Granular Segregation in Hydraulic Ripples.

We report an experimental study of a binary sand bed under : (i) an oscillating flow and (ii) under a circular flow. In both cases, the appearance of a granular segregation is shown to strongly depend on the sand bed preparation. A reproducible protocol is proposed. In the oscillating case, a segregation in volume is observed in the final steady state. The correlation between this phenomenon and the fluid flow is emphasised. In the unidirectional case, a segregation in surface in observed instead of a segregation in volume. The relative difference in size between both sand species is highlighted as the physical parameter leading to the phase segregation. The difference between the characteristic times of appearance of both types of ripples is shown to be the major reason for the different behaviours leading to either a bulk segregation or a surface segregation. [ABSTRACT FROM AUTHOR]