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This paper considers the reconstruction of a defect in a two-dimensional waveguide during non-destructive ultrasonic inspection using a derivative-based optimization approach. The propagation of the mechanical waves is simulated by the Scaled Boundary Finite Element Method (SBFEM) that builds on a semi-analytical approach. The simulated data is then fitted to a given set of data describing the reflection of a defect to be reconstructed. For this purpose, we apply an iteratively regularized Gauss-Newton method in combination with algorithmic differentiation to provide the required derivative information accurately and efficiently. We present numerical results for three different kinds of defects, namely a crack, a delamination, and a corrosion. These examples show that the parameterization of the defect can be reconstructed efficiently and robustly in the presence of noise., 22 pages, 11 figures

In this contribution we present innovative methods for broadband and resonance-free sensing and emitting of ultrasound. The sensing method uses a polyethylene foil and a laser vibrometer as a broadband and resonance-free sound receiver. In general, this method enables absolute measurement of sound particle velocity and sound pressure in arbitrary, laser beam transparent liquids and gases with known density and sound velocity. The resonance-free emitting method is based on the electro-thermo-acoustic principle and enables, contrary to conventional ultrasound transducers, generation of arbitrary shaped acoustic signals without resonances and post-oscillations.

International audience; The structural integrity of wheelset axles of high speed trains as well as freight wagons is of major importance for safety of rail transport. Up to now the inspection of wheelset axles is part of a periodic maintenance which is expensive due to long inspection times and highly influenced by the human factor, especially if those inspections are carried out during night time. Permanently installed SHM-systems at each axle would be able to monitor the structural integrity and could reduce the maintenance effort. However due to mechanical constraints, requirements and regulations the mounting options of such SHM-systems are limited. Approaches with ultrasonic transducers mounted at one end face of the axle can excite guided waves and could be able to monitor the entire component from only one sensor position. The geometry of the axle with many cross-sectional variations results in reflections and changing dispersion relations along the wave propagation direction and lead to complex wave propagation problems hiding flaw echoes. Thus, echo signals are difficult to interpret and correlations between echo signals and flaws are hard to reveal. In the presented paper we show a signal processing approach to extract information on the structural integrity from ultrasonic guided wave echo data. On a mock-up we could show, that the suggested method is able to detect a crack growth reliably. Based on the knowledge of wave propagation and on results from numerical simulations, classification algorithms are developed and applied to additionally enhance the prediction accuracy of crack size and position.

