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91.
The flapping equation for a rotating rigid helicopter blade is typically derived by considering (1) small flap angle, (2) small induced angle of attack and (3) linear aerodynamics. However, the use of nonlinear aerodynamics such as dynamic stall can make the assumptions of small angles suspect as shown in this paper. A general equation describing helicopter blade flap dynamics for large flap angle and large induced inflow angle of attack is derived. A semi-empirical dynamic stall aerodynamics model (ONERA model) is used. Numerical simulations are performed by solving the nonlinear flapping ordinary differential equation for steady state conditions and the validity of the small angle approximations are examined. It is shown that the small flapping assumption, and to a lesser extent, the small induced angle of attack assumption, can lead to inaccurate predictions of the blade flap response in certain flight conditions for some rotors when nonlinear aerodynamics is considered. 相似文献
92.
We present a novel numerical model for analyzing shift margin in spatial spread-spectrum (SSS) multiplexing, in which the
spatial phase of signal beams, not in reference beams, is modulated and demodulated by a random diffuser to multiplex holograms.
The shift margin is determined not by the Bragg effect of a thick hologram, but mainly by the phase correlation property of
the diffuser used. In our model, we simulate signal data patterns and a smooth rough surface of a diffuser using oversampling
and zero-padding, which allows us to cover crosstalk calculations and direct bit-errors calculations, with a fine shift resolution
and a wide diffusion angle range. Calculation results revealed that the practical shift margin that ensures a signal to noise
ratio more than 3.0 and a bit-error rate less than 10−2 is in the range of 0.3ωeff−0.5ωeff, where ωeff is the effective autocorrelation length of the diffuser transmission function. This model will be helpful in designing the
optimum random diffuser profile for SSS multiplexing. 相似文献
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Hanane Ait Hmeid Mustapha Akodad Mourad Baghour Abdelmajid Moumen Ali Skalli Ghizlane Azizi Hicham Gueddari Mostapha Maach Mimoun Aalaoul Ahmed Anjjar Lahcen Daoudi 《Molecules (Basel, Switzerland)》2021,26(18)
The main objective of this work was to contribute to the reduction in the contamination of phenolic compounds contained in margin by an adsorption process on two types of raw bentonite. The margin used in the studies was collected from a semi-modern oil mill located in the Nador–Morocco region. The results of the physico-chemical analyses showed that the effluents of the oil mills showed that they are highly polluted, particularly in terms of the total suspended solids (TSS), chemical oxygen demand (COD), and iron content of around 154.82 (mg/L), and copper content of 31.72 (mg/L). The mineralogy of bentonites studied by X-ray diffraction (XRD) reveals the existence of two types of montmorillonite; theoretically, the diffraction peak (001) of the montmorillonite appears at 15 Å, with a basal spacing that corresponds to a calcium pole, and the diffraction peak (001) appears at 12Å, with a basal spacing that corresponds to a sodium pole. The specific surface area of the bentonite used is characterized by a large specific surface area, varying between 127.62 m2·g−1 and 693.04 m2·g−1, which is due to the presence of hydrated interleaved cations. This surface is likely to increase in aqueous solution depending on the solid/liquid ratio that modulates the degree of hydration. With a high cation exchange capacity (CEC) (146.54 meq/100 g), samples of margin mixed with raw bentonites at different percentages vary between 5% and 100%. The potential of Moroccan bentonite for the phenol adsorption of 9.17 (g/L) from aqueous solutions was investigated. Adsorption tests have confirmed the effectiveness of these natural minerals in reducing phenolic compounds ranging from 8.72% to 76.23% contained in the margin and the efficiency of heavy metal retention through microelements on raw bentonites. The very encouraging results obtained in this work could aid in the application of adsorption for the treatment of margin. 相似文献
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轴流压气机失速初始扰动的研究进展 总被引:7,自引:0,他引:7
在失速主动控制思想的推动下,对轴流压气机失速初始扰动的研究一直是叶轮机械非定常流领域的热点问题之一,同时也是一个尚未认识清楚的难点问题. 本文从失速初始扰动的理论模型、实验研究和机理分析3个方面对轴流压气机失速初始扰动的研究进展进行了回顾.对目前失速初始扰动研究中的集中问题,如低速和高速环境下初始扰动的试验检测方法, 初始扰动类型,以及初始扰动出现的内在流体动力学机理等问题进行了总结. 在此基础上,讨论了叶尖区域的复杂流动特性和失速初始扰动的内在联系,并指出了失速初始扰动研究的发展趋势, 认为今后应进一步深化如下问题:初始扰动形式影响因素的系统研究, 高速压气机中初始扰动新形式研究,初始扰动产生的流体动力学物理机制及其与压气机的设计和运行参数之间的关联性研究. 相似文献
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基于小波变换的离心压气机旋转失速先兆时频分析 总被引:6,自引:2,他引:4
本文采用连续小波变换的方法,对跨音离心压气机机壳壁面动态压力及叶片应变信号进行了时频分析。结果表明,在失速前的先兆阶段存在着两种特征截然不同的先兆成分。即连续增强成分和间断扰动成分。间断扰动成分和间断失速段的失速团具有相近的特性.压力信号中间断失速段的失速团频率比在连续失速段的失速团频率高,而叶片应力信号中则相反,并计算得出连续失速段的失速团比间断失速段的失速团数目少。在间断失速段,压力信号的频率阶跃以及叶片应变信号的频率衰减现象,对应于扰动能量聚集而使失速团数目减少的过程。 相似文献