Date and Place: Thursdays in Room 32349. For detailed dates see below!
Content
In the Scientific Computing Seminar we host talks of guests and members of the SciComp team as well as students of mathematics, computer science and engineering. Everbody interested in the topics is welcome.
List of Talks

Tue03Nov2015
10:30SC Seminar Room 32349
Prof. Andrea Walther, Institut für Mathematik, Universität Paderborn
Title:
On the optimization of piecewise smooth functionsAbstract:
This talk presents the absnormal form for piecewise linear functions, which can be used for the optimization of piecewise linear function. Such an optimization serves as inner loop for the solution of general piecewise smooth optimization problems using successive piecewise linearization (SPL). For the convex as well as the nonconvex case the latest results with respect to the convergence analysis of this new SPL approach will be presented. Furthermore, first numerical results will be shown.

Thu05Nov2015
11:30SC Seminar Room 32349
Simon Gottschalk, Fraunhofer ITWM
Title:
The Oneshot method for optimization problems with underlying PDEsAbstract:
Optimization problems with underlying PDEs appear in disciplines like geophysics, medical imaging, atmospheric science and aerodynamic shape design. Conventional approaches known from the nonlinear optimization theory are in general extremly costly. This talk introduces the Oneshot method in order to solve such optimization problems with less expense. The main idea of this approach is presented in the case of a steady PDE as a constraint. It is explained how the KarushKuhnTucker conditions can be used to find an iterative method which is able to solve this problem. In the second part optimization problems with underlying unsteady PDEs are considered. It is shown how one can transform this problem into a new form such that the Oneshot method is applicable.

Thu19Nov2015
11:30SC Seminar Room 32349
Marc Schwalbach, von Karman Institute (VKI)
Title:
Coldtohot transformation for adjoint optimization of turbomachinery componentsAbstract:
Due to the centrifugal forces experienced by turbomachinery components in operation, structural deformations occur such that the running “hot” structure differs from the resting “cold” structure. Typically, an optimization is performed in the “hot” state and the resulting CAD model has to be deformed to the “cold” state as a postprocessing step for manufacturing. In this project, a coldtohot transformation is to be included in the optimization loop to compute the sensitivities of the objective function with respect to the “cold” CAD parameters directly using adjoints. This talk will give a summary of the overarching project, which involves the CADbased adjoint optimization of turbomachinery components under mechanical constraints, and discuss the coldtohot transformation.

Thu26Nov2015
11:30SC Seminar Room 32349
Junis Abdel Hay, SciComp
Title:
Modelling Real Acoustic Absorbers for Computational Aeroacoustics
Abstract:
Acoustic liners are widely used as acoustic absorbers in the inlet and bypass duct of aeroengines. Commonly used acoustic liners are Helmholtz resonator arrays and consist of a rectangular or honeycomb cell structure which is covered by a perforated facesheet. As far as aeroengines are concerned drainage slots are cut into the resonator walls. Therefore, these resonators are exchanging fluid and are considered to behave nonlocally reacting. The behaviour of locally reacting liners can be described by the acoustic impedance. Currently, there is no method to directly measure the impedance under ﬂow conditions but it can be calculated indirectly by an impedance eduction. Therefore, a liner model is used to include lined surfaces in Computational AeroAcoustic (CAA) simulations. The model’s parameters must be optimized to reproduce the measured liner characteristics. The optimized parameters can be used to predict the liner behaviour for complex geometries and with flow. For some novel liner concepts or nonlocally reacting liners, the behaviour is too complex to be accurately reproduced by models based on simple physical assumptions. This issue is investigated based on impedance eductions for locally and nonlocally reacting liners using different liner models and optimization methods. The effects of the nonlocally reacting behavior will be discussed and the applicability of these different methods is reviewed. An analytical model for two coupled nonlocally reacting Helmholtz resonators is presented and the structure of the model parameters will be discussed. The effects generated by the propagation of sound through the cavities are identified. The model is validated based on CAA simulations of this configuration with normal sound incidence. Furthermore, the results are compared to the experimental data and the model is extended empirically.

Mon07Dec2015
11:30SC Seminar Room 32349
Dr. Torsten Bosse, Argonne National Laboratory
Title:
An asynchronous Oneshot method with loadbalancingAbstract:
We propose a ‘blurred’ oneshot method for the minimization of design optimization problems using a parallel and asynchronous evaluation of the three basic tasks: the state, the adjoint and the controlupdate. In particular, for each task it is allowed to always use and/or override the latest information of another task, i.e., rather than waiting until the fixedpoint iteration provides a new state update it is assumed that parts of the corresponding adjoint iteration already use the latest information from the simulation code. Naturally, this crosscommunication between the three tasks will lead to inconsistencies and any mathematical convergence theory for such an approach is far from being obvious.
Nevertheless, one can expect convergence of the method in certain cases. The key for the success of such a method relies on an optimal distribution of the different tasks for a given amount of available resources on a igh performance cluster. This assignment problem yields a possibility to influence the contraction rates of the primal and dual updates as well as the change in the control variables. Also, it can be be shown that the blurred Oneshot algorithm is a generalization of the previously presented Jacobi and (Multistep) SeidelOneshot method, which can be recovered by a suitable allocation of resources. The blurred method can be applied on (discretized) optimal control problems, which might also include unsteady PDEs.

