We propose finite-time measures to compute the divergence, the curl and the velocity gradient tensor of the point particle velocity for two- and three-dimensional moving particle clouds. For this purpose, a tessellation of the particle positions is performed to assign a volume to each particle. We introduce a modified Voronoi tessellation which overcomes some drawbacks of the classical construction. Instead of the circumcenter we use the center of gravity of the Delaunay cell for defining the vertices. Considering then two subsequent time instants, the dynamics of the volume can be assessed. Determining the volume change of tessellation cells yields the divergence of the particle velocity. Reorganizing the various velocity coefficients allows computing the curl and even the velocity gradient tensor. The helicity of particle velocity can be likewise computed and swirling motion of particle clouds can be quantified. First we assess the numerical accuracy for randomly distributed particles. We find a strong Pearson correlation between the divergence computed with the the modified tessellation, and the exact value. Moreover, we show that the proposed method converges with first order in space and time in two and three dimensions. Then we consider particles advected with random velocity fields with imposed power-law energy spectra. We study the number of particles necessary to guarantee a given precision. Finally, applications to fluid particles advected in three-dimensional fully developed isotropic turbulence show the utility of the approach for real world applications to quantify self-organization in particle clouds and their vortical or even swirling motion.
翻译:我们提出了一种有限时间度量方法,用于计算二维和三维运动粒子云中的点粒子速度的散度、旋度和速度梯度张量。为此,对粒子位置进行镶嵌,以为每个粒子分配一个体积。我们引入了一个修改后的Voronoi图镶嵌算法,克服了经典构造的一些缺点。我们使用Delaunay单元的重心而非外接圆心定义点。然后考虑两个连续的时间点,辨识出体积动力学。确定镶嵌单元的体积变化可得到粒子速度的散度。重组各个速度系数可以计算旋度甚至是速度梯度张量。粒子速度的螺旋度也可以计算,粒子云的旋流运动也可以被量化。首先我们评估了随机分布粒子的数值精度。我们发现使用修改后的镶嵌方法计算的散度值与精确值之间具有很强的皮尔逊相关性。此外,我们还表明该方法在二维和三维中,在空间和时间上都是一阶收敛的。然后,我们考虑以随机速度场对粒子进行搬运,速度场具有指定的幂律能谱。我们研究了保证给定精度所需的粒子数量。最后,应用于三维完全发展各向同性湍流中的流体粒子,展示了该方法在实际应用中量化粒子云的自组织及其旋流甚至是漩涡运动的实用性。