低轨道磁化等离子体中运动航天器等离子体鞘层特性

用二维静电粒子模拟程序研究了在低轨道磁化等离子体中航天器充电过程和等离子体鞘层结构．采用磁化等离子体流经航天器的物理模型,并且考虑零级对流电场的影响．结果表明航天器在低轨道空间飞行时,如果不考虑光电子流,航天器会在短时间内(ｔ＝５４ωｐｅ－１,ωｐｅ是电子等离子体频率)快速充电到平衡电位。在不考虑航天器运动的情况下其周围等离子体鞘层结构是围绕航天器对称的,等离子体鞘层宽度约２．４个德拜（Ｄｅｂｙｅ）长度．当考虑航天器运动时,航天器的平衡负电位绝对值减小．且在背离航天器运动方向上等离子体鞘层宽度为８．３个德拜长度．磁场的增大使负的平衡电位绝对值变小,当航天器运动从平行于磁场方向转向垂直于磁场方向时,航天器负平衡电位的绝对值减小．     

低轨道磁化等离子体中运动航天器等离子体鞘层特性

用二维静电粒子模拟程序研究了在低轨道磁化等离子体中航天器充电过程和等离子体鞘层结构．采用磁化等离子体流经航天器的物理模型,并且考虑零级对流电场的影响．结果表明航天器在低轨道空间飞行时,如果不考虑光电子流,航天器会在短时间内(ｔ＝５４ω_ｐｅ^－１,ω_ｐｅ是电子等离子体频率)快速充电到平衡电位。在不考虑航天器运动的情况下其周围等离子体鞘层结构是围绕航天器对称的,等离子体鞘层宽度约２．４个德拜（Ｄｅｂｙｅ）长度．当考虑航天器运动时,航天器的平衡负电位绝对值减小．且在背离航天器运动方向上等离子体鞘层宽度为８．３个德拜长度．磁场的增大使负的平衡电位绝对值变小,当航天器运动从平行于磁场方向转向垂直于磁场方向时,航天器负平衡电位的绝对值减小．     

Simulation of Atmospheric Boundary Layer Characteristics during Indian Summer Monsoon using Observations from Monsoon Trough Boundary Layer Experiment at Jodhpur, India

The diurnal structure of the boundary layer during Indian summer monsoon period is studied using a one-dimensional meteorological boundary layer model and the observations collected from the Monsoon Trough Boundary Layer Experiment conducted in 1990 at Jodhpur, India. The model was initialized with the observed temperature profiles at 0530 LST on 17 July, 1990 at Jodhpur and was run for 26 hours. The study is carried out with a geostrophic wind speed of 9.5 m s⁻¹ corresponding to the strong wind simulation. The mean thermodynamic and wind structure simulated by the model are in good agreement with those observed from 30 m tower. The computed surface layer characteristics such as the surface fluxes, TKE and standard deviations of velocity components are found to be reasonably in good agreement with those based on turbulence measurements. The shear and buoyancy budget computed from the model are also compared with the turbulence measurements. The integrated cooling budget in the nocturnal boundary layer is examined.     

Simultaneous optical and radar signatures of poleward-moving auroral forms

Dayside poleward moving auroral forms (PMAFs) were detected between 06:30 and 07:00 UT on December 16, 1998, by the meridian scanning photometer and the all-sky camera at Ny Ålesund, Svalbard. Simultaneous SuperDARN HF radar measurements permitted the study of the associated ionospheric velocity pattern. A good general agreement is observed between the location and movement of velocity enhancements (flow channels) and the PMAFs. Clear signatures of equatorward flow were detected in the vicinity of PMAFs. This flow is believed to be the signature of a return flow outside the reconnected flux tube, as predicted by the Southwood model. The simulated signatures of this model reproduce globally the measured signatures, and differences with the experimental data can be explained by the simplifications of the model. Proposed schemes of the flow modification due to the presence of several flow channels and the modification of cusp and region 1 field-aligned currents at the time of sporadic reconnection events are shown to fit well with the observations.     

