Formation of Barchans and Ripples due to Steady Viscous Flow in an Annular Channel

Abstract

An experimental study is made on the pattern formation of the sand bed immersed in a viscous fluid between two concentric cylinders of finite depth, whose channel width is sufficiently large as compared with the particle size. The upper boundary of the fluid is in contact with an annular ring made of transparent acrylic resin, which slides itself at a constant angular velocity, whereas other boundaries are at rest. The velocity field is measured, which revealed that the vertical velocity profile is quite different from the linear shear flow, whereas the effect of the side walls is not significant in the bulk region of our channel. The onset and growth of barchan dunes and sand ripples, time variation of the wavelength and volume of the ripples, and their dependence on the fluid velocity are examined. A simple model that describes the growth of the ripples is proposed.