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951.
A solution to the fixed-time minimum-fuel two-impulse rendezvous problem for the general non-coplanar elliptical orbits is
provided. The optimal transfer orbit is obtained using the constrained multiple-revolution Lambert solution. Constraints consist
of lower bound for perigee altitude and upper bound for apogee altitude. The optimal time-free two-impulse transfer problem
between two fixed endpoints implies finding the roots of an eighth order polynomial, which is done using a numerical iterative
technique. The set of feasible solutions is determined by using the constraints conditions to solve for the short-path and
long-path orbits semimajor axis ranges. Then, by comparing the optimal time-free solution with the feasible solutions, the
optimal semimajor axis for the two fixed-endpoints transfer is identified. Based on the proposed solution procedure for the
optimal two fixed-endpoints transfer, a contour of the minimum cost for different initial and final coasting parameters is
obtained. Finally, a numerical optimization algorithm (e.g., evolutionary algorithm) can be used to solve this global minimization
problem. A numerical example is provided to show how to apply the proposed technique. 相似文献
952.
953.
954.
L. Ma G. Z. Xie T. F. Yi S. B. Zhou K. H. Li X. Zhang H. Dai 《Astrophysics and Space Science》2010,327(1):35-58
We present the results of the monitoring of a sample of 5 HBLs in the B, V and R bands during 2003–2008. All sources in our monitoring project show significant rapid optical variations, except 1ES 1959+650, however; for 1ES 1959+650, the gamma-ray variability on a timescale of ~7 hours has been observed by Holder et al. (Astrophys. J. 583:L9, 2003). Using these variability timescales, we estimate the black hole masses for each source with the Kerr black hole theory. 相似文献
955.
Based on Hinode SOT/NFI observations with greatly improved spatial and temporal resolution and polarization sensitivity, the lifestory of
the intranetwork (IN) magnetic elements are explored in a solar quiet region. A total of 2282 IN elements are followed from
their appearance to disappearance and their fluxes measured. By tracing individual IN elements their lifetimes are obtained,
which fall in the range from 1 to 20 min. The average lifetime is 2.9±2.0 min. The observed lifetime distribution is well
represented by an exponential function. Therefore, the e-fold characteristic lifetime is determined by a least-square fitting
to the observations, which is 2.1±0.3 min. The lifetime of IN elements is correlated closely with their flux. The evolution
of IN elements is described according to the forms of their birth and disappearance. Based on the lifetime and flux obtained
from the new observations, it is estimated that the IN elements have the capacity of heating the corona with a power of 2.1×1028 erg s−1 for the whole Sun. 相似文献
956.
Observed winds,turbulence, and dispersion in built-up downtown areas of Oklahoma City and Manhattan 总被引:2,自引:2,他引:0
Wind and tracer data from the Oklahoma City Joint Urban 2003 (JU2003) and the Manhattan Madison Square Garden 2005 (MSG05)
urban field experiments are being analyzed to aid in understanding air flow and dispersion near street-level in built-up downtown
areas. The mean winds are separately calculated for groups of anemometers having similar exposures such as “near street level”
and “on building top”. Several general results are found, such as the scalar wind speed at street level is about 1/3 of that
at building top. Turbulent standard deviations of wind speed components and temperature, and vertical fluxes of momentum and
sensible heat, are calculated from sonic anemometers near street level at 20 locations in JU2003 and five locations in MSG05,
and from two rooftop locations in MSG05. The turbulence observations are consistent with observations in the literature at
other cities, although the JU2003 and MSG05 data are unique in that many data are available near street level. For example,
it is found that the local (i.e., at the measuring height) averages about 1.5 and the local averages about 0.25 in the two cities, where is the standard deviation of vertical velocity fluctuations, is the friction velocity, and u is the wind speed. The ratio of temperature fluctuations to temperature scale, , averages about −3 in both cities, consistent with similarity theory for slightly unstable conditions, where is the standard deviation of temperature fluctuations, and is the temperature scale. The calculated Obukhov length, L, is also consistent with slightly unstable conditions near street level, even at night during JU2003. The SF6 tracer concentration observations from JU2003 are analyzed. Values of for the continuous releases are calculated for each release and arc distance, where is the 30-min average arc maximum concentration, Q is the continuous source emission rate, and u is the spatial-averaged wind speed in the downtown area. The basic characteristics of the JU2003 plot of averaged agree reasonably well with similar plots for other urban experiments in Salt Lake City and London (i.e., at . A is found to be about 3 during the day and about 10 during the night. 相似文献
957.
High-pressure granulite from Western Kunlun,northwestern China: Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication 总被引:1,自引:0,他引:1
QU JunFeng ZHANG LiFei AI YongLiang L Zeng WANG JianPing ZHOU Hui & WANG ShiYan MOE Key Laboratory of Orogenic Belts Crustal Evolution School of Earth Space Science Peking University Beijing China Henan Polytechnic University Jiaozuo China Geological Survey of Henan Province Zhengzhou China 《中国科学D辑(英文版)》2007,50(7):961-971
High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were ob-tained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic con-dition of these high-pressure granulites is about 760―820℃,1.0―1.2 GPa and the retrograde meta-morphic condition is about 620―720℃,0.7―0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T... 相似文献
958.
Journal of Geographical Sciences - Due to the atmospheric nuclear weapon tests carried out, terrestrial environments have been extensively contaminated by global fallout of plutonium (Pu)... 相似文献
959.
The concentration of gaseous elemental mercury (GEM) in the atmosphere of urban Beijing was measured from October 2003 to September 2004 to investigate the origins and spatial-temporal variations of atmospheric Hg. The mean value of Hg concentration is 17.1 ng·m-3 (n=653). The atmospheric Hg data showed spatial-temporal variations throughout the duration of our observation. The maximum GEM concentration (53.7 ng·m-3) was observed at Gucheng in the western area of urban Beijing. The GEM concentrations increased from the north to the south of the studied area, and were higher in winter than in summer. The highest and lowest monthly averages of GEM concentrations were measured to be 23.3 and 4.1 ng·m-3 in January and July, respectively. In addition, GEM concentrations are higher in the daytime than at night in Autumn and from 14 March to 15 April, but daily GEM variation showed an inverse pattern from 22 April to 22 May. In winter, two peak values of GEM concentrations occurred at 13:30 and 21:30. Daily variation of GEM concentrations in summer was the lowest in the four seasons. Mercury from coal combustion was estimated to be the main source of anthropogenic emissions in Beijing from October 2003 to September 2004. Additionally, Hg emission from natural gas burning was estimated to be another dominant source of atmospheric Hg in Beijing. 相似文献
960.
Fractal kinetic characteristics for dissolving and leaching processes of strontium residue 总被引:1,自引:0,他引:1
The pore structural characteristics of strontium residue were studied with the N2 adsorption method (ASAP2010). The kinetic properties concerning dissolving and leaching strontium waste were described by
determining the concentrations of Sr2+, Ba2+ and soluble sulphides in solutions. The results showed that the specific surface area and pore volume increased with decreasing
granule diameter, and the micropore surface of the residue was fractal. In the dissolving and leaching processes of strontium
residue, soluble ion concentrations increased with decreasing granule diameter of the residue, and the reaction dimension
was lower than the fractal dimension of pore surface. Sr2+ and soluble sulphide concentrations significantly exceeded the defined standard values, while Ba2+ concentrations did not, either in the dissolving or leaching solutions. In addition, dissolving and leaching reactions selectively
occurred on the micropore surface of strontium residue. 相似文献