Events

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.

EU Regional School - Kalidindi Seminar

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

Prof. Dr.Surya Kalidindi - Rigorous Quantification of the Hierarchical Material Structure in a Statistical Framework

Georg W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology, USA

Abstract

A versatile framework for the rigorous quantification of the hierarchical material internal structure will be presented in this lecture along with demonstrations to applications on a broad variety of heterogeneous material structures at different length scales and different materials classes. This new framework is based on the established concepts of n-point spatial correlations that provide a systematic statistical description of the highly complex material structure. The framework takes advantage of toolsets established in digital signal processing, Fourier representations, and principal component analyses to develop high performance computational toolsets needed for performing the computations involved. The versatility of the framework will be demonstrated through the application of a single consistent framework at both atomistic and continuum length scales as well as different materials classes (e.g., multiphase composites, polycrystals, porous membranes).  

I³MS - Möller Seminar

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

Dr. Matthias Möller - Isogeometric Analysis for Compressible Flow Problems in Industrial Applications

Department of Applied Mathematics, Delft University of Technology, Netherlands

Abstract

In this talk, we will present an isogeometric analysis (IgA) approach for the simulation of compressible flows that arise in industrial applications, in particular, in twin-screw rotary compressors.

In the first part of the talk, we present a positivity-preserving high-resolution scheme for compressible flows building upon the generalization of the algebraic flux correction paradigm [2] to isogeometric analysis. Our approach adopts Fletcher's group formulation [1] together with an efficient edge-based formation of system matrices and vectors from pre-computed coefficients to overcome the high computational costs that are typically observed in quadrature-based IgA-assembly algorithms.

Next to this algorithmic approach to achieving high computational efficiency, our implementation in the open-source library G+Smo (https://www.gs.jku.at/gismo) makes use of meta-programming techniques to combine the computational performance of several hardware-optimized linear algebra back-ends like Blaze, Eigen, and VexCL, with ease of implementation offered by the fluid dynamics expression-template library FDBB (https://mmoelle1.gitlab.io/FDBB). Just-in time compilation techniques are used to run the solver in heterogeneous computing environments.

In the second part of the talk, we describe an isogeometric approach for the creation of analysis-suitable multi-patch parameterizations for complex industrial applications and, in particular, for (parts of) twin-screw rotary compressors. Our approach builds on well-established elliptic grid generation techniques, which have been generalized to the IgA framework.

References

[1] C.A.J. Fletcher, The group finite element formulation, Computer Methods in Applied Mechanics and Engineering, 37, 225–244, 1983.

[2] D. Kuzmin, M. Möller, M. Gurris, Algebraic flux correction II. Compressible flow problems. In: Kuzmin et al. (editors) Flux-Corrected Transport: Principles, Algorithms, and Applications, 193–238. Springer, 2nd edition, 2012.