Rayleigh-Taylor-instability induced flow and mixing are of great importance both in nature and engineering scenarios. To capture the underpinning physics, one-way coupled particles are introduced to make a Lagrangian tracking supplement to the discrete Boltzmann simulation. Detailed morphological information of the flow field and thermodynamic nonequilibrium behavior around the interfaces in a two-miscible-fluid system are delineated. Via a defined local mixedness, from densities of two kinds of particles, we quantitatively captured the appearance position and indicated the intensity of Kelvin-Helmholtz instability. The effects of compressibility and viscosity on mixing are investigated separately. Both of them show two-stage effects on mixing process. The underlying mechanism of these two-stage effects is interpreted as that a "hard" system with low compressibility and/or high viscosity is beneficial to the development of large structures at the initial stage and a "soft" system with high compressibility and/or low viscosity is favorable for the generation of small structures at the late stage. At the late stage, for a fixed time, the field averaged mixedness exponentially decreases with the viscosity, which indicates the existence of a characteristic viscosity $\mu_0$. The tracer particles make possible to observe finer structures with clear interfaces in the late mixing stage. One can clearly observe the mixing process and distinguish tracer particles from the heavy and light fluids. Results in this paper are helpful for understanding the mechanism of complex compressible flow with Rayleigh-Taylor instability and present references for related particle tracking experiments.

中文翻译：

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Rayleigh-Taylor不稳定性引起的可压缩流动和混合的粒子跟踪表现

Rayleigh-Taylor 不稳定性引起的流动和混合在自然和工程场景中都非常重要。为了捕捉基础物理，引入了单向耦合粒子，以对离散玻尔兹曼模拟进行拉格朗日跟踪补充。详细描述了流场的详细形态信息和两相混流体系统中界面周围的热力学非平衡行为。通过定义的局部混合，从两种粒子的密度，我们定量地捕获了出现位置并指示了开尔文-亥姆霍兹不稳定性的强度。分别研究了压缩性和粘度对混合的影响。它们都对混合过程表现出两阶段的影响。这些两阶段效应的潜在机制被解释为“硬” 低压缩性和/或高粘度的体系有利于初期大结构的发展，而高压缩性和/或低粘度的“软”体系有利于后期小结构的产生。后期，在一定时间内，场均混合度随粘度呈指数下降，表明存在特征粘度$\mu_0$。示踪粒子可以在后期混合阶段观察具有清晰界面的更精细结构。人们可以清楚地观察混合过程，并将示踪粒子与重流体和轻流体区分开来。