The simulation of surface flow dynamics using a flow-path network model

This paper proposes a flow-path network (FPN) model to simulate complex surface flow based on a drainage-constrained triangulated irregular network (TIN). The TIN was constructed using critical points and drainage lines extracted from a digital terrain surface. Runoff generated on the surface was simplified as ‘water volumes’ at constrained random points that were then used as the starting points of flow paths (i.e. flow source points). The flow-path for each ‘water volume’ was constructed by tracing the direction of flow from the flow source point over the TIN surface to the stream system and then to the outlet of the watershed. The FPN was represented by a set of topologically defined one-dimensional line segments and nodes. Hydrologic variables, such as flow velocity and volume, were computed and integrated into the FPN to support dynamic surface flow simulation. A hypothetical rainfall event simulation on a hilly landscape showed that the FPN model was able to simulate the dynamics of surface flow over time. A real-world catchment test demonstrated that flow rates predicted by the FPN model agreed well with field observations. Overall, the FPN model proposed in this study provides a vector-based modeling framework for simulating surface flow dynamics. Further studies are required to enhance the simulations of individual hydrologic processes such as flow generation and overland and channel flows, which were much simplified in this study.     

Linear irregular wave generation in a numerical wave tank

In the design of any floating or fixed marine structure, it is vital to test models in order to understand the fluid/structure interaction involved. A relatively inexpensive method, compared to physical model testing, of achieving this is to numerically model the structure and the wave conditions in a numerical wave tank. In this paper, a methodology for accurately replicating measured ocean waves in a numerical model at full scale is detailed. A Fourier analysis of the measured record allows the wave to be defined as a summation of linear waves and, therefore, Airy's linear wave theory may be used to input the wave elevation and associated water particle velocities. Furthermore, a structure is introduced into the model to display the ability of the model to accurately predict wave–structure interaction. A case study of three individual measured waves, which are recorded at the Atlantic marine energy test site, off the west coast of Ireland, is also presented. The accuracy of the model to replicate the measured waves and perform wave–structure interaction is found to be very high. Additionally, the absolute water particle velocity profile below the wave from the numerical model is compared to a filtered analytical approximation of the measured wave at a number of time-steps and is in very good agreement.     

Embedded star clusters and the formation of the Oort Cloud

Observations suggest most stars originate in clusters embedded in giant molecular clouds [Lada, C.J., Lada, E.A., 2003. Annu. Rev. Astron. Astrophys. 41, 57-115]. Our Solar System likely spent 1-5 Myrs in such regions just after it formed. Thus the Oort Cloud (OC) possibly retains evidence of the Sun's early dynamical history and of the stellar and tidal influence of the cluster. Indeed, the newly found objects (90377) Sedna and 2000 CR₁₀₅ may have been put on their present orbits by such processes [Morbidelli, A., Levison, H.F., 2004. Astron. J. 128, 2564-2576]. Results are presented here of numerical simulations of the orbital evolution of comets subject to the influence of the Sun, Jupiter and Saturn (with their current masses on orbits appropriate to the period before the Late Heavy Bombardment (LHB) [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]), passing stars and tidal force associated with the gas and stars of an embedded star cluster. The cluster was taken to be a Plummer model with 200-400 stars, with a range of initial central densities. The Sun's orbit was integrated in the cluster potential together with Jupiter and Saturn and the test particles. Stellar encounters were incorporated by directly integrating the effects of stars passing within a sphere centred on the Sun of radius equal to the Plummer radius for low-density clusters and half a Plummer radius for high-density clusters. The gravitational influence of the gas was modeled using the tidal force of the cluster potential. For a given solar orbit, the mean density, 〈ρ〉, was computed by orbit-averaging the density of material encountered. This parameter proved to be a good measure for predicting the properties of the OC. On average 2-18% of our initial sample of comets end up in the OC after 1-3 Myr. A comet is defined to be part of the OC if it is bound and has q>35 AU. Our models show that the median distance of an object in the OC scales approximately as 〈ρ〉^−1/2 when . Our models easily produce objects on orbits like that of (90377) Sedna [Brown, M.E., Trujillo, C., Rabinowitz, D., 2004. Astrophys. J. 617, 645-649] within ∼1 Myr in cases where the mean density is or higher; one needs mean densities of order to create objects like 2000 CR₁₀₅ by this mechanism, which are reasonable (see, e.g., Guthermuth, R.A., Megeath, S.T., Pipher, J.L., Williams, J.P., Allen, L.E., Myers, P.C., Raines, S.N., 2005. Astrophys. J. 632, 397-420). Thus the latter object may also be part of the OC. Close stellar passages can stir the primordial Kuiper Belt to sufficiently high eccentricities (e?0.05; Kenyon, S.J., Bromley, B.C., 2002. Astron. J. 123, 1757-1775) that collisions become destructive. From the simulations performed it is determined that there is a 50% or better chance to stir the primordial Kuiper Belt to eccentricities e?0.05 at 50 AU when . The orbit of the new object 2003 UB₃₁₃ [Brown, M.E., Trujillo, C.A., Rabinowitz, D.L., 2005. Astrophys. J. 635, L97-L100] is only reproduced for mean cluster densities of the order of , but in the simulations it could not come to be on its current orbit by this mechanism without disrupting the formation of bodies in the primordial Kuiper Belt down to 20 AU. It is therefore improbable that the latter object is created by this mechanism.     

