Author(s): Subhasish Dey, Sk Zeeshan Ali
Linked Author(s): Subhasish Dey
Keywords: Fluvial hydraulics, hydrodynamics, turbulent flow, sediment inception, sediment transport.
Abstract: In this paper, the inception of non-cohesive sediment motion under a steady uniform free-surface flow over a plane sediment bed is studied by applying the deterministic concept of force analysis. The mathematical formulation starts with the consideration of the force system along with a proper three-dimensional configuration of bed sediment particles. The hydrodynamic force (drag and lift) on a target sediment particle resting over a compact bed formed by equal sized sediment particles is analyzed from the viewpoint of micro-mechanics. The drag force comprises the form drag due to pressure difference across the particle and the form induced drag due to streamwise pressure gradient arising from the convective streamwise acceleration. The hydrodynamic lift force is constituted by the Saffman lift, the Magnus lift, the centrifugal lift, and the lift due to velocity fluctuations. Special emphasis is rendered to determine the points of action of the force system from the basics of micro-mechanics. The primary mode of inception is considered as rolling mode, where the target particle has a tendency to roll either over the summit of a single bed particle or over the valley formed by the two adjacent bed particles. Thus, the turning moment of the force system on the target particle at the pivoting point forms the governing equation of motion. Three different velocity distributions are considered to analyze the sediment inception under hydraulically smooth, transitional, and rough flow regimes. The decay of the viscous sublayer with shear Reynolds number is incorporated into the mathematical analysis. The effects of turbulent fluctuations are addressed by applying the statistical theory of turbulence. This study reveals that for a hindrance coefficient of 0. 3, the proposed theoretical sediment inception curve (critical Shields parameter versus shear Reynolds number) shows a satisfactory agreement with the experimental data of uniform sediments
Year: 2017