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Place:
Weierstrass-Institute for Applied Analysis and Stochastics
Mohrenstraße 39, 10117 Berlin
Erhard-Schmidt Lecture Room (ESH)
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Stream:
Contact one of the organizers to get access to the credentials of the zoom meeting
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Time:
Every second Monday of the month, 2:00PM - 3:00PM
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Organizers:
Moritz Ebeling-Rump,
Derk Frerichs-Mihov
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Upcoming talks |
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13.06.2022 |
Dr. Matthias Liero
(RG 1)
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Creating personal homepages for academics with jekyll
Abstract:
TBA.
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11.07.2022 |
Maximilian Reiter (RG 4)
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Abstract:
TBA.
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Previous talks |
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09.05.2022 |
Stefanie Schindler
(RG 1)
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The Entropy Method for the Linear Diffusion Equation on the whole real line
Abstract:
In this talk, we investigate the long-time behavior of solutions to the
well-studied linear diffusion equation defined on the whole real line
with non-homogeneous asymptotic boundary values. In other words, we
study solutions that are in equilibria at infinity and answer how they
mix these two stable states when time increases. The idea is to use the
entropy method in order to prove the convergence towards a steady-state
function in parabolic scaling variables. The reason why we use this
approach, that works for nonlinear problems as well, comes from our
research study as a part of the SFB 910. In project A5 we study the
long-time behavior of a nonlinear coupled reaction-diffusion system of
mass-action type on the real line and answer a similar question.
However, in this talk, we focus on the linear diffusion equation for
simplicity.
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11.04.2022 |
Willem van Oosterhout
(RG 1)
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The direct method and application to poro-visco-elastic materials
Abstract:
In this talk, we show how the direct method of the calculus of
variations can be used to prove existence (and uniqueness) of solutions
of PDEs. The main idea is to prove that certain functionals have
minimizers, and that these minimizers satisfy a PDE. Then, we apply the
direct method in combination with a time discretization to prove
existence of solutions for poro-visco-elastic materials.
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14.03.2022 |
Simon Breneis
(RG 6)
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Pricing options under rough volatility
Abstract:
Starting with an introduction of Brownian motion, we discuss the general
goals of mathematical finance and explain the intuition behind the
Black-Scholes model. After discussing option pricing in this standard
framework, we observe some of the shortcomings of the Black-Scholes
model. Finally, to overcome these deficiencies, we introduce stochastic
volatility and rough volatility models.
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21.02.2022 |
Derk Frerichs-Mihov
(RG 3)
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Restic - backups done right!
Abstract:
Computers are in severe danger! Humans worldwide drown computers with
their beverages, they forget them on the trains and install viruses and
other malware. Computers are stolen or hardware problems occur. To
prevent you from hearing "No Backup, no mercy" this talks introduces an
open-source, effective, secure and cross-platform software called
restic with which you can back
up your data.
The talk is a hands-on tutorial how to use restic together with WIAS
resources. You are going to learn how to install restic, how to back up
your data, how to delete old snapshots, useful automations and of course
how to restore your data in the worst case. Buckle up and prepare to
make data loss a thing of the past.
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07.02.2022 |
Alireza Selahi
(RG 7)
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Version control with "Git"
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24.01.2022 |
Anh Duc Vu
(LG 6)
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A Short Introduction to Homogenization
Abstract:
Many fields in mathematics are motivated by physical problems. In our
case, we want to model porous/compound materials. Often, only
a microscopic description is given (e.g. pore structure), while the
macroscopic behaviour is of interest (behaviour of a sponge). The
"limiting" procedure where one transitions from the microscopic to the
macroscopic scale is difficult to handle and lies in the heart of
homogenization theory.
The talk will give a brief overview on the topic as well as the key
insights driving the theory.
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13.12.2021 |
Andrea Zafferi (WG 1)
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GENERIC and its applications to geophysical flows.
Abstract:
The GENERIC (acronym for General Equations of Non-Equilibrium for
Reversible-Irreversible Coupling) formalism is a modelling tool that
uses a variational approach to derive thermodynamically consistent sets
of equations for closed systems. I will introduce formally this method
while showing simple examples related to fluid mechanics. Secondly,
I will discuss transformations that preserve such structure with
applications to more sophisticated problems, i.e., FSI (fluid-solid
interactions) problems, reactive fluid flows or rock de-hydration
processes.
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22.11.2021 |
Dr. Swetlana Giere (felmo GmbH)
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A Conversation about Transition From Science to Data Science
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08.11.2021 |
Dr. Clemens Bartsch (genua GmbH: IT Project Manager)
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Career Opportunities for WIAS Alumni in IT Security? A Personal Case Study.
