共查询到20条相似文献，搜索用时 31 毫秒
1.
Annabel Cartwright Dimitris Stamatellos Anthony P. Whitworth 《Monthly notices of the Royal Astronomical Society》2009,395(4):2373-2380
A simple algorithm is presented which generates a lattice-like, regular spacing of smoothed particle hydrodynamics (SPH) particles in discs, with any prescribed density gradient. Using this for comparison we demonstrate the effect of Poisson noise on SPH estimates of density, pressure and viscous forces when calculated using randomly distributed particles. The standard deviation of density and pressure is typically increased by greater than an order of magnitude. In a disc with a Keplerian velocity profile, the effectiveness of the Balsara switch in reducing the shear component of SPH artificial viscosity is greatly enhanced when the particles are properly spaced, reducing the magnitude of viscosity by two orders of magnitude. Noise problems are exacerbated, not removed, by increasing the numbers of SPH particles, if the number of neighbours used is kept constant. However, comparison of the evolution of a disc created using perfectly spaced particles and a disc with identical parameters but randomly placed particles, reveals very similar results. Although there are subtle differences in the evolution, and the smooth disc takes longer to begin developing structure, in both cases the identical number of objects is created by gravitational collapse. SPH disc simulations do not depend on initial density disturbances to evolve objects by gravitational collapse, which gives added credence to the validity of the results. It also appears that complicated disc settling procedures are unnecessary. 相似文献
2.
利用中国科学院新疆天文台南山观测站26m射电望远镜, 在中心频率1556MHz, 对Crab脉冲星(PSR B0531+21)进行了长达12.6h的连续观测, 观测带宽为512MHz, 时间分辨率为32μs, 研究了巨脉冲辐射的等待时间分布特征. 观测共探测到2097个信噪比大于10的巨脉冲, 对应的流量密度大于100Jy. 巨脉冲的爆发率表现为高度的间歇性, 在较短的时间内具有较高的爆发率, 在相对长的宁静期内巨脉冲的爆发率较低, 尤其是中间脉冲相位内的巨脉冲爆发. 相邻两个巨脉冲的等待时间分布表现为幂律分布特征, 可以用一个非稳态的泊松过程进行模拟, 这表明巨脉冲的爆发是一种独立的随机事件. 此外, 主脉冲和中间脉冲相位上的巨脉冲具有不同的等待时间分布特征, 这意味着脉冲星不同磁极的巨脉冲辐射机制可能是不同的. 这些观测结果对于理解脉冲星的射电辐射机制具有重要意义. 相似文献
3.
本文给出了旋涡星系密度波理论中Poisson积分的一种简化方法。这一简化的最重要应用在于利用它计算旋涡模式时可节省大量计算机时。此外本文还利用这种简化方法对一系列密度——引力势渐近关系进行了系统的数值检验。 用这一简化方法计算核函数比用未简化的核函数形式直接计算节省机时八十四倍。当在数值模式理论研究中大量计算Poisson积分时,应用本文提出的简化将有效地提高计算效率并保证足够高的精度。 相似文献
4.
Jacques Henrard 《Celestial Mechanics and Dynamical Astronomy》1988,45(1-3):245-253
The sheer magnitude of the work involved in the construction of perturbation theories (the astronomical computations) made it inevitable that astronomers would very early become interested in the possibility of constructing them with the aid of computers. Since then general systems for symbolic manipulations have been developed and used widely. Nevertheless there still remain problems for which these general systems are not well adapted and many specialized systems of algebraic manipulation (mostly Poisson series processors) are in use. An attempt is made to review this field by sketching some of the ideas on which these Poisson series processors are built. 相似文献
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J. R. Cherniack 《Celestial Mechanics and Dynamical Astronomy》1973,7(1):107-121
We describe the design of a working Poisson Series Processor that is more general than others in use today. We try to show that the price of generality is worth paying in active research areas in celestial mechanics.This work was supported in part by grant NGR 09-015-002 from the National Aeronautics and Space Administration. 相似文献
8.
