Numerous large‐scale geophysical flows propagate with low‐apparent basal friction coefficients, but the source of such phenomenology is poorly known. Motivated by scarce basal friction data from natural flows, we use numerical methods to investigate the interaction of granular flows with their substrate under idealized conditions. Here we investigate 3‐D monodisperse and polydisperse fluid‐particle granular flow rheology and flow‐substrate interaction using discrete element modeling and coarse‐graining techniques. This combination allows us to calculate the continuum fields of solid fraction, velocity, shear stress, and solid pressure and compare it with force measurements on the substrate. We show that the wall/basal friction coefficient is not constant. Instead, it is a function of the nondimensional slip defined as the ratio of the slip velocity over the slip velocity fluctuations. The scaling of the wall friction with nondimensional slip is independent of air viscosity and density and presence of excess pore pressure. Therefore, the reduction of the basal stress that must occur in mobile natural flows with excess pore pressure is not ascribed to the lowering of wall friction coefficient. Instead, lowering of the normal stress by fluid drag in flows with elevated pore fluid pressure justifies the definition of effective wall and internal friction coefficients to capture the geophysical flow rheology and the forcing on its substrate. These results are fundamental to understand the dynamics of geophysical mass flows including pyroclastic density currents, water‐rich debris flows, and rock and submarine avalanches.

中文翻译：

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有和没有过大孔隙压力的颗粒流的基础摩擦系数：对火山碎屑密度流，富水泥石流以及岩石和海底雪崩的影响

许多大规模的地球物理流以低表观的基础摩擦系数传播，但是这种现象的来源却鲜为人知。受自然流量缺乏的基础摩擦数据的激励，我们使用数值方法研究了理想条件下颗粒流与其底物的相互作用。在这里，我们使用离散元建模和粗粒度技术研究3D单分散和多分散流体颗粒的颗粒流变学和流质相互作用。这种组合使我们能够计算固体分数，速度，剪切应力和固体压力的连续场，并将其与在基材上的力测量值进行比较。我们显示出墙/基摩擦系数不是恒定的。代替，它是无量纲滑移的函数，无量纲滑移定义为滑移速度与滑移速度波动之比。具有无量纲滑动的壁摩擦的缩放与空气粘度和密度以及是否存在过大的孔隙压力无关。因此，在孔隙压力过大的情况下，在流动的自然流动中必须发生的基础应力的减小并不归因于壁摩擦系数的减小。取而代之的是，在孔隙水压力升高的情况下，由于流体阻力引起的流体法向应力的降低证明了有效壁和内摩擦系数的定义是合理的，以捕获地球物理流的流变学及其在基底上的强迫。这些结果对于理解地球物理质量流（包括火山碎屑密度流，富水泥石流，