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11.10.2021 |
Alexander Gerdes
(RG 2)
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Synchronization patterns in globally coupled Stuart-Landau oscillators
Abstract:
We s tudy clusterized states in globally coupled Stuart-Landau
oscillators as a paradigmatic model for patterning processes
[Kemeth2019].
To study 2-Cluster states we set up a reduced model using collective
variables, in which the cluster size ratio [Ott2015] is an additional
bifurcation parameter. In the reduced system one can only observe
longitudinal instabilities leading to complex 2-Cluster behaviour. By
including test oscillators, we can also study instabilities
transversal to the 2-Cluster manifold i.e. changes of the cluster
type. Using numerical bifurcation analysis, we then find stability
regions of cluster solutions of different types. In these, solitary
states serve as primary patterns and allow an analytical treatment.
The identified instabilities can be seen as building blocks of
pathways to complex behaviour such as chimeras [Set2014] and extensive
chaos [KM1994] as well as splay states [Politi2019] occuring for
varying parameters. With the analytical and numerical approach
presented here we identify different transition scenarios from
synchrony to complex behaviour by reducing the coupling strength. We
locate each of these scenarios in regions in the plane of shear
parameters.
[KM1994] N. Nakagawa and Y. Kuramoto, Physica D: Nonlinear Phenomena,
75, Issues 1-3 (1994).
[Set2014] G. Sethia and A. Sen, Phys. Rev. Lett. 112 144101 (2014).
[Ott2015] W. L. Ku, M. Girvan, E. Ott, Chaos 25 , 123122 (2015).
[Kemeth2019] Kemeth, Felix P., Sindre W. Haugland, and Katharina
Krischer. Chaos: An Interdisciplinary Journal of Nonlinear Science
29.2 (2019): 023107.
[Politi2019] P. Clusella and A. Politi, Phys. Rev. E 99, 062201
(2019).
[Kemeth2021] Kemeth, Felix P., et al. Journal of Physics: Complexity
2.2 (2021): 025005.
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13.09.2021 |
Alireza Selahi Moghaddam (RG 7)
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Modelling electrolytes with the Poisson-Nernst-Planck-equation
Abstract:
The capacity of batteries is one of their central quantities and hence
of major interest. In order to calculate it, modelling the behaviour of
electrolytes is necessary. We give a short introduction into some of the
most important concepts in thermodynamics and chemistry, and finally
talk about a state-of-the-art model using the
Poisson-Nernst-Planck-equation.
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09.08.2021 |
Mina Stöhr (RG 2)
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Bifurcations and Instabilities of Temporal Dissipative Solitons in DDE-systems with large delay
Abstract:
We study different bifurcation scenarios and instabilities of Temporal
Dissipative Solitons in systems with time-delayed feedback and large
delay. As these solitons can be described as homoclinic orbits in the
profile equation under the reappearance map, we use homoclinic
bifurcation theory for our comprehension of their bifurcations and
instabilities. We demonstrate our results with the examples of the
FitzHugh-Nagumo system and Morris-Lecar model with time-delayed
feedback.
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12.07.2021 |
Lorenzo Scaglione (RG 3)
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Three months with ParMooN
Abstract:
What does it mean for a research group to implement its own software
library? During my internship at WIAS I discovered the huge world of
ParMooN, a C++ finite element library of the institute. Even if
sometimes it is quite painful to surf through the hundreds of header
and source files of the library, it is somehow fascinating to have an
idea of its complex architecture and of how it can help us to solve
quantitatively problems from the real world. The application of my
code is the simulation of the mechanical behaviour of an elastic
material embedded with a thin vasculature.
In my presentation, I will introduce the physical problem and the
library; then I will show the numerical results I have obtain until
now.
Sophie Luisa Plato (RG 4)
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Biological pest control - Analysis and numerics for a spatial-temporal predator-prey system.
Abstract:
In the production of ornamental plants, as for example roses, it is
desirable to reduce the use of chemical pesticides in order to protect
the environment and the people involved in the production process. This
can be achieved by releasing natural enemies of the pest involved, which
do not harm the plants. A typical example of such a predator-prey pair
is the two-spotted spider mite and the predatory mite.
In this talk we consider a system of two coupled evolution equations
modelling this predator-prey interaction. The first part of the talk is
devoted to the proof of the existence of weak solutions to this model
and in the second part we present our numerical approximations of these
solutions.
