轨道不平顺随机过程的数值模拟

轨道不平顺是列车-轨道系统的动力响应的主要影响因素,不平顺样本是列车-轨道系统动力分析模型中不可缺少的参数.根据随机理论原理推导了轨道不平顺谱的空间、时间域的相互转化关系,得到了时域功率谱的表达式.分析了目前广泛应用几种轨道不平顺随机过程的模拟方法的原理及作法,通过对功率谱快速数值算法的分析给出了一种新三角级数模拟方法,该方法与逆傅氏变换法具有等效性,同时也论证了逆傅氏变换方法样本是高斯过程.最后通过实际算例的分析论证了新三角级数法与傅氏逆变换法在简便、合理等方面的优点.     

A model for the numerical simulation of tephra fall deposits

A simple semianalytical model to simulate ash dispersion and deposition produced by sustained Plinian and sub-Plinian eruption columns based on the 2D advection–dispersion equation was applied. The eruption column acts as a vertical line source with a given mass distribution and neglects the complex dynamics within the eruption column. Thus, the use of the model is limited to areas far from the vent where the dynamics of the eruption column play a minor role. Vertical wind and diffusion components are considered negligible with respect to the horizontal ones. The dispersion and deposition of particles in the model is only governed by gravitational settling, horizontal eddy diffusion, and wind advection. The model accounts for different types and size classes of a user-defined number of particle classes and changing settling velocity with altitude. In as much as wind profiles are considered constant on the entire domain, the model validity is limited to medium-range distances (about 30–200 km away from the source).The model was used to reconstruct the tephra fall deposit from the documented Plinian eruption of Mt. Vesuvius, Italy, in 79 A.D. In this case, the model was able to broadly reproduce the characteristic medium-range tephra deposit. The results support the validity of the model, which has the advantage of being simple and fast to compute. It has the potential to serve as a simple tool for predicting the distribution of ash fall of hypothetical or real eruptions of a given magnitude and a given wind profile. Using a statistical set of frequent wind profiles, it also was used to construct air fall hazard maps of the most likely affected areas around active volcanoes where a large eruption is expected to occur.     

地震海啸传播过程数值模拟研究

运用数值模拟的方法,对常水深4 000 m,8.0级纯倾滑直立断层触发的地震海啸,在大洋中传播的前2 000 s各时刻的形态以及峰值进行研究.结果表明,地震海啸产生后的前1 000 s左右的时间内,存在数值震荡,幅值先快速衰减,再逐渐增大,此后,幅值随着时间的推移缓慢衰减.     

The effect of coastal upwelling on the sea-breeze circulation at Cabo Frio, Brazil: a numerical experiment

The effect of coastal upwelling on sea-breeze circulation in Cabo Frio (Brazil) and the feedback of sea-breeze on the upwelling signal in this region are investigated. In order to study the effect of coastal upwelling on sea-breeze a non-linear, three-dimensional, primitive equation atmospheric model is employed. The model considers only dry air and employs boundary layer formulation. The surface temperature is determined by a forcing function applied to the Earths surface. In order to investigate the seasonal variations of the circulation, numerical experiments considering three-month means are conducted: January-February-March (JFM), April-May-June (AMJ), July-August-September (JAS) and October-November-December (OND). The model results show that the sea-breeze is most intense near the coast at all the seasons. The sea-breeze is stronger in OND and JFM, when the upwelling occurs, and weaker in AMJ and JAS, when there is no upwelling. Numerical simulations also show that when the upwelling occurs the sea-breeze develops and attains maximum intensity earlier than when it does not occur. Observations show a similar behavior. In order to verify the effect of the sea-breeze surface wind on the upwelling, a two-layer finite element ocean model is also implemented. The results of simulations using this model, forced by the wind generated in the sea-breeze model, show that the sea-breeze effectively enhances the upwelling signal.     

