2,094 research outputs found
Generalized Cylinders in Semi-Riemannian and Spin Geometry
We use a construction which we call generalized cylinders to give a new proof
of the fundamental theorem of hypersurface theory. It has the advantage of
being very simple and the result directly extends to semi-Riemannian manifolds
and to embeddings into spaces of constant curvature. We also give a new way to
identify spinors for different metrics and to derive the variation formula for
the Dirac operator. Moreover, we show that generalized Killing spinors for
Codazzi tensors are restrictions of parallel spinors. Finally, we study the
space of Lorentzian metrics and give a criterion when two Lorentzian metrics on
a manifold can be joined in a natural manner by a 1-parameter family of such
metrics.Comment: 29 pages, 2 figure
A streamwise-constant model of turbulent pipe flow
A streamwise-constant model is presented to investigate the basic mechanisms
responsible for the change in mean flow occuring during pipe flow transition.
Using a single forced momentum balance equation, we show that the shape of the
velocity profile is robust to changes in the forcing profile and that both
linear non-normal and nonlinear effects are required to capture the change in
mean flow associated with transition to turbulence. The particularly simple
form of the model allows for the study of the momentum transfer directly by
inspection of the equations. The distribution of the high- and low-speed
streaks over the cross-section of the pipe produced by our model is remarkably
similar to one observed in the velocity field near the trailing edge of the
puff structures present in pipe flow transition. Under stochastic forcing, the
model exhibits a quasi-periodic self-sustaining cycle characterized by the
creation and subsequent decay of "streamwise-constant puffs", so-called due to
the good agreement between the temporal evolution of their velocity field and
the projection of the velocity field associated with three-dimensional puffs in
a frame of reference moving at the bulk velocity. We establish that the flow
dynamics are relatively insensitive to the regeneration mechanisms invoked to
produce near-wall streamwise vortices and that using small, unstructured
background disturbances to regenerate the streamwise vortices is sufficient to
capture the formation of the high- and low-speed streaks and their segregation
leading to the blunting of the velocity profile characteristic of turbulent
pipe flow
Genetic prediction of quantitative traits: a machine learner's guide focused on height
Machine learning and deep learning have been celebrating many successes in
the application to biological problems, especially in the domain of protein
folding. Another equally complex and important question has received relatively
little attention by the machine learning community, namely the one of
prediction of complex traits from genetics. Tackling this problem requires
in-depth knowledge of the related genetics literature and awareness of various
subtleties associated with genetic data. In this guide, we provide an overview
for the machine learning community on current state of the art models and
associated subtleties which need to be taken into consideration when developing
new models for phenotype prediction. We use height as an example of a
continuous-valued phenotype and provide an introduction to benchmark datasets,
confounders, feature selection, and common metrics
Alterações neuroendócrinas causadas por disruptores endócrinos: o exemplo do Bisfenol A
Los contaminantes ambientales, como los disruptores endocrinos,podrĂan provocar profundos cambios en los seres vivos. Aqui se analizan las alteraciones neuroendocrinas y reproductivas en mamĂferos, incluyendo las descriptas en humanos, causadas por una molĂ©cula de origen industrial, el Bisfenol A.Exposure to endocrine disruptors may produce profound alterations in several species. As an example, the neuroendocrine and reproductive alterations due to Bisphenol A in mammals are summarized here.Os contaminantes ambientais, como os disruptores endĂłcrinos, poderiam provocar profundas alterações nos seres vivos. Aqui sĂŁo analisadas as alterações neuroendĂłcrinas e reprodutivas em mamĂferos, incluindo as descritas em humanos, causadas por uma molĂ©cula de origem industrial,o Bisfenol AFil: Bourguignon, Nadia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BiologĂa y Medicina Experimental (i); ArgentinaFil: Fernández, Marina . Universidad de Buenos Aires. Facultad de Medicina; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BiologĂa y Medicina Experimental (i); ArgentinaFil: Lux, Victoria Adela R.. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BiologĂa y Medicina Experimental (i); ArgentinaFil: Libertun, Carlos. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BiologĂa y Medicina Experimental (i); Argentina. Universidad de Buenos Aires. Facultad de Medicina; Argentin
A characterization of Dirac morphisms
Relating the Dirac operators on the total space and on the base manifold of a
horizontally conformal submersion, we characterize Dirac morphisms, i.e. maps
which pull back (local) harmonic spinor fields onto (local) harmonic spinor
fields.Comment: 18 pages; restricted to the even-dimensional cas
Vanishing viscosity limit of navier-stokes equations in gevrey class
In this paper we consider the inviscid limit for the periodic solutions to
Navier-Stokes equation in the framework of Gevrey class. It is shown that the
lifespan for the solutions to Navier-Stokes equation is independent of
viscosity, and that the solutions of the Navier-Stokes equation converge to
that of Euler equation in Gevrey class as the viscosity tends to zero. Moreover
the convergence rate in Gevrey class is presented
Vacuum Spacetimes with Future Trapped Surfaces
In this article we show that one can construct initial data for the Einstein
equations which satisfy the vacuum constraints. This initial data is defined on
a manifold with topology with a regular center and is asymptotically
flat. Further, this initial data will contain an annular region which is
foliated by two-surfaces of topology . These two-surfaces are future
trapped in the language of Penrose. The Penrose singularity theorem guarantees
that the vacuum spacetime which evolves from this initial data is future null
incomplete.Comment: 19 page
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