The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Two-Dimensional Finite-Element Hydraulic Modeling of Bridge Crossings: Research Report

DTFH61-80-Y-3011This report presents the results of a 6-year project, conducted by the US Geological Survey in cooperation with the Federal Highway Administration (FHWA), to develop an accurate, efficient, easy-to-use finite-element surface-water flow model (FESWMS-2DH) for use in analyzing backwater and flow distribution at highway crossings of rivers and flood plains. When lateral variations in water-surface elevation and flow distribution are significant, a two-dimensional approach has advantages over a one-dimensional approach. The finite-element method is ideally suited to modeling two-dimensional flow over complex topography with spatially variable roughness and allows the user great flexibility in defining flow boundaries, channels, and embankments. A large number of alternative flow-equation formulations, interpolation and weighting functions, and schemes for solving the large systems of algebraic equations that arise in applying the finite-element method were tested during the project. Features added to FESWMS-2DH include weir flow (roadway overtopping), culvert flow, linear variation of Manning's n with depth, simple and accurate handling of lateral boundaries, automatic network generation and refinement, and extensive error checking. Sections of the report are devoted to the application of FESWMS-2DH to data from the Geological Survey's Flood Plain Simulation Facility, the use and calibration of FESWMS-2DH, and the use of the model by the highway industr

Unsteady heat transfer topics in gas turbine stages simulations

High pressure gas turbine stages are nowadays working under very challenging conditions. An usual HP stage design is based on transonic highly loaded blades cooled through impingement and film cooling techniques. An important research field for such type of turbine stages is presently represented by the investigation of unsteady performances for loss reduction and heat transfer optimization. Two special issues related to the unsteady stage interaction are addressed in the present work: the first concerns the casing/tip leakage flow, the second the effect and redistribution of inlet temperature hot-spots. The investigation of both requires unsteady modeling since these phenomena are mostly driven by the rotor-stator interaction. High temperature spots, for example, travel through the stator vane as a "hot streaks" of fluid that is mainly redistributed and steered: a simple model of this process is known as Kerrcbrock and Mikolajczak's "segregation effect". A series of steady and unsteady simulations have been made on the HP MT1 turbine stage test rig of QinetiQ. Given an inlet uniform total pressure field, three different total temperature distributions have been simulated. The first is a uniform reference distribution of total temperature, while the other two non-uniform distributions have been obtained from experimental data with a different alignment: with respect to the NGV leading edge. The numerical results have been compared with the experimental values provided by QinetiQ. The comparisons have been discussed focusing on the rotor blade and casing unsteady pressure and heat transfer rate. Copyright © 2006 by ASME

Emerging COVID-19 impacts, responses, and lessons for building resilience in the seafood system

The COVID-19 pandemic and subsequent lockdowns are creating health and economic crises that threaten food and nutrition security. The seafood sector provides important sources of nutrition and employment, especially in low-income countries, and is highly globalized allowing shocks to propagate. We studied COVID-19-related disruptions, impacts, and responses to the seafood sector from January through May 2020, using a food system resilience ‘action cycle’ framework as a guide. We find that some supply chains, market segments, companies, small-scale actors and civil society have shown initial signs of greater resilience than others. COVID-19 has also highlighted the vulnerability of certain groups working in- or dependent on the seafood sector. We discuss early coping and adaptive responses combined with lessons from past shocks that could be considered when building resilience in the sector. We end with strategic research needs to support learning from COVID-19 impacts and responses