青海湖湖陆风的数值研究

用一个含植被参数化的二维中尺度模式研究青海湖夏季的湖陆风,模拟结果再次表明该模式具有重现局地中尺度环境特征,如冷（热）岛环境和湿岛等。同时表明,模式对植被覆盖度是敏感的,即有稀疏植被覆盖（σ＝３０％）情况的湖陆风白天比裸地情况要小,夜间则大,湖岸水平温度（或湿度）梯度随植被覆盖度增加而减小。     

Online numerical simulation: A hybrid simulation method for incomplete boundary conditions

Hybrid simulation is a powerful and cost‐effective simulation technique to evaluate structural dynamic performance. However, it is sometimes rather difficult to guarantee all the boundaries on the physical substructures, especially when the boundary conditions are very complex, due to limited laboratory resources. Lacking of boundary conditions is bound to change the stress state of the structure and eventually result in an inaccurate evaluation of structural performance. A model updating‐based online numerical simulation method is proposed in this paper to tackle the problem of incomplete boundary conditions. In the proposed method, 2 sets of finite element models with the same constitutive model are set up for the overall analysis of the whole structure and the constitutive model parameter estimation of the physical substructure, respectively. The boundary conditions are naturally satisfied because the response is calculated from the overall structural model, and the accuracy is improved as the material constitutive parameters are updated. The effectiveness of the proposed method is validated via numerical simulations and actual hybrid tests on a RC frame structure, and the results show that the negative effect of incomplete boundary conditions is almost eliminated and the accuracy of hybrid simulation is very much improved.     

数值模拟在地球动力学中的研究进展

本文主要针对有限单元法,对近10几年来数值模拟在地球动力学中的应用和发展作了回顾,分为构造应力场模拟;地幔热对流模拟;板块碰撞模拟;岩石圈流变学模拟;以及地震机制与预测模拟等部分.简单阐述了当前地球动力学数值模拟的发展趋势并论述了其面临的主要问题.     

海南岛海风雷暴结构的数值模拟

本文利用高分辨率WRF模式对2012年7月20日发生在海南地区的一次海风雷暴过程进行模拟,探讨了海南岛复杂地形下海风雷暴的结构、发展演变过程及其触发机制.结果表明,海南岛北部向内陆传播的海风与南部受地形阻挡的海风相遇后会形成海风辐合带,辐合带能影响当地的散度和涡旋特征,为雷暴的发生发展提供有利的动力和热力学条件.海南岛受热带海洋的影响较大,当地的水汽条件和对流潜势长期保持着有利于对流发展的状态,自由对流高度始终处于较低的位置,一旦海风辐合带来的抬升运动克服对流抑制到达自由对流高度后,对流就能自主地发展起来,所以单纯的海风辐合也常常能触发当地的强雷暴.雷暴发生发展过程中对流参数存在明显的变化,其演变曲线的突变位置对雷暴的发生有一定的指示作用.海南岛的海风雷暴过程与当地的复杂地形密切相关,地形的动力阻挡作用影响着低层海风的辐合以及对流的发展.     

利用数值模拟方法研究宏观丹霞地貌坡地的演化过程

由于红层岩性、节理发育状态、年代、构造运动特征、气候以及区域地质地理条件等的不同,丹霞地貌的形态在区域上有很大差别,其成因机制十分复杂.从丹霞地貌演化机制的宏观意义着眼,考虑岩性、节理、构造抬升速率及降雨等因素,基于地貌演化理论,利用数值模拟方法定量地展现了丹霞地貌坡地在构造抬升及流水侵蚀这两个主要的内外力作用下的动力学演化过程.地貌演化的幼年期和壮年期,流域输沙能力相对较大,主要表现为流水对基岩的侵蚀过程;直至演化的老年期,流域输沙能力相对较小,大量的风化物淤积,坡度变缓.同时,对不同岩性和垂直节理深度的计算结果进行了对比分析,砾石质地相对坚硬,计算得到的高程较大,表明岩性硬度越高,较容易形成陡壁,而岩性较软,易形成丘陵.垂直节理深度不同,坡地峰值几乎相同,但浅垂直节理较深垂直节理的谷值大,表明流水沿垂直节理进行下切侵蚀的程度不同,对景观的形成有着重要影响.数值模拟结果在物理上是合理的,有利于进一步定性地认识到岩层岩性、节理、构造抬升及流水侵蚀对丹霞地貌坡地演化的作用.     