对一实验数据组进行线性多重回归,如果数据的涨落服从泊松概率分布,常用方法不能给出准确的解。一般地,是假设各泊松分布的形状近似于高斯分布,给出近似解。因此,也存在着人所共知的拟合曲线下的面积亏损问题。本文从最大或然法出发,不加任何假设,严格推导了求解泊松分布线性多重回归的一种方法。按这一方法,“面积亏损”问题在计算精度范围内也得到彻底解决。 相似文献
9.
The formulas for the Poisson bracket of a perturbed two-body problem and a perturbed planetary problem are found in different systems of Keplerian elements. As with canonical parametrization, the Poisson bracket is equal to a linear combination of partial brackets, but it contains coefficients depending on semimajor axis, eccentricity, and inclination. A simple relation between the Poisson brackets and matrices of coefficients of Lagrange-type equations determining the variations of osculating elements is derived. The Poisson bracket of D'Alembertian functions is proved to be a D'Alembertian one by itself. 相似文献
10.
The amount of useful scientific data that a space-borne telescope produces is often limited by the available telemetry of
the platform. General purpose image compression schemes are usually used to compress the image either lossy or losslessly.
These schemes do not take into account the fact that pixel values of typical solar images are only known to within a certain
uncertainty range. We present a preprocessing method to enhance the performance (compression ratio) of any subsequent image
compression scheme. The method uses estimates of the photon shot and thermal noises to compute a recoding look-up table that
maps the initial data into uncertainty intervals. The recoding method is lossy in a mathematical sense but lossless in a physical
sense, since the image alterations are guaranteed to be smaller than the Poisson noise. The performance of any further compression
algorithm is enhanced while achieving a known guaranteed maximum absolute error for each pixel in the case of lossless compression.
This revised version was published online in October 2005 with corrections to one of the authors' names. 相似文献
11.
Tamara Ivanova 《Celestial Mechanics and Dynamical Astronomy》2001,80(3-4):167-176
A specialized Echeloned Poisson Series Processor (EPSP) is proposed. It is a typical software for the implementation of analytical algorithms of Celestial Mechanics. EPSP is designed for manipulating long polynomial-trigonometric series with literal divisors. The coefficients of these echeloned series are the rational or floating-point numbers. The Keplerian processor and analytical generator of special celestial mechanics functions based on the EPSP are also developed. 相似文献
12.
This paper is the third in a series of articles devoted to one of the basic problems of celestial mechanics: the study of the evolution of solar-type planetary systems. In the previous papers a brief review of the history and current state of the problem was given; the plan of the study was outlined; the Jacobi coordinates and the related osculating elements were introduced; the form of the Poisson expansion of the Hamiltonian in all elements was given; and the expansion coefficients for the Hamiltonian of the two-planetary Sun–Jupiter–Saturn problem were obtained (though with impure accuracy) by a simple algorithm that is reduced to the calculation of multiple integrals of elementary functions. In the present paper the expansion of the Hamiltonian of the two-planetary Sun–Jupiter–Saturn problem into the Poisson series in all elements is constructed with the help of the PSP Poisson series processor, which is capable of required accuracy. 相似文献
13.
对一数据涨落服从泊松概率分布的实验数据组进行拟合,如果拟合函数是非线性的,常常只能采用某些近似方法。近似性之一,表现在将拟合函数线性化,或者采用某种参数寻优的方法。近似性之二,则是将泊松问题近似地看作高斯分布问题。本文从最大或然法出发,对两类不同概率分布情况下的曲线拟合进行了比较,给出了对泊松分布问题更严格的求解途径。对于特殊的非线性拟合函数y(x)=ae~(bx~N),直接给出了迭代解。本文对曲线拟合的“面积亏损”问题,也进行了讨论。 相似文献
14.
