Direct Numerical Simulation of Transverse Ripples: 1. Pattern Initiation and Bedform Interactions

• Published in: Journal of geophysical research. Earth surface 2018-03, Vol.123 (3), p.448-477
• Main Authors: Zgheib, N., Fedele, J. J., Hoyal, D. C. J. D., Perillo, M. M., Balachandar, S.
• Format: Article
• Language: English
• Subjects: bedform interactions; Coarsening; Computational fluid dynamics; Computer simulation; coupled bed‐flow simulation; Decay rate; Defects; Deformation mechanisms; Direct numerical simulation; Dynamics; Evolution; Fluid flow; Formability; Interactions; Mathematical models; Numerical simulations; Open channels; pattern formation; Quantitative analysis; Reynolds number; ripples; Sedimentary structures; self‐similarity; Simulation; Structures; Turbulence
• ISSN: 2169-9003; 2169-9011
• DOI: 10.1002/2017JF004398

We present results of coupled direct numerical simulations between flow and a deformable bed in a horizontally periodic, turbulent open channel at a shear Reynolds number of Reτ = 180. The feedback between the temporally and spatially evolving bed and the flow is enforced via the immersed boundary method. Using the near‐bed flow field, we provide evidence on the role of locally intense near‐bed vortical structures during the early stages of bed formation, from the emergence of quasi‐streamwise streaks to the formation of incipient bedform crestlines. Additionally, we take a new look at a number of defect‐related bedform interactions, including lateral linking, defect and bedform repulsion, merging, and defect creation, and show that the underlying mechanisms, in these flow‐aligned interactions, are very similar to each other. Consequently, the interactions are labeled differently depending on the geometry of interacting structures and the outcome of the interaction. In the companion paper, we compare our results to published experimental data and provide an extensive quantitative analysis of the bed, where we demonstrate the importance of neighboring structures, especially upstream neighbors, on bedform dynamics (growth/decay and speed) and wave coarsening. Video files of bed evolution are available in the supporting information. Key Points Mesoscale resolved simulations show the different mechanisms for bedform‐bedform interactions to be very similar to each other Similar to laminar flows over dunes and ripples, a positive phase shift is observed between bed shear stress and topology even in mesoscale‐resolved turbulent flow field Simulations match Coleman and Melville (1996) theory on bedform initiation from a flat bed