Preliminary design of an intermittent smoke flow visualization system

A prototype intermittent flow visualization system that was designed to study vortex flow field dynamics has been constructed and tested through its ground test phase. It produces discrete pulses of dense white smoke consisting of particles of terephthalic acid by the pulsing action of a fast-acting three-way valve. The trajectories of the smoke pulses can be tracked by a video imaging system without intruding in the flow around in flight. Two methods of pulsing the smoke were examined. The simplest and safest approach is to simply divert the smoke between the two outlet ports on the valve; this approach should be particularly effective if it were desired to inject smoke at two locations during the same test event. The second approach involves closing off one of the outlet ports to momentarily block the flow. The second approach requires careful control of valve dwell times to avoid excessive pressure buildup within the cartridge container. This method also increases the velocity of the smoke injected into the flow. The flow of the smoke has been blocked for periods ranging from 30 to 80 milliseconds, depending on the system volume and the length of time the valve is allowed to remain open between valve closings

Concepts of perceptual significance for composition and reproduction of explorable surround sound fields

Recent work in audio and visual perception suggests that, over and above sensory acuities, exploration of an environment is a most powerful perceptual strategy. For some uses, the plausibility of artificial sound environments might be dramatically improved if exploratory perception is accommodated. The composition and reproduction of spatially explorable sound fields involves a different set of problems from the conventional surround sound paradigm, developed to display music and sound effects to an essentially passive audience. This paper is based upon contemporary models of perception and presents proposals for additional spatial characteristics beyond classical concepts of three-dimensional positioning of virtual objects

Instabilities of wave function monopoles in Bose-Einstein condensates

We present analytic and numerical results for a class of monopole solutions to the two-component Gross-Pitaevski equation for a two-species Bose condensate in an effectively two-dimensional trap. We exhibit dynamical instabilities involving vortex production as one species pours through another, from which we conclude that the sub-optical sharpness of potentials exerted by matter waves makes condensates ideal tools for manipulating condensates. We also show that there are two equally valid but drastically different hydrodynamic descriptions of a two-component condensate, and illustrate how different phenomena may appear simpler in each.Comment: 4 pages, 9 figures (compressed figures become legible when zoomed or when paper is actually printed

Effective non-linear dynamics of binary condensates and open problems

We report on a recent result concerning the effective dynamics for a mixture of Bose-Einstein condensates, a class of systems much studied in physics and receiving a large amount of attention in the recent literature in mathematical physics; for such models, the effective dynamics is described by a coupled system of non-linear Sch\"odinger equations. After reviewing and commenting our proof in the mean field regime from a previous paper, we collect the main details needed to obtain the rigorous derivation of the effective dynamics in the Gross-Pitaevskii scaling limit.Comment: Corrected typos, updated reference

Binary Bose-Einstein Condensate Mixtures in Weakly and Strongly Segregated Phases

We perform a mean-field study of the binary Bose-Einstein condensate mixtures as a function of the mutual repulsive interaction strength. In the phase segregated regime, we find that there are two distinct phases: the weakly segregated phase characterized by a `penetration depth' and the strongly segregated phase characterized by a healing length. In the weakly segregated phase the symmetry of the shape of each condensate will not take that of the trap because of the finite surface tension, but its total density profile still does. In the strongly segregated phase even the total density profile takes a different symmetry from that of the trap because of the mutual exclusion of the condensates. The lower critical condensate-atom number to observe the complete phase segregation is discussed. A comparison to recent experimental data suggests that the weakly segregated phase has been observed.Comment: minor change

Spin correlation and Discrete symmetry in Spinor Bose-Einstein Condensates

We study spin correlations in Bose-Einstein condensates of spin 1 bosons with scatterings dominated by a total spin equal 2 channel. We show the low energy spin dynamics in the system can be mapped into an $o(n)$ nonlinear sigma model(NL$\sigma$M). $n=3$ at the zero magnetic field limit and $n=2$ in the presence of weak magnetic fields. In an ordered phase, the ground state has a hidden $Z_2$ symmetry and is degenerate under the group $[U(1)\times S^{n-1}]/Z_2$. We explore consequences of the hidden symmetry and propose some measurements to probe it.Comment: 4 pages; published version in Phys. Rev. Lett. vol 87, 080401-1(2001

Superfluid vs Ferromagnetic Behaviour in a Bose Gas of Spin-1/2 Atoms

We study the thermodynamic phases of a gas of spin-1/2 atoms in the Hartree-Fock approximation. Our main result is that, for repulsive or weakly-attractive inter-component interaction strength, the superfluid and ferromagnetic phase transitions occur at the same temperature. For strongly-attractive inter-component interaction strength, however, the ferromagnetic phase transition occurs at a higher temperature than the superfluid phase transition. We also find that the presence of a condensate acts as an effective magnetic field that polarizes the normal cloud. We finally comment on the validity of the Hartree-Fock approximation in describing different phenomena in this system.Comment: 10 pages, 2 figure

Excitation spectrum and instability of a two-species Bose-Einstein condensate

We numerically calculate the density profile and excitation spectrum of a two-species Bose-Einstein condensate for the parameters of recent experiments. We find that the ground state density profile of this system becomes unstable in certain parameter regimes, which leads to a phase transition to a new stable state. This state displays spontaneously broken cylindrical symmetry. This behavior is reflected in the excitation spectrum: as we approach the phase transition point, the lowest excitation frequency goes to zero, indicating the onset of instability in the density profile. Following the phase transition, this frequency rises again.Comment: 8 pages, 5 figures, uses REVTe

On testing global optimization algorithms for space trajectory design

In this paper we discuss the procedures to test a global search algorithm applied to a space trajectory design problem. Then, we present some performance indexes that can be used to evaluate the effectiveness of global optimization algorithms. The performance indexes are then compared highlighting the actual significance of each one of them. A number of global optimization algorithms are tested on four typical space trajectory design problems. From the results of the proposed testing procedure we infer for each pair algorithm-problem the relation between the heuristics implemented in the solution algorithm and the main characteristics of the problem under investigation. From this analysis we derive a novel interpretation of some evolutionary heuristics, based on dynamical system theory and we significantly improve the performance of one of the tested algorithms