1,175 research outputs found
Triadic resonances in non-linear simulations of a fluid flow in a precessing cylinder
We present results from three-dimensional non-linear hydrodynamic simulations
of a precession driven flow in cylindrical geometry. The simulations are
motivated by a dynamo experiment currently under development at
Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in which the possibility of
generating a magnetohydrodynamic dynamo will be investigated in a cylinder
filled with liquid sodium and simultaneously rotating around two axes. In this
study, we focus on the emergence of non-axisymmetric time-dependent flow
structures in terms of inertial waves which - in cylindrical geometry - form
so-called Kelvin modes. For a precession ratio
the amplitude of the forced Kelvin mode
reaches up to one fourth of the rotation velocity of the cylindrical container
confirming that precession provides a rather efficient flow driving mechanism
even at moderate values of . More relevant for dynamo action might
be free Kelvin modes with higher azimuthal wave number. These free Kelvin modes
are triggered by non-linear interactions and may constitute a triadic resonance
with the fundamental forced mode when the height of the container matches their
axial wave lengths. Our simulations reveal triadic resonances at aspect ratios
close to those predicted by the linear theory except around the primary
resonance of the forced mode. In that regime we still identify various free
Kelvin modes, however, all of them exhibit a retrograde drift around the
symmetry axis of the cylinder and none of them can be assigned to a triadic
resonance. The amplitudes of the free Kelvin modes always remain below the
forced mode but may reach up to 6% of the of the container's angular velocity.
The properties of the free Kelvin modes will be used in future simulations of
the magnetic induction equation to investigate their ability to provide for
dynamo action.Comment: 26 pages, 14 figures, submitted to New J. Phy
Efficient local strategies for vaccination and network attack
We study how a fraction of a population should be vaccinated to most
efficiently top epidemics. We argue that only local information (about the
neighborhood of specific vertices) is usable in practice, and hence we consider
only local vaccination strategies. The efficiency of the vaccination strategies
is investigated with both static and dynamical measures. Among other things we
find that the most efficient strategy for many real-world situations is to
iteratively vaccinate the neighbor of the previous vaccinee that has most links
out of the neighborhood
Electromagnetic induction in non-uniform domains
Kinematic simulations of the induction equation are carried out for different
setups suitable for the von-K\'arm\'an-Sodium (VKS) dynamo experiment. Material
properties of the flow driving impellers are considered by means of high
conducting and high permeability disks that are present in a cylindrical volume
filled with a conducting fluid. Two entirely different numerical codes are
mutually validated by showing quantitative agreement on Ohmic decay and
kinematic dynamo problems using various configurations and physical parameters.
Field geometry and growth rates are strongly modified by the material
properties of the disks even if the high permeability/high conductivity
material is localized within a quite thin region. In contrast the influence of
external boundary conditions remains small. Utilizing a VKS like mean fluid
flow and high permeability disks yields a reduction of the critical magnetic
Reynolds number for the onset of dynamo action of the simplest non-axisymmetric
field mode. However this decrease is not sufficient to become relevant in the
VKS experiment. Furthermore, the reduction of Rm_c is essentially influenced by
tiny changes in the flow configuration so that the result is not very robust
against small modifications of setup and properties of turbulence
Towards a precession driven dynamo experiment
The most ambitious project within the DREsden Sodium facility for DYNamo and
thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf
(HZDR) is the set-up of a precession-driven dynamo experiment. After discussing
the scientific background and some results of water pre-experiments and
numerical predictions, we focus on the numerous structural and design problems
of the machine. We also outline the progress of the building's construction,
and the status of some other experiments that are planned in the framework of
DRESDYN.Comment: 9 pages, 6 figures, submitted to Magnetohydrodynamic
The microgravity environment of the Space Shuttle Columbia payload bay during STS-32
Over 11 hours of three-axis microgravity accelerometer data were successfully measured in the payload bay of Space Shuttle Columbia as part of the Microgravity Disturbances Experiment on STS-32. These data were measured using the High Resolution Accelerometer Package and the Aerodynamic Coefficient Identification Package which were mounted on the Orbiter keel in the aft payload bay. Data were recorded during specific mission events such as Orbiter quiescent periods, crew exercise on the treadmill, and numerous Orbiter engine burns. Orbiter background levels were measured in the 10(exp -5) G range, treadmill operations in the 10(exp -3) G range, and the Orbiter engine burns in the 10(exp -2) G range. Induced acceleration levels resulting from the SYNCOM satellite deploy were in the 10 (exp -2) G range, and operations during the pre-entry Flight Control System checkout were in the 10(exp -2) to 10(exp -1) G range
Systemic Risk and Default Clustering for Large Financial Systems
As it is known in the finance risk and macroeconomics literature,
risk-sharing in large portfolios may increase the probability of creation of
default clusters and of systemic risk. We review recent developments on
mathematical and computational tools for the quantification of such phenomena.
