【作者】 方浩；

【导师】 秦前清；

【作者基本信息】 武汉大学 ， 通信与信息系统， 2013， 博士

【摘要】 随着多媒体技术的飞速发展,人们对数字视觉内容的要求也日益提高,尤其是在自然景象的视效模拟方面,其中对自然灾害等现象的视效模拟越来越受到重视,对其真实感与可信度的要求也大幅提高。为了适应这些对自然灾害视效模拟方面如泥石流视效模拟的要求,研究人员在虚拟现实与图形学领域对流体模拟进行了多年的研究,虽然取得了很多的进展,但是在与泥石流视效模拟相关的大数据量流固耦合模拟方面一直没有大的突破与进展。另一方面,在流体力学领域,对流固耦合方面的数值模拟一直在持续研究,并在某些方面取得了较大的成果。如何借鉴流体力学方面数值模拟的方法与理论,同时又要考虑到视效模拟结合物理真实与视觉欺骗的特点,这是解决泥石流视效模拟这样的大数据量流固耦合模拟问题的一大挑战。本文在甘肃省国土资源厅项目“舟曲特大山洪泥石流灾害成因和防治规划虚拟展示”(No.2010175033)支持下,以舟曲泥石流视效模拟为例对大数据量的流固耦合视效模拟进行了研究。从基于物理的流体视效模拟基础开始,首先对流体视效的实现过程与原则做了阐述。视觉特效是创造、改变或者增强任何真实影像从而得到的活动序列帧连续反映产生的视觉影像,在虚拟现实与图形学领域当中,流体的视效模拟有两个关键点：一是运动形态的准确模拟,二是材质纹理的真实把握。而对其评价的标准由于在客观上图像理解研究的局限,只能从主观上对其进行统计综合评价,本文针对此评价方法进行了研究,提出了对于流体视效模拟的评价因子与权重系数。然后,在充分理解流体动力学Navier-Stokes方程的基础上,针对泥石流视效模拟的情况,对由拉格朗日法发展出来的SPH方法做了改进,提出了一种适合大数据量的流固耦合模拟的新方法,并结合一种新的定点刚体转动方法实现了泥石流视效模拟中流固两相的运动形态模拟,有力地回应了泥石流视效模拟所面临的挑战。本文以泥石流视效模拟为例从两个方面对这种大数据量的流固耦合模拟展开了研究：一方面,以流体动力学的Navier-Stokes方程为基础,使用基于拉格朗日法的SPH方法对大数据量流固耦合模拟进行研究,旨在解决流体部分与刚体部分的平动计算的瓶颈问题,降低视效模拟的计算复杂度；另一方面,基于刚体转动的经典理论,对泥石流中实际刚体的转动模拟进行研究,旨在提高刚体部分的转动计算准确性,提高视效模拟的真实感与可信度。本文的相关工作可以归纳为以下几点：1.提出了一种对流体视效模拟的评价方法。在该评价方法中,重点是对于摄影真实感的评价,视效影像需要符合真实世界的透视关系、动力学方式、照明属性、表面纹理质感等。根据这个原则,确定了流体视效模拟的评价指标体系,设计了流体视效模拟的综合评价方法,确定了评价指标及权重系数。2.SPH方法是在拉格朗日法的基础上发展出的无网格方法,把流体认为足由本质是微团的粒子组成,粒r的物理属性由在某个光滑核半径之内与之发生作用的其余粒子的物理属性插值而得。对于泥石流模拟这样的大数拟量流固耦合模拟,罪重要的是模拟在流体中大量刚体的运动。在这里需要考虑流体与刚体的双向作用,一方面是刚体与流体通过相互碰撞的作用力影响彼此运动,另一方面也要考虑刚体之间的相互作用。如果对其进行完全基于物理的计算,由于这是一个极其复杂的非线性问题,很难得到准确的结果,同时也使得计算量非常大。本文提出一种改进的SPH方法,根据拉格朗目方法的特点,对于流固耦合中的大数据量实际刚体,将其在宏观上抽象化为刚体粒子,通过刚体粒子与流体粒子之间的耦合作用,来准确计算出刚体粒子的运动,极大地降低了计算复杂度。同时,本文还提出了在SPH方法中流体粒子受到漩涡力与碰撞力的概念,提高了泥石流视效模拟中流体部分运动形态的真实感。3.通过本文提出的改进SPH方法得到泥石流视效模拟中做为实际刚体代理的刚体粒子平动之后,为了得到更真实可信的视效模拟效果,实际刚体的转动模拟成为一个重要问题。基于刚体转动的经典理沦,可以将泥石流中实际刚体转动考虑为定点转动问题,但是如果完全按照经典刚体运动理论,只计算与实际刚体发生碰撞的对象,计算过程分的复杂从而难于实现。因此,本文提出一种根据刚体受力域来计算实际刚体转动的方法,在域内的流体粒子都会对实际刚体的转动产生作用力,通过受力球分析来确定实际刚体的转动,这样就提高了泥石流视效模拟中,刚体部分运动形态的真实感。4.本文以舟曲泥石流视效模拟为例,建立泥石流视效模拟的实现框架。在对舟曲泥石流发生的具体情况分析的基础上，提出泥石流中刚体部分与流体部分建模的方法,对流体应用SPH法后农面重建的原理进行了说明。在泥石流与周围环境变互处理上使用查找粒子所处位胃的高度值的简化算法来计算泥石流与通过高度度场来生成的地形之间的碰撞：使用Sweep and Prune算法和边界虚拟粒子法来计算泥石流与建筑物的碰撞作用；在建筑物破碎效果模拟方面,应用了Voronoil图方法设计出泥石流冲毁建筑物的方法。对泥石流刚体部分与流体部分的材质纹理与光照进行了分析,并将之实现；最后,对整个舟曲泥石流的视效模拟使用本文提出方法进行了评价。更多还原

