On the hydrodynamic limit of long rod penetration

According to the quasi-steady-state approximation of long rod penetration, the total penetration efficiency should approach the so called hydrodynamic limit at high impact velocities. However, both experiments and computer simulations show that this is not the case. The total penetration efficiency usually overshoots the hydrodynamic limit at velocities above approximately 3 km/s. We ran computer simulations to investigate the significance of the hydrodynamic limit. We found that the overshoot occurs during the end (or terminal) phase. The end phase starts upon completion of the rod erosion. The kinetic energy trapped in the residual rod and in the target at this stage can cause significant additional deepening of the crater. As the overshoot of the hydrodynamic limit is an edge effect, it is expected to decrease as the aspect ratio of the rod increases.