Thu10Dec2015
11:30SC Seminar Room 32349
Prof. Domenico Quagliarella, Italian Aerospace Research Center (CIRA)
Title:
Risk measures and optimization under uncertaintyAbstract:
Many industrial optimization processes must take account of the stochastic nature of the system and processes to be designed or redesigned and have to consider the random variability of some of the parameters that describe them. Thus it is necessary to characterize the system that is being studied from various points of view related to the treatment of uncertainty. This talk is related to the use of various risk measures in the context of robust and reliability based optimization. We start from the definition of risk measure and its formal setting and then we show how different risk functiontional definitions can lead to different approaches to the problem of optimization under uncertainty. In particular, the application of valueatrisk (VaR) and conditional valueatrisk (CVaR), also called quantiles and superquantiles, is here illustrated. These risk measures originated in the area of financial engineering, but they are very well and naturally suited to reliabilitybased design optimization problems and they represent a possible alternative to more traditional robust design approaches. We will then discuss the implementation of an efficient riskmeasure based optimization algorithm based on the introduction of the Weighted Empirical Cumulative Distribution Function (WECDF) and on the use of methods for changing the probability measure. Finally we will discuss the problems related to the error in the estimation of the risk function and we will illustrate the “bootstrap” computational statistics technique to get an estimate of the standard error on VaR and CVaR. Finally, we will report some simple application examples of this approach to robust and reliability based optimization.

Thu17Dec2015
11:30SC Seminar Room 32349
Thomas Dick, SciComp
Title:
Development of a Discrete Adjoint method for unsteady PDE optimization
Abstract:
In the design and optimization of aerospace systems, there are many problems which are unsteady by nature. For example active flow control, aeroacoustic or turbo machinery. For these problems the assumption, that the underlying physical problem is steady, is not valid. In this talk we will show how one can adapt an adjoint framework for unsteady problems. Therefore we will generalize the first discretize then optimize approach for this case. This will allow us to construct a Lagrangian and to derive the KKT condition, which will ultimately give us a discrete adjoint equation.

Thu28Jan2016
11:30SC Seminar Room 32349
Prof. ClausDieter Munz, IAG, Uni Stuttgart
Title:
High Order Discontinuous Galerkin Schemes for Industrial Flow ProblemsAbstract:
Highorder methods are attractive for simulations of multiscale problems. Due to their low dispersion and dissipation errors, they minimize the number of points, required to resolve a wavelength or another structure with given accuracy. In this talk a survey is given about the construction of the class of spectral element discontinuous Galerkin schemes. Several building aspects in this construction are considered in more detail including shock capturing and dealiasing. Their high computational efficiency, especially on high performance computers is motivated and shown. As applications the fluid flow around a rear mirror of a car is shown and the noise generation by an acoustic feedback identified. Other large scale simulations include the direct numerical simulation of supersonic boundary layers and an example of plasma flow.

Mon08Feb2016
15:00SC Seminar Room 32349
Dr. Roland Ewert, DLR Institut für Aerodynamik und Strömungstechnik
Title:
Strömungsgeräuschsimulation mittels stochastischer QuellenAbstract:
Der Vortrag präsentiert hybride aeroakustische Simulationsmethoden und ihre Anwendung auf Umströmungsgeräuschprobleme der Luftfahrt. In einem ersten Schritt wird das turbulente Umströmungsproblem mit Reynolds gemittelten NavierStokes (RANS) Gleichungen simuliert. Die resultierende Schallgenerierung und Schallpropagation wird in einem zweiten Schritt mit Hilfe von Störungsgleichungen und Methoden der Numerischen Aeroakustik (CAA) gelöst. Die Simulationen verwenden instationäre Geräuschquellen, die stochastisch auf Basis der RANS Turbulenzstatistik realisiert werden. Durch die hohe Simulationseffizienz im Vergleich zu skalenauflösenden Verfahren ist die Analyse einer größeren Anzahl von Konfigurationen möglich. Die Methode bietet damit eine geeignete Grundlage für einen optimierten aeroakustischen Entwurf.
Zur effizienten stochastischen Generierung synthetischer Turbulenz für Schallquellen wird die Fast Random ParticleMesh (FRPM) Methode eingesetzt. Ziel der stochastischen Rekonstruktion ist eine möglichst genaue lokale Reproduktion der verschiedenen statistischen Zielgrößen aus der RANS. Der derzeitige Status der Rekonstruktionsqualität der teilweise gegenseitig abhängigen Zielgrößen wird präsentiert und zukünftige Verbesserungsmöglichkeiten diskutiert.
Verschiedene aeroakustische Propagationsverfahren werden diskutiert. Lineare Störungsgleichungen (APE) können die Refraktion des Schallfeldes im nichtuniformen Strömungsfeld beschreiben. Eine Diskretisierung mit FiniteDifferenzen (DRP Verfahren) auf strukturierten Multiblock Gittern wird einer Diskretisierung mittels der Diskontinuierlichen Galerkin (DG) Methode auf unstrukturierten Gittern gegenüber gestellt. Die Vorhersagequalität wird für generischen Hinterkantenlärm, die Schallabstrahlung von Hochauftriebskonfigurationen und Strahllärm für lärmoptimierte Düsenrandmodifikationen demonstriert.
Des Weiteren wird als Propagationsverfahren ein schnelles Multipole Boundary Element Verfahren für die konvektive Wellengleichung auf der Basis von unstrukturierten Oberflächennetzen vorgestellt, dass die Simulation größerer umströmter Komponenten (z.B. Halbflügel eines Verkehrsflugzeuges) bei vereinfachter Propagationsphysik ermöglicht. Abschließend werden Ergebnisse für skalenauflösende Simulationen mit NavierStokes Gleichungen in Störungsform (NLDE) gezeigt, wobei eine stochastische Quelle als ein aktives BackscatteringModell eingesetzt wird.