Kohei Murotani

Watermarking 3D Polygonal Meshes Using the Singular Spectrum Analysis

Watermarking is to embed a structure called a watermark into the target data such as images. The watermark can be used, for example, in order to secure the copyright and detect tampering. This paper presents a new robust watermarking method that adds a watermark into a 3D polygonal mesh in the spectral domain. In this algorithm, a shape of a 3D polygonal model is regarded as a sequence of vertices called a vertex series. The spectrum of the vertex series is computed using the singular spectrum analysis (SSA) for the trajectory matrix derived from the vertex series. Watermarks embedded by this method are resistant to similarity transformations and random noises.

MPS–FEM PARTITIONED COUPLING APPROACH FOR FLUID–STRUCTURE INTERACTION WITH FREE SURFACE FLOW

Fluid–structure interaction analysis involving free surface flow has been investigated using mesh-based methods or mesh-free particle methods. While mesh-based methods have several problems in dealing with the fragmentation of geometry and moving interfaces and with the instability of nonlinear advective terms, mesh-free particle methods can deal with free surface and moving boundary relatively easily. In structural analyses, the finite element method, which is a mesh-based method, has been investigated extensively and can accurately deal with not only elastic problems but also plastic and fracture problems. Thus, the present study proposes a partitioned coupling strategy for fluid–structure interaction problems involving free surfaces and moving boundaries that calculates the fluid domain using the moving particle simulation method and the structure domain using the finite element method. As the first step, we apply a conventional serial staggered algorithm as a weak coupling scheme. In addition, for the verification of the proposed method, the problem of a breaking dam on an elastic wall is calculated, and the results are compared with the results obtained by other methods.

HIERARCHICAL DOMAIN DECOMPOSITION WITH PARALLEL MESH REFINEMENT FOR BILLIONS-OF-DOF SCALE FINITE ELEMENT ANALYSES

This paper describes a parallel fast generation method of large-scale meshes for a hierarchical domain decomposition method implemented in the open source parallel finite element software ADVENTURE. Since large-scale meshes need to be generated in order to perform various analyses in Japans Petaflops Supercomputer, nicknamed the K computer, a mesh refinement function and a communication table generation function without communication are newly developed and implemented for the hierarchical domain decomposition tool named ADVENTURE_Metis. The developed new version is named ADVENTURE_Metis Ver.2. Since a generation cost of a communication table for sending and receiving data among computational nodes becomes so expensive for the refined large-scale mesh, the present authors have newly developed a parallel algorithm such that the communication tables of vertices, edges and faces are updated each other during mesh refinement after the initial communication tables of vertices, edges and faces are generated for an initial mesh. As a result, the generation of a refined mesh model over billions degrees of freedom (DOFs) from an initial medium-size mesh model of about a million DOFs can be performed in a parallel computer in a short time.

Improvement of boundary conditions for non-planar boundaries represented by polygons with an initial particle arrangement technique

ABSTRACT The boundary conditions represented by polygons in moving particle semi-implicit (MPS) method (Koshizuka and Oka, Nuclear Science and Engineering, 1996) have been widely used in the industry simulations since it can simply simulate complex geometry with high efficiency. However, the inaccurate particle number density near non-planar wall boundaries dramatically affects the accuracy of simulations. In this paper, we propose an initial boundary particle arrangement technique coupled with the wall weight function method (Zhang et al. Transaction of JSCES, 2015) to improve the particle number density near slopes and curved surfaces with boundary conditions represented by polygons in three dimensions. Two uniform grids are utilized in the proposed technique. The grid points in the first uniform grid are used to construct boundary particles, and the second uniform grid stores the same information as in the work by Zhang et al. The wall weight functions of the grid points in the second uniform grid are calculated by newly constructed boundary particles. The wall weight functions of the fluid particles are interpolated from the values stored on the grid points in the second uniform grid. Because boundary particles are located on the polygons, complex geometries can be accurately represented. The proposed method can dramatically improve the particle number density and maintain the high efficiency. The performance of the previously proposed wall weight function (Zhang et al.) with the boundary particle arrangement technique is verified in comparison with the wall weight function without boundary particle arrangement by investigating two example geometries. The simulations of a water tank with a wedge and a complex geometry show the general applicability of the boundary particle arrangement technique to complex geometries and demonstrate its improvement of the wall weight function near the slopes and curved surfaces.

G/sup 1/ surface interpolation for irregularly located data

The purpose of this research is to construct a surface (1) passing through all unorganized data points, (2) with G/sup 1/-continuity and (3) with the minimum square-sum of the principal curvatures K/sub 1//sup 2/+K/sub 2//sup 2/ over the surface. In order to construct surfaces with these three characteristics, we construct the triangular mesh spanning the data points, cover it with Bezier patches, achieve continuity between patches, and minimize the curvature to prevent the surfaces from having flat places and unnecessary undulations. The performance of the proposed method is evaluated by computational experiments.

