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91.
A. Miele T. Wang W. W. Melvin R. L. Bowles 《Journal of Optimization Theory and Applications》1987,53(2):181-217
This paper is concerned with guidance strategies and piloting techniques which ensure near-optimum performance and maximum survival capability in a severe windshear. The take-off problem is considered with reference to flight in a vertical plane. In addition to the horizontal shear, the presence of a downdraft is assumed.First, six particular guidance schemes are considered, namely: constant alpha guidance; maximum alpha guidance; constant velocity guidance; constant absolute path inclination guidance; constant rate of climb guidance; and constant pitch guidance. Among these, it is concluded that the best one is the constant pitch guidance.Next, in an effort to improve over the constant pitch guidance, three additional trajectories are considered: the optimal trajectory, which minimizes the maximum deviation of the absolute path inclination from a reference value, while employing global information on the wind flow field; the gamma guidance trajectory, which is based on the absolute path inclination and which approximates the behavior of the optimal trajectory, while employing local information on the windshear and the downdraft; and the simplified gamma guidance trajectory, which is the limiting case of the gamma guidance trajectory in a severe windshear and which does not require precise information on the windshear and the downdraft.The essence of the simplified gamma guidance trajectory is that it yields a quick transition to horizontal flight. Comparative numerical experiments show that the survival capability of the simplified gamma guidance trajectory is superior to that of the constant pitch trajectory and is close to that of the optimal trajectory.Next, with reference to the simplified gamma guidance trajectory, the effect of the feedback gain coefficient is studied. It is shown that larger values of the gain coefficient improve the survival capability in a severe windshear; however, excessive values of the gain coefficient are undesirable, because they result in larger altitude oscillations and lower average altitude.Finally, with reference to the simplified gamma guidance trajectory, the effect of time delays is studied, more specifically, the time delay 1 in reacting to windshear onset and the time delay 2 in reacting to windshear termination. While time delay 2 has little effect on survival capability, time delay 1 appears to be critical in the following sense: smaller values of 1 correspond to better survival capability in a severe windshear, while larger values of 1 are associated with a worsening of the survival capability in a severe windshear.This research was supported by NASA-Langley Research Center, Grant No. NAG-1-516, and by Boeing Commercial Airplane Company. 相似文献
92.
This paper analyzes the impact of asymmetry between firms on the outcome of price and quality competition from a microeconomic viewpoint. Consumers purchase a product based on not only its price but also its quality level; therefore, two firms compete in determining their prices and quality levels to maximize their profits. The asymmetry arises from the difference in consumers’ loyalty to each firm; that asymmetry then determines a character of differentiation between firms. Our purpose is to show how asymmetry influences competition under varying consumers’ price- and quality-sensitivity. In doing so, we extend earlier work in the area of price and quality competition. We show that in both the moderately quality-sensitive and price-sensitive markets, higher consumers’ sensitivity as well as lower consumers’ loyalty to any firm leads to intense competition, resulting in a decrease of both firms’ equilibrium profits. On the other hand, in highly quality-sensitive market, asymmetry compels the smaller firm to change its competitive strategy. In general, this is more beneficial to the larger firm, as the smaller firm’s profit tends to decline. In the worst case, the smaller firm is driven out of business under equilibrium. 相似文献
93.
Priscilla M. Ujang Christopher J. LawrenceColin P. Hale Geoffrey F. Hewitt 《International Journal of Multiphase Flow》2006
A series of two-phase air–water experiments was carried out in order to study the initiation and the subsequent evolution of hydrodynamic slugs in a horizontal pipeline. Experiments were carried out at atmospheric pressure, 4.0 bar(a) and 9.0 bar(a), and the effects of superficial liquid and gas velocities were investigated. The test section used for these experiments is 37 m in length, with an internal diameter of 0.078 m. To study the interfacial development, measurements of interfacial structures were made at 14 axial locations along the test section, with data acquired at a sampling frequency of 500 Hz. A large number of slugs were initiated within the first 3 m of the test section, with the frequency subsequently reducing towards the fully developed value before the end of the pipe. This reduction in frequency was strongly influenced by the magnitude of the gas and liquid velocities. The frequency of slugging was not strongly affected when the system pressure was changed from 1 atmosphere, to 4.0 and 9.0 bar(a), closely similar values being obtained at the 10 downstream locations. However, higher pressure delayed the onset of slug initiation, with “slug precursors” being formed further downstream as the pressure was increased. The statistical distributions of slug lengths and of the time intervals between slug arrivals were examined in detail and compared to several standard distributions. This showed that slug initiation may be reasonably approximated as an uncorrelated Poisson process with an exponential distribution of arrival times. However, once slugs have developed, there is strong correlation and the arrival time intervals, as well as the lengths, are best represented by the log-normal distribution. 相似文献
94.
Effects of Motion, Implied Direction and Displacement of a Large-Visual-Stimulus on Postural Control
Effects of motion, implied direction in a static stimulus and displacement on postural control were examined independently.
In Experiment 1, rotation of a random-dot stimulus was presented. In Experiments 2 and 3, photographic slides of natural scenes
were used; participants closed their eyes during stimulus rotation to eliminate motion information. In Experiment 2, the stimulus
was presented upright initially, then presented again with a tilt. In Experiment 3, the order was reversed to separate the
effects of implied direction and displacement. Results showed that all information of motion, implied direction, and displacement
had some effect on postural control, although visual information of motion has been presumed to have a principal effect on
postural control. Results suggested that the effects of implied direction might reflect an immediate processing of information.
The effects of displacement and motion might reflect a continuous processing of information. 相似文献
95.
本文给出了模糊质量设计的数学模型 ,并给出了具有模糊约束、模糊约束函数值及双目标两层次的模糊质量设计模型的解法 . 相似文献
96.
This paper presents an analytical and numerical study of transient free convection from a horizontal surface that is embedded in a fluid-saturated porous medium. It is assumed that for time
steady state velocity and temperature fields are obtained in the boundary-layer which occurs due to a uniform flux dissipation rate q
1 on the surface. Then, at
the heat flux on the surface is suddenly changed to q
2 and maintained at this value for
. Firstly, solutions which are valid for small and large
are obtained. The full boundary-layer equations are then integrated step-by-step for the transient regime from the initial unsteady state (
) until such times at which this forward marching approach is no longer well posed. Beyond this time no valid solutions could be obtained which matched the final solution from the forward integration to the steady state profiles at large times
. 相似文献
97.
Boundary-layer analysis is performed for free convection flow over a hot horizontal surface embedded in a porous medium saturated with a gas of variable properties. The variable gas properties are accounted for via the assumption that thermal conductivity and dynamic viscosity are proportional to temperature. A similarity solution is shown to exist for the case of constant surface temperature. Numerical results for the stream function, horizontal velocity, and temperature profiles within the boundary layer as well as for the mass of entrained gas, surface slip velocity, and heat transfer rate at different values of the wall-temperature parameter are presented. Asymptotic solutions for large heating are also available to support the numerical work. 相似文献
98.
99.
Michal Marvan 《Acta Appl Math》2002,72(1-2):51-65
Given a zero-curvature representation , the horizontal gauge cohomology group H
1
is shown to contain obstructions to removability of the 'spectral parameter' of . A method to compute H
1
is suggested. 相似文献
100.