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  • oscillation
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  • hierarchical frequency multiclusters... phase oscillators
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  • spike frequency adaptation... oscillations
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  • Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.,This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.,A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads.,
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  • semiconductor oscillator
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  • softening Duffing oscillator
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  • out-of-plane oscillation
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  • The internal and external flow field of a fluidic oscillator with two feedback channels are examined experimentally within the incompressible flow regime. A scaled-up device with a square outlet nozzle is supplied with pressurized air and emits a spatially oscillating jet into quiescent environment. Time-resolved information are obtained by phase-averaging pressure and PIV data based on an internal reference signal. The temporal resolution is better than a phase angle of 3°. A detailed analysis of the internal dynamics reveals that the oscillation mechanism is based on fluid feeding into a separation bubble between the jet and mixing chamber wall which pushes the jet to the opposite side. The total volume of fluid transported through one feedback channel during one oscillation cycle matches the total growth of the separation bubble from its initial size to its maximum extent. Although the oscillation frequency increases linearly with supply rate, sudden changes in the internal dynamics are observed. These changes are caused by a growth in reversed flow through the feedback channels. The time-resolved properties of the emitted jet such as instantaneous jet width and exit velocity are found to oscillate substantially during one oscillation cycle. Furthermore, the results infer that the jet’s oscillation pattern is approximately sinusoidal with comparable residence and switching times.
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    • Document
  • Self-excited helical flow oscillations are frequently observed in gas turbine combustors. In the present work a new approach is presented tackling this phenomenon with stability concepts. Three reacting swirling flows are investigated. All of them undergo vortex breakdown, but only two of them show self-excited global flow oscillations at well-defined frequencies. The oscillations feature a precession of the vortex core and synchronized Kelvin-Helmholtz instabilities in the shear layers. Based on the mean flow fields, local and global linear hydrodynamic stability analyses are carried out. The dampening effect of the Reynolds stresses is accounted for by an eddy viscosity estimated from the experimental results. Both the local and the global analysis successfully identify linear global modes as being responsible for the large-scale flow oscillations and successfully predict their frequency. However, only the global analysis accurately predicts a globally stable flow field for the case without the oscillation, while the local analysis overpredicts the global growth rate. The predicted spatial distribution of the amplitude functions agree very well to the experimentally identified global mode. This successful application of global and local stability concepts to a complex and practically relevant flow configuration paves the way for the application of theoretically-founded passive and active control strategies.
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  • This thesis discusses the dynamics of quantum-optical systems located in complex, non-markovian photonic environments, using numerical simulations as well as analytical derivations. The main focus lies on time-delayed quantum-coherent feedback effects of the Pyragas type, in analogy to effects well-known in classical physics. Time-delayed feedback is treated as a specific kind of a non-markovian environment. Additionally, the control of qubit entanglement in optical cavities is studied. The thesis consists of three main parts: After an overview of the important concepts of quantum optics, advanced methods for the description and numerical treatment of time-delayed quantum-coherent feedback are presented. The main focus lies on the development of a pseudomode-based approach, which describes time-delayed feedback as the coupling to a complex network of lossy harmonic oscillators. Additionally, further methods for numerical simplification are presented, in particular nonlinear expectation value dynamics for the treatment of arbitrary non-markovian reservoirs, as well as the use of input-output theory for time-delayed feedback. In the second part, applications of time-delayed feedback to control quantum statistics are discussed. First it is demonstrated that time-delayed feedback can be used to create and sustain entanglement between coupled qubits. This approach will then be extended to larger networks of qubits. Afterwards, feedback control is applied to nonlinear quantum-optical systems: It is shown how to use it for the stabilization of Fock states in a cavity containing a Kerr medium, and this analysis is expanded to a cavity containing a two-level system. Furthermore, it is shown how time-delayed feedback can be used to control and enhance the entanglement of photons emitted in a biexciton cascade. In the third and last part, the non-equilibrium dynamics of qubits coupled to a photonic environment consisting of high-Q cavities subject to periodic driving are discussed. First, the connection between bistability and entanglement in a cavity containing two qubits is analyzed. Next a setup of two coupled cavities, containing a qubit each, is examined. The influence of a third cavity in between the two other cavities, simulating a delay line, is analyzed. Furthermore, a protocol is developed on how to overcome dephasing between the two qubits, using a resonant Raman scattering process. It is shown that this protocol can create and sustain high values of entanglement, and is ready to be extended to systems of more than two qubits.,Diese Arbeit behandelt die Dynamik quantenoptischer Systeme in komplexen, nicht-markovschen photonischen Umgebungen mit Hilfe numerischer Simulationen und analytischer Rechnungen. Der Schwerpunkt liegt auf Effekten von zeitverzögerter quanten-koheränter Rückkopplung vom Pyragas-Typ, in Analogie zur Pyragas-Kontrolle in klassischen nichtlinearen Systemen. Zeitverzögerte Rückkopplung wird in dieser Arbeit als eine spezielle Art einer nicht-markovschen Umgebung behandelt. Am Ende der Arbeit wird außerdem die Kontrolle von Qubit-Verschränkung in optischen Kavitäten behandelt. Diese Arbeit ist in drei Teile gegliedert: Nach einer Übersicht über die wichtigsten Konzepte der Quantenoptik werden neue fortgeschrittene Methoden zur Beschreibung und numerischen Simulation von zeitverzögerter Rückkopplung präsentiert. Ein Pseudomoden-basierter Ansatz wird entwickelt, der Rückkopplung als Kopplung zu einem Netzwerk von harmonischen Oszillatoren beschreibt. Zudem wird eine Methode präsentiert, mit der durch nichtlineare Terme die numerische Simulation nicht-markovscher Umgebungen vereinfacht werden kann. Auch die Verwendung der Input-Output-Theorie für zeitverzögerte Rückkopplung wird behandelt. Im zweiten Teil werden Anwendungen von zeitverzögerter Rückkopplung diskutiert. Zuerst wird die Erzeugung von Verschränkung in zwei gekoppelten Qubits analysiert. Diese Methode wird auf Qubit-Netzwerke verallgemeinert. Zeitverzögerte Rückkopplung wird daraufhin auf nichtlineare quantenoptische Systeme angewendet und die Stabilisierung von Fock-Zuständen in einer Kavität mit Kerr-Medium gezeigt. Die Resultate werden auf eine Kavität mit angekoppeltem Zweiniveausystem übertragen. Schließlich wird der Einfluss von zeitverzögerter Rückkopplung auf die Verschränkung von Photonen aus einer Biexzitonkaskade diskutiert. Im dritten Teil der Arbeit werden Qubits, die an eine oder mehrere Kavitäten gekoppelt sind, behandelt. Der Zusammenhang von Verschränkung und Bistabilität im Falle zweier Qubits in einer Kavität wird analysiert. Daraufhin wird ein System aus zwei gekoppelten Kavitäten, die je ein Qubit beinhalten und kohärent gepumpt werden, analysiert. Der Einfluss einer dritten Kavität, die eine verzögerte Kopplung modelliert, wird diskutiert. Schließlich wird ein Protokoll präsentiert, mit dem, basierend auf resonanter Raman-Streuung, die Dephasierung zwischen Qubits verringert werden kann. Es wird gezeigt, dass dieses Protokoll zu hohen Qubit-Verschränkungen führt und auch auf mehr als zwei Qubits erweitert werden kann.,
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