María del Carmen Martínez Juan F. Navarro José Manuel Ferrándiz 《Celestial Mechanics and Dynamical Astronomy》2007,99(1):59-68
The efficiency in the computation of circular functions, such as cos(u) or sin(u), where u is a Poisson series, is important to derive accurate solutions of many problems of Celestial Mechanics, for instance, the
orbital or rotational perturbed motion of natural or artificial bodies, since expansions in terms of Legendre functions and
multiple Fourier series appear almost everywhere. Therefore, it is worth searching for alternative algorithms with lower computational
cost. In this article, we propose a method based on the idea of elimination, which was originally applied to solve numerical
problems, mainly in the case of matrix functions. Our comparisons with the traditional Taylor expansion prove that this new method can be more efficient when applied to compute the sine and cosine of a Poisson series. 相似文献
15.
Peter J. Shelus William H. Jefferys III 《Celestial Mechanics and Dynamical Astronomy》1975,11(1):75-78
Eliminating many of the trigonometric function calls by a suitable series transformation has resulted in a substantial reduction of on-line computing time while very long Poisson series are being evaluated. A further reduction has been realized by applying a short SNOBOL processor to the FORTRAN coding of the transformed series which eliminates many of the multiplication operations during the course of series evaluation. 相似文献
16.
D. L. Richardson 《Celestial Mechanics and Dynamical Astronomy》1988,45(1-3):267-274
PARSEC is a PC-based, interactive algebraic manipulation package designed to manipulate series of the kind frequently found in Celestial Mechanics applications and perturbation procedures. The system is fundamentally an input/command interpreter which allows the user to enter algebraic expressions and procedures and to control the flow of series manipulations interactively. The system is designed to allow easy manipulations of polynomial-trigonometric series within the environment of an electronic scratchpad. 相似文献
17.
R. A. Broucke 《Celestial Mechanics and Dynamical Astronomy》1988,45(1-3):255-265
The present article describes the design and applications of our Poisson series subroutine package developed and maintained since 1968. The programs are written in standard FORTRAN-77 and are almost independent of the wordlength of the particular computer. The system has no restriction on the number of polynomial and angular variables and the storage allocations are completely automatic, invisible to the user.The nucleus of the system consists of about 20 basic traffic subroutines that handle the terms of the different series. Besides these subroutines, we have a number of I/O routines as well as arithmetic subroutines and a large number of Celestial Mechanics applications such as the classical expansions of the Kepler Problem and several expansions of Disturbing Functions. A preprocessor has also been built, allowing the user to write code in a high-level language, such as Jefferys' Trigman, and then translate it in our call-statements.The system was developed on several different computers: first on the Univac 1108 at the Jet Propulsion Laboratory in 1968, then the IBM 360-91 at UCLA, Los Angeles, California and finally the CDC 6600 and CYBER 170/750 at the University of Texas, Austin, Texas. The latest version is entirely geared towards the IBM-PC and compatibles. 相似文献
18.
《Chinese Astronomy and Astrophysics》1987,11(2):105-112
We give here a method of simplifying the evaluation of the Poisson integral in the spiral density wave theory, which will save computer time in the evaluation of the kernal function by a factor of 84. A systematic check of the method has been made for a series of asymptotic density — gravitational potential relations. 相似文献
19.
基于泊松模型的X射线脉冲星信号的最大似然TOA估计 总被引：1，自引：0，他引：1
讨论了X射线脉冲星光子到达的周期平稳Poisson模型和到达时刻(Time of arrival,TOA)估计问题.在此基础上,得到了脉冲到达时刻的最大似然估计和克拉美-罗限(Cramer-Rao boundary,CRB),并推导了低信噪比情况下的似然函数近似表达和克拉美-罗限.利用解析的脉冲轮廓,对PSR B1821-24的TOA估计进行了Monte Carlo仿真,讨论了不同观测时间和信噪比下的估计误差,给出了相应的信噪比门限.研究表明:该分析方法能够有效估计X射线脉冲星的TOA定时精度,有利于评估其在各种应用中的性能. 相似文献
20.
综述日冕物质抛射的观测和持性，简短的前言之后，给出ＣＭＥ的发现经过及统计特性，着重介绍ＣＭＥ与其他种类太阳活动的相关。然后介绍ＣＭＥ的一般特性，包括可能与ＣＭＥ相关的一些物理过程的观测特性。初步结论是：ＣＭＥ是一种演变中的磁结构现象。 相似文献