高纬向阳侧磁层顶通量传输事件的特性研究——通量管轴线方位及运动分析

本文分析了2001年2月和3月期间Cluster Ⅱ穿越磁层顶前后的观测资料,检测到13个通量传输事件（FTEs）.用多颗卫星磁场测量资料的最小方差分析（MVAB）方法确定FTE的管轴方向（其中6个方向较可靠）.FTE管轴方向的分布和低纬处不同,在磁顶法线坐标系LMN中对M轴有较大偏离,比较靠近L轴.deHoffmann-Teller(HT)分析指出,13个FTEs都存在一个很好的HT参考系,表明它们以一个准稳的MHD结构运动.对垂直于管轴方向的运动分析表明FTEs并不一定和背景等离子体一起对流,它们可快于或慢于背景流,但FTEs的运动和背景流基本沿相同方向,其间可有一不大的夹角.在HT坐标系中,10个FTEs的等离子体速度接近零, 其他3个FTEs的等离子体速度约为局地Alfven波速的14%,都不符合Walen关系.其中北半球事件的Walen曲线为正斜率,南半球事件为负斜率,这说明等离子体沿磁力线（北半球顺着磁场,南半球逆着磁场）流向磁层.     

VELOCITY DETERMINATION AND ERROR ANALYSIS FOR THE SEISMIC REFRACTION METHOD*

A careful examination of the seismic refraction technique details the general assumptions and processes on which it is based. It is apparent that the normal interpretive process of determining velocity by hand fitting time breaks, while necessary to identify the refractors, is subject to imprecision. We describe a digital technique that calculates velocity and its precision from the time break measurements. The technique also facilitates examining for the possibility of systematic errors. Having observed the kind of imprecision that was apparent with velocity we extended our study to layer thicknesses (and thus depth) and found that the imprecision was significantly magnified. The technique and our claims for it are supported by an example of its application to a refraction survey in the Sturt Block, Western Australia.     

The magnetic field experiment onboard Equator-S and its scientific possibilities

The special feature of the ringcore fluxgate magnetometer on Equator-S is the high time and field resolution. The scientific aim of the experiment is the investigation of waves in the 10–100 picotesla range with a time resolution up to 64 Hz. The instrument characteristics and the influence of the spacecraft on the magnetic field measurement will be discussed. The work shows that the applied pre- and inflight calibration techniques are sufficient to suppress spacecraft interferences. The offset in spin axis direction was determined for the first time with an independent field measurement by the Equator-S Electron Drift Instrument. The data presented gives an impression of the accuracy of the measurement.     

An experiment to study and control the Langmuir sheath around INTERBALL-2

The satellite INTERBALL-2 has an orbit with high inclination (62.8°), covering the altitude range between a few hundred and about 20000 km. The ambient plasma conditions along this orbit are highly variable, and the interactions of this plasma with the spacecraft body as well as the photo-electron sheath around it are considered to be interesting topics for detailed studies. The electric potential of the spacecraft with respect to the ambient plasma that develops as a result of the current equilibrium reacts sensitively to variations of the boundary conditions. The measurement and eventual control of this potential is a prerequisite for accurate measurements of the thermal plasma. We describe the purpose and technical implementation of an ion emitter instrument on-board INTERBALL-2 utilising ion beams at energies of several thousand electron volts in order to reduce and stabilise the positive spacecraft potential. First results of the active ion beam experiments, and other measures taken on INTERBALL-2 to reduce charging are presented. Furthermore, the approach and initial steps of modelling efforts of the sheath in the vicinity of the INTERBALL-2 spacecraft are described together with some estimates on the resulting spacecraft potential, and effects on thermal ion measurements. It is concluded that even moderate spacecraft potentials as are commonly observed on-board INTERBALL-2 can significantly distort the measurements of ion distribution functions, especially in the presence of strongly aniso-tropic distributions.     

