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排序方式: 共有768条查询结果,搜索用时 31 毫秒
51.
Giuseppe Colla Alberto Battistelli Simona Proietti Stefano Moscatello Youssef Rouphael Mariateresa Cardarelli Marco Casucci 《Microgravity science and technology》2007,19(5-6):118-121
Producing sprouts directly during space missions may represent an interesting opportunity to offer high-quality fresh ready to eat food to the astronauts. The goal of this work was to compare, in terms of growth and nutritional quality, rocket (Eruca sativa Mill.) seedlings grown in the International Space Station during the ENEIDE mission with those grown in a ground-based experiment (in presence and absence of clinorotation). The rocket seedlings obtained from the space-experiment were thinner and more elongated than those obtained in the ground-based experiment. Cotyledons were often closed in the seedlings grown in the space experiment. Quantitative (germination, fresh and dry weight) and qualitative (glucose, fructose, sucrose and starch) traits of rocket seedling were negatively affected by micrograv-ity, especially those recorded on seedlings grown under real microgravity conditions The total chlorophyll, and carotenoids of seedlings obtained in the space experiment were strongly reduced in comparison to those obtained in the ground-based experiment (presence and absence of clinorotation). The results showed that it is possible to produce rocket seedlings in the ISS; however, further studies are needed to define the optimal environmental conditions for producing rocket seedlings with high nutritional value 相似文献
52.
Wireless sensor networks (WSN) have attracted much attention in recent years due to their potential use in many applications such as border protection and combat field surveillance. Given the criticality of such applications, maintaining a dependable operation of the network is a fundamental objective. However, the resource-constrained nature of sensor nodes and the ad hoc formation of the network, often coupled with an unattended deployment, pose non-conventional challenges and motivate the need for special techniques for dependable design and management of WSN. In this article, we highlight the potential of careful positioning of the base station (BS), which acts as a sink node for the collected data, as a viable means for increasing the dependability of WSN. We categorize published work on optimal positioning of BS in WSN. Referring to such work as static positioning, we further introduce dynamic schemes that reposition the BS during the network operation. We show that dynamic BS positioning can be very effective in optimizing the network functional and non-functional performance objectives and in coping with dynamic changes in the environment and available network resources 相似文献
53.
Christoph M. Wintersteiger Youssef Hamadi Leonardo de Moura 《Formal Methods in System Design》2013,42(1):3-23
In recent years, bit-precise reasoning has gained importance in hardware and software verification. Of renewed interest is the use of symbolic reasoning for synthesising loop invariants, ranking functions, or whole program fragments and hardware circuits. Solvers for the quantifier-free fragment of bit-vector logic exist and often rely on SAT solvers for efficiency. However, many techniques require quantifiers in bit-vector formulas to avoid an exponential blow-up during construction. Solvers for quantified formulas usually flatten the input to obtain a quantified Boolean formula, losing much of the word-level information in the formula. We present a new approach based on a set of effective word-level simplifications that are traditionally employed in automated theorem proving, heuristic quantifier instantiation methods used in SMT solvers, and model finding techniques based on skeletons/templates. Experimental results on two different types of benchmarks indicate that our method outperforms the traditional flattening approach by multiple orders of magnitude of runtime. 相似文献
54.
