Mechanics of particle entrainment from a rectangular cavity

Yee-Meng Chiew, Prakash Agarwal

Monday 29 june 2015

16:15 - 16:30h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 3A. Sediment - Erosion

Laboratory experiments are conducted in an open-channel flume to investigate the mechanics of particle entrainment from a rectangular cavity under various test conditions. The motivation of this study is the sediment entrainment from gaps between the large concrete blocks used in grade control structures, which are constructed to prevent pier-scour during typhoon-induced floods in Taiwan. Video recordings of particle movement in the cavity reveal that particle entrainments are significantly influenced by the random fluctuations of the flow field; with particles being entrained at widely varying time durations ranging from as short as 10 seconds to as long as 10 minutes and beyond, even under the same experimental condition. Particle trajectories inside the cavity are observed to be predominantly affected by the cavity aspect ratio and the initial release position of the particle. This is because the flow field undergoes significant changes with different cavity dimensions, causing the particle to be subjected to different flow structures depending on its position in the cavity. The presence of the rectangular cavity in the open channel leads to a sudden geometrical expansion, causing the 2-dimensional flow to transition into a 3-dimensional flow in the cavity. The new flow field affects both the lateral and vertical movements of the particle; and eddies that form at the downstream cavity edge are observed to be the agent primarily responsible for directly entraining the particle out of the cavity. In summary, the mechanics of particle entrainment from a rectangular cavity in an open channel flow involves interaction of the particle with the flow field and the random fluctuations in the flow. When a particle experiences a favorable, albeit sporadic sequence of events, entrainment of the particle occurs.