Mean flow and turbulence structure across the ripple-dune transition: An experiment under mobile bed conditions

Original research is presented that is designed to improve our understanding of flow-bed coupling across the ripple-dune transition. Flume experiments were conducted in order to monitor changes in flow and turbulence structure at seven stages across the transition over a mobile sand bed at three different flow heights. High spatial and temporal resolution three-dimensional velocity measurements were obtained using an acoustic Doppler velocimeter (ADV). Suspended sediment concentration was indirectly monitored using ADV signal amplitude.; Time-averaged turbulence statistics and quadrant analysis support the contention that shear-related low-magnitude, high-frequency vortex shedding dominates turbulence production across transition. Turbulence intensities and Reynolds stress increase over each stage until the formation of two-dimensional dunes. Thereafter, results indicate that shear layer related turbulence is dampened as topographically accelerated negative flowlines acting from the point of separation to reattachment become increasingly influential in suppressing vortex-shedding in the depth limited flow. This invokes a degree of flow-bed stability as dunes develop into three-dimensional forms. Activity at reattachment increases upon the onset of dampening as a result of high-velocity fluid reaching the bed and/or wake flapping under higher downstream flow velocity. Quadrant analysis reveals that quadrant 2 events dominate over quadrant 4 in terms of event frequency, but that the reverse is true in terms of magnitude. Strong relationships between activity in quadrant 1 and quadrant 4, and quadrant 2 and quadrant 3, suggest that mass and momentum exchanges are dependent upon interrelated activity in all four quadrants. Regression analysis shows that vertical velocity fluctuations invoke more sediment suspension than fluctuations in downstream velocity, and that Reynolds stress does not exhibit a strong relationship with suspended sediment concentration.; Ultimately, the results are synthesized with existing information to yield an improved model for the physical and fluid dynamic mechanisms that facilitate the ripple-dune transition.