Development of a Three-Dimensional Multiscale Octree SBFEM for Viscoelastic Problems of Heterogeneous Materials

The multiscale method provides an effective approach for the numerical analysis of heterogeneous viscoelastic materials by reducing the degree of freedoms(DOFs).A basic framework of the Multiscale Scaled Boundary Finite Element Method(MsSBFEM)was presented in our previous works,but those works only addressed two-dimensional problems.In order to solve more realistic problems,a three-dimensional MsSBFEM is further developed in this article.In the proposed method,the octree SBFEM is used to deal with the three-dimensional calculation for numerical base functions to bridge small and large scales,the three-dimensional image-based analysis can be conveniently conducted in small-scale and coarse nodes can be flexibly adjusted to improve the computational accuracy.Besides,the Temporally Piecewise Adaptive Algorithm(TPAA)is used to maintain the computational accuracy of multiscale analysis by adaptive calculation in time domain.The results of numerical examples show that the proposed method can significantly reduce the DOFs for three-dimensional viscoelastic analysis with good accuracy.For instance,the DOFs can be reduced by 9021 times compared with Direct Numerical Simulation(DNS)with an average error of 1.87%in the third example,and it is very effective in dealing with three-dimensional complex microstructures directly based on images without any geometric modelling process.

Improved lazy theta algorithm based on octree map for path planning of UAV

This paper investigates the path planning method of unmanned aerial vehicle(UAV)in threedimensional map.Firstly,in order to keep a safe distance between UAV and obstacles,the obstacle grid in the map is expanded.By using the data structure of octree,the octree map is constructed,and the search nodes is significantly reduced.Then,the lazy theta*algorithm,including neighbor node search,line-of-sight algorithm and heuristics weight adjustment is improved.In the process of node search,UAV constraint conditions are considered to ensure the planned path is actually flyable.The redundant nodes are reduced by the line-of-sight algorithm through judging whether visible between two nodes.Heuristic weight adjustment strategy is employed to control the precision and speed of search.Finally,the simulation results show that the improved lazy theta*algorithm is suitable for path planning of UAV in complex environment with multi-constraints.The effectiveness and flight ability of the algorithm are verified by comparing experiments and real flight.

Database-oriented storage based on LMDB and linear octree for massive block model

Data organization requires high efficiency for large amount of data applied in the digital mine system. A new method of storing massive data of block model is proposed to meet the characteristics of the database, including ACID-compliant, concurrency support, data sharing, and efficient access. Each block model is organized by linear octree, stored in LMDB（lightning memory-mapped database）. Geological attribute can be queried at any point of 3D space by comparison algorithm of location code and conversion algorithm from address code of geometry space to location code of storage. The performance and robustness of querying geological attribute at 3D spatial region are enhanced greatly by the transformation from 3D to 2D and the method of 2D grid scanning to screen the inner and outer points. Experimental results showed that this method can access the massive data of block model, meeting the database characteristics. The method with LMDB is at least 3 times faster than that with etree, especially when it is used to read. In addition, the larger the amount of data is processed, the more efficient the method would be.

基于TIN-Octree的三维地质模型构建方法研究

在三维地质模型中,不能有效的分析地层与断层之间的形态与结构,提出了基于TIN-Octree混合空间数据模型的精细三维地质体构建方法。分析两者的地质形态与连接状态,形成良好的数据映射剖面,并利用Octree的编码特性,增加对象的描述精度以及存储性能,可以有效地为油气勘探开发提供可视化效果。并结合实际地质地震勘探解释成果资料的应用,表明该模型构建方法可以有效地建立地质体三维模型,具有良好的应用价值。

Volumetric extraction of porous materials based on octree algorithm

Through the octree data structure analysis,a volumetric dataset of closed-cell porous materials is converted into a dataset of hierarchical octree nodes,and then the specific traversal search algorithm on the octree nodes is depicted in details,which is involved in six steps of the volume growth model and one step of the volume decomposition model.Moreover,the conditions of both the proceeding traversal and three possibilities of terminating are given,and the traversal algorithm of completeness is proved from a theoretical perspective.Finally,using a simulated volumetric dataset of columnar pores,the extracting effectiveness of the octree traversal algorithm is verified.The results show that the volume and the distribution information of pores can be successfully extracted by the proposed algorithm,which builds a solid foundation for a more effective performance analysis of porous materials.

