Abstract

Sand ripples, familiar from ocean floors, are studied by oscillating an annular sand-water cell. Empirical rules for the onset and disappearance of ripples are uncovered and compared with simple dimensional arguments, and with prior experimental and theoretical work. In addition the dynamical behavior of sand ripples are investigated, and the existence of a secondary instability where small and large wavelengths coexist in space is shown. Finally, by visualizing the fluid flow above the sand ripples the paper describes vortices which are an integral pan of the ripple formation and show that there is a strong coupling between two different types of vortices and the sand ripple pattern itself. © 1999 American Institute of Physics.