliten zu unterstützen. In diesem Simulator können alle Satellitenkomponenten modelliert und die On-board Software somit auf funktionaler Ebene verifiziert werden. Auf diese Weise können aufwendige und kostenintensive Entwicklungsmodelle einzelner Subsysteme und Schlüsseltechnologien wegfallen. Insbesondere sei hier die Verifizierung der Genauigkeit des Lageregelungssystems angesprochen. Als technologische Voraussetzung bringt gerade diese Entwicklungstechnologie budgetschwache Kleinsatellitenprojekte der Realisierung einen deutlichen Schritt näher und erhöht gleichzeitig die Güte des Systemdesigns des Satelliten. Im Rahmen einer Kooperation wurde diese Simulationsumgebung dem Institut für Raumfahrtsysteme zur Adaption und Anwendung im Stuttgarter Kleinsatellitenprogramm zur Verfügung gestellt. Zur Anwendung der Systemsimulation im Kleinsatellitenprojekt Flying Laptop sind alle relevanten Satellitenkomponenten im Simulator durch entsprechende, detaillierte Softwaremodelle abgebildet worden. Der sukzessiven Entwicklung der Komponentenmodelle ist durch den schrittweisen Aufbau von immer komplexeren Testständen der Software-Verifikations-Einrichtung Rechnung getragen worden. Infolgedessen erweiterten sich die Simulationsfähigkeiten von ersten Orbitsimulationen mit wenigen Komponentenmodellen, aber mit geschlossenem Simulationskreislauf, über die Simulation von Energie- und Thermalbilanzen unter Betriebsbedingungen bis hin zu vollständigen Systemsimulationen unter Einbindung der realen On-board Kontrollalgorithmen auf einem FPGA-Entwicklungsboard. Gerade dieses charakteristische FPGA-basierte On-board Computer System mit den darauf betriebenen Kontrollalgorithmen führt zu einer großen Herausforderung bei der modellhaften Repräsentation derselbigen im Systemsimulator. In diesem Zusammenhang stellt die Einbindung der realen On-board Kontrollalgorithmen auf einem FPGA-Entwicklungsboard in den geschlossenen Simulationskreislauf eine erfolgreich realisierte technische Neuerung dar. Zum Zeitpunkt der Fertigstellung dieser Arbeit steht dem Projekt Flying Laptop durch die Einbindung der realen On-board Kontrollalgorithmen auf dem FPGA-Entwicklungsboard ein umfangreicher Teststand mit großem Potential zur Entwicklung und zum Funktionsnachweis der On-board Kontrollalgorithmen zur Verfügung. Dieser wurde, wie die vorgestellten Simulationsergebnisse zeigen, bereits intensiv zum Testen der ersten Lageregelungsalgorithmen genutzt und kann auch zukünftig sukzessive mit Funktionserweiterungen der On-board Kontrollalgorithmen angewandt werden. Typische Missionsszenarien wie die Transition zwischen zwei Betriebsmodi des Satelliten oder der automatische Übergang in den SAFE Mode bei einem Fehler können jetzt simuliert und getestet werden. Die Ergebnisse dieser Simulationen dienen nicht nur dem reinen Funktionsnachweis der On-board Kontrollalgorithmen, sondern fließen direkt in die Optimierung der Kontrollalgorithmen zurück und führen so zu einem iterativem Verbesserungsprozess. Der Systemsimulator ist mit seiner Datenbank als Teil seiner Infrastruktur ganz gezielt so implementiert worden, dass einzelne Testszenarien einfach und ohne wiederholten Konfigurationsaufwand reproduziert werden können. Somit unterstützt der Systemsimulator den sich in der Softwareentwicklung typischerweise iterativ wiederholenden Verifikationsprozess in einer optimalen Form.Item Open Access New methods for 3D reconstructions using high resolution satellite data(2021) Gong, Ke; Fritsch, Dieter (Prof. Dr.-Ing. habil. Prof. h.c.)Item Open Access Spectroscopic characterization of extrasolar planets from ground-, space- and airborne-based observatories(2010) Angerhausen, Daniel; Krabbe, Alfred (Prof. Dr. rer. nat.)This thesis deals with techniques and results of observations of exoplanets from several platforms. In this work I present and then attempt solutions to particular issues and problems connected to ground- and space-based approaches to spectroscopic characterization of extrasolar planets. Furthermore, I present the future prospects of the airborne observatory, SOFIA, in this field of astronomy. The first part of this thesis covers results of an exploratory study to use near-infrared integral-field-spectroscopy to observe transiting extrasolar planets. I demonstrate how adaptive-optics assisted integral field spectroscopy compares with other spectroscopic techniques currently applied, foremost being slit spectroscopy. An advanced reduction method using elements of a spectral-differential decorrelation and optimized observation strategies is discussed. This concept was tested with K-Band time series observations of secondary eclipses of HD 209458b and HD 189733b obtained with the SINFONI at the Very Large Telescope (VLT), at spectral resolution of R~3000. In ground-based near infrared (NIR) observations, there