Thirty Years of Turnstiles and Transport

To characterize transport in a deterministic dynamical system is to compute exit time distributions from regions or transition time distributions between regions in phase space. This paper surveys the considerable progress on this problem over the past thirty years. Primary measures of transport for volume-preserving maps include the exiting and incoming fluxes to a region. For area-preserving maps, transport is impeded by curves formed from invariant manifolds that form partial barriers, e.g., stable and unstable manifolds bounding a resonance zone or cantori, the remnants of destroyed invariant tori. When the map is exact volume preserving, a Lagrangian differential form can be used to reduce the computation of fluxes to finding a difference between the action of certain key orbits, such as homoclinic orbits to a saddle or to a cantorus. Given a partition of phase space into regions bounded by partial barriers, a Markov tree model of transport explains key observations, such as the algebraic decay of exit and recurrence distributions.Comment: Updated and corrected versio

Grain refinement in a AlZnMgCuTi alloy by intensive melt shearing: A multi-step nucleation mechanism

This is a post-print version of the article. Copyright @ 2010 Elsevier B.V.Direct chill (DC) cast ingots of wrought Al alloys conventionally require the deliberate addition of a grain refiner to provide a uniform as-cast microstructure for the optimisation of both mechanical properties and processability. Grain refiner additions have been in widespread industrial use for more than half a century. Intensive melt shearing can provide grain refinement without the need for a specific grain refiner addition for both magnesium and aluminium based alloys. In this paper we present experimental evidence of the grain refinement in an experimental wrought aluminium alloy achieved by intensive melt shearing in the liquid state prior to solidification. The mechanisms for high shear induced grain refinement are correlated with the evolution of oxides in alloys. The oxides present in liquid aluminium alloys, normally as oxide films and clusters, can be effectively dispersed by intensive shearing and then provide effective sites for the heterogeneous nucleation of Al3Ti phase. As a result, Al3Ti particles with a narrow size distribution and hence improved efficiency as active nucleation sites of alpha-aluminium grains are responsible for the achieved significant grain refinement. This is termed a multi-step nucleation mechanism.Funding was obtained from the EPRSC

The Impact of Turfgrass Pest Management System Techniques on Surface and Ground Water Quality

ReportThe impact of various turfgrass pest management strategies (PMS) on water quality has recently become a concern for many golf course superintendents, sports facilities managers and homeowners. With water quality standards becoming increasingly stringent, management practices have had to follow suit. Uses of alternative control strategies have become increasingly important. This includes the use of biological, cultural and preventative control practices to reduce pest pressure, as well as environmental impacts. Turfgrass is, no doubt, a beneficial addition to most ecosystems, yet when mis-managed can cause harm as well. Mis-management of the turfgrass ecosystem can greatly influence the nitrogen, phosphate and pesticide levels in surface and ground water, causing problems for communities that depend on clean water for consumption as well as recreation. Aquatic ecosystems as well can be severely harmed by increased levels of nitrogen and phosphate, which can cause algal bloom, decreased dissolved oxygen levels, and eutrophication, which in turn has an impact on nearly all ecosystems. Pesticides that find their way into surface or ground water pose a problem to exposed species ranging from fish to humans. When managed correctly, turfgrass provides many positive attributes, including increased UV absorption, CO2 remediation, soil stabilization, ground and surface water filtration, and aesthetic benefits. It is our goal to test the impact of three of the most commonly implemented turfgrass pest management systems, (preventative, IPM, and organic systems) on surface and ground water quality and turfgrass performance. The results will hopefully provide answers on how to produce acceptable turfgrass quality while benefiting the environment

The contribution of constitutional supercooling to nucleation and grain formation

The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by Rutter and Chalmers in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller et al. On that basis, Winegard and Chalmers further considered 'supercooling and dendritic freezing of alloys' where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS, recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy

Current Space Station Experiments Investigating Component Level Electronics Repair

The Soldering in a Reduced Gravity Experiment (SoRGE) and Component Repair Experiment (CRE)-1 are tests performed on the International Space Station to determine the techniques, tools, and training necessary to allow future crews to perform manual electronics repairs at the component level. SoRGE provides information on the formation and internal structure of through-hole solder joints, illustrating the challenges and implications of soldering in reduced gravity. SoRGE showed a significant increase in internal void defects for joints formed in low gravity compared to normal gravity. Methods for mitigating these void defects were evaluated using a modified soldering process. CRE-1 demonstrated the removal, cleaning, and replacement of electronics components by manual means on functional circuit boards. The majority of components successful passed a post-repair functional test demonstrating the feasibility of component-level repair within the confines of a spacecraft. Together, these tasks provide information to recommend material and tool improvements, training improvements, and future work to help enable electronics repairs in future space missions

Component Repair Experiment-1: An Experiment Evaluating Electronic Component-Level Repair During Spaceflight

The Component Repair Experiment-1 (CRE-1) examines the capability for astronauts to perform electronics repair tasks in space. The goal is to determine the current capabilities and limits for the crew, and to make recommendations to improve and expand the range of work that astronauts may perform. CRE-1 provided two-layer, functional circuit boards and replacement components, a small tool kit, written and video training materials, and 1 hr of hands on training for the crew slated to perform the experiment approximately 7 months prior to the mission. Astronauts Michael Fincke and Sandra Magnus performed the work aboard the International Space Station (ISS) in February and March 2009. The astronauts were able to remove and replace components successfully, demonstrating the feasibility of performing component-level electronics repairs within a spacecraft. Several unsuccessful tasks demonstrated areas in need of improvement. These include improved and longer training prior to a mission, an improved soldering iron with a higher operating temperature and steady power source, video training and practice boards for refresher work or practice before a repair, and improved and varied hand tools and containment system

Imaging and Analysis of Void-defects in Solder Joints Formed in Reduced Gravity using High-Resolution Computed Tomography

As a part of efforts to develop an electronics repair capability for long duration space missions, techniques and materials for soldering components on a circuit board in reduced gravity must be developed. This paper presents results from testing solder joint formation in low gravity on a NASA Reduced Gravity Research Aircraft. The results presented include joints formed using eutectic tin-lead solder and one of the following fluxes: (1) a no-clean flux core, (2) a rosin flux core, and (3) a solid solder wire with external liquid no-clean flux. The solder joints are analyzed with a computed tomography (CT) technique which imaged the interior of the entire solder joint. This replaced an earlier technique that required the solder joint to be destructively ground down revealing a single plane which was subsequently analyzed. The CT analysis technique is described and results presented with implications for future testing as well as implications for the overall electronics repair effort discussed