Turbulent passive scalar field of a small prandtl number

The statistical-mechanics theory of the passive scalar field convected by turbulence, developed in an earlier paper [Phys. Fluids 28, 1299 (1985)], is extended to the case of a small molecular Prandtl number. The set of governing integral equations is solved by the equation-error method. The resultant scalar-variance spectrum for the inertial range is F(k)~x−5/3/[1+1.21x1.67(1+0.353x2.32)], where x is the wavenumber scaled by Corrsin's dissipation wavenumber. This result reduces to the − (5)/(3) law in the inertial-convective range. It also approximately reduces to the − (17)/(3) law in the inertial-diffusive range, but the proportionality constant differs from Batchelor's by a factor of 3.6.

一种3自由度并联柔索驱动机器人的精度研究

研究了一种3自由度并联柔索驱动机器人精度分析和精度综合的问题.分析了影响机器人位姿精度的主要因素,推导建立了关节误差及柔索误差模型.提出了一种精度综合算法,并基于给定的机器人关节允差,综合分析出装配误差及柔索误差的最大取值.通过仿真验证了误差模型的正确性.样机试验表明,利用柔索误差模型可以提高机器人的运动精度.

一种平面运动位姿的并联组合测量方法研究

基于特殊的测量环境需要,提出了用于平面运动位姿测量的并联组合测量方法。介绍了并联组合测量方法的测量机构组成和测量原理,并进行了可行性论证;通过建立误差模型,对几何误差源与原始测量参数的映射关系及其对最终位姿测量误差的影响进行了分析,仿真结果和实际应用测量数据验证了分析的正确性。所述并联组合测量方法构思新颖,结构合理,适用于具有一定特殊测量条件的高精度平面大范围运动过程位姿测量。如果在工程应用中有效地控制几何误差源的影响,该方法则具有一定的推广应用价值。

一种新型平面组合传感器的误差建模与分析

设计了一种新型三自由度位姿测量平面组合传感器装置,用于完成对平面运动的两个移动自由度和一个转动自由度的动态测量。介绍了传感器的机构构成和测量原理,利用微分法原理建立了误差模型,对误差产生原因进行分析,得出了机构误差对测量精度的影响曲线,试验和仿真验证了新型平面组合传感器机构的合理性。新型平面组合传感器机构简单,测量精度高,适用于特定环境下的高精度平面运动位姿测量。

Direct numerical test of the B-G-K model equation by the DSMC method

In the present paper the rarefied gas how caused by the sudden change of the wall temperature and the Rayleigh problem are simulated by the DSMC method which has been validated by experiments both in global flour field and velocity distribution function level. The comparison of the simulated results with the accurate numerical solutions of the B-G-K model equation shows that near equilibrium the BG-K equation with corrected collision frequency can give accurate result but as farther away from equilibrium the B-G-K equation is not accurate. This is for the first time that the error caused by the B-G-K model equation has been revealed.

A lattice Boltzmann equation for waves

We propose a lattice Boltzmann model for the wave equation. Using a lattice Boltzmann equation and the Chapman-Enskog expansion, we get 1D and 2D wave equations with truncation error of order two. The numerical tests show the method can be used to simulate the wave motions.

Correcting the systematic error of the density functional theory calculation: the alternate combination approach of genetic algorithm and neural network

The alternate combinational approach of genetic algorithm and neural network (AGANN) has been presented to correct the systematic error of the density functional theory (DFT) calculation. It treats the DFT as a black box and models the error through external statistical information. As a demonstration, the AGANN method has been applied in the correction of the lattice energies from the DFT calculation for 72 metal halides and hydrides. Through the AGANN correction, the mean absolute value of the relative errors of the calculated lattice energies to the experimental values decreases from 4.93% to 1.20% in the testing set. For comparison, the neural network approach reduces the mean value to 2.56%. And for the common combinational approach of genetic algorithm and neural network, the value drops to 2.15%. The multiple linear regression method almost has no correction effect here.

Simulation on Gain Recovery of Quantum Dot Semiconductor Optical Amplifiers by Rate Equation

The gain recoveries in quantum dot semiconductor optical amplifiers are numerically studied by rate equation models. Similar to the optical pump-probe experiment, the injection of double optical pulses is used to simulate the gain recovery of a weak continuous signal for the QD SOAs. The gain recoveries are fitted by a response function with multiple exponential terms. For the pulses duration of 10 ps, the gain recovery can be described by three exponential terms with the time constants, and for the pulse with the width of 150 fs, the gain recovery can be described by two exponential terms, the reason is that the short pulse does not consume lot of carriers.