3D Simulation of Flow with Free Surface Based on Adaptive Octree Mesh System

The technique of adaptive tree mesh is an effective way to reduce computational cost through automatic adjustment of cell size according to necessity. In the present study, the 2D numerical N-S solver based on the adaptive quadtree mesh system was extended to a 3D one, in which a spatially adaptive oetree mesh system and multiple particle level set method were adopted for the convenience to deal with the air-water-structure multiple-medium coexisting domain. The stretching process of a dumbbell was simulated and the results indicate that the meshes are well adaptable to the free surface. The collapsing process of water column impinging a circle cylinder was simulated and from the results, it can be seen that the processes of fluid splitting and merging are properly simulated. The interaction of second-order Stokes waves with a square cylinder was simulated and the obtained drag force is consistent with the result by the Morison＇s wave force formula with the coefficient values of the stable drag component and the inertial force component bein~ set as 2.54.

树网格离散优化三维瞬变电磁有限体积正演算法

三维瞬变电磁正演算法作为研究处理与解释方法的重要基础,如何加速计算过程,减少计算机内存消耗尤为重要.为此,本文采用有限体积算法在八叉树(octree)网格上对时间域Maxwell方程组进行空间离散,相比于交错六面体网格,octree网格在局部区域网格细度相同的条件下可以显著减少细化区域外的网格数量,对复杂几何体边界的模拟更加灵活,而相比于非结构四面体网格,octree网格单元位置排列更加规律.通过octree网格离散三维正演模型使计算网格规模显著降低,减少了待求解方程的未知数,降低了物理内存消耗.空间离散后,瞬变电磁正演响应可以表示为关于初始磁场的矩阵指数函数,采用位移逆Krylov子空间模型降阶算法实现瞬变电磁场的求解,只需对系数矩阵进行一次矩阵分解和多次回代即可获得一系列时间序列的瞬变电磁场正演结果.本文算法在空间离散和方程求解两方面优化三维瞬变电磁正演过程,数值算例结果验证了本文算法的精度和高效性.

基于改进PointNet++的大规模猪体点云部位分割

PointNet++点云分割网络能直接处理点云并有良好的分类和分割效果,然而对于自由运动状态下获取的大型牲畜点云,存在较大姿态差异,PointNet++无法难以直接捕捉其局部特征,对牲畜体点云各部位分割效果不佳。针对此问题论文采用Octree结构改进了PointNet++集合抽象层中的分组与采样层,使得网络能够更好地捕捉目标点云的非刚性变化,充分提取不同层次下的局部信息,实现对大规模牲畜点云数据的自动分割。采用Octree改进的PointNet++模型分割活体猪点云数据的头部、耳朵、尾部、躯体和四腿。实验表明,500组猪体点云数据,采用320组不同姿态下的猪体点云进行网络训练,180组分割测试结果的平均mIoU达到了88.06%,OA达到了96.57%。

基于动态四叉树的数控铣削加工仿真的研究

提出采用基于自相似动态四叉树(Quadtrees)的思想进行实体造型,实现数控仿真加工毛坯实体的动态建模,从而在一定程度上解决数控加工仿真过程中毛坯除料算法的运算速度缓慢而造成的真实感差的问题,并通过一个零件加工仿真验证了该算法的可行性和有效性。

大量点云数据跨平台可视化实时交互方法

针对逆向工程中大量三维点云数据的跨平台可视化交互难以实时渲染的问题,提出一种多层次动态调度绘制方法。对传统八叉树改进后对点云数据建立邻域空间索引编码,实现空间整体对点云分层划分,使用"分区-子块-点云"预加载调度策略提高海量点云交互效率,结合细节层次(level-of-detail,LOD)控制技术和浏览器Ajax异步调度点云数据控制策略,算法提高了空间查询效率,加快了点云绘制速度,并有效的降低视点移动时的过渡走样现象。实验结果表明:使用算法在高层次细节渲染帧率最终维持在25fps左右,浏览器内存消耗比未处理数据完全加载降低48.5%,实验结果验证了该方法的可行性并取得良好的网络交互三维可视化体验。

基于Shear-warp的交互式海量数据场体绘制算法

提出了基于min-max Octree快速分类Shear-warp的交互式海量数据场体绘制算法。主要包括根据海量数据的特征,快速读入数据以及设计适当的不透明度传递函数;建立Summed-Area表与min-max Octree数据结构,并对体数据进行快速分类,然后,利用分类的结果进行快速体绘制。实验证明该方法不仅效率高,而且显示效果好。

基于点云配准的曲面三维差异检测

在工件打磨任务中,实测数据与标准模型数据之间的差异性比较是实现机器人自动化打磨的关键研究问题之一。针对如何快速精确地提取陶瓷素胚工件曲面的三维差异数据问题,设计了一种曲面配准流程。首先,针对点云的粗对齐步骤,采用体素网格滤波进行下采样,采用了Hough投票法进行粗配准,将粗配准步骤运行时间控制在0.3 s以内。其次,针对点云的精确配准步骤,利用了CUDA加速比较耗时的ICP配准算法,将精确配准耗时控制在0.5 s以内。最后,针对差异提取步骤,通过引入Octree数据结构实现了差异点云的快速提取。最终,将模型曲面点云与扫描曲面点云之间的RMS误差控制在0.5 mm内,总运行时间控制在1 s之内,实现了两个曲面的快速精确对齐以及差异提取。