Observations of interplanetary scintillation during the 1998 Whole Sun Month: a comparison between EISCAT, ORT and Nagoya data

Observations of interplanetary scintillation (IPS) allow accurate solar wind velocity measurements to be made at all heliographic latitudes and at a range of distances from the Sun. The data may be obtained with either single, double or multiple antennas, each requiring a different method of analysis. IPS data taken during the 1998 whole sun month (30th July–31st August 1998) by EISCAT, the ORT (Ooty Radio Telescope), India, and the Nagoya IPS system, Japan, allow the results of individual methods of analysis to be compared. Good agreement is found between the velocity measurements using each method, and when combined an improved understanding of the structure of the solar wind can be obtained.On leave from the Physical Research Laboratory, Ahmedabad 380 009, India     

三维逆时偏移GPU/CPU机群实现方案研究

叠前逆时偏移是当前最为准确的地震成像方法,由于计算量大、存储量大等原因需要合适的实现策略和高效的计算平台.本文以高阶有限差分逆时偏移为基础,重点讨论了在GPU上实现需要解决的显存不足问题和人工边界问题.利用区域分解技术可以在当前GPU上高效地实现任意生产规模的三维逆时偏移成像,不会受到GPU显存规模的制约.常规最佳匹配层边界条件边界区域控制方程与内部区域差异较大,不适于GPU高速运算.本文在GPU上实现近似最佳匹配层(NPML)边界条件,使得高阶有限差分计算不需要分支判断,边界区域辅助波场的存储量也较低,保证了在GPU上进行波场传播的高效性.三维理论数据和实际资料成像结果表明了本文方法的正确性.     

BED SHEAR IN EVOLVING SCOUR AT A CYLINDER:A THEORETICAL APPROACH

ＢＥＤＳＨＥＡＲＩＮＥＶＯＬＶＩＮＧＳＣＯＵＲＡＴＡＣＹＬＩＮＤＥＲ：ＡＴＨＥＯＲＥＴＩＣＡＬＡＰＰＲＯＡＣＨＳｕｂｈａｓｉｓｈＤＥＹ１ａｎｄＳｒｉｊｉｂＫ．ＫＡＲ２Ａｂｓｔｒａｃｔ：Ｔｈｅｔｅｍｐｏｒａｌｖａｒｉａｔｉｏｎｏｆｓｈｅａｒｓｔｒｅｓｓ...     

MYJ和YSU方案对WRF边界层气象要素模拟的影响

研究新一代中尺度气象模式WRF中两种大气边界层方案(MYJ,YSU)对沈阳冬季大气边界层结构模拟的影响,重点分析温度层结、低层风场、边界层高度等对污染物扩散有重要影响的气象要素.和观测数据的比对表明WRF基本能够模拟出温度风速的日变化特征,但模拟风速偏大.YSU方案由于模拟的边界层顶卷挟和边界层内混合作用较强,夜间接地逆温强度低于MYJ方案,逆温维持时间比MYJ方案短4小时,同时模拟边界层高度也高于MYJ方案,有利于污染物垂直扩散.边界层高度的3种计算方法中,湍流动能方法计算的边界层高度最高,Richardson数方法次之,位温方法得到的高度最低.Richardson数方法对临界值的选取较敏感.     

MYJ和YSU方案对WRF边界层气象要素模拟的影响

研究新一代中尺度气象模式WRF中两种大气边界层方案(MYJ,YSU)对沈阳冬季大气边界层结构模拟的影响,重点分析温度层结、低层风场、边界层高度等对污染物扩散有重要影响的气象要素.和观测数据的比对表明WRF基本能够模拟出温度风速的日变化特征,但模拟风速偏大.YSU方案由于模拟的边界层顶卷挟和边界层内混合作用较强,夜间接地逆温强度低于MYJ方案,逆温维持时间比MYJ方案短4小时,同时模拟边界层高度也高于MYJ方案,有利于污染物垂直扩散.边界层高度的3种计算方法中,湍流动能方法计算的边界层高度最高,Richardson数方法次之,位温方法得到的高度最低.Richardson数方法对临界值的选取较敏感.     

