The role of near wall turbulent structures on sediment transport

Abstract

In this paper, we present the results of an experimental investigation designed to analyze the interaction between near wall coherent structures (or burst) and bedload transport in an open channel flow. Laser doppler velocimetry (LDV) measurements of the instantaneous velocity near the wall were coupled with real time measurements of sand particle trajectories on the bottom of an hydraulic flume. We will first give a description of the bursting phenomenon and we will present the experimental design. Then, we will give some details about the specific signal processing techniques we used in this work to detect coherent structures on the velocity signal. The third part of this paper will present our results. We will show that the period between two consecutive displacements of a solid particle can be compared to the mean period between two consecutive ejections. This would suggest that these structures are involved in bedload transport. For small displacements, the time distribution between two consecutive movements shows two modes. One would be associated to particles whose mean deposit time corresponds to the ejection period, the second would be associated to particles whose mean deposit time is close to the sweep period. We made the assumption that there exist two transport modes at the wall: the dominant one was transport by ejections, but sweeps can be involved in the removal of particles whose friction coefficient with the wall is high.