Turbulent-laminar transitions in flows laden with cohesive sediment

Abstract:

Cohesive sediments, including fine clays, silts and biologically modulated coarser sediments, are ubiquitous in natural environments. The cohesive properties of clay allow the formation of clay flocs or gels and therefore relatively small clay concentrations can suppress turbulence in a flow. Increasing clay concentration can result in transient behaviour. These transitional flows are characterized by a region of strong turbulence near the bed with a region of reduced turbulence above it, referred to as plug flow. The region of the plug flow develops towards the bed with increasing clay concentration for equilibrium-state flows. In natural environments, changing channel geometry, tidal forcing or fluvial discharge results in flow acceleration or deceleration, i.e. unsteady flow. However, the dynamics of unsteady cohesive sediment laden flows are poorly understood. New experiments of clay suspension flows were conducted in a tapering flume whose geometry forces flow acceleration or deceleration depending on flow direction. The tapering provides controlled spatial changes in flow velocity and thus flow regime. Decelerating flow shows a velocity profile deferred from the standard logarithmic profile and a convex profile of velocity fluctuations with initial increased intensity. The reduced flow velocity of decelerating flows is expected to enhance formation of clay bonds, but is initially prevented by the increase in velocity fluctuations. The increased turbulence fluctuations allows the flow to transport the sediment further downstream while adjusting to the change in velocity. Accelerating flow on the other hand contains a concave profile of velocity fluctuations, with the maximum found near the bed. The initial reduced turbulence intensities reduces the capacity of the flow to hold sediment in suspension and causes a delay in floc breakup. The constant shifting balance between turbulent and cohesive forces regulate the dynamic structure of transitional flows. After deceleration or acceleration of the flow, this balance needs time, thus distance, to reach an equilibrium. Increasing clay concentration enhances the difference in turbulence levels for accelerating and decelerating flows.