EXPERIMENTAL STUDY ON THE COHERENT STRUCTURE OF THE TURBULENT BOUNDARY LAYER IN OPEN-CHANNEL FLOW

1 INTRODUCTION The coherent structure of turbulence is the most important discovery of turbulence studies in the last several decades. The coherent structure is a joint spatial state (Liang, 1999). In this space, there exist all kinds of motions that are organized and related. Namely, strip structures, large vortical structures, and other organizing flows are present. In 1932, researchers found that turbulence wasn抰 a fully random phenomenon and came to realize that the repeated emergen…     

基于单台加速度记录的混合全局优化HVSR反演场地浅层速度结构

利用水平与竖向谱比（HVSR）方法反演场地速度结构是国际上迅速发展的研究领域.HVSR反演计算实质是一个土层场地模型空间搜索的全局优化问题,当模型搜索空间的复杂程度增大时,目前常用的搜索算法收敛速度慢,计算效率较低.本文实现了一种结合遗传和模拟退火方法优点的混合全局优化HVSR反演算法,通过理论模型和竖向台阵实测数据的检验,表明该算法能获得很好的反演效果,较好地解决了蒙特卡罗方法收敛速度慢,遗传算法收敛早熟和模拟退火算法搜索效率低的问题.本文在此基础上讨论了单台加速度S波记录用于场地速度结构HVSR反演的适用性,为基于单个地震台的地震观测记录反演浅层速度结构提供了一种高效且较为准确的反演方法.

     

Estimation of mean water transit time on a steep hillslope in South Korea using soil moisture measurements and deuterium excess

A method for estimating daily mean transit time (DMTT) within a soil layer was proposed using field measurements of soil moisture. Vertical profiles of soil moisture time series were used for storage estimation. Water fluxes were evaluated through matrix and bypass flow. Variations in soil moisture and soil thickness were used to evaluate matrix flow. Exponential decay in depth of macropores was also used for bypass flow approximation. DMTT evaluation was compared to results obtained from a stable water isotope model using two years of data acquired on a steep granite hillslope in the Sulmachun watershed, South Korea. Various uncertainties in transit time evaluation such as model structure, non‐stationary assumption and data acquisition of existing approaches can be accounted for in the proposed methodology, and the flowpath contribution can be further configured in conjunction with hydrometric measurements. Probability density functions of isotope analyses were partially explained by transit time distributions that were based on soil moisture measurements. Supplementary sensitivity analyses for uncertainty configurations indicate that matrix flow is the primary process in determining transit time distribution while the impact of bypass flow is minor. The feasibility of a DMTT approach over isotope‐based methodologies highlights not only the strength of this proposed method, both in cost and time, but also its further application potential for existing soil moisture measurements. Copyright © 2013 John Wiley & Sons, Ltd.     

Spacecraft potential control aboard Equator-S as a test for Cluster-II

The payload of Equator-S was complemented by the potential control device (PCD) to stabilise the electric potential of the spacecraft with respect to the ambient plasma. Low potentials are essential for accurate measurements of the thermal plasma. The design of PCD is inherited from instruments for Geotail and Cluster and utilises liquid metal ion sources generating a beam of indium ions at several keV. The set-up of the instrument and its interaction with the plasma instruments on board is presented. When the instrument was switched on during commissioning, unexpectedly high ignition and operating voltages of some ion emitters were observed. An extensive investigation was initiated and the results, which lead to an improved design for Cluster-II, are summarised. The cause of the abnormal behaviour could be linked to surface contamination of some emitters, which will be monitored and cured by on-board procedures in future. The mission operations on Equator-S were not at all affected, because of the high redundancy built into the instrument so that a sufficient number of perfectly operating emitters were available and were turned on routinely throughout the mission. Observations of the effect of spacecraft potential control on the plasma remained limited to just one event on January 8, 1998, which is analysed in detail. It is concluded that the ion beam lead to the predicted improvement of the particle measurements even outside the low density regions of the magnetosphere where the effect of spacecraft potential control would have been much more pronounced, and that the similar instruments for the four Cluster-II spacecraft to be launched in 2000 will be very important to ensure accurate plasma data from this mission.     