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14.06.2021 |
Jacob Gorenflos (FMP / Leibniz PhD Network)
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A glimps into doing a Ph.D. in the Leibniz Association
Abstract:
Data is the best way to start a discussion. We, in the Leibniz PhD
Network, regularly discuss with Leibniz policy and politicians on how to
improve the situation of doctoral researchers. Surprisingly, there is
a significant lack of knowledge on how their situation really is.
Therefore, we started the working group survey in 2017. This kicked of
the biennial cycle in which we now survey you: 2017, 2019 and we are
currently finishing the development of the 2021 questionnaire. Since
2017, the survey and its results have been the basis of our work.
Here, you will be presented with the data of the 2019 survey and
a perspective on what to expect of the 2021 survey.
Lasse Ermoneit (RG 2)
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Semi-analytical approach to determine the timing jitter of a mode-locked laser with opto-electronic feedback
Abstract:
Passively mode-locked lasers are an important device among semiconductor lasers and are used to generate high-frequency regular pulses. Their mathematical description can be done via delay differential equations to avoid the more complex consideration with partial differential equations. Since any laser system is usually subject to noise due to quantum effects, this is incorporated into the system of equations and a nonlinear differential equation with delay and stochastic white noise is obtained.. The timing jitter is the measure for the irregularity of the pulses of the laser: It corresponds to the standard deviation of the noise influenced pulses to an external clock.
Here, an approach is presented that makes it possible to shortcut a large part of the fully numerical non-parallelizable computations in order to get to this key quantity, the timing jitter.
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10.05.2021 |
Dilara Abdel (LG 5)
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Modelling charge transport in perovskite solar cells: Potential-based and limiting ion depletion
Abstract:
Perovskite solar cells (PSCs) have become one
of the fastest growing photovoltaic technologies within the last
few years. However, their commercialization is still in its early
stages and several challenges need to be overcome. For this reason it
is paramount to understand the charge transport in perovskites better
via improved modelling and simulation. Unfortunately, there is a
discrepancy in the adequate modeling of the additional ionic transport
within the perovskite material with drift-diffusion equations. Thus,
we present a new charge transport model which is, unlike other models
in the literature, based on quasi Fermi potentials instead of
densities. This allows to easily include nonlinear diffusion (based on
for example Fermi-Dirac, Gauss-Fermi or Blakemore statistics) as well
as limit the ion depletion (via the Fermi-Dirac integral of
order -1). We present numerical finite volume simulations to underline
the importance of limiting ion depletion.
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03.05.2021 |
Artur Stephan (RG 1)
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PhD defense rehearsal |
Coarse-graining for gradient systems and Markov processes
Abstract:
Coarse-graining is a well-established tool in mathematical and natural
sciences for reducing the complexity ofa physical system and for
deriving effective models. In the talk, we consider several examples
that originate from interacting particle systems and describe reaction
and reaction diffusion systems. The aim is twofold: first,provide
mathematically rigorous results for physical coarse-graining. Secondly,
the so derived systems can be formulated in a mathematically equivalent
way, which provides new modelling insights.
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12.04.2021 |
Leonie Schmeller (RG 7)
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Phase-field model with nonlinear elasticity (Modelling and Numerical
aspects)
Abstract:
In this talk, I will present a phase field model for a Neo-Hookian
(nonlinear) elastic material, which is then coupled to formulate
a Cahn-Hillard type dynamic system. After introducing the individual
components of the problem, a coupling is discussed. We set up a weak
formulation and show a strategy to implement and solve the problem
numerically.
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15.03.2021 |
Heide Langhammer (RG 5)
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Instead of 08.03.2021 |
Inhomogeneous Random Graphs: A large deviations result for their cluster sizes and its implications.
Abstract:
An inhomogeneous random graph consists of a fixed number of vertices
that are connected via random edges. The edge probabilities depend on an
additional parameter that we call the vertex type. We want to study how
such a random graph decomposes into its connected components. In
particular, we want to understand conditions for the existence of
macroscopic components, whose size is proportional to the total number
of vertices. Once the model parameters surpass a certain threshold,
a (unique) macroscopic component appears with high probability. I will
explain how this phase transition can be studied via large deviations
theory which reformulates the probabilistic calculus of the model into
an optimization problem for a certain function.
I will also discuss in which ways the inhomogeneous random graph model
can be linked to models of coagulation that I will only briefly sketch.
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08.02.2021 |
David Sommer (RG 4)
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Dynamic Programming Approach for Robust Receding Horizon Control in Continuous Systems
Abstract:
There is still little connection in the literature between the field of
Model-based Reinforcement Learning (MB-RL) and the field of Continuous
Optimal Control. In Continuous Optimal Control, the ODE model,
describing a real physical process, is usually regarded as ground truth.
