CeNoS Kolloquium im Sommersemester 2015

Vortragsreihe: Ehemalige im Beruf

Simulierte Motoren mit Hilfe Neuronaler Netze: Echzeitfähige Motormodelle am HiL-Prüfständen

Michael Grevenstette, Bertrandt AG Ehningen
Einladender: O. Kamps

Die Elektrifizierung des Automobils hat in den vergangenen Jahrzehnten deutlich zugenommen. 60 und mehr elektronische Steuergeräte (Electronic Control Units - ECUs) für die einzelnen Komponenten (Motor, Getriebe, ABS, ESP, usw.) sind in modernen Autos keine Seltenheit. Die funktionale Entwicklung und Absicherung dieser Steuergeräte geschieht nicht mehr nur im realen Fahrzeug, sondern immer öfter auch an HiL (Hardware-in-the-Loop)-Simulatoren. Auf diesen läuft dann ein möglichst genaues Simulations-Modell des Gesamtfahrzeugs bzw. der zu testenden Komponente, welches die Steuersignale der ECU auswertet und dann dem Steuergerät in Echtzeit die entsprechenden Sensorsignale zur Verfügung stellt, analog zur realen Komponente.

Am Beispiel eines Motor-HiLs möchte ich einmal die Grundzüge einer Motor-Simulation mit Hilfe eines echtzeitfähigen, neuronalen Prozessmodells vorstellen und des Weiteren aufzeigen, welche Möglichkeiten diese Simulation bietet und wo die Grenzen zur genauen physikalischen Abbildung liegen.

Prof. Dr. Krassimir Panajotov, B-Phot Brussels Photonics Team
Einladende: S. Gurevich

We discuss recently discovered deterministic chaos in Vertical-Cavity Surface-Emitting Lasers (VCSELs). Our quantum dot (QD)-VCSEL emits linearly polarized light at threshold and undergoes a pitchfork bifurcation when increasing injection current that creates two symmetrical elliptically polarized solutions. Then, a sequence of bifurcations brings it to two single-scroll chaotic attractors. Finally, the two chaotic attractors merge into a double-scroll "butterfly" chaotic attractor. Experimentally, this "butterfly" attractor is observed as a mode hopping between two elliptically polarized modes. The dwell time exponentially decreases with injection current which is opposite to noise-induced polarization mode hopping in quantum well VCSELs. We report on bistability between two limit cycles, which originate from a bifurcation on the two elliptically polarized states. Asymmetry in the system, namely a misalignment between the VCSEL phase anisotropy and dichroizm explains well all experimental features. Finally, we demonstrate the physical generation of random bits at high bit rates (> 100 Gb/s) using the optical chaos from the solitary QD VCSEL.

Sondertermin

Dynamics of localized structures in cavity nonlinear optics subject to injection and delayed feedback

Dr. Mustapha Tlidi, Université Libre de Bruxelles
Einladende: S. Gurevich

Localized structures in dissipative media have been observed in various field of nonlinear science such as fluid dynamics, optics, laser physics, chemistry, and plant ecology. Localized structures consist of isolated or randomly distributed spots surrounded by regions in the uniform state They may consist of peaks or dips embedded in the homogeneous background. They are often called spatial solitons, dissipative solitons, localized patterns, cavity solitons, or auto-solitons depending on the physical contexts in which their were observed.

We investigate a control of the motion of localized structures of light by means of delay feedback in the transverse section of a broad area nonlinear optical systems. The delayed feedback is found to induce a spontaneous motion of a solitary localized structure that is stationary and stable in the absence of feedback. In the absence of the delay feedback we present an experimental evidence of stationary localized structures in a 80 μm aperture vertical cavity surface emitting laser. The spontaneous formation of localized structures takes place above the lasing threshold and under optical injection. Then, we consider the effect of the time delayed optical feedback and investigate analytically the role of the phase of the feedback and the carrier lifetime on the motion of the localized structures. We show that these two parameters affect strongly the space time dynamics of two- dimensional localized structures.

Prof. Dr. Vincenzo Capasso, University of Milano
Einladender: M. Burger

In the   mathematical modeling of tumor-driven angiogenesis, the strong coupling   between  the kinetic parameters of the relevant stochastic branching-and-growth of the capillary network, and  the family of interacting underlying fields is a major source of complexity from both the analytical and computational point of view.

Our main goal is thus to address the mathematical problem of reduction of the complexity of such systems by taking advantage of its intrinsic multiscale structure; the (stochastic) dynamics of cells will be described at their natural scale (the microscale), while the (deterministic) dynamics of the underlying fields will be described at a larger scale (the macroscale)

Prof. Dr. Markus A. Schmidt, Leibniz Institute of Photonic Technology, Fiber Sensors Research Group, Jena and Max Planck Institute for the Science of Light, Erlangen
Einladende: C. Denz

Hybrid optical fibers are microstructured optical fibers which include axially invariant nanostructures in the form of nanowires. Such nanowires can modify the properties of the fibers in a unprecedented way leading to novel devices with applications in various fields of science and technology. In this talk I will report on our latest results in fiber-based plasmonics (e.g., plasmonic hybridization in arrays of gold nanowires or the development of monolithic near field nanoprobes) and in nonlinear optics using hybrid chalcogenide-silica waveguides (e.g., coherent mid-IR supercontinuum generation).

