68th Annual Meeting of the APS Division of Fluid Dynamics (November 22, 2015 — November 24, 2015)

V0011: Genesis & evolution of barchan dunes

Authors

• Ali Khosronejad, St. Anthony Falls Laboratory, University of Minnesota
• Dionysios Angelidis, St. Anthony Falls Laboratory, University of Minnesota
• David Porter, Minnesota Supercomputing Institute, University of Minnesota
• Xiaolei Yang, St. Anthony Falls Laboratory, University of Minnesota
• Fotis Sotiropoulos, St. Anthony Falls Laboratory, University of Minnesota

DOI: https://doi.org/10.1103/APS.DFD.2015.GFM.V0011

Barchans, crescent-shape patches of non-cohesive sediment material, are the most important signature of turbulent atmospheric and aquatic flow on earth surface where the supply of sediment is scarce. Previous attempts in the past decades to identify the mechanisms that lead to their initiation, growth and ultimately forming such unique geometrical form have constantly failed. Lack of sufficient spatial and temporal resolution in the past studies has been the most important contributor to this failure. We herein employ a highly advanced computational model (Khosronejad & Sotiropoulos, JFM, 2014) to investigate details of barchan field evolution: starting from a flatbed to their fully developed 3D crescentic shape under a uni-modal flow direction. Our computational model employs a fully coupled hydro-morphodynamics approach to link the fluid and sediment phases and take into account their interactions throughout the simulation.

Here in this video we show the mechanisms that lead to initiation of micro-scale crescentic features rising from an initial flatbed, development of 3D barchans, and equilibration of subaqueous barchan field. To do so, we utilize the a mega-database obtained from the high resolution coupled flow and morphodynamics simulation.  We elucidate to processes such as forming bed instabilities over mobile bed and merging of smaller-faster patches of sediment to the larger-slower ones to create crescent-shaped features. Our simulation results clearly illustrate migration and merging of individual barchans. Moreover, availability of thicker sediment material induces the formation of transverse ripples on the horns of barchans. These transverse ripples, influenced by energetic coherent turbulent structures, migrate over the horns and after reaching the end of the horn give birth to smaller-scale barchans through process known as calving.