Die vorliegende Arbeit beschreibt die Untersuchungen verschiedener Aspekte der Luftschallausbreitung in kleinen, teilweise offenen Hohlräumen, die mit aktiven und passiven Einbauten gefüllt sind. Die Untersuchungen werden unterteilt in die Beschreibung der Schallquellen im Hohlraum einerseits und die Vorhersage der Transferimpedanz als Bindeglied zwischen den Quellen und den Feldgrößen an den Schnittstellen zur Umgebung andererseits. Es ist zu berücksichtigen, dass die Struktur des Schallfeldes im Hohlraum stark abhängig ist vom Verhältnis zwischen der Wellenlänge des Luftschalls und den Abmessungen des betrachteten Systems. Drei ausgeprägte Frequenzbereiche werden unterschieden, für die die bekannten Vorhersagemethoden angepasst bzw. neuartige Modellierungsansätze entwickelt werden. Die statistische Energieanalyse (SEA), die im Bereich hoher Frequenzen geeignete Methoden liefert, wird durch einen Korrekturterm ergänzt, der Quell- und Empfängerpositionen in der Nähe der Raumbegrenzungen berücksichtigt. Dieser Korrekturterm berücksichtigt die ersten Reflexionen an den Begrenzungswänden mit Hilfe von Spiegelschallquellen. Für den Bereich mittlerer Frequenzen werden zwei neuartige Ansätze zur Vorhersage des Schallfeldes vorgeschlagen. Die präsentierten Ansätze basieren auf wahrscheinlichkeitstheoretischen Beschreibungen der Systemgeometrie und ermöglichen so auch Aussagen über die Zuverlässigkeit der Berechnungsergebnisse. Die vorgeschlagenen Methoden basieren zum Einen auf der Modellierung des freien Fluidvolumens im Hohlraum und zum Anderen auf der Modellierung der Einbauten als einfach geformte Streukörper. Wenngleich die Gültigkeitsbereiche der vorgeschlagenen Verfahren unterschiedlichen Beschränkungen unterliegen, liefern beide Methoden vergleichbare Ergebnisse im interessierenden Frequenzbereich. Die Gültigkeit und Anwendbarkeit der vorgeschlagenen Verfahren wird durch den Vergleich mit den numerischen Berechnungen überprüft. Verschiedene Ansätze werden vorgestellt, um die Schallquellen in das Modell zu integrieren. Als Bindeglied zwischen Quellbeschreibung und Schallausbreitung wird die Zerlegung der komplex geformten Quellen in Multipole verschiedener Ordnung vorgeschlagen. Bei der Zerlegung komplex geformter Quellen unter Anwendung der sphärischen Nahfeldholographie (NAH) sind häufig die Gültigkeitsbedingungen der Rayleigh-Hypothese nicht erfüllt. Für diesen Fall wird ein Näherungsverfahren vorgestellt, dass eine Reihenentwicklung in eine begrenzte Anzahl von Gegenbauer-Polynomen verwendet, um die Probleme durch Schallfeldsingularitäten zu beseitigen. Die Gültigkeit und Anwendbarkeit der vorgestellten Verfahren, Methoden und Ergebnisse, werden durch den Vergleich mit experimentell gewonnenen Ergebnissen validiert. In the study presented different aspects are investigated of air borne sound propagation inside a partially open enclosure densely packed with active and passive installations. Primarily the work is aimed at predicting the sound field at the interfaces of the enclosure to the adjacent fluid space. The prediction procedure is subdivided into the description of the sound sources inside the enclosure and the prediction of the transfer impedance linking the sound sources to the sound field at the interface to the adjacent fluid. Depending on the relation between wavelength and the geometrical dimension of the system the sound field structure inside the enclosure is found to vary markedly with frequency. Common analysis tools are adapted for their applicability in the given context. The Statistical Energy Analysis (SEA), which is assumed to be applicable in the high frequency region, is supplemented by an approach including the influence of source and receiver positions close to boundaries or field discontinuities. Therefore a correction is integrated into the SEA considering the first order reflections by including the corresponding image sources. From testing the applicability of common analysis tools it is shown a lack of appropriate analysis tools for the intermediate frequency range, which also provide information about the reliability of the analysis results. Two novel approaches are presented to predict the sound field in a frequency region of low modal density. The approaches are based on probabilistic descriptions of the system geometry, which appear promising with regard to the engineering practice. It is shown that the acoustical behaviour can be extracted by either modelling the remaining free fluid space or the installations inside the enclosure. Although the validity limitations of the approaches presented are different, both approaches produce similar results in the considered frequency region. The validity and applicability of the probabilistic approaches is demonstrated by comparing the results with those from a numerical calculation. Strategies are presented for integrating the sound sources into the modelling. For the coupling between source characterisation and sound propagation it is suggested to use a decomposition of complex shaped sound sources into multipoles of different order. The decomposition in terms of spherical Nearfield Holography (NAH), however, is found to be limited to simple source geometries. For complex shaped sound sources, not fulfilling the validity conditions of the Rayleigh hypothesis a method using a truncated series of Gegenbauer polynomials is proposed to overcome the problems arising from singularities in the sound field. The validity of the proposed approaches, methods and simplifications is verified by comparing the results with those of dedicated experiments.

Charged species occur in all combustion systems. Chemical pathways which involve ions are known to contribute to the formation of air pollutants like soot and aerosols. The appearance of electrical charges, which are closely related to the combustion process itself, offers the opportunity of their use for sensing purposes in a variety of applications. Reliable models for the chemical reaction network and the transport processes are required for numerical simulations in these fields of research. Laminar flat flames have proven to be suitable systems for the development and validation of chemical reaction mechanisms. In this work, the concentrations of charged species along a flat, fuel-lean, and laminar methane-oxygen flame are calculated and compared to experimental results. For the first time, these simulations also include negative ions. Existing software was enhanced to enable the inclusion of these ions. The chemical reaction mechanism of the charged species is compiled from different sources found in according literature. Altogether, the model contains 65 reversible reactions involving 11 charged species. Also special emphasis is put on the diffusion processes of the ions. A model is developed and discussed which describes the mutual interactions of the charged species during diffusion. It allows an arbitrary fraction of negative ions, because it does not depend on the assumption that the electrons dominate this process. The simulations are used to validate the reaction mechanism. Reaction flow analyses show which chemical pathways are taken. Reactions which were suggested in the literature are discussed quantitatively. The influence of charged species diffusion on the simulation results as well as their sensitivities to uncertainties in the input data of the transport model are analyzed.

Über die Notwendigkeit eines Bürokratieabbaus sind sich Bürger, Politiker und Wissenschaftler grundsätzlich einig. Doch über die Details einer Entbürokratisierung wird häufig gestritten. Wie sind Bürokratiekosten zu messen? Welche Formen von Bürokratieproblemen gibt es? Wie sollte eine exzessive Bürokratisierung bekämpft werden?