Liquid acrobatics

We experiment with injecting a continuous stream of gas into a shallow liquid, similar to how one might blow into a straw placed at the bottom of a near-empty drink. By varying the angle of the straw (here a metal needle), we observe a variety of dynamics, which we film using a high-speed camera. Most noteworthy is an intermediate regime in which cyclical jets erupt from the air-liquid interface and breakup into air-born droplets. These droplets trace out a parabolic trajectory and bounce on the air-liquid interface before eventually coalescing. The shape of each jet, as well as the time between jets, is remarkably similar and leads to droplets with nearly identical trajectories. The following article accompanies the linked fluid dynamics video submitted to the Gallery of Fluid Motion in 2008.Comment: Accompanies video submission to APS DFD 2008 Gallery of Fluid Motion, low http://ecommons.library.cornell.edu/bitstream/1813/11469/3/Bird_DFD2008_mpeg1.mpg , and high resolution http://ecommons.library.cornell.edu/bitstream/1813/11469/2/Bird_DFD2008_mpeg2.mp

Smoothed Particle Hydrodynamics Calculations of Stellar Interactions

Smoothed Particle Hydrodynamics is a multidimensional Lagrangian method of numerical hydrodynamics that has been used to tackle a wide variety of problems in astrophysics. Here we develop the basic equations of the SPH scheme, and we discuss some of its numerical properties and limitations. As an illustration of typical astrophysical applications, we discuss recent calculations of stellar interactions, including collisions between main sequence stars and the coalescence of compact binaries.Comment: 21 pages, invited review paper to appear in Journal of Computational and Applied Mathematics (JCAM). Fig. 2 is available from the authors in hardcopy form only. Revised, slightly abbreviated versio

Control of Complex Dynamic Systems by Neural Networks

This paper considers the use of neural networks (NN's) in controlling a nonlinear, stochastic system with unknown process equations. The NN is used to model the resulting unknown control law. The approach here is based on using the output error of the system to train the NN controller without the need to construct a separate model (NN or other type) for the unknown process dynamics. To implement such a direct adaptive control approach, it is required that connection weights in the NN be estimated while the system is being controlled. As a result of the feedback of the unknown process dynamics, however, it is not possible to determine the gradient of the loss function for use in standard (back-propagation-type) weight estimation algorithms. Therefore, this paper considers the use of a new stochastic approximation algorithm for this weight estimation, which is based on a 'simultaneous perturbation' gradient approximation that only requires the system output error. It is shown that this algorithm can greatly enhance the efficiency over more standard stochastic approximation algorithms based on finite-difference gradient approximations

Records for Bittacus Hangingflies and Panorpa Scorpionflies (Mecoptera: Bittacidae and Panorpidae) in Florida

We provide new county records for four species of panorpids (Panorpa americana Swederus, Panorpa lugubris Swederus, Panorpa sp. undetermined, and one undescribed species from the Panorpa rufescens Rambur species group) and two species of bittacids (Bittacus pilicornis Westwood, Bittacus punctiger Westwood), and call attention to a previously published county record for another species, Panorpa rufa Gray, in Florida. Additionally, we reject a previously published record for the scorpionfly Panorpa claripennis Hine in Florida, which we overlooked in our 2008 preliminary checklist of Florida mecopterans. The record for P. lugubris in Miami-Dade County is the southernmost record for any panorpid in the continental United States

Gamma-Ray Spectral Characteristics of Thermal and Non-Thermal Emission from Three Black Holes

Cygnus X-1 and the gamma-ray transients GROJ0422+32 and GROJ1719-24 displayed similar spectral properties when they underwent transitions between the high and low gamma-ray (30 keV to few MeV) intensity states. When these sources were in the high gamma-ray intensity state (gamma-2, for Cygnus X-1), their spectra featured two components: a Comptonized shape below 200-300 keV with a soft power-law tail (photon index >3) that extended to ~1 MeV or beyond. When the sources were in the low-intensity state (gamma-0, for Cygnus X-1), the Comptonized spectral shape below 200 keV typically vanished and the entire spectrum from 30 keV to ~1 MeV can be characterized by a single power law with a relatively harder photon index ~2-2.7. Consequently the high- and low-intensity gamma-ray spectra intersect, generally in the ~400 keV - 1 MeV range, in contrast to the spectral pivoting seen previously at lower (~10 keV) energies. The presence of the power-law component in both the high- and low-intensity gamma-ray spectra strongly suggests that the non-thermal process is likely to be at work in both the high and the low-intensity situations. We have suggested a possible scenario (Ling & Wheaton, 2003), by combining the ADAF model of Esin et al. (1998) with a separate jet region that produces the non-thermal gamma-ray emission, and which explains the state transitions. Such a scenario will be discussed in the context of the observational evidence, summarized above, from the database produced by EBOP, JPL's BATSE earth occultation analysis system.Comment: 6 pages, 3 figures, accepted for publication in Proceedings of 2004 Microquasar Conference, Beijing, China, Chinese Journal of Astronomy and Astrophysics, minor corrections per refere

Preliminary Checklist of the Mecoptera of Florida: Earwigflies, Hangingflies, and Scorpionflies

We provide the first species checklist of Mecoptera indigenous to Florida, based upon preliminary data gathered primarily from specimens housed in the Florida State Collection of Arthropods. There are 11 described and one undescribed species of mecopterans, representing three families, inhabiting the state of Florida. These include the recently discovered meropeid (earwigfly), Merope tuber Newman, four species of bittacids (hangingflies), represented by the genus Bittacus Latreille, and 7 species (one undescribed) of panorpids (scorpionflies), represented by the genus Panorpa Linnaeus. We are not certain if one of these, Bittacus texanus Banks, is indigenous, represents a nonindigenous occurrence, or is simply erroneous. Two of the species on our list, Bittacus stigmaterus Say and Panorpa venosa Westwood, represent first state records and one species, Panorpa floridana Byers, is endemic. Six of the species which have been recorded in Florida, M. tuber, Bittacus punctiger Westwood, Panorpa rufa Gray, Panorpa pachymera Byers, Panorpa lugubris Swederus and P. venosa, represent the southernmost records for these species in the continental United States. Perceived diversity and abundance of mecopteran species in Florida are limited by climate, biogeography, and collection bias