Parabolic resonances and instabilities in near-integrable two degrees of freedom Hamiltonian flows

When an integrable two-degrees-of-freedom Hamiltonian system possessing a circle of parabolic fixed points is perturbed, a parabolic resonance occurs. It is proved that its occurrence is generic for one parameter families (co-dimension one phenomenon) of near-integrable, t.d.o. systems. Numerical experiments indicate that the motion near a parabolic resonance exhibits new type of chaotic behavior which includes instabilities in some directions and long trapping times in others. Moreover, in a degenerate case, near a {\it flat parabolic resonance}, large scale instabilities appear. A model arising from an atmospherical study is shown to exhibit flat parabolic resonance. This supplies a simple mechanism for the transport of particles with {\it small} (i.e. atmospherically relevant) initial velocities from the vicinity of the equator to high latitudes. A modification of the model which allows the development of atmospherical jets unfolds the degeneracy, yet traces of the flat instabilities are clearly observed

Symmetry breaking perturbations and strange attractors

The asymmetrically forced, damped Duffing oscillator is introduced as a prototype model for analyzing the homoclinic tangle of symmetric dissipative systems with \textit{symmetry breaking} disturbances. Even a slight fixed asymmetry in the perturbation may cause a substantial change in the asymptotic behavior of the system, e.g. transitions from two sided to one sided strange attractors as the other parameters are varied. Moreover, slight asymmetries may cause substantial asymmetries in the relative size of the basins of attraction of the unforced nearly symmetric attracting regions. These changes seems to be associated with homoclinic bifurcations. Numerical evidence indicates that \textit{strange attractors} appear near curves corresponding to specific secondary homoclinic bifurcations. These curves are found using analytical perturbational tools

Effectiveness of the Blue Crab Spawning Sanctuary in Chesapeake Bay

The blue crab spawning stock in Chesapeake Bay sustained a severe and persistent decline beginning in 1992. As part of the effort to enhance the spawning stock, the spawning sanctuary in lower Chesapeake Bay was enlarged to over 240,000 ha. This marine reserve and corridor prohibits exploitation of mature females en route to or in the spawning grounds during the summer spawning season (1 June-15 September). To assess the effectiveness of the sanctuary, we tagged terminally molted, mature females inside and outside the sanctuary during three sanctuary seasons (2002-2004). Crabs were captured throughout the Bay and its tributaries, measured, tagged, and released on site. Recaptures of tagged crabs were reported by commercial and recreational fishers. Probability of recapture for crabs released outside of the sanctuary was 6.3, 5.2, and 2.8 times the probability of recapture for crabs tagged inside the sanctuary for 2002, 2003 and 2004, respectively. Consequently, a significant proportion of adult female blue crabs remains in the sanctuary to spawn and is not captured by the fishery. Hence, the marine reserve and corridor for the blue crab spawning stock in Chesapeake Bay is an effective means of protecting females migrating to or residing in the spawning grounds. This investigation serves as one of the few empirical tests to date of the effectiveness of a marine reserve designed to protect spawning stock

Reef height drives threshold dynamics of restored oyster reefs

Nonlinear threshold responses to biotic or abiotic forcing may produce multiple population trajectories dependent upon initial conditions, which can reinforce population recovery or drive local ex - tinction, yet experimental tests of this phenomenon are lacking in marine ecosystems. In field experiments at 4 sites in 2 tributaries of lower Chesapeake Bay, we examined demographic responses (density and survival) of eastern oyster Crassostrea virginica populations to reef height and associated gradients in sediment deposition and habitat complexity. After 2 yr, oyster reefs exhibited diverging trajectories to ward either degradation or persistence, dependent upon initial reef height. Reefs higher than 0.3 m supported greater oyster density, survival, and reef complexity, whereas sediment deposition was reduced. Reefs lower than 0.3 m experienced heavy sediment deposition and were eventually buried. These observations (1) provide experimental evidence for threshold dynamics in marine species, (2) suggest that the collapse of oyster populations was largely due to anthropo - genic habitat degradation that eliminated positive feed - backs and which may have created an alternative reef trajectory towards local extinction, and (3) indicate an avenue by which oyster restoration is achievable

Stickiness in Hamiltonian systems: from sharply divided to hierarchical phase space

We investigate the dynamics of chaotic trajectories in simple yet physically important Hamiltonian systems with non-hierarchical borders between regular and chaotic regions with positive measures. We show that the stickiness to the border of the regular regions in systems with such a sharply divided phase space occurs through one-parameter families of marginally unstable periodic orbits and is characterized by an exponent \gamma= 2 for the asymptotic power-law decay of the distribution of recurrence times. Generic perturbations lead to systems with hierarchical phase space, where the stickiness is apparently enhanced due to the presence of infinitely many regular islands and Cantori. In this case, we show that the distribution of recurrence times can be composed of a sum of exponentials or a sum of power-laws, depending on the relative contribution of the primary and secondary structures of the hierarchy. Numerical verification of our main results are provided for area-preserving maps, mushroom billiards, and the newly defined magnetic mushroom billiards.Comment: To appear in Phys. Rev. E. A PDF version with higher resolution figures is available at http://www.pks.mpg.de/~edugal

Evaluation of a ducted-fan power plant designed for high output and good cruise fuel economy

Theoretical analysis of performance of a ducted-fan power plant designed both for high-output, high-altitude operation at low supersonic Mach numbers and for good fuel economy at lower fight speeds is presented. Performance of ducted fan is compared with performance (with and without tail-pipe burner) of two hypothetical turbojet engines. At maximum power, the ducted fan has propulsive thrust per unit of frontal area between thrusts obtained by turbojet engines with and without tail-pipe burners. At cruise, the ducted fan obtains lowest thrust specific fuel consumption. For equal maximum thrusts, the ducted fan obtains cruising flight duration and range appreciably greater than turbojet engines

NASA space station automation: AI-based technology review

Research and Development projects in automation for the Space Station are discussed. Artificial Intelligence (AI) based automation technologies are planned to enhance crew safety through reduced need for EVA, increase crew productivity through the reduction of routine operations, increase space station autonomy, and augment space station capability through the use of teleoperation and robotics. AI technology will also be developed for the servicing of satellites at the Space Station, system monitoring and diagnosis, space manufacturing, and the assembly of large space structures

Maintaining a way of life: trials and tribulations of farmers’ market families

Never before in our nation’s history has there been so many ways for consumers to purchase food. From grocery stores, to super centers such as Wal-Mart and Costco, convenience stores, online purchases, community supported agriculture (CSA), and farmers’ markets, Americans have a multitude of venues to choose from. Although many Americans currently purchase their foods from grocery stores, a growing number of them are buying locally at their farmers’ markets and from CSAs. As the sustainability movement takes a greater foothold in the American household, local products and local foods are becoming ever more important and prevalent. Yet with all of the statistics surrounding local agriculture, the human element is often lost. A majority of small farmers and their spouses, often the ones who sell at a local level, have to work full time both on and off farm to support their families and farms. This case study examines the professional lives of five local farm families who choose to sell their products at the Fayetteville, Arkansas farmers’ market. It seeks to understand farmers’ reasons for farming and selling locally, as well as their biggest challenges and rewards. In addition, it seeks to fill gaps in literature regarding farmers’ motivations for selling at a local level