Personnel: Prof. Ugo Piomelli, Mr. Mohammad Omidyeganeh
When a fluid flows over a mobile sand bed the sediment transport generated by the interaction of the flow field with the bed often results in the periodic deformation of the bed in the form of dunes. Small irregularities on a flat bed tend to grow until a stable bed form is established. Once the equilibrium shape is reached, dunes slowly migrate in the direction of the flow, as sand is lifted in the high-shear regions, and redeposited in the separated-flow area.
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| Low-pressure isosurfaces visualize the coherent vortices over a dune. |
The accurate prediction of bed deformation is important: dune formation may affect navigation, erosion of bridge piles and other structures, as well as dispersion of contaminants, and requires accurate modeling of the sediment flux and fluid dynamics computation Our aim is to attempt to connect the flow-field characteristics with the bed deformation.
First, we aim to understand the flow characteristics for a typical dune geometry and the sensitivity of the fluid dynamics to geometric parameters such as dune height and flow depth, and second apply a reliable bed deformation model to trace the bed forms.
We have performed LES of flow over a typical dune geometry where the ratios of the wavelength and average flow depth to the dune height are 20 and 3.5 respectively. The Reynolds number based on the average flow depth and mean velocity is 18,900. Two other simulations have been done with dune heights half and double the typical height while the Reynolds number is kept constant. The flow separates at the crest and reattaches on the bed farther downstream, as a new boundary layer develops along the upward slope. In the separated shear layer spanwise vortices are generated (perhaps by a Helmholtz-like instability) that convect downstream. These structures weaken faster for smaller dune heights. Horseshoe vortices exist in the separation zone.
