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501.
Smith S.M. An P.E. Holappa K. Whitney J. Burns A. Nelson K. Heatzig E. Kempfe O. Kronen D. Pantelakis T. Henderson E. Font G. Dunn R. Dunn S.E. 《Oceanic Engineering, IEEE Journal of》2001,26(4):453-465
The Advanced Marine Systems Lab at Florida Atlantic University has developed a new ultramodular plastic mini autonomous underwater vehicle (AUV), called the Morpheus, for littoral military and coastal oceanographic sampling, survey, and mapping. The name Morpheus was chosen because the Greek god Morpheus could change shape or "morph." The higher degree of modularity of the Morpheus AUV allows it to "morph" or change its size and components for different applications. This vehicle is composed of modular injection-molded plastic pressure vessels and a cabling system that allow the modules to be rearranged without rewiring bulkheads. The plastic pressure vessels are inexpensive, inherently mass-producible, extremely corrosion-resistant, and have low magnetic signatures. The pressure vessels are small but are sized to fit most standard electronic board standards. The mini AUV can be anywhere from 4 to 10 ft in length, depending on its mission. The vehicle architecture is an adaptation of the Ocean Explorer AUV system and uses an ANSI 709.1 (LonTalk) distributed control network for connecting all sensors and actuator subsystems as smart nodes. The modularity in containers, control, and power makes this vehicle rapidly reconfigurable and easy to repair or upgrade. This paper will present details of the motivation, design, and construction of the new mini AUV. The Morpheus was deployed during the summer of 2000 in field exercises for very shallow and shallow water mine counter measures. Some results from these tests will be presented 相似文献
502.
Abstract— We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 φ units or 570 μm, much smaller than the previous rough estimate of ~900 μm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross‐section of 1.60 φ units or 330 μm. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/ chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine‐pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet‐textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent‐body chondrule‐fragmentation events and not to chondrule‐formation processes in the solar nebula. 相似文献
503.
A. Treves S. Campana M. Chieregato A. Moretti T. Nelson M. Orio 《Astrophysics and Space Science》2007,308(1-4):167-169
Blank field sources (BFS) are good candidates for hosting dim isolated neutron stars (DINS). The results of a search of BFS
in the ROSAT HRI images are revised. We then focus on transient BFS, arguing that they belong to a rather large population.
The perspectives of future research on DINS are then discussed.
相似文献
504.
Roberto Millan-Nuñez Saul Alvarez-Borrego David M. Nelson 《Estuarine, Coastal and Shelf Science》1982,15(3):317-335
Sea level, salinity, temperature, nitrate, nitrite, phosphate, silicate, chlorophylls a, b and c and their phaeophytins, phytoplankton abundance and phytoplankton productivity time series were generated for the mouth and three interior locations of Bahia San Quintin, Baja California, Mexico, for 10 days during summer of 1979. The samples were taken once every 2 h. This was done to describe space and time variability of these ecological properties and to elucidate the main factors that cause this variability. Upwelling events bring nutrient reach waters near the bay mouth and tidal currents propagate those waters throughout the bay. Nutrient remineralization at the sediments and the effect of turbulence induced by tidal currents and winds increase nutrient concentrations in the interiors of the bay. In comparison with available information on nutrients limited growth of planktonic algae, nutrients seemed not to be limiting to phytoplankton growth during the sampling period. Phytoplankton cell abundances at the extremes of the lagoon are an order of magnitude lower than at the mouth due to greater turbidity. Chlorophyll concentrations at the extremes are about one-third of those of the mouth. Primary productivity decreases from the mouth to the interiors in the same manner as chlorophyll does. There is not a significant difference in cell size between phytoplankton at the bay mouth and those at the extremes of the bay. Primary productivity in the bay is comparable to the productivity maxima of other upwelling areas. There is no clear permanent dominance of diatoms over dinoflagellates, or vice versa, at any location in the bay. The alternation of upwelling and non-upwelling played an important role, together with that of the spring-neap tide cycle, in producing low frequency (< 0.01 cycles h?1) temporal variability of ecological properties throughout the bay. 相似文献
505.