太湖风生流的三维数值模拟

建立了太湖三维风生流数值模型,并用差分法求解;垂直方向上采用了坐标变换技术,把任一节点的水深转换成无量纲水深,从而有效地消除了因风力作用造成的自由水面波动和湖底不规则的影响;水平面上采用锯齿网格处理,对于四周以闭边界为主的湖泊水域,显得比较合理。计算结果表明,湖泊风生流沿垂直、水平方向都有较大变化,流向上下、水平也并不一致,这是湖泊水流区别于其它水域水流之所在。计算模拟显示所建模型的有效性和可操作性。     

洪泽湖吞吐流二维数值模拟

吞吐流是湖泊中湖水运动的主要形式之一。它通过水体交换,进而对湖泊污染物质扩散,迁移,泥沙冲淤以及湖水浓度场的变化产生影响。根据洪泽湖的具体特点,本文构造了一个二维开边界数值模拟,对洪泽湖吞吐淮河的特征进行数值实验,研究其吞吐流过程。     

鄱阳湖典型洪泛区地下水数值模拟研究

受地表河湖系统水情变化干扰,高度动态和异质性的洪泛区地下水文对河湖水资源、水污染以及生态环境功能等方面具有重要影响和贡献。鄱阳湖洪泛区湿地在长江中下游具有重要区位优势和研究特色,但变化环境下其水动力特征和水量交换情况等仍存在许多不确定性。本文以鄱阳湖典型洪泛区为研究区,采用地下水流二维数值模型,开展了洪泛区地表地下水转化作用与水量变化的模拟研究。结果表明,鄱阳湖季节性水位变化很大程度上决定了主湖区与周边地下水之间的动态补排模式,即洪泛区地下水补给湖泊主要发生在枯水和退水时期,而湖泊补给地下水主要发生在涨水和高洪水位时期。一般情况下,整个洪泛区地下水位与湖水位的年内变化态势基本一致,主湖区附近的地下水位年内变幅较大,而大部分洪泛区的地下水位变幅相对较小。北部地下水流速明显大于南部,主湖区附近地下水流速明显大于洪泛区,地下水流速基本小于1～2 m/d。水均衡分析发现,洪泛区地下水系统以接受降雨输入（52%）和主湖区补给（39%）为主,以地下水蒸发输出（72%）和向湖排泄（24%）为主,但补给主要发生在春、夏季,而排泄则发生在秋、冬季。地形地貌对洪泛区地下水位分布以及流速场演化具有主控作用,...     

A new approach to numerical simulation of source development of earthquake

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空间-波数域三维大地电磁场积分方程法数值模拟

地球物理勘探中, 地下复杂地质体电磁场三维数值模拟计算量大、存储要求高.针对这一问题, 本文提出一种空间-波数域三维电磁场数值模拟方法, 该方法利用电磁场积分方程为卷积的特点, 将积分沿水平方向进行二维傅里叶变换, 将电磁场三维空间域卷积问题转换为不同波数相对独立的多个一维积分问题, 由此计算量和存储需求大大减少, 易于实现并行计算.采用有限单元法中的形函数进行一维数值积分, 一维积分离散为多个单元积分之和, 每个单元采用二次形函数表征电流变化, 可得出单元积分的解析表达式; 保留垂向为空间域, 优势之一在于可根据实际情况合理调整单元疏密程度, 准确模拟任意复杂地形和导电率异常, 兼顾计算精度与计算效率, 优势之二是用形函数拟合求得积分的解析解, 计算精度和效率高; 最后引入压缩算子, 采用迭代求得电磁场的数值解.本文方法充分利用不同波数之间一维积分高度并行性、一维形函数积分的高精度及快速傅里叶变换的高效性, 实现电磁场三维高效高精度数值模拟.设计模型将本文方法数值模拟结果和软件INTEM3D的数值模拟结果对比, 验证了方法的正确性; 设计高阻和低阻异常体, 研究了异常电导率与背景电导率差异对迭代收敛速度的影响; 改变计算规模, 随着计算网格增多, 算法耗时与存储呈近似线性增长; 设计复杂模型与目前主流数值模拟方法对比, 本文算法速度快1个数量级以上, 且计算规模越大, 算法效率的优势越明显.研究结果表明, 本文提出的空间-波数域三维电磁场数值模拟方法理论和方法正确, 计算效率高, 对计算机存储要求低, 算法高度并行, 适合任意复杂条件大规模三维电磁场高效、高精度数值模拟.