Ince-Strutt Stability Charts for Ship Parametric Roll Resonance in Irregular Waves

Ince-Strutt stability chart of ship parametric roll resonance in irregular waves is conducted and utilized for the exploration of the parametric roll resonance in irregular waves. Ship parametric roll resonance will lead to large amplitude roll motion and even wreck. Firstly, the equation describing the parametric roll resonance in irregular waves is derived according to Grim’s effective theory and the corresponding Ince-Strutt stability charts are obtained. Secondly, the differences of stability charts for the parametric roll resonance in irregular and regular waves are compared. Thirdly, wave phases and peak periods are taken into consideration to obtain a more realistic sea condition. The influence of random wave phases should be taken into consideration when the analyzed points are located near the instability boundary. Stability charts for different wave peak periods are various. Stability charts are helpful for the parameter determination in design stage to better adapt to sailing condition. Last, ship variables are analyzed according to stability charts by a statistical approach. The increase of the metacentric height will help improve ship stability.     

不规则波与开孔沉箱相互作用数值模拟

采用三步有限元法对N-S方程进行离散,同时借助CLEAR-VOF方法追踪流体自由表面,利用主动吸收式造波等手段改进了二维不规则波浪数值水槽,使得水槽中的波浪谱与目标靶谱吻合较好。进而建立了不规则波浪与开孔沉箱作用一种新的数值模式,分析研究不规则波作用下开孔沉箱的反射率,并与现有的物模结果和数模结果进行了对比,为不规则波与开孔沉箱作用问题的研究,探求了一种新的数值手段。     

Placement Effect on the Stability of Tetrapod Armor Unit on Breakwaters in Irregular Waves

Tetrapod, one of the well-known artificial concrete units, is frequently used as an armor unit on breakwaters. Two layers of tetrapod units are normmaly placed on the breakwaters with different placement methods. In this study, the stability of tetrapod units with two different regularly placement methods are investigated experimentally in irregular waves. Stability coefficients of tetrapod units for both placement methods are obtained. The important characteristic wave parameters of irregular waves causing the same damage ratio as those of the regular waves are also determined. It reveals that the average of one-tenth highest wave heights within the wave train (H_1/10) causes the similar damage as regular waves.     

全空间下并行矢量空间分析研究综述与展望

新一代并行空间分析将面临空间大数据分析和实时空间分析服务的挑战。矢量空间计算作为GIS系统中的重要组成部分,在并行化算法设计中存在负载不均,并行扩展性差,IO性能低等技术瓶颈。本文首先从应用需求和技术发展的演变历史回顾了矢量空间分析算法发展过程;然后,从研究现状的角度详细阐述了并行矢量空间分析计算的研究成果,总结了并行空间分析算法的算法特征和技术瓶颈,对不同并行编程模型进行了对比,并提出了并行空间分析算法的研发流程;最后,从发展前景的角度预测了全空间信息系统中基于多粒度时空对象的空间数据模型和计算方法的发展趋势,提出了以内存计算等技术实现存算一体化的新型空间数据模型和分析方法的技术趋势。     

面向全空间信息系统的多粒度时空对象数据模型描述框架

全空间信息系统是一种面向从微观到宏观的动态复杂世界的空间信息系统,其理论基础是多粒度时空对象数据模型。为了确定多粒度时空对象数据模型的具体内容,需要确定描述多粒度时空对象特征的基本框架;为了开展多粒度时空对象的实际建模,需要明确多粒度时空对象数据模型的建模过程。为此,本文首先从数据模型、数据管理、可视化、空间分析和实际应用5个方面,分析了全空间信息系统与传统GIS的联系与区别,从空间范畴、动态变化、复杂关系、认知与行为、可视化技术、时空大数据分析6个方面,分析了传统GIS空间数据模型存在的不足;在此基础上,提出了多粒度时空对象的多粒度、多类型、多形态、多参照系、多元关联、多维动态、多能自主7个特点,确定了由时空参照、空间位置、空间形态、组成结构、关联关系、认知能力、行为能力和属性特征8项内容构成的多粒度时空对象数据模型描述框架;最后在分析了传统GIS空间数据模型建模过程的基础上,提出了多粒度时空对象数据模型的建模过程和思路。     

基于Supermap Objects的正射影像数据库管理系统的设计与实现

通过莆田市正射影像数据库管理系统的建设实践,论述述了基于Supermap Objects和Oracle的数字正射影像数据库管理系统的总体结构、数据库和管理系统功能设计,详细阐述了数据库的逻辑结构、影像管理方式的选择、影像数据组织、系统功能的实现。     

一种基于三角网扩张法的Delaunay三角网逐块归并算法

本文中提出一种基于三角网扩张法的不规则三角网的逐块归并算法,它采用按横向或纵向对离散点集切割分块,对各子块用三角网扩张法构建Delaunay三角网,最后用三角网扩张法依次将相邻的子网合并。该算法采用的子集分块的做法使构网时的搜索范围减小,在数据量较大时避免了计算时间随点数的指数次增加,同时在构网时也能保证三角形邻接关系的正确维护。