平板边界层转捩的数值模拟

对于自由湍流引起的压力梯度为零的平板边界层转捩过程的数值模拟,本文提出了一个单方程湍流模型 (VL M模型),对湍流粘性系数和湍流长度尺度都进行了修正以正确引入低雷诺数效应。数值计算表明,该模型在自由来流湍流度为 0.03～8%的大范围内能正确模拟边界层转捩全过程,包括转捩区起点位置和转捩区长度、边界层内的平均及脉动参数分布等。本文还提出了上述转捩过程的时均化五阶段模型。      

Optical observations of the X-ray source 2S0921-630

We report photometric observations of the optical counterpart of the X-ray source 2S0921-630. The data, obtained at the South African Astronomical Observatory during 3 weeks in 1980 and 1981, are consistent with a 17.9 day periodic modulation of the flux in the B band. Correlated variability of the (B-V) and (U-B) colour indices with the B mag. is demonstrated and quantified. The observed B mag. and colours at maximum and minimum light are used to compute some of the system parameters. A model of 2S0921-630 is proposed in terms of a binary system in which the variable inclination of a luminous accretion disc produces the long-term modulation of the optical flux.     

Maximum Likelihood Elevation Angle Estimates of Radar Targets Using Subapertures

The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface.     

Study of rapid variability in Cygnus X-1

The EXOSAT Medium Energy instrument has been used with the new HER6 mode to study the rapid variability of Cygnus X-1. It was found that the fluctuations in the higher energy band (5–14 keV) lagged behind those in the lower band (2–5 keV) by about 6 milliseconds. This confirms predictions of the inverse Compton model and gives an upper limit to the scale-size of the hot gas clouds.     

Postdetection Integration Loss for Logarithmic Detectors

The effect of a logarithmic receiver on the detection performance of a radar has been determined by Green by numerical computations for up to 100 pulses integrated. It was further suggested that the loss in decibels as compared to a square-law detector would appear to be proportional to the logarithm of the number of pulses integrated. In this correspondence we show that this ?conjecture? is false, and that the additional loss when N becomes very large approaches a constant value of 2.2 dB.     

A Hybrid Navigation Concept Using a Spinning Satellite?Borne Interferometer and Self?Contained Equipment

This paper analyzes a hybrid navigation concept that uses signals from a radio interferometer mounted on a spinning geostationary satellite, preliminary position estimates from self-contained equipment, and stored a priori information on the past performance of this equipment. The craft-borne processor, optimum in the maximum a posteriori (MAP) sense, is designed to estimate position coordinates using only the incoming radio signals, although improved estimates result if the other two items are available. An error analysis starts with the derivation of an estimation error covariance matrix, whose elements depend on additive receiver noise and the physical parameters of the system. A minimum 1? estimation error in position is obtained by trading off these parameters. The effects of other major error sources, such as tropospheric phase fluctuations, multipath, and craft altitude uncertainty, are added to the estimation error to give a total 1? position error on the order of 3.7 to 5.6 km.     

Evolution of the sun''s near-surface asphericities over the activity cycle

Solar oscillations provide the most accurate measures of cycle dependent changes in the sun, and the Solar and Heliospheric Observatory/Michelson Doppler Imager (MDI) data are the most precise of all. They give us the opportunity to address the real challenge — connecting the MDI seismic measures to observed characteristics of the dynamic sun. From inversions of the evolving MDI data, one expects to determine the nature of the evolution, through the solar cycle, of the layers just beneath the sun's surface. Such inversions require one to guess the form of the causal perturbation — usually beginning with asking whether it is thermal or magnetic. Matters here are complicated because the inversion kernels for these two are quite similar, which means that we don't have much chance of disentangling them by inversion. However, since the perturbation lies very close to the solar surface, one can use synoptic data as an outer boundary condition to fix the choice. It turns out that magnetic and thermal synoptic signals are also quite similar. Thus, the most precise measure of the surface is required.

We argue that the most precise synoptic data come from the Big Bear Solar Observatory (BBSO) Solar Disk Photometer (SDP). A preliminary analysis of these data implies a magnetic origin of the cycle-dependent sub-surface perturbation. However, we still need to do a more careful removal of the facular signal to determine the true thermal signal.