Unified understanding of physics on granular drag force

由于其离散性，大块颗粒物质的行为与通常的流体或固体有很大的不同。因此，对颗粒物质独特行为的基本认识还不够。特别是，我们专注于颗粒曳力了解颗粒物理的基本原理，在这个研究项目。近年来，一些研究小组独立地进行了颗粒曳力实验，得到了各种各样的结果。然而，还没有统一的概念。我们进行了两种实验，以解决这一绝望的局面。一个是颗粒层和液滴之间的碰撞。另一个是通过固体穿透颗粒柱同时测量阻力和壁面压力。在前一个实验中，我们发现了一些在其他撞击实验中从未观察到的新颖的撞击坑形状。此外，我们还揭示了使用无量纲参数的火山口半径的标度关系。从后者，另一方面，我们已经找到了非线性标度律慢颗粒阻力和诱导壁压。通过对实验结果的详细分析，我们得出结论，非线性标度的起源与晶界处的衰减有关。通过这些实验结果，我们对颗粒曳力有了一些基本的认识。

Due to its discreteness, the behavior of bulk granular matter is much different from usual fluids or solids. Therefore, the fundamental understanding of the unique behavior for granular matter has not been enough yet. Particularly, we focus on the granular drag force to understand the fundamentals of the granular physics, in this research project. Recently, some research groups have independently performed granular drag force experiments, and obtained various results. However, there has not been any unifying concept. We carried out two kinds of experiments in order to solve this desperate situation. The one is the impact between a granular layer and a liquid droplet. The other one is the simultaneous measurement of drag force and wall pressure by a solid penetration into a granular column. From the former experiment, we have found some novel crater shapes that have never observed in other impact experiments. In addition, we have revealed the scaling relation for the crater radius using a dimensionless parameter. From the latter one, on the other hand, we have found nonlinear scaling laws for slow granular drag force and induced wall pressures. By detailed analyses of the experimental results, we have concluded that the origin of nonlinearity of the scaling relates to the attenuation at grains boundaries. Through these experimental results, we have now obtained some fundamental understandings for the granular drag force.