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321.
P.J. t Hoen 《Ultrasonics》1982,20(5):231-236
There is considerable interest in aperture apodization for enhancing image quality. With continuous wave excitation, the influence of apodization can quite easily be predicted: the directivity function in the far-field corresponds to the Fourier transform of the aperture function. We, however, analyse a practical situation — short acoustic pulse emission focused in the near-field. Nine types of aperture function (for example, Tukey, Parzen, Hamming) are compared.It appears that with the pulsed mode a considerable reduction in off-axis intensity can be obtained by judicious choice of the aperture function. Side-lobes can be completely eliminated.As a tool, an approximate relationship is established between the Fourier transform of the aperture function and the main-lobe width of the directivity function. Further, an optimum balance is sought between the main-lobe width and the width at the ?25 dB level of the directivity function. 相似文献
322.
323.
Michael Coopersmith 《Journal of statistical physics》1979,20(2):175-179
The original Kac ring model is examined for the situation in which some of the scatterers are non-time-reversal invariant. It is shown that the system tends to absolute equilibrium (no fluctuations) even in the limit of very small density of these anomalous scatterers. 相似文献
324.
Summary In this paper the relativistic second-harmonic generation of a high-power laser radiation in a laser-produced plasma has been
studied theoretically in the presence of a self-generated magnetic field. The relativistic Vlasov equation has been employed
for the nonlinear response of the electrons in the hot magnetized plasma. It is observed that the power conversion efficiency
of the generated second harmonic wave is much higher for relativistic calculations than that for nonrelativistic calculations.
To speed up publication, the authors of this paper have agreed to not receive the proofs for correction. 相似文献
325.
强磁场中Rb D2线法拉第反常色散滤光器特性研究 总被引:3,自引:2,他引:1
报道强磁场中RbD_2线反常色散光学滤波器的理论模型与实验研究,讨论了强磁场中RbD-2线法拉第反常色散秀射谱及其磁场,温度调谐特性. 相似文献
326.
In this paper we use techniques from Ito excursion theory to analyze Brownian motion on generalized combs. Ito excursion theory is a little-known area of probability theory and we therefore present a brief introduction for the uninitiated. A general method for analyzing transport along the backbone of the comb is demonstrated and the specific case of a comb whose teeth are scaling branching trees is examined. We then present a recursive method for evaluating the distribution of the first passage times on hierarchical combs. 相似文献
327.
328.
The freezing point depression of a solvent, imbibed by crosslinked polyisoprene networks, was studied. It is argued that in a swollen vulcanizate, polymer chains subdivide the solvent, creating zones too small for nucleation on cooling to occur at the normal freezing point of the solvent. The freezing point of the solvent is identified with the temperature at which nuclei can form in the swollen network. A linear relationship between freezing point and the volume fraction of rubber in the swollen gel is predicted for a uniformly crosslinked network. Such a relationship is shown to hold for gamma-irradiated polyisoprene, but not for peroxide-cured samples, where deviations from linearity are interpreted as indicative of nonuniformity in the network. © 1993 John Wiley & Sons, Inc. 相似文献
329.
Heterogeneity, as it occurs in porous media, is characterized in terms of a scaling exponent, or fractal dimension. A feature of primary interest for two-phase flow is the mixing length. This paper determines the relation between the scaling exponent for the heterogeneity and the scaling exponent which governs the mixing length. The analysis assumes a linear transport equation and uses random fields first in the characterization of the heterogeneity and second in the solution of the flow problem, in order to determine the mixing exponents. The scaling behavior changes from long-length-scale dominated to short-length-scale dominated at a critical value of the scaling exponent of the rock heterogeneity. The long-length-scale-dominated diffusion is anomalous. 相似文献
330.
We consider a two-dimensional electron with an anomalous magnetic moment, g>2, interacting with a nonzero magnetic field B perpendicular to the plane which gives rise to a flux F. Recent results about the discrete spectrum of the Pauli operator are extended to fields with the
decay at infinity: we show that if
exceeds an integer N, there is at least
bound states. Furthermore, we prove that weakly coupled bound states exist under mild regularity assumptions also in the zero flux case. 相似文献