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Experimental and numerical investigation of the coupling of turbulence and sediment transport over dunes

Collaborators: Dr. Mark Schmeeckle (Arizona State University)

June 2016-Present

Project Summary: We investigate the spatiotemporal coupling of sediment transport over dunes using a turbulence- and particle-resolving numerical model and high-speed video in a laboratory flume. The model utilizes the Large Eddy Simulation (LES) for the fluid turbulence and a Discrete Element Method (DEM) simulation for the sediment. Previous experiments assessing the affects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitutde transport events, called permeable splat events, play an important role in both downstream and cross-stream transport near flow reattachment. The flume was lined with 17 concrete ripples that had a 2 cm high crest and were 30 cm long.  A high-speed camera observed sediment transport along the entirety of the bedform at 250 Hz. Downstream and vertical fluid velocity was observed at 1mm and 3 mm above the bed using Laser Doppler Velocitmetry (LDV) at 15 distances along bedform profile. As observed in our previous backward-facing step experiments and simulations, mean downstream fluid velocity increases nonlinearly with increasing distance along the ripple. Observed sediment transport, however, increases linearly with increasing distance along the ripple with an exception at the crest of the bedform, where both mean downstream fluid velocity and sediment transport decrease significantly. Previous experiments assessing only the affect of flow separation showed that calculating sediment transport as a function of boundary shear stress  using a Meyer-Peter Müller type equation, produced a zone of underestimated transport near flow reattachment. Results reported here show that calculating sediment transport in this way underestimates observed sediment transport along the entire profile of the bedform, not just near flow reattachment. Sediment transport time-series data show a zone of high-magnitude cross-stream transport near flow reattachment. Manual particle tracking data illustrate that near flow reattachment, splat events are present and responsible for large cross-stream transport events. These combined data suggest that permeable splat events are still playing an important role in the pattern of sediment transport along bedforms.

The Importance of Splat Events on the Spatiotemporal Pattern of Bedload Transport over Bedforms: Laboratory Experiments Downstream of a Backward-Facing Step

Published at JGR: Earth Surface: Leary_Schmeeckle_2017

Collaborators: Dr. Mark Schmeeckle (Arizona State University)

May 2014-2017

Project Summary: Despite numerous experimental and numerical studies investigating transport over bedforms in rivers, the spatiotemporal details of the pattern of transport over bedforms remain largely unknown. Here we report turbulence-resolving, simultaneous measurements of bedload motion and near-bed fluid velocity downstream of a backward facing step in a laboratory flume. Two synchronized high-speed video cameras simultaneously observed bedload motion and the motion of neutrally buoyant particles in a laser light sheet 6 mm above the bed at 250 frames/s downstream of a 3.8 cm backward-facing step. Particle imaging velocimetry algorithms were applied to the laser sheet images to obtain two-dimensional field of two-dimensional vectors while manual particle tracking techniques were applied to the video images of the bed. As expected, the experiments exhibit a strong positive correlation between sediment flux and near-bed fluid velocity. Experimentally observed sediment transport is compared to sediment transport modeled as a function of boundary shear stress using a Meyer-Peter Müller type equation. Modeled sediment transport underestimates observed sediment transport near flow reattachment. Localized, intermittent, high-magnitude transport events are more apparent near flow reattachment than farther downstream. These events are composed of downstream and cross-stream sediment transport of comparable magnitudes. Transport pattern and fluid velocity data are consistent with the existence permeable “splat events”, wherein a volume of fluid moves toward and impinges on the bed. The substantial effects of splat events on transport over bedforms cannot be modeled using simple bedload transport equations and must be included in future models of bedform evolution.

Findings from our 2014 backstep experiments are in review at the Journal of Geophysical Research: Earth Surface.

All images and text on this site © 2016 Kate Leary