     

提高浅层地震反射静校正精度的一种方法与数值模拟

浅层地震反射法是一种常用的勘探方法.在浅层地震资料处理中,静校正的精度直接影响速度反演的结果和叠加剖面的质量,在地形平缓时,固定基准面静校正可以满足勘探精度的要求,但在复杂地形条件下,其存在较大误差,即使采用浮动基准面,仍会由于地表一致性假设而残余静校正量,不能消除地形起伏引起的影响,为了提高浅层地震反射静校正的精度必须在常规静校正后进行一次剩余静校正,本文给出起伏地形条件下,滑动基准面(过共中心点的水平面即为该共中心点的滑动基准面)的剩余静校正量,该校正量与炮检距、反射层埋深、地层波速以及炮点和接收点高程有关,适用于单一介质和层状介质情况,本文通过对典型地形起伏的3个水平均匀层状介质理论地质模型的速度谱计算和分析,阐明在复杂地形条件下,应用本文提出的剩余静校正方法可以消除地形起伏的影响,提高静校正精度,在此基础上做动校正可以得到高质量的水平叠加剖面.     

CSAMT三维正演数值模拟研究进展

可控源大地电磁(CSAMT)资料的三维正、反演问题已成为国际地球内部电磁感应领域研究的前沿课题.本文介绍了可控源音频大地电磁法中三种主要的数值模拟正演方法,即有限元法、有限差分法,积分方程法,比较了这三种方法的优缺点.积分方程法只需对异常区进行剖分,仅需计算小体积异常区的场,计算速度快,但只适合模拟简单模型,精确度也较低;有限元法与有限差分法虽然精确度较高,但要求对全部区进行离散化,占用的计算机容量较大,计算时间长,因此基于并行算法的三维电磁场正演研究可能是可控源音频大地电磁法未来的发展趋势.     

A numerical laboratory for simulation and visualization of seismic wavefields1

Computational seismic modelling (CSM) plays an important role in the geophysical industry as an established aid to seismic interpreters. Numerical solution of the elastic wave equations has proved to be a very important tool for geophysicists in both forward modelling and migration. Among the techniques generally used in CSM, we consider the finite-element method (FEM) and investigate its computational and visualization requirements. The CSMFEM program, designed for this purpose and developed on an IBM 3090 computer with vector facility, is described in detail. It constitutes a numerical laboratory for performing computer experiments. Two Newmark type algorithms for time integration are compared with other time integration schemes, and both direct and iterative methods for solving the corresponding large sparse system of linear algebraic equations are analysed. Several numerical experiments to simulate seismic energy propagation through heterogeneous media are performed. Synthetics in the form of common shot gathers, vertical seismic profiles and snapshots are suitably displayed, since with the large amounts of data obtained from CSM research, methods for visualization of the computed results must be developed. The FEM is compared with other numerical tools, such as finite-difference and pseudo-spectral methods.     

High-resolution numerical simulation of turbulence in natural waterways

We develop an efficient and versatile numerical model for carrying out high-resolution simulations of turbulent flows in natural meandering streams with arbitrarily complex bathymetry. The numerical model solves the 3D, unsteady, incompressible Navier-Stokes and continuity equations in generalized curvilinear coordinates. The method can handle the arbitrary geometrical complexity of natural streams using the sharp-interface curvilinear immersed boundary (CURVIB) method of Ge and Sotiropoulos (2007) [1]. The governing equations are discretized with three-point, central, second-order accurate finite-difference formulas and integrated in time using an efficient, second-order accurate fractional step method. To enable efficient simulations on grids with tens of millions of grid nodes in long and shallow domains typical of natural streams, the algebraic multigrid (AMG) method is used to solve the Poisson equation for the pressure coupled with a matrix-free Krylov solver for the momentum equations. Depending on the desired level of resolution and available computational resources, the numerical model can either simulate, via direct numerical simulation (DNS), large-eddy simulation (LES), or unsteady Reynolds-averaged Navier-Stokes (URANS) modeling. The potential of the model as a powerful tool for simulating energetic coherent structures in turbulent flows in natural river reaches is demonstrated by applying it to carry out LES and URANS in a 50-m long natural meandering stream at resolution sufficiently fine to capture vortex shedding from centimeter-scale roughness elements on the bed. The accuracy of the simulations is demonstrated by comparisons with experimental data and the relative performance of the LES and URANS models is also discussed.