  The behavior of an ultrasonic shot peening process is observed and analyzed
by using a model of inelastic hard spheres in a gravitational field that are
fluidized by a vibrating bottom wall (sonotrode) in a cylindrical chamber. A
marked heterogeneous distribution of impacts appears when the collision between
the shot and the side wall becomes inelastic with constant dissipation. This
effect is one order of magnitude larger than the simple heterogeneity arising
from boundary collision on the cylinder. Variable restitution coefficients
bring the simulation closer to the general observation and allows the
investigation of peening regimes with changing shot density. We compute within
this model other physical quantities (impact velocities, impact angle,
temperature and density profile) that are influenced by the number $N$ of
spheres.
