40 research outputs found
On the relevance of subcritical hydrodynamic turbulence to accretion disk transport
Hydrodynamic unstratified keplerian flows are known to be linearly stable at
all Reynolds numbers, but may nevertheless become turbulent through nonlinear
mechanisms. However, in the last ten years, conflicting points of view have
appeared on this issue. We have revisited the problem through numerical
simulations in the shearing sheet limit. It turns out that the effect of the
Coriolis force in stabilizing the flow depends on whether the flow is cyclonic
(cooperating shear and rotation vorticities) or anticyclonic (competing shear
and rotation vorticities); keplerian flows are anticyclonic. We have obtained
the following results: i/ The Coriolis force does not quench turbulence in
subcritical flows; ii/ The resolution demand, when moving away from the
marginal stability boundary, is much more severe for anticyclonic flows than
for cyclonic ones. Presently available computer resources do not allow
numerical codes to reach the keplerian regime. iii/ The efficiency of turbulent
transport is directly correlated to the Reynolds number of transition to
turbulence , in such a way that the Shakura-Sunyaev parameter . iv/ Even the most optimistic extrapolations of our numerical data show
that subcritical turbulent transport would be too inefficient in keplerian
flows by several orders of magnitude for astrophysical purposes. v/ Our results
suggest that the data obtained for keplerian-like flows in a Taylor-Couette
settings are largely affected by secondary flows, such as Ekman circulation.Comment: 21 pages, 17 figures, accepted in Astronomy and Astrophysic
On the monotone stability approach to BSDEs with jumps: Extensions, concrete criteria and examples
We show a concise extension of the monotone stability approach to backward
stochastic differential equations (BSDEs) that are jointly driven by a Brownian
motion and a random measure for jumps, which could be of infinite activity with
a non-deterministic and time inhomogeneous compensator. The BSDE generator
function can be non convex and needs not to satisfy global Lipschitz conditions
in the jump integrand. We contribute concrete criteria, that are easy to
verify, for results on existence and uniqueness of bounded solutions to BSDEs
with jumps, and on comparison and a-priori -bounds. Several
examples and counter examples are discussed to shed light on the scope and
applicability of different assumptions, and we provide an overview of major
applications in finance and optimal control.Comment: 28 pages. Added DOI
https://link.springer.com/chapter/10.1007%2F978-3-030-22285-7_1 for final
publication, corrected typo (missing gamma) in example 4.1
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
A visual analytics approach for understanding biclustering results from microarray data
Abstract Background Microarray analysis is an important area of bioinformatics. In the last few years, biclustering has become one of the most popular methods for classifying data from microarrays. Although biclustering can be used in any kind of classification problem, nowadays it is mostly used for microarray data classification. A large number of biclustering algorithms have been developed over the years, however little effort has been devoted to the representation of the results. Results We present an interactive framework that helps to infer differences or similarities between biclustering results, to unravel trends and to highlight robust groupings of genes and conditions. These linked representations of biclusters can complement biological analysis and reduce the time spent by specialists on interpreting the results. Within the framework, besides other standard representations, a visualization technique is presented which is based on a force-directed graph where biclusters are represented as flexible overlapped groups of genes and conditions. This microarray analysis framework (BicOverlapper), is available at http://vis.usal.es/bicoverlapper Conclusion The main visualization technique, tested with different biclustering results on a real dataset, allows researchers to extract interesting features of the biclustering results, especially the highlighting of overlapping zones that usually represent robust groups of genes and/or conditions. The visual analytics methodology will permit biology experts to study biclustering results without inspecting an overwhelming number of biclusters individually.</p
Report on the Implementation of International Humanitarian Law with regards to the Protection of Cultural Heritage in the Occupied Tskhinvali Region, Georgia
COMET - A submission to the 2020 update of the European Strategy for Particle Physics on behalf of the COMET collaboration
The search for charged lepton flavour violation (CLFV) has enormous discovery potential in probing new physics Beyond the Standard Model (BSM). Among the muonic CLFV processes, conversion is one of the most important processes, having several advantages compared to other such processes. We describe the COMET experiment, which is searching for conversion in a muonic atom at the J-PARC proton accelerator laboratory in Japan. The COMET experiment has taken a staged approach; the first stage, COMET Phase-I, is currently under construction at J-PARC, and is aiming at a factor 100 improvement over the current limit. The second stage, COMET Phase-II is seeking another 100 improvement (a total of 10,000), allowing a single event sensitivity (SES) of with seconds of data-taking. Further improvements by one order of magnitude, which arise from refinements to the experimental design and operation, are being considered whilst staying within the originally-assumed beam power and beam time. Such a sensitivity could be translated into probing many new physics constructions up to TeV energy scales, which would go far beyond the level that can be reached directly by collider experiments. The search for CLFV conversion is thus highly complementary to BSM searches at the LHC