341 research outputs found
Topological Entropy of Braids on the Torus
A fast method is presented for computing the topological entropy of braids on
the torus. This work is motivated by the need to analyze large braids when
studying two-dimensional flows via the braiding of a large number of particle
trajectories. Our approach is a generalization of Moussafir's technique for
braids on the sphere. Previous methods for computing topological entropies
include the Bestvina--Handel train-track algorithm and matrix representations
of the braid group. However, the Bestvina--Handel algorithm quickly becomes
computationally intractable for large braid words, and matrix methods give only
lower bounds, which are often poor for large braids. Our method is
computationally fast and appears to give exponential convergence towards the
exact entropy. As an illustration we apply our approach to the braiding of both
periodic and aperiodic trajectories in the sine flow. The efficiency of the
method allows us to explore how much extra information about flow entropy is
encoded in the braid as the number of trajectories becomes large.Comment: 19 pages, 44 figures. SIAM journal styl
Topological Chaos in Spatially Periodic Mixers
Topologically chaotic fluid advection is examined in two-dimensional flows
with either or both directions spatially periodic. Topological chaos is created
by driving flow with moving stirrers whose trajectories are chosen to form
various braids. For spatially periodic flows, in addition to the usual
stirrer-exchange braiding motions, there are additional
topologically-nontrivial motions corresponding to stirrers traversing the
periodic directions. This leads to a study of the braid group on the cylinder
and the torus. Methods for finding topological entropy lower bounds for such
flows are examined. These bounds are then compared to numerical stirring
simulations of Stokes flow to evaluate their sharpness. The sine flow is also
examined from a topological perspective.Comment: 18 pages, 14 figures. RevTeX4 style with psfrag macros. Final versio
Braids, Conformal Module and Entropy
The conformal module of conjugacy classes of braids implicitly appeared in a
paper of Lin and Gorin in connection with their interest in the 13. Hilbert
Problem. This invariant is the supremum of conformal modules (in the sense of
Ahlfors) of certain annuli related to the conjugacy class. This note states
that the conformal module is inverse proportional to a popular dynamical braid
invariant, the entropy. The entropy appeared in connection with Thurston's
theory of surface homeomorphisms. An application of the concept of conformal
module to algebraic geometry is given.Comment: Research announcement, 4 pages Minor revision, misprints correcte
Estimating topological entropy from the motion of stirring rods
Stirring a two-dimensional viscous fluid with rods is often an effective way
to mix. The topological features of periodic rod motions give a lower bound on
the topological entropy of the induced flow map, since material lines must
`catch' on the rods. But how good is this lower bound? We present examples from
numerical simulations and speculate on what affects the 'gap' between the lower
bound and the measured topological entropy. The key is the sign of the rod
motion's action on first homology of the orientation double cover of the
punctured disk.Comment: 10 pages, 20 figures. IUTAM Procedia style (included). Submitted to
volume "Topological Fluid Dynamics II.
On the stability of topological phases on a lattice
We study the stability of anyonic models on lattices to perturbations. We
establish a cluster expansion for the energy of the perturbed models and use it
to study the stability of the models to local perturbations. We show that the
spectral gap is stable when the model is defined on a sphere, so that there is
no ground state degeneracy. We then consider the toric code Hamiltonian on a
torus with a class of abelian perturbations and show that it is stable when the
torus directions are taken to infinity simultaneously, and is unstable when the
thin torus limit is taken
Fluid Stirring on a Sphere -- a Topological Approach
The stirring and mixing of a fluid with moving rods is vital in many physical applications in order to achieve homogeneity within the mixture. These rods act as an obstacle that stretches and folds together fluid elements. Over time, the permutation of these rods comprise a mathematical braid whose properties dictate a minimum topological entropy, a number to describe the total disorder or chaos of a system. A braid whose topological entropy is greater than one exhibits chaotic behavior which guarantees an optimal mixing pattern. These rod stirring braids have been previously studied on both the disk as well as the two dimensional torus. The trajectory of fluid mixing on a sphere poses an intriguing starting inquiry to overall mixing on spherical surfaces like the ocean, stars, etc. We use a recipe established by Yvon Verberne to create pseudo-Anosov maps on a punctured sphere using Dehn twist in order to construct similar maps on a 4-times punctured sphere. Since a quotient of the torus under a hyperelliptic involution of torus is the 2-sphere with 4 marked points, we are able to use various methods in order to estimate the topological entropy of a stirring protocol on a 4-times punctured sphere
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