Modification to the Hardisty equation, regarding the relationship between sediment transport rate and particle size

Bagnold-type bed-load equations are widely used for the determination of sediment transport rate in marine environments. The accuracy of these equations depends upon the definition of the coefficient k(1) in the equations, which is a function of particle size. Hardisty (1983) has attempted to establish the relationship between k(1) and particle size, but there is an error in his analytical result. Our reanalysis of the original flume data results in new formulae for the coefficient. Furthermore, we found that the k(1) values should be derived using u(1) and u(1cr) data; the use of the vertical mean velocity in flumes to replace u(1) will lead to considerably higher k(1) values and overestimation of sediment transport rates.

VTI各向异性叠前深度偏移及速度分析

Theoretical research, laboratory test and field observation show that most of sediment rock has anisotropic features. It will produce some notable errors when applying isotropic methods such as prestack depth migration and velocity analysis to dada acquired under anisotropic condition; it also has a bad effect on geologic interpretation. Generally speaking, the vertical transverse isotropic media is a good approximation to geologic structure, thus it has an important realistic meaning for anisotropic prestack depth migration theory researching and precise complex geologic imaging if considering anisotropic effect of seismic wave propagation. There are two indispensable parts in prestack depth migration of realistic records, one is proper prestack depth migration algorithm, and the other is velocity analysis using prestack seismic data. The paper consists of the two aspects. Based on implicit finite difference research proposed by Dietrich Ristow et al （1997） about VTI media prestack depth migration, the paper proposed split-step Fourier prestack depth migration algorithm （VTISSF） and Fourier finite difference algorithm （VTIFFD） based on wave equation for VTI media, program are designed and the depth migration method are tested using synthetic model. The result shows that VTISSF is a stable algorithm, it generally gets a good result if the reflector dip is not very steep, while undermigration phenomena appeared in steep dips case; the VTIFFD algorithm bring us better result in steep dips with lower efficiency and frequency dispersion. For anisotropic prestack depth migration velocity analysis of VTI media, The paper discussed the basic hypothesis of VTI model in velocity analysis algorithm, basis of anisotropic prestack depth migration velocity analysis and travel time table calculation of VTI media in integral prestack depth migration. Then , analyzed the P-wave common imaging gather in the case of homogeneous velocity and vertically variable velocity . studied the residual correction in common imaging gather produced by media parameter error, analyzed the condition of flat event and correct depth in common imaging gather . In this case, the anisotropic model parameter vector is , is vertical velocity of a point at top surface, is vertical velocity gradient, and are anisotropic parameter. We can get vertical velocity gradient from seismic data; then the P-wave common imaging gather of VTI media whose velocity varies in vertical and horizontal direction, the relationship between media parameter and event residual time shift of common image gather are studied. We got the condition of flattening common imaging gather with correct depth. In this case the anisotropic model parameter vector is , is velocity gradient in horizontal direction. As a result, the vertical velocity grads can be decided uniquely, but horizontal velocity grads and anisotropic parameter can’t be distinguished if no priori information available, our method is to supply parameter by velocity scanning; then, as soon as is supplied we can get another four parameters of VTI media from seismic data. Based on above analysis, the paper discussed the feasibility of migration velocity analysis in vertically and horizontally varied VTI media, synthetic record of three models are used to test the velocity analysis method . Firstly, anisotropic velocity analysis test is done using a simple model with one block, then we used a model with multiple blocks, thirdly, we analyzed the anisotropic velocity using a part of Marmousi model. The model results show that this velocity analysis method is feasible and correct.