Limiting analysis such as law of large numbers and central limit theorems allow
to approximate the distribution in large systems and study quantities such as
the loss distribution in large portfolios. Large deviations analysis allow us
to study the tail of the loss distribution and to identify pathways to default
clustering. Sensitivity analysis allows to understand the most likely ways in
which different effects, such as contagion and systematic risks, combine to
lead to large default rates. Such results could give useful insights into how
to optimally safeguard against such events.Comment: in Large Deviations and Asymptotic Methods in Finance, (Editors: P.
Friz, J. Gatheral, A. Gulisashvili, A. Jacqier, J. Teichmann) , Springer
Proceedings in Mathematics and Statistics, Vol. 110 2015
Synthetic proteinâprotein interaction domains created by shuffling Cys(2)His(2) zinc-fingers
Cys(2)His(2) zinc-fingers (C2H2 ZFs) mediate a wide variety of proteinâDNA and proteinâprotein interactions. DNA-binding C2H2 ZFs can be shuffled to yield artificial proteins with different DNA-binding specificities. Here we demonstrate that shuffling of C2H2 ZFs from transcription factor dimerization zinc-finger (DZF) domains can also yield two-finger DZFs with novel proteinâprotein interaction specificities. We show that these synthetic proteinâprotein interaction domains can be used to mediate activation of a single-copy reporter gene in bacterial cells and of an endogenous gene in human cells. In addition, the synthetic two-finger domains we constructed can also be linked together to create more extended, four-finger interfaces. Our results demonstrate that shuffling of C2H2 ZFs can yield artificial protein-interaction components that should be useful for applications in synthetic biology
Influence of high permeability disks in an axisymmetric model of the Cadarache dynamo experiment
Numerical simulations of the kinematic induction equation are performed on a
model configuration of the Cadarache von-K\'arm\'an-Sodium dynamo experiment.
The effect of a localized axisymmetric distribution of relative permeability
{\mu} that represents soft iron material within the conducting fluid flow is
investigated. The critical magnetic Reynolds number Rm^c for dynamo action of
the first non-axisymmetric mode roughly scales like
Rm^c({\mu})-Rm^c({\mu}->infinity) ~ {\mu}^(-1/2) i.e. the threshold decreases
as {\mu} increases. This scaling law suggests a skin effect mechanism in the
soft iron disks. More important with regard to the Cadarache dynamo experiment,
we observe a purely toroidal axisymmetric mode localized in the high
permeability disks which becomes dominant for large {\mu}. In this limit, the
toroidal mode is close to the onset of dynamo action with a (negative)
growth-rate that is rather independent of the magnetic Reynolds number. We
qualitatively explain this effect by paramagnetic pumping at the fluid/disk
interface and propose a simplified model that quantitatively reproduces
numerical results. The crucial role of the high permeability disks for the mode
selection in the Cadarache dynamo experiment cannot be inferred from
computations using idealized pseudo-vacuum boundary conditions (H x n = 0).Comment: 16 pages, 9 Figures, published in New Journal of Physics 14(2012),
05300
The influence of current collectors on Tayler instability and electro-vortex flows in liquid metal batteries
The Tayler instability is a kink-type flow instability which occurs when the
electrical current through a conducting fluid exceeds a certain critical value.
Originally studied in the astrophysical context, the instability was recently
shown to be also a limiting factor for the upward scalability of liquid metal
batteries. In this paper, we continue our efforts to simulate this instability
for liquid metals within the framework of an integro-differential equation
approach. The original solver is enhanced by multi-domain support with
Dirichlet-Neumann partitioning for the static boundaries. Particular focus is
laid on the detailed influence of the axial electrical boundary conditions on
the characteristic features of the Tayler instability, and, secondly, on the
occurrence of electro-vortex flows and their relevance for liquid metal
batteries.Comment: 26 pages, 16 figure
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