Size segregation, which is a robust feature of sheared granular mixtures and geophysical mass flow deposits, is found to diminish in the presence of a viscous fluid. We study this inhibitive effect through coupled fluid‐particle simulations of granular flows fully immersed in different ambient fluids. Granular‐fluid mixture flows are modeled according to three distinct flow regimes—free fall, fluid inertial, and viscous—at different angles of inclination. Each flow regime corresponds to distinct flow dynamics and segregation behaviors. We find that segregation is indeed weaker and slower in the presence of an ambient fluid, which is more so as the flow becomes more viscous. The ambient fluid affects segregation in two major ways. First, buoyancy reduces the contact pressure gradients which are needed to drive large particles up, while at the same time reduces the particles' apparent weight. On the other hand, the streamwise drag force substantially changes the flow rheology, specifically the shear rate profile, thereby modifying the segregation behavior in the normal direction. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not affect segregation in a direct manner.

中文翻译：

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不同环境流体在颗粒质量流中的粒度分离

发现尺寸分离是剪切的颗粒混合物和地球物理质量流沉积物的强大特征，在粘性流体的存在下，尺寸分离减小了。我们通过完全浸没在不同环境流体中的颗粒流的耦合流体-颗粒模拟来研究这种抑制作用。颗粒流体混合物的流动是根据三种不同的流动状态（自由下落，流体惯性和粘性）在不同的倾斜角度进行建模的。每个流态对应于不同的流动力学和偏析行为。我们发现，在环境流体存在的情况下，隔离确实更弱，更慢，随着流动变得更粘稠，分离会更加严重。环境流体以两种主要方式影响隔离。首先，浮力降低了将大颗粒向上推动所需的接触压力梯度，同时减少了颗粒的表观重量。另一方面，沿流的拖曳力实质上改变了流变性，特别是剪切速率分布，从而改变了法向方向上的偏析行为。出人意料的是，与流体粘度无关，在法向方向上的流体阻力可以忽略不计，并且不直接影响偏析。