Nilsen Christopher, Andersson Helge I
Department of Energy and Process Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Oct;90(4):043005. doi: 10.1103/PhysRevE.90.043005. Epub 2014 Oct 10.
We use synthetic turbulence simulations to study how inertial particles cluster in a turbulent flow, for a wide range of Stokes numbers. Two different types of synthetic turbulence are used: one Gaussian, where the time evolution of the velocity field is a simple phase shift, and one non-Gaussian, where convection is used to evolve the velocity field in time. In both flow types we observe significant particle clustering over a wide range of scales and Stokes numbers. The clustering found at low Stokes numbers can be attributed to the vortex centrifuge effect, where heavy particles are expelled from regions dominated by vorticity. This mechanism is much more effective in the non-Gaussian turbulence, because local flow structures are convected with the particles. The preferential sampling of regions with low vorticity is almost negligible in the Gaussian turbulence. At higher Stokes numbers, caustics are formed in a very similar manner in both Gaussian and non-Gaussian synthetic turbulence. In non-Gaussian turbulence, heavy particles cluster in regions of low fluid kinetic energy, while the opposite is true in Gaussian turbulence. Our results show that synthetic simulations cannot correctly predict how the particle clustering correlates with local fluid flow properties, without including convection.
我们使用合成湍流模拟来研究惯性粒子在湍流中如何聚集，涵盖了广泛的斯托克斯数范围。使用了两种不同类型的合成湍流：一种是高斯型的，其中速度场的时间演化是一个简单的相移；另一种是非高斯型的，其中对流用于随时间演化速度场。在这两种流动类型中，我们都观察到在广泛的尺度和斯托克斯数范围内存在显著的粒子聚集现象。在低斯托克斯数下发现的聚集现象可归因于涡旋离心机效应，即重粒子从以涡度为主的区域被排出。这种机制在非高斯湍流中更为有效，因为局部流动结构与粒子一起对流。在高斯湍流中，对低涡度区域的优先采样几乎可以忽略不计。在较高的斯托克斯数下，高斯和非高斯合成湍流中焦散的形成方式非常相似。在非高斯湍流中，重粒子聚集在流体动能较低的区域，而在高斯湍流中情况则相反。我们的结果表明，如果不包括对流，合成模拟无法正确预测粒子聚集与局部流体流动特性之间的相关性。
