In this study, the characteristics of near-bed turbulence were experimentally investigated for three distinct roughness regimes, namely the isolated, wake interference, and skimming. Incipient flow conditions prevailed throughout the experiments. Spherical particles of the same size and density were placed upon a rough particle bed to simulate the three regimes.Experimental runs for the aforementioned regimes were performed in a tilting water-recirculating flume. Flow measurements atop the spherical particles were performed by means of a 3-D Laser Doppler Velocimeter (LDV). The aim of the tests was to determine the contribution of various turbulent stress components to the initial entrainment of spherical particles. Analysis of the constructed joint frequency distributions of  and  reveals a significant variation in the magnitude and duration of sweeps (u>0, w<0), ejections (u<0, w>0), inward (u<0, w<0), and outward interactions (u>0, w>0) for each of the three roughness regimes considered here. Along the same lines, time series plots of the instantaneous normal and shear stresses suggest that the normal stress U² should be considered as the most dominant stress responsible for particle entrainment. This result is consistent with other reported turbulent measurements at flow conditions well above those corresponding to particle incipient conditions.

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Abstract

 1. Introduction

 2. Experimental Facility

 3. Methodology - Results

 4. Conclusions

 Acknowledgements

 References