  Granular surfaces tend to develop lateral ripples under the action of surface
forces exerted by rolling wheels, an effect known as washboard or corrugated
road. We report the results of both laboratory experiments and soft-particle
direct numerical simulations. Above a critical speed, the ripple pattern
appears as small patches of traveling waves which eventually spread to the
entire circumference. The ripples drift slowly in the driving direction.
Interesting secondary dynamics of the saturated ripples were observed, as well
as various ripple creation and destruction events. All of these effects are
captured qualitatively by 2D soft particle simulations in which a disk rolls
over a bed of poly-disperse particles in a periodic box. These simulations show
that compaction and segregation are inessential to the ripple phenomenon. We
also discuss a simplified scaling model which gives some insight into the
mechanism of the instability.