is considerable likelihood of confusion between telluric absorption features and spectral features in the targeted object. I describe a detailed method that can cope with such confusion by a forward modelling approach employing Earth transmission models. In space-based transit spectroscopy with Hubble's NICMOS instrument, the main source of systematic noise is the perturbation in the instrument's configuration due to the near Earth orbital motion of the spacecraft. I present an extension to a pre-existing data analysis sequence that has allowed me to extract a NIR transmission spectrum of the hot-Neptune class planet GJ 436b from a data set that was highly corrupted by the above mentioned effects. Satisfyingly, I was able to obtain statistical consistency in spectra (acquired over a broad wavelength grid) over two distinct observing visits by HST. Earlier reductions were unable to achieve this feat. This work shows that systematic effecting the spectrophotometric light-curves in HST can be removed to levels needed to observe features in the relatively small scale-height atmospheres of hot Neptune class planets orbiting nearby stars. In the third and final part of this thesis, I develop and discuss possible science cases for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) in the field of detection and characterization of extrasolar planets. The principle advantages of SOFIA and its suite of instrumentation is illustrated and possible targets are introduced. Possible next generation instrumentation (dedicated to exoplanetary science) is discussed.Item Open Access Improved simulation techniques for modelling impact and crash behaviour of composite structures(2010) Aktay, Levent; Kröplin, Bernd (Prof. Dr.-Ing. habil.)Composite elements such as glass- and carbon-fibre-reinforced tubes in axial crush exhibit high specific energy absorption (SEA) and a challenge for the engineer is to utilise these properties in structural applications. Numerical tools based on the Finite Element Method (FEM) are unable to simulate accurately the observed crush fronts in brittle composite composed of delaminations with fibre and matrix debris, which have a strong influence on energy absorption (EA). As alternative Meshless Methods replace finite element(FE)s by a set of nodes or particles and have the potential to simulate the fragmentation behaviour. However, they are computationally less efficient and costly and appropriate material models and failure criteria are not yet established. Therefore there is an increasing interest in combining the two different numerical methods. Such coupling could exploit the potential of each method while avoiding their deficiencies. In this work the meshless Smooth Particle Hydrodynamics (SPH) Method was used for modelling of composite damage phenomena under crash and impact loads. The computational studies were conducted in the explicit numerical tool PAM-CRASH. Because SPH Method is computationally expensive depending on the dynamic neighbouring search algorithm, new numerical techniques were suggested to reduce the computational time. In these techniques the failure region was modelled using a discrete particle formulation (direct coupling) or the FE mesh was converted into discrete particles (semi-adaptive coupling) to model the debris in the propagating crush front. The coupling was applied through a sliding interface condition. The applicability of SPH Method was firstly tested via benchmark simulation examples, which helped understanding the numerical response of discrete modelling under different loading conditions. The results of the benchmark study were extended to high velocity impact (HVI) simulations of sandwich composite panels. These numerical simulations were conducted for different sandwich panel configurations, impactor shapes and impact velocities. The sandwich panels consisted of carbon fibre/epoxy facings and a core of polyetherimide (PEI) or aramid paper honeycomb (Nomex). The facings were modelled with standard layered shell elements whilst for the PEI core discrete particles were used. In the case of the Nomex core, the core material was modelled with standard solid elements in which the discrete particles were embedded. The numerical results, such as contact force, core deformation and maximum penetration, obtained by using FEM, SPH Method, direct- and semi-adaptive coupled FEM/SPH Method were compared with the experimental results of gas gun impact tests and good agreement was observed. The coupled techniques showed that they can describe more accurately the physical phenomena involved in the EA in the core, where element distortion occurs. Due to the increase of computational time with increasing number of particles, the coupled techniques showed to be adequate since SPH Method was only employed in