SEDIMENT SUSPENSION EVENTS FROM RIPPLE BEDS IN OSCILLATORY FLOW: EXPERIMENTS

Abstract

An experimental sediment flume is used to investigate sediment transport mechanics within an oscillatory turbulent boundary layer over a mobile sediment bed in the ripple bed regime. Two-phase PIV is utilized to simultaneously capture data from each phase, allowing examination of suspension mechanisms, carrier phase stresses, and to obtain statistics to describe the momentum exchange between the phases. The technique employs median filtering, as well as size and brightness criteria to separate and accurately identify each phase. Independent well-conditioned tests have been conducted to improve the algorithm to account for the imaging conditions encountered in the vicinity of a mobile bed in order to minimize cross-talk between the phases and allow quantification of the dispersed phase concentration. Results show that large-scale vortical structures are responsible for the ejection of sediment from the bed into the outer flow. These structures are a significant source of turbulent transport, but their overall contribution to the bed stress is small compared to the mean flow. Triple decomposition of the equations of motion show that long time averaged sediment flux is of similar magnitude to cyclic fluctuations and the time averaged flow consists of two counter rotating cells. Turbulent kinetic energy created at flow reversal advects over the sediment bed and keeps particles suspended in the flow. Calculation of the vertical particle drag, body force, and convection terms revels that at flow reversal the body force terms are larger than the drag causing the particles on average to settle. The particle convection terms are small compared to particle drag and body force terms. Some of the terms most significant in the particle drag are the fluctuating components indicating that the turbulence is keeping the particles suspended in the outer flow.