1,085 research outputs found
Modulated rotating waves in the magnetized spherical Couette system
We present a study devoted to a detailed description of modulated rotating
waves (MRW) in the magnetized spherical Couette system. The set-up consists of
a liquid metal confined between two differentially rotating spheres and
subjected to an axially applied magnetic field. When the magnetic field
strength is varied, several branches of MRW are obtained by means of three
dimensional direct numerical simulations (DNS). The MRW originate from parent
branches of rotating waves (RW) and are classified according to Rand's (Arch.
Ration. Mech. Anal 79:1-37, 182) and Coughling & Marcus (J. Fluid Mech.
234:1-18,1992) theoretical description. We have found relatively large
intervals of multistability of MRW at low magnetic field, corresponding to the
radial jet instability known from previous studies. However, at larger magnetic
field, corresponding to the return flow regime, the stability intervals of MRW
are very narrow and thus they are unlikely to be found without detailed
knowledge of their bifurcation point. A careful analysis of the spatio-temporal
symmetries of the most energetic modes involved in the different classes of MRW
will allow in the future a comparison with the HEDGEHOG experiment, a
magnetized spherical Couette device hosted at the Helmholtz-Zentrum
Dresden-Rossendorf.Comment: Contains 3 tables and 8 figures. Published in the Journal of
Nonlinear Scienc
Induction in a von Karman flow driven by ferromagnetic impellers
We study magnetohydrodynamics in a von K\'arm\'an flow driven by the rotation
of impellers made of material with varying electrical conductivity and magnetic
permeability. Gallium is the working fluid and magnetic Reynolds numbers of
order unity are achieved. We find that specific induction effects arise when
the impeller's electric and magnetic characteristics differ from that of the
fluid. Implications in regards to the VKS dynamo are discussed.Comment: 14 pages, 7 figure
Analytic results and weighted Monte Carlo simulations for CDO pricing
We explore the possibilities of importance sampling in the Monte Carlo
pricing of a structured credit derivative referred to as Collateralized Debt
Obligation (CDO). Modeling a CDO contract is challenging, since it depends on a
pool of (typically about 100) assets, Monte Carlo simulations are often the
only feasible approach to pricing. Variance reduction techniques are therefore
of great importance. This paper presents an exact analytic solution using
Laplace-transform and MC importance sampling results for an easily tractable
intensity-based model of the CDO, namely the compound Poissonian. Furthermore
analytic formulae are derived for the reweighting efficiency. The computational
gain is appealing, nevertheless, even in this basic scheme, a phase transition
can be found, rendering some parameter regimes out of reach. A
model-independent transform approach is also presented for CDO pricing.Comment: 12 pages, 9 figure
Impact of time-dependent non-axisymmetric velocity perturbations on dynamo action of von-K\'arm\'an-like flows
We have performed numerical simulations of the kinematic induction equation
in order to examine the dynamo efficiency of an axisymmetric
von-K\'arm\'an-like flow subject to time-dependent non-axisymmetric velocity
perturbations. The numerical model is based on the setup of the French
Von-K\'arm\'an-Sodium dynamo (VKS) and on the flow measurements from a model
water experiment conducted at the University of Navarra in Pamplona, Spain. Our
simulations show that the interactions of azimuthally drifting flow
perturbations with the fundamental drift of the magnetic eigenmode (caused by
the inevitable equatorial symmetry breaking of the basic flow) essentially
determine the temporal behavior of the dynamo state. We find two distinct
regimes of dynamo action that depend on the (prescribed) drift frequency of an
() vortex-like flow perturbation. For comparatively slowly drifting
vortices we observe a narrow window with enhanced growth-rates and a drift of
the magnetic eigenmode that is synchronized with the perturbation drift. The
resonance-like enhancement of the growth-rates takes place when the vortex
drift frequency roughly equals the drift frequency of the magnetic eigenmode in
the unperturbed system. Outside of this small window, the field generation is
hampered compared to the unperturbed case, and the field amplitude of the
magnetic eigenmode is modulated with approximately twice the vortex drift
frequency. The abrupt transition between the resonant regime and the modulated
regime is identified as an spectral exceptional point where eigenvalues
(growth-rates and frequencies) and eigenfunctions of two previously independent
modes collapse.Comment: 14 pages, 14 Figures. Minor changes to match the published versio
A simple mean field model for social interactions: dynamics, fluctuations, criticality
We study the dynamics of a spin-flip model with a mean field interaction. The
system is non reversible, spacially inhomogeneous, and it is designed to model
social interactions. We obtain the limiting behavior of the empirical averages
in the limit of infinitely many interacting individuals, and show that phase
transition occurs. Then, after having obtained the dynamics of normal
fluctuations around this limit, we analize long time fluctuations for critical
values of the parameters. We show that random inhomogeneities produce critical
fluctuations at a shorter time scale compared to the homogeneous system.Comment: 37 pages, 2 figure
Terminus-driven retreat of a major southwest Greenland tidewater glacier during the early 19th century : insights from glacier reconstructions and numerical modelling
Peer reviewedPublisher PD
Counter-selectable marker for bacterial-based interaction trap systems
Counter-selectable markers can be used in two-hybrid systems to search libraries for a protein or compound that interferes with a macromolecular interaction or to identify macromolecules from a population that cannot mediate a particular interaction. In this report, we describe the adaptation of the yeast URA3/5-FOA counter-selection system for use in bacterial interaction trap experiments. Two different URA3 reporter systems were developed that allow robust counter-selection: (i) a single copy F\u27 episome reporter and (ii) a co-cistronic HIS3-URA3 reporter vector. The HIS3-URA3 reporter can be used for either positive or negative selections in appropriate bacterial strains. These reagents extend the utility of the bacterial two-hybrid system as an alternative to its yeast-based counterpart
Dopaminium perchlorate
In the title compound [systematic name: 2-(3,4-dihydroxyphenyl)ethanaminium perchlorate], C8H12NO2
+·ClO4
−, the cations and anions are linked into three-dimensional structure via intermolecular N—H⋯O and O—H⋯O hydrogen bonds
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