The impact of a sphere with velocity $u_0$ on a fine, loose granular system under the acceleration due to gravity has been studied by fast video photography. The behavior of the granular bed is found to be similar to a fluid during initial impact, followed by a cavity drag during projectile penetration. From the trajectory of the projectile it is found that the drag on the projectile can be well described by adding a bulk frictional force $f$ to the hydrostatic force $\kappa z$ where $\kappa$ is a constant and $z$ denotes the penetration depth. Both $\kappa$ and $f$ are $u_0$ dependent. This form of the drag force suggests that fluidlike viscous dissipations in the bed can be neglected in these three-dimensional (3D) experiments. However, due to the imposed boundary this hydrodynamic term of the drag force is found to be not negligible in quasi-2D granular beds.

• Received 23 August 2005

DOI:https://doi.org/10.1103/PhysRevE.72.062301