DEVELOPMENT OF A HYBRID MODEL FOR THREE-DIMENSIONAL GIS

This paper presents a hybrid model for three-dimensional Geographical Information Systems which is an integration of surface- and volume-based models. The Triangulated Irregular Network (TIN) and octree models are integrated in this hybrid models. The TIN model works as a surface-based model which mainly serves for surface presentation and visualization. On the other hand, the octree encoding supports volumetric analysis. The designed data structure brings a major advantage in the three-dimensional selective retrieval. This technique increases the efficiency of three-dimensional data operation.

Some improvements of the marching cubes algorithm for the rendering of an orebody

Orebody-rendering techniques are developed using the marching cubes (MC) algorithm. The shape of an orebody is viv- idly displayed in real time and can be used to guide mining design as well. The MC algorithm has been improved in two aspects. By analyzing the principles of the MC algorithm, as well as the features of the specific application, improvements were developed to: eliminate ambiguities by using a unified isosurface constructing method in the voxels, and improve the operating efficiency of the MC algorithm by incorporating an octree structure. The analytical results of the examples demonstrate the effectiveness of our proposal.

Finite cell method compared to h-version finite element method for elasto-plastic problems

The finite cell method （FCM） combines the high-order finite element method （FEM） with the fictitious domain approach for the purpose of simple meshing. In the present study, the FCM is used to the Prandtl-Reuss flow theory of plasticity, and the results are compared with the h-version finite element method （h-FEM）. The numerical results show that the FCM is more efficient compared to the h-FEM for elasto-plastic problems, although the mesh does not conform to the boundary. It is also demonstrated that the FCM performs well for elasto-plastic loading and unloading.

SIMULATION SYSTEM FOR FIVE-AXIS NC MACHINING USING GENERAL CUTTING TOOL

A simulation system for five axis NC machining using general cutting tools is presented This system differs from other simulation system in that it not only focuses on the geometric simulation but also focuses on the collision detection which is usually not included in NC machining simulation Besides all of these, estimating cutting forces is also discussed In order to obtain high efficiency, all algorithms use swept volume modeling technique, so the simulation system is compact and can be performed efficiently

复杂外形飞机绕流数值模拟研究

飞机复杂外形的绕流数值模拟在飞机型号研制中有重要的作用 ,也是数值模拟中比较复杂的问题之一。本文采用基于Octree技术的方法生成非结构四面体网格 ,用耦合隐式离散的方法进行有限体积离散 ,利用此方法对某型飞机的绕流特性进行了数值模拟 ,收到较好效果。

Visibility Culling Algorithm with Hierarchical Adaptive Bounding Setting

Hierarchical adaptive bounding setting （HABS）, a novel algorithm to reduce potentially visible set, is designed to be Used for various geometry shape character in complex simulation scene to greatly improve geometry spatial storage precision. A spatial hierarchy tree is used to represent the topology of the model, and then the visibility geometry set from the viewpoint is determined by processing the hierarchy tree and frustum detection. In this process, HABS improves the viewpoint-to-region visibility detection efficiently. The algorithm is well-suited for complex models whose shape characters are various.

Spatio-temporal model for soil characteristic of reclamation land

The development of spatio-temporal data model is introduced. According to the soil characteristic of reclamation land, we adopt the base state with amendments model of multi-layer raster to organize the spatio-temporal data, using the combined data structure on linear quadtree and linear octree to code. The advantage of this model is that it can easily obtain the information of certain layer and integratedly analyze the data with other methods. Then, the methods of obtain and analyses are introduced. The method can provide a tool for the research of the soil characteristic change and spatial distribution in reclamation land.

Efficient Hierarchical Structure of Wavelet-Based Compression for Large Volume Data Sets

With volume size increasing, it is necessary to develop a highly efficient compression algorithm, which is suitable for progressive refinement between the data server and the browsing client. For three-dimensional large volume data, an efficient hierarchical algorithm based on wavelet compression was presented, using intra-band dependencies of wavelet coefficients. Firstly, after applying blockwise hierarchical wavelet decomposition to large volume data, the block significance map was obtained by using one bit to indicate significance or insignificance of the block. Secondly, the coefficient block was further subdivided into eight sub-blocks if any significant coefficient existed in it, and the process was repeated, resulting in an incomplete octree. One bit was used to indicate significance or insignificance, and only significant coefficients were stored in the data stream. Finally, the significant coefficients were quantified and compressed by arithmetic coding. The experimental results show that the proposed algorithm achieves good compression ratios and is suited for random access of data blocks. The results also show that the proposed algorithm can be applied to progressive transmission of 3D volume data.