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脉动风场中烟雾模拟方法的优化研究
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摘要
在计算机图形学中,真实的再现烟雾等自然现象一直是最具有挑战性研究方向之一。传统的粒子系统和纹理映射等方法模拟出烟雾真实感较差,无法满足高真实性要求。随着流体力学领域中对流体物理特性的研究日益成熟,基于物理的方法得到了广泛的应用,它从烟雾的内在物理特性出发,建立精确的物理模型,模拟出高精确度的烟雾动画。本文采用基于物理的方法进行烟雾模拟,并针对烟雾模拟的实时性和真实性要求进行研究。
     首先,在提高实时性方面,采用非粘性不可压缩Navier-Stokes方程组建立烟雾物理模型,利用有限差分法结合GPU加速技术求解烟雾物理方程,将求解区域离散化,并对烟雾基本物理方程进行简化,减少计算量。同时,应用MacCormack方法求解对流项,减少传统方法的求解步骤,提高计算速度并保证求解过程的无条件稳定性。
     其次,在提高真实性方面,引入风场的作用,使烟雾的形态和运动更加逼真自然。采用优化的卡门谱建立风场的互谱密度矩阵,引入特征正交分解(POD)技术求解矩阵,并利用快速傅立叶变换(FFT)技术求出脉动风在频域空间的速度场,将脉动风场以外力的形式加入到描述烟雾运动的物理方程中,实现风场与烟雾的交互作用。
     再次,将烟雾的重力、热浮力和漩涡约束力加入到影响烟雾运动的特征因素集中,改善烟雾的运动细节,增强运动的真实性。此外,考虑烟雾的密度和温度随速度场的变化,以准确的计算烟雾的重力和热浮力。
     最后,利用VC++语言和开放性图形接口OpenGL设计烟雾的实时仿真系统,对改进后的烟雾模拟算法进行验证,实验证明:利用本文的算法可以实时的生成在脉动风环境下逼真自然的烟雾。
The simulation of natural phenomena such as smoke, has been one of the most challenging research areas in computer graphics. The smoke image generated by traditional particle system and the texture mapping is poor and can hardly meet the requirements of high authenticity. As extensive research in fluid dynamics has been done, physically based method has been widely used to simulate smoke. It analyses the inherent physical characteristics of the smoke, and establishes an accurate physical model to obtain the realistic smoke. We adopt the physical method to simulate the smoke, and focus on our researches on the real-time and the reality of the simulation.
     Firstly, in order to improve the real-time quality, we use non-viscous incompressible Navier-Stokes equations to establish the physical model of the smoke, solve physical equations by the finite difference method and GPU acceleration, discrete the computational domain, and simplify the basic physics equations to reduce the computational time. In addition, we adopt MacCormack method to solve the advection item of the Navier-Stokes equation, so as to reduce the solving steps of traditional methods, improve the computational speed and ensure the solving processes are unconditional stable.
     Secondly, for the purpose of improving the authentic performance, we introduce the role of wind field, so that the shape and movement of the smoke will be more realistic and natural. We use optimized Karman spectrum to establish cross-spectral density matrix of the wind field, then introduce Proper Orthogonal Decomposition(POD) technique for solving the matrix, and then calculate the fluctuating wind velocity in the frequency domain space by the Fast Fourier Transform(FFT). Finally, the wind fields are considered as the outside forces to be added into the physical equations of the smoke, so the interaction of the fluctuating wind and the smoke is realized.
     Thirdly, the gravity, thermal buoyancy and vorticity confinement force, which greatly affect the smoke’s movement, are added into the smoke field, to improve the details and the reality of the smoke’s movement. Moreover, we also consider that the smoke’s density and temperature changes with the velocity field, to ensure the smoke’s gravity and buoyancy are calculated accurately.
     Finally, in order to verify our improved smoke simulation algorithms, we use the VC++ language and Open Graphics Library to design the real-time smoke simulation system. Experiment results show: our algorithms can run in real-time and the smoke effects are of great reality and nature in the turbulent wind environment.
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