Sand ripples in an oscillating annular sand-water cell

Sand ripples, familiar from ocean doors, are studied by oscillating an annu lar sand-water cell. Empirical rules for the onset and disappearance of rip ples are uncovered and compared with simple dimensional arguments, and with prior experimental and theoretical work. In addition the dynamical behavio r of sand ripples are investigated, and the existence of a secondary instab ility where small and large wavelengths coexist in space is shown. Finally, by visualizing the fluid flow above the sand ripples the paper describes v ortices which are an integral part of the ripple formation and show that th ere is a strong coupling between two different types of vortices and the sa nd ripple pattern itself. (C) 1999 American Institute of Physics. [S1070-66 31(99)00601-7].