https://orcid.org/0000-0003-2684-1070
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ABSTRACT
The present paper concerns an experimental study of externally generated turbulence and its impact on the bed load sediment transport in an open channel flow. The external turbulence was generated by vertically oscillating a submerged grid, hereafter referred to as turbulence generator, in the flume with a clearance of 11 cm from the bed. Two kinds of experiments were performed: rigid flat-bed and plane sand-bed. One two-dimensional (2-D) LDA was set up to measure the velocity profiles for different values of flow discharge, amplitude and period of oscillations of the turbulence generator. The mean velocities, turbulence intensities, Reynolds shear stress, and their impacts on the rate of sediment transport have been analyzed. The fractional contributions of burst-sweep cycles to the Reynolds shear stress have also been studied. The turbulence generated due to a vertically oscillating-submerged grid is an analogous to the production of turbulence beneath the roller located on the front of broken wave, and the impact thus lies on the sediment transport. The Shields parameter was found to be correlated with the sediment transport rate, and the transport rate as a function of turbulence level is found to increase with turbulence level.
Acknowledgments
One of the authors (BSM) would like to express his sincere thanks to Professors Jorgen Fredsoe and B. M. Sumer for their financial support to carry out this research at the Technical University of Denmark, MEK, Coastal and River Engineering Section. The first author (SPO) contributed in this study while he was working as a research fellow at Indian Statistical Institute, Kolkata, India. We express our sincere thanks to Professor Sumer for his important contributions through discussion during the preparation of the paper. We must acknowledge Dr. Krishnendu Barman for his help in drawing the quadrant analysis figures. We also acknowledge the anonymous reviewers and Editor for their constructive comments and suggestions to improve the paper.
Disclosure statement
No potential conflict of interest was reported by the authors.
Notations
| A = | = | Cross-sectional area; |
| a = | = | amplitude of turbulence generator; |
| d50 = | = | median diameter of sediment particles; |
| e = | = | clearance from the edge of turbulence generator to the bed; |
| g = | = | acceleration due to gravity; |
| h = | = | flow depth; |
| H = | = | threshold level (hole size); |
| M = | = | line indicates Meyer-Peter and Muller (Citation1948); |
| Q = | = | flow rate; |
| q = | = | measured bed load sediment transport rate (sediment trap method); |
| qu = | = | measured bed load sediment transport rate for undisturbed condition (sediment trap method, plane-bed experiments only); |
| Rf = | = | grain Reynolds number (Rf = u*d50/ν); |
| s = | = | relative density; |
| SiH = | = | stress fraction at ith quadrant; |
| T = | = | wave period of turbulence generator; |
| Ti = | = | test runs for i = 1, 2, 3,….17. |
| u, v, w = | = | instantaneous velocity components; |
| = | mean velocity components | |
| (u΄, v΄, w΄) = | = | fluctuating components of velocity; |
| u* = | = | friction velocity; |
| u+ = | = | dimensionless velocity; |
| y = | = | distance from the bed; |
| y+ = | = | dimensionless distance from the bed; |
| θb = | = | Shields parameter corresponding to bed friction velocity; |
| ν = | = | kinematic viscosity; |
| ρ = | = | density of fluid; |
| σg = | = | geometric standard deviation (= |
| τ = | = | instantaneous shear stress; |
| τ΄ = | = | fluctuating shear stress; |
| = | dimensionless bed load discharge. |