The plasma two-stream instability is proposed as a matter-antimatter separation mechanism for the Alfvén-Klein cosmology. 相似文献
506.
507.
The age of the universe is computed for spatially homogeneous and isotropic cosmological models of the relativistic Wesson's 5D variable mass theory of gravity. It is shown that if the pressure in the mass dimension is negligible, ages old enough to agree with the observations can only be obtained if the cosmological constant is different from zero. In addition, unlike the standard FRW models in Einstein's 4D theory, the age value itself is not affected by the nature of the matter content (matter and radiation dominated models give the same age). 相似文献
508.
509.
Christopher F. DElia David M. Nelson Walter R. Boynton 《Geochimica et cosmochimica acta》1983,47(11):1945-1955
Silicic acid (H4SiO4) flux from the sediment, H4SiO4 concentration and river flow were used to obtain an annual dissolved silicon budget for Chesapeake Bay. H4SiO4 concentrations vary seasonally in the estuary: for a 12-year period, mean H4SiO4 concentrations in the mesohaline region were high both in spring and in late summer to early fall, and were low in late spring—occasionally approaching levels potentially limiting to diatom growth. Most of the annual allochthonous H4SiO4 supply to the estuary derives from the three major rivers, but regenerative H4SiO4 flux from the sediment to the water column exceeds the total riverine input by a factor of at least five. Sediment H4SiO4 efflux exhibits seasonality and averages approximately 2–3 mol Si m?2 yr?1. The high rates of sediment dissolution and efflux appear to maintain high levels of H4SiO4 in the mesohaline region, and Si-limitation of diatom growth there seems unlikely. The relative rates of biogenic silica formation and dissolution do not vary synchronously: seasonal variations in diatom productivity, sedimentary release of H4SiO4 and river flow all contribute to the observed late winter and late summer seasonal maxima and late spring minimum in water column H4SiO4 concentrations. If the only source of Si to support sedimentary H4SiO4 efflux is biogenic particulate silica recently deposited from the water column and this silica in turn was produced by diatoms in a ratio of 8C:1 Si, the minimum annual primary production by diatoms is at least 260 g C m?2, approximately half of annual total plankton primary production. This estimate would be revised upwards according to the amount of particulate biogenic silica dissolving in the water column. Burial of biogenic silica amounts to from 2 to 84% of the sediment efflux of H4SiO4, depending on location in the bay. On an annual basis, burial represents from 60 to 100% of fluvial H4SiO4-Si inputs. 相似文献
510.
M. Kusakabe T.L. Ku J.R. Southon J.S. Vogel D.E. Nelson C.I. Measures Y. Nozaki 《Earth and Planetary Science Letters》1987,82(3-4)
The vertical distributions of10Be and9Be at three locations in the Pacific (25°N, 170°E; 17°N, 118°W; 3°S, 117°W) are presented. The results show that both isotopes exhibit nutrient-like profiles. From the surface to the bottom, the increase for10Be is two- to threefold and that for9Be is about fivefold. While the inter-station variations in surface water concentrations may reach a factor of two, deep-water values tend to be much more uniform averaging about 2000 atoms/g for10Be and 30 pM for9Be. A similar situation applies to the10Be/9Be ratio; it varies approximately from 1 to 3 × 10−7 (atom/atom) at shallow depths but tends toward a value close to 1.1 × 10−7 in the deep ocean. The variation of10Be/9Be can be viewed as resulting from the fact that10Be in a given parcel of water consists of two components: recycled and primary. The recycled component is that part of10Be which has reached tracer equilibrium with9Be, as opposed to the primary component which, upon entering the sea from the atmosphere, has yet to equilibrate with9Be through particle cycling and mixing processes. It is estimated that 70% to nearly 100% of10Be at the three stations are being recycled, and the recycled beryllium bears an atomic ratio of10Be/9Be close to 1 × 10−7. The oceanic residence time of Be is of the order of 1000–4000 years, comparable to or slightly longer than the ocean mixing time. 相似文献