基于非分裂CFS-PML边界条件的频散介质GPR时域有限元模拟

通过数值模拟研究探地雷达（GPR）高频电磁波在频散介质中的传播规律,对提高实测资料的解释精度具有重要意义.复频移完全匹配层边界条件（CFS-PML）以其优越的吸收特性被广泛用于一阶电磁波动方程的GPR时域有限差分数值模拟中,其实现过程大都涉及电磁场的卷积计算,辅助变量较多,降低计算效率.为此,本文从复拉伸坐标系下的Debye频散介质电磁波动方程出发,通过合理构造辅助微分方程,推导了二阶Debye频散介质电磁波动方程的非分裂CFS-PML边界条件实现公式,避免了电磁波场的分裂和卷积计算.在此基础上,利用Galerkin法和Newmark-β差分法推导了基于非分裂CFS-PML边界条件的GPR有限元方程及其时域差分离散格式.两个GPR模型的模拟结果表明：本文提出的基于辅助微分方程的非分裂CFS-PML边界条件实现方法可有效地吸收大角度入射的低频虚假反射波,提高模拟精度;相比于非频散介质,高频电磁波在频散介质中传播衰减更强、子波持续时间增大、分辨率和传播速度降低、直达波和反射波的主频更小,分析结果有助于提高实测GPR资料的解译精度.

     

Neural network modelling for mean velocity and turbulence intensities of steep channel flows

The main purpose of this study is to evaluate the potential of simulating the profiles of the mean velocity and turbulence intensities for the steep open channel flows over a smooth boundary using artificial neural networks. In a laboratory flume, turbulent flow conditions were measured using a fibre‐optic laser doppler velocimeter (FLDV). One thousand and sixty‐four data sets were collected for different slopes and aspect ratios at different locations. These data sets were randomly split into two subsets, i.e. training and validation sets. The multi‐layer functional link network (MFLN) was used to construct the simulation model based on the training data. The constructed MFLN models can almost perfectly simulate the velocity profile and turbulence intensity. The values of correlation coefficient (γ) are close to one and the values of root mean square error (RMSE) are close to zero in all conditions. The results demonstrate that the MFLN can precisely simulate the velocity profiles, while the log law and Reynolds stress model (RSM) are less effective when used to simulate the velocity profiles close to the side wall. The simulated longitudinal turbulence intensities yielded by the MFLN were also fairly consistent with the measured data, while the simulated vertical turbulence intensities by the RSM were not consistent with the measured data. Copyright © 2007 John Wiley & Sons, Ltd.     

A lattice Boltzmann model coupled with a Large Eddy Simulation model for flows around a groyne

This present paper proposes a two-dimensional lattice Boltzmann model coupled with a Large Eddy Simulation (LES) model and applies it to flows around a non-submerged groyne in a channel. The LES of shallow water equations is efficiently performed using the Lattice Boltzmann Method (LBM) and the turbulence can be taken into account in conjunction with the Smagorinsky Sub-Grid Stress (SGS) model. The bounce-back scheme of the non-equilibrium part of the distribution function is used to determine the unknown distribution functions at inflow boundary, the zero gradient of the distribution function is set normal to outflow boundary to obtain the unknown distribution functions here and the bounce-back scheme, which states that an incoming particle towards the boundary is bounced back into fluid, is applied to the solid wall to ensure non-slip boundary conditions. The initial flow field is defined firstly and then is used to calculate the local equilibrium distributions as initial conditions of the distribution functions. These coupled models successfully predict the flow characteristics, such as circulating flow, velocity and water depth distributions. The comparisons between the simulated results and the experimental data show that the model scheme has the capacity to solve the complex flows in shallow water with reasonable accuracy and reliability.