2 9 Ju n 19 99 Velocity statistics in excited granular media

We present an experimental study of velocity statistics for a partial layer of inelastic colliding beads driven by a vertically oscillating boundary. Over a wide range of parameters (accelerations 3-8 times the gravitational acceleration), the probability distribution P (v) deviates measurably from a Gaus-sian for the two horizontal velocity components. It can be described by P (v) ∼ exp(−|v/v c | 1.5), in agreement with a recent theory. The characteristic velocity v c is proportional to the peak velocity of the boundary. The granular temperature, defined as the mean square particle velocity, varies with particle density and exhibits a maximum at intermediate densities. On the other hand, for free cooling in the absence of excitation, we find an exponential velocity distribution. Finally, we examine the sharing of energy between particles of different mass. The more massive particles are found to have greater kinetic energy. 1 We determine the statistical properties of particles in a vibrated granular medium experimentally. While many similarities between ordinary gases and excited granular media have been found, a fundamental difference is that collisions between particles in granular matter are inelastic. As a consequence, the velocity distribution deviates measurably from a Gaussian, but can be described by P (v) ∼ exp(−|v/v c | 1.5) for a large range of parameters where external excitation is sufficiently frequent, in agreement with a recent theory.