【Abstract】 With the rapid development of multimedia technology, people today gradually require more from digital vision, especially in the aspect of natural landscape visual simulation. People pay more and more attention to the visual simulation of the natural disaster phenomenon etc., and require much more reality and the reliability. To reach the request towards visual effects simulation of natural disaster such as debris flow, researchers have done research for many years about fluid simulation in the domain of virtual reality and graphics. They have made many progresses, but still not made much progress in fluid solid coupling of debris flow visual simulation. On the other hand, in the field of fluid mechanics, researches have done consistent research about the numerical simulation of fluid solid coupling, and have made some great achievement. It’s a big challenge to solve the problem of great data quantity fluid solid coupling simulation like debris flow visual simulation. While using the method and theory of fluid mechanics numerical simulation, both physical reality and visual deceive of visual simulation should be considered.This article is under the support of Gansu Province Department of land and resources project "Zhouqu devastating Mountain torrents debris flow disaster origin and preventing and controlling plan virtual demonstration"(No.2010175033), and takes Zhouqu debris flow visual effects simulation as an example and does research on great data quantity fluid solid coupling simulation. Based on the physical fluid visual effects simulation, it states the realization process and principle of the fluid visual effects. Visual effect is the visual images produced by active sequence frame continuous reflection when creating, changing or enhancing any actual images. In the domain of virtual reality and graphics, the visual simulation of the fluid has two key points:the first is the accurate simulation of the movement shape, and the second is the accurate simulation of the material quality texture. Because of the limitation of objective image understanding research, it can only be evaluated subjectively. This article has carried on the exploration into the evaluation method, proposing the evaluation factor and weight coefficient of fluid visual simulation. With full understanding of fluid dynamics Navier-Stokes equation and the situation of debris flow visual effects simulation, it develops the SPH method derived from Lagrange method and proposes one kind of new method suitable for great data quantity fluid solid coupling simulation, realize the fluid solid coupling simulation combining with a new method of fixed-point rotation of rigid body, and at last powerfully solves the problems that debris flow visual effects simulation faces.This article takes debris flow visual effects simulation as an example and launches the research from two aspects towards the great data quantity fluid solid coupling simulation:on the one hand, based on Navier-Stokes equation in fluid mechanics, it does research in a SPH way to investigate the great data quantity fluid solid coupling simulation in order to solve the bottleneck problem of parallel motion computing efficiency of the fluid part and rigid body part and accelerate the speed of visual effects simulation, reducing the complexity of visual simulation calculation; on the other hand, based on the classical rigid body rotation theory, it does research towards the rotation simulation of the actual rigid body in debris flow, with the purpose of enhancing the rotation computational accuracy of the rigid body and the validity of the visual effects. The related work of this article can be induced into the following several points:1. An evaluation method of fluid simulation is put forward. The focal point of this method is the evaluation of photorealistic. The visual image must accord with the real-world perspective relation, the dynamic functions, the lighting attributes, and the texture, etc. Following this principle, the evaluation system of visual fluid simulation is set up, the comprehensive evaluation method is designed, and the evaluation index and weight coefficients are determined.2. SPH is a non-grid method based on the Lagrangian method, expressing fluid made up by the particles (parcel). The physic attributes of a particle are calculated by the interpolations of other particles’attributes in the smooth length of it. The simulation of debris flow is a large dataset fluid-solid coupling, it’s important to simulate the movement of large amounts of rigid in the fluid. It must be considered as the coupling of two kinds of movement:the movement of the fluid and the movement of the rigid, which include interactions not only between the rigid and the fluid but also between the rigid. It is an extremely complicate nonlinear problem if completely based on physic. It brings about large amount of computation and is hard to get an accurate result. This paper gives an improved SPH method based on the characters of the Lagrangian method. For the large dataset actual rigid, they are abstracted into the rigid particles. By the interaction between the rigid and fluid particles, the movement of the rigid is calculated accurately, so it lowers the complexity of the calculation. Meanwhile, this paper brings the vortex force and impact that the fluid particles are affected by in SPH method, improving the reality of the fluid motion in the debris visual simulation.3. In the method this paper gives, the translation of the rigid particles acting for the actual rigid is calculated, then it comes to simulate the rotation of the actual rigid in order to get more realistic visual effect. The rotation of the actual rigid in debris flow can be thought as a fixed-point rotation based on classical theory of rotation. But it’s sufficiently complex to calculate the rotation by the crashes between the rigid and other objects completely following the classical rigid movement theory. Since that, this paper gives a method of calculating the rotation of the actual rigid by force domain. All the fluid particles in the force domain of an actual rigid will act on the rotation of the rigid. The rotation of the actual rigid is determined by the force ball, which develop the realistic of the rigid body motion in the visual simulation of debris flow.4. Taking the visual effect simulation of Zhouqu debris flow as an example, this paper forms a simulation framework of the debris flow visual effect. Based on the specific analysis of the Zhouqu debris flow, the paper puts forward a modeling method used for the rigid body part and fluid part in the debris flow, meanwhile,it gives a detailed description about the surface reconstruction principle with SPH method applying in the fluid research. For debris flow interacting with the surrounding environment, the crash between debris flow and the topographic map made up by height field is calculated in a simplified algorithm by searching the location of the particles, and the crash between debris flow and the buildings in Sweep and Prune algorithm and boundary virtual particles method. For the breaking down simulation of the buildings, a method of Voroni map is used to design the simulation of debris flow washing off the buildings. Besides, the material texture and lighting of the rigid body part and fluid part in the debris flow are analyzed, as well as the implementation. Finally, the whole visual effect simulation of the Zhouqu debris flow is evaluated in the method that this paper puts forward.更多还原