This may lead to catastrophic failures when the resulting control is
applied to the real system, due to errors in the model. In MB-RL, this
issue is often addressed by keeping and continuously updating
a posterior over model parameters, but successful applications so far
are mostly limited to Markov Decision Processes which are discrete in
time. We propose a model-based decision-time planning agent for
continuous optimal control problems of arbitrary horizon length.
Continuous updates of the model parameters during the online phase
enable handling of complex unknown dynamics even with simple linear
models. During planning, robust feedback-control laws are computed in
a Dynamic Programming sense by utilizing Bellman's principle.
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11.01.2021 |
Alexandra Quitmann (RG 5)
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Spin systems and random loops
Abstract:
Random loop models are systems of statistical mechanics whose
configurations can be viewed as collections of closed loops living in
higher dimensional space. They are interesting objects on its own and
further have a close connection to other important statistical mechanics
models such as spin systems. In this talk, I will introduce random loop
models, discuss a conjecture about the occurrence of macroscopic loops
and explain its role as alternative formulation of spin systems.
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07.12.2020 |
Moritz Ebeling-Rump
(RG 4)
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Topology Optimization subject to a Local Volume Constraint
Abstract:
The industry sector of additive manufacturing has shown remarkable
growth in previous years and is predicted to continue growing at a rate
of 15% in the coming years. It progressed from prototyping to actual
production. Topology Optimization and Additive Manufacturing have been
called a "match made in heaven", because Topology Optimization
can aid engineers to take advantage of the newfound design freedom.
Commonly a perimeter term is incorporated which avoids checker-boarding,
but also counteracts the desired creation of infill structures. By
incorporating a local volume constraint mesoscale holes are introduced.
Analytically, the existence of unique solutions is shown. Apart from
better cooling properties and a larger resilience to local material
damage, these structures demonstrate an improved nonlinear material
behavior. One observes an increased critical buckling load
- a potentially catastrophic failure mode that would not be taken into
account if only considering linear elasticity.
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11.11.2020 |
Derk Frerichs
(RG 3)
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The very basics of numerical analysis - or what am I doing here when I'm
not drinking coffee?
Abstract:
When we drink coffee, caffeine spreads in our body through our blood.
The flow of particles inside a media, e.g. the caffeine inside the
blood, can be described with the so called convection-diffusion-reaction
equations that are often approximated using numerical algorithms. In
this talk the basic concepts of numerical analysis are explained with
the help of a conforming Courant finite element discretization of the
convection-diffusion-reaction equations. Afterwards a short outlook is
given that explains my current research activities.
Numerical examples round up the presentation.
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22.11.2018 |
Markus Mittnenweig (RG 1)
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Entropy methods for quantum and classical evolution equations
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27.08.2018 |
Clemens Bartsch (RG 3)
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Post-quantum cryptography and the first quantum-safe digital signature scheme
Abstract:
In May 2018 news spread far beyond the cryptologist community:
a group of German, Dutch and American computer scientists had published
the first quantum-resilient digital signature scheme as an internet standard (RFC 8391),
thus taking a major step towards arming digital signature against future attacks
with quantum computers.
The proposed XMSS scheme (eXtended Merkle Signature Scheme) makes use of
cryptographic hash functions, which are considered quantum-safe.
In this talk we want to lead the audience towards an understanding of the importance
and mode of operation of digital signature schemes, the threat that quantum computers
might in the near future pose to them, and how the newly standardized scheme offers
resilience against quantum computer attacks.
We will start with a general introduction of digital signature and an explanation
of a basic version of the widespread RSA algorithm and its major weaknesses,
focusing on factorization attacks. Then we will introduce the basics of quantum computing,
show how Shor's algorithm enables them to very efficiently perform factorization attacks,
thus breaking RSA, and finally introduce XMSS and give an explanation for why it is
supposed to be safe against quantum-aided attacks.
Code examples and examples of quantum computations performed with a
prototypical 5-qubit processor (IBM Q Experience) will be included in the talk.
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19.02.2018 |
Thomas Frenzel (RG 1)
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Working with Wasserstein gradient flows
Abstract:
This talk explains what the Wasserstein distance is, how it generates a gradient flow for the heat equation and how to pass to the limit in a sandwich model with thin plates.
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19.06.2017 |
Artur Stephan (Guest of RG 1)
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starts at 1:00 PM |
On approximations of solutions of evolution equations using semigroups
Abstract:
In the talk, some results of my master thesis will be discussed. We approximate the solution of a non-autonomous linear evolution equation in the operator-norm topology. The approximation is derived using the Trotter product formula and can be estimated. As an example, we consider the diffusion equation perturbed by a time dependent potential.
