基于面网格的物理仿真方法及其在数字化制造中的应用

发布时间：2018-07-07 23:32

本文选题：流体仿真 + 表面张力　；参考：《浙江大学》2016年博士论文

【摘要】：物理仿真是基于真实世界物理规律,在计算机中虚拟地重现流体运动、弹性体形变等各种物理现象的过程。由于物理仿真能够达到很高的精度,而成本比物理实验要低,因此被广泛应用于航空航天、机械制造、生物医药等诸多领域的前期研究。影视制作中许多难以实际拍摄的场景,也是通过对流水、风沙、火焰等现象进行物理仿真,并结合高质量的渲染器,制作出的视觉特效。近年来,随着3D打印技术的发展和普及,数字化制造再次成为前沿的研究领域。将物理仿真应用于3D打印,对模型进行结构力学分析,可以及时发现设计缺陷,保证制造出模型的结构稳定;通过运动学分析,可以制造出具有特殊运动规律的模型。物理仿真的应用极大提高了数字化制造的水平。面网格在物理仿真中有着非常重要的作用。首先,薄层结构的物体(如纸张、布料、板材、气泡等)通常都是用面网格来直接表示。其次,面网格可以表示模型的表面形状,因此被广泛用于碰撞检测以及场景的实时显示。对于流体等无固定形状的物质,还可以用动态面网格追踪其自由液面。尽管自然界中真实物体都具有一定的体积,但是体网格拓扑结构和计算往往都比较复杂。因此,如果能用面网格对计算模型进行近似,就可以显著减少运算量,提高仿真速度。本文用面网格对水滴、粘性流体薄膜、塑性薄膜等三种物质进行了建模和仿真。水滴体积很小,其形状主要受表面张力的影响,因此可以忽略水滴内部的流体运动,通过面网格的直接形变进行仿真。水滴之间的融合与分裂,可以通过面网格的布尔运算及网格优化来实现。粘性流体薄膜和塑性薄膜都是薄层结构,因此用面网格可以很好地对其进行仿真。我们将其仿真技术应用于两种传统制造工艺的虚拟化:水转印和热塑成型,提出“可计算水转印刷”和“可计算热塑成型”的方法,使其能够用于三维曲面全彩着色。我们研制了这些新型工艺的原型系统,并通过实验证明了其实用性和可靠性。本文的主要贡献如下:·提出了一种基于面网格的实时水滴仿真方法。通过对水滴表面网格直接形变,对水滴的运动、水滴与亲水表面接触,以及水滴在固体表面滑动的现象进行仿真。该方法把流体仿真从三维体素简化为面网格的形变,从而使计算量大幅减少,不仅能够实时仿真,还可以让用户与水滴进行交互。·通过面网格对粘稠流体薄膜漂浮在水面上的现象进行仿真,模拟物体经过薄膜浸入水中,薄膜被拉伸并贴在物体表面的过程。我们将该现象的仿真应用于曲面着色工艺——水转印的虚拟化流程,提出“可计算水转印刷”的方法,解决了传统水转印工艺中对复杂曲面着色时,图案与模型难以精确对齐的问题。我们搭建了计算水转印的原型系统,实现了虚拟仿真、系统标定、运动控制的集成。针对复杂模型水转印过程中,水转印膜拉伸过大导致颜色失真的缺陷,我们提出了多次转印的方法。每次只在一个方向对模型局部着色,多次转移之后图案相叠加,在模型表面上共同形成目标纹理。·通过面网格对塑性薄膜进行建模,模拟软化塑料片在大气压力下产生形变,并贴在物体表面的过程。我们将该现象的仿真应用于塑料成型工艺——真空热塑成型的虚拟化流程,提出“可计算热塑成型”的方法,并结合3D打印,将带纹理的虚拟数字化模型制作成为实物模型。首先,将数字化模型作为模具,对真空成型的过程进行虚拟仿真。根据塑料片形变的仿真结果,计算出预形变图案,将其打印在透明塑料片上。通过对抽真空过程的仿真,找出塑料片与模具之间可能出现的气体空腔,据此在模型上设置排气孔,并通过3D打印制作出带有排气孔的模具。我们对一台小型真空成型机进行了改造,使塑料片上的图案能够与物体精确对齐。经过真空成型,打印的纹理就贴在了模具的表面,同时塑料片也为图案覆盖了一层透明保护壳,最终得到与数字化模型相同的实物模型。
[Abstract]:Physical simulation is a process based on real world physical laws to reproduce a variety of physical phenomena, such as fluid motion, elastic body shape, and so on in a computer. Physical simulation can achieve high precision, and the cost is lower than physical experiment. So it is widely used in the early stage of aerospace, mechanical manufacturing, biological medicine and many other fields. Research. Many scenes which are difficult to be photographed in film and television production are also physical simulation through the phenomena of water, wind sand, flame and so on. The visual effects are made with high quality renderers. In recent years, with the development and popularization of 3D printing technology, digital manufacturing has become the frontier of research again. The physical simulation is applied to the 3D The structural mechanics analysis of the model can detect the design defects in time and ensure the structure stability of the model. Through the kinematic analysis, a model with special motion laws can be produced. The application of physical simulation greatly improves the level of digital manufacturing. The surface grid has a very important role in the physical simulation. First, thin structure objects (such as paper, cloth, plate, bubble, etc.) are usually expressed directly by surface mesh. Second, the surface mesh can represent the surface shape of the model, so it is widely used for collision detection and real-time display of the scene. The free surface can be traced with a dynamic surface mesh for materials such as fluid and so on. Although the real objects in the natural world have a certain volume, the topology and calculation of the body mesh are often complicated. Therefore, if the surface mesh can be used to approximate the calculation model, the computation can be reduced and the speed of the simulation can be improved. In this paper, three materials, such as water droplets, viscous fluid film, plastic film and so on, are used in this paper. The volume of water droplets is very small and its shape is mainly influenced by the surface