Flexible body dynamic stability for high performance aircraft

Dynamic equations which include the effects of unsteady aerodynamic forces and a flexible body structure were developed for a free flying high performance fighter aircraft. The linear and angular deformations are assumed to be small in the body reference frame, allowing the equations to be linearized in the deformation variables. Equations for total body dynamics and flexible body dynamics are formulated using the hybrid coordinate method and integrated in a state space format. A detailed finite element model of a generic high performance fighter aircraft is used to generate the mass and stiffness matrices. Unsteady aerodynamics are represented by a rational function approximation of the doublet lattice matrices. The equations simplify for the case of constant angular rate of the body reference frame, allowing the effect of roll rate to be studied by computing the eigenvalues of the system. It is found that the rigid body modes of the aircraft are greatly affected by introducing a constant roll rate, while the effect on the flexible modes is minimal for this configuration

Some remarks on the visible points of a lattice

We comment on the set of visible points of a lattice and its Fourier transform, thus continuing and generalizing previous work by Schroeder and Mosseri. A closed formula in terms of Dirichlet series is obtained for the Bragg part of the Fourier transform. We compare this calculation with the outcome of an optical Fourier transform of the visible points of the 2D square lattice.Comment: 9 pages, 3 eps-figures, 1 jpeg-figure; updated version; another article (by M. Baake, R. V. Moody and P. A. B. Pleasants) with the complete solution of the spectral problem will follow soon (see math.MG/9906132

Feedback in a cavity QED system for control of quantum beats

Conditional measurements on the undriven mode of a two-mode cavity QED system prepare a coherent superposition of ground states which generate quantum beats. The continuous system drive induces decoherence through the phase interruptions from Rayleigh scattering, which manifests as a decrease of the beat amplitude and an increase of the frequency of oscillation. We report recent experiments that implement a simple feedback mechanism to protect the quantum beat. We continuously drive the system until a photon is detected, heralding the presence of a coherent superposition. We then turn off the drive and let the superposition evolve in the dark, protecting it against decoherence. At a later time we reinstate the drive to measure the amplitude, phase, and frequency of the beats. The amplitude can increase by more than fifty percent, while the frequency is unchanged by the feedback.Comment: 13 pages, 5 figures, ICAP 2012 23rd International Conference on Atomic Physic

Air hydrodynamics of the ultrafast laser-triggered spark gap

We present space and time resolved measurements of the air hydrodynamics induced by ultrafast laser pulse excitation of the air gap between two electrodes at high potential difference. We explore both plasma-based and plasma-free gap excitation. The former uses the plasma left in the wake of femtosecond filamentation, while the latter exploits air heating by multiple-pulse resonant excitation of quantum molecular wavepackets. We find that the cumulative electrode-driven air density depression channel initiated by the laser plays the dominant role in the gap evolution leading to breakdown

Ascaroside Signaling Is Widely Conserved among Nematodes

Background: Nematodes are among the most successful animals on earth and include important human pathogens, yet little is known about nematode pheromone systems. A group of small molecules called ascarosides has been found to mediate mate finding, aggregation, and developmental diapause in Caenorhabditis elegans, but it is unknown whether ascaroside signaling exists outside of the genus Caenorhabditis. Results: To determine whether ascarosides are used as signaling molecules by other nematode species, we performed a mass spectrometry-based screen for ascarosides in secretions from a variety of both free-living and parasitic (plant, insect, and animal) nematodes. We found that most of the species analyzed, including nematodes from several different clades, produce species-specific ascaroside mixtures. In some cases, ascaroside biosynthesis patterns appear to correlate with phylogeny, whereas in other cases, biosynthesis seems to correlate with lifestyle and ecological niche. We further show that ascarosides mediate distinct nematode behaviors, such as retention, avoidance, and long-range attraction, and that different nematode species respond to distinct, but overlapping, sets of ascarosides. Conclusions: Our findings indicate that nematodes utilize a conserved family of signaling molecules despite having evolved to occupy diverse ecologies. Their structural features and level of conservation are evocative of bacterial quorum sensing, where acyl homoserine lactones (AHLs) are both produced and sensed by many species of gram-negative bacteria. The identification of species-specific ascaroside profiles may enable pheromone-based approaches to interfere with reproduction and survival of parasitic nematodes, which are responsible for significant agricultural losses and many human diseases worldwide