Physical distinction between rolling-grain ripples and vortex ripples: an experimental study.

We have performed an experimental study of the transition of rolling-grain ripples toward vortex ripples. In particular, we have looked for the influence of the grains' diameter, the frequency of oscillation, and the grains' cohesion. We demonstrate that the rolling-grain ripples are transient patterns which do appear as soon as we are close to the threshold for grain motion, whereas vortex ripples are always the final patterns observed and are the only patterns observed if one is far from the threshold for grain motion. Our results show that the "elasticity" (i.e., the tendency to modify the wavelength by either compression or dilatation) of the vortex ripples explains several discrepancies with respect to the observed evolutions and measurements reported so far in the literature.