An advanced understanding of particle entrainment is required to optimise the design and maintenance of numerous open channel hydraulic systems and structures placed in these systems; including river channels and canals. This study is on particle entrainment (defined as the movement of a particle from a stationary position to being mobile in the flow). Three aspects of particle entrainment were identified as the focus of this work: First, the waiting time for an exposed particle to entrain under constant flow conditions. Second, the flow features responsible for the entrainment of an individual exposed particle. Third, the motion of an entrained particle immediately after entrainment. Waiting time was found to be highly sensitive to protrusion, with a small increase in protrusion resulting in a significant decrease in waiting time. Contrary to previous suggestions the waiting time to entrainment was found to be poorly described by an exponential distribution; instead Weibull or gamma distributions provide an improved fit in both qualitative and quantitative terms. Ensemble averaged flow fields at the point of entrainment were computed to determine the features responsible for entrainment. The data from the transverse vertical PIV plane indicated the presence of two counter-rotating vortices, with the boundary between the vortices located directly over the entrainment particle. The streamwise vertical PIV measurements showed the presence of a structure extending for a considerable distance in the streamwise direction, the length of which appeared to be independent of submergence. Further, the inclination of the downstream end of the structure appeared to increase with submergence. From the point of entrainment particle dffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. From the point of entrainment particle diffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. The results highlight the clear difference in the local scale between the diffusion of an already mobile particle with one starting from a position of rest.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641478 |
| Date | January 2015 |
| Creators | Witz, Matthew J. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=225690 |
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