Tsunami Run-Up Simulation

Ishinomaki city was severely damaged by the tsunami of the Great East Japan Earthquake on 2011. Our target is to simulate impact by the tsunami run-up with many floating objects on coastal areas and an urban area of Ishinomaki. Zoom-up analysis by three analyses stages is adopted to solve a large area from an epicenter to an urban area. In the first stage, the two-dimensional shallow-water analysis is solved from the epicenter to the coastal areas. In the second and third stages, the three-dimensional tsunami run-up analyses are solved for the coastal areas using the hierarchical domain decomposition explicit moving particle simulation (MPS) method. Since our target is the tsunami run-up analysis in the urban area in the third stage, we performed three kinds of tsunami analyses. The first analysis was the two tanks floating between buildings. The second analysis was the analysis of 431 floating objects. The third analysis was the elastic analysis for buildings by fluid pressure.

High-Accuracy Electromagnetic Field Simulation Using Numerical Human Body Models

We have investigated a high-accuracy analysis for the electromagnetic field of numerical human body models using the finite-element method. The numerical human body models generated from computed tomography images are represented as voxel data, and composed of skin layers, blood vessels, bones, internal organs, and so on. In this paper, we propose a mesh smoothing technique to reduce the noise caused by reflection and scattering of the electric fields around material boundaries. Therefore, using a supercomputer, we successfully evaluated the electromagnetic field distribution inside the whole body model with a high accuracy.

A Hybrid Finite Element and Mesh-Free Particle Method for Disaster-Resilient Design of Structures

The MPS-FE method, which is a hybrid method for Fluid-Structure Interaction (FSI) problems adopting the Finite Element method (FEM) for structure computation and Moving Particle Semi-implicit/Simulation (MPS) methods for free surface flow computation, was developed to utilize it in disaster-resilient design of important facilities and structures. In general free-surface flow simulation using the MPS method, wall boundaries are represented as fixed particles (wall particles) set as uniform grids, so the interface of fluid computation does not correspond to the interface structure computation in the conventional MPS-FE method. In this study, we develop an accurate and robust polygon wall boundary model, named Explicitly Represented Polygon (ERP) wall boundary model, in which the wall boundaries in the MPS method can be represented as planes that have same geometries as finite element surfaces.

Inundation Simulation Coupling Free Surface Flow and Structures

Water-related disasters such as tsunamis, storm surges, and floods involve fluid–structure interaction (FSI) problems with free surface flow. Since failures of artifacts are caused due to inundation, water forces and impact forces by floating objects, simulation of such problems has great importance to design for safety and robustness.In this chapter, we present a robust and efficient coupled method for fluid–structure interaction with violent free surface flow, named the MPS-FE method and its improved method. The MPS-FE method adopts the finite element (FE) method for structure computation and the moving particle semi-implicit/simulation (MPS) method for fluid computation involving free surface flow. The conventional MPS-FE method, in which MPS wall boundary particles and finite elements are overlapped in order to exchange information at fluid–structure interface, is not versatile and reduces the advantages of software modularity. We developed a non-overlapping approach in which the interface in the fluid computation corresponds to that in the structure computation through an MPS polygon wall model. The accuracy of the improved MPS-FE method was verified by solving a dam break problem with an elastic obstacle and by comparing the result obtained with that of the conventional MPS-FE method and other methods.

Parallel Analysis System for Fluid–Structure Interaction with Free-Surfaces Using ADVENTURE_Solid and LexADV_EMPS

In this chapter, we present a parallel analysis system for fluid–structure interaction (FSI) analysis with free-surfaces. It is based on a method that uses the moving-particle semi-implicit/simulation (MPS) method for flow computations and the finite element (FE) method for structural computations. The MPS-FE method is an efficient and robust approach for FSI problems involving free-surface flow. To develop the system presented herein, we use two existing open-source software modules: ADVENTURE_ Solid, a large-scale FE solver for structural computations; and LexADV_ EMPS, a library for large-scale MPS computations for free-surface flow. The explicitly represented polygon (ERP) wall boundary model employed in LexADV_ EMPS is accurate and stable, and it expresses wall boundaries as a set of arbitrarily shaped triangular polygons with appropriately imposed boundary conditions. Thus, when the ERP is used, in both the fluid and the structure computations, the fluid–structure interfaces are matched, and therefore, preprocessing of the data for FSI analysis is greatly facilitated. We demonstrate the applicability of the developed system by solving a dam-break problem with an elastic obstacle.