the impact damage zone. The crashworthy response of aluminum (Al) and composite energy absorbers made of carbon fibre composites were also studied. For both crash absorbers, the tube walls were modelled with shell elements. In the case of metallic tubes, the discrete and the coupled FE/discrete element (DE)s were used to model the polystyrene (PS) foamfiller. For the composite energy absorbers, the crushing response of tube segments specially developed for crushing studies was investigated. These segments could be easily built in thermoset and thermoplastic composite materials with the advantage of giving an indication on the crushing behaviour of bigger structural component made of the same material. For their modelling, the layered shell model was used. The resin material, which showed elastic-plastic material response during crushing, was modelled with discrete particles while bonding and delamination with tied-contact algorithm. Numerical results were also compared with experimental data obtained by crush tests. They showed good agreement for the Al absorbers. Since these tubes are, from the point of view of particles, a closed domain, the experimentally observed deformation modes could be successfully numerically reproduced. This mainly relied on the reproducibility of the crushing behaviour of the metallic structures and their well-known standard material parameters. Composite specimens on the other hand presented different deformation patterns.Item Open Access Stability of a laminar streaky boundary-layer behind a roughness element(2015) Shin, Yong-su; Rist, Ulrich (apl. Prof. Dr.-Ing.)Analysis of flow instability is of importance to understand laminar-turbulent transition which is a crucial factor for aerodynamic performance. The present study deals with influences of a roughness element on the flow instability of a laminar boundary-layer. Roughness elements in laminar boundary-layers generate localized disturbances. They grow transiently and formulate streamwise elongated streaky structures downstream. Spanwise periodicity of these streaky structures disturbs the streamwise development of two-dimensional Tollmien-Schlichting waves in a laminar boundary-layer. In this way, a delay of the laminar-turbulent transition is achieved (flow stabilization). On the other side, physically unavoidable velocity reduction behind the roughness elements brings on high shear layers in wall-normal direction at the same time. Also, separations or strong vortical structures occur occasionally depending on both shape of the roughness elements and flow conditions. They cause a flow destabilization and sometimes trigger a bypass transition. Because these two opposite phenomena happen concurrently and interact with each other, it is difficult to precisely understand the instability mechanisms provoked by the roughness elements. Therefore, the goal of the present work is to study the stability of a laminar streaky layer induced by a roughness element. This work consists of two parts: Bi-global linear stability analysis and experimental measurements. A complex instability procedure of the three-dimensional streaky layers arranged parallel in streamwise and periodical in spanwise direction can be analysed by a bi-global approach. Corroborating experiments were conducted in the laminar water channel at the University of Stuttgart. Simultaneous operation of two hot-film probes and signal processing enabled to find the theoretically calculated unstable eigenmodes in practical flow. In addition, observations of both velocity distribution in a complex flow field and nonlinear vortex structures were carried out by a flow visualization using a hydrogen-bubble method and Particle-Image-Velocimetry measurements. As a result, a streaky layer which includes streamwise elongated high- and low-speed streaks and a separation behind the roughness element was found by a CFD computation using a laminar solver and confirmed by time-averaged experimental velocity components. The bi-global LST identified two highly unstable eigenmodes. These eigenmodes oscillate symmetrically and asymmetrically with respect to the spanwise coordinates and were accordingly termed varicose and sinuous mode, respectively. Their streamwise evolution depends mainly on a streamwise development of the streaks. Experimental results confirmed the presence of these two unstable modal modes. The varicose mode dominants flow instability, and the sinuous mode has a smaller signal-to-noise ratio. Additionally, an external forcing was tried to increase the initial amplitude of the smaller sinuous mode with respect to the varicose one. Despite some deficiencies of the experimental setup, a separate artificial amplification of a specific eigenmode, i.e. the sinuous mode, was possible. In the latter part of the present study, the nonlinear behaviour of the streaky layer further downstream and the breakdown under an over-critical condition were explored. Because the linear theory cannot calculate nonlinear instability, complex three-dimensional flows and vortical structures were investigated by experimental flow visualization methods, and an evolution from nonlinear streaks to hairpin vortices was detected.Item Open Access Potentials and limitations of the a priori data-augmentation of turbulence closure models(2026) Mandler, Hannes; Weigand, Bernhard (Prof. Dr.