Stability analysis of the elliptic cylinder wake

This paper presents the results of numerical stability analysis of the wake of an elliptical cylinder. Aspect ratios where the ellipse is longer in the streamwise direction than in the transverse direction are considered. The focus is on the dependence on the aspect ratio of the ellipse of the various bifurcations to three-dimensional flow from the two-dimensional Kármán vortex street. It is shown that the three modes present in the wake of a circular cylinder (modes A, B and QP) are present in the ellipse wake, and that in general they are all stabilized by increasing the aspect ratio of the ellipse. Two new pertinent modes are found: one long-wavelength mode with similarities to mode A, and a second that is only unstable for aspect ratios greater than approximately 1.75, which has similar spatiotemporal symmetries to mode B but has a distinct spatial structure. Results from fully three-dimensional simulations are also presented confirming the existence and growth of these two new modes in the saturated wakes.

Directed Transport of Atoms in a Hamiltonian Quantum Ratchet

Classical ratchet potentials, which alternate a driving potential with periodic random dissipative motion, can account for the operation of biological motors. We demonstrate the operation of a quantum ratchet, which differs from classical ratchets in that dissipative processes are absent within the observation time of the system (Hamiltonian regime). An atomic rubidium Bose-Einstein condensate is exposed to a sawtooth-like optical lattice potential, whose amplitude is periodically modulated in time. The ratchet transport arises from broken spatiotemporal symmetries of the driven potential, resulting in a desymmetrization of transporting eigenstates (Floquet states). The full quantum character of the ratchet transport was demonstrated by the measured atomic current oscillating around a nonzero stationary value at longer observation times, resonances occurring at positions determined by the photon recoil, and dependence of the transport current on the initial phase of the driving potential.

COLLECTIVE DIRECTIONAL TRANSPORT AND COUPLED BROWNIAN RATCHETS

Recent progresses on collective directional transport in coupled systems are overviewed. By considering the mutual interactions among elements, the directed ratchet motion exhibits complicated and novel cooperative dynamics. The directional transport can be induced not only by breaking the spatial symmetry of the substrate potential, but also by breaking the coupling symmetry and the spatiotemporal symmetries.

Bifurcation from relative periodic solutions

Relative periodic solutions are ubiquitous in dynamical systems with continuous symmetry. Recently, Sandstede, Scheel and Wulff derived a center bundle theorem, reducing local bifurcation from relative periodic solutions to a finite-dimensional problem. Independently, Lamb and Melbourne showed how to systematically study local bifurcation from isolated periodic solutions with discrete spatiotemporal symmetries.In this paper, we show how the center bundle theorem, when combined with certain group theoretic results, reduces bifurcation from relative periodic solutions to bifurcation from isolated periodic solutions. In this way, we obtain a systematic approach to the study of local bifurcation from relative periodic solutions.

CONTROL OF SPATIOTEMPORAL CHAOS IN NEURONAL NETWORKS

Different strategies for the control of chaotic neuronal networks and physical systems are presented. The variables of the dynamics span phase-spaces with very high dimensions. Novel control techniques are introduced that handle these high-dimensional cases. Depending on the system, several orbits with different spatiotemporal symmetries may be stabilized out of the chaotic attractor. The role of chaos control in information processing is pointed out.