Events

I³MS - Dedé Seminar

Location: AICES Seminar Room 115, 1st floor, Schinkelstr. 2, 52062 Aachen

Prof. Dr. Luca Dedé - Isogeometric Analysis of High Order, Surface, and Geometric Partial Differential Equations with Applications

Department of Mathematics, Politecnico di Melano, Italy

Abstract

In this talk, we consider the spatial approximation of high order, surface, and geometric Partial Differential Equations (PDEs) by means of Isogeometric Analysis (IGA). Specifically, we discretize the PDEs by means of NURBS-based IGA in the framework of the Galerkin method and we show that IGA is particularly suitable for approximating these classes of PDEs. Indeed, NURBS-basis IGA straightforwardly encapsulates the exact representation of the computational domain (the geometry) in the numerical approximation of the PDE, thus significantly enhancing the accuracy of the solution of surface and geometric PDEs. Moreover, we consider trial spaces of NURBS basis functions with high degree of continuity in the computational domain, a feature that is particularly efficient for approximating high order PDEs. We show the efficiency and accuracy of the method by solving high order PDEs on both open and closed surfaces and phase field problems driven by the Cahn-Hilliard and crystal growth equations. Moreover, we approximate by means of NURBS-based IGA some benchmark geometric PDEs, specifically the mean curvature and Willmore flow problems and the Canham-Helfrich curvature model. The latter is indeed suited to model the shape of biomembranes and vescicles as red blood cells. Finally, we present and discuss a dynamical model for the simulation of the interaction of the biomembranes with the fluid.

CHARLEMAGNE DISTINGUISHED LECTURE SERIES - Cremers Seminar

Location: SUPERC, 6th Floor, Generali Room, 52062 Aachen

Prof. Dr. Daniel Cremers - Novel Algorithms for 3D Computer Vision

Department of Computer Science, Technical University of Munich

Abstract

The reconstruction of the 3D world from a moving camera is among the
central challenges in computer vision.  While traditional approaches
have been focused on computing correspondence and 3D structure for a
sparse set of feature points, more recent approaches aim at directly
computing dense geometric surfaces using all available image data.  In
my talk, I will present some recent developments on convex
formulations for dense reconstruction from multiple images or multiple
videos.  Furthermore, I will present real-time capable direct methods
for reconstructing the world from handheld color or RGB-D cameras.
Applications include 3D photography, free-viewpoint television and
driver assistance.

I³MS - Gupta Seminar

Location: AICES Seminar Room 115, 1st floor, Schinkelstr. 2, 52062 Aachen

Prof. Dr. Shakti Gupta - Carbon nanostructures: Molecular Simulations, Continuum Models and Some Related Issues

Department of Mechanical Engineering, Indian Institute of Technology, India

Abstract

Continuum hypothesis based properties, for example, elastic modulli or
thermal conductivity of a material at small lengths scale can be derived
efficiently using molecular mechanics or dynamics. While doing so one
makes a few key assumptions and develops what are called as equivalent
continuum structures (ECSs). Accuracy of the derived quantity for a given
structure thus depends strongly on its ECS. In this talk we will first
present development of ECSs for single-walled carbon nanotubes (SWCNTs)
and graphene based on the theory of linear vibrations and show instances
when these ECSs may fail or behave counterintuitively. Subsequently,
results from two methods leading to conflicting values of critical
buckling strain in SWCNTs under compression will be presented. Lastly, we
will present some very recent results on instabilities in carbon nanocone
stacks motivating to development of continuum models.

I³MS - Verstraete Seminar

Location: AICES Seminar Room 115, 1st floor, Schinkelstr. 2, 52062 Aachen

Prof. Dr. Matthieu Verstraete - Ab Initio Phonon Limited Transport

QMAT CESAM Research Unit, Department of Physics, Université de Liege, Belgium

Abstract

We revisit the thermoelectric (TE) transport properties of two champion materials, PbTe and SnSe, using fully first principles methods. In both cases the performance of the material is due to subtle combinations of structural effects, scattering, and phase space reduction. In PbTe anharmonic effects are completely opposite to the predicted quasiharmonic evolution of phonon frequencies and to frequently (and incorrectly) cited extrapolations of experiments. This stabilizes the material at high T, but also tends to enhance its thermal conductivity, in a non linear manner, above 600 Kelvin. This explains why PbTe is in practice limited to room temperature applications. SnSe has recently been shown to be the most efficient TE material in bulk form. This is mainly due to a strongly enhanced carrier concentration and electrical conductivity, after going through a phase transition from 600 to 800 K. We calculate the transport coefficients as well as the defect concentrations ab initio, showing excellent agreement with experiment, and elucidating the origin of the double phase transition as well as the new charge carriers. If I have time I will show you something weird about transition metal dichalcogenides.

  

References 

  • [1] Hellman, IA Abrikosov, and SI Simak, PRB 84 180301 (2011)
  • [2] AH Romero, EKU Gross, MJ Verstraete, and O Hellman PRB 91, 214310 (2015)
  • [3] A Dewandre, S Bhattacharya, O Hellman, AH Romero, GKH Madsen, MJ Verstraete PRL 117 276601 (2016)

I³MS - Pauli Seminar

Location: AICES Seminar Room 115, 1st floor, Schinkelstr. 2, 52062 Aachen

Dr. Lutz Pauli - Stabilized Finite Element Methods for Computational Design of Blood-Handling Devices

Chair for Computational Analysis of Technical Systems, RWTH Aachen University

Abstract

The development of reliable blood damage (hemolysis) models is a key issue for the virtual design of ventricular assist devices (VADs). Commonly used stress-based hemolysis models assume an instantaneous deformation of red blood cells. Therefore, a strain-based model is considered, which is able to compute the time-dependent (viscoelastic) deformation of the cells.
The flow and hemolysis quantities are computed by stabilized finite element methods. The stabilization theory is critically reviewed and tailored to the individual problem statements. Efficient and accurate variational multi-scale formulations for anisotropic meshes, in combination with discontinuity-capturing, will be presented. Furthermore, we will discuss turbulence modeling with large eddy simulation and the handling of rotating objects with multiple reference frames or moving mesh techniques.
For the hemolysis estimations, we will discuss a logarithm transformation for a viscoelastic tensor equation that is able to improve the convergence of the equation system significantly. The numerical methods will be applied to benchmark devices and state-of-the-art VADs.