The motion of fine sand particles in turbulent open channel shear flows over porous bed conditions
The current study aims to investigate the physical mechanisms controlling fine sediment transport within open channel shear flows over porous beds, with particular emphasis on the role of flow turbulence in particle settling and deposition processes. Preliminary visualisation experiments used a VHS camera to observe the near-bed motion of sand particles and their behaviour within the surface layer of a rhombically-packed bed of uniform spheres. Measurement of near-bed particle trajectories indicate that turbulent particle fall velocities w's are generally larger than fall velocities measured in still water ws, most notably for finer sand grades. Distinctive modes of particle behaviour observed at the bed interface also suggest that flow-separation eddies, generated within surface interstices, have a primary influence on subsequent particle motion, i.e. deposition or re-entrainment. Similar particle behaviour is also displayed in a natural gravel bed. A more detailed analysis of sand particle motion in turbulent open channel flow was carried out employing a high-speed camera and particle-tracking technique to record and analyse particle trajectories within different flow regions. The non-dimensional ratio of measured particle fall velocity w's and still water fall velocity ws was used to indicate the relative enhancement of vertical particle motion within the turbulent flow conditions. Experiment-averaged values of this ratio reveal that particle fall velocities are generally enhanced (i.e. w's/ws > 1) in recorded near-bed and intermediate flow regions (z/H 0.5) and hindered (i.e. w's/ws < 1) in a recorded outer flow region (z/H 0.5). The ratio w's/ws also reveals a general tendency to increase with decreasing grain size di. Vertical profiles of the normalised particle fall velocity w's/u* are shown to be analogous to turbulence intensity distributions (u' rms/u* and w'rms/u*), with the highest values of w's/u* occurring in the near-bed region and coinciding approximately with the regions of highest turbulence activity. This clearly implies the existence of turbulence-enhanced particle fall velocities within the flow conditions considered. Application of a quadrant analysis technique reinforces this notion, revealing further similarities between conditioned turbulent fluid fluctuations and particle motions, in particle, the dominance of 'inrush' events (quadrant 4) in the near-bed flow and 'ejection' events (quadrant 2) away from the bed.