408 research outputs found
Computational Method for Phase Space Transport with Applications to Lobe Dynamics and Rate of Escape
Lobe dynamics and escape from a potential well are general frameworks
introduced to study phase space transport in chaotic dynamical systems. While
the former approach studies how regions of phase space are transported by
reducing the flow to a two-dimensional map, the latter approach studies the
phase space structures that lead to critical events by crossing periodic orbit
around saddles. Both of these frameworks require computation with curves
represented by millions of points-computing intersection points between these
curves and area bounded by the segments of these curves-for quantifying the
transport and escape rate. We present a theory for computing these intersection
points and the area bounded between the segments of these curves based on a
classification of the intersection points using equivalence class. We also
present an alternate theory for curves with nontransverse intersections and a
method to increase the density of points on the curves for locating the
intersection points accurately.The numerical implementation of the theory
presented herein is available as an open source software called Lober. We used
this package to demonstrate the application of the theory to lobe dynamics that
arises in fluid mechanics, and rate of escape from a potential well that arises
in ship dynamics.Comment: 33 pages, 17 figure
In-plane magnetic reorientation in coupled ferro- and antiferromagnetic thin films
By studying coupled ferro- (FM) and antiferromagnetic (AFM) thin film
systems, we obtain an in-plane magnetic reorientation as a function of
temperature and FM film thickness. The interlayer exchange coupling causes a
uniaxial anisotropy, which may compete with the intrinsic anisotropy of the FM
film. Depending on the latter the total in-plane anisotropy of the FM film is
either enhanced or reduced. Eventually a change of sign occurs, resulting in an
in-plane magnetic reorientation between a collinear and an orthogonal magnetic
arrangement of the two subsystems. A canted magnetic arrangement may occur,
mediating between these two extremes. By measuring the anisotropy below and
above the N\'eel temperature the interlayer exchange coupling can be
determined. The calculations have been performed with a Heisenberg-like
Hamiltonian by application of a two-spin mean-field theory.Comment: 4 pages, 4 figure
A study of low-energy transfer orbits to the Moon: towards an operational optimization technique
In the Earth-Moon system, low-energy orbits are transfer trajectories from
the earth to a circumlunar orbit that require less propellant consumption when
compared to the traditional methods. In this work we use a Monte Carlo approach
to study a great number of such transfer orbits over a wide range of initial
conditions. We make statistical and operational considerations on the resulting
data, leading to the description of a reliable way of finding "optimal" mission
orbits with the tools of multi-objective optimization
Spin Reorientations Induced by Morphology Changes in Fe/Ag(001)
By means of magneto-optical Kerr effect we observe spin reorientations from
in-plane to out-of-plane and vice versa upon annealing thin Fe films on Ag(001)
at increasing temperatures. Scanning tunneling microscopy images of the
different Fe films are used to quantify the surface roughness. The observed
spin reorientations can be explained with the experimentally acquired roughness
parameters by taking into account the effect of roughness on both the magnetic
dipolar and the magnetocrystalline anisotropy.Comment: 4 pages with 3 EPS figure
Metastable Random Field Ising model with exchange enhancement: a simple model for Exchange Bias
We present a simple model that allows hysteresis loops with exchange bias to
be reproduced. The model is a modification of the T=0 random field Ising model
driven by an external field and with synchronous local relaxation dynamics. The
main novelty of the model is that a certain fraction f of the exchange
constants between neighbouring spins is enhanced to a very large value J_E. The
model allows the dependence of the exchange bias and other properties of the
hysteresis loops to be analyzed as a function of the parameters of the model:
the fraction f of enhanced bonds, the amount of the enhancement J_E and the
amount of disorder which is controlled by the width sigma of the Gaussian
distribution of the random fields.Comment: 8 pages, 11 figure
A biophysical model of cell adhesion mediated by immunoadhesin drugs and antibodies
A promising direction in drug development is to exploit the ability of
natural killer cells to kill antibody-labeled target cells. Monoclonal
antibodies and drugs designed to elicit this effect typically bind cell-surface
epitopes that are overexpressed on target cells but also present on other
cells. Thus it is important to understand adhesion of cells by antibodies and
similar molecules. We present an equilibrium model of such adhesion,
incorporating heterogeneity in target cell epitope density and epitope
immobility. We compare with experiments on the adhesion of Jurkat T cells to
bilayers containing the relevant natural killer cell receptor, with adhesion
mediated by the drug alefacept. We show that a model in which all target cell
epitopes are mobile and available is inconsistent with the data, suggesting
that more complex mechanisms are at work. We hypothesize that the immobile
epitope fraction may change with cell adhesion, and we find that such a model
is more consistent with the data. We also quantitatively describe the parameter
space in which binding occurs. Our results point toward mechanisms relating
epitope immobility to cell adhesion and offer insight into the activity of an
important class of drugs.Comment: 13 pages, 5 figure
Mechanical Systems with Symmetry, Variational Principles, and Integration Algorithms
This paper studies variational principles for mechanical systems with symmetry and their applications to integration algorithms. We recall some general features of how to reduce variational principles in the presence of a symmetry group along with general features of integration algorithms for mechanical systems. Then we describe some integration algorithms based directly on variational principles using a
discretization technique of Veselov. The general idea for these variational integrators is to directly discretize Hamilton’s principle rather than the equations of motion in a way that preserves the original systems invariants, notably the symplectic form and, via a discrete version of Noether’s theorem, the momentum map. The resulting mechanical integrators are second-order accurate, implicit, symplectic-momentum algorithms. We apply these integrators to the rigid body and the double spherical pendulum to show that the techniques are competitive with existing integrators
Analysis of Capture Trajectories into Periodic Orbits About Libration Points
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77033/1/AIAA-33796-456.pd
Spacecraft Formation Dynamics and Design
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76961/1/AIAA-13002-440.pd
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