ONGOING
PROJECTS
Impacts
of Solid Objects on Fluid Surface  Physics of Skipping Stones Shinichiro
Nagahiro, May 2005

At a lakeside, we sometimes
see
timewasters throwing a stone on water and counting the number of
bounces the stone was able to make. Our intuition gives us some rules
for successful bounces: a stone must be flat and thin in shape, and one
should throw it with small incident angle with water's surface [1].
Could
we explain these empirical rules with the help of classical mechanics
and hydrodynamics? The
answer is, of course, "yes" but it is very difficult. A stone splashes
water and largely deforms its surface. To solve the NavierStokes
equation with such boundary condition is almost impossible. Hence, the
studies on "soldfluid impacts" have been relied on experiments for
long years. Recently, C. Clanet, H. Hersen and L. Bocquet performed an experiment of stone skipping using an aluminium disk as a model stone, and revealed a secret for a successful bounce: throwers should tilt the stone about 20 degrees to water's surface [2]. The "magic angle" 20 [deg.] minimizes the required speed necessary for a bounce (see figure 1). To better understand their experiments, we numerically and theoretically studied diskwater impact. For numerical simulation, we used the technique of Smoothed Particle Hydrodynamics (SPH). SPH describes flow with a set of fluid particles which moves in accordance with the NavierStokes equation. Because the method does not require a grid for computation, and thus has an advantage to treat free surface motion (see a mpeg video of our SPH simulation). Neglecting the effect of viscosity, surface tension and deformation of water's surface, we considered a model of ordinary differential equation (ODE) for a further analysis [3]. Both approaches successfully agreed with the experiment and our numerical simulation showed that the exact value of the "magic angle" varies slightly with the precise geometry of the impact. But, in most cases of stoneskipping, it remains around 20 [deg.]. Even though our ODE model was seemingly too simple to reproduce the experiment, the model could give a qualitative explanation for the diskwater impact. Hence we expect that impact of stone on water could be understood as a very simple dynamic process. [1] F. Jerdone Coleman "The Secrets of Stone Skipping", Stone Age Sports Publications (1996) [2] C. Clanet, H. Hersen and L. Bocquet, Nature (London) 427, 29 (2004) [3] S. Nagahiro and Y. Hayakawa, Phys. Rev. Lett. 94, 174501 (2005) 