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# Isogeometric | ||
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## Isogeometric Anylasis | ||
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等几何分析始于Thomas Hughes教授参与关于从CAD模型创建有限元模型的研究。研究的要点表达了这样一个主题:尽管对网格生成进行了多年的研究,但CAD到Mesh创建问题依然是有效使用FEA(有限元分析)的重大瓶颈,对于复杂的工程设计来说,这个问题似乎越来越严重。 | ||
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CAD/CAE领域中的等几何分析,等几何分析目前是被视作CAE最具革命性的突破。 | ||
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CAD的开花理论, | ||
由Bezier Spline,推广未B样条B-Spline曲线和曲面,目前非均匀有理B样条曲面成为工业标准,2000Tom Sederberg博士进一步推广为T样条曲面。 | ||
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在传统NURBS(non-uniform rational B-Spline surface)上,节点细化是全局的,即我们在一处添加节点,都需要保持节点构成四边形网格,因此需要沿着曲面上的一条曲线添加节点。很多时候,我们只希望局部改变NURBS的节点结构,不希望影响全局 | ||
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几乎所有的物理定律都是由偏微分方程来描述。各种物理现象的数值模拟等价于在几何模型上求解各种偏微分方程。计算机辅助工程 CAE(Computer Aided Engineering)的核心就是在几何体上进行多物理场数值模拟。1940年代由克朗(Courant)等应用数学家发展起来的有限元方法(Finite Element Method)是CAE中最为普遍的方法,我国的数学家冯康也独立地奠定了有限元的理论基础。 | ||
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传统的有限元方法是将几何体进行三角剖分,然后在三角剖分上建立基函数。有限元算法中,我们在每个单元(四面体)上计算刚度矩阵,然后再组装成整体的刚度矩阵。传统的CAD工业中,所有的几何体都被表示成NURBS曲面;而在传统的CAE工业中,几何体被表示为网格剖分,很多时候是三角剖分。将CAD模型转换成三角剖分是整个数字模拟仿真流程中最为耗时、耗费计算资源的步骤,一般占所有计算时间的以上。同时,网格离散会带来几何误差,这是影响仿真问题精度的一个关键因素。 | ||
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2005年,Tom Hughes博士提出了等几何分析的思想(isogeometric analysis),核心是采用统一的样条语言来表达几何形状和物理场:利用平面NURBS参数曲面或者体NURBS参数体积作为计算域,计算单元为节点区间对应的曲面单元或者体单元;以NURBS基函数作为物理场的基函数(形函数),以控制顶点的物理数学分量作为未知变量。这样,形状几何与分析几何在同一样条空间,避免了数据交换;在精确CAD几何上进行仿真计算,避免了几何离散误差;在同样自由度下,达到较高计算精度,或者在同样的精度下,需要较少的自由度。同时,物理仿真保持了光滑性。从工程角度而言,等几何分析与传统的有限元方法兼容,等几何分析可以集成到现有的有限元环境之中。 | ||
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虽然等几何分析具有很多传统方法无法比拟的优点,但是等几何分析并没有被工业界广泛采纳,核心在于传统CAD工业中的样条曲面无法满足等几何分析的要求。传统NURBS大量应用裁剪样条,裁剪样条无法分解成规则单元,需要重新构造;传统NURBS缺乏水密性,存在大量缝隙,因此有很多弥补缝隙的碎片样条,需要融合成整片样条;最为致命的问题是传统CAD模型只表达了几何体的边界曲面,而等几何分析需要将整个实体的内部也表达成样条。 | ||
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## 参考 | ||
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- [等几何分析:从CAD到CAE的另一条路](https://mp.weixin.qq.com/s/ELmpeERmjF8Tou7OaS3P0A) |
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# 问题集 | ||
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## 2024-7-19 | ||
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接手公司项目时,使用他们的代码拷贝过来,产生了一个问题,就是渲染时牙龈边沿处的跳动。 | ||
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牙龈根据牙齿数据来生成,目前存在牙龈穿过牙齿的现象。 | ||
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### flickering | ||
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渲染边沿出现跳动flicker的现象通常由几个常见的原因组成 | ||
- z-fighting, 是webgl常见的问题,当两个表面或多个表面非常接近时,它们的深度值可能会重叠,导致在渲染时不知道切换那个表面,可以增加一个偏移来避免 |
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