Souhail Boussetta Gianpaolo Balsamo Anton Beljaars Tomas Kral Lionel Jarlan 《International journal of remote sensing》2013,34(9-10):3520-3542
The leaf area index (LAI), defined as the one-sided green leaf area per unit ground area, is used in many numerical weather prediction (NWP) models as an indicator of the vegetation development state, which is of paramount importance to characterize land evaporation, photosynthesis, and carbon-uptake processes. LAI is often simply represented by lookup tables, dependent on the vegetation type and seasons. However, global LAI datasets derived from remote sensing observations have more recently become available. These products are based on sensors such as the Advanced Very High Resolution Radiometer (AVHRR) or the Moderate Resolution Imaging Spectroradiometer (MODIS), onboard polar orbiting satellites that can cover the entire globe within typically 3 days and with a spatial resolution of the order of 1 km. We examine the meteorological impact of satellite-derived LAI products on near-surface air temperature and humidity, which comes both from the stomatal transpiration of leaves and from the intercepted water on the surface of leaves, re-evaporating into the atmosphere. Two distinct monthly LAI climatology datasets derived respectively from AVHRR and MODIS sensors are tested. A set of forecasts and data assimilation experiments with the integrated forecasting system of the European Centre for Medium-range Weather Forecasts is performed with the monthly LAI climatology datasets as opposed to a vegetation-dependent constant LAI. The monthly LAI is shown to improve the forecasts of near-surface (screen-level) air temperature and relative humidity through its effect on evapotranspiration, with the largest impact obtained over needleleaf forests, crops, and grassland. At longer time-scales, the introduction of the monthly LAI is shown to have a positive impact on the model climate particularly during the boreal spring, where the LAI climatology has a large seasonal cycle. 相似文献
55.
The surface properties of two perfluoroalkylethyl acrylic copolymers—aqueous, Zonyl®329 and solvent‐based, Zonyl®225—were studied. Zonyl®329 is a water‐based dispersion and Zonyl®225 a solvent‐based copolymer solution; both polymers have the same perfluoroalkyethyl side chains [F(CF2)nCH2CH2? ] but have different comonomer compositions. Thin films, prepared by dip coating onto mica and quartz, with and without annealing, were characterized by contact angle and by X‐ray photoelectron spectroscopy (XPS). The contact angle measurements showed little variation with polymer and with substrate, consistent with the supposition that the perfluoroalkylethyl chains aggregate on the surface and thus dominate surface properties, irrespective of the composition of the rest of the polymer. XPS revealed only small variations in surface chemistry for studied films. Annealed films showed improved segregation for solvent‐based Zonyl®225, which has both hydrocarbon alkyl and perfluoroalkylethyl side chains; the presence of hydrocarbon alkyl chains enables the perfluoroalkylethyl chains to reorganize after annealing. Depending on the external conditions, this thermal treatment can enable more perfluoroalkylethyl chains to reach the film surface (solid/air interface), leading to a reduction in the dispersive‐dominant surface and enhancement in perfluoroalkylethyl segregation. This suggested that perfluoroalkylethyl side chains dominate the surface properties, which are thus not dependent on substrate, backbone composition, or formulation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009 相似文献
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59.
Boulos Youssef Abed Elkader Dehbi Ahmed Hamou Jean Marc Saiter 《Materials & Design》2008,29(10):2017-2022
The effects of natural ageing in the north Algeria of tri-layer films lay down hot greenhouse made of low-density polyethylene are presented in this work. Ageing was monitored by observing the changes of physical, mechanical and structural effects on the various greenhouse faces. It has been shown that the structural change occur on the outside face of the films, while the inside faces are protected by the dye. The study shows that the measured parameters are directly related to the criteria of evaluation of the greenhouse effectiveness. The lifetime of these films under natural conditions in north Algerian was estimated to be 10 months. 相似文献
60.
We present a comparison between two intermediate viscoplastic approaches: the self-consistent approach and the ?-model for the texture development in face-centered cubic polycrystals. In these models, the interactions between a grain and its surroundings are taken into account in different ways. We performed different tests to simulate axisymmetric tension/compression and plane-strain compression for different values of the parameter ? and for different formulations of the self-consistent model. Predicted results (stress–strain, texture, slip activity, local stress and strain rate deviations) are compared and conclusions are drawn. The ?-model allows for fairly good predictions, in comparison to existing experimental results, while it is much easier to formulate and to implement numerically. The ?-model is also able to predict texture transition from the copper type to the brass type. Using the same boundary conditions and spherical grains, the self-consistent model is unable to predict this texture transition and only a copper type texture is obtained. 相似文献