Creep motion and fluidization in a granular pile

我们对具有稳定表面流的颗粒桩进行了实验研究。在表面流动之下，即使是散体深处的颗粒也表现出非常缓慢的流动，并且这种运动可以在任意深度被探测到。蠕变运动的平均速度随深度呈指数衰减，特征衰减长度近似等于颗粒大小，与流速无关。在重新定标的时间尺度上分析了深层颗粒的扩散，并用高速相机观测了地表水流，得到了速度分布。当土层足够薄时，蒸发效率可以用水的平均体积分数的函数来很好地描述。该函数对于几种类型的土具有共同的形式，并且可以通过单个参数进行参数化。该值取决于土壤类型和风速。后一种依赖关系可以用水表面的蒸发阻力来强有力地描述。这些结果可望普遍适用于颗粒团聚体，并成为系统地计算任意深度土壤蒸发效率的基础。

We investigated experimentally granular piles exhibiting steady surface flow. Below the surface flow, even the particles in layers deep in the bulk exhibit very slow flow and that such motion can be detected at an arbitrary depth. The mean velocity of the creep motion decays exponentially with depth, and the characteristic decay length is approximately equal to the particle-size and independent of the flow rate. We analyzed particle diffusion in deep layer in rescaled time scale, and also observed the surface flow using highspeed camera to obtain the velocity profile.Evaporation rate of water from several soil types were measured under controlled conditions. When the layer of soil is sufficiently thin, the evaporation efficiency is well described by a function of the mean volume fraction of water. The function has a common form for several types of soil and can be parameterized by a single parameter. The value depends on the soil type and also on the wind speed. The latter dependence can be robustly described in terms of the evaporation resistance for the watered surface. These findings are expected to hold commonly for granular assemblies and to become a basis for calculating the evaporation efficiency for the soil with an arbitrary depth in a systematic way.