A.G. Vladimirov, Weierstraß-Institut für Angewandte Analysis und Stochastik, Berlin
Einladende: S. Gurevich

An approach to the modeling of nonlinear dynamics in multimode semiconductor lasers using delay differential equations (DDEs) is discussed. We consider DDE models of different multimode laser devices: passively mode-locked semiconductor lasers generating short optical pulses with high repetition rates, frequency swept lasers used in optical coherence tomography, and multi-stripe laser arrays with off-axis optical coherent feedback. We present the results of numerical simulations of different dynamical states in these lasers and discuss asymptotic approaches to the stability analysis of stationary and periodic operation regimes. In particular, using a DDE model of a multimode ring laser we provide a theoretical interpretation of the dynamical behavior observed experimentally in long- and short-cavity frequency swept lasers operating in the Fourier domain and sliding frequency mode-locked regimes, respectively.

Data-driven model reduction in the Loewner framework

Athanasios Antoulas, Rice Univ. & Jacobs Univ. Bremen
Einladender: M. Ohlberger

Interpolatory model reduction methods have matured quickly in the last decade and have been adopted by an ever-growing number of researchers. They have emerged as one of the leading choices for truly large scale problems. These methods have their roots in numerical analysis and linear algebra and are related to rational interpolation and Pade approximation. In the case of linear dynamical systems, the main idea behind these methods is to generate a reduced-model whose transfer function interpolates that of the original system at select interpolation points. Recently, major advances showed how to apply interpolation methods to nonlinear systems. The resulting approach turns out to global,
in other words no small inputs are required.

In this talk we will give an overview of recent advances in model reduction of linear and nonlinear dynamical systems by means of interpolatory methods and in particular the  Loewner framework. Several examples illustrating the theory will also be presented.

A few references

A.C. Antoulas, S. Lefteriu, and C.A. Ionita,
A tutorial introduction to the Loewner Framework for Model Reduction
In 'Model Reduction and Approximation for Complex Systems', Edited by P. Benner, A. Cohen, M. Ohlberger, and K. Willcox, Birkhauser, ISNM Series (2015).

S. Gugercin, C.A. Beattie and A.C. Antoulas,
Data-driven and interpolatory model reduction
Book in preparation, SIAM (2015).

Nematic elastomers: a biomimetic material

Len Pismen, Israel Institute of Technology, Haifa 
Einladender: U. Thiele

Nematic elastomers, made of cross-linked polymeric chains with embedded mesogenic structures, combine orientational properties of liquid crystals with solid elasticity. Their specific feature is a strong coupling between the director orientation, that can be affected by chemical reactions and light, and mechanical deformations. This property enables their usage in actuators and self-propelling devices. The feedback interactions between nematic order, elastic stress, and composition lead to a variety of patterns in nemato-elastic films and their spontaneous bending into three-dimensional forms, which are strongly affected by topological defects of nematic order.

Dr. Andreas Menzel, Institut für Theoretische Physik II - Soft Matter , Heinrich-Heine-Universität Düsseldorf
Einladender: U. Thiele

Mid-IR-driven Electron Recollision: Molecular Diffraction Imaging and Attosecond soft-X-rays

Prof. Dr. Jens Biegert, ICFO - The Institute of Photonics Sciences, Mediterranean Technology Park, Barcelona
Einladender: H. Zacharias

Electron recollision in an intense laser field is at the centre of attoscience research and gives rise to a variety of phenomena, ranging from electron diffraction to coherent soft X-ray emission. We have, over the years, developed intense sources of waveform controlled mid-IR light, i.e. few-cycle duration and carrier to envelope phase stable pulses, to exploit ponderomotive scaling, quantum diffusion and quasi-static photo emission. I will briefly highlight the laser technology that enables this new direction of strong field research and our recent achievements in sub-Angstrom resolution imaging of an entire polyatomic molecule, the generation of isolated attosecond pulses at the carbon K-shell edge (284 eV) and application to soft X-ray absorption spectroscopy in condensed matter.

Interfacial phenomena in thin liquid films: mathematical modeling and scientific computation

Prof. Dr. Te-Sheng Lin, Department of Applied Mathematics,National Chiao Tung University
Hsinchu, Taiwan
Einladender: U. Thiele

The mechanics of thin liquid films can be modeled by a fourth-order nonlinear partial differential equation, the so-called thin film equation, which describes the evolution of the film thickness. In the first part of the talk we will discuss two approaches in the model derivation: one is the long-wave model derived through asymptotic expansion of the full governing equations and the other one is the gradient dynamics formulation based on an underlying free energy functional. As an example we discuss the model for a thin film of nematic liquid crystal in the limit of strong anchoring at the free surface and at the substrate. We show that the two derivations should agree with each other.

In the second part of the talk we will discuss several computational methods in investigating the models. One is the alternating-direction-implicit type finite difference solver to look for the time dependent solution for a given intial condition. The other one is to take advantages of the numerical continuation method to look for the stable or unstable steady state solutions and the time-periodic solutions. As a result one is able to construct the full bifurcation diagram of the solutions. We show the behavior of partially wetting liquids on a rotating cylinder as an example.