The flow of segregated bidisperse assemblies of particles is of major importance for geophysical flows and bedload transport in particular. In the present paper, the mobility of strictly bidisperse segregated particle beds is studied with a coupled fluid discrete element method (DEM). Large particles are initially placed above small ones in a three-dimensional domain inclined at a slope of $10\%$. A gravity-driven water free surface flow induces a downslope shear-driven granular flow of the erodible bed. It is observed that, for the same water flow conditions, the bedload transport rate is higher in the bidisperse configuration than in the monodisperse one. Depending on the Shields number and on the depth of the interface between small and large particles, different transport phenomenologies are observed, ranging from no influence of the small particles to small particles reaching the bed surface due to diffusive remixing. In cases where the small particles hardly mix with the overlying large particles and for the range of studied size ratios $(r<4)$, it is shown that the increased mobility is not a bottom roughness effect, that would be due to the reduction of roughness of the underlying small particles, but a granular flow effect. This effect is analyzed within the framework of the $\mu \left(I\right)$ rheology, modeling the stress to strain rate relation for dense granular flows. It is demonstrated that the buried small particles are more mobile than larger particles and play the role of a “conveyor belt” for the large particles at the surface. Based on rheological arguments, a simple predictive model is proposed for the additional transport in the bidisperse case. It reproduces quantitatively the DEM results for a large range of Shields numbers and for size ratios smaller than 4. The results of the model are used to identify four different transport regimes of bidisperse mixtures, depending on the mechanism responsible for the mobility of the small particles. A phenomenological map is proposed for bidisperse bedload transport and, more generally, for any granular flow on an erodible bed.

中文翻译：

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床载运输过程中双分散混合物的流动性

颗粒的分离的双分散集合体的流动对于地球物理流动和尤其是床载运输尤其重要。本文采用耦合流体离散元方法（DEM）研究了严格双分散离析颗粒床的迁移率。最初将大粒子放置在小粒子上方的三维域中，该三维域的斜度为$10％$。重力驱动的无水表面流引起了易蚀床的下坡剪切驱动的颗粒流。观察到，对于相同的水流条件，在双分散构型中的床载输送速率比在单分散构型中的高。根据Shields数和大小颗粒之间界面的深度，观察到不同的传输现象，范围从小颗粒的无影响到由于扩散性再混合而到达床表面的小颗粒。在小颗粒几乎不与上面的大颗粒混合的情况下，并且在所研究的尺寸比范围内$（\mathrm{[R}<4）$，显示出增加的迁移率不是底部粗糙度效应，这是由于下面的小颗粒的粗糙度降低，而是颗粒流动效应。这种影响是在$\mu （\mathrm{一世}）$流变学，模拟了稠密颗粒流的应力与应变率的关系。结果表明，被埋入的小颗粒比大颗粒具有更大的运动性，并且对表面的大颗粒起着“输送带”的作用。基于流变学的观点，提出了一种简单的预测模型，用于双分散情况下的附加传输。它定量地再现了大范围的Shields数和小于4的尺寸比的DEM结果。该模型的结果用于识别双分散混合物的四种不同传输方式，具体取决于负责小颗粒迁移的机理。提出了一种现象学图谱，用于双分散床载运输，更一般地，用于易蚀床上的任何颗粒流。