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11.
Abstract Flow through a blade trailing edge passage has been experimentally investigated in this article. This rectangular passage is divided into two channels by either a straight or a 120° wavy partition. Fluid in the first channel was injected into the second channel via 14 openings (holes) distributed uniformly and in the crests and troughs for the wavy configuration. Particle image velocimetry and a charge coupled device camera were used in experiment, with Reynolds number varying from 1,800 to 6,700. Experimental results show that the wavy configuration produces an oscillation flow in the second channel. The cause for the oscillation may due to the 120° wavy partition causing shear-layer instabilities and the formation of spanwise eddies by the wavy board. Pictures from the charge coupled device camera revealed strong mixing in the trailing edge, and this mixing could enhance heat transfer, which was confirmed in previous research. The amplitude of oscillation depends on Reynolds number and hole positions. It was observed that the injection flow was not symmetric to the openings, and it was skewed to the direction of the first channel main flow. The ratio of flow rate through the outlet and the bleeding holes was from 1:1 to 1:3. 相似文献
12.
尾缘凹陷轴流风轮内流特性分析与降噪研究 总被引:1,自引:0,他引:1
为减少空调室外机用低压轴流风轮叶片的颤振,改善音质,降低噪音和电机功率。本文设计了尾缘凹陷新型三叶轴流风轮,同时除对前缘局部加厚处理外,对叶片其他区域整体减薄。文中采用Navier-Stokes方程对新风轮和原风轮的内流及气动特性进行了三维数值模拟,同时对两者的流量压力曲线和噪音频谱进行了实验研究。研究表明:对轴流风轮叶片尾缘进行凹陷设计,对前缘以外的区域减薄设计,能够减轻风轮重量,降低电机负荷同时减弱转子尾迹;对叶轮前缘进行局部加厚处理可以减小叶尖颤振,降低叶片旋转频率峰值噪音,从整体上降低噪音,改善音质。 相似文献
13.
In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr-Purcell-Meiboom-Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal pi rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor (15)N nuclei I=N (D) and S=N(A) in RNA Watson-Crick base pairs with homonuclear scalar couplings J(IS)=(2h)J(N(D), N(A)) across the hydrogen bonds. 相似文献
14.
Kiruluta AJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2006,183(2):308-314
The principal advantage of NMR at high field is the concomitant increase in signal-to-noise ratio (SNR). This can be traded for improved spatial resolution and combined with parallel imaging to achieve higher temporal resolution. At high field strength, the RF-wavelength and the dimension of the human body complicate the development of NMR coils. For example, at 7 T, the wavelength in free space corresponds to about 1m. The dielectric constant in tissue with a high water content can be as high as 70 and at a larmor frequency of 300 MHz, this corresponds to a wavelength inside tissue of less than 15 cm. The operating wavelength is thus comparable to the diameter of most body parts. To this end, both temporal and spatial variations of the excitation field must be taken into account in addition to the expected increase in conductivity. For all these reasons, we find the propagation of radiation at ultra high fields (>4 T) new phenomena commonly observed in quantum optics but traditionally negligible in NMR such as phase modulation of the excitation field such that the identity between pulse area and flip angle is no longer valid. In this paper, the emergence of field propagation phenomena in NMR experiments is analytically and numerically demonstrated. It is shown that in addition to the well-studied dielectric resonance phenomena at high magnetic fields (>4 T), field propagation effects transform the excitation pulse into an adiabatic excitation. The high field strength also mean that nonlinear effects such as self-induced transparency are now possible in NMR experiments. 相似文献
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本文通过数值计算方法系统研究了正向阴燃波的两种典型结构,即反应滞后结构和反应前导结构,比较了当来流空气流量发生变化时两种结构的主要特征参数的变化规律。在此基础上,通过数值方法分析了这两种结构的线性稳定性。结果表明,在绝热条件下,对于文中所选参数取值,两种结构的正向阴燃波都是线性稳定的。 相似文献
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18.
Tozoni JR Teles J Auccaise R Oliveira-Silva R Rivera-Ascona C Vidoto EL Guimarães AP Oliveira IS Bonagamba TJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,212(2):265-273
In this paper we present a series of high-resolution zero-field NMR spectra of the polycrystalline intermetallic compound GdAl2. The spectra were obtained with the sample at 4.2 K in the ordered magnetic state and in the absence of an external static magnetic field. Using a sequence composed of two RF pulses, we obtained up to five multi-quantum echoes for the 27Al nuclei, which were used to construct the zero-field NMR spectra. The spectra obtained from the FID observed after the second pulse and the even echoes exhibited higher resolution than the odd ones. In order to explain such behavior, we propose a model in which there are two regions inside the sample with different inhomogeneous spectral-line broadenings. Moreover, with the enhanced resolution from the FID signal, we were able to determine quadrupolar couplings with great precision directly from the respective spectra. These results were compared with those obtained from the quadrupolar oscillations of the echo signals, and showed good agreement. Similar data were also obtained from 155Gd and 157Gd nuclei. 相似文献
19.
An MR imaging technique has been developed producing head and body images of diagnostic quality in only a few seconds acquisition time. The Fourier type imaging technique uses excitation with relatively small excitation angels, echoes produced by gradient inversion, and extremely fast profile repetition. A typical result at 0.5 T is an artifact-free head image of 128 x 128 resolution, 10 mm slice thickness in an acquisition time of 2 seconds. 相似文献
20.