tension. Therefore, the fluid movement inside the water droplets can be ignored and the direct deformation of the surface mesh is simulated. The fusion and splitting of water droplets can be realized by the Boolean operation and the mesh optimization of the surface mesh. Both viscous fluid film and plastic film can be achieved. It is a thin layer structure, so the surface mesh can be well simulated. We apply its simulation technology to the virtualization of two traditional manufacturing processes: water transfer and thermoplastic molding, the method of "computable water transfer printing" and "computable thermoplastic molding", so that it can be used in full color coloring of 3D surfaces. We developed this method. Some new technology prototype systems have been proved to be practical and reliable. The main contributions of this paper are as follows:. A real-time water drop simulation method based on surface mesh is proposed. By direct deformation of the surface grid of water droplets, the movement of water droplets, the contact of water droplets with the surface of hydrophilic surface, and the sliding of water droplets on the solid surface. Simulation. This method simplifies the fluid simulation from the three-dimensional voxel to the deformation of the surface mesh, so that the calculation can be reduced greatly. It can not only simulate the real time, but also allow users to interact with the water droplets. The process of being stretched and attached to the surface of an object. We applied the simulation of the phenomenon to a surface coloring process, a virtual process of water transfer, and proposed a "computable water transfer printing" method to solve the problem of precise alignment between patterns and models in the process of coloring complex surfaces in the traditional water transfer process. The prototype system has realized the virtual simulation, the system calibration and the integration of motion control. In view of the defects of the color distortion caused by the overstretching of the water transfer film during the process of water transfer to the complex model, we put forward a multiple transfer method. Each time the model is localized in one direction only, the pattern is superimposed after multiple transfers, and the model surface is on the model surface. The plastic film is modeled by the surface mesh to simulate the deformation of the plastic film under atmospheric pressure and to be attached to the surface of the object. We apply the simulation to the plastic molding process, the virtual flow process of vacuum thermoplastic molding, and put forward the method of "calculable thermoplastic molding". Combined with 3D printing, the virtual digital model with texture is made into a physical model. First, the digital model is used as a mold to simulate the process of vacuum forming. According to the simulation results of the plastic sheet deformation, the preformed pattern is calculated and printed on the transparent plastic sheet. By simulation of the vacuum process, the plastic is found out. A gas cavity may appear between the die and the mold. According to this, the exhaust holes are set on the model, and the mold with the vent hole is produced by 3D printing. We have reformed a small vacuum forming machine to make the pattern on the plastic sheet aligned with the object accurately. After vacuum molding, the print texture is attached to the surface of the mold. At the same time, the plastic sheet also covered a transparent protective shell for the pattern, and finally got the same physical model as the digital model.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TP391.9

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