Scientific Computing Seminar

Date and Place: Thursdays in Room 32-349. 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

  • Thu
    02
    Nov
    2017

    11:30SC Seminar Room 32-349

    Max Sagebaum, SciComp

    Title: Overview and introduction to MeDiPack

    Abstract:

    The correct Algorithmic Differentiation (AD) of message passing is by itself straight forward. Nevertheless, the handling of the different concepts in MPI like asynchronous request or collective communication and the combination with AD concepts like an index management scheme make the task quite involved. With MeDiPack (Message Differentiation Package) a novel approach is taken that uses code generation to reduce the duplication of implementations. This makes bugfixes or changes straight forward, since only one location needs to be changed and not every method where the MPI or AD concept is used.

    The presentation presents the main challenges for the handling of adjoint message passing and introduces the approach taken for MeDiPack.

  • Thu
    16
    Nov
    2017

    11:30SC Seminar Room 32-349

    Title:
    Selected shape optimization problems from industry and how to solve them with modern HPC techniques

    In this talk we like to give a small insight into shape optimization problems from industry that we were involved in the last couple of months. We will discuss two of them – one coming from aerodynamics and one from hydrodynamics – in detail and especially why they show a need for modern techniques and the use of HPC.

    Most of the all the algorithms and programs that we apply throughout an optimization loop are developed either by our team members or in cooperation with universities from all over the world, the most important ones being the open-source CFD code SU2 and the algorithmic differentiation tool CoDiPack.

    Within the scope of this talk, we will however stress their application and the technical side of the topics rather than their in depth development.

  • Thu
    07
    Dec
    2017

    11:30SC Seminar Room 32-349

    Raju Ram, ITWM

    Title:
    Parallel Deflated Conjugate Gradient Method to Model Groundwater Flow in a Layered Grid

    Abstract:

    Groundwater, present beneath the earth’s surface in soil pore spaces, is the primary source of fresh water that we use in day to day life. Hydrologists at Dutch research institute Deltares are developing large groundwater models to support water managers in their decision-making process. These models use a Deltares accelerated version of MODFLOW called iMODFLOW. Together with the United State Geological Survey (USGS), Deltares has developed the Parallel Krylov Solver (PKS) package, which has recently been incorporated into iMODFLOW. It was observed that for the larger number of subdomains, the Preconditioned Conjugate Gradient (PCG) solver in PKS deteriorates the number of iterations.
    We have implemented the deflation preconditioner with constant and linear deflation vectors in the PCG solver. These vectors approximate the eigenvectors that are slowing down convergence. The groundwater simulation time can be reduced by a factor of 4 using deflation in iMODFLOW. This speedup is achieved due to a decrease in the PCG iterations. The iteration drop is highest using linear deflation vectors.
    In this talk, we present the mathematics behind deflation, implementation on a parallel computer and discuss the results.

  • Thu
    14
    Dec
    2017

    11:30SC Seminar Room 32-349

    Prof. Dr. Claudia Schillings, Institut für Mathematik, Universität Mannheim

    Title:
    Uncertainty Quantification for Inverse Problems

    Abstract:

    Uncertainty Quantification (UQ) is an interesting, fast growing research area aimed at developing methods to address the impact of parameter, data and model uncertainty in complex systems. In this talk we will focus on the identification of parameters through observations of the response of the system – the inverse problem. The uncertainty in the solution of the inverse problem will be described via the Bayesian approach. In cases, where the model evaluations are prohibitively expensive, ad hoc methods such as the Ensemble Kalman Filter (EnKF) for inverse problems are widely and successfully used by practitioners in order to approximate the solution of the Bayesian problem.
    The low computational costs, the straightforward implementation and their non-intrusive nature make them appealing in various areas of application, but, on the downside, they are underpinned by very limited theoretical understanding. In this talk, we will discuss an analysis of the EnKF based on the continuous time scaling limits, whichallows to derive estimates on the long-time behaviour of the EnKF and, hence, provides insights into the convergence properties of the algorithm. In particular, we are interested in the properties of the EnKF for a fixed ensemble size. Results from various numerical experiments supporting the theoretical findings will be presented.

  • Thu
    14
    Dec
    2017

    12:15SC Seminar Room 32-349

    Jonas Kusch, Karlsruhe Institute of Technology

    Title:
    An approximate Newton Smoothing Method for Shape Optimization

    Abstract:

    In this talk, we derive a smoothing method for shape optimization in Stokes and Navier-Stokes flows. The smoothing routine automatically picks a spatially dependent smoothing parameter in such a way that the optimization process is accelerated, turning the smoothing routine into an approximate Newton method.
    This task is achieved by analytically deriving the symbol of the Hessian for the Stokes equations. We numerically investigate the Hessian symbol for convective flows and demonstrate the applicability of the symbol for the Navier-Stokes equations.
    The constructed preconditioner approximates the derived symbol using windowed Fourier transform and thereby accelerates the optimization process while yielding a smooth search direction. Due to the fact that the smoothing is performed locally, the method will identify areas in which a non-smooth design is physically meaningful and will automatically turn off smoothing in these regions.

  • Thu
    11
    Jan
    2018

    11:30SC Seminar Room 32-349

    Johannes Blühdorn, TU Kaiserslautern

    Title:
    Efficient Solution of the Unit Cell Problem

    Abstract:

    For materials with complex microstructure, analytical descriptions of the effective (macroscopic) behaviour are usually not available. In the context of mathematical homogenization, the effective behaviour is approximated based on a simulation of a material unit cell on the microscopic scale. There, one has to solve an elliptic PDE. The best approximation is usually obtained with periodic boundary conditions. While the unit cell problem can traditionally be solved by finite element methods, Moulinec and Suquet proposed a more efficient algorithm that needs no meshing and can work directly on data obtained from CT images. It relies upon the reformulation of the original problem in terms of the periodic Lippmann-Schwinger equation. The solution to this integral equation is approximated in the space of trigonometric polynomials by means of a fixed point iteration. In each iteration, a PDE with constant coefficients must be solved, which is done efficiently in Fourier space. In this talk, we derive the method for linear elasticity at small deformations. We outline how it extends to nonlinear material laws and large deformations and discuss its advantages, limitations and more recent adaptions.

  • Thu
    18
    Jan
    2018

    11:30SC Seminar Room 32-349

    Dr. Stefan Görtz, DLR Braunschweig, Institut für Aerodynamik und Strömungstechnik

    Title:
    TBA

    Abstract:

    TBA

  • Thu
    25
    Jan
    2018

    11:30SC Seminar Room 32-349

    Max Stein, TU Kaiserslautern

    Title:
    TBA

    Abstract:

    TBA

  • Thu
    01
    Feb
    2018

    11:30SC Seminar Room 32-349

    Johannes Blühdorn, TU Kaiserslautern

    Title:
    TBA

    Abstract:

    TBA

  • Thu
    08
    Feb
    2018

    11:30SC Seminar Room 32-349

    Falko Schneider, ITWM

    Title:
    TBA

    Abstract:

    TBA