-Ing. habil.)Turbulent flows occur in numerous technical applications. In some applications, turbulence is deliberately exploited to increase their efficiency. In others, the efficiency can be increased by suppressing turbulence to the greatest extent possible. The ability to accurately predict turbulent flows is, therefore, of immense importance. Nowadays, mainly numerical simulation methods are used for this purpose. As solving the Navier-Stokes equations would be far too costly for most applications of practical interest, the Reynolds-averaged Navier-Stokes equations are typically considered instead. However, their solution requires closure models to describe the influence of the turbulence on the mean flow. As a result of structural and parametric deficiencies of existing models, especially the popular eddy viscosity models, the accuracy of the predicted flow fields often no longer meets the current quality requirements. One way to address these deficiencies is to replace the empirical but often constant model coefficients by functions of the local mean flow field. Unlike the classical modeling approach, which seeks to derive such functional dependencies from theory and physical reasoning, leveraging machine learning instead allows for extracting the desired coefficient functions from publicly available DNS data. The models could, therefore, be calibrated for applications that are still simple but exceed the complexity of the traditional calibration cases, e.g., applications governed flow separation and reattachment. This thesis investigates the merits of this approach with respect to the accuracy of the flow field predictions and the possibility of developing more universal closure models. To this end, an a priori augmentation method for existing closure models was developed. A two-stage procedure was proposed to find appropriate functions for the closure coefficients. First, using the DNS data, the extended closure model is inverted to obtain the spatial distribution of the optimal coefficients for a particular training case. These allow the optimal structure of the constitutive equation to be determined in order to prevent any structural deficiencies. By subsequently solving a regression problem, functions represented by neural networks can be inferred that predict those optimal values of the coefficients as a function of the local mean flow state. Based on three examples, namely the flows through a plane channel, a plane channel with periodic hills, and a square duct, the data-augmented development of such model corrections was demonstrated. The errors in the prediction of the velocity field for the respective training cases could be reduced by up to 65%. The accuracy achieved with this method is typically unmatched even for significantly more complex existing closure models. In addition, it was proven that the extended models provide at least equivalent, but often more accurate predictions than the baseline model for a wide range of Reynolds numbers. The same applies to applications that differ geometrically, but not phenomenologically from the training case. However, if the test case was characterized by different flow phenomena than the training case, a sometimes considerable decrease in the predictive accuracy compared to the baseline model was observed. The obvious strategy of dealing with this loss of universality, i.e., deriving the coefficient functions from a more diverse training data set, proved to be ineffective. This is considered to be due to the complexity of the structure of the coefficient functions, which is limited for stability reasons and, hence, usually not sufficient to actually reflect the diversity of the training data. The method developed in this work for the data-driven augmentation of existing closure models is representative of a number of similar approaches that seek to improve flow field predictions via a more accurate description of the Reynolds stress tensor. In summary, such methods are suitable for developing highly specialized models that achieve the desired accuracy gains for a class of not too complex and phenomenologically similar flows. These two limitations could probably be remedied by CFD-integrated training and well-designed combinations of many such expert models.