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12.06.2017 |
Clemens Bartsch (RG 3)
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starts at 11:00 AM |
A mixed stochastic-numeric algorithm for transported interacting particles
Abstract:
A coupled system of population balance and convection-diffusion equations is solved numerically, employing stochastic and finite element techniques in combination. While the evolution of the particle population is modelled as a Markov jump process and solved with a stochastic simulation algorithm, transport of temperature and species concentration are subject to a finite element approximation. We want to briefly introduce both the stochastic and the deterministic approach and discuss some difficulties to overcome when combining them. A proof of concept simulation of a flow crystallizer in 2D is presented.
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08.05.2017 |
Sibylle Bergmann (RG 7)
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An atomistically informed phase-field model for describing the solid-liquid interface kinetics in silicon
Abstract:
An atomistically informed parametrization of a phase-field model for describing the anisotropic mobility of liquid-solid interfaces in silicon is presented. The model is derived from a consistent set of atomistic data and thus allows to directly link molecular dynamics and phase field simulations. Expressions for the free energy density, the interfacial energy and the temperature and orientation dependent interface mobility are systematically fitted to data from molecular dynamics simulations based on the Stillinger-Weber interatomic potential. The temperature-dependent interface velocity follows a Vogel-Fulcher type behavior and allows to properly account for the dynamics in the undercooled melt.
Our three dimensional simulations reproduce the expected physical behavior of a silicon crystal in a melt, e.g. the critical nucleation radius and the experimentally observed equilibrium shape.
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24.10.2016 |
Johannes Neumann (RG 4)
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The phase field approach for topology optimization
Abstract:
In this talk I will present an approach on topology optimization
based on the phase field model from [Blank et. al., 2014] which utilizes
the Allan-Cahn gradient flow. This method natively includes changes to the
topology during the optimization and replaces sharp interfaces with
boundary layers for smoothness. Instead of the prime-dual active set method a Lagrangian
approach is considered.
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10.10.2016 |
Swetlana Giere (RG 3)
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A Walk to a Random Forest
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05.09.2016 |
Alexander Weiß (GetYourGuide: Head of Data Science)
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Talk on professional experience in the field of data science
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22.08.2016 |
Michael Hofmann (RG 2)
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Einfluss dynamischer Resonanzen auf die Wechselwirkung optischer Femtosekunden-Pulse mit transparenten Dielektrika
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27.06.2016 |
Florian Eichenauer (RG 1)
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starts at 4:00 PM |
Analysis for Dissipative Maxwell-Bloch Type Models
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20.06.2016 |
Paul Helly (Guest of RG 1)
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A structure-preserving finite difference scheme for the Cahn-Hilliard equation
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25.01.2016 |
Alena Moriakova (Guest of RG 2)
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Analysis of periodic solutions of the Mackey-Glass equation
Abstract:
The Mackey-Glass equation is the nonlinear time delay differential
equation, which describes the formation of white blood cells.
We study the possibility of simultaneous existence of several stable
attractors (periodic solutions) in this equation.
As a research method we use method of uniform normalization.
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11.01.2016 |
Thomas Frenzel (RG 1)
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(Evolutionary) Gamma-Convergence and micro-macro limits
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23.11.2015 |
Sina Reichelt (RG 1)
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Two-scale homogenization of systems of nonlinear parabolic equations
Abstract:
We consider two different classes of systems of nonlinear parabolic equations, namely, reaction-diffusion systems and Cahn-Hilliard-type equations. While the latter class admits a gradient structure, the former does in general not admit one. The equation's coefficients are periodically oscillating with a period which is proportional to the characteristic microscopic length scale. Using the method of two-scale convergence, we rigorously derive effective (upscaled or homogenized) equations for the limit of smaller and smaller periods. Therefore, depending on the class of systems under consideration, we use either suitable Gronwall-type estimates (for Lipschitz continuous reaction terms) or Gamma-convergence (for energy functionals).
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09.11.2015 |
Mayya Zhilova (RG 6)
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Bootstrap confidence sets under model misspecification
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26.10.2015 |
Dmitry Puzyrev (RG 1)
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starts at 10:00 AM (Erhard-Schmidt lecture room) |
Delay Induced Multistability and Zigzagging of Laser Cavity Solitons
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21.09.2015 |
Clemens Bartsch (RG 3)
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starts at 2:30 PM |
An Assessment of Solvers for Saddle Point Problems Emerging from the Incompressible Navier-Stokes equations
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