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Computing Flow and Sediment Transport over Bedforms

Computing Flow and Sediment Transport over Bedforms. Jonathan Nelson Aaron Burman Yasuyuki Shimizu Steve McLean Ron Shreve Mark Schmeeckle. Department of the Interior U.S. Geological Survey. Objective.

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Computing Flow and Sediment Transport over Bedforms

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  1. Computing Flow and Sediment Transport over Bedforms Jonathan Nelson Aaron Burman Yasuyuki Shimizu Steve McLean Ron Shreve Mark Schmeeckle Department of the Interior U.S. Geological Survey

  2. Objective • A practical computational morphodynamics model for predicting the behavior of bedforms • Predict geometry of bedforms accurately • Predict pressure with sufficient accuracy to determine form drag • Suitable for initiation, development to equilibrium states in steady flows • Suitable for predicting adjustment in time-varying flows

  3. Morphodynamic Modeling • Flow Model • Sediment-transport Model • Exner equation for morphologic change

  4. Flow Model • Large-eddy simulation, solves the Navier Stokes equations with subgrid-scale closure,CIP technique • Unsteady two-dimensional model with nonlinear k-epsilon closure

  5. Instantaneous streamwise velocity at the bed Both graphs are vertically exaggerated (real scale: dunes are 4cm tall, 90 cm wide and 80cm long) Time-averaged and vertically-averaged streamwise velocity (50 sec)

  6. Large-eddy Model • Works well for two-d dunes for both velocity and pressure • Works almost as well more complex three-d dunes • Takes too long for morphodynamics modeling of realistic geometry Need to use a closure model

  7. Sediment-transport Model • Direct simulation of particle motion using model developed by Schmeeckle • Pickup/deposit functions from Nakagawa and Tsujimoto

  8. Flow and sediment transport downstream of a backward step

  9. Development of bedforms from an initially perturbed bed- Sinusoidal perturbation,10-cm wavelength, 1-cm height

  10. Conclusions • Large-eddy simulation models are the future of computational morphodynamics, but we’re not quite there in terms of computational speed. • Simpler closure type models are not as accurate, but appear to be accurate enough for bedform morphodynamics. • Experiments and numerical results strongly suggest that local flow variability is critically important for predicting transport in complex flows. • Next step is temporal response to varying flows, bar/bedform interactions, intermediate features.

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