Bed forms due to mass transfer in turbulent flows: a kaleidoscope of phenomena

Abstract

This paper gives a brief account and interpretation of bed forms produced during mass transfer from beds of hardened Plaster of Paris (calcium sulphate) exposed to turbulent streams of water. Defects within or upon the initially plane plaster beds perturbed the profile of mass-transfer (solution) rate as the result of the setting up of separated flows. Those defects that exceeded a certain critical dimension set by flow conditions grew in length, breadth and depth into heel-shaped hollows of parabolic plan calledflutes.The continued growth and eventual interference of coextensive flutes gave rise to a bed withscallops, an assemblage of intersecting, saucer-shaped depressions. Defects which fell below the critical dimension set by flow conditions grew initially in length and breadth to give long, narrowgrooveslying parallel with flow, but after a sufficient time were erased so that the bed reverted to plane. Many plane beds became fashioned into continuous systems of very longfurrows and ridgesaligned parallel with flow and coupled with paired longitudinal vortices resembling Taylor–Görtler vortices klinematically. The dimensions and orientation of this structure suggested that Klineian streaks, which on rigid beds occur randomly in space and time, had become stabilized in space in the new context of the deformable beds. The furrows commonly experienced a higher-order instability which resulted in the production ofsecondary flutesandscallopssimilar to those resulting from pre-existing defects. All of the bed forms described are associated with separated flows, and it is suggested that separation of flow may be a phenomenon necessary for their maintenance and, in the case of flutes, perhaps also for their origin.