基于广义Lippmann-Schwinger方程的地震波场模拟算法与应用

In the last several decades, due to the fast development of computer, numerical simulation has been an indispensable tool in scientific research. Numerical simulation methods which based on partial difference operators such as Finite Difference Method (FDM) and Finite Element Method (FEM) have been widely used. However, in the realm of seismology and seismic prospecting, one usually meets with geological models which have piece-wise heterogeneous structures as well as volume heterogeneities between layers, the continuity of displacement and stress across the irregular layers and seismic wave scattering induced by the perturbation of the volume usually bring in error when using conventional methods based on difference operators. The method discussed in this paper is based on elastic theory and integral theory. Seismic wave equation in the frequency domain is transformed into a generalized Lippmann-Schwinger equation, in which the seismic wavefield contributed by the background is expressed by the boundary integral equation and the scattering by the volume heterogeneities is considered. Boundary element-volume integral method based on this equation has advantages of Boundary Element Method (BEM), such as reducing one dimension of the model, explicit use the displacement and stress continuity across irregular interfaces, high precision, satisfying the boundary at infinite, etc. Also, this method could accurately simulate the seismic scattering by the volume heterogeneities. In this paper, the concrete Lippmann-Schwinger equation is specifically given according to the real geological models. Also, the complete coefficients of the non-smooth point for the integral equation are introduced. Because Boundary Element-Volume integral equation method uses fundamental solutions which are singular when the source point and the field are very close，both in the two dimensional and the three dimensional case, the treatment of the singular kernel affects the precision of this method. The method based on integral transform and integration by parts could treat the points on the boundary and inside the domain. It could transform the singular integral into an analytical one both in two dimensional and in three dimensional cases and thus it could eliminate the singularity. In order to analyze the elastic seismic wave scattering due to regional irregular topographies, the analytical solution for problems of this type is discussed and the analytical solution of P waves by multiple canyons is given. For the boundary reflection, the method used here is infinite boundary element absorbing boundary developed by a pervious researcher. The comparison between the analytical solutions and concrete numerical examples validate the efficiency of this method. We thoroughly discussed the sampling frequency in elastic wave simulation and find that, for a general case, three elements per wavelength is sufficient, however, when the problem is too complex, more elements per wavelength are necessary. Also, the seismic response in the frequency domain of the canyons with different types of random heterogeneities is illustrated. We analyzed the model of the random media, the horizontal and vertical correlation length, the standard deviation, and the dimensionless frequency how to affect the seismic wave amplification on the ground, and thus provide a basis for the choice of the parameter of random media during numerical simulation.

磁层－电离层电流的暴时变化以及主磁场对空间环境的影响

Two problems are studied in this thesis, the relationship of the magneto-spheric - ionospheric current systems during storms, and the effects of the main field to the space environment. The thesis includes three parts. 1. Magnetic disturbances caused by magnetospheric - ionospheric current systems Transient variations of the geomagnetic field at middle-low latitudes are mainly caused by the ionospheric dynamo current (IDC), the symmetric ring current (SRC), the partial ring current-region II field-aligned current-ionospheric current system (PRFI), and the region I field-aligned current-ionospheric current system (FACI). The storm on May 1 ～ 6, 1998 is analyzed. Firstly, the S_q-field caused by IDC current is removed by using the modified Hibberd's method in which the effect of SRC is considered. The neglect of SRC-field can give as much as 40% error in S_q-field evaluation. Secondly, the disturbance fields at the middle and low latitudes are separated according to their origins. As a result, the disturbance caused by FACI-current is an important part of the asymmetrical depression of H-component in middle and low latitudes during storms. The results show that the relative intensity of the Sq-field increases in the main phase of the storm and decreases in the recovery phase. The latitudinal gradient of the Sq-field is positive during the whole storm. The storm of May 1 ～ 6, 1998 contains two events. In the first event on May 2, the SRC-field is similar to Dst index. But in the second event on May 4 ～ 5, the SRC-field delays to Dst index, and the SRC-field depresses while the PRFI- and FACI-fields recovery. 2. Analysis of S_q~p variation in CGM coordinates In order to study the conjugation of geomagnetic variations between northern and southern hemispheres, we use the corrected geomagnetic coordinates (CGM) instead of the geomagnetic coordinates (GM) to analyze the S_q~P equivalent current system. The CGM coordinates are built up by International Geomagnetic Reference Field (IGRF) model. The S_q~p variations and equivalent current systems in the northern and southern polar regions are more symmetrical in CGM coordinates than in GM co-ordinates. This fact implies that the current distributions in polar regions are governed by the configuration of the geomagnetic field lines. As the elaborate structure of S_q~p current system in quiet time is obtained, we summarize the seasonal variation of the electrojet in quiet time. 3. The magnetospheric configuration of non-parallel-dipole model The magnetospheric configurations are calculated for two possible geomag-netic field models during the geomagnetic field reversals. These models are the dipole field with the axis to the sun and the quadrupole field model. We use the finite element method to solve the magnetic equation, and use the surface evolution method to solve the equilibrium equation. The results show that the main field greatly affects the space environment.