Research And Implementation Of Grid Design Method For Free-Form Grid Structures

Due to the complex shape, grid design of free-form spatial grid structures is quite difficult. However, most of the research about grid design aims at one particular project, and cannot be applied to other structures. Therefore, grid design method for free-form structures is studied in this dissertation. Some new methods are proposed to surface grid generation, and some existing methods are developed to design grids on point cloud data.Based on the former study, the concept of free-form spatial grid structures is explained in this dissertation, and the application of free-form spatial grid structures is introduced. Geometry design methods of free-form structures and tasks of grid design are summarized. An overview of grid generation and grid optimization method is given, and the content of this dissertation is defined.Free-form surface modeling technique is based on B-spline method. The definition and properties of B-spline,. B-spline curve and surface and Non-Uniform Rational B-Splines (NURBS) are stated, and the calculation methods for projection and reverse projection of B-spline curves and surfaces are introduced in the dissertation. A data connector for IGES files is developed to obtain surface information from kinds of modeling software systems.Based on the acquirement of spatial grid structures, a new method for free-form surface grid generation, named isoparametric line dividing method, is proposed. The fundamental of the method is explained, which is to extract isoparametric lines from surfaces and divide these lines to create grids. The algorithm process for dividing a isoparametric line is specified.Grid generation on simple free-form surfaces and complex free-form surfaces is performed using isoparametric line dividing method. Simple surfaces are classified into three types according to their outline shapes, which is quadrilateral-outlined surface, triangular-outlined surface, and circle-outlined. surface. And grid patterns are designed aiming at different surface types. The approach to generate complex surfaces and some important concepts related is introduced. The boundary lines are sorted, and the relationship between the boundary lines and associate lines are explained. Fundamental and optimization operation for grid generation is specified in detail.Grid generation method for3D point cloud is studied. The Advancing Front Technique (AFT) widely used in FEM is applied. Searching box data structure is brought in and procedure for point cloud boundary detection is stated. The algorithm process for creating new nodes during front advancing is explained. And at the end grid generation algorithm is concluded.After grid generation by AFT, grid optimization is studied, including Delaunay optimization and Laplacian optimization. The fundamental for both method is introduced and the improved Laplacian method is stated. Grid modification methods on behalf of point constraint and line constraint are proposed. And physical models are made by two ways to show the overall process of grid design based on3D point cloud.According to the theories and algorithms mentioned above, grid design program for free-form grid design is developed. It achieved the function of grid design of free-form surfaces and free-form point cloud. At the end of this paper, conclusions and issues that should be studied further are summarized.